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- Herausgeber: Bentham Science Publishers
- Kategorie: Fachliteratur
- Sprache: Englisch
Silver Nanoparticles: Synthesis, Functionalization and Applications
[Provided by Editors]
The book “SILVER NANOPARTICLES: Synthesis, functionalization and Applications” presents a detailed investigation of the various methods of synthesizing AgNPs via chemical and green methods. The surface engineered silver nanoparticles present immense applications in the contemporary era starting from theranostics, biosensors, next generation antimicrobials, fabrication of solar cells, to the effective soil management leading to a sustainable agriculture. The widespread utility of AgNPs necessitates a comprehensive understanding of the operational strategies revolving around their surface modification and conjugation. This book epigrammatically discusses the candidature of engineered AgNPs as the material of future
[Edited by Taimur]
Silver Nanoparticles: Synthesis, Functionalization and Applications presents detailed information about the range of methods of synthesizing silver nanoparticles (AgNPs). The book systematically delves into the subject with an introductory chapter before moving to chemical synthesis of AGnPs and fabrication methods which help in assigning functional properties for useful nanomaterials. Basic and advanced synthetic methods like surface functionalization and bioconjugation are covered. Additionally, the book informs about impactful applications of AGNPs across a range of industries. Through this book, readers will be able to understand the importance of silver nanoparticles as a futuristic material in scientific investigations and gain a comprehensive understanding of the operational strategies revolving around their surface modification and conjugation.
Key Features:
-Covers the basics of silver nanoparticle (AGNP) synthesis
-Focuses on green methods of AGNPs
- Covers information about surface modification and functionalization of AGNPs with different molecules (including biomolecules)
-Covers a range of applications of AGNPs
-Includes advanced applications of AGNPs in next-generation antibiotics
Silver Nanoparticles: Synthesis, Functionalization and Applications a handy reference for scholars in advanced chemical engineering, materials science and pharmacology programs as well as anyone who wants to know all about silver nanoparticles.
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Seitenzahl: 254
Veröffentlichungsjahr: 2022
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FOREWORD
The book “SILVER NANOPARTICLES: Synthesis, functionalization and Applications” highlights the glorious journey traversed by AgNPs in the contemporary era where they have cemented their place as the most extensively exploited material of the present generation. The most popular theranostic application, tumor annihilating properties, and their prevalence as next-generation antibiotics have been presented epigrammatically. The silver nanoparticles serve as ubiquitous material of the present generation with applications in a wide-array of disciplines that include renewable energy, optoelectronics, image contrast agents, tomography, magnetic imaging, bioprobes, diagnostic kits, gene delivery, and molecular medicine. Among their concurrent applications, the biological interventions of AgNPs have revolutionized the pharma sector by making a significant contribution starting from the drug delivery and ending at bioimaging. The surface engineering of AgNPs causes them to develop functional head-groups that serve as precursors for the chemical transformations to therapeutically relevant molecules with value-added physicochemical and physiological properties in the form of optical, electrical, and magnetic characteristics. This book is a rich blend of professional writing and a raw understanding of the key details of AgNPs in terms of their surface engineering and applications. Apparently, being the most extensively exploited material of the generation with ubiquitous applications, the present readership must inculcate the basic understanding of the synthesis and further utilization of AgNPs, the need that this book mainly caters.
PREFACE
The book “SILVER NANOPARTICLES: Synthesis, functionalization and Applications” presents a succinct coverage of the current synthesis protocols, functionalization techniques, and state-of-the-art applications of surface functionalized AgNPs in the medical field. The presented book traverses the journey of AgNPs from synthesis paradigm to functionalization, which further extends to the key biological applications. This book focuses on a wider audience, including medicinal chemists, drug design experts, biological and translational researchers, and physical chemists working in the field of biological nanoscience. The book provides a rich experience of understanding the synthesis, functionalization, and applications of AgNPs in a single reading enriched with self-explanatory images, illustrations, and tables wherever necessary. The book provides useful information for the postgraduate (Master’s and research higher degree) doctoral students and postdoctoral research fellows working on the development of advanced nanostructures and nanopharmaceuticals. This book intends to cover both the academic and research requirements of the students for pursuing course work in various academic degree programs, including Physical Sciences, Pharmaceutical Sciences, and various disciplines under the Natural Sciences.
CONSENT FOR PUBLICATION
Not applicable.
CONFLICT OF INTEREST
The author declares no conflict of interest, financial or otherwise.
ACKNOWLEDGEMENTS
Declared none.
Silver Nanoparticles: A Ubiquitous Material of Future
Parteek Prasher,Mousmee SharmaAbstract
Silver nanoparticles (AgNPs) offer ubiquitous applications in diverse fields. The materials based on AgNPs provide profitable solutions to the contemporary exigencies such as energy harvesting, future antibiotics, molecular sensors, tracers, image enhancers, tomography contrast agents, antimicrobial fabrics, smart wearables, and many more. The unique physicochemical and optoelectronic properties of AgNPs play a central role in offering these wide applications. The synthesis and stabilization of AgNPs via physiologically benevolent molecules, and biomolecules; in addition to the surface engineering of AgNPs to obtain tailored properties, further support the diverse applications of silver-based nanomaterials. The green synthesis of AgNPs by plant extracts, microbial exopolysaccharides, and supernatant provides an economical approach for large-scale generation of AgNPs. The silver nanotechnology popularized after its utilization in household appliances, including water purifiers, washing appliances. However, excessive usage with limited regulations led to the manifestation of increased toxicity in the ecosystem, and successive accumulation in autotrophs and higher trophic levels in a food chain. Therefore, the ubiquitous applications of AgNPs must judiciously consider the environmental impact and influence on the ecosystem to ensure sustainable development. This chapter presents important highlights on the various applications of AgNPs in the modern era.
1. Introduction
The term ‘nano’ refers to materials having at least one dimension in the range 1-100 nanometer. Metallic particles of spherical, cylindrical, triangular, or cubic shape with dimensions in the same range represent metal nanoparticles, which due to their unique physicochemical and optical properties, present numerous applications. Compared to the bulk matter, the nanoparticles demonstrate a high fervor in developing advanced materials. The noble metal nanoparticles possess unique surface plasmon resonance (SPR) that affords characteristic optical properties useful in biological probing and for developing lab-on-a-chip sensors. The nano-size of the metallic particles enables their entry to the microbial cells
and tissues where the triggering of redox imbalance by the nanoparticles results in the microbicidal action. In addition to oxidative stress, the metallic nanoparticles instigate a cascade of processes in vivo thereby displaying the oligodynamic effect, useful for the antimicrobial activity. Among most of the noble metals, the silver nanoparticles (AgNPs) present ubiquitous applications in diverse fields making them as the most exploitable material of the contemporary era. Starting from optoelectronics to smart devices, from cosmetics to textiles, from fertilizers to pesticides, from solar cells to energy efficient materials, and from next generation antibiotics to drug delivery vectors, the AgNPs dominate over other metallic nanoparticles. However, most of the nanoformulations of AgNPs suffer limitations such as oxidation to Ag+ ions that exhibit toxicity to the living organisms and their complex physicochemical transformations in environment adversely effects the agroecosystems. Though surface functionalization and fabrication mitigate the toxicity of AgNPs to some extent, however the subsequent loss of surface capping results in direct exposure of AgNPs thereby producing inevitable effects. In this chapter, we will discuss about the ubiquitous applications of AgNPs via a via toxicity and disposal limitations.
Fig. (1)) Contemporary applications of silver nanoparticles (AgNPs).The shape, size, and surface coating or functionality decide the physicochemical characteristics of AgNPs, which further cater to multiple applications in catalysis, sensing, theranostics, biocides, and energy harvesting. The diverse shapes and size of nanoparticles arise from the synthesis methodology, including several critical physical parameters. Mainly, the presence of a stabilizing agent that preferentially binds to a particular crystal surface arrests the isotropic growth thereby resulting in unidirectional crystal growth and anisotropic shapes. Likewise, the spherical nanoparticles adopt a variety of diameters in the presence of same reducing and capping agent, depending on the temperature, pH of the solution and nature of the solvent. Similarly, the electrical, optical, magnetic, and mechanical properties of the nanoparticles differ markedly as compared to the bulk matter.
1.1. Electrical Properties of AgNPs
The electric properties of AgNPs differ from that of the bulk material due to the restricted movement of electrons at the nanoscale that results in the changes in electric properties of nanomaterial. As such, the semiconductor materials behave as conductors at nanoscale. Similarly, an increase in the concentration of AgNPs improves their electrical conductivity, however the AgNPs of size <10 nm do not conduct electricity, compared to that of the silver metal. Importantly, the conversion of AgNPs to silver nanowire markedly improved the electrical properties and power transfer. The incorporation of AgNPs to polymers such as polyaniline improves the electrical conductivity of the latter due to enhanced electrical mobility due to the presence of AgNPs. Decoration of graphene nanosheets with AgNPs improves the electrical properties of the former by acting as the nanospacer that increases the interlayer distance and improves the electrical conductance between the layers [1]. The decoration of polyester fabric with AgNPs dramatically improves its electrical conductivity due to the ‘silver colloid effect”. The enhanced conductivity fabric experienced six-fold increase in the conductivity in the presence of AgNPs due to the rearrangement and quantum tunneling effect of AgNPs at nanoscale [2]. Similarly, the addition of AgNPs to electronically conducting adhesives prevents their agglomeration thereby improving the electrical conductivity of the latter [3]. Enhanced electrical performance of low temperature screen-printed AgNPs presented high frequency electronic applications due to a superior packing of AgNPs, thereby reducing the surface roughness by three-fold [4]. The decoration of carbon nanotubes with AgNPs filler significantly improved the electrical conductivity of the CNT-polymer composites by four-fold compared to the pristine and functionalized carbon nanotubes. These findings confirm the unique electrical conductivity of AgNPs for diverse applications [5]. Singh et al. (2014) reported the zeta potential and electrical properties of multishaped AgNPs obtained by using maltose as reducing agent in the presence of microwaves. The spherical and anisotropic AgNPs displayed electrical conductivity in the range 2.04 x 10-4 and 1.49 x 10-3 respectively, with anisotropic AgNPs exhibiting superior enhancement in current compared to the spherical AgNPs mainly due to their sharp vertices that promote enhancement in the electrical field [6]. Bhagat et al. (2015) reported electrical properties of green synthesized AgNPs dispersed in distilled water and DPPH solution. Notably, the AgNPs dispersed in DPPH solution displayed a rapid increase in the current with respect to the applied voltage, compared to the dispersion of AgNPs in distilled water. The current-voltage characteristics suggested that the changes in current depends on the concentration of DPPH and the current increases with the increase in the antioxidant characteristics of AgNPs. AgNPs@polydopamine core-shell nanoparticles presented applications as fillers into poly(vinylidene difluoride) matrix to prepare dielectric composites, where the core-shell nanoparticles improved the dielectric constant of composites. The AgNO3/dopamine ratio and pH of the dopamine solution displayed significant effect on dielectric properties of composites, with highest dielectric constant achieved at 25 wt percent filled loading at 100 Hz. Besides, the AgNPs display significant thermoelectric properties [7]. Wang et al. (2014) reported the thermoelectric properties of AgNPs-polyaniline hybrid nanocomposites, where the electrical performance of the nanocomposite improved markedly on addition of AgNPs. The presence of AgNPs lowered seebeck coefficient, whereas the electric conductivity improved significantly. Negligible enhancement appeared for the thermal conductivity even on increasing the content of AgNO3, which resulted in an improvement of the figure of merit of the nanocomposites, with a maximum value 5.73 x 10-5, compared to that for the pure polyaniline nanocomposites. These investigations validated the hybridization of conducting polymers with AgNPs as an effective strategy for obtaining improved thermoelectric properties [8]. Shivananda et al. (2020) reported the electrical properties of composite films obtained by hybridization of AgNPs with silk-fibroin. The AgNPs reportedly improve the dielectric and AC conductivity of the silk-fibroin film and extend their biosensing applications as implantable thermoelectric wireless switching devices. The AgNPs adopt a spherical shape and crystalline FCC structure with a uniform distribution in the silk-fibroin matrix. The electrical properties of composite film further improved on increasing the concentration of AgNPs, without altering the thermal and mechanical properties of the former [9]. Zhang et al. (2016) reported the dielectric properties of polymer composites loaded with polydopamine@AgNPs core-satellite particles, where the presence of AgNPs and the size of polydopamine core markedly enhanced the dielectric constant of the former. The AgNPs reportedly scattered on the polydopamine surface while forming the core-satellite structure, which prevents aggregation of AgNPs and provides stability to the structure [10]. Song et al. (2016) reported the effect of in situ formed AgNPs on electrical properties of silver nanowires loaded epoxy resin. The AgNPs imparted isotropic electrical properties in epoxy resins containing AgNWs as fillers, whereas the absence of AgNPs demonstrated anisotropic electrical properties to the nanocomposite. Similarly, the presence of AgNPs in combination with boron nitride nanosheets synergistically influences the electrical conductance of epoxy nanocomposites. The epoxy nanocomposites loaded with AgNPs and boron nitride nanosheets display an excellent thermal conductivity, superior insulation strength, low dielectric loss and lesser permittivity [11]. As compared to the pure epoxy, the nanocomposites containing binary nanofillers afford a commendable enhancement in breakdown voltage mainly due to the formation of AgNPs conducting channels that significantly increase the breakdown path. The nanofillers with appreciable electrical conductivity do not affect the insulation properties of nanocomposites upon blocking of the former with electrically insulating nanofillers [12]. The doping of carbon nanofibers with AgNPs reportedly improves the electrical conductivity of the former, which depends on the average diameter of nanofibers and the concentration of doped AgNPs. The total average current improved by five-fold with the increase in the concentration of AgNPs from 30-40 mmol. The increase in electrical conductivity mainly appeared due to the lowering of electrical resistance of carbon nanofibers on adding AgNPs [13]. Gnidakouong et al. (2019) appraised the influence of low-temperature sintering on the electrical performance of AgNP-CNT nanocomposites. Reportedly, at a given temperature, the melted aggregates of AgNPs facilitate the joining of CNT ropes, thereby improving their surface electrical properties and reducing the interfacial resistance and tunneling resistance due to π-π electronic interactions. Similarly, the point-like AgNP fillers and advanced sintering resulted in lowering of surface electrical resistance mainly due to the reduction in porosity caused by the coalescence of AgNPs [14]. Bhadra et al. (2019) reported the influence of humidity on the electrical properties of AgNP based nanocomposites. The electrical properties of silver-polyaniline/ polyvinyl alcohol nanocomposites displayed uniform changes in resistivity with an increase in humidity. Mechanistically, the water molecules present in humidity donate electrons to the valence bond of polyaniline/ polyvinyl alcohol molecules, thereby increasing the bandgap by lowering the number of holes. In addition, the hydrophobic nature of polyvinyl alcohol causes a swelling effect, which increases the inter-particle distance between the conducting fillers, thereby reducing the overall conductivity of nanocomposites [15].
1.2. Optical Properties of AgNPs
AgNPs absorb and scatter light uniquely depending on the shape, size, and refractive index of the nanoparticles. The AgNPs display surface plasmon resonance (SPR) occurring due to the electromagnetic field induced oscillation of the electrons present in the conduction band. The unique optical properties offer optical biosensing applications to AgNPs to monitor the complex biological processes and for investigating biomolecular interactions. The biosensors based on SPR display extraordinary sensitivity and responsiveness to minute changes occurring in the refractive index of the analyte. Such biosensors provide applications for the detection of biomacromolecules including nucleotides, peptides, antibodies, and enzymes. The optical properties of the AgNPs and their unique localized-SPR bands enable the prediction of nanoparticle size [16]. Similarly, the surface coating of AgNPs fine-tunes their optical properties for enhanced biological applications with reduced toxicity [17]. In addition to the biological applications, the tunable optical properties of hydrophilic AgNPs enable the detection of heavy metals in water. The wavelength of SPR peak for AgNPs displayed a weak dependence on the heavy metal ion concentration in water, where the broadening of SPR band and high background absorption restricted the detection of ions. Tuning of the AgNPs surface overcomes these limitations. Mainly, the hydrophilic AgNPs displayed enhanced sensitivity and selectivity towards Ni+2 ions, with a sensitivity of 0.3 ppm [18]. Sivanesan et al. (2011) reported citrate functionalized AgNPs with tunable SPR properties for applications in protein analysis and the detection of specific protein cofactors such as cytochrome c, in nanomolar concentration with the help of surface enhanced resonance Raman (SERR) [19]. Raj et al. (2017) developed a localized SPR- based dopamine sensor based on L-tyrosine-capped AgNPs. The nanoparticles demonstrated a lowering in fluorescence intensity and an increase in the absorption spectra with an increase in the concentration of dopamine from 0-50 µM. The sensor exhibited a superior sensitivity and selectivity towards dopamine, compared to the other biomolecules with a detection limit of 0.16 µM [20, 21]. Ajitha et al. (2016) reported the role of capping agent in the optical properties of AgNPs for the detection of hydrogen peroxide. The loading of polyvinyl alcohol functionality on AgNPs provided localized-SPR based sensor for the detection of H2O2 with a detection limit of 10-7 M. The strength of localized-SPR changed with changes in the concentration of H2O2 and reaction time due to the catalytic degradation of AgNPs [22]. Li et al. (2017) investigated the optical limiting properties of AgNPs hybridized with polydimethylsiloxane (PDMS). The optimal limiting effect mainly arises due to non-linear optical adsorption and refraction. Notably, the AgNPs-PDMS hybrid sheets demonstrated superior optical limiting properties compared to the Ag@SiO2 solution mainly due to the early onset of limiting and a wide reduction in transmittance [23]. Adamiv et al. (2014) reported the non-linear optical properties of AgNPs. The AgNPs in size range 14-18 nm upon annealing with Li2B4O7:Ag glass locate themselves in the thin near-surface layer, thereby forming the interface region, which transforms the positive character of non-linear refraction of Li2B4O7:Ag glass to negative. The change markedly enhances its non-linear properties due to plasmon resonance [24]. Nisha et al. (2019) further reported the optical limiting behavior of AgNPs. The nanoparticles displayed a non-linear refractive index of 7.15 x 10-8 cm2/W and the non-linear absorption coefficient of 0.04 x 10-4 cm2/W, whereas the third order non-linear susceptibility appeared as 4.30 x 10-6 esu [25]. Pandey et al. (2012) reported biopolymer-AgNPs nanocomposites for the optical detection of ammonia. The SPR-based sensor displayed a response time of 2s, and exhibited a detection limit of 1ppm towards ammonia solution at room temperature. The localized-SPR properties of the dispersion of AgNPs in the polymer matrix showed calorimetric sensing applications with a high reproducibility and fast response time. The optical sensor based on AgNPs displayed application for the physiological detection of ammonia in biological fluids, including plasma, cerebrospinal fluid, saliva, and sweat [26]. Edison et al. (2016) developed an AgNPs based optical sensor for the detection of dissolved ammonia. The appearance of SPR peak and yellowish color revealed the formation of AgNPs in control solution and ammonia containing solutions. Further, the SPR absorbance of AgNPs in ammonia containing solutions increased with the formation of diamine silver complex and in the presence of ammonium phenolate ions. These events increased the rate of AgNPs nucleation and yielded small-sized nanoparticles of average diameter 30 nm with a distorted spherical shape, in the absence of ammonia. The presence of 100-ppm ammonia solution, however yielded spherical shaped AgNPs with an average diameter of 5 nm [27]. Bhutto et al. (2018) reported the plasmonic properties of AgNPs obtained from the phenolic compounds as reducing agents. Reportedly, the AgNPs display higher plasmonic response, which depends on the antioxidant properties of phenolic acids and the degree of hydroxylation of the latter. The higher degree of hydroxylation resulted in ameliorated scavenging property of the phenolate compounds and a higher potency to reduce Ag+ ions to AgNPs. Similarly, the rate of reaction and reducing power depended on the nature of phenolate and its substituent pattern [28]. Singha et al. (2014) reported the synthesis of high optical quality AgNPs by the reduction of ascorbic acid at room temperature. The obtained AgNPs displayed extremely sharp and intense SPR bands with narrow bandwidth, superior to the conventional bands obtained from the AgNPs synthesized by common reducing agents [29]. Kemper et al. (2017) studied the effect of LED irradiation on the optical properties of AgNPs in the polyethylenimine thin films. The tailoring of AgNPs in the transparent polymer matrix provided novel applications, including the adaptable light filters for the perspective lab-on-a-chip applications. Reportedly, the AgNPs begin reshaping their morphology upon irradiation, thereby resulting in the changes in absorption signals and SPR properties. Green light irradiation leads to the forced plasmon oscillation by the excited regions on AgNPs, resulting in the photoreduction of redundant Ag+ ions [30]. Wang et al. (2018) reported the development of polydopamine capped AgNPs based SPR biosensor for a highly sensitive, regenerative and stable detection of horse IgG with a detection limit of 0.625 µg/mL. The AgNPs acted as signal enhancing labels with a 2/4-fold higher detection limit compared to the gold nanoparticles. The SPR based biosensor displayed a high selectivity towards the horse IgG, with a bonding constant of 2.93 x 107 L mol-1 to the antibody, as detected by the polydopamine loaded AgNPs based SPR biosensor. The desired performance by polydopamine loaded AgNPs film-sensing platform occurred due to the effective antibody immobilization by polydopamine, whereas the presence of AgNPs improves the sensitivity of the biosensor owing to the electronic coupling between AgNPs, hence amplifying the SPR response [31]. Raj et al. (2017) developed a highly sensitive and selective SPR-based fiber optic sensor based on AgNPs for cysteine detection. The sensor displayed a detection limit of 7.7 nM for cysteine among various biomolecules. While increasing the cysteine concentration, the resonance wavelength lowered and shifted slightly towards the lower wavelength. Similarly, the localized-SPR resonance peak intensity lowered on adding cysteine, thereby suggesting the event of transduction pertaining to the interactions of cysteine with capped AgNPs [20, 21]. Mota et al. (2020) suggested that the low-intensity polychromatic light irradiation markedly governs the optical properties of AgNPs. Reportedly, the resonance between SPR bands of AgNPs governs the self-limiting growth process of anisotropic AgNPs. Importantly, the light emitting diode irradiation wavelength decides the final morphology of the AgNPs due to the shape-dependence of the plasmonic spectrum of AgNPs. These investigations suggested a photoinduced control of the morphology and the plasmonic properties of the AgNPs in the presence of low-intensity light emitting diode [32]. Karimzadeh et al. (2010) investigated the non-linear optical properties of AgNPs in water with a continuous wave laser irradiation at 532 nm. The closed Z-scan measurements suggested the thermal effect on the non-linear refractive index of AgNPs. The aberrant thermal lens model follows in agreement with the Z-scan investigations for AgNPs, with a nonlinear refractive index of -1.0 x 10-8 cm2/W. and thermo-optic coefficient as -0.99 x 10-4 W/mK. These investigations revealed that the thermal nonlinear effects play a significant role in deciding the photonic applications of AgNPs and for appraising their nonlocal nonlinear processes [33]. Pugazhendi et al. (2015) reported nonlinear optical properties of AgNPs obtained from Alpinia calcarata. Nonlinear optical studies performed by single beam Z-scan setup optimized the nonlinear refractive index of the order 10-8 cm2W-1, whereas the nonlinear absorption coefficient and the third order nonlinear susceptibility appeared as 10-3 cmW-1 and 10-3 esu. These investigations suggested that AgNPs demonstrate a superior optical non-linearity as evidenced by the Z-scan technique [34]. Zhang et al. (2019) investigated the regulation of optical properties of triphenylamine-capped AgNPs based on the SPR effect. The nanoparticles display a red shift in the UV-Vis absorption upon interfacial coordination caused due to an increase in the electron withdrawing strength presented by Ag atom. The SPR effect of AgNPs of size 6nm improves the single photon fluorescence emission and two-photon absorption. The triphenylamine-AgNPs hybrids display a higher cross-section for two-photon resonance hence presenting excellent applications in optical power limiting with a threshold value of 0.49 J/cm-2. The reported interfacial coordination induced hybrids afford a favorable approach for effectively regulating the linear optical properties and for optimizing the nonlinear performance [35].
1.3. Catalytic Properties
The AgNPs present excellent photocatalytic properties, which present several applications, including the degradation of organic pollutants and dyes. Jiang et al. (2005) reported the catalytic applications of AgNPs supported on silica nanospheres. The supporting of nanoparticles on silica spheres avoids their flocculation during the catalytic activity in the solution, thereby enabling an optimal catalytic activity. Notably, the presence of surfactants lowered the catalytic activity of AgNPs by inhibiting the adsorption of reactant molecules on the surface of nanoparticles. Similarly, the presence of electrolytes in the solution possesses the ability to enhance the rate of migration of the reactants in the solution resulting in the enhancement of the rate of catalytic reaction. However, the electrolyte may also restrict the adsorption of reactant molecules on the surface of AgNPs hence lowering the catalytic activity. Anchoring of AgNPs on silica particles prevents their aggregation and avoids the deactivation or poisoning of catalyst during the catalytic reaction [36]. Chandraker et al. (2019) presented photocatalytic properties of biogenic AgNPs obtained from Ageratum conyzoides. 2h exposure of AgNPs to methylene blue resulted in the degradation of the dye in the presence of sunlight. Visual detection of color change from blue to colorless confirmed the dye degradation. Mainly, photons contained in the incident solar radiation excited the electrons present on the surface of AgNPs, which further are accepted by the dissolved molecules of oxygen present in the solution. The free electrons convert dioxygen to oxygen anion radicals that eventually catalyze the conversion of dye into simpler organic molecules, hence causing its degradation. AgNPs reportedly displayed photocatalytic oxidation of nitric oxide over the nanoparticle-loaded carbon fiber cloths [37]. The modification of TiO2 by AgNPs stabilized the photoefficiency of the composites during the five consecutive cycles of nitric oxide photooxidation, thereby hindering the formation of NO2. The carbon fiber cloths containing 3.7 wt% of AgNPs displayed the highest removal rate for nitric oxide. The maximum and minimum removal rates of nitric oxide appeared to be 80% and 95%, respectively [38]. Zhang et al.
