green synthesis of nanoparticles using plant extracts research paper

DOI: 10.1007/s10311-020-01074-x
Corpus ID: 221110996

Green synthesis of nanoparticles using plant extracts: a review

Sapana Jadoun , Rizwan Arif , +1 author R. Meena
Published in Environmental Chemistry… 13 August 2020
Environmental Science, Chemistry

507 Citations

Methods for green synthesis of metallic nanoparticles using plant extracts and their biological applications - a review, green synthesis of metal nanoparticles, green synthesis of platinum nanoparticles for biomedical applications.

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Green Synthesis of Nanoparticles Using Different Plant Extracts and Their Characterizations

First Online: 23 July 2021

Cite this chapter

green synthesis of nanoparticles using plant extracts research paper

Lina M. Alnaddaf 4 ,
Abdulsalam K. Almuhammady 5 ,
Khaled F. M. Salem 6 nAff7 ,
Maysaa T. Alloosh 8 ,
Maysoun M. Saleh 9 &
Jameel M. Al-Khayri 10

1361 Accesses

5 Citations

The green nanoparticles synthesis is a modern field that currently resonates compared to other preparation methods due to its characteristics that make it used in all fields. This chapter briefly explained traditional and biological methods for preparing nanomaterials and mentioned the advantage and disadvantage to these methods, then explained in more detail the phytofabrication of nanoparticles from different parts of the plant, which are considered a good source for biological molecules that act as reducing agents and modifies metal ions into nanoparticles that have unique properties. It also illustrates the green methods for preparing nanoparticles such as silver, zinc oxide and copper in some detail and their reaction conditions which influence the size, shape and structure of NPs. In addition to mechanisms of their formation and the different biomolecules that contribute to its synthesis.

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Green synthesis of nanoparticles using plant extracts: a review

Green Synthesis of Nanoparticles: An Emerging Phytotechnology

Plant-Mediated Green Synthesis of Nanoparticles

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Khaled F. M. Salem

Present address: Department of Biology, College of Science and Humanitarian Studies, Shaqra University, Qwaieah, Saudi Arabia

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Biotechnology and Molecular Biology, Faculty of Agriculture, Al-Baath University, Homs, Syria

Lina M. Alnaddaf

Arab Center for Nanotechnology, Cairo, Egypt

Abdulsalam K. Almuhammady

Department of Plant Biotechnology, Genetic Engineering and Biotechnology Research Institute (GEBRI), University of Sadat City, Sadat City, Egypt

Faculty of Pharmacy, Department of Biochemistry and Microbiology, Al-Baath University, Homs, Syria

Maysaa T. Alloosh

Genetic Resources Department, General Commission for Scientific Agricultural Research (GCSAR), Damascus, Syria

Maysoun M. Saleh

Department of Agricultural Biotechnology, College of Agriculture and Food Sciences, King Faisal University, Al-Ahsa, Saudi Arabia

Jameel M. Al-Khayri

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Alnaddaf, L.M., Almuhammady, A.K., Salem, K.F.M., Alloosh, M.T., Saleh, M.M., Al-Khayri, J.M. (2021). Green Synthesis of Nanoparticles Using Different Plant Extracts and Their Characterizations. In: Al-Khayri, J.M., Ansari, M.I., Singh, A.K. (eds) Nanobiotechnology . Springer, Cham. https://doi.org/10.1007/978-3-030-73606-4_8

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Green Synthesis of Nanomaterials Using Plant Extract: A Review

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1 Department of Computer Science and Electronics, Guru Nanak College of Science, Ballarpur, Chandrapur - 442701, India.
2 Department of Physics, R.T.M., Nagpur University, Nagpur - 44003, India.
3 Institute of Optoelectronics, Military University of Technology, Kaliskiego 2 Str., 00-908 Warsaw, Poland.
PMID: 33208069
DOI: 10.2174/1389201021666201117121452

For the last two decades, extensive research is conducted on metal and metal oxide nanoparticles and their application in the field of medical, cosmetics, catalysts, packaging, photonics, agriculture and electronics. However, these nanoparticles show toxicity to the environmental, human and animal health. The toxicity effects of nanoparticles are mainly due to their size, which can easily pass through physiological barriers and also due to the synthesis procedure. The toxicity due to their size cannot be avoided, but toxicity due to the synthesis process can be nullified by adopting the biosynthesis process. Bacteria, fungus, fish scales, plant extracts and algae are used to synthesize metal and metal- oxide nanoparticles such as silver, gold, iron-oxide, zinc-oxide, zirconia, etc. For the last few years, researchers have been working on synthesis methods of plant extracts to produce stable, cost-effective and economical nanoparticles. In this review, we focus on the biosynthesis of nanoparticles using different parts of plant extracts. The review contains a summary of selected papers from 2018-20 with a detailed description of the process of synthesis, mechanism, characterization and their application in various fields of biosynthesized metal and metal oxide nanoparticles.

Keywords: Green synthesis; antibacterial applications and photocatalytic applications.; biosynthesis; metal nanoparticles; metal-oxide nanoparticles; nanoparticles characterizations.

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Published: 30 October 2018

‘Green’ synthesis of metals and their oxide nanoparticles: applications for environmental remediation

Jagpreet Singh 1 ,
Tanushree Dutta 2 ,
Ki-Hyun Kim 3 ,
Mohit Rawat 1 ,
Pallabi Samddar 3 &
Pawan Kumar ORCID: orcid.org/0000-0003-0712-8763 4

Journal of Nanobiotechnology volume 16 , Article number: 84 ( 2018 ) Cite this article

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In materials science, “green” synthesis has gained extensive attention as a reliable, sustainable, and eco-friendly protocol for synthesizing a wide range of materials/nanomaterials including metal/metal oxides nanomaterials, hybrid materials, and bioinspired materials. As such, green synthesis is regarded as an important tool to reduce the destructive effects associated with the traditional methods of synthesis for nanoparticles commonly utilized in laboratory and industry. In this review, we summarized the fundamental processes and mechanisms of “green” synthesis approaches, especially for metal and metal oxide [e.g., gold (Au), silver (Ag), copper oxide (CuO), and zinc oxide (ZnO)] nanoparticles using natural extracts. Importantly, we explored the role of biological components, essential phytochemicals (e.g., flavonoids, alkaloids, terpenoids, amides, and aldehydes) as reducing agents and solvent systems. The stability/toxicity of nanoparticles and the associated surface engineering techniques for achieving biocompatibility are also discussed. Finally, we covered applications of such synthesized products to environmental remediation in terms of antimicrobial activity, catalytic activity, removal of pollutants dyes, and heavy metal ion sensing.

Introduction

Over the last decade, novel synthesis approaches/methods for nanomaterials (such as metal nanoparticles, quantum dots (QDs), carbon nanotubes (CNTs), graphene, and their composites) have been an interesting area in nanoscience and technology [ 1 , 2 , 3 , 4 , 5 , 6 , 7 , 8 , 9 ]. To obtain nanomaterials of desired sizes, shape, and functionalities, two different fundamental principles of synthesis (i.e., top down and bottom up methods) have been investigated in the existing literature (Fig. 1 ). In the former, nanomaterials/nanoparticles are prepared through diverse range of synthesis approaches like lithographic techniques, ball milling, etching, and sputtering [ 10 ]. The use of a bottom up approach (in which nanoparticles are grown from simpler molecules) also includes many methods like chemical vapor deposition, sol–gel processes, spray pyrolysis, laser pyrolysis, and atomic/molecular condensation.

Different synthesis approaches available for the preparation of metal nanoparticles

Interestingly, the morphological parameters of nanoparticles (e.g., size and shape) can be modulated by varying the concentrations of chemicals and reaction conditions (e.g., temperature and pH). Nevertheless, if these synthesized nanomaterials are subject to the actual/specific applications, then they can suffer from the following limitation or challenges: (i) stability in hostile environment, (ii) lack of understanding in fundamental mechanism and modeling factors, (iii) bioaccumulation/toxicity features, (iv) expansive analysis requirements, (v) need for skilled operators, (vi) problem in devices assembling and structures, and (vii) recycle/reuse/regeneration. In true world, it is desirable that the properties, behavior, and types of nanomaterials should be improved to meet the aforementioned points. On the other hand, these limitations are opening new and great opportunities in this emerging field of research.

To counter those limitations, a new era of ‘green synthesis’ approaches/methods is gaining great attention in current research and development on materials science and technology. Basically, green synthesis of materials/nanomaterials, produced through regulation, control, clean up, and remediation process, will directly help uplift their environmental friendliness. Some basic principles of “green synthesis” can thus be explained by several components like prevention/minimization of waste, reduction of derivatives/pollution, and the use of safer (or non-toxic) solvent/auxiliaries as well as renewable feedstock.

‘Green synthesis’ are required to avoid the production of unwanted or harmful by-products through the build-up of reliable, sustainable, and eco-friendly synthesis procedures. The use of ideal solvent systems and natural resources (such as organic systems) is essential to achieve this goal. Green synthesis of metallic nanoparticles has been adopted to accommodate various biological materials (e.g., bacteria, fungi, algae, and plant extracts). Among the available green methods of synthesis for metal/metal oxide nanoparticles, utilization of plant extracts is a rather simple and easy process to produce nanoparticles at large scale relative to bacteria and/or fungi mediated synthesis. These products are known collectively as biogenic nanoparticles (Fig. 2 ).

Key merits of green synthesis methods

Green synthesis methodologies based on biological precursors depend on various reaction parameters such as solvent, temperature, pressure, and pH conditions (acidic, basic, or neutral). For the synthesis of metal/metal oxide nanoparticles, plant biodiversity has been broadly considered due to the availability of effective phytochemicals in various plant extracts, especially in leaves such as ketones, aldehydes, flavones, amides, terpenoids, carboxylic acids, phenols, and ascorbic acids. These components are capable of reducing metal salts into metal nanoparticles [ 11 ]. The basic features of such nanomaterials have been investigated for use in biomedical diagnostics, antimicrobials, catalysis, molecular sensing, optical imaging, and labelling of biological systems [ 12 ].

Here, we summarized the current state of research on the green synthesis of metal/metal oxide nanoparticles with their advantages over chemical synthesis methods. In addition, we also discussed the role of solvent systems (synthetic materials), various biological (natural extracts) components (like bacteria, algae, fungi, and plant extracts) with their advantages over other conventional components/solvents. The main aim of this literature study is to provide detailed mechanisms for green synthesis and their real world environmental remediation applications. Overall, our goal is to systematically describe “green” synthesis procedures and their related components that will benefit researchers involved in this emerging field while serving as a useful guide for readers with a general interest in this topic.

Biological components for “green” synthesis

Innumerable physical and chemical synthesis approaches require high radiation, highly toxic reductants, and stabilizing agents, which can cause pernicious effects to both humans and marine life. In contrast, green synthesis of metallic nanoparticles is a one pot or single step eco-friendly bio-reduction method that requires relatively low energy to initiate the reaction. This reduction method is also cost efficient [ 13 , 14 , 15 , 16 , 17 , 18 , 19 ].

Bacterial species have been widely utilized for commercial biotechnological applications such as bioremediation, genetic engineering, and bioleaching [ 20 ]. Bacteria possess the ability to reduce metal ions and are momentous candidates in nanoparticles preparation [ 21 ]. For the preparation of metallic and other novel nanoparticles, a variety of bacterial species are utilized. Prokaryotic bacteria and actinomycetes have been broadly employed for synthesizing metal/metal oxide nanoparticles.

The bacterial synthesis of nanoparticles has been adopted due to the relative ease of manipulating the bacteria [ 22 ]. Some examples of bacterial strains that have been extensively exploited for the synthesis of bioreduced silver nanoparticles with distinct size/shape morphologies include: Escherichia coli , Lactobacillus casei , Bacillus cereus , Aeromonas sp. SH10 Phaeocystis antarctica , Pseudomonas proteolytica , Bacillus amyloliquefaciens , Bacillus indicus , Bacillus cecembensis , Enterobacter cloacae , Geobacter spp., Arthrobacter gangotriensis , Corynebacterium sp. SH09, and Shewanella oneidensis . Likewise, for the preparation of gold nanoparticles, several bacterial species (such as Bacillus megaterium D01, Desulfovibrio desulfuricans , E. coli DH5a, Bacillus subtilis 168, Shewanella alga , Rhodopseudomonas capsulate , and Plectonema boryanum UTEX 485) have been extensively used. Information on the size, morphology, and applications of various nanoparticles is summarized in Table 1 .

Fungi-mediated biosynthesis of metal/metal oxide nanoparticles is also a very efficient process for the generation of monodispersed nanoparticles with well-defined morphologies. They act as better biological agents for the preparation of metal and metal oxide nanoparticles, due to the presence of a variety of intracellular enzyme [ 23 ]. Competent fungi can synthesize larger amounts of nanoparticles compared to bacteria [ 24 ]. Moreover, fungi have many merits over other organisms due to the presence of enzymes/proteins/reducing components on their cell surfaces [ 25 ]. The probable mechanism for the formation of the metallic nanoparticles is enzymatic reduction (reductase) in the cell wall or inside the fungal cell. Many fungal species are used to synthesize metal/metal oxide nanoparticles like silver, gold, titanium dioxide and zinc oxide, as discussed in Table 1 .

Yeasts are single-celled microorganisms present in eukaryotic cells. A total of 1500 yeast species have been identified [ 26 ]. Successful synthesis of nanoparticles/nanomaterials via yeast has been reported by numerous research groups. The biosynthesis of silver and gold nanoparticles by a silver-tolerant yeast strain and Saccharomyces cerevisiae broth has been reported. Many diverse species are employed for the preparation of innumerable metallic nanoparticles, as discussed in Table 1 .

Plants have the potential to accumulate certain amounts of heavy metals in their diverse parts. Consequently, biosynthesis techniques employing plant extracts have gained increased consideration as a simple, efficient, cost effective and feasible methods as well as an excellent alternative means to conventional preparation methods for nanoparticle production. There are various plants that can be utilized to reduce and stabilize the metallic nanoparticles in “one-pot” synthesis process. Many researchers have employed green synthesis process for preparation of metal/metal oxide nanoparticles via plant leaf extracts to further explore their various applications.

Plants have biomolecules (like carbohydrates, proteins, and coenzyme) with exemplary potential to reduce metal salt into nanoparticles. Like other biosynthesis processes, gold and silver metal nanoparticles were first investigated in plant extract-assisted synthesis. Various plants [including aloe vera ( Aloe barbadensis Miller), Oat ( Avena sativa ), alfalfa ( Medicago sativa ), Tulsi ( Osimum sanctum ), Lemon ( Citrus limon ), Neem ( Azadirachta indica ), Coriander ( Coriandrum sativum ), Mustard ( Brassica juncea ) and lemon grass ( Cymbopogon flexuosus )] have been utilized to synthesize silver nanoparticles and gold nanoparticles, as listed in Table 2 . The major part of this type of research has explored the ex vivo synthesis of nanoparticles, while metallic nanoparticles can be formed in living plants (in vivo) by reducing metal salt ions absorbed as soluble salts. The in vivo synthesis of nanoparticles like zinc, nickel, cobalt, and copper was also observed in mustard ( Brassica juncea ), alfalfa ( Medicago sativa ), and sunflower ( Helianthus annuus ) [ 27 ]. Also, ZnO nanoparticles have been prepared with a great variety of plant leaf extracts such as coriander ( Coriandrum sativum ) [ 28 ], crown flower ( Calotropis gigantean ) [ 29 ], copper leaf ( Acalypha indica ) [ 30 ], China rose ( Hibiscus rosa - sinensis ) [ 31 ], Green Tea ( Camellia sinensis ) [ 32 ], and aloe leaf broth extract ( Aloe barbadensis Miller) [ 33 ]. Readers can refer to the work of Iravani [ 34 ] for a comprehensive overview of plant materials utilized for the biosynthesis of nanoparticles.

Solvent system-based “green” synthesis

Solvent systems are a fundamental component in the synthesis process, whether it is “green” synthesis or not. Water is always considered an ideal and suitable solvent system for synthesis processes. According to Sheldon, “the best solvent is no solvent, and if a solvent is desirable then water is ideal” [ 35 ]. Water is the cheapest and most commonly accessible solvent on earth. Since the advent of nanoscience and nanotechnology, the use of water as a solvent for the synthesis of various nanoparticles has been carried out. For instance, synthesized Au and Ag nanoparticles at room temperature using gallic acid, a bifunctional molecule, in an aqueous medium [ 36 ]. Gold nanoparticles were produced via a laser ablation technique in an aqueous solution. The oxygen present in the water leads to partial oxidation of the synthesized gold nanoparticles, which finally enhanced its chemical reactivity and had a great impact on its growth [ 37 ].

In the literature, “green” synthesis consists of two major routes:

Wherein water is used as a solvent system.

Wherein a natural source/extract is utilized as the main component.

Both of these routes have been covered in the coming section according to the present literature. Hopefully, our efforts will help researchers gain a better knowledge of ‘green’ synthesis methods, the role of toxic/non-toxic solvents (or components), and renewable resources derived from natural sources. Ionic and supercritical liquids are one of the best examples in this emerging area. Ionic liquids (ILs) are composed of ions that have melting points below 100 °C. Ionic liquids are also acknowledged as “room temperature ionic liquids.” Several metal nanoparticles (e.g., Au, Ag, Al, Te, Ru, Ir, and Pt) have been synthesized in ionic liquids [ 38 , 39 , 40 , 41 ]. The process of nanoparticle synthesis is simplified since the ionic liquid can serve as both a reductant and a protective agent.

ILs can be hydrophilic or hydrophobic depending on the nature of the cations and anions. For example, 1-butyl-3-methyl imidazolium (Bmim) hexafluorophosphate (PF6) is hydrophobic, whereas its tetrafluoroborate (BF4) analogue is hydrophilic. Since both species are ionic in nature, they can act as catalysts [ 40 , 42 , 43 , 44 , 45 ]. Bussamara et al. have performed a comparative study by controlling the synthesis of manganese oxide (Mn 3 O 4 ) nanoparticles using imidazolium ionic liquids and oleylamine (a conventional solvent). They found that smaller sized nanoparticles (9.9 ± 1.8 nm) were formed with better dispersity in ionic liquids than in the oleylamine solvent (12.1 ± 3.0 nm) [ 46 ]. Lazarus et al. synthesized silver nanoparticles in an ionic liquid (BmimBF4). The synthesized nanoparticles were in both smaller isotropic spherical and large-sized anisotropic hexagonal shaped forms [ 47 ]. An electrochemical method was employed for this purpose [ 48 ]. Ionic liquid was used in the electrolytic reaction as a substitute for water without mechanical stirring. For the first time, Kim et al. developed a one-phase preparation technique for gold (Au) and platinum (Pt) nanoparticles by means of thiol-functionalized ionic liquids (TFILs). TFILs acted as a stabilizing agent to produce crystalline structures with small sizes [ 49 ]. Dupont et al. used 1-n-butyl-3-methylimidazolium hexafluorophosphate (which is room temperature ionic liquid) for synthesizing Ir(0) nanoparticles by Ir(I) reduction. The average size of synthesized nanoparticles was ~ 2 nm. Interestingly, the ionic liquid medium is impeccable for the production of recyclable biphasic catalytic systems for hydrogenation reactions [ 50 ].

The benefits of using ionic liquids instead of other solvents include the following. (a) Many metal catalysts, polar organic compounds, and gases are easily dissolved in ILs to support biocatalysts. (b) ILs have constructive thermal stabilities to operate in a broad temperature range. Most of these melt below room temperature and begin to decompose above 300 or 400 °C. As such, they allow a broader synthesis temperature range (e.g., three to four times) than that of water. (c) The solubility properties of IL can be modulated by modifying the cations and anions associated with them. (d) Unlike other polar solvents or alcohols, ILs are non-coordinating. However, they have polarities comparable to alcohol. (e) ILs do not evaporate into the environment like volatile solvents because they have no vapor pressure. (f) ILs have dual functionality because they have both cations and anions. The problems associated with the biodegradability of ionic liquids make them not acceptable for synthesis of metallic nanoparticles. To diminish these non-biodegradability issues, many new potentially benign ionic liquids are being developed with maximum biodegradation efficiency [ 51 , 52 , 53 , 54 ]. The innumerable ILs are used to synthesize various metallic nanoparticles as listed in Table 3 .

Likewise, ordinary solvents can be converted into super critical fluids at temperatures and pressures above critical point. In the supercritical state, solvent properties such as density, thermal conductivity, and viscosity are significantly altered. Carbon dioxide is the most feasible super critical, non-hazardous, and inert fluid [ 55 , 56 ]. Also, supercritical water can serve as a good solvent system for several reactions. As, water has critical temperature of 646 K and pressure of 22.1 MPa [ 57 ]. Silver and copper NPs can be synthesized in supercritical carbon dioxide [ 58 ]. Sue et al. suggested that decreasing the solubility of metal oxides around the critical point can lead to super saturation and the ultimate formation of nanoparticles [ 59 ]. Kim et al. synthesized tungsten oxide (WO 3 ) and tungsten blue oxide nanoparticles by using sub- and supercritical water and methanol [ 60 ].

Stability and toxicity of the nanoparticles

The environmental distribution and transport of released nanoparticles depend on their ability to make metastable aqueous suspensions or aerosols in environmental fluids. The stability of the nanoparticles in the environment can therefore be evaluated by estimating their propensity to aggregate or interact with the surrounding media. Aggregation is a time-dependent phenomena associated with the rate of particle collision while the stability of the suspension is largely determined by the size of the particles and affinity toward other environmental constituents. The “green” synthesis of AgNPs from tea leaf extraction was found to be stable after entering the aquatic environment [ 61 ]. Likewise, the stability of AgNPs (in aqueous medium) manufactured using plant extracts and plant metabolites was confirmed from the resulting material [ 62 ]. Surface complexation is also reported to affect the intrinsic stability of nanoparticles by regulating its colloidal stability. The nature and stability of nanoparticles were theoretically predicted through a mechanistic understanding of the surface complexation processes [ 63 ]. The colloidal stability (or rate of dissolution) of nanoparticles can be regulated by controlling the particle size and surface capping or through functionalization techniques [ 64 , 65 ]).

Transformation of nanoparticles is an essential property to consider when assessing their environmental impact or toxicity. For instance, sulfurization of AgNPs greatly reduced their toxicity due to the lower solubility of silver sulfide [ 66 ]. For similar reasons, the use of biocompatible stabilizing agents (e.g., biodegradable polymers and copolymers) have opened up a “greener” avenue of nanomaterial surface engineering. Such techniques can impart remarkable stability, e.g., in situ synthesis of AuNPs capped with Korean red ginseng root [ 67 ]. Apart from surface chemistry, other key structural features determining the nanomaterial toxicity are the size, shape, and composition of the nanomaterials [ 68 ]. Toxicity analysis of AgNP synthesized using plant leaf extracts showed enhanced seed germination rates in the AgNP chemical treatment for activation than the corresponding control treatments [ 69 ]. However, the mechanism of such rate enhancement effects was not reported.

Mechanism of “green” synthesis for metals and their oxide nanoparticles

Microorganism-based mechanism.

There are different mechanisms for the formation of nanoparticles using different microorganisms. First, metallic ions are captured on the surface or inside the microbial cells, and then these arrested metal ions are reduced into metal nanoparticles by the action of enzymes. Sneha et al. [ 70 ] described the mechanism of microorganism-assisted silver and gold nanoparticles formed via Verticillium sp. or algal biomass based on the following hypothesis. (a) First, the silver or gold ions were captured on the surface of fungal cells via electrostatic interactions between ions and negatively charged cell wall enzymes. (b) Then, silver or gold ions were bioreduced into silver or gold nuclei, which subsequently grew. The two key aspects in the biosynthesis of nanoparticles are NADH (nicotinamide adenine dinucleotide) and NADH-dependent nitrate reductase. Kalishwaralal et al. [ 71 ] demonstrated that the nitrate reductase was responsible for the production of bioreduced silver nanoparticles by B. licheniformis . Nonetheless, the bioreduction mechanisms associated with the production of metal salt ions and the resulting metallic nanoparticles formed by microorganisms remain unexplored.

Plant leaf extract-based mechanism

For nanoparticle synthesis mediated by plant leaf extract, the extract is mixed with metal precursor solutions at different reaction conditions [ 72 ]. The parameters determining the conditions of the plant leaf extract (such as types of phytochemicals, phytochemical concentration, metal salt concentration, pH, and temperature) are admitted to control the rate of nanoparticle formation as well as their yield and stability [ 73 ]. The phytochemicals present in plant leaf extracts have uncanny potential to reduce metal ions in a much shorter time as compared to fungi and bacteria, which demands the longer incubation time [ 74 ]. Therefore, plant leaf extracts are considered to be an excellent and benign source for metal as well as metal oxide nanoparticle synthesis. Additionally, plant leaf extract play a dual role by acting as both reducing and stabilizing agents in nanoparticles synthesis process to facilitate nanoparticles synthesis [ 75 ]. The composition of the plant leaf extract is also an important factor in nanoparticle synthesis, for example different plants comprise varying concentration levels of phytochemicals [ 76 , 77 ]. The main phytochemicals present in plants are flavones, terpenoids, sugars, ketones, aldehydes, carboxylic acids, and amides, which are responsible for bioreduction of nanoparticles [ 78 ].

Flavonoids contain various functional groups, which have an enhanced ability to reduce metal ions. The reactive hydrogen atom is released due to tautomeric transformations in flavonoids by which enol-form is converted into the keto-form. This process is realized by the reduction of metal ions into metal nanoparticles. In sweet basil ( Ocimum basilicum ) extracts, enol- to keto-transformation is the key factor in the synthesis of biogenic silver nanoparticles [ 79 ]. Sugars such as glucose and fructose exist in plant extracts can also be responsible for the formation of metallic nanoparticles. Note that glucose was capable of participating in the formation of metallic nanoparticles with different size and shapes, whereas fructose-mediated gold and silver nanoparticles are monodisperse in nature [ 80 ].

An FTIR analysis of green synthesized nanoparticles via plant extracts confirmed that nascent nanoparticles were repeatedly found to be associated with proteins [ 81 ]. Also, amino acids have different ways of reducing the metal ions. Gruen et al. [ 82 ] observed that amino acids (viz cysteine, arginine, lysine, and methionine are proficient in binding with silver ions. Tan et al. [ 83 ] tested all of the 20 natural α-amino acids to establish their efficient potential behavior towards the reduction of Au 0 metal ions.

Plant extracts are made up of carbohydrates and proteins biomolecules, which act as a reducing agent to promote the formation of metallic nanoparticles [ 34 ]. Also, the proteins with functionalized amino groups (–NH 2 ) available in plant extracts can actively participate in the reduction of metal ions [ 84 ]. The functional groups (such as –C–O–C–, –C–O–, –C=C–, and –C=O–) present in phytochemicals such as flavones, alkaloids, phenols, and anthracenes can help to generate metallic nanoparticles. According to Huang et al. [ 85 ], the absorption peaks of FTIR spectra at (1) 1042 and 1077, (2) 1606 and 1622, and (3) 1700–1800 cm −1 imply the stretching of (1) –C–O–C– or –C–O–, (2) –C=C– and (3) –C=O, respectively. Based on FTIR analysis, they confirmed that functional groups like –C–O–C–, –C–O–, –C=C–, and –C=O, are the capping ligands of the nanoparticles [ 86 ]. The main role of the capping ligands is to stabilize the nanoparticles to prevent further growth and agglomeration. Kesharwani et al. [ 87 ] covered photographic films using an emulsion of silver bromide. When light hit the film, the silver bromide was sensitized; this exposed film was placed into a solution of hydroquinone, which was further oxidized to quinone by the action of sensitized silver ion. The silver ion was reduced to silver metal, which remained in the emulsion.

Based on the chemistry of photography, we assumed that hydroquinone or plastohydroquinone or quinol (alcoholic compound) serve as a main reducing agent for the reduction of silver ions to silver nanoparticles through non-cyclic photophosphorylation [ 87 ]. Thus, this experiment proves that the biomolecules and heterocyclic compounds exist in plant extract were accountable for the extracellular synthesis of metallic nanoparticles by plants. It has already been well established that numerous plant phytochemicals including alkaloids, terpenoids, phenolic acids, sugars, polyphenols, and proteins play a significant role in the bioreduction of metal salt into metallic nanoparticles. For instance, Shanakr et al. [ 88 ] confirmed that the terpenoids present in geranium leaf extract actively take part in the conversion of silver ions into nanoparticles. Eugenol is a main terpenoid component of Cinnamomum zeylanisum (cinnamon) extracts, and it plays a crucial role for the bioreduction of HAuCl 4 and AgNO 3 metal salts into their respective metal nanoparticles. FTIR data showed that –OH groups originating from eugenol disappear during the formation of Au and Ag nanoparticles. After the formation of Au nanoparticles, carbonyl, alkenes, and chloride functional groups appeared. Several other groups [e.g., R–CH and –OH (aqueous)] were also found both before and after the production of Au nanoparticles [ 89 ]. Thus, they proposed the possible chemical mechanism shown in Fig. 3 . Nonetheless, the exact fundamental mechanism for metal oxide nanoparticle preparation via plant extracts is still not fully tacit. In general, there are three phases of metallic nanoparticle synthesis from plant extracts: (1) the activation phase (bioreduction of metal ions/salts and nucleation process of the reduced metal ions), (2) the growth phase (spontaneous combination of tiny particles with greater ones) via a process acknowledged as Ostwald ripening, and (3) the last one is termination phase (defining the final shape of the nanoparticles) [ 90 , 91 ]. The process of nanoparticle formation by plant extract is depicted in Fig. 4 [ 92 ].

Schematic for the reduction of Au and Ag ions [ 89 ]

Mechanism of nanoparticle formation by plant leaf extract [ 228 ]

Environmental remediation applications

Antimicrobial activity.

Various studies have been carried out to ameliorate antimicrobial functions because of the growing microbial resistance towards common antiseptic and antibiotics. According to in vitro antimicrobial studies, the metallic nanoparticles effectively obstruct the several microbial species [ 93 ]. The antimicrobial effectiveness of the metallic nanoparticles depends upon two important parameters: (a) material employed for the synthesis of the nanoparticles and (b) their particle size. Over the time, microbial resistance to antimicrobial drugs has become gradually raised and is therefore a considerable threat to public health. For instance, antimicrobial drug resistant bacteria contain methicillin-resistant, sulfonamide-resistant, penicillin-resistant, and vancomycin-resistant properties [ 94 ]. Antibiotics face many current challenges such as combatting multidrug-resistant mutants and biofilms. The effectiveness of antibiotic is likely to decrease rapidly because of the drug resistance capabilities of microbes. Hence, even when bacteria are treated with large doses of antibiotics, diseases will persist in living beings. Biofilms are also an important way of providing multidrug resistance against heavy doses of antibiotics. Drug resistance occurs mainly in infectious diseases such as lung infection and gingivitis [ 95 ]. The most promising approach for abating or avoiding microbial drug resistance is the utilization of nanoparticles. Due to various mechanisms, metallic nanoparticles can preclude or overwhelm the multidrug-resistance and biofilm formation, as described in Figs. 5 and 6 .

Schematic for the multiple antimicrobial mechanisms in different metal nanoparticles against microbial cells [ 96 ]

Various mechanisms of antimicrobial activity of metal nanoparticles [ 93 ]

Various nanoparticles employ multiple mechanisms concurrently to fight microbes [e.g., metal-containing nanoparticles, NO-releasing nanoparticles (NO NPs), and chitosan-containing nanoparticles (chitosan NPs)]. Nanoparticles can fight drug resistance because they operate using multiple mechanisms. Therefore, microbes must simultaneously have multiple gene mutations in their cell to overcome the nanoparticle mechanisms. However, simultaneous multiple biological gene mutations in the same cell are unlikely [ 96 ].

Multiple mechanisms observed in nanoparticles are discussed in Table 4 . Silver nanoparticles are the most admired inorganic nanoparticles, and they are utilized as efficient antimicrobial, antifungal, antiviral, and anti-inflammatory agents [ 97 ]. According to a literature survey, the antimicrobial potential of silver nanoparticles can be described in the following ways: (1) denaturation of the bacterial outer membrane [ 98 ], (2) generation of pits/gaps in the bacterial cell membrane leading to fragmentation of the cell membrane [ 99 , 100 ], and (3) interactions between Ag NPs and disulfide or sulfhydryl groups of enzymes disrupt metabolic processes; this step leads to cell death [ 101 ]. The shape-dependent antimicrobial activity was also examined. According to Pal et al. [ 102 ], truncated triangular nanoparticles are highly reactive in nature because their high-atom-density surfaces have enhanced antimicrobial activity.

The synthesis of Au nanoparticles is highly useful in the advancement of effective antibacterial agents because of their non-toxic nature, queer ability to be functionalized, polyvalent effects, and photo-thermal activity [ 103 , 104 , 105 ]. However, the antimicrobial action of gold nanoparticles is not associated with the production of any reactive oxygen species-related process [ 106 ]. To investigate the antibacterial potential of the Au nanoparticles, researchers attempted to attach nanoparticles to the bacterial membrane followed by modifying the membrane potential, which lowered the ATP level. This attachment also inhibited tRNA binding with the ribosome [ 106 ]. Azam et al. [ 107 ] examined the antimicrobial potential of zinc oxide (ZnO), copper oxide (CuO), and iron oxide (Fe 2 O 3 ) nanoparticles toward gram-negative bacteria ( Escherichia coli , Pseudomonas aeruginosa ) and gram-positive bacteria ( Staphylococcus Aureus and Bacillus subtilis ). Accordingly, the most intense antibacterial activity was reported for the ZnO nanoparticles. In contrast, Fe 2 O 3 nanoparticles exhibited the weakest antibacterial effects. The order of antibacterial activities of nanoparticles was found to be as ZnO (19.89 ± 1.43 nm), CuO (29.11 ± 1.61 nm), and Fe 2 O 3 (35.16 ± 1.47 nm). These results clearly depicts that the size of the nanoparticles also play a momentous role in the antibacterial potential of each sample [ 107 ]. The anticipated mechanism of antimicrobial action of ZnO nanoparticles is: (1) ROS generation, (2) zinc ion release on the surface, (3) membrane dysfunction, and (4) entry into the cell. Also, the antimicrobial potential of ZnO nanoparticles is concentration and surface area dependent [ 108 ]. Mahapatra et al. [ 109 ] determined the antimicrobial action of copper oxide nanoparticles towards several bacterial species such as Klebsiella pneumoniae , P. aeruginosa , Shigella Salmonella paratyphi s. They found that CuO nanoparticles exhibited suitable antibacterial activity against those bacteria. It was assumed that nanoparticles should cross the bacterial cell membrane to damage the crucial enzymes of bacteria, which further induce cell death. For instance, green synthesized nanoparticles show enhanced antimicrobial activity compared to chemically synthesized or commercial nanoparticles. This is because the plants [such as Ocimum sanctum (Tulsi) and Azadirachta indica (neem)] employed for synthesis of nanoparticles have medicinal properties [ 110 , 111 ]. For example, green synthesized silver nanoparticles showed an efficient and large zone of clearance against various bacterial strains compared to commercial silver nanoparticles (Fig. 7 ) [ 112 ].

Schematic for the antimicrobial activity for the five bacterial strains: a Staphylococcus aureus , b Klebsiella pneumonia , c Pseudomonas aeruginosa , d Vibrio cholera , and e Proteus vulgaris . Numbers of 1 through 6 inside each strain denote: (1) nickel chloride, (2) control ciprofloxacin, (3) Desmodium gangeticum root extract, (4) negative control, (5) nickel NPs prepared by a green method, and (6) nickel NPs prepared by a chemical method [ 229 ]

Catalytic activity

4-Nitrophenol and its derivatives are used to manufacture herbicides, insecticides, and synthetic dyestuffs, and they can significantly damage the ecosystem as common organic pollutants of wastewater. Due to its toxic and inhibitory nature, 4-nitrophenol is a great environmental concern. Therefore, the reduction of these pollutants is crucial. The 4-nitrophenol reduction product, 4-aminophenol, has been applied in diverse fields as an intermediate for paracetamol, sulfur dyes, rubber antioxidants, preparation of black/white film developers, corrosion inhibitors, and precursors in antipyretic and analgesic drugs [ 113 , 114 ]. The simplest and most effective way to reduce 4-nitrophenol is to introduce NaBH 4 as a reductant and a metal catalyst such as Au NPs [ 115 ], Ag NPs [ 116 ], CuO NPs [ 117 ], and Pd NPs [ 118 ]. Metal NPs exhibit admirable catalytic potential because of the high rate of surface adsorption ability and high surface area to volume ratio. Nevertheless, the viability of the reaction declines as a consequence of the substantial potential difference between donor (H 3 BO 3 /NaBH 4 ) and acceptor molecules (nitrophenolate ion), which accounts for the higher activation energy barrier.

Metallic NPs can promote the rate of reaction by increasing the adsorption of reactants on their surface, thereby diminishing activation energy barriers [ 119 , 120 ] (Fig. 8 ). The UV–visible spectrum of 4-nitrophenol was characterized by a sharp band at 400 nm as a nitrophenolate ion was produced in the presence of NaOH. The addition of Ag NPs (synthesized by Chenopodium aristatum L. stem extract) to the reaction medium led to a fast decay in the absorption intensity at 400 nm, which was concurrently accompanied by the appearance of a comparatively wide band at 313 nm, demonstrating the formation of 4-aminophenol [ 121 ] (Fig. 9 ).

Schematic of the metallic NP-mediated catalytic reduction of 4-nitrophenol to 4-aminophenol [ 120 ]

UV-visible spectra illustrating Chenopodium aristatum L. stem extract synthesized Ag NP-mediated catalytic reduction of 4-NP to 4-AP at three different temperatures a 30 °C, b 50 °C, and c 70 °C. Reduction in the absorption intensity of the characteristic nitrophenolate band at 400 nm accompanied by concomitant appearance of a wider absorption band at 313 nm indicates the formation of 4-AP [ 121 ]

Removal of pollutant dyes

Cationic and anionic dyes are a main class of organic pollutants used in various applications [ 122 ]. Organic dyes play a very imperative role due to their gigantic demand in paper mills, textiles, plastic, leather, food, printing, and pharmaceuticals industries. In textile industries, about 60% of dyes are consumed in the manufacturing process of pigmentation for many fabrics [ 123 ]. After the fabric process, nearly 15% of dyes are wasted and are discharged into the hydrosphere, and they represent a significant source of pollution due to their recalcitrance nature [ 124 ]. The pollutants from these manufacturing units are the most important sources of ecological pollution. They produce undesirable turbidity in the water, which will reduce sunlight penetration, and this leads to the resistance of photochemical synthesis and biological attacks to aquatic and marine life [ 125 , 126 , 127 ]. Therefore, the management of effluents containing dyes is one of the daunting challenge in the field of environmental chemistry [ 128 ].

The need for hygienic and safe drinking water is increasing day by day. Considering this fact, the use of metal and metal oxide semiconductor nanomaterials for oxidizing toxic pollutants has become of great interest in recent material research fields [ 129 , 130 , 131 ]. In the nano regime, semiconductor nanomaterials have superior photocatalytic activity relative to the bulk materials. Metal oxide semiconductor nanoparticles (like ZnO, TiO 2 , SnO 2 , WO 3 , and CuO) have been applied preferentially for the photocatalytic activity of synthetic dyes [ 31 , 132 , 133 , 134 ]. The merits of these nanophotocatalysts (e.g., ZnO and TiO 2 nanoparticles) are ascribable to their high surface area to mass ratio to enhance the adsorption of organic pollutants. The surface energy of the nanoparticles increases due to the large number of surface reactive sites available on the nanoparticle surfaces. This leads to an increase in rate of contaminant removal at low concentrations. Consequently, a lower quantity of nanocatalyst will be required to treat polluted water relative to the bulk material [ 135 , 136 , 137 , 138 ]. Like metal oxide nanoparticles, metal nanoparticles also show enhanced photocatalytic degradation of various pollutant dyes; for example, silver nanoparticles synthesized from Z. armatum leaf extract were utilized for the degradation of various pollutant dyes [ 127 ] (Fig. 10 ).

Schematic for the reduction of a safranine O, b methyl red, c methyl orange, and d methylene blue dyes using silver NPs synthesized from Z. armatum leaf extract by metallic nanoparticles [ 136 ]

Heavy metal ion sensing

Heavy metals (like Ni, Cu, Fe, Cr, Zn, Co, Cd, Pb, Cr, Hg, and Mn) are well-known for being pollutants in air, soil, and water. There are innumerable sources of heavy metal pollution such as mining waste, vehicle emissions, natural gas, paper, plastic, coal, and dye industries [ 139 ]. Some metals (like lead, copper, cadmium, and mercury ions) shows enhanced toxicity potential even at trace ppm levels [ 140 , 141 ]. Therefore, the identification of toxic metals in the biological and aquatic environment has become a vital need for proper remedial processes [ 142 , 143 , 144 ]. Conventional techniques based on instrumental systems generally offer excellent sensitivity in multi-element analysis. However, experimental set ups to perform such analysis are highly expensive, time-consuming, skill-dependent, and non-portable.

Due to the tunable size and distance-dependent optical properties of metallic nanoparticles, they have been preferably employed for the detection of heavy metal ions in polluted water systems [ 145 , 146 ]. The advantages of using metal NPs as colorimetric sensors for heavy metal ions in environmental systems/samples include simplicity, cost effectiveness, and high sensitivity at sub ppm levels. Karthiga et al. [ 147 ] synthesized AgNPs using various plant extracts used as colorimetric sensors for heavy metal ions like cadmium, chromium, mercury, calcium, and zinc (Cd 2+ , Cr 3+ , Hg 2+ , Ca 2+ , and Zn 2+ ) in water. Their as-synthesized Ag nanoparticles showed colorimetric sensing of zinc and mercury ions (Zn 2+ and Hg 2+ ). Likewise, AgNPs synthesized using mango fresh leaves and dried leaves (fresh, MF-AgNPs and sun-dried, MD-AgNPs) exhibited selective sensing for mercury and lead ions (Hg 2+ and Pb 2+ ). Also, AgNPs prepared from pepper seed extract and green tea extract (GT-AgNPs) showed selective sensing properties for Hg 2+ , Pb 2+ , and Zn 2+ ions [ 147 ] (Fig. 11 ).

Schematic of metal removal using metal oxides prepared by green synthesis. Left— a digital images and b absorption spectra of neem bark extract-mediated silver NPs (NB-AgNPs) with different metal ions and concentration-dependent studies of c Hg 2+ and d Zn 2+ . Right— a digital images and b absorption spectra of fresh mango leaf extract-mediated silver NPs (MF-AgNPs) with different metal ions and c concentration-dependent studies of Pb 2+ removal [ 147 ]

Conclusion and future prospects

‘Green’ synthesis of metal and metal oxide nanoparticles has been a highly attractive research area over the last decade. Numerous kinds of natural extracts (i.e., biocomponents like plant, bacteria, fungi, yeast, and plant extract) have been employed as efficient resources for the synthesis and/or fabrication of materials. Among them, plant extract has been proven to possess high efficiency as stabilizing and reducing agents for the synthesis of controlled materials (i.e., controlled shapes, sizes, structures, and other specific features). This review article was organized to encompass the ‘state of the art’ research on the ‘green’ synthesis of metal/metal oxide nanoparticles and their use in environmental remediation applications. Detailed synthesis mechanisms and an updated literature study on the role of solvents in synthesis have been reviewed thoroughly based on the literature available to help encounter the existing problems in ‘green’ synthesis. In summary, future research and development of prospective ‘green’ materials/nanoparticle synthesis should be directed toward extending laboratory-based work to an industrial scale by considering traditional/present issues, especially health and environmental effects. Nevertheless, ‘green’ material/nanoparticle synthesis based on biocomponent-derived materials/nanoparticles is likely to be applied extensively both in the field of environmental remediation and in other important areas like pharmaceutical, food, and cosmetic industries. Biosynthesis of metals and their oxide materials/nanoparticles using marine algae and marine plants is an area that remains largely unexplored. Accordingly, ample possibilities remain for the exploration of new green preparatory strategies based on biogenic synthesis.

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JS, KHK and PK made substantial contributions to interpretation of literature; drafted the article and revised it critically. All made substantial contributions to draft the article and revised it critically for important intellectual content and gave approval to the submitted manuscript. All authors read and approved the final manuscript.

Acknowledgements

The corresponding author (KHK) acknowledges a supporting Grant from the National Research Foundation of Korea (NRF) funded by the Ministry of Science, ICT, & Future Planning (No. 2016R1E1A1A01940995). Dr. Pawan Kumar would like to thank SERB and UGC, New Delhi, for the ‘Empowerment and Equity Opportunities for Excellence in Science’ video file No. EEQ/2016/00484 and the UGC-BSR Start Up-Research Project.

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Singh, J., Dutta, T., Kim, KH. et al. ‘Green’ synthesis of metals and their oxide nanoparticles: applications for environmental remediation. J Nanobiotechnol 16 , 84 (2018). https://doi.org/10.1186/s12951-018-0408-4

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Applications of Green Synthesized Metal Nanoparticles — a Review

Seerengaraj vijayaram.

1 Fisheries College, Jimei University, Xiamen, 361021 China

Hary Razafindralambo

2 ProBioLab, Teaching and Research Centre, Gembloux Agro-Bio Tech, University of Liege, Liège, Belgium

3 BioEcoAgro Joint Research Unit, TERRA Teaching and Research Centre, Microbial Processes and Interactions, Gembloux AgroBio Tech/Université de Liège, Gembloux, Belgium, University of Liege, Liège, Belgium

Yun-Zhang Sun

Seerangaraj vasantharaj.

4 Department of Biotechnology, Hindusthan College of Arts and Science, Coimbatore, 641028 Tamil Nadu India

Hamed Ghafarifarsani

5 Department of Fisheries, Faculty of Natural Resources, Urmia University, Urmia, Iran

Seyed Hossein Hoseinifar

6 Department of Fisheries, Faculty of Fisheries and Environmental Sciences, Gorgan University of Agricultural Sciences and Natural Resources, Gorgan, Iran

Mahdieh Raeeszadeh

7 Department of Basic Sciences, Sanandaj Branch, Islamic Azad University, Sanandaj, Iran

Associated Data

The datasets generated during and/or analyzed during the current study are available from the corresponding author on reasonable request.

All applicable international, national, and/or institutional guidelines for the care and use of animals were followed.

Green nanotechnology is an emerging field of science that focuses on the production of nanoparticles by living cells through biological pathways. This topic plays an extremely imperative responsibility in various fields, including pharmaceuticals, nuclear energy, fuel and energy, electronics, and bioengineering. Biological processes by green synthesis tools are more suitable to develop nanoparticles ranging from 1 to 100 nm compared to other related methods, owing to their safety, eco-friendliness, non-toxicity, and cost-effectiveness. In particular, the metal nanoparticles are synthesized by top-down and bottom-up approaches through various techniques like physical, chemical, and biological methods. Their characterization is very vital and the confirmation of nanoparticle traits is done by various instrumentation analyses such as UV–Vis spectrophotometry (UV–Vis), Fourier transform infrared spectroscopy (FT-IR), scanning electron microscope (SEM), transmission electron microscopy (TEM), X-ray diffraction (XRD), atomic force microscopy (AFM), annular dark-field imaging (HAADF), and intracranial pressure (ICP). In this review, we provide especially information on green synthesized metal nanoparticles, which are helpful to improve biomedical and environmental applications. In particular, the methods and conditions of plant-based synthesis, characterization techniques, and applications of green silver, gold, iron, selenium, and copper nanoparticles are overviewed.

Introduction

Green synthesis of nanoparticles using living cells through biological pathways is more efficient techniques and yields a higher mass when compared to other related methods. Plants are the sources of several components and biochemicals that can role as stabilizing and reducing agents to synthesize green nanoparticles. The green synthesized methods are eco-friendly, non-toxic, cost-effective, and also more stable when compared to other biological, physical, and chemical methods [ 1 ]. Green synthesis of nanoparticles is categorized into three groups, viz. extracellular, intracellular, and phytochemicals. The nanoparticle synthesis from plant extract is an inexpensive process and it results in higher yield due to the huge quantity of phytochemical components in the plant extract that can also act as reducing and stabilizing agents converting metal ions into metal nanoparticles [ 2 ]. Green synthesized metal and metal oxide nanoparticles have emerging applications in the biomedical sector like diagnostics, wound healing, tissue treatment, immunotherapy, regenerative medicine, dentistry, and biosensing platforms. Biotoxicology and its antimicrobial, antifungal, and antiviral characteristics were passionately contested [ 3 ]. Plant-mediated copper oxide nanoparticles synthesized from various plant extracts play a various role like diverse biological activities, environmental remediation, photocatalysis, catalytic reduction, sensing, energy storage, and several organic transformations such as coupling, reduction, and multicomponent reactions [ 4 ]. Green synthsized selenium nanoparticles are helpful to improve the activity in antioxidant, catalysis, anticancer, photocopiers, xerography, rectifiers, and solar cells [ 5 ]. Green synthesized cerium oxide nanoparticles have potential photocatalytic dye degradation, antioxidant activity, antidiabetic, anticancer antibacterial, and antifungal activity properties [ 6 ]. Green synthesized stannic oxide nanoparticles have potential photocatalytic, antioxidant, and antibacterial activity, and these nanoparticles are helpful to enhance the environment and human health applications [ 7 ].Green synthesized silver chloride nanoparticles are used to develop environmental and biomedical applications [ 8 ]. Green synthesized metal nanoparticles are produced from different parts of the plants and also these methods are eco-friendly, non-toxic, and cost-effective method. Green synthesized nanoparticles have more active performance to remove dyes, antibiotics, and metal ions compared to other physical and chemical methods [ 9 ]. Green synthesis method is the best method for the preparation of nanoparticles, and these methods are helpful to reduce the toxicity, increase the stability, eco-friendly, and cost-effective methods. Green synthesis methods have more beneficial response in environmental and biomedical applications [ 10 ]. Plants contain many types of phytochemical compounds like phenolics, terpenoids, polysaccharides, and flavonoids that possess oxidation–reduction capabilities. Thus, they are preferably utilized for the green synthesis of nanoparticles [ 11 ]. Phytochemical compound synthesis of nanoparticles is not a general procedure as essential knowledge of the exact phytochemical components is needed for the synthesis of stabilized nanoparticles [ 12 ]. It is a general point of view that plants’ secondary metabolites (polyphenols) are the mainly significant elements playing a very important function in the progression of the green synthesis of nanoparticles. The green synthesis practice is more advanced, safe and cost-effective, easily reproduce, and stable [ 13 ]. There are some positive impacts on the plant-based green synthesis of nanoparticles when compared to the other related biological methods using bacteria, fungi, actinomycetes, and algae [ 14 ]. Diverse plant parts (roots, stem, leaf, seed, and fruit) are concerned with such synthesized green nanoparticles because of the presence of notable phytochemicals [ 15 ]. Plant-synthesized nanoparticles in dissimilar parts of plants involve washing the particular part with tap or distilled water followed by squeezing, filtering, and adding particular salt solutions. Then, metallic salt was added and nanoparticles are extracted. A variety of metallic nanoparticles are synthesized by this method. The green nanoparticles are used in personal care, medicine, nano-enabled devices, food, aquaculture sciences, and agricultural products. Green synthesis of nanoparticles is eco-friendly; this method is generally used for the industrial production of metal nanoparticles. Green synthesized metal nanoparticles are produced from physicochemical methods [ 16 ]. A biosynthesis approach is a vital mechanism to avoid harmful by-products via eco-friendly and sustainable development. The biosynthesis process is involved in several biological structures, namely plant extracts, bacteria, yeast, seaweeds, and algae to produce metal and metal oxide nanoparticles [ 17 ]. Sharma et al. (2015) reported the green synthesis, characterization, and application of a variety of metal nanoparticles, which can eliminate pollutant dyes from water such as azo dyes, acid dyes, and cationic dyes. The metallic nanoparticles are used to remove water pollutants from water bodies such as rivers, lakes, and other water streams, thereby enhancing aquatic life. Green synthesized nanoparticles can be applied to treat environmental pollutants. Earlier studies report that the catalytic characteristics of some nanoparticles can reduce the toxicity of environmental pollutants [ 18 , 19 ]. Green synthesis processes to metal nanoparticles are eco-friendly, non-toxic, and cost-effective. Also, these methods are playing a very significant role in the pharmaceutical industry [ 20 ]. The broad application of metallic nanoparticles utilized in various fields like biology, medicine, pharma, and other fields has led to high demand for these nanoparticles and thereby resulting in a significant need for better production of such nanoparticles. The efficiency of metallic nanoparticles used against human pathogenic microbes has made nanoparticles attractive in biomedical fields [ 21 ]. Particularly, metal nanoparticles have elevated outside the region and more attractive sites to support quicker response which enhances production yields. These metallic nanoparticles are generally divided into two groups, namely noble and non-noble metallic groups based on nanoparticle types. The metallic nanoparticles are inexpensive, eco-friendly, non-toxic, and reduce the accumulation of hazardous wastes. Green syntheses of metallic nanoparticles are safer for biomedical and environmental applications [ 22 , 23 ]. A notable perspective of different metal nanoparticles is to be used as antimicrobial medicine (antiviral, antibacterial, fungicidal, antiparasitic, and pesticide agents, etc.) alongside some plant diseases [ 24 ]. A cancer nanobullet is significant in treating cancer because it is very effective and safe. The use of cancer nanodrugs considerably increased drug delivery to the target when compared to the conventional drug administration system. It notably improves the safety and competence of the regularly utilized anticancer drugs. Effective targeting, delayed release, long-lasting half-life, and reduced toxicity are the major positive effects of nanomedicine delivery systems [ 25 ]. This review paper overviews the only green synthesis and characterization of metal nanoparticles, namely silver, gold, iron, selenium, and copper, and the applications of such nanoparticles are used to treat antimicrobial, anticancer, reduce metal toxicity, dye degradation, and wastewater treatment and not for other biological, chemical, and physical methods. The green synthesized different metal nanoparticles are eco-friendly, non-toxic, cost-effective, and also more stable when compared to other ones as well as these nanoparticles have more active performance compared to other methods.

Nanoparticle Synthesis Methods

Top-down and bottom-up approaches.

The nanoparticles synthesized through a biological system have numerous advantages like non-toxicity, high yield production, easy scaling up, and well-defined morphology. Hence, it has to turn into an innovative way of nanoparticle production. The green synthesis technique has been established to synthesize extremely effective nanoparticles. The green synthesized nanoparticles are safe, eco-friendly, and easy to handle [ 26 – 28 ]. Different practices of green synthesis of nanoparticles have been reviewed by Saratale et al. [ 29 ] and their biomedical and agricultural applications have been summarized (Saratale et al. 2018a). The synthesis of green nanoparticles is categorized into two classes, namely “top-down” and “bottom-up” based on the way of nanoparticle formation (Fig. 1 ). In the “top-down” approach, the dimension of nanoparticles was larger and hence a mechanical method or the additions of acids are necessary to decrease the particle size of the nanoparticles. Generally, the top-down approach requires the use of complex analysis (thermal decomposition method, mechanical method/ball-milling method, lithographic methods, laser ablation, sputtering). The “bottom-up” approach was quite different from the top-down process and was commenced at the atomic level via forming molecules. The bottom-up methods are carried out using different manners (chemical vapor deposition (CVD) method, sol–gel method, spinning, pyrolysis) [ 30 ].

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Summary of the top-down and bottom-up approaches to green synthesizing nanoparticles

Factors Influencing the Green Synthesis of Various Nanoparticles

The green synthesis of nanoparticle morphology characterization can be adapted by different specifications such as pH, temperature, reaction time, and reactant concentration. These parameters have majorly recognized the impact of environmental factors on the synthesis of nanoparticles as well as these elements may play an imperative function during the optimization of metallic nanoparticle synthesis [ 31 ].

Temperature

Different levels of study reports are being undertaken worldwide to learn the control of temperature over nanoparticles. Temperature is the most important factor in disturbing the dimension and form of the nanoparticles and their level of synthesis. Dissimilar types of shapes (triangle, octahedral platelets, spherical, and rod) and the dimension of the synthesis of nanoparticles can be tailored as a function of temperature. As the temperature level increases, the reaction response rate is also strengthening the formation of nucleation centers [ 32 ]. During the green synthesis of nanoparticles, the reaction time is a major factor that plays the most important function in influencing the shape, size, and yield of synthesized nanoparticles [ 33 , 34 ].

The pH of response plays a significant function in the structure of nanoparticles. Namely, pH and temperature also control the formation of nucleation centers. The pH level increases mean automatically enlarged the number of nucleation centers, which is most important to boost the formation of metal nanoparticles. It has been recognized that pH takes a significant function in formulating the structural morphology and size of the nanoparticles [ 35 ]. The medium pH reaction is a major function in the formation of nanoparticles [ 36 ].

Reaction Time

The reaction time is the most important aspect that controls the structural morphology of nanoparticles along with temperature and pH. Karade et al. [ 37 ] mentioned that reaction time plays a crucial role in the synthesis of magnetic nanoparticles [ 38 ].

Characterization of Nanomaterials

The nanoparticle characterization can be classified according to the physical and chemical instrumentation analysis used (Fig. 2 ), including UV–Vis spectroscopy, Fourier transforms infrared spectroscopy (FT-IR), scanning electron microscope (SEM), X-ray diffraction (XRD), atomic force microscopy (AFM), dynamic light scattering (DLS), surface-enhanced remain spectroscopy (SERS), atomic absorption spectroscopy (AAS), energy dispersive spectroscopy (EDS), ray photoelectron spectroscopy (XPS), and high angle annular dark-field (HAADF) [ 39 , 40 ].

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Summary of the various techniques characterizing metal and metal oxide nanoparticles

In FTIR analysis, infrared red rays are passed through the sample, some IR rays are engrossed by the sample, and the remaining rays pass through it. The spectrum provides absorption or transmission as a function of wavelength, which characterizes the sample substances [ 41 ]. FTIR analysis is a suitable, cost-effective, simple, and non-invasive practice to recognize the function of biomolecules in the decrease of nanoparticles (silver nitrate to silver) [ 42 ].

UV–Vis Spectrophotometry

In the characterization of nanoparticles by using UV–Vis absorption spectroscopy, the nanoparticle size level is varying from several metals while the size range is 2–100 nm. Generally, the nanoparticles confirmed by UV–Vis absorption spectroscopy are analyzed with wavelengths ranging from 300 to 800 nm. The metallic nanoparticles synthesized under particular salt conditions have strong absorption to give a point spectrum in the noticeable area [ 43 ]. Previous study reports exposed that absorption of wavelength 200–800 was appropriate for the categorization of nanoparticle size range 2–100 nm [ 44 ].

Scanning Electron Microscope (SEM)

Nanoparticles can be characterized by SEM. This instrumentation investigation is used to identify the shape, size, morphology, and distribution of synthesized nanoparticles [ 39 , 40 ]. The SEM analysis assessed the alteration of a morphological structure before and after treatment. Previous studies reported that noticeable modifications in cell shape and perforations of nanoparticles in the cell wall have been used as indicators of the antimicrobial action of nanoparticles [ 45 , 46 ].

X-ray Diffraction (XRD)

Material atomic structures can be analyzed XRD. This system is helpful to identify the qualitative and quantitative levels of materials. XRD investigation was used to recognize and confirm crystalline nanoparticle size and structure [ 39 , 40 ]. To analyze the particle dimension of nanomaterials from XRD data, the Debye–Scherrer formula was applied by ruling the width of the Bragg reflection law according to the equation: d = Kλ/β cos θ , where d is the particle size (nm), K is the Scherrer constant, λ is the wavelength of X-ray, β is the full width half maximum, and θ is the diffraction angle (half of Bragg angle) that corresponds to the lattice plane [ 47 ].

Atomic Force Microscopy (AFM)

Atomic force microscopy (AFM) classified and confirmed the size, shape, and outside the region of synthesized nanoparticles [ 39 , 40 ].

Transmission Electron Microscopy (TEM)

Transmission electron microscopy (TEM) categorized and confirmed the crystal structure and particle size of material at the nanoscale level [ 39 , 40 ].

Annular Dark-Field Imaging (HAADF)

Annular dark-field imaging (HAADF) recognized the interaction of nanoparticles with bacteria while providing information on the size distribution of nanoparticles interacting with each type of bacteria [ 39 , 40 ].

Intracranial Pressure (ICP)

Intracranial pressure (ICP) spectrometry was confirmed by the metal concentration in deionized and original nanoparticle solutions. Experimentally, coupled plasma mass spectrometry (ICP-MS) and coupled emission spectroscopy (ICP-ES) are used to calculate the concentration of metal nanoparticles [ 39 , 40 ].

Green Synthesis of Metal Nanoparticles and Their Applications

Researchers affirmed that biologically synthesized nanoparticles are more pharmacological active compared to other physicochemical methods. Plant extract synthesized metallic nanoparticles are stable and monodispersed easily by controlling different influencing factors such as pH, temperature, retention time, and mixing ratio. Green metal nanoparticles synthesized by different plants such as neem leaves ( Azadirachta indica ), tulsi leaves ( Ocimum tenuiflorum ), curry leaves ( Murraya koenigii ), guava leaves ( Psidium guajava ), and mango leaves ( Mangifera indica ) [ 48 ]. The metallic nanoparticles green synthesized using diverse medicinal plants have shown the most important therapeutic properties, namely antimicrobial activity, insecticidal activity, antioxidant activity, wound healing properties, antidiabetic activity, immunomodulatory activity, hepatoprotective activity, and anticancer activity (Fig. 3 ). The medicinal plant-based synthesized metallic nanoparticles have a huge beneficiary effect in the sector of biomedicine [ 49 ]. The major significant concept of using green nanotechnology in agriculture reduced harmful environmental effects and the high expenditure of fertilizers, while green nanoparticles (GNPs) synthesized from dissimilar plants reduced the harmful emission of carbon dioxide, nitrous oxide, and methane. Also, green nanoparticles are used to increase productivity in agriculture and lower health risks concerned agricultural farmers [ 50 ]. Naturally, plants contain numerous phytochemical components (Table (Table1). 1 ). These factors are eco-friendly and inexpensive. Green synthesized nanoparticles are exposed to the significant importance of heavy metal detoxification from the environment. In view of the number of heavy metals polluting soil and water, green nanoparticles are helpful to reduce metal toxicity in the environment [ 51 ]. As the different parts of plants roots, stem, leaf, seed, and fruit contain many phytochemical substances, the green synthesis metallic nanoparticles approach is cost-effective, non-toxic, and eco-friend and is more efficient compared to other biological methods [ 14 ]. For the synthesis of green nanoparticles, it is important for selected plant parts to be washed with tap or distilled water, after squeezing, filtering, and adding particular salt solutions. The color modification of the solution confirms the synthesized nanoparticles. Phytochemical components (phenolic acids: ellagic acid, caffeic acid, protocatechuic acid, and gallic acid) play an important role in the synthesis of metal nanoparticles. The phytochemical factors are reducing and stabilizing agents of synthesized metal nanoparticles [ 52 ].

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Green synthesis of metal and metal oxide nanoparticles and their applications

List of green synthesized metal nanoparticles and their response

Plants (scientific name)	Plants (common name)	Source of the plant (leaf, stem, root, flower, fruit)	Metal salts or powder	Phytochemical components	Applications	References
	Five-leaved chaste tree	Leaf extract	Silver	Alkaloids, glycosides, flavonoids, phenolic compounds, reducing sugars, resin, tannins	Broad spectrum antibacterial response	[ ]
	Banana	Peel extract	Silver	Alkaloids, glycosides, steroids, saponins, tannins, flavanoids, and terpenoids	Antibacterial response	[ ]
	Pawpaw	Leaf extract	Silver	Alkaloids, saponin, tannin, flavonoids, anthraquinone (free and bound), phlobatannin, cardiac glycosides, terpenoids, and proanthocyanidin	Antiviral activity	[ , ]
	Bitter weed	Leaf extract	Silver	Diterpenoids, flavonoids, and polyphenols	Antiviral activity	[ , ]
	Black pepper	Leaf extract	Silver	Palmitic, hexadecenoic, stearic, linoleic, oleic, higher saturated acids, arachidic, and behenic acids	Anticancer activity	[ , ]
	European cranberry	Fruit extract	Silver	Polyphenol anthocyanins	Antiinflammatory activity	[ ]
	Drumstick tree	Leaf extract	Silver	Phenols, Β-sitosterol, caffeoylquinic acid, quercet in, kaempferol	Antibacterial response	[ , ]
	Banyan	Leaf extract	Silver	Flavonoids, phenols, terpenoids, and terpenes	Antimicrobial response	[ , ]
	Java plum	Seed extract	Silver	Gallic acid, P-coumaric acid, quercet in, 3,4-dihyroxybenzoic acid	Antifungal activity	[ , ]
	Korean ginseng	Leaf extract	Silver	Ginsenosides, polysaccharides, alkaloids, glucosides, and phenolic acids	Anticancer activity	[ , ]
	Mugwort	Leaf extract	Silver	Sabinene, Β-thujone, chrysanthenone, camphor, borneol, and germacrene D	Antibacterial, antifertility, antimalarial, antitumor	[ , ]
	False waterwillow	Leaf extract	Silver	Two new 2′-oxygenated flavonoids and two new phenyl glycosides	Antibacterial activity	[ , ]
	Arabian coffee	Seed extract	Silver	Caffeic, P-coumaric, vanillic, ferulic, and protocatechuic acids	Antibacterial activity	[ , ]
	Hollyhock	Flower extract	Silver	Ferulic acid, caffeic acid, tricin, luteolin-3′,4′-dimethyl ether	Antibacterial activity	[ , ]
	Gotu kola	Leaf extract	Silver	Isoprenoids and phenylpropanoid derivatives	Antimicrobial activity	[ , ]
	Yellow milfoil	Flower extract	Silver	Flavonoids, phenolic acids, coumarins, terpenoids, and sterols	Anticancer activity	[ , ]
and	Tulasi and Indian lilac	Leaf extract	Silver	Oleanolic acid, rosmarinic acid, ursolic acid eugenol,, linalool, carvacrol, β elemene, β caryophyllene, germacrene and nimbolide, azarirachtin, and gedunin	Antiplasmodium activity	[ ]
	Dill	Leaf extract	Silver	Anethine, phellandrene, and d-limonene, and its leaves are rich in tannins, steroids, terpenoids, and flavonoids	Antiparasitic activity	[ ]
	Wild fennel flower	Leaf extract	Gold	Arachidonic, eicosadienoic, linoleic, and linolenic), campesterol, stigmasterol, Β-sitosterol, Α-spinasterol, ( +)-limonene, and ( +)-citronellol	Antioxidant, antibacterial, anticancer activity	[ , ]
	Pomegranate	Fruit extract	Gold	Phenolic acids, hydrolysable tannins, and flavonoids	Antibacterial activity	[ , ]
	Peppermint	Leaf extract	Gold	Diterpenes, steroids, tannin, flavonoids, cardial glycosides, alkaloids, phenols, coumarin, and saponin	Antibacterial activity	[ , ]
	Cashew nut	Leaf extract	Gold	Phenols, alkaloids, anthraquinolones, flavonoids, glycosides, tannins, glycoside, terpenoids, and tannins	Antibacterial activity	[ , ]
	Gale of the wind	Leaf extract	Gold	Flavonoids, alkaloids, terpenoids, lignans, polyphenols, tannins, coumarins, and saponins	Anticancer activity	[ , ]
	Tanjong tree	Leaf extract	Gold	Tannin, some caoutchouc, wax, coloring matter, starch, and ash-forming inorganic salts	Anticancer activity	[ , ]
	Onion	Extract	Gold	Flavonoids, carbohydrates, glycosides, proteins, alkaloids, saponins, acid compounds, reducing sugars, and oils	Anticancer activity	[ , ]
	Clove buds	Bud extract	Gold	Sesquiterpenes, monoterpenes, hydrocarbon, and phenolic compounds	Anticancer activity	[ , ]
	Wrinkled marsh weed	Leaf extract	Gold	Phenolics, flavonoids, terpenoids, and amino acids	Catalytic activity in the reduction of different nitrophenols	[ , ]
	Lantana	Leaf extract	Gold	Flavonoids, carbohydrates, proteins, alkaloids, glycosides, saponins, steroids, triterpenes, and tannin	Antibacterial activity and dye degradiation	[ , ]
	Common reed	Leaf extract	Gold	Tanin, terpenoids, glycosides, and flavonoids	Anticancer activity	[ , ]
	Mock buckthorn	Leaf extract	Gold	Friedeline syringic acid, bet a-sitosterol, daucosterol, gluco-syringic acid, and taraxerol	Antibacterial activity and antioxidant activity	[ , ]
	Rose apple	Leaf extract	Iron	Flavonoids, ellagitannins, phloroglucinols, and phenolic acids	Removal of chromium metal from environment	[ , ]
	Mediterranean cypress	Leaf extract	Iron	Cosmosiin, caffeic acid, and P-coumaric acid cupressuflavone, amentoflavone, rutin, quercitrin, quercet in, myricitrin	Wastewater treatment	[ , ]
	Tea plant	Leaf extract	Iron	Epigallocatechingallate (EGCG) ranged from 117 to 442 mg/L, epicatechin 3-gallate (EGC) from 203 to 471 mg/L, epigallocatechin (ECG) from 16.9 to 150 mg/L, epicatechin (EC) from 25 to 81 mg/L and catechin (C) from 9.03 to 115 mg/L	Reduction of bromophenol blue indicator	[ , ]
	Gum trees	Leaf extract	Iron	Saponins, tannins, phenols, and glycosides	Wastewater treatment	[ , ]
	Flat-crown	Leaf extract	Iron	Apocarotenoids, chalcone, dipeptide, elliptosides, essential oils, fatty acids, flavonoids, histamine, imidazolyl carboxylic acid, prosapogenins, steroids, triterpene saponins, and triterpenoids	Anticancer activity	[ , ]
	Babchi	Leaf extract	Iron	Coumarins, flavonoids, and meroterpenes	Antitumor activity	[ , ]
	Green tea	Leaf extract	Iron	Alkaloids, flavonoids, steroids, terpenoids, carotenoids, benzoic acid, ascorbic acid, tocopherols, folic acid, and tannins consisting of catechin (flavonol), and gallic acids	Dye degradiation	[ , ]
	Mock buckthorn	Leaf extract	Iron	Alkaloids, flavonoids, steroids, terpenoids	Antibacterial activity	[ ]
	Flat-crown	Leaf extract	Iron	Apocarotenoids, chalcone, dipeptide, elliptosides, essential oils, fatty acids, flavonoids, histamine, imidazolyl carboxylic acid, prosapogenins, steroids, triterpene saponins, and triterpenoids	Anticancer activity	[ , ]
	Hop bush	Leaf extract	Iron	Flavonoids such as tannins, santin, pendlet on, saponins, and pinocembrin	Antibacterial activity	[ , ]
	Gum trees	Leaf extract	Iron	Quinones, saponins, carbohydrates, tannins, phenols, flavonoids	Wastewater treatment	[ , ]
	Garlic	Bud extract	Selenium	Alkaloid, saponins, flavonoids, glycoside, anthraquinones, tannin, and terpenoids	Antioxidant activity	[ , ]
	Horseshoe geranium	Leaf extract	Selenium	Linalool, citronellol and geraniol, and their esters, menthone, nerol, isomenthone, rose oxides, terpineol, pinene, and myrcene	Antibacterial and antifungal activity	[ , ]
	Bombay ebony	Leaf extract	Selenium	Alkaloids, flavonoids, tannin, terpenoids, and essential oils	Antibacterial, antioxidant and anticancer activity	[ , ]
	Garlic	Bud extract	Selenium	Alkaloid, saponins, flavonoids, glycoside, anthraquinones, tannin, and terpenoids	Anticancer activity	[ , ]
	True	Leaf extract	Selenium	Flavonoids, steroids, terpenoids, proteins, phenols, carbohydrates, reducing sugar, starch, tannins, glycosides	Antimicrobial activity	[ , ]
	Graveolens	Leaf extract	Selenium	Phenoilic compounds and flavonoids	Anticancer activity	[ , ]
	Dentate clausena	Leaf extract	Selenium	Coumarins, carbazole alkaloid, and sesquiterpenes	Larvicidal activity	[ , ]
	Common guava	Leaf extract	Selenium	Quercet in, avicularin, apigenin, guaijaverin, kaempferol, hyperin, myricet in, gallic acid, catechin, epicatechin, chlorogenic acid, epigallocatechin gallate, and caffeic acid	Antibacterial and anticancer activity	[ , ]
	Shaggy button weed	Leaf extract	Selenium	Β-sitosterol, ursolic acid, quercet in, dalspinin, rutin, kaempferol, tannic acid, and epigallocatechin	Antibacterial, antioxidant, antiinflammatory, and anticancer activity	[ , ]
	Cocoa	Seed extract	Selenium	Procyanidins, theobromine, (-)-epicatechin, catechins, and caffeine	Antibacterial activity	[ , ]
	Clove	Bud extract	Copper	Sesquiterpenes, monoterpenes, hydrocarbon, and phenolic compounds	Antimicrobial and antioxidant	[ , ]
	Kobus magnolia	Leaf extract	Copper	Flavonoids, phenols, citric acid, ascorbic acid, polyphenolic, terpenes, alkaloids, and reductase	Antibacterial activity	[ ]
	China-rose	Leaf extract	Copper	Proteins, vitamin C, organic acids (essentially malic acid), flavonoids, anthocyanins	Antibacterial activity	[ , ]
	Golden dewdrop	Fruit extract	Copper	Flavonoids, phenols, saponins, sterols, tannins, alkaloids	Water treatment process	[ , ]
	Fish poison bush	Leaf extract	Copper	Alkaloids, saponin, steroids, tannin, coumarin, flavonoids, diterpenes, cardial glycosides, phenols, and phytosterol	Antidiabetic	[ , ]
	Citron	Fruit extract	Copper	Iso-limonene, citral, limonene, phenolics, flavonones, vitamin C, pectin, linalool, decanal, and nonanal	Antibacterial activity	[ , ]
	Dog’s tongue	Leaf extract	Copper	Glycosides, alkaloids, saponins, flavonoids, phenols, tannins, carbohydrates	Antibacterial activity and antioxidant	[ , ]
	Ceylon caper	Leaf extract	Copper	Fatty acids, flavonoids, tannins, alkaloids, E-octadec-7-en-5-ynoic acid, saponins glycosides, terpenoids, saponin, P-hydroxybenzoic, syringic, vanillic, ferulic, and P-coumanic acid	Antibacterial activity and antioxidant	[ , ]
	Mountain knot grass	Leaf extract	Copper	Phenol and alkaloids	Antimicrobial and catalytic properties	[ , ]
	Guduchi	Leaf extract	Copper	Tinosponone, tinosporin, berberine, palmitine, choline, tembet arine, isocolumbin and tetrahydropalmatine and other alkaloids, steroids, lactones, glycosides, and sesquiterpenoids	Antimicrobial activity	[ , ]
	Calotrope	Leaf extract	Copper	Cardenolides, flavonoids, and saponins	Antitumor activity	[ , ]

Green Synthesis of Silver (Ag) Nanoparticle and Its Applications

Different kinds of plant parts like root, stem, latex, leaf, and seed are utilized for the synthesis of metal nanoparticles. Earlier studies have reported that silver nanoparticles are synthesized by plant extract [ 23 , 54 , 168 , 169 ]. López-Miranda et al. [ 170 ] pointed out that plant synthesized silver nanoparticle by plant extract French tamarisk ( Tamarix gallica ). Chinnappan et al. [ 171 ] described that butterfly tree ( Bauhinia purpurea ) flower extract synthesized silver nanoparticles as a simple and cost-effective method. Lakshmanan et al. [ 172 ] pointed out that plant extract of Asian spider flower ( Cleome viscose ) synthesized by silver nanoparticles. This has an enhanced ability to reduce silver nitrate into metallic silver. Similarly, plant oregano ( Origanum vulgare L .) synthesized by silver nanoparticles in an aqueous solution. The synthesis of the nanoparticle is based on the reduction of Ag + ions and also color changes (light brown to dark brown) of the nanoparticle solution [ 173 ]. The methanolic leaf extract of a five-leaved chaste tree ( Vitex negundo ) is synthesized by spherical shape silver nanoparticles; this nanoparticle has a broad-spectrum antibacterial response [ 174 ]. Rodrıguez-Luis et al. [ 175 ] pointed out that licorice ( Glycyrrhiza glabra ) root extract and cuachalalate ( Amphipterygium adstringens ) bark extract synthesized by the spherical shape of silver nanoparticles in the size range 3–9 nm (TEM). Ibrahim [ 176 ] mentioned that banana ( Musa paradisiaca Linn .) peel extract synthesized by spherical and crystalline silver nanoparticles in the size range of 23.7 nm, and showed that the same material was a better capping and reducing agent by characterizing nanoparticles with various instrumentation techniques such as TEM, SEM, FE-SEM, DLS, and SAED.

Silver nanoparticles are broadly explored in the structure range 1–100 nm. Primarily, the silver nanoparticle is the alternative way to improve biomedical applications such as drug delivery, wound healing action, tissue scaffolding, and protective coating applications. In addition, nanosilver has a notable accessible surface area that allows the binding of any ligands. Silver nitrate is commonly used in the form of antimicrobial activity. A silver nanoparticle is a unique and emerging field against harmful microbes [ 177 ]. A such nanoparticle is of significant importance in its physical, chemical, and biological properties. Silver nanoparticles are considerably more positive effects such as broad-spectrum antimicrobial response, non-toxic, anticancer properties, and other therapeutic purposes, and also capable to form unique, diverse nanostructures and low-cost production [ 178 – 180 ]. Several researchers reported that silver nanoparticles are strong antibacterial effects against E. coli , S. aureus , S. typhus , P. aeruginosa , V. cholera , and B. subtilis [ 40 , 181 , 182 ]. A silver nanoparticle is used to break down the cell wall of organisms and interrupt the whole synthesis process [ 183 , 184 ]. Martinez et al. (2018) examined that silver nanoparticles are good bactericidal properties depending on the size and shape of the nanoparticle, the size range of the nanoparticle decreasing and increasing the antibacterial response. The silver nanoparticle has different shapes like spherical, rod-shaped, and truncated triangular. The truncated triangular shape of the silver nanoparticle has strong antibacterial effects against E. coli . Truncated triangular silver nanoplates have a high contact area and surface reaction [ 185 , 186 ]. Green synthesis of silver nanoparticle structure and dimension also plays a most important role in catalyzing various types of dyes and photocatalytic degradation [ 134 ] and in the wastewater treatment process [ 187 ]. Green synthesized and characterization of silver nanoparticles used to treat antibacterial, antifungal, antimycobacterial, and antimalarial activity against Pseudomonas aeruginosa MTCC-1688, Streptococcus pyogenes MTCC-442, Staphylococcus aureus MTCC-96, Escherichia coli MTCC-44, Candida albicans MTCC 227, Aspergillus niger MTCC 282, Aspergillus clavatus MTCC 1323, Mycobacterium tuberculosis H 37 RV, and Plasmodium falciparum [ 188 ]. Green synthesized ( Leucaena leucocephala ) silver nanoparticles have potential antibacterial, antimycobacterial, and antimalarial activity against Streptococcus pyogenes , Pseudomonas aeruginosa , Bacillus subtilis , Staphylococcus aureus , Escherichia coli , Salmonella typhi , Mycobacterium tuberculosis , and Plasmodium falciparum [ 189 ].

The silver nanoparticle is considered to be a successful and novel pharmacological agent that regulates efficient antiviral activity against Influenza virus , Human parainfluenza virus type 3 , Chikungunya virus , Herpes simplex virus , Norovirus , bovine Herpes virus , Dengue virus , Adenovirus , feline coronavirus (FCoV), and HIV [ 55 , 190 – 199 ]. The silver nanoparticle was used to suppress antiviral effects in a dose-dependent manner by in vitro methods. It also showed that at 9.3 g/mL (EC50 concentration) for 2-h incubation time, the silver nanoparticle can only affect virally infected cells, and not toxic to uninfected cells [ 197 ]. Likewise, silver nanoparticles have decreased the transmission of viruses in personal protection instruments. Graphene-silver nanocomposite was inhibited against feline coronavirus infection 4.7 × 10 4 TCID50 (tissue culture infective dose required to kill 50% of the infected host) at a lower concentration (0.1 mg/mL) of nanoparticle [ 198 ]. The red flush flower ( Lampranthus coccineus ) extract synthesis of silver nanoparticles has significant antiviral activity against Coxsackievirus infection at a lower concentration (12.74 μg/mL). In addition, a molecular docking report pointed out that green synthesized silver nanoparticle was an active binding Coxsackievirus 3c protease against antiviral infection [ 200 ]. Kaushik et al. [ 201 ] showed that a plant pawpaw ( Carica papaya ) leaf extract synthesized silver nanoparticles (125 μg/mL and 62.5 μg/mL) performed as the antiviral factor against chikungunya infection using Vero cells. A lower-level concentration of silver nanoparticles (62.5 μg/mL) was inhibited by 52% against the chikungunya virus . Sharma et al. [ 202 ] reported that bitterweed ( Andrographis paniculata ) leaf extract synthesized silver nanoparticle has significant antiviral activity against chikungunya viral infection. Maximum non-toxic dose concentration 31.25 μg/mL (MNTD) and 15.63 μg/m (½MNTD) showed that level of inhibition (75–100% and 25–49%) was a cytopathogenic response, respectively.

Cancer is one of the mainly dangerous diseases in the world. The several side effects performed in classical cancer therapy and their poor tolerance performance became the purpose for an enormous level of search for novel drugs of natural origin that are capable to regulate the disease’s progress and heal it. The silver nanoparticle is a wide noteworthy factor for cancer analysis. The silver nanoparticles were stimulated to activate the p53 tumor suppressor. In addition, silver nanoparticles are performed to treat higher toxic response against cancer cells compared to non-cancer fibroblasts [ 203 ]. The plant catmint ( Nepeta deflersiana ) synthesized silver nanoparticle provoked apoptosis and cell death through cell necrosis HeLa stops the sub G1 cell cycle [ 61 ]. Jacob et al. [ 204 ] examined that pepper black pepper ( P. nigrum ) leaf extract synthesized silver nanoparticle has induced cytotoxic activity against cancer cells. The Indian gooseberry ( Phyllanthus emblica ) leaf extract synthesized silver nanoparticle has significant anticancer properties against Hepatocellular carcinoma (HCC) [ 205 ]. The two different size variations of silver nanoparticles demonstrated anticancer activity against MCF-7 and T-47D cells. The silver nanoparticle induced endoplasmic reticulum stress (EPR) through unfolded protein, and also increased the activation of caspase 9 and caspase 7, causing cell death [ 206 ]. The silver nanoparticles are recogniith an antiinflammatory reaction via playing a role in the wound healing progression, due to interleukin 1 and TNF-ά interferons and also suppression of COX-2 and MMP-3 expressions. The silver nanoparticle has significantly reduced the function of TNF-ά during the inflammatory process [ 207 – 210 ]. The black pepper ( Piper nigrum ) leaf extract synthesized silver nanoparticle has a selected cytokine inhibitory factor for IL-1 and IL-6 [ 211 ]. European cranberry ( Viburnum opulus ) bush fruit extracts synthesizing Ag nanoparticle was noted to show an antiinflammatory response by in vitro and in vivo methods, the Ag nanoparticles being used to enhance the potential therapeutic analysis for the treatment of inflammation [ 182 ]. Ag nanoparticle is an efficient tool for numerous biomedical applications such as antimicrobial, antifungal, antiviral, catalysis, wound healing and dressing, implanted materials, tissue engineering, anticancer therapy, and medical devices (catheters, prostheses, vascular grafts). In addition, an Ag nanoparticle is used in some diagnostic processes like antipermeability factors, bio-sensing, and dental preparations [ 212 ].

Green Synthesis of Gold (Au) Nanoparticle and Its Applications

Green synthesized Au nanoparticles are in natural form. They also have chemical and thermal functionality. Gold-based nanoparticles can join photosensitizers for photodynamic antimicrobial chemotherapy [ 118 , 213 ]. Green synthesized gold nanoparticle was a good catalytic reaction without a capping agent and non-toxic carrier, the gold nanoparticle was used to improve the drug and gene delivery applications [ 214 ]. With these systems, the gold core provides stability to the congregation through the single layer recognition to change the surface properties like charge and hydrophobicity. In addition, a characteristic feature of gold nanoparticles was connected to thiols, as long as the efficiency of controlled intracellular release [ 215 ]. The gold nanoparticles contain different kinds of characteristics; the distinguishing features can be changed in size, shape, and aspect ratio of nanoparticles. The gold nanoparticle is non-toxic and biocompatible for drug delivery and gene therapeutic applications. The gold nanoparticle has various aspects of the biomedical progression such as easily detecting and diagnosis heart disease, cancer, and infectious factor [ 216 , 217 ].

The pummelo ( Citrus maxima ) aqueous extract solution synthesized gold nanoparticles [ 218 ]. The Indian long pepper ( Piper longum ) extract synthesized gold nanoparticle size range of 56 nm was confirmed by the DLS particle size analyzer [ 219 ]. The onion ( Allium cepa ) extract synthesized gold nanoparticles in the size range of 100 nm [ 220 ]. The wild spinach ( Chenopodium album ) leaf extract synthesized gold nanoparticle sizes ranging from 10 to 30 nm and its shape is quasi-spherical [ 221 ]. The Japanese Honeysuckle ( Lonicera Japonica ) flower extract synthesized gold nanoparticle size ranging from 8.02 nm (average diameter) [ 222 ]. The gold nanoparticle detected, diagnose, and healing of many diseases. Au nanoparticle is a very important tool for biomedical applications [ 223 , 224 ]. The green synthesized gold nanoparticles are considered through various instrumentation techniques such as UV–Vis, Fourier transforms infrared (FTIR), SEM, and high-resolution transmission electron microscopy (HRTEM), and also the gold nanoparticle is mainly concerned to enhance various biomedical applications such as antibacterial, drugs delivering factor for therapeutic cancer level from chemical hazardous to biomolecules, catalysis, photonics, electronics, and sensing [ 84 , 225 – 228 ]. The view-rakot ( Nepenthes khasiana ) leaf extract synthesized gold nanoparticles categorized by different instrumental analyses like XRD, SEM, and TEM after analysis to confirm the size range of 50–80 nm of nanoparticle [ 229 ]. The influencing factor is significantly important in the synthesis of nanoparticles; the gold nanoparticle factors range as follows: pH (6–9), temperature (20 to 80 °C), the concentration of mixture (5 to 10 mL), UV–Vis spectrum wavelength of 550 nm. These ranges are helpful to verify and synthesis gold nanoparticles [ 230 – 232 ].

Lysenko et al. [ 233 ] examined the synthesis of two gold nanoparticles coated with silicon dioxide shells and gold–silicon dioxide carrier nanoparticles; these gold nanoparticles reduced the toxicity of gold ions, and also both gold nanoparticles are strong antiviral effects against adenovirus. The synthesized gold nanoparticles were treated to antiviral activity against the dengue virus and were associated with the suppress action of RNA against the dengue virus. The gold nanoparticles performed to go into the infected Vero cells and reduced the response of dengue virus serotype 2 (DENV-2) replication and infectious virion release; these treatments were carried out during post- and pre-infection conditions [ 234 ]. The novel vaccine with hybrid coated gold nanoparticles and domain III of the viral envelope protein (EDIII) response antiviral activity against the dengue virus. The complex coated hybrid gold nanoparticle response differs, depending on the size and concentration [ 235 ]. The ethanolic and hydroalcoholic extract of gold nanoparticles showed antitubercular activity, and two different types of concentrations are identified as the response such as MIC (2.5 μg/mL and 20 μg/mL) and the highest concentration level (50 μg/mL and 75 μg/mL), respectively [ 236 ]. The wild fennel flower ( Nigella arvensis ) (NA- gold nanoparticles) leaf extract synthesized gold nanoparticles looked into that different applications such as antioxidant, catalytic activities antibacterial, and cytotoxicity. The plant-mediated synthesis of gold nanoparticles improved cytotoxic activity against cancer cell lines (H1299 and MCF-7) with an IC50 value of (10 and 25 μg/mL) and a catalytic reaction against methylene blue was 44%, respectively [ 237 ]. Au nanoparticles with size and shape range have some excellent characteristic features of biomedical applications such as antimicrobial and antibacterial activities, antiviral treatments, anticancer therapy, targeted drug delivery, medical imaging, molecular imaging in living cells, photothermal therapy, hyperthermic property to treat tumors, biocatalysis and biomarkers, biosensors, and intracellular analysis [ 238 ].

Green Synthesis of Iron (Fe) Nanoparticles and Their Applications

The green synthesized zero-valent iron nanoparticles are produced from different plants such as thyme ( Thymus vulgaris (TV)), Damask rose ( Rosa damascene (RD)), and stinging nettle ( Urtica dioica (UD)) [ 239 ]. Green synthesized iron nanoparticles from various plants such as common water-hyacinth ( Eichhornia crassipes ), lantana ( Lantana Camara ), and sensitive plants ( Mimosa pudica ) . This plant synthesized iron nanoparticle standard size ranges 20–60 nm and morphological structure is irregular and aggregated quasi-spherical shape respectively [ 240 ] . However, the green synthesis of iron nanoparticles was a major process in enhancing the physical, chemical, and biological properties of nanoparticles [ 241 , 242 ]. Plant extract synthesized iron nanoparticle has antioxidant capability following some procedures like 2, 2-diphenyl- 1-picryl-hydroxyl (DPPH) radical scavenging assay, Folin-Ciocalteu method, and ferric reducing antioxidant power (FRAP) [ 243 – 245 ]. Different phytochemical components (polyphenols, flavonoids, and amino acids) played an imperative role in the synthesis of iron nanoparticles. These phytochemical synthesized nanoparticles are efficient antioxidant activity [ 108 , 246 , 247 ].

The green synthesis approach in developing zero-valent iron nanoparticles was an important process for the treatment of brominated organic compounds, pesticides, azo dyes, alkaline-earth metals, malachite green, nitrate, monochlorobenzene, antibiotics, and conversion of some metals like chromium, cobalt, and copper [ 112 , 247 – 255 ]. The aqueous solution of rose apple ( Syzygium jambos (L.) Alston ) leaf extract synthesized zerovalent iron nanoparticles eliminated to hexavalent chromium metal. Complete removal of chromium was based upon nanoparticles dosage, temperature, and pH. The appropriate pH and concentration of nanomaterials are considerable factors in the removal of chromium metal [ 256 ]. However, the coconut husk extract synthesized magnetite iron nanoparticles absorbed a low level of calcium and cadmium [ 257 ]. The aqueous solution leaf extract at various ratios and temperatures synthesized zero-valent iron nanoparticles. The process confirmed by color changes of the nanosolution as well as phyto components was the significant agent for reducing and stabilizing factors in the synthesis of zero-valent iron nanoparticles [ 250 , 258 , 259 ]. Similarly, zero-valent iron nanoparticles are characterized by the following methods such as SEM, TEM, and zeta potential. The classification analysis was confirmed by the shape, size, and stability of nanoparticles. The zero-valent iron nanoparticle was helpful in the removal of lead based on time duration and concentration (low quantity of lead removal takes lesser time and low concentration higher quantity removal of lead takes high concentration and more time) [ 260 , 261 ]. Nanotechnology and iron nanomaterials have different kinds of applications such as drug delivery [ 262 , 263 ], electronics, biotechnology, catalysis [ 110 , 264 , 265 ], environmental remediation [ 114 , 266 ], cosmetics, space industry, anticancer and drug delivery [ 116 , 267 , 268 ], and materials science. Wei et al. [ 220 ] pointed out that those citrus maxima peel extract synthesized iron nanoparticles played a major role in waste minimization and well-organized resource utilization. The most comprehensive study reports pointed out that the synthesis of iron nanoparticles was utilized for remediation of water and soil, and the zero-valent iron nanoparticles are efficient catalytic properties [ 269 ].

The Mediterranean cypress ( Cupressus sempervirens ) aqueous leaf extract synthesized iron nanoparticle was important effects to removing dye from wastewater depending on time and concentration manner. The synthesized iron nanoparticle was used to remove methyl orange (95% at 6 h) [ 270 ]. In addition, tea plant ( Camellia sinensis) leaf extracts synthesized iron nanoparticle was utilized for the reduction of bromophenol blue pH indicator [ 250 ]. The green leaf extract synthesized iron nanoparticle was used to treat malachite green dye degradation [ 271 ]. The gum trees ( Eucalyptus globules ) leaf extract synthesized iron nanoparticles demonstrated that the marvelous reduction possible against eutrophic wastewater [ 272 ]. The tangerine ( Citrus reticulate ) peel extract synthesized iron nanoparticles utilized to remove the cadmium in the water system [ 273 ]. The green synthesis approach to developing iron nanoparticles has reduced the level of chromium depending on the concentration manner (1 mg of iron nanoparticle reduces 500 mg of chromium) [ 274 ].

Iron nanoparticles are more significant in the biomedical field. The flat-crown ( Albizia adianthifolia ) leaf extract synthesized iron nanoparticles utilized to treat MCF-7 and AMJ-13 cancer cell lines and cause apoptosis [ 275 ]. In addition, sugar apple ( Annona squamosa ) leaf extract synthesized iron nanoparticles showed an efficient cytotoxic reaction against cancer cell line (HepG2) [ 274 ]. Similarly, Babchi ( Psoralea corylifolia ) leaf extract synthesized iron nanoparticle exhibited antitumor activity against renal carcinoma cell line (Caki-2 cells) [ 276 ]. The green synthesis approach to developing nanoparticles was of significant importance to treat human pathogens [ 277 ].

Green Synthesis of Selenium (Se) Nanoparticle and Its Applications

Green synthesis of selenium nanoparticles is environmentally safe, cost-effective, non-toxic, and easily produced in large quantities [ 278 , 279 ]. The plant-mediated synthesis of selenium nanoparticles is non-toxic and cost-effective method as well as the plant phytochemical components (polyphenols, flavones, carboxylic acids aldehydes, amides, and ketones) were a significant role in the reduction of metal ions during the synthesis of selenium nanoparticle [ 132 , 280 – 286 ]. Different types of plant parts such as leaf extracts, fruit, seed, fruit peels, and root synthesized selenium nanoparticle in different shapes and sizes [ 129 , 132 , 287 – 291 ].

The plant-mediated synthesis of selenium nanoparticles performs to remove the heavy metal from the contaminated solution depending on the size and shape of the nanoparticles. In addition, green synthesized selenium nanoparticles carry out to remove heavy metals (zinc, copper, and nickel) from the soil, and also another study reported that selenium nanoparticles performed to remove elemental mercury from soil and air [ 281 , 292 , 293 ].

The selenium nanoparticle applications in different ways include as follows: (1) increasing shelf life of food; (2) antioxidant and antimicrobial response in preserved food; (3) maintenance of health and growth [ 283 , 294 – 296 ]. The selenium nanoparticle is the most imperative food supplement as well as increased the bioavailability, controlled release of selenium in organisms. Selenium nanoparticle medicine is a major biotherapeutic agent without any side effects [ 297 ]. The selenium nanoparticle has various kinds of physiological responses in humans like antioxidant action, preventing tumor formation, and regulating the immune system [ 298 , 299 ]. Vyas and Rana [ 300 ] pointed out that cultivated garlic ( Allium sativum ) bud extract synthesized selenium nanoparticles demonstrated an antioxidant response following assays FRAPS, ABTS, and DPPH. The tea extract synthesized selenium nanoparticle was established for its antioxidant activity by using different methods such as ABTS and DPPH assays [ 301 ].

The cultivated garlic ( Allium sativum ) buds extract synthesized selenium nanoparticles investigated for its antibacterial response against Bacillus subtilis and Staphylococcus aureus by using the disc diffusion method [ 302 ]. The horseshoe geranium ( Pelargonium zonale ) leaf extract synthesized selenium nanoparticle was revealed antibacterial and antifungal activity against pathogenic bacteria, namely Escherichia coli , Staphylococcus aureus , and fungi, namely black dot of potato ( Colletotrichum coccodes ) and green mold ( Penicillium digitatum ) [ 136 ]. The bombay ebony ( Diospyros montana ) leaf extract synthesized selenium nanoparticles expressed by antibacterial activity against Escherichia coli , Aspergillus niger , and Staphylococcus aureus , anticancer activity in (MCF-7) concentration manner, and antioxidant activity (DPPH), respectively [ 291 ]. Selenium nanoparticle was significantly active against enterovirus. Combined with oseltamivir, the combined drugs strongly inhibited enterovirus and also reduced ROS production in astrocytoma cells [ 303 ]. The selenium nanoparticle was indicated to suppress p38 kinase, ROS production, and Jun amino-terminal kinase (JNK) signaling pathway. The selenium nanoparticles reduced the viral protein and viral yield, and control the ROS production [ 304 , 305 ]. Ulva fasciata (UF extract) synthesized selenium nanoparticles have strong antibacterial activity against S. aureus and P. aeruginosa and also potential anticancer agents [ 306 ]. Green synthesized ( Solanum nigrum ) selenium nanoparticles have strong inhibitory action against gram-positive and gram-negative bacteria as well as antioxidant and anticancer (inhibition of breast cancer cells) efficacy posed by selenium nanoparticles [ 307 ].

Cancer nanobiotechnology is a new way to detect, diagnose, and treatment of cancer. Green synthesized selenium nanoparticle is a rapid technique in different cancer cells, namely human colon adenocarcinoma, liver cancer cells, human breast cancer cells, and Ehrlich ascites carcinoma [ 291 ]. The cultivated garlic ( Allium sativum ) bud extract synthesized selenium nanoparticles demonstrated a positive cytotoxic response against the Vero cell line [ 308 ]. The smaller size of selenium nanoparticles connected with cancer cells through the action of DNA breakage ultimately leads to cause cell death and also performs cytotoxic activity against cancer cells [ 309 ]. The green synthesized selenium nanoparticle exhibited anticancer properties in some cancer cells (human cervical carcinoma cells, liver cancer, and lung cancers) whereas a few cancer cells (ovarian cancer, leukemia cancer, colon cancer, skin cancer, prostate cancer) are still needed to be investigated [ 310 ]. The green synthesized selenium nanoparticle still needs to be investigated in the therapeutic approaches in glioma, and also the size and spherical shape of the selenium nanoparticle were a significant response to anticancer activity [ 311 ].

Green Synthesis of Copper (Cu) Nanoparticle and Its Applications

The green synthesis technique is a significantly imperative approach for the production of nanoparticles. The process is an effective and efficient tool compared to other methods, while being non-toxic, eco-friendly, and cost-effective. The copper nanoparticles are utilized to enhance biomedical applications such as antibacterial, antifungal, and antiviral activity [ 312 ]. Plant-mediated synthesized copper nanoparticles from various types of plants such as fire lily ( Gloriosa superba L .), common grape ( Vitis vinifera, Nerium ), Nerium ( Nerium oleander ), Ceylon caper ( Capparis zeylanica ), and jackfruit-Champa ( Artabotrys odoratissimus ) [ 147 , 152 , 154 , 313 , 314 ]. The fire lily ( Gloriosa Superba L ) leaf extract and bark extract of pomegranate ( Punica granatum ) synthesized copper nanoparticle enhanced reducing and capping agents size range of nanoparticle 23 nm [ 315 – 317 ]. Different kinds of plant extract, namely Magnolia ( Nag Champa ) ( Artabotrys Odoratissimus ) and angel’s trumpet ( Datura innoxia ) synthesized by copper nanoparticles in 8–10 min and size range of 4–100 nm [ 152 ]. Saranyaadevi et al. [ 154 ] showed the classification confirmation of green synthesized copper nanoparticles through the UV–Vis absorption peak 531 nm, XRD — particle size 5 nm, and TEM — size (50–100 nm).

Copper is an essential micronutrient for plants, and also 70% of total copper presence in chloroplasts as well as copper plays a significant role in the synthesis of plant pigments, chlorophyll, carbohydrate metabolism, and amino acids [ 150 ]. Copper is an important mineral for humans with dissimilar kinds of functions: strength of the skin, blood vessels, the connective tissue of the body, production of hemoglobin, myelin, and melanin, and regulation of the thyroid functions [ 318 ]. The copper nanoparticles role in many fields such as agricultural, industrial engineering, and technological fields. The green synthesis of the copper nanoparticle is a cost-effective, cheap, non-toxic, and eco-friendly method. The copper nanoparticles were confirmed to provide an efficient antibacterial response in agricultural research fields [ 319 ]. Copper nanoparticles have numerous types of properties such as mechanical, thermal, magnetic, and electrical and are also used for antimicrobial coatings for surgical tools, water treatment, and heat transfer processes [ 320 ].

Golden dewdrop ( Duranta erecta ) fruit extract synthesized copper nanoparticle reduced toxic dye (methyl orange, azo dyes, Congo red) degradation from water [ 321 ]. The fish poison bush ( Gnidia glauca ) leaf extract and Ceylon leadwort ( Plumbago zeylanica ) leaf extract synthesized copper nanoparticle’s response as an antidiabetic factor [ 322 ].

The clove ( Syzygium aromaticum ) bud extract synthesized copper nanoparticles are a more effective response in antimicrobial and antioxidant function [ 323 ]. The copper nanoparticles act as a fungicide agent against different plant pathogens, namely black rot ( Alternaria alternata ), basal rot ( Fusarium oxysporum ), crown and root rot ( Rhizoctonia solani ), blue mold ( Penicillium italicum ), fruit and stem rot ( Phoma destructive ), basal rot ( Fusarium sp.) green mold ( Penicillium digitatum ), and black kernel ( Curvularia lunata ) [ 324 ]. Similarly, Kobus magnolia ( Magnolia kobus ) leaf extract synthesized copper nanoparticle was a significant antibacterial response against E. coli [ 325 ]. Green synthesized copper oxide nanoparticles have strong antibacterial and antifungal effects against Staphylococcus aureus ( S. aureus ), Streptococcus pyogenes ( S. pyogenes ), Pseudomonas aeruginosa ( P. aeruginosa ), and Escherichia coli ( E. coli ), and good antifungal performance against Aspergillus niger ( A. niger ), Candida albicans ( C. albicans ), Aspergillus clavatus ( A. clavatus ), and Epidermophyton floccosum ( E. floccosum ) [ 326 ]. Green synthesized copper nanoparticles have potential antibiofilm, antibacterial, and antioxidant activity, and these nanoparticles used as an alternative therapy for microorganisms that have been improved antibiotic resistance [ 327 ]. Green synthesized ( Sesbania aculeata leaf extract) copper nanoparticles are helpful to increase plant growth of Brassica nigra at lower dose (25 and 30 mg/100 mL) and also play a good antimicrobial agent, and these nanoparticles are helpful to enhance protection and production of plants [ 328 ].

Several studies report strongly that copper nanoparticle was a good antiviral agent [ 329 ]. The green synthesis of copper nanoparticle size about 160 nm demonstrated the antiviral activities against influenza virus of swine-origin by plate titration assay. The copper nanoparticle reduced the viral protein so these nanoparticles are utilized to produce face masks and kitchen cloths. The copper nanoparticle (40–120 μg/mL) size (39.3 nm) and spherical shape were significant cytotoxic effects against human cancer cell lines such as human skin carcinoma cells (B16F10) and normal mouse embryonic fibroblasts (NiH3T3) within 24 h [ 330 ]. The China-rose ( Hibiscus rosa-Sinensis ) leaf extract synthesized copper nanoparticles are confirmed that an effective antimicrobial response against clinical pathogens like E. coli and Bacillus subtilis , and also this element reaction is an achievement against lung cancer [ 53 ].

Toxicity of Metal Nanoparticle Aspects

The toxicity of metal nanoparticles is associated with oxidative stress reactions and intracellular reactive oxygen species (ROS) production, as well as the activation of pro-inflammatory mediators. As a result of this, DNA and protein damage, lysosomal hydrolases, mitochondrial dysfunction, apoptosis, cell membrane damage, cytoplasm disorder, changes in ATP, and cell membrane permeability can be considered, which finally includes cell dysfunction. However, depending on the size, type of particles, individual particles, and mixtures, it can change the toxic effects (Fig. 4 ) [ 56 ].

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Mechanism of metal nanoparticles in biomedical and environmental applications

Nanoparticles are an effective tool in different fields such as food, agriculture, medicine, micro-wiring, electronics, and energy harvesting. The synthesis of nanoparticles uses physical, chemical, and biological methods. Green synthesis ways appear more effective and efficient compared to other related methods. The green synthesis method is an eco-friendly, non-toxic, and cost-effective method. In this review, we summarize especially information about various syntheses, characterization, and applications of plant-based synthesized metal and metal oxide nanoparticles are utilized to analyze antibacterial, antifungal, antimalarial antioxidant, anticancer, photocatalytic, and metaltoxicity properties. These studies strongly recommended green synthesis approach to develop metal and metal oxide nanoparticles more beneficial response in environmental and biomedical applications. In the future, our research group focuses to synthesis different types of green nanoparticles utilize to develop the application different sectors like pharamaceutical, medical, environment, aquaculture, and agriculture. This study result enlightens the direction of future research in the green nanoparticle development in environmental and biomedical sectors.

Author Contribution

SV did the supervision, resources, writing — original draft of the paper. HR, YZS, HG, SHH, and MR performed writing — review and editing the paper. SV did the process of investigation and conceptualization. SV wrote and prepared the original draft. SV had close supervision on the process of preparing paper, too. HR, YZS, and SHH did the project administration. All authors have read and agreed to the published version of the manuscript.

We wish to acknowledge the National Natural Science Foundation of China (Grant No. 32072990), Xiamen Marine and Fisheries Development Fund (Grant No. 19CZP018HJ04), and Industry-University Cooperation Project of Fujian Province (Grant No. 2018N5011) for supporting this research work.

Data Availability

Declarations.

The authors declare no competing interests.

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Contributor Information

Yun-Zhang Sun, Email: moc.361@gnahznuynusumj .

Hamed Ghafarifarsani, Email: ri.ca.tu.inmula@irafahg_demah .

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Published: 06 August 2024

Green synthesis of magnetite iron oxide nanoparticles using Azadirachta indica leaf extract loaded on reduced graphene oxide and degradation of methylene blue

Muhammad Shahbaz Akhtar 1 ,
Sania Fiaz 1 ,
Sohaib Aslam 1 ,
Shinho Chung 1 ,
Allah Ditta 2 , 3 ,
Muhammad Atif Irshad 4 ,
Amal M. Al-Mohaimeed 5 ,
Rashid Iqbal 6 , 7 ,
Wedad A. Al-onazi 5 ,
Muhammad Rizwan 8 &
Yoshitaka Nakashima 9

Scientific Reports volume 14 , Article number: 18172 ( 2024 ) Cite this article

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Biomaterials
Environmental sciences

In the current arena, new-generation functional nanomaterials are the key players for smart solutions and applications including environmental decontamination of pollutants. Among the plethora of new-generation nanomaterials, graphene-based nanomaterials and nanocomposites are in the driving seat surpassing their counterparts due to their unique physicochemical characteristics and superior surface chemistry. The purpose of the present research was to synthesize and characterize magnetite iron oxide/reduced graphene oxide nanocomposites (FeNPs/rGO) via a green approach and test its application in the degradation of methylene blue. The modified Hummer's protocol was adopted to synthesize graphene oxide (GO) through a chemical exfoliation approach using a graphitic route. Leaf extract of Azadirachta indica was used as a green reducing agent to reduce GO into reduced graphene oxide (rGO). Then, using the green deposition approach and Azadirachta indica leaf extract, a nanocomposite comprising magnetite iron oxides and reduced graphene oxide i.e., FeNPs/rGO was synthesized. During the synthesis of functionalized FeNPs/rGO, Azadirachta indica leaf extract acted as a reducing, capping, and stabilizing agent. The final synthesized materials were characterized and analyzed using an array of techniques such as scanning electron microscopy (SEM)-energy dispersive X-ray microanalysis (EDX), Fourier transform infrared spectroscopy (FT-IR), X-ray diffraction analysis, and UV–visible spectrophotometry. The UV–visible spectrum was used to evaluate the optical characteristics and band gap. Using the FT-IR spectrum, functional groupings were identified in the synthesized graphene-based nanomaterials and nanocomposites. The morphology and elemental analysis of nanomaterials and nanocomposites synthesized via the green deposition process were investigated using SEM–EDX. The GO, rGO, FeNPs, and FeNPs/rGO showed maximum absorption at 232, 265, 395, and 405 nm, respectively. FTIR spectrum showed different functional groups (OH, COOH, C=O), C–O–C) modifying material surfaces. Based on Debye Sherrer's equation, the mean calculated particle size of all synthesized materials was < 100 nm (GO = 60–80, rGO = 90–95, FeNPs = 70–90, Fe/GO = 40–60, and Fe/rGO = 80–85 nm). Graphene-based nanomaterials displayed rough surfaces with clustered and spherical shapes and EDX analysis confirmed the presence of both iron and oxygen in all the nanocomposites. The final nanocomposites produced via the synthetic process degraded approximately 74% of methylene blue. Based on the results, it is plausible to conclude that synthesized FeNPs/rGO nanocomposites can also be used as a potential photocatalyst degrader for other different dye pollutants due to their lower band gap.

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Introduction.

Global freshwater resources are limited and declining day by day. Water quality is challenged due to point and non-point sources of pollution. Freshwater resources are contaminated with industrial discharges/effluents and domestic wastes with nuisance organic and inorganic pollutants such as polyaromatic hydrocarbons (PAHs), persistent organic compounds (POPs), dyes, metals, and metalloids 1 , 2 . For safe and clean water, noxious pollutants should be removed from water by economically viable and efficient approaches. Rapid population expansion, increasing industrialization, urbanization, and pervasive agricultural practices have produced wastewater, which has rendered the water not just polluted but also toxic. On a global scale, millions of people die each year from illnesses caused by the consumption of water contaminated with dangerous microorganisms and toxic pollutants 3 . The treatment of wastewater has become a real global challenge. The requirement for chemicals, the synthesis of disinfection byproducts, the length of the process, and economics are the limiting factors for the use of different wastewater treatment techniques that have been reported over the last few decades 4 .

Aquatic ecosystems are being contaminated with dyes used in the textile industry. Approximately, 15% of globally produced dyes end up as toxic effluents and contaminants 5 . During textile chemical processes, artificial dyes such as azo dyes undergo rapid decomposition and become toxic to the environment due to the formation of aromatic amines because of double bonds in nitrogen 6 . Thus, effluents discharged from industrial chemical processes result in the coloration of water and the addition of toxic/hazardous substances that are a real threat to aquatic ecosystems 7 , 8 . Therefore, International Environmental Standards (IES) have been imposed to create awareness among the public about the threats of effluents released from industry 8 . Different physicochemical and biological approaches are reported in pertinent literature for the removal of pollutants e.g., dyes from the wastewater 9 , 10 . Conventional methods such as flocculation/coagulation, electrochemical and advanced oxidative processes, activated sludge process, reverse osmosis technique, and sorption are thoroughly investigated for the removal of pollutants 11 , 12 , 13 , 14 , 15 , 16 , 17 . Recently, the photocatalytic degradation technique emerged as a promising approach by using catalysts and light sources. In this technique, the generation of photo-induced OH radicals accelerates the photodegradation of organic effluents into non-toxic chemical species such as water and carbon dioxide without reliance on any separation technique 18 , 19 , 20 , 21 , 22 , 23 . The treated wastewater can be reused in agricultural, chemical, and textile sectors.

These days, nanotechnology is emerging as a powerful interdisciplinary tool and gaining much interest because of its performance and efficacy. Its extraordinary ability to build new atomic structures has already sparked the development of cutting-edge tools and materials with a diverse variety of applications 24 . Nowadays, multifunctional next-generation nanomaterials and nanocomposites are being synthesized by exploiting the physicochemical characteristics of nanostructures. These physicochemical properties include low density, high adsorptive surface area, more functional groups, differential shapes/orientations, and chemical fitness and suitability 25 , 26 . Along with other diverse applications, these nanostructures can treat water/wastewater. Because of their high surface-to-volume ratio, high sensitivity and reactivity, high adsorption capacity, and ease of functionalization, nanomaterials and nanocomposites are particularly well suited for application in wastewater treatment. Different wastewater treatment techniques include adsorption/biosorption, nanofiltration, photocatalysis, disinfection, and sensor technologies 27 . More recently, the use of nanocomposite photocatalysts is gaining much interest as a cutting-edge technique for the breakdown of pollutants present in wastewater. This technology has shown tremendous potential to treat wastewater since it uses nanocomposite photocatalysts made of doped graphene 28 .

The superior and wonderful substance known as graphene (G) has a two-dimensional (2D) hexagonal honeycomb lattice structure with a benzene (C 6 H 6 ) ring. Pristine graphene is referred to be a semi-metal or zero-bandgap material since there is no energy difference between its valence and conduction bands. Graphene can be synthesized by two major routes i.e., bottom-up and top-down routes 29 . The discovery of graphene has had a profound impact on different scientific, engineering, agricultural, medical, and environmental disciplines, particularly after the investigation of Novoselov et al. 30 . Graphene is characterized by exceptional traits such as high electro-thermal conductivity, low density, and excellent mechanical, carrier, and optical properties 31 , 32 , 33 . To improve the solubility, conductivity, and physicochemical properties of G, functionalization (covalent or non-covalent) and fabrication of G are done to synthesize modified graphene-based materials such as graphene oxide (GO), reduced GO, G-based derivates, and nanocomposites. These G-based nanomaterials/nanocomposites are superior in morphological and physicochemical characteristics 25 , 26 , 34 , 35 .

At the advent of the Industrial Revolution, point and non-point releases of industrial effluents and hazardous wastes have increased environmental risks. Both colored and non-colored industrial wastewater effluents have detrimental effects on human health because of the presence of different pollutants. Several methods, such as adsorption onto adsorbents like activated carbon, flocculation, chemical oxidation, ultrafiltration, and clays are used to treat industrial wastewater. Due to the high cost of these technologies, photocatalysis is considered one of the most straightforward and ecologically benign methods for the treatment of wastewater. The key advantage of this approach is that water pollution is broken down using solar energy and a straightforward laboratory setup 36 .

Recently, a greener approach has been developed to synthesize nanoparticles and nanocomposites by using green chemistry. Conventional methods for synthesizing nanoparticles have a variety of negative effects on the environment and human life. When synthesizing nanoparticles/nanocomposites with traditional methods, toxic/hazardous chemicals and high temperatures are frequently used. To meet this challenge, concepts of green chemistry are manipulated in science and engineering that have resulted in different eco-friendly fields such as green nanotechnology. This approach helps in establishing green processes that are clean, safe, and environment friendly that can replace in practice chemical and physical processes producing nanomaterials and nanocomposites. More recently, green synthesis has been gaining much interest because it is eco-friendly, cost-effective, natural, renewable, and safe with minimal generation of toxic/hazardous substances and uses no or less strong oxidizing or reducing 37 , 38 , 39 , 40 . Plant-based extracts are enriched with different compounds and substances such as terpenoids and phenolic compounds that can be deployed for the reduction of metallic salts to nanoparticles and can hinder nanoparticle aggregation due to capping or stabilizing characteristics 41 , 42 . Azadirachta indica used in the present investigation is enriched in diverse phytochemicals and compounds such as carotenoids, terpenoids, triterpenoid (nimbin), flavonoids, glycosides, alkaloids, salannin, tannin and phenolic substances 37 , 43 , 44 , 45 . Phytochemicals derived from Azadirachta indica have the potential to sorb on metallic nanoparticle surfaces in addition to their capping property. Furthermore, reducing sugars in the extract of Azadirachta indica is capable of reducing metallic ions and can form metallic nanoparticles 46 , 47 , 48 . Natural products and their derivatives, including wine, different amino acids, glucose, and plant extracts, all include polyphenols, which function as reducing, capping, and stabilizing agents. Similarly, microorganisms including bacteria, yeast, and algae can also be used for the ecologically benign synthesis of nanoparticles 49 . The present study was conducted to synthesize and characterize iron oxide/reduced graphene oxide (FeNPs/rGO) nanocomposites synthesized via the green approach and tested its application in photocatalytic degradation of methylene blue.

Materials and methods

Fresh leaves of Azadirachta indica were taken from the botanical garden of Forman Christian College University, Lahore (31°32′58.9′′ N 74°20′37.0′′ E). Reagent grade Iron (III) chloride hexahydrate (FeCl 3 ·6H 2 O), methylene blue, sodium hydroxide (NaOH), graphite powder, concentrated sulfuric acid (H 2 SO 4 ), potassium permanganate (KMnO4), hydrogen peroxide (H 2 O 2 ), and hydrochloric acid (HCl) were obtained from Sigma-Aldrich and utilized without additional purification.

Preparation of leaf extract

The branches of Azadirachta indica were stripped of their leaves after collection from the botanical garden of Forman Christian College, University, Lahore. To remove the dirt and debris, collected leaves were washed with tap water (once) and distilled water (twice). After washing, the leaves were air-dried for a week in the shade. Using an electronic grinder, dried leaves of Azadirachta indica were ground into a fine powder, which was then sieved through a fine mesh sieve. In a 500 mL beaker, 1.35 g of plant powder and 200 mL of distilled water were added. On a hot plate, stirring was done continuously for two hours in a beaker. The plant extract was filtered using Whatman no. 40 filter paper after two hours of stirring. After filtration, 160 mL of brown-colored plant extract was collected. This was in line with the method reported by Bhuiyan et al. 50 .

Synthesis of graphene oxide

One gram of graphite powder and 50 mL of concentrated H 2 SO 4 were added to a 500 mL beaker and agitated for about 30 min to synthesize graphene oxide (GO) using a modified Hummer technique. The graphite powder gave the solution its dark color. For one hour, the mixture was cooled to below 5 °C. Six grams of KMnO 4 were added to the reaction mixture and then continuously agitated for two hours at a temperature below 15 °C. Then, 90 mL of distilled water was added dropwise to start the oxidation process. After the reaction, the liquid turned dark brown, and the temperature was constantly maintained below 30 °C and stirred for two hours. Then, 280 mL of distilled water and 6 mL of H 2 O 2 were added to terminate the reaction and to remove any extra KMnO 4 . When the solution was forcefully swirled, a brilliant yellow hue resulted from the reaction. The reaction mixture was filtered before being cleaned with 10% HCl and distilled water. The final product was oven-dried for eight hours at 80 °C. The resultant GO displayed a similar appearance and properties as reported by Ishtiaq et al. 51 .

Green synthesis of reduced graphene oxide using plant extract

Reduced graphene oxide (rGO) was synthesized via a green approach by using Azadirachta indica plant extract as a capping agent. One gram of synthetic graphene and 5 g of plant extract were combined in a 500 mL beaker, and the mixture was then continuously swirled at 70 °C for an hour to accomplish full reduction. Centrifuged after full reduction, and then rinsed with distilled water and ethanol to achieve pH neutrality following an oven drying process at 80 °C for about 8 h. The method was followed as reported by Anwar et al. 52 .

Synthesis of magnetite iron nanoparticles

Ferric chloride hexahydrate (FeCl 3 .6H 2 O) was used as a precursor material to synthesize magnetite Fe 2 O 3 nanoparticles. At room temperature, a 1:1 mixture of 50 mL of Azadirachta indica plant leaf extract and 50 mL of 0.1 M FeCl 3 .6H 2 O solution was added dropwise. Then, 1 M of NaOH was added till the pH reached 11. The production of an intensely black-colored solution after stirring the resulting mixture for about 30 min with a magnetic stirrer demonstrated the synthesis of iron oxide nanoparticles (FeNPs). Centrifugation of the synthesized nanoparticles at 4500 rpm was done for 15 min and washed thrice with ethanol and distilled water and separated these FeNPs. The FeNPs were dried in an oven for 24 h at 65 °C. After drying, magnetite iron nanoparticles were obtained, and their synthesis was in line with Bhuiyan et al. 50 .

Synthesis of magnetite iron oxides/reduced graphene oxide nanocomposites using Azadirachta indica plant leaf extract

Green deposition was used for producing iron oxide/reduced graphene oxide (FeNPs/rGO) nanocomposites. Iron oxide and reduced graphene oxide were used at a weight ratio of 1:2. Iron oxides (0.08 g) and reduced graphene oxide (0.16 g) were collected individually in 100 mL beakers with 10 mL of distilled water. Both solution combinations were sonicated in an ultrasonicator for 30 min to improve suspension. For three hours, a solution of reduced graphene oxide was stirred continuously on a hot plate while a suspension of iron oxides was fed into it at a rate of around 0.5 mL every ten minutes. Following that, the solution was centrifuged at 4500 rpm for 10 min, and washing was carried out in three phases using distilled water and ethanol to remove any undesirable contaminants. The product was then heated up for 8 h at 65 °C to dry it. The dried product was ground into a fine powder containing magnetite iron oxides and reduced graphene oxide nanocomposite (FeNPs/rGO).

Characterization techniques

For optical evaluations, the absorption spectra of synthesized nanocomposites were analyzed using UV–visible spectroscopy (Cary 50) in the 200–800 nm range. Agilent technology (Cary 630) FT-IR in the 650–4000/cm range was used to investigate functional groups. The crystal phase composition was studied using X-ray diffraction (Bruker D2 Phaser) in the 2θ range 10–80°. The mean particle size of all the synthesized nanomaterials by employing the deby Sherer’s equation. Morphology and elemental analyses were conducted using SEM–EDX (S-3400 N), Hitachi.

Photocatalytic activity

The photocatalytic activity of each sample was assessed against methylene blue (MB) with and without the presence of the catalyst. A 100 mL beaker was filled with 25 ml of a 20 ppm MB solution, which was then stirred for 30 min. After 20 min, 5 mg of nanocomposite was added to this solution and swirled. The absorbance at a certain wavelength (665 nm) was then measured by adding a small quantity of this solution to a cuvette every 5 min. The dye inside the nanocomposite had completely broken down after one hour. The % dye degradation was calculated using the equation suggested by Anwar et al. 52 .

Ethical approval

The collection of plant materials used in this study complies with relevant institutional, national, and international guidelines and legislation.

Results and discussion

Optical properties.

For evaluations of optical properties, absorption spectra of synthesized nanomaterials and nanocomposites were analyzed using UV–visible spectroscopy in the 200–800 nm range. The maximum absorbance of synthesized nanomaterials and nanocomposites i.e., GO, rGO, FeNPs, and FeNPs/rGO was observed at 232, 265, 395, and 405 nm, respectively, confirming the synthesis of graphene-based nanocomposites of magnetite iron oxides with reduced graphene oxide. Absorption peaks between 230 and 240 nm are characteristic peaks of GO confirming the synthesis of GO. The obtained 232 nm absorption peak in the case of GO can be ascribed to π–π* transitions of the remaining sp 2 C=C bonds 53 , 54 that was shifted to 265 nm in the case of rGO after reduction of GO by green method using Azadirachta indica leaf extract. During GO reduction, an increase in π conjugation network shifting of absorption towards longer wavelength region is due to less requirement of energy 55 , 56 . Similarly, absorption spectra of FeNPs and FeNPs/rGO were observed at 395 and 405 nm, respectively, confirming the synthesis of graphene-based nanoparticles and nanocomposites of magnetite iron oxides with reduced graphene oxide. Band gap values were calculated from UV–visible data and were represented in Fig. 1 using Wood and Tauc plots 57 .

Wood and Tauc plots for ( a ) GO ( b ) rGO, ( c ) FeNPs, and ( d ) FeNPs/rGO nanocomposite using the green deposition method.

Fourier transform infrared spectroscopy

Functional groups of GO, rGO, and FeNPs/rGO nanocomposite were observed using FTIR spectrum in the range of 500–4000/cm as shown in Fig. 2 a,b. FTIR is a valuable spectroscopic method for characterizing various functional groups, particularly functional groups containing oxygen. The presence of several functional groups including O–H, C–OH, COOH, and C–O in the FTIR spectrum demonstrated that the precursor graphite had been successfully oxidized and GO had been successfully synthesized. A distinctive peak of the stretching mode of O–H functional groups is a large peak of the spectrum between 3500 and 2500 cm −1 58 , 59 , 60 . The 1573 cm −1 peak is attributed to the stretching of the C=C from the unoxidized domain of graphite, whereas the 1730 cm −1 peak is attributed to the stretching of the carboxyl group 58 , 60 . The stretching vibration of C–O from C–O–C is responsible for the 1017 cm −1 peak. The reduction of GO into rGO is visible in the FTIR spectrum of rGO, which is depicted in Fig. 2 . This is because the O-containing functionalities' peak intensities are less intense than GO's peak intensities. These results demonstrated the reduction of GO caused by ascorbic acid.

FTIR spectrum of GO synthesized via the chemical method, and rGO synthesized via a green method ( a ), and FTIR spectrum of magnetite iron oxide/reduced graphene oxide (FeNPs/rGO) nanocomposite synthesized by green deposition method ( b ).

The existence of several functional groups in rGO, however, showed that functional groups are still present in the synthesized material despite its close resemblance to pristine graphene which is in agreement with Andrijanto et al. 61 .

FTIR spectrum (Fig. 3 ) showed a characteristic peak at 577 cm −1 which confirmed the synthesis of FeNPs/rGO nanocomposites synthesized by the green deposition method. The absorption band around 577 cm −1 was attributed to FeO (indicating Fe 3 O 4 ) 62 and confirmed the synthesis of FeNPs/rGO nanocomposites. These results are in line with those reported by Sodipo et al. 63 .

XRD spectrum and mean particle size of ( a ) GO NOs, ( b ) rGO NPs, ( c ) FeNPs, ( d ) FeGO, and ( e ) represents the FerGO nanocomposite synthesized via green deposition method.

X-ray diffraction analysis

The X-ray diffraction pattern of FeNPs/rGO nanocomposites is shown in Fig. 3 a–e. The diffraction peak of rGO was about 2 θ = 26° while the diffraction peak of magnetite iron oxide was about 2 θ = 44 confirming the synthesis of FeNPs/rGO nanocomposites synthesized via the green deposition method. The diffraction peak of rGO was about 2 θ = 26° indicating the production of restacked rGO sheets after reduction of GO 64 , 65 . The JCPDS number for metallic iron nanoparticles, like α-Fe (alpha iron), is often 06–0696, reflecting their crystal structure in X-ray diffraction. However, variations can occur based on factors like nanoparticle size, shape, and surface characteristics. Based on Debye Sherrer's equation, the mean particle size of all synthesized materials was calculated, which follows the standard calculation of nanomaterials. As can be seen in Fig. 3 a–e, GO nanoparticles have the particle size of (60–80), rGO nanoparticles are (90–95), FeNPs have the particle size of (70–90); Fe/GO nanocomposites have a particle size (40–60) nm, whereas Fe/rGO nanocomposites are of (80–85) nm. Several studies have examined the synthesis and properties of reduced graphene oxide (rGO) and ferric composites, such as Ma 66 , which presented a controllable rGO/Fe 3 O 4 composite film. Supriya 67 investigated the alteration of crystal symmetry in cobalt ferrite-reduced graphene oxide nanocomposites. Studying the synthesis and properties of reduced graphene oxide (rGO) and ferric composites is significant because it can lead to the development of advanced materials with unique properties and applications. These composites have the potential to be used in various fields such as energy storage, catalysis, sensors, and biomedical applications, making them a subject of great interest in scientific research. Sagadevan 68 presented a chemically stabilized rGO/ZrO2 nanocomposite synthesis with improved electrical properties. Additionally, Singh 69 discussed the outstanding electromagnetic interference shielding capabilities of a lightweight rGO-Fe3O4 nanoparticle composite. As a result of these studies, rGO and ferric composites have been demonstrated to be effective across a wide range of applications, including magnetoelectronics and electromagnetic shielding.

Scanning electron microscopy-energy dispersive X-ray analysis

The morphological and elemental distribution of all the synthesized nanoparticles was verified by scanning electron microscopy attached to the energy dispersive spectroscopy examination. Figure 4 a,b reveals the EDX elemental mapping of FeNPs and Fe/rGO composites only respectively. It was observed that the percentage of Fe is 55.61%, carbon is 4.51% and oxygen is 35.72% as shown in Fig. 4 a. The binding energies of Fe are related to characteristic peaks around 0.9, 6.1, and 7 keV along with the characteristic peak of oxygen at 0.5 keV. Therefore, the EDX analysis confirmed that both iron & oxygen are present in all the nanocomposites. Results are in agreement with those reported by Rahman et al. and Sayed et al. 70 , 71 .

EDX pattern of FeNPs synthesized via green approach ( a ), and EDX graph of FeNPs/rGO nanocomposites synthesized via green deposition method ( b ).

The surface morphology and texture analysis of the synthesized nanocomposites were verified by SEM examination (Fig. 5 a–e). Scanning electron microscopy demonstrated the rough surfaces of all the synthesized materials. All types of particles possessed cohesively manifested clustered and spherical shapes. Interactions between graphene-based nanomaterials and magnetite nanoparticles in the nanocomposites are influenced by functional groups and surface chemical properties. Based on the present surface morphologies of the synthesized nanomaterials, it has been established that they can be used for better applications in the removal of dyes and metals from wastewater due to their porous and rough surfaces. These nanomaterials can also be used for the removal of pesticides and other pollutants from contaminated water. These composites of FeNPs are gaining much interest because of their unique optical, electric, and magnetic properties 72 .

Surface morphology of GO NPs ( a ), rGO NPs ( b ), Fe NPs ( c ), Fe/GO ( d ), and Fe/rGO ( e ) nanocomposites, respectively.

Photocatalytic activity of FeNPs, and Fe/rGO nanocomposite against methylene blue

The percentage degradation of methylene blue (MB) in samples is represented in Fig. 6 a,b. The activity was done with FeNPs nanoparticles and with FeNPs/rGO nanocomposites to estimate their relative performance in the degradation of toxic pollutants. The FeNPs alone showed about 16% degradation (Fig. 6 a) while when it was combined with reduced graphene oxide to make FeNPs/rGO nanocomposite, then it showed enhanced photocatalytic activity and degradation was about 75% as shown in Fig. 7 b. These results agree with those reported by Abid et al. 73 and Sadhukhan et al. 74 .

Degradation of methylene blue against time ( a ) FeNPs nanoparticles ( b ) FeNPs/rGO nanocomposites.

A plot of ln(A − A∞) versus time for rate constant.

The first order rate Eq. 2 was used to draw the graph between ln(A − A∞) versus time to conduct the kinetic investigation. Figure 7 illustrated this relationship by stating that the slope gave the value of the first order rate constant, k (min −1 ), as

The properties and applications of reduced graphene oxide (rGO) and its composites with ferric have been studied in a variety of studies. According to Shahid et al. 75 and Iftikhar et al. 76 , graphene enhances the photocatalytic properties of rGO composites with different ferric materials. An rGO/Fe 3 O 4 composite film was examined by Ma et al. 66 , demonstrating its potential for a variety of applications. As a demonstration of the versatility of rGO composites, Deepi et al. 77 synthesized a novel rGO-SCO nanocomposite with high specific capacitance. The nanocomposite also demonstrated excellent stability in aqueous electrolytes, making it a promising material for energy storage applications. Furthermore, rGO composites have also shown potential for water treatment, corrosion inhibition, and biosensing.

Conclusions

Graphene-based nanomaterials such as graphene oxide (GO), reduced graphene oxide (rGO), iron nanoparticles (FeNPs), and magnetite iron oxide/G-based nanocomposites (FeNPs/rGO) were successfully synthesized using Azadirachta indica leaf extract via green deposition method . Synthesized materials were successfully characterized by UV–vis spectrophotometry, Fourier Transform Infra-Red spectroscopy, XRD, and SEM–EDX techniques. The final synthesized FeNPs/rGO nanocomposites showed remarkable photocatalytic activity in the removal of toxic pollutants such as methylene blue dye from the wastewater. FeNPs/rGO nanocomposites showed 75% degradation against methylene blue which was attributed to its lower band gap. Based on these results, it is plausible to conclude that newly synthesized FeNPs/rGO nanocomposites can also be deployed as a potential photocatalyst degrader for photocatalytic degradation of other different dye pollutants due to lower band gap.

Data availability

The datasets used and/or analyzed during the current study are available from the corresponding author upon reasonable request.

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Acknowledgements

The authors extend their appreciation to the Researchers supporting project number (RSP2024R469), King Saud University, Riyadh, Saudi Arabia. The authors greatly acknowledge the financial support by HEC, Pakistan to pursue this research work. The authors also acknowledge the support of NRF and MIST, Korea.

Open Access funding enabled and organized by Projekt DEAL. This research is funded by the Higher Education Commission (HEC) Pakistan through the NRPU Project (No. 20-16094/NRPU/R&D/HEC/2021 2021). The authors extend their appreciation to the Researchers supporting project number (RSP2024R469), King Saud University, Riyadh, Saudi Arabia. This work was also supported by the National Research Foundation of Korea (NRF) grant funded by the Korean Government (MSIT) (No.NRF-2022K1A3A9A05036564).

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Conceptualization, M.S.A., S.F., S.A., S.C., A.D., M.A.I., A.M.A., R.I., W.A.A., M.R., and Y.N.; Data curation, M.S.A., and S.F.; Formal analysis, M.S.A., S.F., S.A., S.C., A.D., M.A.I., A.M.A., R.I., W.A.A., M.R., and Y.N.; Funding acquisition, M.S.A., M.R., and A.M.A.; Investigation, M.S.A., and S.F.; Methodology, M.S.A., S.A., M.V., S.C., A.D., A.M. A., and R.I.; Project administration, M.S.A., M.R., and A.D.; Resources, M.S.A., S.F., S.A., S.C., A.D., M.A.I., A.M.A., R.I., W.A.A., M.R., and Y.N.; Software, M.S.A., S.F., S.A., S.C., A.D., M.A.I., A.M.A., R.I., W.A.A., M.R., and Y.N.; Supervision, M.S.A.; Validation, M.S.A., S.F., S.A., S.C., A.D., M.A.I., A.M.A., R.I., W.A.A., M.R., and Y.N.; Visualization, M.S.A., S.F., S.A., S.C., A.D., M.A.I., A.M.A., R.I., W.A.A., M.R., and Y.N.; Writing—original draft, M.S.A., and S.F.; Writing—review & editing, M.S.A., S.F., S.A., S.C., A.D., M.A.I., A.M.A., R.I., W.A.A., M.R., and Y.N. All authors have read and agreed to the published version of the manuscript.

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Akhtar, M.S., Fiaz, S., Aslam, S. et al. Green synthesis of magnetite iron oxide nanoparticles using Azadirachta indica leaf extract loaded on reduced graphene oxide and degradation of methylene blue. Sci Rep 14 , 18172 (2024). https://doi.org/10.1038/s41598-024-69184-y

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DOI : https://doi.org/10.1038/s41598-024-69184-y

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Green Nanoparticles Synthesis Using Plants Extracts and Biomedical Applications

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Published Papers

A special issue of Plants (ISSN 2223-7747). This special issue belongs to the section " Phytochemistry ".

Deadline for manuscript submissions: closed (31 December 2023) | Viewed by 23140

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Dear Colleagues,

“Nanotechnology is the application of science to control matter at the molecular level”. It is a promising technology applied in all areas of science. Nanoparticles have multifunctional properties and very interesting applications in various fields, such as medicine, nutrition, energy, agriculture and the environment. Nanoparticles are of great interest due to their extremely small size and large surface-to-volume ratio, which contribute to both chemical and physical differences in their properties (e.g., mechanical, biological and sterical properties, catalytic activity, thermal and electrical conductivity, optical absorption and melting point) compared to most materials with same chemical composition. The biogenic synthesis of monodispersed nanoparticles with specific sizes and shapes has challenged biomaterial science. Synthesizing metal nanoparticles using green technology (green synthesis methods) via microorganisms, plants, viruses, etc., has been extensively studied recently due to its recognition as a green and efficient way to exploit biological system as convenient nanofactories. The biological synthesis of nanoparticles is increasingly regarded as a rapid, ecofriendly and easy-to-scale-up technology. Biological nanoparticles were found to be more pharmacologically active than physico-chemically synthesized nanoparticles. Biosynthesized nanoparticles have various applications, especially in the field of biomedical research (for specific delivery of drugs, such as paclitaxel, methotrexate and doxorubicin; tumor detection; angiogenesis; genetic disease and genetic disorder diagnosis; photoimaging; and photothermal therapy.) These developments are aimed at enhancing the uptake and bioavailability of nano-sized nutrients and supplements and improving the taste, consistency, stability, texture and safety of food products, as well as the development of nano-delivery systems for bioactive compounds. Therefore, nanotechnology may provide an opportunity for researchers of plant science and other fields to develop new tools for synthesis of environmentally friendly and cost-effective nanoparticles for applications in various fields. This Special Issue will highlight the latest findings and developments in this field worldwide in the form of research, reviews and mini reviews on all aspects related to the future prospects of green synthesis of nanoparticles and their biomedical applications.

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green nanoparticle synthesis
plant-based nanoparticle synthesis
biological synthesis of nanoparticles
plant metabolites used in the synthesis of nanoparticles
application of green nanotechnology in the agricultural field
application of green-synthesized nanomaterials in biomedical fields
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Green mediated synthesis of cerium oxide nanoparticles by using Oroxylum indicum for evaluation of catalytic and biomedical activity

First published on 13th August 2024

The present perspective emphasizes the green synthesis of CeO 2 -NPs using Oroxylum indicum fruit extract. The synthesized NPs were characterized utilizing analytical techniques, including FT-IR, UV-vis, XRD, SEM-EDX, and VSM. Of them, XRD analysis ratifies the cubic fluorite crystal structure along with a particle size of 23.58 nm. EDX results support the presence of cerium and oxygen in a proper ratio. The surface morphology of NPs, however, was scrutinized using SEM. The lower IC 50 value (20.8 μg mL −1 ) of NPs compared to the reference substance, ascorbic acid (33.2 μg mL −1 ), demonstrates CeO 2 -NPs to be a compatible antioxidant. Moreover, the drug-releasing capability of CeO 2 -NPs was a buffer pH-dependent parameter. The acidic pH solution was 20.5%, while the basic pH solution was 16.9%. The drug-releasing capability was analyzed using the Higuchi model and Korsmeyer–Peppas kinetics. The values of the determination coefficient ( R 2 ) were found to be 0.9944 and 0.9834, respectively. The photocatalytic activity of CeO 2 -NPs was evaluated, considering methylene blue as a model dye. The degradation percentage was attained up to 56.77% after it had been exposed for 150 min. Apart from this, the synthesized NPs were screened against two fungus species, Bipolaris sorokiniana and Fusarium . The percentage of growth was measured at 56% and 49%, respectively.

1. Introduction

Metal oxide nanocarriers are usually synthesized using sol–gel, microwave, hydrothermal, biological parts, and co-precipitation techniques that are included as physical, chemical, and biological approaches. 10–12 Recently, green synthesis, a type of biological method, has drawn the attention of scientists in nanotechnology due to the myriad sources of plants, microorganisms, and animals that are comparatively biologically active. Green synthesis requires less use of chemicals, is environmentally benign in nature, is easy to synthesize, and has low toxicity. 13–15 Nature as a biological laboratory provides new species of fungi, bacterium cells, algae, and plants that possess various phytochemicals, including tannins, flavonoids, and terpenoids. 16 Surprisingly, the phytochemicals function as reducing agents as well as stabilizing agents and capping agents of metal ions and nanoparticles, respectively. 16 Moreover, the stabilizing agents impede nanocarriers from aggregation and provide various modes of bonding with nanoparticles. 17 In this work, the green synthesis of cerium oxide nanoparticles from the aqueous extract of Oroxylum indicum , a plant species with medicinal properties, has been employed for the synthesis of bioactive CeO 2 -NPs. 18–20 Oroxylum indicum contains a wide range of bioactive substances that have been identified as potentially useful in the treatment of pneumonia, piles, cardiac conditions, diarrhea, and tannins, glycosides, saponins, phenols, and quines. In addition to these benefits, the compounds have made it easier to synthesize CeO 2 -NPs in an environmentally friendly manner. They also function as capping agents, stabilizing agents, and reduce toxicity while enhancing dispersion, stability, and prevention of agglomeration. Finally, the compounds maintain a higher surface area for catalytic activity and encourage a more controlled release of their antioxidant properties. 17,21

There are two distinct oxidation states for cerium: tetravalent Ce 4+ and trivalent Ce 3+ . Consequently, depending on the material's composition, cerium oxide can exist as two distinct oxides, such as Ce 2 O 3 (Ce 3+ ) and CeO 2 (Ce 4+ ). CeO 2 -NPs have a cubic fluorite crystal structure, and Ce 3+ and Ce 4+ coexist on their surface. CeO 2 -NPs have reducing reactive oxygen species (ROS) as an antioxidant activity. Those nanoparticles have a wide range of applications in different fields due to their two distinct oxidation states. 18,19,22 Basically, the electropositive charge present in the surface cavities of CeO 2 -NPs enables them to interact with free charge-carrying species and lessen internal oxidative stress. 23 These NPs have the potential to function as antimicrobial agents because they cause oxidative stress in fungi and bacteria. Another prominent application of CeO 2 -NPs was assessing the drug delivery approach along with Metronidazole (MTZ), an effective antimicrobial agent. 24–26 It is surprising that in contact with aerobic bacteria, the antimicrobial agents exert negligible or no antibacterial activity. Furthermore, the drug has been classified as IV in the biopharmaceutical classification system (BCS) owing to its low permeability and incredibly low water solubility (1 mg mL −1 ), resulting in poor absorption and low bioavailability. 27,28 This finding was certainly confirmed through the application of nanotechnology because of the alteration of physical and chemical properties. 29 Beside theses, CeO 2 -NPs have significant antifungal activity, bolstered by various previous studies. Shahbaz et al. (2022) synthesized CeO 2 -NPs using Acorus calamusas rhizomes extract and screened against the Puccinia striiformis fungus, which revealed a positive response. 30 In another study, Costa et al. (2023) conducted research against Bipolaris sorokiniana , a fungus that create root diseases in wheat. 31

In the last few decades, environmental pollution has become a burning issue all over the world, and essential parts of the environment are frequently polluted through anthropogenic and natural process. The indiscriminate use of toxic materials, textile dyes, pesticides, and aromatic compounds is defiling the aqueous medium. The dumping of waste materials containing organic dyes can exert an adverse impact on biota and polluted water. To get rid of this impact, various analytical techniques have been initiated recently to remove or degrade the organic dye into less harmful chemical species. The available methods include chemical oxidation, photo-degradation, electrochemical oxidation, coagulation, flocculation, etc. The various limitations of the methods, such as lack of efficiency, complexity, and high energy consumption, make them comparatively less popular. On the contrary, NPs as photo-catalysts draw an appeal to eliminate the toxic dye from an aqueous medium owing to their reusability, efficiency, and visible light.

As a result, the co-precipitation technique was used in this study to create CeO 2 -NPs through green synthesis utilizing fruit extract from Oroxylum indicum . The elements found in the fruit extract of Oroxylum indicum were utilized as efficient reducing and capping agents. In an indirect mechanism, the plant extracts help to facilitate the oxidation of Ce 3+ to Ce 4+ by functioning as complexing agents instead of as traditional reducers. It also investigates these nano carriers' antioxidant activity. It was demonstrated that CeO 2 -NPs had photocatalytic characteristics as they may degrade methylene blue (MB) dye. Additionally, CeO 2 -NPs have antifungal efficacy against B. sorokiniana and Fusarium plant pathogens. Moreover, the aim of this work was to examine how metal oxide nanocarriers, especially CeO 2 -NPs, affected the loading and release of MTZ, a model drug.

2. Experimental

2.1 chemicals and reagents, 2.2 preparation of plant extract.


	Diagram demonstrating the procedure for extracting phytochemicals.

2.3 Synthesis of CeO 2 NPs


Ce(NO ) ·6H O + phytochemicles (green source) → Ce (OH)	(1)


4Ce(OH) + O → 4CeO + 6H O	(2)


	Diagrammatic representation of the environmentally friendly synthesis of CeO -NPs.

2.4 Characterization of CeO 2 NPs

2.5 evaluation of in vitro antioxidant activity of green synthesized ceo 2 nps.


	(3)

2.6 Preparation of MTZ-loaded CeO 2 -NPs

2.7 estimating the drug loading capacity and the entrapment efficiency.


	(4)


	(5)

2.8 In vitro MTZ drug release study of CeO 2 -NPs


	(6)


	A schematic representation of the loading and release of CeO -NPs using metronidazole (MTZ).

2.9 Photocatalytic activity green synthesized CeO 2 -NPs


	(7)

2.10 Antifungal activity of CeO 2 -NPs


	(8)

3. Results and discussion

3.1 characterization of nps.


	(9)


	Shows (a) UV-visible spectrum and (b) band gap of CeO -NPs.


	FTIR spectrum of (a) Oroxylum indicum fruit extract, and (b) CeO -NPs.


	(a) Shows the SEM of CeO -NPs, (b) the average particle size distribution graph in nm, and (c) EDX analysis.


	XRD results of the CeO -NPs.

Sample name	Parameters	Equation	Equation number
CeO -NPs	Lattice parameter, a (nm)		(10)
	Unit volume (a ) nm	V = a	(11)
	Average crystallite size (D) nm		(12)
	Micro strain (ε) × 10		(13)
	Dislocation density (δ) × 10		(14)

Different sources of synthesis CeO -NPs	Lattice parameter, a (nm)	Unit volume (a ) nm	Average crystallite size (D)nm	Micro strain (ε) × 10	Dislocation density (δ) × 10	Ref.
Gloriosa superba L. leaf extract	0.5416	0.158867	24	—	1.73
Salvadora persica bark extract	0.5431	0.16019	5.66	0.86	0.312
Chemically	0.5404	0.15808	18.66	4.1	6.74
Chemically	0.5450	0.161878	10.98	2.9105	8.2945
Oroxylum indicum fruit extract	0.5405	0.157901	23.58	1.48	1.79	This work


	Shows the (a) Rietveld refinement graph; (b) electron density mapping in 2D counter form; (c) electron density mapping in 3D form; (d) crystal structure of CeO -NPs in the single unit cell; (e) crystal structure of CeO -NPs in a single layer; and (f) crystal structure in a polyhedral structure.


	Emphasize the VSM of CeO -NPs.

The pHpzc was determined by graphing the starting pH (pH I ) of the suspensions against their final pH (pH F ) in Fig. 10(a) , resulting in the bisector displayed. Before pHpzc, the suspension's final pH was higher than pHpzc. For CeO 2 -NPs, the pHpzc value was 8.71 at the point where this curve crossed the bisector. Plotting this curve, ΔpH (the difference between pH I and pH F ), against pH I is shown in Fig. 10(b) . The pHpzc value is the point on the curve where ΔpH is zero, or the point where the curve crosses the x -axis. According to both plots, the pHpzc of synthesized CeO 2 -NPs is roughly 8.7. 55


	(a and b) Shows typical plots used to determine the pHpzc of CeO -NPs.

3.2 Antioxidant activity green synthesized CeO 2 -NPs


	Schematic mechanism of DPPH scavenging by CeO -NPs.


	shows DPPH radical scavenging action of CeO -NPs and ascorbic acid.

The IC 50 values in Table 3 for ascorbic acid and CeO 2 -NPs were measured at 20.8 μg mL −1 and 33.2 μg mL −1 , respectively. As per the report by, 9 the ascorbic acid and CeO 2 -NPs had IC 50 values of 9.36 μg mL −1 and 15.47 μg mL −1 , respectively.

Concentration μg mL	% DPPH radical scavenging activity
Concentration μg mL	CeO -NPs	Control
20	45.6 ± 2.28	47.6 ± 2.38
40	53.4 ± 2.67	57.6 ± 2.88
60	55.6 ± 2.78	63 ± 3.15
80	60.8 ± 3.04	65.8 ± 3.29
100	63.4 ± 3.17	72.2 ± 3.61

3.3 Drug loading and pH-responsive drug release of MTZ from CeO 2 -NPs


	Shows the release profile at different pH solutions of MTZ-loaded CeO -NPs.

3.4 Application of kinetic models

Equation no.	Model	Equation
(15)	Zero-order model	Q = Q + kt
(16)	First-order model
(17)	Higuchi model	Q = kt
(18)	Korsmeyer–Peppas model	log = log + n

In vitro , the drug release kinetic were analyzed using various models and equations, including zero-order, first-order, Higuchi, and Korsemeyer–Peppas models ( Table 5 ). To determine the kinetic model of the MTZ release from CeO 2 -NPs release, the obtained release data (until 180 min) were fitted to the above-mentioned equations. The obtained plots at pH 1.2 and pH 7.4 are depicted in Fig. 14 and 15 .

Kinetics model	Constants		Correlation coefficients (R )
Kinetics model	pH 1.2	pH 7.4	pH 1.2	pH 7.4
Zero-order model	k = 0.0166 (mg mL min )	k = 0.0238 (mg mL min )	0.9915	0.8991
First-order mode	k = 0.0004 (min )	k = 0.0007 (min )	0.9879	0.8828
Higuchi model	k = 0.3032 (mg mL min )	k = 0.4473 (mg mL min )	0.9944	0.9399
Korsmeyer–Peppas model	k = 1.1612 (mg mL min ), n = 0.0646	k = 0.9484 (mg mL min ), n = 0.1264	0.9583	0.9834


	(a) Zero-order, (b) first-order, (c) Higuchi, and (d) Korsmeyer–Peppas kinetic models for the release of MTZ from CeO -NPs at pH 1.2.


	(a) Zero-order, (b) first-order, (c) Higuchi, and (d) Korsmeyer–Peppas kinetic models for the release of MTZ from CeO -NPs at pH 7.4.

The Higuchi model kinetics equation for acidic buffer pH 1.2 yielded the highest value of the determination coefficient ( R 2 ), which was 0.9944. The Korsmeyer–Peppas kinetic model was the most appropriate for basic buffer pH 7.4, and its coefficient ( R 2 ) was 0.9834. When it comes to drug release systems with one-dimensional diffusion, the Higuchi model offers a foundation for comprehending the kinetic mechanism. As we looked at the in vitro MTZ release CeO 2 -NPs results, we saw that the correlation coefficients ( R 2 ) of the Higuchi model of MTZ release CeO 2 -NPs at pH 1.2 were close to 1. This means that the model fits the data the best. Furthermore, the value of the correlation coefficients ( R 2 ) of the Korsmeyer–Peppas kinetic models of MTZ release CeO 2 -NPs at pH 7.4 was found to be close to 1, and the calculated diffusional exponent “ n ” for the release of MTZ in CeO 2 -NPs by the Korsmeyer–Peppas model is 0.1264. According to Korsmeyer–Peppas results n < 0.43, it was proved that the release mechanism is controlled by Fickian diffusion for CeO 2 -NPs. The drug molecules in this investigation are bonded to and adsorbed onto the active regions of the nanoparticles. 61,62 In addition, different papers reported that MTZ drug release kinetics by nano carriers followed the previously mentioned models ( Table 6 ).

Sample	Acidic buffer	Model name	Basic buffer	Model name	Reference
Chitosan/polyvinylpyr-rolidone	0.9604	Higuchi	0.9753	Korsmeyer–Peppas
Chitosan/graphene oxide	0.9127	Korsmeyer–Peppas	0.9503	Korsmeyer–Peppas
CeO	0.9944	Higuchi	0.9834	Korsmeyer–Peppas	This work

3.5 Photocatalytic activity green synthesized CeO 2 -NPs


	Degradation of methylene blue dye by CeO -NPs under UV irradiation.

3.6 Mechanism of photocatalytic decay


	Mechanism of photodegradation of MB dyes using CeO -NPs.

Superoxide radicals (O 2 − ) are created when electrons oxidize attractive oxygen, forming hydroxyl radicals (˙OH) that form the pore water molecules ions. Significantly affecting the degradation of the dye was the formation of apertures and electrons in the valence and conduction bands. The following is the photocatalysis reaction mechanism: 42,66


	(19)


CeO (h ) + H O → OH˙ + H	(20)


CeO (e ) + O → O ˙	(21)


O ˙ + MBdye → CO + H O	(22)


OH˙ + MBdye → CO + H O	(23)

The oxygen and water molecules are adsorbed on the photocatalyst's surface. These molecules react with the electron–hole pairs ( eqn (20) and (21) ) to produce the unstable hydroxyl radicals (OH˙) and superoxide ions (O 2 ˙ − ), which oxidize the organic pollutants into inorganic compounds ( eqn (22) and (23) ).

3.7 Antifungal effect of CeO 2 -NPs


	In vitro control of (a) B. sorokiniana and (b) Fusarium using the green synthesized CeO -NPs. (c) Mycelial growth inhibition against fungi.

4. Conclusions

Data availability, author contributions, conflicts of interest, acknowledgements.

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Green synthesis of nanoparticles using plant extracts: a review

2020, Environmental Chemistry Letters

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Advances in Green Synthesis

Hassnain Khan

Nanomaterials

faryad khan

The key pathways for synthesizing nanoparticles are physical and chemical, usually expensive and possibly hazardous to the environment. In the recent past, the evaluation of green chemistry or biological techniques for synthesizing metal nanoparticles from plant extracts has drawn the attention of many researchers. The literature on the green production of nanoparticles using various metals (i.e., gold, silver, zinc, titanium and palladium) and plant extracts is discussed in this study. The generalized mechanism of nanoparticle synthesis involves reduction, stabilization, nucleation, aggregation and capping, followed by characterization. During biosynthesis, major difficulties often faced in maintaining the structure, size and yield of particles can be solved by monitoring the development parameters such as temperature, pH and reaction period. To establish a widely accepted approach, researchers must first explore the actual process underlying the plant-assisted synthesis of a metal...

Emerging Research on Bioinspired Materials Engineering

Hairul Islam

International Journal for Research in Applied Science and Engineering Technology

PRATIK KUMAR JAGTAP

Ebrahim Mostafavi

Metal nanoparticles (MNPs) produced by green approaches have received global attention because of their physicochemical characteristics and their applications in the field of biotechnology. In recent years, the development of synthesizing NPs by plant extracts has become a major focus of researchers because of these NPs have low hazardous effect in the environment and low toxicity for the human body. Synthesized NPs from plants are not only more stable in terms of size and shape, also the yield of this method is higher than the other methods. Moreover, some of these MNPs have shown antimicrobial activity which is consistently confirmed in past few years. Plant extracts have been used as reducing agent and stabilizer of NPs in which we can reduce the toxicity in the environment as well as the human body only by not using chemical agents. Furthermore, the presence of some specific materials in plant extracts could be extremely helpful and effective for the human body; for instance, polyphenol, which may have antioxidant effects has the capability for capturing free radicals before they can react with other biomolecules and cause serious damages. In this article, we focused on of the most common plants which are regularly used to synthesize MNPs along with various methods for synthesizing MNPs from plant extracts.

jamil ahmed

This review focuses on the green synthesis of silver nanoparticles using various plant sources. Nano biotechnology focus on the use of living organisms plants for engineering nanoparticles and its biomedical, pharmaceutical applications. Plants extracts provide rapid, cost effective and eco-friendly sources for fabrication of metallic nanoparticles. Green biological method of synthesizing nanoparticles has materialized as alternative to overcome the curb of conventional methods such as synthesized by several physical and chemical methods including chemical reduction of ions in aqueous solution with or without stabilizing agent and reduction in inverse micelles or thermal decomposition in organic solvents. Employing plants towards synthesis of nanoparticles has advantageous over non biological methods as with the presence of broad variability of bio-molecules in plants can act as capping and reducing agents and thus increases the rate of reduction and stabilization of nanoparticles. Thus biosynthesized metallic nanoparticles of variable size and shape have broad potential applications in life and science. Keyword: Biosynthesized Nanoparticles, Green Source, Biofabrication, Ecofriendly, Applications

Biocatalysis and Agricultural Biotechnology

Pragati Kumari

Acta naturae

Igor Yaminsky

While metal nanoparticles are being increasingly used in many sectors of the economy, there is growing interest in the biological and environmental safety of their production. The main methods for nanoparticle production are chemical and physical approaches that are often costly and potentially harmful to the environment. The present review is devoted to the possibility of metal nanoparticle synthesis using plant extracts. This approach has been actively pursued in recent years as an alternative, efficient, inexpensive, and environmentally safe method for producing nanoparticles with specified properties. This review provides a detailed analysis of the various factors affecting the morphology, size, and yield of metal nanoparticles. The main focus is on the role of the natural plant biomolecules involved in the bioreduction of metal salts during the nanoparticle synthesis. Examples of effective use of exogenous biomatrices (peptides, proteins, and viral particles) to obtain nanopart...

Kavitha Kumar

Journal of Chemical Technology & Biotechnology

Vineet Kumar

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Bioinspired green synthesis of copper, nickel, and hybrid nanoparticles using Myristica Fragrans seeds: Biomedical applications and beyond

Ullah, Asad
Rehman, Ubaid Ur
Ahmad, Riaz
Rahman, Fazal

Nanotechnology focuses on materials at the molecular and atomic levels, with sizes ranging from 0.1 to 100 nm. This study explores the synthesis and characterization of copper oxide (CuO), nickel oxide (NiO), and hybrid nanoparticles using an aqueous seed extract from Myristica fragrans. The nanomaterials underwent comprehensive characterization employing various techniques: UV analysis, FTIR spectroscopy, XRD, TGA, EDX and SEM. We explored their biological applications through antioxidant and antibacterial assays. UV analysis determined the optical absorption spectra values for CuO, NiO and hybrid nanoparticles. FTIR analysis confirmed functional groups in the plant extract responsible for capping and reducing the reaction medium. XRD and SEM analysis demonstrated the crystalline nature and morphology of the nanoparticles. CuO nanoparticles exhibited polyhedral morphology, while NiO nanoparticles were primarily spherical with some agglomeration. The CuO-NiO hybrid nanoparticles showed a wurtzite morphology with significant agglomeration and larger mean size than CuO and NiO nanoparticles. EDX indicated higher quantities of Cu and Ni. XRD spectra revealed the average particle sizes of nanoparticles. TGA indicated the thermal stability of the nanoparticles, with hybrid nanoparticles being the most stable. The nanoparticles exhibited excellent antioxidant activity, with hybrid nanoparticles showing the highest values in measuring total antioxidant capacity, total reducing power (TRP), ABTS assay, and DPPH-free radical scavenging assay at 400 μg/mg. Antibacterial assays against multidrug-resistant bacterial strains demonstrated that antibiotics-coated hybrid nanoparticles exhibited potent antibacterial properties against Staphylococcus aureus, Escherichia coli, and Pseudomonas aeruginosa. In conclusion, CuO, NiO, and CuO-NiO hybrid nanoparticles mediated by Myristica fragrans showcase promising characteristics for various applications, especially in biomedical and clinical settings. The nanoparticles eco-friendly synthesis and biocompatible nature make them attractive candidates for future research and development.

nanoparticles;
Myristica fragrans;
nanotechnology

IMAGES

6 Workflow for green synthesis of nanoparticles using plant leaf
Green synthesis of nanomaterials using plant extract [55]
The general steps of green synthesis of inorganic nanoparticles using
Plant-Mediated Nanoparticle Synthesis
(PDF) Green Synthesis (Using Plant Extracts) of Ag and Au Nanoparticles
(PDF) Green synthesis of nanoparticles using plant extracts: a review

COMMENTS

Green synthesis of nanoparticles using plant extracts: a review
Green synthesis of nanoparticles has many potential applications in environmental and biomedical fields. Green synthesis aims in particular at decreasing the usage of toxic chemicals. For instance, the use of biological materials such as plants is usually safe. Plants also contain reducing and capping agents. Here we present the principles of green chemistry, and we review plant-mediated ...
Green synthesis of silver nanoparticles using plant extracts and their
Synthesis of metal nanoparticles using plant extracts is one of the most simple, convenient, economical, and environmentally friendly methods that mitigate the involvement of toxic chemicals. Hence, in recent years, several eco-friendly processes for the rapid synthesis of silver nanoparticles have been reported us
Recent advances in green synthesized nanoparticles: from production to
Here, we provide an overview of the general mechanisms involved in the green synthesis of nanoparticles using plant extracts, with relevant references. Reducing Agents: Plant extracts contain a diverse range of bioactive compounds, including polyphenols, flavonoids, and terpenoids, which possess reducing properties.
Green synthesis of nanoparticles using plant extracts: a review
Flowchart for synthetic route, characterization and applications of green synthesis of palladium and platinum nanoparticles from plant's extract. Reprinted from Siddiqi and Husen (2016) with ...
Green synthesis and characterization of silver nanoparticles using
However, there is no such information about the synthesis of silver nanoparticles and any of their biological applications from the plant Eugenia roxburghii.
Green synthesis of silver nanoparticles using plant extracts and their
This review summarizes and elaborates the new findings in this research domain of the green synthesis of silver nanoparticles (AgNPs) using different plant extracts and their potential applications as antimicrobial agents covering the literature since 2015.
Green synthesis of nanoparticles: Current developments and limitations
In this review, processes involved in the green synthesis of nanomaterials were summarized, and the relevant limitations were evaluated. This review hopes to point out the major issues and challenges in green synthesis of nanoscale metallic nanoparticles, and put forward the prospects for future research direction.
Green synthesis of silver nanoparticles using medicinal plants
This paper reviews recent developments in the green synthesis, optimization conditions, mechanism, and characterization techniques for AgNPs, particularly using medicinal plants extracts, along with considering the effect of different parameters that affect green synthesis.
Green synthesis of nanoparticles using plant extracts: a review
Here we present the principles of green chemistry, and we review plant-mediated synthesis of nanoparticles and their recent applications. Nanoparticles include gold, silver, copper, palladium, platinum, zinc oxide, and titanium dioxide.
Green synthesis of silver nanoparticles using plant extracts
This review provides a useful and comprehensive presentation regarding the synthesis of silver nanoparticles using these plant extracts, describing their main physical-chemical properties and some ...
Plant-Based Green Synthesis of Nanoparticles: Production
The growing demand for green chemistry and nanotechnology has pushed for the development of green synthetic methods for the production of nanomaterials using plants, microbes, and other natural resources. Researchers have been focusing on the green synthesis of NPs, using an environmentally favorable technique.
Green Synthesis of Nanoparticles Using Different Plant Extracts and
The green nanoparticles synthesis is a modern field that currently resonates compared to other preparation methods due to its characteristics that make it used in all fields. This chapter briefly explained traditional and biological methods for preparing...
Green Synthesis of Nanomaterials Using Plant Extract: A Review
In this review, we focus on the biosynthesis of nanoparticles using different parts of plant extracts. The review contains a summary of selected papers from 2018-20 with a detailed description of the process of synthesis, mechanism, characterization and their application in various fields of biosynthesized metal and metal oxide nanoparticles.
'Green' synthesis of metals and their oxide nanoparticles: applications
Among the available green methods of synthesis for metal/metal oxide nanoparticles, utilization of plant extracts is a rather simple and easy process to produce nanoparticles at large scale relative to bacteria and/or fungi mediated synthesis.
Green synthesis of silver nanoparticles using plant extracts and their
Synthesis of metal nanoparticles using plant extracts is one of the most simple, convenient, economical, and environmentally friendly methods that mitigate the involvement of toxic chemicals. Hence, in recent years, several eco-friendly processes for ...
Green synthesis of silver nanoparticles by using leaf extracts from the
Over the last few years, the green synthesis of nanoparticles (NPs) using plant extracts has emerged as a promising methodology for the fabrication of metallic NPs (especially silver, copper, and g...
Applications of Green Synthesized Metal Nanoparticles
Introduction Green synthesis of nanoparticles using living cells through biological pathways is more efficient techniques and yields a higher mass when compared to other related methods. Plants are the sources of several components and biochemicals that can role as stabilizing and reducing agents to synthesize green nanoparticles. The green synthesized methods are eco-friendly, non-toxic, cost ...
Green synthesis of magnetite iron oxide nanoparticles using Azadirachta
Green synthesis of reduced graphene oxide using plant extract. Reduced graphene oxide (rGO) was synthesized via a green approach by using Azadirachta indica plant extract as a capping agent. One ...
Recent advances in the nanoparticles synthesis using plant extract
In this paper, we provide a general overview on properties, synthesis methods and applications of nanoparticles NPs prepared from plant extract. Indeed, different techniques of green synthesis of NPs by plant extract were discussed and presented.
Green Nanoparticles Synthesis Using Plants Extracts and Biomedical
Green nanoparticle synthesis is considered the most efficient and safe nanoparticle synthesis method, both economically and environmentally. The current research was focused on synthesizing zinc oxide nanoparticles (ZnONPs) from fruit and leaf extracts of Citrullus colocynthis.
Green mediated synthesis of cerium oxide nanoparticles by using
The present perspective emphasizes the green synthesis of CeO 2-NPs using Oroxylum ... salt (3.72 g) was dissolved in 10 mL of distilled water with constant stirring at room temperature for 30 min. The plant extract ... Green synthesis of cerium oxide nanoparticles using Acorus calamus extract and their antibiofilm activity ...
Green Synthesis of Nanoparticles Using Different Plant Extracts and
The green nanoparticles synthesis is a modern field that currently resonates compared to other preparation methods due to its characteristics that make it used in all fields.
Applications of Green Synthesized Metal Nanoparticles
It has been hailed as "green" technology when plant extracts, bacteria, fungi, and algae are used to create nanoparticles. As green synthesis evolves, its potential to change numerous industries ...
Green synthesis of nanoparticles and its key ...
It has been exemplified in various papers that biomolecules (proteins, vitamins), plant extracts (flavonoids, terpenoids), and microorganisms (bacteria, fungi and yeast) are accountable for the green synthesis of different forms of nanoparticles [3].
Green synthesis of nanoparticles using plant extracts: a review
The literature on the green production of nanoparticles using various metals (i.e., gold, silver, zinc, titanium and palladium) and plant extracts is discussed in this study. The generalized mechanism of nanoparticle synthesis involves reduction, stabilization, nucleation, aggregation and capping, followed by characterization.
Bioinspired green synthesis of copper, nickel, and hybrid nanoparticles
Nanotechnology focuses on materials at the molecular and atomic levels, with sizes ranging from 0.1 to 100 nm. This study explores the synthesis and characterization of copper oxide (CuO), nickel oxide (NiO), and hybrid nanoparticles using an aqueous seed extract from Myristica fragrans.
Green Synthesis of Zinc Oxide Nanoparticles Using Currant Extracts and
The present work has reported a green chemistry‐based approach for the synthesis of crystalline metal oxide nanoparticle using plant extract to reduce metal ions.
Green Synthesis of Gold Nanoparticles Using Plant Extracts as
Biogenic approaches, mainly the plant-based synthesis of metal nanoparticles, have been chosen as the ideal strategy due to their environmental and in vivo safety, as well as their ease of synthesis.

Green synthesis of nanoparticles using plant extracts: a review

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‘Green’ synthesis of metals and their oxide nanoparticles: applications for environmental remediation

Introduction

Biological components for “green” synthesis

Solvent system-based “green” synthesis

Stability and toxicity of the nanoparticles

Mechanism of “green” synthesis for metals and their oxide nanoparticles

Plant leaf extract-based mechanism

Environmental remediation applications

Catalytic activity

Removal of pollutant dyes

Heavy metal ion sensing

Conclusion and future prospects

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Journal of Nanobiotechnology

Applications of Green Synthesized Metal Nanoparticles — a Review

Hary Razafindralambo

Yun-Zhang Sun

Hamed Ghafarifarsani

Seyed Hossein Hoseinifar

Mahdieh Raeeszadeh

Associated Data

Introduction

Nanoparticle Synthesis Methods

Factors Influencing the Green Synthesis of Various Nanoparticles

Temperature

Reaction Time

Characterization of Nanomaterials

UV–Vis Spectrophotometry

Scanning Electron Microscope (SEM)

X-ray Diffraction (XRD)

Atomic Force Microscopy (AFM)

Transmission Electron Microscopy (TEM)

Annular Dark-Field Imaging (HAADF)

Intracranial Pressure (ICP)

Green Synthesis of Metal Nanoparticles and Their Applications

Green Synthesis of Silver (Ag) Nanoparticle and Its Applications

Green Synthesis of Gold (Au) Nanoparticle and Its Applications

Green Synthesis of Iron (Fe) Nanoparticles and Their Applications

Green Synthesis of Selenium (Se) Nanoparticle and Its Applications

Green Synthesis of Copper (Cu) Nanoparticle and Its Applications

Toxicity of Metal Nanoparticle Aspects

Author Contribution

Data Availability