Open Access Article
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Facile green synthesis and applications of silver nanoparticles: a state-of-the-art review

Nazia Tarannum*a, Divyaa and Yogendra K. Gautamb
aDepartment of Chemistry, Chaudhary Charan Singh University, Meerut 250004, India. E-mail: naz1012@gmail.com
bSmart Materials and Sensor Laboratory, Department of Physics, Chaudhary Charan Singh University, Meerut 250004, India

Received 2nd June 2019 , Accepted 7th October 2019

First published on 29th October 2019


Abstract

In the field of nanotechnology, the development of reliable and eco-friendly methods for the synthesis of NPs is crucial. The conventional methods for the synthesis of NPs are costly, toxic, and not ecofriendly. To overcome these issues, natural sources such as plant, bacteria, fungi, and biopolymers have been used to synthesize AgNPs. These natural sources act as reducing and capping agents. The shape, size, and applications of AgNPs are prominently affected by the reaction parameters under which they are synthesized. Accessible distributed data on the synthesis of AgNPs include the impact of different parameters (temperature and pH), characterization techniques (DLS, UV-vis, FTIR, XRD, SEM, TEM and EDX), properties and their applications. This review paper discusses all the natural sources such as plants, bacteria, fungi, and biopolymers that have been used for the synthesis of AgNPs in the last ten years. AgNPs synthesized by green methods have found potential applications in a wide spectrum of areas including drug delivery, DNA analysis and gene therapy, cancer treatment, antimicrobial agents, biosensors, catalysis, SERS and magnetic resonance imaging (MRI). The current limitations and future prospects for the synthesis of inorganic nanoparticles by green methods are also discussed herein.


1. Introduction

Nanoscience and nanotechnology are highly interdisciplinary branches conducted at the nanoscale, which is about 1 to 100 nm. The physicist Richard Feynman presented a talk entitled “There is Plenty of Room at the Bottom” on December 29, 1959 at the California Institute of Technology at a meeting of the American Physical Society Feynman RP. There's plenty of room at the bottom: an invitation to enter a new field of physics.1 In his keynote address, Feynman described manipulation technology at the atomic scale. After a decade, in his expedition of ultraprecise fabrication, Professor Norio Taniguchi framed the term nanotechnology. Nanotechnology has turned into a mainstream and vital innovation in recent years. Nanotechnology itself addresses NPs that are nuclear or atomic aggregates described by a size of under 100 nm. Nanotechnology alludes to the term for the assembling, depiction, control, and utilization of structures to control the size and shape at the nanoscale.2 Materials in the nanoscale have exceptional contrasting properties to that of similar materials in bulk. These distinctions are due to the basic and physical properties of metal molecules and surface-to-volume proportion to nanotechnology progression, where countless nanomaterials display characteristic properties.3

Around 5000 years back, numerous Egyptians, Persians, Greeks and Romans utilized silver in several structures to store nourishment items.4 During ancient periods, silverware was used in household daily activity due to its antimicrobial activity. There are records regarding the therapeutic applications of silver in the literature as early as 300 BC. Until the revelation of antimicrobials by Alexander Flemming, silver was ordinarily utilized as an antimicrobial specialist. In the Hindu religion, to date, silverware is favored for making the “panchamrit” utilizing Ocimum sanctum, curd, and different ingredients. The restorative properties of different metals are referenced in the old Indian Ayurvedic prescription book named “Charak Samhita5”.

In the past, AgNPs have attracted considerable attention from analysts. Due to the uncommon attributes of AgNPs, they are used in different fields such as biomedical (fast diagnosis, imaging, tissue regeneration and drug delivery, and development of new medical products),7 textile industry,8 food packaging,9 cosmetic industry,10 catalysis,11 sensors,12 biology, coatings,13 plasmonics (SERS),14 optoelectronics,15 antimicrobial activities,16 DNA sequencing,17 SERS,18 climate change and contamination control,19 clean water technology,20 energy generation, and information storage. Also, due to their remarkable protection against a wide scope of microorganisms and medicinal properties, AgNPs are utilized as anti-infection agents, tranquilizers conveyance agents, water treatment, farming, etc.21 Furthermore, due to their high conductivity, AgNPs have found application in electronic devices, inks, adhesives, pastes, etc.22 Generally, the synthesis of AgNPs is carried out using physiochemical techniques such as autoclaving, gamma-ray radiation, use of microemulsions, electrochemical techniques, chemical reduction, laser ablation, microwave irradiation, and photochemical reduction.23–33 Fig. 1 presents the various techniques used for the synthesis of NPs.


image file: c9ra04164h-f1.tif
Fig. 1 Representation of various techniques for the synthesis of NPs.6

The above methods have a high yield, but simultaneously they have limitations such as the use of toxic chemicals, and high functional cost and energy requirement. To overcome the limitations of physiochemical methods, alternative cost-effective methods involving plant extracts, microorganisms and natural polymers have been used for the synthesis of AgNPs. The combination of green chemistry and nanotechnology has extended the range of cytogenetically and biologically compatible metallic NPs.4

Over the previous decade, few review concentrating on the green synthesis of AgNPs have been published.34 The majority of them concentrated on a few plants (aloe leaf,35 cherry extract,36 Coffea arabica seed,37 Trianthema decandra,38 Macrotyloma uniflorum,39 and Rosa rugosa40), biopolymers41 (chitosan42,43) and microbial sources44 for the synthesis of AgNPs. Several characterization procedures (DLS, UV-vis, FTIR, XRD, SEM, TEM and EDX) have been employed to investigate information regarding the source, shape, size and properties of AgNPs with respect to different applications. The present review, in contrast to the prior reviews, focuses on the synthetic methods, parameters, characterization techniques, applications, and anticipated antibacterial components from different green ways for the synthesis of AgNPs.

2. Green synthesis

The basic requirement for the green synthesis of AgNPs is silver nitrate and a natural reducing agent.10–30 Generally, a natural reducing agent or different components present in the cell work as stabilizing or capping agents, and thereby the need to include these agents from outside is minimized.4 The traditional strategies for the production of NPs are costly, harmful, not environment-friendly. Thus, to overcome these issues, specialists have adopted green methods for the synthesis of NPs.45 Natural resources and their constituents have been utilized to synthesize NPs. Green synthesis can be classified as: (a) from plants and their extracts, (b) from bacteria, (c) from fungi and (d) from biopolymers. Various reducing agents have been used for the synthesis of AgNPs, which are shown in Table 1 with the general mechanism for the synthesis of AgNPs. The green synthesis via plants and plant extracts, bacteria, fungi, and biopolymers is described in the next sections of this review.
Table 1 Various constituents of plant, bacteria, fungi, and biopolymer responsible for the reduction of silver nitrate to AgNPs
Source Components responsible for the reduction of silver nitrate Mechanism for the synthesis
Plants Flavanoids, terpenoids, alkaloids, polyphenols, alcohol, phenolic acids, antioxidants, vitamins Electrostatic interaction between the functional groups of respective constituent of plant extract and Ag+ ion
Fungi Proteins, enzymes, NADH, NADPH, peptides, nitrogenous biomacromolecules, napthoquinones, anthraquinones Intracellular and extracellular synthesis of AgNPs
Biopolymers Chitosan, lignin, polypeptides, alginate, cellulose, protein Electrostatic interaction between Ag+ ion and polar groups attached to polymer


2.1 Synthesis of AgNPs from plants

The plant-based synthesis of AgNPs is generally adopted more compared to methods that use microorganisms since it can be improved easily, less bio-threatening and do not include the step of cell culture growth.46–50 All the parts of a plant (leaves, fruits, roots, seeds, and stems) contain biomolecules (e.g. enzymes, alkaloids, polysaccharides, tannins, terpenoids, phenols, and vitamins), which are of great therapeutic value and, despite their complex structures, are good for the environment.51 Plant extract replaces all toxic chemicals such as trisodium citrate and sodium borohydride (NaBH4). The extract from plants assists well in the synthesis of NPs due to the formation of AgNPs stabilized by the flavonoid and terpenoid components present in leaf broth, while the reduction of silver ions is favored by the polyol and water-soluble heterocyclic components of leaf broth.52 The extract of plant Salvia spinosa grown under in vitro conditions was used for the first time to synthesize AgNPs.53 The first report on the formation of AgNPs by a living plant system Alfalfa sprouts was presented by Gardea Torresdey et al. (2003). Alfalfa roots can absorb Ag from agar medium and transfer them in the same oxidation state to the shoots of the plant. These Ag atoms are converted to AgNPs in the shoots.54 Harekrishna Bar et al. (2009) reported the use of the latex of Jatropha curcas as the reducing and capping agent to synthesize AgNPs.55 Sithara et al. synthesiszed AgNPs by using leaf extract of Acalypha hispida and these AgNPs were used for the detection of Mn2+ ions.56 Gavhane et al. (2012) reported the use of the extract of Neem and Triphala to synthesize AgNPs, which were characterized using EDX, TEM, and NTA. TEM and NTA revealed the size of the AgNPs was in the range of 43 nm to 59 nm and they were spherical in shape.57 Ahmad and Sharma (2012) utilized (pineapple juice) Ananas comosus as a stabilizing and reducing agent for the synthesis of AgNPs.58 Charusheela Ramteke et al. (2012) synthesized antibacterial AgNPs using the leaf extract of (Tulsi) Ocimum sanctum.59 Roy et al. (2014) used Malus domestica fruit extract as a reducing and capping agent to synthesize AgNPs with an average diameter of 20 nm. The formation of the NPs was characterized by UV-vis spectroscopy, their morphology and distinctive phases were analyzed by TEM and XRD, and biomolecules for the reduction and stabilization of NPs were identified via FTIR spectroscopy.60 Velmurugan et al. (2015) synthesized AgNPs using peanut shell extract and compared their antifungal activity and characteristics with that of commercial AgNPs.61

Prem Jose Vazhacharickal et al. (2015) synthesized AgNPs using Curry leaf (Murraya koenigii) as the reducing and capping agent, which exhibited good antibacterial activity.62 M. Firdaus et al. (2017) reported the synthesis of AgNPs using aqueous fruit extract from (Carica papaya) papaya as the reductant under sunlight irradiation without additional capping agents. The AgNPs were characterized via UV-vis spectrophotometry and FTIR spectroscopy. A green environmental sensor was developed due to the good selectivity of AgNPs towards the hazardous heavy metal mercury in aqueous solution.63 Jerushka S. Moodley et al. (2018) reported the antimicrobial potential of synthesized AgNPs using leaf extracts of Moringa oleifera and utilized sunlight irradiation as the primary source of energy.64 Yu C. et al. (2019) synthesized AgNPs from the leaf extract of Eriobotrya japonica (Thunb) and utilized them in the catalytic degradation of reactive dyes.65 The most suitable choice to synthesize AgNPs is plant-like angiosperms. Medicinally important plants such as Boerhaaviadiffusa,66 Tinosporacordifolia,67 Terminalia chebula,68 aloe vera69, Ocimumtenuiflorum,70 Catharanthus roseus,71 Emblica officinalis72, Azadirachtaindica,73 common spices Piper nigrum,74 Cocos nucifera75, Cinnamon zeylanicum76 and some tropical weeds such as Partheniumhystero-phorus77 have been utilized to synthesize AgNPs. Plants that produce essential oils (Mentha piperita) and alkaloids (Papaver somniferum) have also been used to synthesize AgNPs. The have been a few cases in which chemicals such as sodium-dodecyl sulfate were used externally to stabilize AgNPs.78 All the plant extracts act as both reducing agents and capping agents. The proteins metabolites79 and chlorophyll80 present in the extract of plants act as stabilizing agents to synthesize AgNPs. Fig. 2 presents the mechanism for the synthesis of AgNPs from plants. Other synthetic procedures, conditions, characterization and application of AgNPs are discussed below (Table 2).


image file: c9ra04164h-f2.tif
Fig. 2 Mechanism for the synthesis of AgNPs from plant sources.
Table 2 Synthesis using plant extracts generates NPs with well-defined shapes, structures, and morphologies compared to that obtained using the bark, tissue, and entire plant
S. no. Reducing agents (green sources) Applications Operating conditions Characterization techniques used Particle characteristics Reference
1 Aqueous leaf extract of aloe vera Antibacterial activity AgNO3 (1 mM), extract: AgNO3 (1[thin space (1/6-em)]:[thin space (1/6-em)]10), stir 20 min, incubated for 24 h UV-vis, FTIR, TEM, Size: (36.61 ± 4.88 nm), shape: spherical 35
2 Cherry extract Antioxidant AgNO3 (1 mM), extract: AgNO3 (1[thin space (1/6-em)]:[thin space (1/6-em)]10) FTIR, UV-vis, DLS, TEM, XRD, TGA and DTA Size: (61.1 ± 39.2 nm) (for blue light), shape: spherical, crystal - FCC 36
3 Coffea arabica seed extract Antibacterial activity on E. coli and S. aureus AgNO3 (0.02 M, 0.05 M, and 0.1 M), volume of extract kept constant for each solution SEM-EDX, TEM, XRD, UV-vis, DLS Size: (20 to 30 nm), shape: spherical and ellipsoidal 37
4 Plant extract of Trianthema decandra Antimicrobial activity AgNO3 (1 mM), extract: AgNO3 (1[thin space (1/6-em)]:[thin space (1/6-em)]5, 1[thin space (1/6-em)]:[thin space (1/6-em)]10, 1[thin space (1/6-em)]:[thin space (1/6-em)]15) EDX, FTIR, UV-vis, SEM Size: (36 to 74 nm) 38
5 Seed extract of Macrotyloma uniflorum   AgNO3 (0.59 mM), extract: AgNO3 (1[thin space (1/6-em)]:[thin space (1/6-em)]60) TEM, XRD, UV-vis, FTIR Size: (12 nm) 39
6 Fruit extract of Tanacetum vulgare   AgNO3 (1 mM), extract: AgNO3 (1.8[thin space (1/6-em)]:[thin space (1/6-em)]50) TEM, XRD, EDX, FTIR, zeta potential, UV-vis, FTIR Size: (16 nm), shape: spherical 40
7 Leaf extracts of Rosa rugosa   AgNO3 (1 mM), extract: AgNO3 (2.5[thin space (1/6-em)]:[thin space (1/6-em)]60) UV-vis, TEM, XRD, FTIR, zeta potential, EDX Size: (12 nm), shape: spherical 40
8 Saccharum officinarum High antimicrobial activity against biofilm-forming bacteria and fungi. Reduce cytotoxicity on mammalian somatic and tumoral cells AgNO3 (14, 24, 52, and 104 mM), CS[thin space (1/6-em)]:[thin space (1/6-em)]AgNO3[thin space (1/6-em)]:[thin space (1/6-em)]VC (5[thin space (1/6-em)]:[thin space (1/6-em)]1[thin space (1/6-em)]:[thin space (1/6-em)]1), stirred for 12–15 h with heat UV-vis, TEM, EDS Size: (<10 nm) 42
9 Seed extract of Nelumbo nucifera Antibacterial and antifungal AgNO3 (1 mM), seed extract: AgNO3 (1[thin space (1/6-em)]:[thin space (1/6-em)]19) UV-vis, FTIR, TEM, XRD, SEM Size: (5.03 to 16.62 nm), shape: spherical 43
10 Eriobotrya japonica (Thunb.) leaf extract Catalytic degradation of reactive dyes Different ratios of leaf extract and silver salt solution (1[thin space (1/6-em)]:[thin space (1/6-em)]1, 1[thin space (1/6-em)]:[thin space (1/6-em)]2, and 1[thin space (1/6-em)]:[thin space (1/6-em)]10, v/v) UV-vis, XRD, TEM, SEM, FTIR, EDX Size at different temperatures: 20 °C: 9.26 ± 2.72, 50 °C: 13.09 ± 3.66, 80 °C: 17.28 ± 5.78 nm 65
11 Sapota pomace extract (Manilkara zapota) Good antibacterial activity against Gram-positive and Gram-negative bacteria AgNO3 (7 mM), mixed with extract in ratio 1[thin space (1/6-em)]:[thin space (1/6-em)]0.5 (v/v), temperature 20 °C UV-vis, XRD, FTIR, DLS, TEM, zeta potential Size: (8 to 16 nm), shape: spherical, moderate stability (zeta potential of −13.41) 81
12 Pomegranate peel extract (Punica granatum) Antibacterial activity against Staphylococcus, Pseudomonas aeruginosa and Escherichia coli pathogens AgNO3 (1 mM), mixed with extract (incubated for 24 h) UV-vis, FTIR, SEM Size: (5 to 50 nm), UV-vis: 371 nm 82
13 Saccharum officinarum extract and chitosan Antibacterial against Bacillus subtilis, Klebsiella planticola, Streptococcus faecalis, Pseudomonas aeruginosa and Escherichia coli AgNO3 (1 mM), extract: AgNO3 (9[thin space (1/6-em)]:[thin space (1/6-em)]1), incubated at 37 °C, till change in color UV-vis, TEM, SEM, EDS, FTIR Size: (10 to 60 nm), UV-vis: 460 nm 83
14 Arbutus unedo (strawberry) leaf extract Cost effectiveness, medical and pharmaceutical applications AgNO3 (1 mM), AgNO3:extract (1[thin space (1/6-em)]:[thin space (1/6-em)]1); temperature 80 °C, stir at 1000 rpm UV-vis, EDS, TEM, XRD Size: (2 to 20 nm), shape: spherical, geometry: FCC 84
15 Pomegranate leaf extract Antibacterial, anticancer activity on human cervical cancer cells86 AgNO3 (1 mM), AgNO3:extract (9[thin space (1/6-em)]:[thin space (1/6-em)]1) UV-vis, FTIR, SEM, XRD, EDX Size: (10 to 30 nm), geometry cubic 85
16 Walnut seed extract Used in photocatalytic degradation of effluent dye AgNO3 (1 mM), AgNO3[thin space (1/6-em)]:[thin space (1/6-em)]Extract (10[thin space (1/6-em)]:[thin space (1/6-em)]1) UV-vis, XRD, FTIR, TEM Size: (80 to 90 nm), shape: spherical, UV-vis: 420 nm, crystalline 87
17 Cinnamomum camphora leaf extract   AgNO3 (1 mM), heat: 30 °C, stir: 150 rpm UV-vis, XRD, TEM, SEM, AFM, FTIR Size: (55 to 80 nm), shape: spherical and triangular 88
18 Pomegranate peel extract Photocatalytic degradation of methylene blue AgNO3 (1 mM), pH: 8, temperature: (21 ± 5 °C) UV-vis, XRD, FTIR, EDS Size: (57.7 to 142.4 nm) 89
19 Azadirachta indica aqueous leaf extract Antibacterial activity against Staphylococcus aureus and Escherichia coli AgNO3 (1 mM to 5 mM) (1–5 mL) of extract was added to 10 mL of AgNO3 solution FTIR, UV-vis, DLS, photoluminescence, TEM Size: (34 nm), shape: spherical and irregular 90
20 Grape (Vitis vinifera) fruit extract Antibacterial activity against Bacillus subtilis and Escherichia coli AgNO3 (20 mM) extract: AgNO3 solution (1[thin space (1/6-em)]:[thin space (1/6-em)]1) UV-vis, DLS, EDX, TEM Size: (19 nm), shape: spherical 91
21 Alpinia katsumadai seed extract Free radical scavenging, antibacterial and antioxidant AgNO3 (10 mM) extract: AgNO3 (1[thin space (1/6-em)]:[thin space (1/6-em)]10) pH: 10, stir: 200 rpm for 90 min UV-vis, FETEM, EDX, SAED, XRD, FTIR Size: (12.6 nm), shape: spherical 92
22 Apple extract Antibacterial against Gram-negative and Gram-positive bacteria with MIC of 125 mg mL−1 AgNO3 (0.1 M) extract: AgNO3 (1[thin space (1/6-em)]:[thin space (1/6-em)]9) stir and heat at 80 °C XRD, DLS, FTIR, UV-vis Size: (30.25 ± 5.26 nm), crystalline 93
23 Berberis vulgaris leaf and root extract Antibacterial activity against Escherichia coli and Staphylococcus bacteria AgNO3 (0.5, 1, 3, 10 mM) extract: (3, 5, 10, 15, 30 mL) contact time: (1, 2, 6, 12, 24 h) XRD, TEM, UV-vis, DLS, Size: (30 to 70 nm), shape: spherical 94
24 Cinnamon zeylanicum bark extract and powder Bactericidal activity 100, 500 and 1000 mg of CBP added to 50 mL of 1 mM aqueous AgNO3 solution and incubated in the dark at 25 °C and shaken at 160 rpm. For CBPE, 1, 2.5 and 5 mL extract added to 50 mL of 1 mM aqueous AgNO3 solution UV-vis, TEM, EDX, XRD, zeta potential Size: (31 and 40 nm), quasi-spherical, and small, rod-shaped 95
25 Lippia nodiflora aerial extract Antioxidant and antibacterial against human pathogenic bacteria, cytotoxic against MCF-7 breast cancer cell lines AgNO3 (1 mM), extract: AgNO3 solution (1[thin space (1/6-em)]:[thin space (1/6-em)]19), heat from 30 °C to 95 °C for 10 min UV-vis, FTIR, XRD, SEM-EDX, TEM, zeta potential Size: (30 to 60 nm) 96
26 Andean blackberry fruit extract Antioxidant AgNO3 (1 mM), extract: AgNO3 solution (1[thin space (1/6-em)]:[thin space (1/6-em)]10), keep at 25 °C UV-vis, TEM, DLS, XRD, FTIR Size: (12 to 50 nm), shape: crystalline and spherical 97
27 Aqueous broccoli extract High toxicity against MCF-7 cell line AgNO3 (1 mM), extract: AgNO3 solution (1[thin space (1/6-em)]:[thin space (1/6-em)]19), pH: (6 to 7) UV-vis, FTIR, XRD, SEM, TEM, EDAX Size: (40 to 50 nm), FCC structure 98
28 Pinus merkusii cone flower extract   AgNO3 (0.1 M), extract: AgNO3 solution (2[thin space (1/6-em)]:[thin space (1/6-em)]1), heated at 60 °C FTIR, UV-vis, TEM Size: (9 to 23 nm), shape: spherical 99
29 Curcuma longa tuber (turmeric) powder and extract Immobilization on cotton cloth for bactericidal activity 100, 500 and 1000 mg of CLP added to 50 mL of 1 mM aqueous AgNO3 solution and incubated in the dark at 25 °C in a rotary shaker at 160 rpm. 1, 2.5 and 5 mL extract added to 50 mL of 1 mM aqueous AgNO3 solution UV-vis, TEM, XRD Size: (21 and 30 nm) 100
30 Garlic extract (Allium sativum) Nontoxic to VSMCs and NIH 3T3 fibroblasts AgNO3 (0.98 mM), extract solution (1.0 mL to 2.5 mL) added to 51 mL of AgNO3 solution TEM, UV-vis, EDX, ATR-FTIR, zeta potential, HPLC Size: (4 to 6 nm) 101
31 Ginger extract (Zingiber officinale) Antibacterial activity against Escherichia coli, Klebsiella pneumoniae, Pseudomonas aeruginosa, Bacillus cereus and Proteus vulgaris AgNO3 (1 mM) extract (20%): AgNO3 solution (1[thin space (1/6-em)]:[thin space (1/6-em)]9), temperature (27 ± 2 °C) UV-vis, XRD Size: (2.89 nm), shape: spherical 102
32 Aqueous seed extract of Manilkara zapota (L.) Antimicrobial activity against Candida species 10% concentration of MZSE was added to 0.01 M AgNO3, heated at 80 °C EDX, DLS, TEM, XRD, UV-vis Size: (40 to 100 nm) 103
33 Leaf extract of avocado Antibacterial activity AgNO3 (5 mM), extract: AgNO3 solution (1[thin space (1/6-em)]:[thin space (1/6-em)]9), kept in the dark for 24 h FTIR, XRD, SEM, UV-vis Size: (35.6 nm), shape: spherical 104
34 Origanum vulgare L. plant extract Antimicrobial activity AgNO3 (0.5 mM), 1 mL of plant extract added to 49 mL of AgNO3 solution and stirred for 2 h at 85–90 °C FTIR, UV-vis, XRD, TEM, EDX Size: (12 nm), FCC structure 105
35 Root extract of Croton sparsiflorus Antimicrobial activity AgNO3 (1 M) UV-vis, SEM Size: (30 to 50 nm), shape spherical 106
36 Roots extract of Coleus forskohlii Antimicrobial activity AgNO3 (1 mM), extract: AgNO3 solution (1[thin space (1/6-em)]:[thin space (1/6-em)]20), incubated for 24 h at 28 °C UV-vis, EDS, FTIR, SEM, XRD Size: (82.46 nm), shape: needle 107
37 Lemon leaf extract Antimicrobial activity AgNO3 (2 mM), extract: AgNO3 solution (1[thin space (1/6-em)]:[thin space (1/6-em)]9), keep in the dark at room temp FTIR, UV-vis, TEM, SEM, AFM Size: (Smaller than 100 nm range), shape: multi-shaped 109
38 Banana peel extract Antimicrobial activity AgNO3 (1.75 mM), extract: AgNO3 (1[thin space (1/6-em)]:[thin space (1/6-em)]50 (v/v)) UV-vis, XRD, SEM, EDX Size: (23.7 nm), crystalline 110
39 Valerian officinalis aqueous extract   AgNO3 (5 mM), plant powder (0.25, 0.50, 0.75 and 1.0 g) 50 mL distilled water UV-vis, XRD, SEM, TEM Size: (22 nm), shape: spherical, crystalline 111
40 Tectona grandis seed extract Antimicrobial activity against microorganisms AgNO3 (1 mM), AgNO3:seed extract (1[thin space (1/6-em)]:[thin space (1/6-em)]9) UV-visible XRD, FTIR SEM/EDS, FESEM, TEM Size: (10 to 30 nm), shape: spherical, crystalline 112
41 Extracts of Ananas comosus   AgNO3 (10 mM), pineapple juice: AgNO3 (1[thin space (1/6-em)]:[thin space (1/6-em)]10) XRD, UV-vis, EDAX, TEM Size: (12 nm), FCC crystalline 113
42 Extract of saffron (Crocus sativus L.) Antibacterial activity AgNO3 (2 mM), extract: AgNO3 solution (1[thin space (1/6-em)]:[thin space (1/6-em)]4) UV-vis, FTIR, XRD, TEM Size: (15 nm), shape: spherical 114
43 Onion (Allium cepa) extract Antibacterial activity AgNO3 (0.1 mM), extract: AgNO3 solution (1[thin space (1/6-em)]:[thin space (1/6-em)]10), constant stirring at 50–60 °C UV-vis, DLS, TEM Size: (33.6 nm), shape: spherical 115
44 Thymus kotschyanus plant extract Antioxidant, antibacterial and cytotoxic effects AgNO3 (1 mM), extract: AgNO3 (1[thin space (1/6-em)]:[thin space (1/6-em)]10), stir for 30 min in the dark UV-vis, FTIR, EDX, XRD, TGA, SEM, TEM, AFM Size: (50 to 60 nm) 116
45 D. carota (carrot) extract   AgNO3 (0.5 mM), extract: AgNO3 (1[thin space (1/6-em)]:[thin space (1/6-em)]6) XRD, UV-visible FTIR, TEM Size: (20 nm), shape: spherical 117
46 Garcinia mangostana stem aqueous extract Antimicrobial activity AgNO3 (1 mM), extract: AgNO3 (3[thin space (1/6-em)]:[thin space (1/6-em)]17) UV-vis, XRD, SEM, EDX Size: (30 nm) 118
47 Olive leaf extract Antibacterial activity AgNO3 (1 mM), extract (2–9 mL) added to AgNO3 solution TEM, UV-vis, FTIR, TG, XRD Size: (20 to 25 nm), shape: spherical 119
48 Extract of[thin space (1/6-em)]Chenopodium ambrosioides   AgNO3 (1 mM and 10 mM), extract: AgNO3 (0.5, 1, 2, 3 and 5 mL[thin space (1/6-em)]:[thin space (1/6-em)]5) UV-vis, TEM, FTIR Size: (4.9 ± 3.4 nm), FCC 120
49 Aqueous leaf extract of Acalypha indica Antifungal effect against Phytopathogen Colletotrichum capsici AgNO3 (1 mM), extract: AgNO3 (1[thin space (1/6-em)]:[thin space (1/6-em)]9), incubate at 37 °C UV-vis, antifungal   121
50 Ficus benghalensis leaf extract Antibacterial activity   UV-vis, TEM-EDX, XRD   122
51 Litchi chinensis leaf methanolic extract Strong muscle relaxant, analgesic and anti-inflammatory activities AgNO3 (1 M), extract: AgNO3 (1[thin space (1/6-em)]:[thin space (1/6-em)]1 and 1[thin space (1/6-em)]:[thin space (1/6-em)]10) UV-vis   123
52 Salvia leriifolia leaf extract Antibacterial activity against 9 bacteria AgNO3 (1 mM), extract: AgNO3 (1[thin space (1/6-em)]:[thin space (1/6-em)]24) SEM, AFM, XRD, FTIR Size: (27 nm), shape: pherical 124
53 Glycyrrhiza glabra root extract Treatment of gastric ulcer AgNO3 (1 mM), extract: AgNO3 (1[thin space (1/6-em)]:[thin space (1/6-em)]49) UV-vis, TEM, XRD, FTIR Size: (19 nm), crystalline 125
54 Pimpinella anisum seed extract Antimicrobial activity and cytotoxicity on human neonatal skin stromal cells and colon cancer cells AgNO3 (3 mM), extract: AgNO3 (1[thin space (1/6-em)]:[thin space (1/6-em)]100) UV-vis, FTIR, XRD, EDX, TEM Size: (15 nm) 126
55 Glycyrrhiza uralensis root extract Antimicrobial agent against Escherichia coli, Staphylococcus aureus, Pseudomonas aeruginosa and Salmonella enterica AgNO3 (1 mM), extract: AgNO3 (1[thin space (1/6-em)]:[thin space (1/6-em)]1), heated in oil bath at 80 °C, change in color is observed UV-vis, TEM, SAED, XRD, DLS, FTIR Size: (5 to 15 nm), shape: spherical 127
56 Orange peel Antimicrobial activity AgNO3 (1 mM), AgNO3:orange peel extract (1[thin space (1/6-em)]:[thin space (1/6-em)]1), pH above 7 UV-vis, FTIR, DLS, XRD, zeta potential, TEM Size: (48.1 ± 20.5 nm) 128
57 Citrus recticulata fruit peel extract Antibacterial activity against Streptococcus pyogenes, Staphylococcus aureus, Bacillus subtilis, Escherichia coli, Salmonella Paratyphi and Klebsiella pneumoniae AgNO3 (1 mM), extract: AgNO3 (1[thin space (1/6-em)]:[thin space (1/6-em)]20) UV-vis, FTIR, XRD, SEM, EDX Size: (24 nm) 129


2.2 Green synthesis of AgNPs from bacteria

NPs of noble metals such as Ag and Gold have been synthesized utilizing either intra or extracellular inorganic materials created by bacteria.130 Fig. 3 shows the synthetic procedure for the synthesis of AgNPs from the biomass of bacteria. Slawson et al. (1992) reported that AgNPs are biocompatible in a few bacteria, which are Ag-resistant.131 Pooley (1982) reported that bacteria aggregate Ag on the bacterial cell walls, and recommended the use of bacteria to industrially recover Ag from ore.132 First, Klaus et al. (1999) synthesized AgNPs using the biomass of the Pseudomonas stutzeri AG259 bacteria (Ag resistant). The amount of AgNPs accumulated by the bacteria cells was up to 200 nm.133 Kalimuthu et al. (2008) reported the synthesis of AgNPs with a size of 50 nm by adding silver nitrate aqueous solution to the biomass of B. licheniformis. A whitish-yellow to brown color confirmed the formation of AgNPs stabilized by the nitrate of enzymes.134 Nanda and Saravanan (2009) also synthesized AgNPs utilizing culture supernatants of Staphylococcus aureus. However, the culture supernatants from Enterobacteriaceae can be used for the quick synthesis of AgNPs.135 Shivaji et al. (2011) synthesized AgNPs utilizing culture supernatants of psychrophilic bacteria.136 Monowar et al. (2018) reported the extracellular synthesis of AgNPs utilizing the extract of an endophytic bacterium from Pantoeaananatis.137 Samadi et al. (2009) reported the use of Proteus mirabilis PTCC 1710 bacteria to synthesize AgNPs. During the incubation of bacteria, different types of broth are used for advancement in extracellular and intracellular synthesis. The mechanism for the green synthesis of AgNPs from bacteria is shown Fig. 3.
image file: c9ra04164h-f3.tif
Fig. 3 Mechanism for the green synthesis of AgNPs from bacteria.

The choice of bacteria in the green synthesis of AgNPs and an appropriate method are important for their large scale production.138 Mokhtari et al. (2009) reported the synthesis of AgNPs via photosynthesis by adding a solution of silver nitrate to the culture supernatant of Klebsiella pneumonia, and showed visible-light irradiation prompted the synthesis of AgNPs with a size of 3 nm139. According to reports by Lee and Shehata and Marr (Lee 1996; Shehata and Marr 1971), AgNPs were also produced via the reduction of silver ions using culture supernatants of bacteria. It should be noted that the growth of bacteria depends on the nutrients in the culture medium (glucose, phosphate or tryptophan).140 Shahverdi et al. (2007) reported the use of Enterobacter cloacae (Enterobacteriaceae), Escherichia coli and Klebsiella pneumonia for the fast synthesis of AgNPs, which formed AgNPs within a few minutes of Ag ions reacting with the cell filtrate.141 Kharissova et al. (2013) highlighted that bacteria kept on growing after the synthesis of AgNPs. However, compared to conventional methods, the utilization of bacteria for the reduction of Ag+ ions leads to a slow formation rate and limited range of shapes and sizes of AgNPs.142 Therefore, fungi-based NPs and reducing agents involving plants and plant extracts have been investigated for the synthesis of AgNPs (Table 3).

Table 3 Synthetic conditions, applications, size and characterization techniques for AgNPs using various strains of bacteria
S. no. Bacteria Application Conditions Characterization Size Reference
1 Psychrophilic bacteria Stable for 8 months in the dark 1 mL of 1 mM AgNO3 was added to 25 mg of the washed cell, and incubated under a fluorescent lamp (CFL) of 9 W UV-vis spectroscopy, transmission electron microscopy, atomic force microscopy Size: (6 to 13 nm) 136
2 Endophytic bacterium, Pantoea ananatis Antimicrobial against multi-drug resistant bacteria Reaction mixture of cell free extract and 100 mL of 0.1 mM AgNO3 solution (2%, v/v) exposed to bright sunlight, pH (7) UV-vis, TEM, SEM, FTIR, zeta potential Size: (8.06 to 91.32 nm) 137
3 Culture supernatant of Klebsiella pneumonia   AgNO3 (1 mM), supernatant 1% (v/v) XRD, UV-vis, TEM, EDS Size: (3 nm) 139
4 Culture supernatants of Enterobacteriaceae   AgNO3 (1 mM), supernatant (1%, v/v) UV-vis, EDS, TEM Size: (52.5 nm) 141
5 Biomass of bacterial exopolysaccharide Used in degradation of azo dye AgNO3, (9 mM) UV-vis, TEM, SEM, AFM, XRD, TGA-DTA, Raman spectroscopy Size: (35 nm), shape: spherical 143


2.3 Synthesis of AgNPs from fungi and yeast

Organic matter provides unique traits for the synthesis of NPs with advanced properties. Fungi are the main choice of microorganisms for the synthesis of NPs due to their vast range of advantages over yeast, bacteria, plants, and physicochemical techniques.144 Fig. 4 presents the mechanism for the synthesis of AgNPs using fungi.
image file: c9ra04164h-f4.tif
Fig. 4 Mechanism for the synthesis of AgNPs using fungi.

Fungi can synthesize metal NPs since they secrete enzymes and proteins, which are used to reduce metal salts. The large-scale synthesis of NPs from distinct fungal strains has been implied due to their growth even in vitro145. Xue B. et al. (2016) reported morphological and molecular methods to synthesize AgNPs under optimized conditions, i.e., the substrate concentration of 1.5 mM, alkaline pH, reaction temperature of 55 °C, and reaction time of 10 h, utilizing the fungal strain of Arthroderma fulvum. The synthesized AgNPs were found to be crystalline in nature and the particle size was optimized to be ∼15.5 ± 2.5 nm. Antifungal activity was observed against fungal strains, including Candida, Fusarium, and Aspergillus.146 Honary S. et al. (2013) evaluated a green synthetic method for the extracellular production of AgNPs using Penicillium citrinum isolated from soil. The synthesized NPs were found to be spherical in shape with an average diameter of 109 nm.147 A controlled and up-scalable green method for the synthesis of AgNPs with a well-defined morphology utilizing the cell-free aqueous filtrate of a non-pathogenic and suitable biocontrol agent Trichoderma asperellum was reported for the first time.148 Verma VC et al. (2010) prepared AgNPs utilizing Aspergillus clavatus and demonstrated their antimicrobial potential.149 AgNPs were synthesized by Li G. et al. (2012) using culture supernatants of Aspergillus terreus for the reduction of Ag ions.150

Subashini G. and Bhuvaneswari S. (2018) reported the synthesis of AgNPs from fungi and their applications in various fields of biology.151 AgNPs synthesized using Fusarium oxysporum were optimized by Birla SS et al. (2013) using different media, pH, temperature, light intensity, filtrate volume, salt concentration, and quantity of biomass.152 Neethu S. et al. (2018) reported the extracellular green synthesis of AgNPs utilizing the biomass of Penicillium polonium.153 Khan MN et al. (2015) utilized aqueous Raphanussativus root extract as a reducing and capping agent for the synthesis of silver nanomaterials for the first time.154 Ma L. et al. (2017) utilized the supernatant of the fungus strain Penicillium aculeatum Su1 to synthesize extracellular AgNPs.155 Al-Bahrani R. et al. (2017) reported the green synthesis of AgNPs utilizing the aqueous extract of basidiocarps of oyster mushroom, Pleurotus stratus156. Jalal M. et al. (2018) studied the extracellular green synthesis of AgNPs using the supernatant of Candida glabrata isolated from oropharyngeal mucosa of human immunodeficiency virus (HIV) patients and evaluated them for antibacterial and antifungal potential against human pathogenic bacteria and fungi.157 Eugenio M. et al. (2016) reported the biosynthesis of Ag NPs using yeast strains.158 Otari SV et al. (2014) synthesized AgNPs utilizing the culture supernatant of phenol degraded broth as the reducing agent.159 Ishida K. et al. (2014) studied the synthesis and antifungal activity of AgNPs synthesized utilizing the aqueous extract of the fungus Fusarium oxysporum.160 More details on the synthesis of AgNPs from fungi and yeast are discussed in Table 4.

Table 4 Synthetic conditions, applications, size and characterization techniques for the synthesis of AgNPs using various strains of fungi
S. no. Reducing fungus Application Optimization conditions Characterization techniques Shape and size Reference
1 Arthroderma fulvum[thin space (1/6-em)] Antifungal against Candida, Aspergillus spp., and Fusarium spp. AgNO3 (1.5 mM) alkaline pH, reaction temperature 55 °C, and reaction time of 10 h[thin space (1/6-em)] UV-vis, XRD, TEM Size: (15.5 ± 2.5 nm) crystalline 146
2 Penicillium citrinum isolated from soil Presence of amide linkage groups found in the fungal extract Dark compartment at 28 °C, 24 h, membrane filter (0.45 ) FTIR, photon correlation spectroscopy (PCS), SEM, fluorescence spectroscopy, UV-vis Size: (109 nm), shape: spherical 147
3 Trichoderma asperellum AgNPs formed were highly stable for 6 months AgNO3 (1 mM), 5 days incubated at 25 °C with biomass of Trichoderma asperellum UV-vis, FTIR, TEM, XRD, SERS Size: (13–18 nm) 148
4 Aspergillus clavatus (AzS-275), an endophytic fungus Antimicrobial against Candida albicans, Pseudomonas fluorescens and Escherichia coli AgNO3 (1 mM), cell biomass: AgNO3 (1[thin space (1/6-em)]:[thin space (1/6-em)]9), incubated at 25 °C on a rotary shaker (150 rpm) for 72 h UV-vis, FTIR, XRD, TEM, AFM Size: (10 to 25 nm) extracellular, polydispersed spherical or hexagonal 149
5 Biomass of Aspergillus terreus Antifungal and antibacterial AgNO3 10 mM, NADH, biomass: AgNO3 solution (5[thin space (1/6-em)]:[thin space (1/6-em)]1), incubated for 24 h at 28 °C XRD, TEM, UV-vis Size: (1 to 20 nm), shape: spherical 150
6 Raphanus sativus Antimicrobial activity Raphanus sativus root extract, (1.0–6.0 mL) added to AgNO3 solution DLS, TEM, EDX, XRD, FTIR, SEM Size: (3.2 to 6 nm) 154
7 Cell-free filtrate of the fungus strain penicillium aculeatum Su1 Antimicrobial activity, drug delivery vehicle or anticancer drug for clinical treatment. AgNO3 (10 mM) TEM, XRD, FTIR Size: (4 to 55 nm), FCC crystalline 155
8 Pleurotus ostreatus Inhibitory activity against pathogenic bacteria (1–6 mg mL−1) of aqueous extract of P. ostreatus was added to 5 mL, of 1 mM aqueous silver nitrate, kept at 28 ± 2 °C in the dark and incubated for 6, 12, 18, 24, 30, 36 and 40 h SEM, TEM, EDX, FTIR Size: (<40 nm) 156
9 Candida glabrata Antimicrobial activity against clinical strains of bacteria and fungi AgNO3 (1 mM) supernatant (20 mL) kept at room temperature overnight FTIR, UV-vis, TEM Size: (2 to 15 nm) 157
10 Biomass of Trichoderma viride (fungi) Antibacterial activity against human pathogenic bacteria AgNO3 (10 mM), biomass: AgNO3 (5[thin space (1/6-em)]:[thin space (1/6-em)]1), incubated for 24 h at 25 °C UV-vis, TEM, SEM, Size: (1 to 50 nm), shape: globular 158
11 Biomass of thermophilic Bacillus sp. AZ1 Antimicrobial activity against human pathogenetic bacteria AgNO3 (1 mM) SEM, EDX, TEM, UV-vis Size: (7 to 31 nm), shape: spherical 161
12 Biomass of Aspergillus niger Antimicrobial activity AgNO3 (10 mM), biomass of fungi:AgNO3 (5[thin space (1/6-em)]:[thin space (1/6-em)]1), incubated for 24 h at 28 °C UV-vis, XRD, TEM Size: (1 to 20 nm), shape: spherical 162


2.4 Green synthesis of AgNPs using biopolymers

Nearly all of the wide varieties of biopolymers used for the synthesis of AgNP play the dual role of reducing and stabilizing agent except for the use of starch as a capping agent.163 Fig. 5 presents the synthesis of AgNPs from various sources of biopolymers. Leung TC et al. (2010) synthesized AgNPs within 10–15 min by utilizing carboxymethylated-curdlan or fucoidan as reducing and stabilizing agents. Heating the reaction mixture at 100 °C led to the formation of AgNPs with a size in the range of 40–80 nm.164 Regiel Futyra A et al. (2017) reported that biopolymers enhanced the antimicrobial activity of AgNPs. Chitosan and ascorbic acid were utilized as the reducing and capping agent, respectively, for the synthesis of AgNPs with a size smaller than 10 nm.165 Biogenic AgNPs were synthesized using Nigella sativa extract (NSE), which exhibited potential antioxidant activity. The TEM image showed biphasic spherical AgNPs with an average particle size of 8 nm. The effect of the AgNPs on the sustained release and film-forming capacity of chitosan was then evaluated.166 Ahmad MB et al. (2011) synthesized AgNPs in an aqueous medium. The reduction of AgNO3 was carried out using chitosan and polyethylene glycol (PEG). PEG and chitosan were utilized as the polymeric stabilizer and solid support, respectively.167 Vasileva et al. (2011) synthesized stable and uniform starch-stabilized silver NPs with an average diameter of 14.4 ± 3.3 nm using ultrasound-mediated silver nitrate reduction by D-glucose. UV-vis spectroscopy, HR-TEM, XRD, TG/DTA, and DSC were used to characterize the starch-stabilized silver NPs completely. These NPs exhibited catalytic activity for the reduction of H2O2. Induced by the catalytic decomposition of H2O2, the degradation of the AgNPs caused a significant change in the absorbance strength of the localized surface resonance band depending on the concentration of H2O2. Subsequently, the optical sensor based on improvised plasmon resonance was characterized and calibrated. Good sensitivity and a linear response over the wide concentration range of 10−1 to 10−6 mol L−1 H2O2 were established.168
image file: c9ra04164h-f5.tif
Fig. 5 Procedure for the synthesis of AgNPs using biopolymers.

Atta A et al. (2014) reported a green synthetic method involving the reduction of Ag+ ions in aqueous acidic solution in the presence of polyvinyl alcohol modified with thiol groups (PVA-SH). AgNPs were stabilized by coating different types of citrate-reduced AgNPs with different weight ratios of PVSH derivatives (1–3 wt%). The as-prepared AgNPs were characterized via UV-vis spectroscopy, TEM/EDS, DLS and XRD combined with Rietveld analysis. The changes in the particle size, shape and hydrodynamic diameter of the AgNPs were determined using TEM, XRD and UV-visible spectroscopy after different durations of exposure to synthetic stomach fluid (SSF) and 1 M HCl. The data showed that for more than 90 days, these AgNPs were highly stable against SSF, which was not previously reported in the literature.169 Wu Q. et al. (2008) synthesized glutathione-capped AgNPs with adjustable sizes. These particles could be bound covalently to other functional molecules and displayed sensitive optical properties to particle size and surface modification. The AgNPs with a diameter of ∼6 nm prevented the proliferation of human K562 cells with leukemia, implying their potential cancer activity.170 The procedure for the synthesis of AgNPs using biopolymers is shown in Fig. 5.

Si S. et al. (2007) synthesized AgNPs at pH 11 utilizing synthetic oligopeptides containing tryptophan residue at the C-terminus. The tryptophan residue in the peptides, possibly through electron transfer, is responsible for reducing metal ions to the respective metals.171 Oligopeptides based on L-valine with the chemical structure Z–(L-Val) 3–OMe and Z–(L-Val) 2–L-Cys(S-Bzl)–OMe formed stable organogels in butanol. Both peptides are effective gelators, but they crystallize more readily than Z–(L-Val) 2–L-Cys(S-Bzl)–OMe. These two peptides are capable of forming mixed fibers, including gel butanol. The fibers can be mineralized using DMF as a reducing agent with AgNPs. The Z–(L-Val) 2–L-Cys(S-Bzl)–OMe fraction of the sulfur-containing peptide controlled the shape and size of the resulting NPs. Small spherical particles were distributed throughout the fiber at a high Z–(L-Val) 2–L-Cys(S-Bzl)–OMe content. A lower Z–(L-Val) 2–L-Cys(S-Bzl)–OMe content led to an increase in particle size and more complex forms such as plate-like and silver-like raspberry particles. The interactions between peptide and silver ions or silver particles occur through the complexation of silver ions to the sulfur atom of the thioether moiety and were shown to be the key interaction in controlling the formation of the particles.172

Kasthuri J. et al. (2009) reported the synthesis of quasi-sphere AgNPs using apiine as the reduction and stabilization agent. The size and shape of the NPs could be controlled by varying the ratio of metal salts to apiine compound in the reaction medium. UV-vis-NIR, TEM, FTIR spectroscopy, XRD and TGA were used to characterize the synthesized NPs. The interaction between the NPs and the carbonyl group of the apiine compound was confirmed using FT-IR spectroscopy. The average size of the AgNPs was found to be 39 nm via TEM invetigation.173 Safaepour M. et al. (2009) synthesized evenly dispersed AgNPs with a uniform size and shape in the range of 1 to 10 nm using geraniol. The cytotoxicity analysis of the AgNPs showed a direct dose–response relationship, where higher concentrations resulted in increased cytotoxicity. The AgNPs were able to inhibit the growth of the Fibrosarcoma-Wehi 164 cell line by less than 30% at a concentration of 1 μg mL−1.174 The aqueous solution of AgNPs exhibited different SPR when prepared at different pH values. PEG was used as a reducing and stabilizing agent to synthesize AgNPs since it is ecofriendly, which produced monodispersed particles with a diameter of less than 10 nm. The colloids exhibited activity against Gram-positive and Gram-negative bacteria and fungi. Biodegradable starch played the role of a capping agent in the synthesis of AgNPs. The analysis showed that a starch layer was coated on NPs. The diameters of the particles ranged from 5–20 nm. XRD analysis showed the face-centered cubic structure of the NPs. In many fields of science, ion-exchangeable polymers act as capping agents. These often-used polymers contain phosphonic acid groups with a low molecular weight. Polymer complexation to Ag+ occurs, and then the metal ions are reduced to NPs. In the presence of an ion-exchange polymer, AgNPs were stabilized. The morphology of the surface indicated the formation of cubes and rectangular prism structures. Copolymers such as CD, grafted with PAA, helped to synthesize AgNPs initiated by potassium persulfate.175

Maity D. et al. (2011) used poly(methyl vinyl ether co maleic anhydride) (PVM/MA) as a reducing and capping agent. The synthesized NPs were stable for a month at room temperature and surrounded by 5–8 nm sheath of PVM/MA.176 A variety of factors influenced the formation of NPs, such as acidity, initial concentration of starting materials, and molar ratio of reactants. Some dispersing agents prevented the accumulation of NPs and helped in the analysis of morphology, particle size, composition of elements, etc. The NPs were non-aggregated, and possessed a face-centered cubic (FCC) structure, and spherical shape. Ascorbic acid or citrate was used to reduce ions, which resulted in an average particle size of approximately 10.2–13.7 nm. The zeta potential ranged from 40–42 mV and was primarily influenced by the acidity and size of the NPs.177 When reacted with ammonium hydroxide, formaldehyde produced a polymer that affected the way silver was bound to the substrate. In unfavorable conditions for the synthesis of the polymer, the NPs formed were concentrated and possessed a gold-silver plasmon resonance (498 nm).177

3. Synthetic mechanism and characteristics of AgNPs

The synthesis of AgNPs can be carried out using natural sources such as carbohydrates, fat, phenols flavonoids, proteins, enzymes and coenzymes, terpenoids, gum, alkaloids, and sugars to reduce Ag+ ions. Depending on the organism/extract used, the active ingredient responsible for the reduction of Ag+ ions varies. Electrons are required for the nano transformation of AgNPs from acid (ascorbic acid) dehydrogenation and keto–enol conversion in mesophytes or both mechanisms in xerophyte plants. A similar reduction process can be performed by microbial cellular and extracellular oxidoreductase enzymes. The major constituents of fungi and bacteria such as quinones, NADH, nitroreductase enzyme, and proteins are responsible for the reduction and stabilization of AgNPs. It is assumed that the electrostatic interactions between the carboxylic group attached to the surface of fungal cell and Ag+ ions result in the formation of AgNPs, and proteins prevent the AgNPs from agglomerating.178 The major source of nitrogen used by bacteria is nitrate, which is reduced to nitrite by the enzyme nitrate reductase and NADH. This metabolic activity for the formation of ammonium and nitrile can be utilized in the green synthesis of AgNPs via the intracellular reduction of Ag+ ions. Proteins effectively prevent the agglomeration and increase in particle size caused by particle collisions, which maintain the high stability of colloidal AgNPs.155 Fig. 6 shows a schematic of the reaction mechanism for the synthesis of AgNPs from various sources. Based on the tautomerization in flavanoids, the possible mechanism for the synthesis of AgNPs is shown in Fig. 6(a). Fig. 6(b) presents the reaction mechanism for the synthesis of AgNPs involving the reduction of Ag+ ions due to the oxidation of NADH to NAD+.
image file: c9ra04164h-f6.tif
Fig. 6 (a) Reaction mechanism for the synthesis of AgNPs due to the flavanoids92 present in plant extract. (b) Reaction mechanism for the synthesis of AgNPs due to NADH present in fungi and bacteria.

3.1 Separation of AgNPs from suspension and their characterization

Researchers mainly use the centrifugation technique to obtain the pellet or powder form of synthesized AgNPs. AgNPs suspensions have also been dried to obtain the product in powder form. Some common techniques for the characterization of AgNPs include UV-vis spectroscopy, SEM, TEM, FTIR, XRD, and EDAX. DLS study is used for AgNPs synthesized from bio-polymers rather than plant extracts and microorganisms. UV-visible spectroscopy is considered the primary characterization technique to monitor the synthesis and stability of synthesized AgNPs. Due to the unique optical properties of AgNPs, they show strong interaction with light at specific wavelengths. The band gap in AgNPs is very low, and as a result electrons move freely, causing an SPR band due to the collective oscillation of electrons of AgNPs in resonance with light.179–181 Zeta potential values show the stability of synthesized AgNPs. Dubey et al. reported that AgNPs show a lower zeta potential value in acidic pH, and higher values in more basic pH solutions. It was observed that the absorbance peak became sharp with an increase in reaction time.40 The XRD analysis of the AgNPs shows diffraction peaks at 38.13°, 44.21°, 64.47°, 77.37°, 81.47°, 98.01°, 110.56° and 114.80°, which confirms the FCC structure of AgNPs.40,43,93 XRD and EDAX study also confirm the purity of the synthesized AgNPs. FTIR analysis reveals the functional groups responsible for the reduction and stabilization of AgNPs. TM Nguyen et al. analyzed the presence of protein via FTIR in the seeds of Nelumbo nucifera. The synthesized spherical AgNPs using the seed extract of Nelumbo nucifera showed cytotoxicity against Gram-negative bacteria.43 ZA Ali et al. reported that AgNPs synthesized using apple extract were stable due to the presence of the ethylene group in apple extract. These AgNPs showed antibacterial activity against multidrug-resistant bacteria.93 Honary S. et al. studied the presence of amide and ester linkages in Penicillium citrinum, which are responsible for the formation of AgNPs. The presence of a fluorescence emission band at 414 nm showed that the AgNPs were bound with protein and the protein also present in its native form in solution.147 DLS study on the suspension of AgNPs was used to calculate the average particle size and particle distribution of the synthesized AgNPs. SEM, TEM and AFM were used to study the surface morphology, size and various shapes of AgNPs.36,109,116 Koyla H. et al. synthesized globular polycrystalline AgNPs using the leaf extract of spinach.108 Hamelian M. et al. synthesized plate- or rod-shaped AgNPs using the plant extract of Thymus kotschyanus.116 Fig. 8 presents micrographs of the AgNPs synthesized by various methods under different optimization conditions. TGA was used to determine the effect of AgNO3 and L-cystine on the organic composition of the AgNPs. It was also used to determine the amount of organic material in the synthesized AgNPs and to predict their thermal stability.

3.2. Factors affecting the microstructure and application of silver nanoparticles

It has been reported that material properties are influenced by the structure (micro or nano) of the materials.182,183 The major physical and chemical parameters that affect the AgNP synthetic process are the temperature of the reaction, concentration of metal salt, content of extracts, pH of the reaction mixture, duration of the reaction and agitation. Parameters such as metal ion concentration, extract composition and reaction period have a major impact on the size, shape and morphology of AgNPs, where different experimental conditions lead to changes in the color of the reaction mixture. Yangqing He et al. reported that with an increase in the concentration of silver nitrate from 1 to 10 mM, the intensity of SPR peaks increased, which implies that at higher precursor salt concentration, more AgNPs were formed. A minor blue shift was observed for a higher concentration of Ag+ ions in the range of 425 to 418 nm. FTIR analysis revealed the presence of flavonoids and proteins, which were responsible for the reduction and stabilization of the AgNPs. These nanoparticles showed cytotoxicity against human gastric carcinoma, acted as free radical scavengers and showed antimicrobial activity.92 AgNPs synthesized from grape and tomato showed good antioxidant antibacterial and protein kinase inhibitory activity.91 Most authors reported the suitability of basic medium for the synthesis of AgNPs due to the better stability of the synthesized NPs observed.89,92,98 Some other benefits reported under basic pH are fast growth rate, good yield and monodispersity, as well as enhanced reduction process. By altering the pH, nearly spherical shaped AgNPs are converted into spherical AgNPs. The AgNPs formed in more basic pH (>11) and in acidic pH (<7) are unstable and agglomerate in the medium. Synthetic conditions such as stirring time and temperature are significant. Many researchers have used temperatures ranging from 20–100 °C to synthesize AgNPs from biopolymers and plant extracts; however, microorganisms die at high temperatures, and therefore they require a temperature of up to 40 °C. The rate of synthesis of AgNPs increases with an increase in temperature (30–90 °C) and encourages the production of small-size NPs. On average, the temperature range of 25–37 °C is considered suitable for the synthesis of AgNPs. Fig. 7 presents SEM images of AgNPs synthesized from different sources.95,99,116 Several reports demonstrated that AgNPs absorb electromagnetic radiation in the visible range from 380 to 450 nm, which is known as LSPR excitation. Honary S. et al. synthesized spherical monodispersed AgNPs with a size of 109 nm using Penicillium citrinum. PCS spectra show a polydispersity index (PDI) of 0.01. For a broad size distribution of AgNPs, the PDI is greater than 0.7, and the PDI should be between 0.01 and 0.5 for good monodispersity of AgNPs.147 Based on the symmetry in the shape of AgNPs, many researchers reported with a decrease in symmetry, charge separation increases, and consequently the primary SPR peak shows a red shift, while due to the snipping of the corners of asymmetric AgNPs, a blue shift observed. In general for spherical AgNPs, they have more SPR peaks than irregular AgNPs.88,97
image file: c9ra04164h-f7.tif
Fig. 7 SEM images of AgNPs synthesized from different sources.4

3.3 Indication for the formation of AgNPs

The literature review has reported the appearance of colorless AgNO3 solution to yellow or brown-yellow solution as the indication that AgNPs have been synthesized. UV-vis data showed the maximum absorbance wavelength for the synthesized AgNPs to be in the range of 400–460 nm.10–30 The absorbance data also helps to analyze the effect of pH, concentration of metal ions, and extract content on the size and stability of AgNPs. In most of the studies, SEM morphological analysis revealed spherical AgNPs, whereas few authors reported irregular, triangular, hexagonal, isotropic, polyhedral, flower, pentagonal, anisotropic and rod structures.37,40,88,91,92,149 Fig. 7 shows the SEM images of AgNPs with different shapes. The formation of face-centered cubic (FCC) crystalline-structured AgNPs has been reported by nearly all researchers using XRD studies. In some cases, AgNPs have been reported to show cubic and hexagonal structures also. EDAX is used to determine the elemental composition in nanomaterials. Depending on the reducing agent and other operating conditions, the stability of AgNPs may vary from 1 day to 1 year. Compared to plant extracts, the reaction mixture for the synthesis of AgNPs using microorganisms and bio-polymers is continually agitated to prevent agglomeration. The agitation of the reaction mixture achieved through the application of an external mechanical force can accelerate the formation of NPs.

4. Applications of silver nanoparticles

AgNPs have been used extensively as anti-bacterial agents in the health industry, food storage, textile coatings and a number of environmental applications. It is important to note that despite decades of use, the evidence for the toxicity of silver is still unclear. Products made with AgNPs have been approved by a variety of accredited bodies, including the US FDA, US EPA, and Japan SIAA, and in Korea.181 In addition, AgNPs are incorporated into nanoscale sensors due to their electrochemical properties, which can provide faster response times and lower detection limits. AgNPs are used as antibacterial agents, ranging from disinfecting medical devices and home appliances to water treatment. The textile industry has been encouraged to use AgNPs in various textile fabrics. Silver nanocomposite fibers with AgNPs incorporated into the fabric have been prepared in this direction.184 Cotton fibers with AgNPs are highly antibacterial against Escherichia coli. The catalytic activities of NPs differ from that of bulk materials, for example, high catalytic activity for the decoloration of monochrome black T dye in the presence of sodium borohydride (from 19.74% to 86.05%) and a light source (from 41.96% to 80.11%).185 The optical properties of metallic NPs depend primarily on their surface plasmon resonance, where the plasmon refers to the collective oscillation of free electrons within the metallic NPs.193 The plasmon resonant peaks and line widths are well known to be sensitive to the size and shape of the NPs. AgNPs are used in agriculture to increase crop production, plant nutrition, and defend against diseases.189 The various applications of AgNPs are presented in Fig. 8.
image file: c9ra04164h-f8.tif
Fig. 8 Various applications of AgNPs.

Chen Yu et al. reported the application of AgNPs in catalysis, which enhanced the reduction rate of NaBH4 in the reduction of azo dye.65,186 Due to the enhanced electromagnetic field on the surface of AgNPs, AgNPs are broadly used in nanomedicine including diagnostics, biomedicines, nanoelectronics and molecular imaging. AgNPs act as nanoantennas due to the increase in their resonant SPR peak with an increase in the intensity of the electromagnetic field. AgNPs act as sensors with Raman spectroscopy to identify any molecule due specific vibrational modes.50,187 Due to the antimicrobial action of AgNPs they are used in food packaging to prevent microbial infections.188,191 AgNPs are used in nanosensors to analyze contaminations, colors or flavors, drinking water and for clinical diagnostics.189 AgNPs have found application in agriculture also. Plant productivity can be enhanced via the communication of nanotechnology-based smart plant sensors with actuate electronic device, where these sensors optimize and automate water and agrochemical allocation, and enable high-throughput plant chemical phenotyping. Ag NPs are used in plant nutrition and defense against diseases,192 where AgNPs can be delivered with pesticides to crops to enhance the production of crops in agriculture.190 AgNPs are extensively used as therapeutic agents as antifungal, antimicrobial, anti-inflammatory and antiviral agents. Due to the antimicrobial action of AgNPs, they can be used in drug delivery to reduce the dose of drugs, improve specificity and decrease toxicity.88,193

5. Conclusion

In conclusion, considering environmental concerns for the synthesis of AgNPs, the green approach is preferred over conventional methodologies. The conventional synthetic methods for AgNPs require a huge amount of energy and hazardous chemicals (hydrazine or borohydride as reduction agents) and may lead to the formation of hazardous by-products. The use of biodegradable polymers, sugars or polyphenols from plant extracts, enzymes and bacteria under ambient conditions may lead to the sustainable synthesis of AgNPs with a uniform size. AgNPs formed in more basic pH (>11) are stable and in acidic pH (<7) are unstable and agglomerated. This implies that the size and stability of AgNPs are dependent on pH. Using composites based on PEG and Ag CMC, Ag nanorods exhibiting luminescent properties, specific size and shape can be easily obtained using microwave irradiation. The synthesis of AgNPs is quite easy and simple using plants and their extract compared to other sources such as fungi and bacteria. The size and morphology of AgNPs varies with a variation in reaction parameters. The simple use of vitamins such as vitamin B2, B1, and C may produce NPs at ambient temperature in aqueous media. In addition, new biomimetic techniques have proven to be beneficial for the preparation of AgNPs, although there are still some inherent safety concerns. The green methods for the synthesis of AgNPs using bio-renewable materials seems to be promising because they require non-toxic chemicals for the reduction of silver salt. This review provides a wide spectrum of all the natural resources such as plants, bacteria, fungi, and biopolymers for the production of AgNPs in the last ten years.

Abbreviations

AgNPsSilver nanoparticles
NPsNanoparticles
PEGPolyethylene glycol
SERSSurface-enhanced Raman scattering
DLSDynamic light scattering
TEMTransmission electron microscopy
SEMScanning electron microscopy
XRDX-ray diffraction spectroscopy
EDAXEnergy-dispersive X-ray spectroscopy
FTIRFourier transform infrared spectroscopy
CDCyclodextrin
PAAPolyacrylic acid
CMCCarboxy methyl cellulose
NTANP tracking analysis
PCSPhoton correlation spectroscopy
SPRSurface plasmon resonance
TGAThermo-gravimetric analysis

Conflicts of interest

The authors do not have any conflict of interest.

Acknowledgements

The authors would like to acknowledge Department of Science and Technology (DST) and University fund for financial assistance.

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