Environmental Applications of Biosynthesized Silver Nanoparticles Complimentary Copy

Abstract

Nanoparticles, exhibit significantly changed physical, chemical, and biological properties due to their high surface-to-volume ratio, which makes them valuable for a wide range of applications. Among the various metal nanoparticles, silver nanoparticles (AgNPs) have attained a pinnacle position due to their enhanced electrical, optical, and thermal properties. AgNPs have proven application in various fields like chemistry, medicine, agriculture, space, electronics, and environment. These NPs have been synthesized by various physical, chemical, and biological routes. The chemical methods, although very common, versatile, and inexpensive to produce AgNPs in definite sizes and shapes but are toxic and non-eco-friendly. The physical methods, although

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In: Applications of Silver Nanoparticles
Editor: Elliot Conley
ISBN: 979-8-88697-842-1
© 2023 Nova Science Publishers, Inc.
Chapter 1
Environmental Applications of Biosynthesized
Silver Nanoparticles
Ravi P. Srivastava1
Shalini Pareek2
Navneet Kumar3
Swati Verma4
and Divya Shrivastava2,
1Department of Materials Science and Engineering, Yonsei University,
Seoul, South Korea
2School of Life Sciences, Jaipur National University, Jaipur, Rajasthan, India
3Department of Electronics Engineering, Hanyang University, Seoul, South Korea
4Department of Civil and Environmental Engineering, Hanyang University,
Seoul, South Korea
Abstract
Nanoparticles, exhibit significantly changed physical, chemical, and
biological properties due to their high surface-to-volume ratio, which
makes them valuable for a wide range of applications. Among the various
metal nanoparticles, silver nanoparticles (AgNPs) have attained a
pinnacle position due to their enhanced electrical, optical, and thermal
properties. AgNPs have proven application in various fields like
chemistry, medicine, agriculture, space, electronics, and environment.
These NPs have been synthesized by various physical, chemical, and
biological routes. The chemical methods, although very common,
versatile, and inexpensive to produce AgNPs in definite sizes and shapes
but are toxic and non-eco-friendly. The physical methods, although
 Corresponding Author’s Email: shrivastavadivyajnu@gmail.com.
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superior in terms of greenness, but require very high temperatures,
consume enormous amounts of energy, and are expensive, which makes
them unsuitable for synthesis. On the other hand, the biosynthesis of
AgNPs using plant extracts, microbes, algae, fungi, etc. has opened an
eco-friendly, cost-effective, and safer way to synthesize AgNPs.
Biosynthesis methods have the least impact on human health and
environment and can produce uniform-sized AgNPs of definite shape
without any contamination. In this chapter, we have compiled and
discussed the applications of biosynthesized AgNPs for various
environmental purposes such as the removal of organic pollutants from
water, removal of toxic metal ions, for food packaging bioplastic, an
alternative to chemical pesticides and fertilizers, biosensors, and solar
cells, etc. The chapter also provides a comprehensive description of the
latest research on the biosynthesis of AgNPs, and includes mechanistic
insight into biosynthesis methods, properties of biosynthesized AgNPs
and scopes for further improvements.
Keywords: silver nanoparticles, biosynthesis, environment
Introduction
The applications of metal nanoparticles (NPs) have emerged in various fields
of day-to-day life due to their exceptional biological and physicochemical
properties. In the present era of nanotechnology silver nanoparticles (AgNPs)
have a significant contribution (Sharifi-Rad et al., 2021). AgNPs have shown
their potential and importance in various fields, majorly the food packaging,
instrumentation, medicine, textile, engineering, energy, cosmetics, surgery,
electronics, agriculture, and environment (Kasi et al., 2014; Akter et al., 2018).
The physicochemical properties of AgNPs like the large surface area to
volume ratio, capability to bind with different extents with a ligand, easy
customizations, and catalytic behaviors are among some properties that make
them highly applicable in diverse fields (Jain et al., 2008; Akter et al., 2018).
There are several approaches to synthesize AgNPs - physical, chemical,
photochemical, and biological. Each technique has its advantages and
disadvantages. The physical methods use radiations which do not involve the
use of any chemical but on the other hand, these are expensive, and the yield
is less along with high energy requirements (Sharma et al., 2022). The most
frequently used technique is the chemical approach which uses silver metal
precursor (AgNO3), reducing agents like ethylene glycol, and lastly stabilizing
or capping agents like polyvinyl pyrrolidone. The use of costly, toxic, and non-
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eco-friendly chemicals in the process is a serious concern for living beings and
the environment (Tran et al., 2018). Photochemical methods involve the use
of photochemically generated intermediates. It is a clean and convenient
process but the high cost and requirement of environment-controlled
experimental conditions reduce its frequency of use. Therefore, the greener
routes for the synthesis of AgNPs have gained wider attention (Philip et al.,
2009).
For the biological synthesis of AgNPs, the reducing and stabilizing agents
have been reported from bacteria, fungi, algae, amino acids, sugars, vitamins,
plants, and animals (Huang et al., 2006; Khan et al., 2010; Jacob et al., 2011;
Rafey et al., 2011; Nidya et al., 2015; deMatos and Courrol, 2017). The
reduction of AgNO3 using biologically derived chemicals has advantages like
non-toxicity, an abundance of raw material, renewability, avoidance of
hazardous waste generation, cost-effective production in an ambient
environment, and eco-friendly way of synthesizing AgNPs for various
applications (Mocanu et al., 2009). Out of the biological sources, used to
derive reducing agents, the plant parts like leaves, roots, flowers, bark, and
fruits do not require special isolation techniques, cultures, and culture
conditions that are needed for microbial and animal-based sources. In addition,
synthesis of plant based AgNPs is highly economic and easier for bulk
synthesis of AgNPs (Iravani, 2011; Sharifi-Rad et al., 2021).
AgNPs have a wide range of applications in various fields. The present
chapter focuses on the applications of AgNPs in solving environmental issues
such as the removal of organic pollutants from water, removal of toxic metal
ions, use in bioplastic for food packaging, an alternative to chemical pesticides
and fertilizers, biosensors, solar cells, etc. AgNPs-based water filters are used
to disinfect the usable water. The catalytic activity of AgNPs has also been
reported for the degradation of dye contamination in water. Removal of heavy
metals (like Hg2+, Cr3+, Co2+, Pb2+, and Ni2+) from the water has also been
accomplished using AgNPs and their conjugates (Pradeep, 2009).
Biosynthesized AgNPs have also been used in producing food-grade
bioplastic for providing antimicrobial and mechanical strength to it (Youssef
et al., 2014; Ediyilyam et al., 2021). For biosensing devices, AgNPs have also
proved their environmental importance in detecting toxic substances in the
environment (Remya et al., 2022). The properties of nanopesticides like target
specificity, slow and effective release, improved adhesion, and dispersion on
plant surface make them a better choice compared to conventional pesticides.
Now a days, the use of biosynthesized AgNPs as nanopesticides is much more
eco-friendly and user-friendly (Deka et al., 2021).
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Solar energy generated by solar cells is the most potential, green, and eco-
friendly alternative to nonrenewable sources of energy like coal and
petroleum. Silicon solar cells are commercial solar cells but research in
different other types of solar cells such as heterojunction polymer solar cells,
dye-sensitized solar cells, and perovskite solar cells is still in progress. In these
solar cells AgNPs are used to generate plasmonic effects, to improve the
optical and photovoltaic properties, and efficiency of solar cells (Prasad et al.
2013). Various researchers have used biosynthesized AgNPs for use in solar
cells, observing the importance of their eco-friendly method of generation. In
this chapter, we will focus on the synthesis and properties of biosynthesized
AgNPs and discuss their applications for the environment followed by future
prospects of the research in this area.
Methods of Biosynthesis of AgNPs
The biosynthesis of metal NPs has the advantage of being eco-friendly, non-
toxic, and low-cost compared to the synthesis by chemical and physical
methods. In general, the biological extracts are mixed with a metal precursor
solution and the color change indicates the formation of nanoparticles. The
biological extract source can be plants, animals, fungi, bacteria, algae, or
biomolecules like vitamins, polysaccharides, and amino acids (Mittal et al.,
2013). Here the biological sources are the sources of polyphenols,
biomolecules, or metabolites that act as reducing (reduces the metal ion from
positive to zero oxidation state), capping, and stabilizing agents for the
synthesis of metal nanoparticles. In common, the parameters that decide the
shape, size and amount of metal nanoparticles using biological sources are the
concentration of biological extract, reaction time, temperature, pH, salt, etc.
(Roy et al., 2019). Here we will summarize the biosynthesis methods of
AgNPs from the mentioned sources. The studies related to the use of
biomolecules directly (as brought from the market) are mainly concentrated to
produce functionalized AgNPs and using them as biosensors rather than their
direct involvement in the synthesis of AgNPs.
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Synthesis of AgNPs Using the Following
Plants
Synthesis of AgNPs using plant parts like leaves, roots, stem, flowers, and
bark help in easy scale-up, eco-friendly, non-pathogenic, and inexpensive
production as compared to physical and chemical methods (Huang et al.,
2007). In 2003, the production of AgNPs using alfa-alfa was the first study
reporting the use of plant material for the synthesis of AgNPs (Gardea-
Torresdey et al. 2003). The common protocol for the synthesis of AgNPs from
plants includes the collection of required plant parts and washing with double
distilled water 2-3 times. Washed plant parts are dried and boiled in double
distilled water for 10-15 min followed by filtering the obtained solution. In the
collected filtrate AgNO3 solution is added and preserved at room temperature.
Change in the color from pale yellow to dark brownish green indicates the
formation of AgNPs which is further confirmed by UV-Vis spectrophotometer
(Rajeshkumar, 2016). The plants used for the synthesis of AgNPs are mostly
medicinal plants due to their richness in phytochemicals. These plants majorly
include Azadirachta indica, Ocimum species, Viburnum nervosum, Cinnamon,
Cocos nucifera, Catharanthus roseus, Eucluptus, Tinospora, Camellia, etc.
(Mukunthan et al., 2011; Ramteke et al., 2013; Mariselvam et al., 2014;
Gauthami, 2015; Mohammed, 2015; Ahmed et al., 2016; Selvam et al., 2017;
Zahoor et al. 2021). In the majority of AgNPs synthesis using plant extracts,
the phytochemicals act as reducing, capping, and stabilizing agents. Proteins
present in the plant extracts are reported to trap Ag+ ions on their surface due
to electrostatic interactions leading to a change in the secondary structure of
proteins and the formation of Ag nuclei. On the other hand, some studies have
also reported the role of phytochemicals like amides, aldehydes, ketones,
terpenes, flavonoids, alkaloids, and flavones as reducing agents of Ag+ ions.
In xerophytes, anthraquinone and emodins are reported to undergo
tautomerization to form AgNPs (Li et al., 2007).
Microbes
Microorganisms are proven source for the biosynthesis of AgNPs. The two
modes of microbial biosynthesis are intra and extracellular (Das et al., 2014).
In intracellular mode, the silver ions accumulate inside the cell and nucleate
the formation of AgNPs which increases with the growth of the microbe.
Nanoparticles are harvested from the live bacterial cells with special
treatments at optimum temperature. In an extracellular mode of synthesis, the
secretory products of bacteria like enzymes, vitamins, amino acids, and sugars
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are used to synthesize AgNPs (Roy et al., 2019). In most accepted mechanisms
it is shown that nitrate reductase (NADH- dependent enzyme which is a part
of the electron transport chain) converts nitrate (NO3−) into nitrite (NO2−). The
electrons released in the process are used to reduce Ag+ to Ag0 (Kumar et al.,
2007). Besides this bacterial cell wall is also reported to reduce the Ag+ to
make AgNPs. Majorly used bacterial species are Streptomyces spp.,
Pseudomonas spp., and Pantoea spp., (Slawson et al., 1992; Fayaz et al.,
2010).
Fungi
In comparison to bacteria, the production of AgNPs from fungi have the
advantage of over-production due to the large availability of enzymes,
proteins, and reducing agents on its cell surface required to synthesize AgNPs
(Xu et al., 2020). Fungi have unique properties like metal tolerance, bio-
concentration, fast growth, and extracellular secretion of enzymes, which
make them fit for the production of AgNPs. Depending upon the location, the
fungi-mediated synthesis of AgNPs can be of two types’ intracellular and
extracellular (Tiwari et al., 2015; Majeed et al., 2015). Intracellularly, AgNPs
can be obtained from mycelia while extracellularly AgNPs can be obtained
from cell-free filtrate. In comparison to intracellular, extracellular synthesis is
preferred due to the easy collection and downstream processing. The most
used fungi for the synthesis of AgNPs are Fusarium spp., Phomopsis spp.,
Penicillium spp., Trichoderma spp., and Aspergillus spp. (Salaheldin et al.,
2016; Ottoni et al., 2017; Neethu et al., 2018; Seetharaman et al., 2018). For
the extracellular synthesis of AgNPs, the fungal extract is purified by washing
off or precipitating the fungal components. Fungi trap the Ag+ on their cell
surface and reduce them to Ag0 using the enzymes (like xylanase and nitrate
reductase), naphthoquinone, and anthraquinone (Devi and Joshi, 2015;
Elegbede et al., 2018). In addition to these enzymes, fungal proteins are also
reported to act as a capping agent in the synthesis of AgNPs. Various factors
like temperature, nitrate source and light source provided to fungus act to
control the synthesis, shape, and size of AgNPs (Hamedi et al., 2017).
Animals
There are some reports which have shown the synthesis of AgNPs using
animal parts like cockroach wings, honey from honeybees, cow milk, cobweb,
goat fur, paper wasp net, etc. (Lee et al., 2013; Lateef et al., 2016a; Lateef et
al., 2016b; Balasooriya et al., 2017; Khatami et al., 2019). The biomolecules,
especially proteins from these animal parts have been reported to act as
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reducing and capping agent in the synthesis of AgNPs. In general, the animal
parts used are first hydrolyzed with alkali, centrifuged and the extract thus
obtained is used for the synthesis of AgNPs (Morones et al., 2005).
Algae
Algae are one of the important sources for AgNPs biosynthesis due to fast
growth, metal accumulation ability, and presence of various biologically
active molecules. The various active biomolecules like pigments, enzymes
polysaccharides, proteins, secondary metabolites, sulfated agents, amino
acids, oils, fats, antioxidants, phycobilins, etc. are present in algae. These act
as reducing and capping agents in the synthesis of eco-friendly, size and shape-
controlled AgNPs (Asmathunisha and Kathiresan, 2013; Michalak and
Chojnacka, 2015). The most commonly used algae for the synthesis of AgNPs
are the members of Cynophyceae, Rhodophyceae, Chlorophyceae, and
Phaeophyceae. Specific factors like temperature, pH, incubation period, and
the ratio of algal extract and AgNO3 solution determine the shape, size, and
amount of AgNPs during synthesis. The general process of biosynthesis of
AgNPs using a precursor solution is to mix the algal extract with the AgNO3,
the specific change in color indicates the formation of AgNPs (Khanna et al.,
2019). There are two modes for the synthesis of AgNPs from algae viz.,
intracellular, and extracellular. The intracellular mode needs no pre-treatment
of the algae as it relies on the internal processes going on inside the algae like
respiration, photosynthesis, and nitrogen fixation. In this case, the reducing
agents which reduce Ag+ to Ag0 are NADPH or NADPH-dependent reductase
enzymes generated during photosynthesis or respiration (Sicard et al., 2010;
Sharma et al., 2016). On the other hand, the extracellular mode refers to the
use of algal cell exudates like lipids, proteins, enzymes, metabolites, and other
biomolecules for the synthesis of AgNPs. In this, the algal biomass undergoes
the pre-treatments like washing and mixing (Vijayan et al., 2014; Dahoumane
et al., 2016).
Biomolecules
The direct use of biomolecules like amino acids, polysaccharides, and
vitamins has been reported for the biosynthesis of AgNPs. Amino acids like
cysteine have been used to produce nanocrystalline silver sol (Roy et al.,
2012). Amino acids attached to phenolic compounds were used to synthesize
AgNPs with spherical and prism morphologies by Kumar et al. (2013b).
Tyrosine and tryptophan have been used by Shankar and Rim (2015) to
synthesize AgNPs. These two amino acids worked both as capping and
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reducing agents. AgNPs are not only synthesized rather functionalized using
amino acids for bio-sensing activities. A simple, fast, and cost-effective
method of biosynthesis of AgNPs, using twenty one types of amino acids with
white light illumination without any other additive was investigated by de
Matos and Courrol (2016). Chandra and Singh (2018) used alanine,
tryptophan, histidine, glutamic acid, aspartic acid, and methionine to
functionalize AgNPs for catalytic and oxygen-sensing activities. Similarly,
Khalkho et al. functionalized the AgNPs by using L-cysteine for probing the
Vitamin B1 (Khalkho et al., 2020), while the AgNPs functionalized with
Vitamin B12 were used for detection of iron (Fe+3) in food and baby products
(Harke et al., 2022). The biological activity of AgNPs was modulated by
functionalizing with sugars like D-glucose, D-mannose, and D-galactose.
These surface modifications reduce the toxicity of synthesized AgNPs
(Kennedy et al., 2014). Moreover, AgNPs functionalized with carbohydrate
derivative, glutathione-lactose (GSH-Lac) were used to detect thiram pesticide
in agricultural samples (Dhavle et al., 2021).
Characterization of AgNPs
Among the various metal nanoparticles, AgNPs due to their unique properties
have gained significant interest and found application in various fields of
science and technology. The AgNPs can be synthesized by physical, chemical,
or biological methods. The different synthesis methods can lead to different
properties of the nanoparticles. Therefore, the characteristics of synthesized
nanoparticles must be carefully investigated to explore their full potential for
application in different fields. A very basic, and usual way is visual inspection
i.e., to check the color change of the solution from yellow to brown, indicating
the formation of AgNPs. It can be further confirmed with various
characterization techniques, which not only reveal the formation of AgNPs but
also provide the qualitative and quantitative information of different
characteristics in a precise manner. It helps to suggest the best-fit application
of the synthesized nanoparticles. The structural, electrical, optical, and
compositional properties of the synthesized AgNPs can be studied with
different characterization techniques, such as X-ray diffraction, scanning
electron microscopy, transmission electron microscopy, EDAX, FTIR, and
UV-Vis spectroscopy. We will discuss these characterization techniques in
brief, before proceeding for the applications of the AgNPs.
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X-ray Diffraction (XRD)
X-ray diffraction is a powerful and versatile tool to study the crystal structure
of nanoparticles. The technique works on Bragg’s law and can provide
qualitative as well as quantitative information of the material. The incoming
X-rays after diffraction from atomic planes interfere with each other resulting
in a diffraction pattern. The obtained diffraction pattern is fingerprint for a
material, which can be compared with the standard diffraction patterns by the
Joint Committee on Powder Diffraction Standards (JCPDS) to confirm the
identity of the synthesized material. Figure 1a shows the XRD diffraction
pattern for the biosynthesized AgNPs obtained from the leaf extract of
Ipomoea aquatica (Khan et al., 2020).
In the diffraction patterns (Figure 1a) peaks corresponding to 2θ values of
38.01°, 44.18°, 64.26°, 77.16° and 81.29° can be assigned to (111), (200),
(220), (311), and (222) crystallographic planes and confirm the formation of
face-centered cubic AgNPs (Khan et al., 2020). The additional peaks
appearing in the diffraction pattern were supposed to be related to the
crystallization of bioorganic phase on the surface of AgNPs, originating from
leaf extract. The average crystallite size of the nanoparticles can also be
estimated from the broadening of diffraction peaks using Debye-Scherrer’s
equation.
D = (Kλ)/(βcosθ) (1)
Here, K (0.94) is a dimensionless constant (Jain et al., 2017), β is the full
width at half maximum (FWHM) of the peak and λ is the wavelength of X-
ray radiation. The lattice parameter ‘a’ of AgNPs can also be calculated with
the following equation.
𝑑 = 𝑎
√ℎ2+ 𝑘2+ 𝑙2 (2)
Here, (hkl) represents miller indices of the plane, and ‘d’ represents
interplanar spacing. Moreover, information about the strain present in the
material can also be obtained with this technique.
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Figure 1. (a) X-ray diffraction spectrum of AgNPs biosynthesized using leaf extract
of Ipomoea aquatica, (b) UV-Visible spectrum of AgNPs biosynthesized using leaf
bud extract of Rhizophora mucronata, (c) FTIR spectrum of AgNPs biosynthesized
using leaf extract of Coleus aromaticus, and (d) EDAX spectrum of AgNPs
biosynthesized using Aspergillus terreus.
UV-Visible Spectroscopy
UV-Visible spectroscopy is a primary, fast, and reliable technique to confirm
the synthesis of AgNPs. The technique also provides the information about the
stability of synthesized nanoparticles. In this technique a beam of light is
passed through the sample and the intensity of transmitted light is compared
with the reference. Usually, a wavelength range of 400 to 800 nm is used for
the characterization of AgNPs. Figure 1b shows the UV-visible spectrum
obtained for the AgNPs biosynthesized using leaf bud extract of Rhizophora
mucronata (Umashankari et al., 2012). The optical properties of the
nanoparticles are sensitive to their size, shape, and distribution which makes
optical characterization an important tool to probe the properties of
nanoparticles and suggest their applications. In a certain range of wavelengths
AgNPs induce surface plasmon resonance (SPR). From the intensity and
position of SPR peaks the quality of synthesized AgNPs can be analyzed
(Prathna et al., 2011). A broad absorption peak appearing at a longer
ab
cd
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wavelength suggests the formation of large-size or aggregated particles,
whereas a narrow peak appearing at a low wavelength resembles the small size
of the nanoparticles (Smitha et al., 2008). Additionally, the stability of the
green synthesized AgNPs can also be estimated with the SPR. The SPR peak
appearing at the same wavelength indicates the stability of green synthesized
AgNPs maintained for several months (Zhang et al., 2016).
Fourier Transform Infrared (FTIR) Spectroscopy
The role of biological molecules in the green synthesis and stabilization of
synthesized AgNPs can be studied with FTIR spectroscopy. It is very valuable
and economic technique. The identification of biomolecules involved in the
reduction of Ag+ ions as well as functional groups responsible for the synthesis
of AgNPs can be studied with this technique (Anandalakshmi et al., 2016). In
the qualitative analysis, the presence of different types of chemical bonds is
identified using infrared radiation. The FTIR spectrum exhibits various peaks
depending on different kinds of chemical bonds and the interaction of various
functional groups (alkanes, ketones, and amines) with infrared radiation
(Palithya et al., 2022). A comparative study of the FTIR spectrum of medicinal
plant extract and biosynthesized AgNPs can help in the identification of
functional groups involved in the surface coating and stabilization of the
synthesized nanoparticles (Akintelu et al., 2020). FTIR spectrum obtained for
AgNPs biosynthesized using the leaf extract of Coleus aromaticus is shown
in Figure 1c (Vanaja et al., 2013). The FTIR spectrum shows different peaks
resembling the various functional molecules associated with AgNPs.
Scanning Electron Microscopy (SEM)
The morphology of synthesized nanoparticles has a significant effect on the
properties and therefore, visualization of the size and shape of nanoparticles
is very important. In scanning electron microscopy an intense beam of
electrons interacts with the material and produces various signals. After
interaction of primary electron beam with the material, the generated
secondary and backscattered electrons reveal useful information about the
material. SEM imaging can provide information of size, morphology, particle
distribution, and particle aggregation. It does not provide information about
the internal structure of the material. A typical SEM for AgNPs synthesized
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using leaf extract of fern-Dryopteris barbigera is shown in Figure 2a (Jan et
al., 2022).
Figure 2. (a) FE-SEM micrograph of the AgNPs synthesized using leaf extract of
fern-Dryopteris barbigera; (b) TEM (c) lattice fringes and (d) SAED pattern of
AgNPs synthesized using yeast extract.
Transmission Electron Microscopy (TEM)
Transmission electron microscopy is another very useful and advanced
electron microscopy technique to characterize nanoparticles. In TEM, the
imaging is accomplished with transmitted electrons. A few microliters of the
synthesized solution is dropped on a carbon grid for TEM imaging. The
technique provides much improved spatial resolution in comparison to SEM
and is capable to provide more in-depth information on nanomaterials. Figure
2c shows high-resolution transmission electron microscope (HRTEM) image
demonstrating the capability of the technique to image lattice fringes (Shu et
al., 2020). Moreover, the selected area (electron) diffraction (SAED) study
ab
c d
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provides much useful information about the crystallinity of the material.
Figure 2 (b, d) shows a typical TEM micrograph along with the SAED pattern
for the biosynthesized AgNPs using yeast extract (Shu et al., 2020).
Dynamic Light Scattering (DLS)
Dynamic light scattering, also known as photon correlation spectroscopy, is
one of the most important and useful technique to characterize the particle
size, distribution, and surface charge of nanoparticles. It is a non-destructive
technique to probe the particle size of nanoparticles in an aqueous solution.
The technique can probe a wide range of particles extending from submicron
to one nanometer. The method basically depends on the interaction of particles
with electromagnetic radiation. The light scattered at certain angles is recorded
by a detector. The measurement of light intensity fluctuations over the time,
due to Brownian motion, allows the determination of the hydrodynamic radius
of the particles. The size obtained by DLS is usually larger than that measured
by TEM, due to the influence of Brownian motion (Zhang et al., 2016).
Zeta Potential
Zeta potential is a very informative and critical parameter to study the effective
electric surface charge, dispersion, and stability of synthesized AgNPs.
Depending on the electrical charge, particles can have different speeds and
directions under the applied electric field. The zeta potential is determined
through the velocity measurement in the suspension medium under the
application of an electric field. The value of zeta potential provides
information on the stability of synthesized nanoparticles. Particles with a high
positive or negative charge have an increased tendency of repulsion which
lowers their aggregation leading to stable particles. The zeta potential values
over 20-30 mV, in general, suggest a stable suspension, whereas the lower
values of zeta potential indicate aggregation of AgNPs (Jastrzębska et al.,
2015).
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Energy-Dispersive X-ray Spectroscopy (EDAX)
The elemental composition of synthesized AgNPs can be ascertained with the
EDAX. The EDAX is a very quick technique and is usually equipped with
electron microscopes. In EDAX the characteristic X-rays emitted after
interaction with nanoparticles are used to determine the chemical composition
of the nanoparticles. When an incident electron knocks out a core-shell, such
as a K shell electron, the transition of another electron from an outer shell
(such as M shell) to this empty shell takes place and the energy difference
between these two shells is released in the form of X-ray radiation. Each
element present in the sample emits characteristic X-rays which are detected
and compared with the standard database to identify it. The EDAX spectra of
biosynthesized AgNPs using Aspergillus terreus is shown in Figure 1d (Lotfy
et al. 2021).
Environmental Applications of Biosynthesized AgNPs
Biosynthesized AgNPs have shown their potential in various environmental
applications making them safe and clean to live. Here we will discuss the role
and mechanism of AgNPs in wastewater treatment (like removing dyes and
sensing heavy metal ions), in making food grade bioplastic, in making sensors
to detect toxins in the environment, as well as their potential to replace harmful
chemical pesticides and fertilizers. Their role in the fabrication of solar cells
for clean energy generation will be explored in detail. A diagrammatic
representation showing the role of biosynthesized AgNPs in various
environmental applications is depicted in Figure 4.
Dye Removal from Water
The role of AgNPs synthesized using green routes has been studied for a
prolonged period towards various applications like wastewater treatment,
antimicrobial studies, solar cells, catalysis, sensing, optoelectronics,
biomedical, etc. In particular, the use of AgNPs in removing organic dye
molecules from wastewater via adsorption and photocatalysis is remarkable.
Adsorption of organic pollutants by AgNPs is a two-step physicochemical
process. In the first step, organic dye molecules diffuse into the outer surface
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of AgNPs followed by their diffusion into the interior of AgNPs in the second
step (Obayomi et al., 2022).
Photocatalytic Degradation
Photocatalytic mineralization of toxic organic dye pollutants by AgNPs is
triggered by the in-situ generation of reactive oxygen species (ROS). These
ROS play a significant role in the conversion of toxic organic compounds into
non-toxic organic compounds. During photocatalysis, electrons are excited
from the valence band to the conduction band upon light irradiation to produce
dynamic electron-hole pairs. These electron-hole pairs interact with oxygen
and water molecules present in the system to produce ROS. Consequently,
superoxide and hydroxyl radicals are generated as a potent oxidizing agent to
mineralize toxic dye molecules into non-toxic byproducts including CO2 and
H2O (Kumar et al., 2022). The band gap energy of AgNPs is ~2.5 eV, which
is significantly lower than the indirect band gap energy (~3.2 eV) of most used
metal oxide photocatalysts, i.e., TiO2 (Aziz et al., 2018). Due to the lower
energy gap AgNPs, display excitation in sunlight or visible region.
The key mechanism in photocatalysis is the excitation of an electron from
the valence band (VB) to the conduction band (CB) under illumination with
light energy equivalent to the energy band gap of photocatalyst, as shown in
Figure 3. Consequently, photo-generated electron-hole pairs are formed in the
domain of photocatalysts. The bonding and atomic energy values of a material
are analogous and can be used for determining the electronegativity of
constituting atoms in view of fermi-level distribution (Nethercot, 1974). The
determined electro-negativity values can be used to gain insights of the
spectroscopic information of a semiconductor material with an equal
possibility to predict bond formation energy with high accuracy. It is critical
to determine the position of CB and VB with respect to the redox potentials of
O2/O2•− and •OH/H2O radicals, to investigate the workability of photo
generated e¯/h+ pairs at the photocatalyst surface (Kumar et al., 2022). The
potential energies of the CB and VB of a semiconductor photocatalyst material
can be estimated with Eq. 3 and 4 while the electro-negativities of the elements
constituting a semiconductor photocatalyst can be obtained from Eq. 5
(Nethercot, 1974; Yuan et al., 2014a).
2
CB g H
E = χ-0.5E + E
(3)
VB CB g
E = E + E
(4)
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Ravi P. Srivastava, Shalini Pareek, Navneet Kumar et al.
16
1
a b c (a+b+c)
χ = [χ(A) χ(B) χ(C) ]
(5)
Here, ECB and EVB are the energy positions (vs. NHE) of CB and VB,
respectively; Eg is the band gap energy determined from UV-DRS
measurements; EH2 is the free electron energy (-4.5 eV vs. H2), χ is the absolute
electronegativity (AE) of the photocatalyst material; χ(A), χ(B), and χ(C) are
electro negativities of the constituting A, B, and C elements respectively; and
a, b, and c are the number of atoms of A, B, and C elements that form
photocatalyst.
Figure 3. Schematic representation of the mechanism of visible light-assisted
mineralization of organic dye molecules by AgNPs.
Silver Nanoparticles in Photocatalysis
Various studies have been reported on the photocatalytic degradation of toxic
organic dye molecules from aqueous solutions. Kumar et al. (2013a) reported
the biosynthesis of AgNPs using Ulva lactuca (seaweed) for the photocatalytic
mineralization of methyl orange (MO) dye under visible light illumination.
Batch experiments showed that 20 mg of the spherical Ag photocatalysts
caused~90% mineralization of 10 ppm aqueous solution of MO dye in 10 h.
Yola et al. (2014) synthesized spherical AgNPs composite with colemanite ore
waste (COW) and tested for the adsorptive-photocatalytic degradation of
reactive yellow 86 (RY 86) and reactive red 2 (RR 2) dyes. In this study, 50
mg of Ag-COW photocatalyst showed ~98 and ~95% removal efficiency of
100 ppm aqueous solutions of RY 86 and RR 2 dyes in 1 h under visible light
O2
Ag
photo-
catalyst
O2•
H++ •OH
H2O
O2•‾
•OOH
•OH
H+
Organic dyes CO2+ Water
SUN
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Environmental Applications of Biosynthesized Silver Nanoparticles
17
illumination. Similarly, Tamuly et al. (2014) synthesized AgNPs using
pedicellamide (A), isolated from Piper pedicellatum leaf, and used for the
photocatalytic mineralization of methyl red (MR) dye under UV light. In this
typical study, 0.01 mol % of AgNPs solution caused ~98% photocatalytic
degradation of MR dye (10 ppm) in 10 min. Sinha and Ahnaruzzaman (2015)
reported the utilization of eggshell of Anas platrhynchos as stabilizing and
reducing agents for the synthesis of Ag and Au-Ag core-shell NPs. As-
synthesized AgNPs were tested for the photocatalytic degradation of various
dyes like Rose Bengal (RB), methyl violet 2B (MV), and methylene blue (MB)
under solar light illumination. Degradation experiments showed that 10 mg of
AgNPs could degrade ~ 97%, ~98%, and ~96% of RB, MV, and MB (10 ppm)
dyes in 90, 105, and 90 minutes, respectively. Saha et al. (2017) reported the
biogenic synthesis of AgNPs using fruit extract of Gmelina arborea and used
an array of characterization techniques like TEM, XRD, and UV-Visible
spectroscopy to study their characteristics. The efficiency of as-prepared
AgNPs was tested for the reductive removal of MB dye under visible light. In
the removal experiment, 3 mL of AgNPs solution displayed ~99% removal of
10 ppm, 30 mL of MB dye solution in 10 min. Phuyal et al. (2022) synthesized
AgNPs from an aqueous extract of Rhododendron arboretum leaves under
alkaline conditions (pH= 9). AgNPs synthesized from this method were
thoroughly characterized by XRD, TEM, SEM, ATR-FTIR, and UV-Visible
spectroscopy. Batch experiments showed ~81% degradation of 10 ppm MB
dye in 24 h for solar photoreaction using 1 mg of AgNPs. A few more studies
on the green route synthesis of AgNPs and their use towards organic dye
removal from wastewater are summarized in Table 1.
Detection of Heavy Metal Ions
Heavy metals are the metals that severely affect living organisms and
surroundings when present in a density of more than 5 g cm-3. Heavy metals,
like chromium (Cr), lead (Pb), mercury (Hg), cadmium (Cd), antimony (Sb),
and arsenic (As) are non-essential, non-degradable, toxic, and carcinogenic.
Heavy metals enter the environment through industrial means, human
activities, and natural ways (Aravinthan et al., 2015). Heavy metals are taken
up by the plants from contaminated soil and subsequently pass to herbivorous
animals via the food chain. Consumption of toxic-heavy-metal-contaminated
food plants and meat may adversely affect human health (Bar et al., 2009).
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Table 1. Green routes for the synthesis of AgNPs and their use towards organic dyes removal from wastewater
S. No.
Target pollutant
Reducing
agent
Source of Ag
Nature of
light
Catalyst
dose (mg)
Time
(min)
Volume of
the solution
(mL)
Initial
concentration of
pollutant (ppm)
%
Removal
Reference
1.
Methyl Orange
NA
Ulva lactuca
Vis-light
20
600
50
10
90
Kumar et al.,
2013a
2.
Reactive yellow 86
NA
Colemanite ore
waste
Vis-light
50
60
50
100
97.7
Yola et al., 2014
Reactive red 2
NA
Colemanite ore
waste
Vis-light
50
60
50
100
95.2
3.
Coomassie Brilliant
Blue G-250
NA
Coccinia grandis
leaf
UV light
10
90
50
10
85
Arunachalam et
al., 2012
4.
Methyl red
NA
Pedicellamide
UV light
0.01 mol%
10
10
10
98
Tamuly et al.,
2014
5.
Methyl orange,
methylene blue, and
eosin Y
NaBH4
Trigonella
foenum-graecum
Vis-light
0.5, 2, and
2 mL
20
15
10
~95 all
Vidhu et al.,
2014a
6.
Reactive turquoise
blue
NA
Black Tulsi
NA
50
120
100
60
96.8
Banerjee et al.,
2014
7.
Methylene blue
NaBH4
Saraca indica
flower
NA
2 mL
12
10
10
99
Vidhu et al.,
2014b
8.
Methylene blue
NA
Salvadora
persica stem
UV-light
8
80
70
15
96
Tahir et al.,
2015
9.
Methyl violet,
safranin, eosin
methylene blue, and
methyl orange
NA
Indian screw tree
Sunlight
5
180
NA
50
~95
Bhakya et al.,
2015
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S. No.
Target pollutant
Reducing
agent
Source of Ag
Nature of
light
Catalyst
dose (mg)
Time
(min)
Volume of
the solution
(mL)
Initial
concentration of
pollutant (ppm)
%
Removal
Reference
10.
Congo red
Ethanolic
NaBH4
Amaranthus
gangeticus Linn
leaf
80μL
15
20
10
Kolya et al.,
2015
11.
Rose Bengal, methyl
violet 2B, and
methylene blue
Egg shell
Sunlight
10
90, 105,
and 90
200
10
97.3,
97.6,
and 96
Sinha and
Ahmaruzzaman
et al., 2015
12.
Methylene blue
Saccharomyces
cerevisiae
Sun light
10
360
100
10
80
Roy et al.,
2015a
13.
Methyl orange
Hypnea
musciformis
Sunlight
10
60
50
10
95
Ganapathy and
Sivakumar,
2015
14.
Crystal violet
NA
Azadirachta
indica
NA
10
60
100
20
97.2
Satapathy et al.,
2015
15.
Methylene blue and
congo red
Cordia
dichotoma
Sunlight
50
600 and
20
100
10 and 100
82 and
98
Kumari et al.,
2016
16.
Methylene blue
NaBH4
Caulerpa
racemosa extract
NA
5 mL
30
-
5
~99
Edison et al.,
2016a
17.
Safranine O, methyl
red, methyl orange,
and methylene blue
Zanthoxylum
armatum leaves
NA
1
2400
10
10
~80, 78,
96, and
85
Kumari and
Singh, 2016
18.
Congo red and
methyl orange
NaBH4
Anacardium
occidentaletesta
NA
0.01 mL
25 and
15
-
~99 and
86
Edison et al.,
2016b
19.
Methylene blue
NaBH4
Gmelina arborea
NA
3 mL
10
30
10
~99
Saha et al., 2017
20.
Crystal violet
NA
Carissa carandas
UV-light
25
150
100
10
~99
Anupama and
Madhumitha,
2017
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Table 1. (Continued)
S. No.
Target pollutant
Reducing
agent
Source of Ag
Nature of
light
Catalyst
dose (mg)
Time
(min)
Volume of
the solution
(mL)
Initial
concentration of
pollutant (ppm)
%
Removal
Reference
21.
Methyl orange
NA
Solanum nigrum
Sunlight
10
360
20
10
~80
Malaikozhundan
et al., 2017
22.
Methylene blue
Prosopis farcta fruit
Vis-light
10
30
20
10
70.2
Miri et al., 2018
23.
Brilliant blue
NaBH4
Gardenia
jasminoides Ellis
1 mL
240
12
-
92
Saravanakumar
et al., 2018
24.
Methylene blue
NA
Punica granatum
Sunlight
10
4800
100
10
89
Joshi et al.,
2018
25.
Methylene blue
NA
Cauliflower waste
Sunlight
5
150
50
1
97.57
Kadam et al.,
2020
26.
Methylene blue,
acridine orange, and
rose Bengal
Citrus aurantium
Sunlight
-
5880 and
240
-
-
95, 87,
and 90
Ringwal et al.,
2021
27.
Brilliant Blue R
Cichorium intybus
18.57
30
20
20
82
Sidorowicz et
al., 2021
28.
Methylene blue
Eucalyptus leaves
Sunlight
1 mL
1440
20
10
71.34
Rabeea et al.,
2021
29.
Methylene blue
Rhododendron
arboreum
Sunlight
1
1440
50
10
81
Phuyal et al.,
2022
30.
Methyl orange
NaBH4
Calotropis procera
0.01 mL
10
98
Chandhru et al.,
2022
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Environmental Applications of Biosynthesized Silver Nanoparticles
21
Heavy metals entering in the human body, combine with enzymes and
proteins, make strong chemical bonds and inhibit regular body functioning.
Biosynthesized AgNPs are used to construct heavy metal ion sensors to sense
heavy metal contamination in the environment. Using the leaf extract of
Dahlia pinnata AgNPs were synthesized to selectively sense Hg2+ ions in
water in a broad range of pH (Roy et al., 2015b). Kumar et al. (2015) used an
eco-friendly approach for photo-induced synthesis of stable AgNPs using an
aqueous extract of Murraya koenigii. The obtained AgNPs colloid selectively
detected the Hg2+ and showed a linear relation between SPR band intensity
and range of Hg2+ concentrations (50 nm to 500 μM). These workers also
proposed an oxidation-reduction mechanism for this detection process.
According to this, when Hg2+ is added to the AgNP solution, an oxidation-
reduction reaction occurs between Ag0 and Hg2+ ions. In this reaction AgNPs
get oxidized and converted to Ag+ whereas Hg2+ ions get reduced to Hg0 atom.
SPR band also shifted to the blue region and fully vanished with increased
Hg2+ concentration.
AgNPs biosynthesized using Citrullus lanatus were found highly
sensitive to sense Cu+2 and Hg+2 contaminations in the water. Detection of
Cu+2 ions was based on changing absorbance due to the complex formation of
the metal ions with AgNPs. It was represented by a new peak at around 770
nm, in addition to the peak at 406 nm for AgNPs. The AgNPs functionalized
with 3-mercapto-1, 2-propanediol (MPD) were used for the detection of Hg+2
in water via the colorimetric method. When Hg+2 solution was added to MPD-
AgNP, a new peak around 606 nm was observed along with the peak at 404
nm for AgNPs. This new peak was predicted due to the aggregations of Hg+2
ions by MPD-AgNP via dipropionate ions (Maiti et al., 2016). Simultaneous
recognition of Hg2+ and Pb2+ by a highly selective and cost-effective
colorimetric sensor was reported by Ahmed et al. (2020) using biosynthesized
AgNPs from the roots extract of Bistorta amplexicaulis. This AgNPs-based
colorimetric sensor was found highly sensitive toward Hg2+ with a limit of
detection (LOD) of 8.0 × 10−7 M and for Pb2+with LOD of 2.0 × 10−7 M.
Similarly, for the detection of Hg+2 colorimetric analyses using Artemisia
vulgaris-mediated AgNPs without modifying aqueous solution was proposed
by Adhikari et al. (2022). In this study, the AgNPs colloidal solution turned
colorless with the addition of 380 μL of 1 mM Hg2+.
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Ravi P. Srivastava, Shalini Pareek, Navneet Kumar et al.
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Use in Biodegradable Food Packaging Plastic
Plastic is one of the unavoidable needs of today’s world but on the other hand,
it has created a huge problem due to its non-degradability. This problem has
attracted the attention of most environmentalists and scientists towards
making biodegradable plastic which easily degrades in the environment by the
action of microorganisms. Such plastics are called bioplastics or biopolymers;
and these are especially plant-based polymers (Avella et al., 2005). Food
packaging in plastic is an important issue concerning to food safety.
Nanotechnology is being used to improve the quality of food packaging
components with confirmation of food safety. The evaluation parameters like
tensile strength, elasticity, strength, toughness, and antimicrobial properties of
food packaging plastic are important considerations. Polyvinyl alcohol (PVA)
films have increasing demand due to their use in food packaging, so increasing
their mechanical strength and antimicrobial properties is a great concern.
Researchers have shown that the incorporation of AgNPs has helped in
achieving both objectives. Youssef et al. (2014) have shown the method to
incorporate the biosynthesized AgNPs from Bacillus subtilis into chitosan to
prepare chitosan-silver nanocomposite films. These films showed
biodegradable and antimicrobial properties, which are good for a food
packaging material. Similarly, Mohanta et al. (2017) showed the potential of
the plant Protium serratum leaf extract to prepare AgNPs and reported their
application as an antimicrobial agent against Pseudomonas aeruginosa,
Escherichia coli, and B. subtilis in food packaging materials. The AgNPs
synthesized using peel extract of black grapes (Vitis vinifera) were used to
prepare PVA based nanofibers. The film prepared using these AgNPs-PVA
nanofibers showed the antimicrobial properties and better storage of lemons
and strawberries wrapped in them for up to 10 days compared to the control
which showed a shelf life of 3 days only at room temperature (Kowsalya et
al., 2019). Fan et al. (2019) have shown the incorporation of Ag-loaded nano
cellulose to PVA films. Cellulose nanocrystals (CNCs) act as carriers and
dispersants of AgNPs. This hybrid of CNC-PVA-AgNPs showed nearly two-
fold increase in film tensile strength, reduced moisture content absorption, and
showed good antibacterial properties. Similarly, the mechanical strength,
extensibility, and physical resistance of thermoplastic maize starch films were
enhanced by adding AgNPs synthesized using the watery mixture of
pomegranate (Punica granatum L.) seed extract (Mohseni et al., 2020).
The food packaging industry ensures the use of economically feasible,
eco-friendly, and non-toxic materials for food packaging. Due to the properties
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Environmental Applications of Biosynthesized Silver Nanoparticles
23
like packaging functionality and easy biodegradability, the biopolymers like
gelatin, cellulose, starch, and chitosan are used to replace conventional
plastics. The incorporation of AgNPs into chitosan and gelatin for making
novel packaging material resulted in improved physicochemical and
biological properties of the film matrix (Ediyilyam et al., 2021). It was found
that the cross-linking movement of AgNPs reduced the degree of swelling,
moisture retaining capacity, and transmission of water vapors. On adding
0.0075% of AgNPs to pure chitosan-gelatin film the tensile strength increased
from 24.4 ± 0.03 to 25.8 ± 0.05 MPa. The addition of AgNPs to gelatin-
chitosan films also lowered the bacterial contamination compared to plastic
polyethylene films which increased the shelf life of food material. The hybrid
antimicrobial composite film of AgNPs with cellulose nanofibrils and PVA
prepared by Madivoli et al. (2022) showed excellent mechanical strength and
antimicrobial properties against Gram-positive and negative bacteria and
fungus used in the investigation.
Use in Nanofertilizers and Nanopesticides
Nanofertilizers enrich the crop quality and balance the crop nutrients (Mathur
et al. 2022). Improvements in plant growth and yield against drought, salinity,
heat and cold stress by biosynthesized AgNPs-based nanofertilizers have been
reviewed by Alabdallah and Hasan (2021). For the formation of nano-
biopesticides the biological extracts producing high amounts of
phytochemicals and metabolites are mixed with silver salts. These
phytochemicals act as reducing, capping, or stabilizing agents (Lade et al.,
2017). The mechanism of action of AgNPs as nanofertilizers is shown by
proteomic experiments. AgNPs when exposed to Bacillus thuringiensis
affected the aggregation of envelope protein precursors explaining their role
for proton motive force (Lok et al., 2006). Silver ions are also reported to
inhibit DNA replication (Klaine et al., 2008). Yang et al. (2009) reported the
impairment of DNA replication in E. coli when AgNPs were found bound to
DNA in the cytoplasm of E. coli. Foliar spray of AgNPs was found to enhance
the antimicrobial activity resulting in increased potato tuber yield and healthier
plants for a long time (Tahmasbi et al., 2011). Nano-sized silica silver particles
were found to have anti-fungal effect on the powdery mildew of pumpkin at
10 ppm. At this concentration, the beneficial microbes of pumpkin were not
significantly affected by these particles (Park et al., 2006). Euphorbia
prostrate leaf extract mediated synthesis of AgNPs showed pesticidal activity
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Ravi P. Srivastava, Shalini Pareek, Navneet Kumar et al.
24
against rice weevil, Sitophilus oryzae L with LD50 value 44.69 mg/Kg. One
interesting finding was that no new insect infestations were found in AgNPs-
treated stored rice even after 2 months of the treatment (Zahir et al., 2012).
AgNPs synthesis mediated by leaf extract of seaweed Hypnea musciformis T
resulted in larvicidal activity against the cabbage pest Plutella xylostella with
LD50 24.5 to 38.23 ppm (Roni et al., 2015). Similarly, AgNPs biosynthesis
using the endophytic bacterium Bacillus siamensis strain C1 from coriander
plant showed a strong antibacterial effect against bacterial leaf blight and
bacterial brown stripe of rice. These NPs also increased rice plant length, dry
and fresh weight of root and shoot (Ibrahim et al., 2019).
Figure 4. Diagrammatic representation of environmental applications
of biosynthesized AgNPs with their role.
Environmental Toxins Biosensing Activity
Biosynthesized AgNPs have been used for the detection of toxic molecules in
water and food samples, pesticides in agriculture, and environmental samples.
AgNPs synthesized from Rumex roseus plant extract along with reduced
graphene oxide were immobilized on a glassy carbon electrode (GCE) to
detect H2O2, in the concentration range of 35 μM to 1.95 mM at conditions of
pH 7 and 0.1 M phosphate-buffered saline (PBS) electrolyte (Chelly et al.
2021). Similarly, nitrobenzene (NB), as a precursor, is employed by various
dyes, colors, explosives, herbicides, pesticides, and aniline. Human systems
Environmental
Applications of
AgNPs
Nanopesticides
and
nanofertilizers
In food
grade
bioplastic
Biosensors
Dye removal
from waste
water
Solar cells
Removal of
heavy metal
ions
Photocatalytic
degradation
Oxidation-
reduction
Antimicrobial and
provide mechanical
strength
Photoanode
Provide bonding
sites and increase
surface adsorption
Impair DNA
replication in pest
cells; help in
nutrient uptake by
plant
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Environmental Applications of Biosynthesized Silver Nanoparticles
25
are affected by short and long-term exposure and inhalation of NB which
causes headache, nausea, dizziness, anemia, etc. Unfortunately, companies
making dyes, pesticides, drugs, etc. release this NB into water and soil which
is a threat to environmental safety. Various methods have been employed to
detect NB precisely and easily in environmental samples. These methods
include spectrophotometric, chromatographic, and electrochemical methods.
Eucalyptus extract was used as a reducing and stabilizing agent by
Shivakumar et al. (2020) to synthesize eco-friendly AgNPs. The
biosynthesized AgNPs were used to modify a GC electrode, to detect NB
quantitatively with a good sensitivity of 2.262 AM-1 cm-2 and LOD of 0.027
M. The material of this electrode showed practically good results for the
detection of NB in the tap water and lake water. Similarly, the GC electrode
was modified using Camellia japonica leaf extract synthesized AgNPs to
increase the sensitivity and detection limit (0.012 M) for NB (Karthik et al.
2017). Karuppiah et al. (2015) used reduced graphene oxide and Justicia
glauca leaf extract synthesized AgNPs to construct an electrochemical sensor
for detecting NB in the concentration range of 0.5 to 900 M. The constructed
sensors showed a detection limit of 0.261 M and sensitivity of 0.836 A/M cm-
2 for NB.
Chandra and Singh (2018) used alanine, tryptophan, histidine, glutamic
acid, aspartic acid, and methionine to functionalize AgNPs for catalytic and
oxygen-sensing activities. The multiple functional groups of amino acids on
surface of AgNPs provide bonding sites for electrostatic interactions and H-
bonds with reagents, enhancing surface adsorption. The good catalytic activity
of so-formed functionalized AgNPs is due to the small size of AgNPs which
results in a high surface-to-volume ratio presenting more atoms on the surface
to act as catalytic sites. Similarly, L-cysteine functionalized AgNPs (L-cys
capped AgNPs) were used for probing the Vitamin B1 in food and
environmental samples (Khalkho et al., 2020). AgNPs functionalized with
carbohydrate derivative, glutathione-lactose (GSH-Lac), produced GSH-Lac-
Ag NPs which were used to detect thiram pesticide in agricultural samples
(Dhavle et al., 2021). Functionalization of biosynthesized AgNPs using
Vitamin B12 was reported by Harke et al. (2022) for bio-sensing of iron (Fe+3)
in food and baby products.
The biosensing activity of biosynthesized AgNPs using neem leaves
extract for sensing MCZ fungicide showed a linear response with the
concentration of MCZ and sensitivity 39.1 nm/mM. The photocatalytic
activity of AgNPs using 0.5 mM MCZ solution, resulted in the broadening of
the absorbance peak at 290 nm which indicated damage and accumulation of
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Ravi P. Srivastava, Shalini Pareek, Navneet Kumar et al.
26
MCZ molecules (Alex et al., 2020). Jabril et al. showed the leaf extract of five
different plants Basil, Geranium, Eucalyptus, Melia, and Rutato as reducers,
stabilizers, and cappers in the synthesis of AgNPs (Jabril et al., 2021). In this
study, Melia azedarach synthesized AgNPs (AgNPs-M) resulted in smaller
size which is useful for electrochemical applications. GCE modified by
AgNPs-M showed high sensitivity and good electrolytic activity for phenol
determination in tap and mineral water and showed LOD to be 0.42 mM for
phenol.
Use in Solar Cells
There is continuous and rapid growth in the energy demand around the globe.
The fossil fuel (coal, gas, and oil) based energy sources are limited in amount
and create pollution which is harmful to the environment as well as to human
lives. Therefore, considering the present and future scenario of energy demand
alternative ways to produce clean energy from renewable energy sources are
of utmost importance. Among the different renewable energy sources, solar
energy with an unlimited supply of sunlight is one of the most attractive
sources. To convert the incident sunlight into electricity a device called solar
cell is required. A solar cell absorbs the solar radiation incident on it and
generates the electron-hole pairs which are separated due to an internal built-
in field and further collected at two electrodes leading to the production of
electricity. Metal NPs, such as silver and gold, on their interaction with
electromagnetic radiation, induce surface plasmon resonance (SPR) in the
visible region, which improves the light trapping ability and make them useful
candidates for the light-harvesting applications (Kreibig et al., 1995,
Standridge et al., 2009). This property of AgNPs attracted researchers to
explore their application in solar cells. Prasad et al. employed AgNPs,
prepared from a facile green synthesis route, in the quantum dye-sensitized
solar cells (QDSSCs) (Prasad et al., 2013). AgNPs prepared using banana sap,
were coated onto cadmium sulfide quantum dots sensitized titania (CdS-TiO2)
porous photoanode. A schematic diagram of devices fabricated with
polysulfide electrolyte and graphite as a counter electrode, is shown in Figure
5a. The devices made with AgNPs showed 50% improvement in the
photocurrent, and a significant improvement of 150% in the power conversion
efficiencies (Figure 5b). The incorporation of AgNPs not only doubled the fill
factors but also reduced series resistance and increased the shunt current,
resulting in improved current density leading to high power conversion
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Environmental Applications of Biosynthesized Silver Nanoparticles
27
efficiency. Graphene is a miracle 2D material with unique electrical and
optical properties. The nanocomposites based on AgNPs and graphene oxide
(GO)/(reduced GO) have attracted much attention in photovoltaic devices due
to their superior electrical and optical properties. Yuan et al. proposed the
incorporation of solution-processed GO-AgNPs in photovoltaic solar cells
(PSCs) and obtained an improved power conversion efficiency (PCE) of up to
7.54% for PBDTTT-C-T: PC71BM-based PSCs (Yuan et al., 2014b). In this
context, Sutradhar et al. performed one-pot green synthesis of AgNPs and
investigated for photovoltaic devices. They employed an aqueous extract of
Sapodilla fruit as an eco-friendly and nontoxic reducing material for the
synthesis of AgNPs. The biosynthesized AgNPs were incorporated into the
graphene oxide and investigated for the PEDOT: PSS-based polymer cells
(Sutradhar et al., 2016). The TEM image of synthesized AgNPs/GO
nanocomposites, device structure, and band diagram for the fabricated devices
are shown in Figure 5c-e. The AgNPs and GO layer played an important role
in the improved device performance. On illumination with light, the electrons
generated from AgNPs were excited to the LUMO energy level and reached
to ITO cathode through the GO layer.
The current density versus voltage (J−V) curves for fabricated devices are
shown in Figure 5f. From the figure it can be observed that the addition of
PEDOT: PSS in AgNPs-GO nanocomposites has significantly enhanced the
PCE of the fabricated devices. For GO-based devices Jsc, Voc, and FF values
were 5.66 mA cm−2, 0.603 V, and 54%, respectively, which lead to a PCE of
1.84%. For GO/PEDOT: PSS-based devices PCE increased to 2.93% owing
to increased Jsc and FF (7.48 mAcm−2 and 56%, respectively). For the devices
fabricated with AgNPs/GO the Jsc, Voc, and FF values of 8.28 mA cm−2, 0.722
V, and 59% were recorded, yielding a PCE of 3.52%. Interestingly,
AgNPs/GO/PEDOT: PSS-based devices showed a dramatic increase in PCE
(3.98%) compared to devices fabricated with only AgNPs/GO. A high value
of Jsc (9.20 mA cm−2), Voc (0.722 V), and FF (60%) were recorded for these
devices. A strong anchoring effect of AgNPs on the GO and PEDOT: PSS
surface and the synergistic effect of nanocomposite film induced higher
mobility of the charge carriers toward both electrodes. A strong coupling
between the SPR effect of AgNPs/GO/PEDOT: PSS and incident light,
improved the light absorption and corresponding exciton generation, along
with charge collection, which is reflected in the significant enhancement in the
Jsc and PCE (Sutradhar et al., 2016).
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Ravi P. Srivastava, Shalini Pareek, Navneet Kumar et al.
28
Figure 5. (a) Schematic device structure and (b) J-V characteristics of QDSSC
solar cells fabricated with AgNPs. (c) TEM image of AgNPs/GO nanocomposites;
(d) band diagram; (e) device structure and (f) J-V curves for devices fabricated with
AgNPs-GO-PEDOT: PSS nanocomposites. (g) schematic device structure (h) J-V
curves and (i) short circuit current density and power conversion efficiency against
Ag doping amounts for DSSCs with AgNPs mixed TiO2-coated photoanode.
In another study, Sarvanan et al. treated the silver ions with Peltophorum
pterocarpum flower extract at room temperature, to synthesize the uniform
AgNPs (Sarvanan et al., 2017). Green synthesized AgNPs, using different Ag
content (1, 2, and 3 wt%), were mixed with P25-TiO2 nanoparticles to prepare
the plasmonic nanocomposite, which was used as photoanodes in DSSCs. The
schematic of the DSSCs fabricated with the Ag-incorporated TiO2-coated
photoanodes is shown in Figure 5g. The absorbance in the visible region was
significantly influenced by the addition of AgNPs with P25-TiO2, due to the
surface plasmon resonance band of AgNPs. The absorbance in the visible
region (420 to 750 nm) increased about twice with AgNPs. After the
incorporation of 2 wt % of the AgNPs, due to the plasmonic effect of the
modified electrodes, an approximately 28% increase in the PCE leading to a
change from 2.83 to 3.62% was observed (Figure 5h-i). Addition of 2 wt % of
the AgNPs resulted in a maximum increase in open-circuit voltage (up to
12.1%), short-circuit current density (up to 10.7%), and FF (up to 7.2%). A
rapid interfacial charge transfer arising from the AgNPs on the TiO2 surface
a b c
def
ghi
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Environmental Applications of Biosynthesized Silver Nanoparticles
29
and plasmonic effect were proposed for the enhanced photovoltaic
performance of the fabricated devices.
Conclusion
Nanotechnology is a prime area of importance in various fields. As compared
to chemical and physical synthesis of nanoparticles biological synthesis is
gaining high demand as it is non-toxic, eco-friendly, and cost-effective
method. This chapter reviewed biosynthesis methods, properties, and
environmental applications of AgNPs. The literature revealed that the size and
shape of biosynthesized AgNPs can be affected by various parameters like pH,
temperature, biological source, reaction time, etc. during the synthesis. AgNPs
have potential role in removing heavy metals and dyes for wastewater
treatment. Synthesis of nanofertilizers and nanopesticides based on
biosynthesized AgNPs has reduced the risk of environmental problems caused
by chemical pesticides and fertilizers. The use of these NPs in the production
of bioplastic for food packaging has led to environmental safety from non-
degradable plastic. Sensors based on biosynthesized AgNPs have been used to
construct biosensing devices to detect and sense environmental toxins in water
and soil. Research is also going on to use biosynthesized AgNPs in fabricating
solar cells and making the process eco-friendlier. The present research to
control the morphology, distribution, particle size, and other general properties
of biosynthesized AgNPs may further lead to explore their applications for
creating and making the environment more safe and friendly to live.
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