Environmental applications of silver nanoparticles: state-of-the-art review and emerging trends

Review

1 Introduction

Environmental pollution has intensified globally due to rapid industrialisation, technological advancements, urbanisation, and unsustainable agricultural practices. This causes significant concern and disrupts ecosystems and food cycles, ultimately affecting living beings. Anthropogenic activities generate all environmental contaminants, and industries, medical and research institutions, household waste, and agricultural practices are major contributors [1]. Pollution arises from any chemical, biological, physical, or radiological substance that impacts soil, water, air, or living beings. These include metals (e.g., lead, mercury, arsenic, cadmium, copper), particulate matter (PM2.5 and PM10), pesticides (organochlorines and phosphates), micro- and nanoplastics, antibiotics, gases, organic compounds (e.g., dyes or nitrophenols), and microorganisms [2-5]. The concentration of these pollutants in the environment is proportional to the world population and their demand for advanced technology, healthcare, and household products. Therefore, monitoring and remediation of these contaminants from the environment is the need of the hour, with cost-effective, sensitive, and selective strategies or systems foremost. Indeed, various high-throughput techniques, as well as chemical and biological methods, are available to address these pollutants, but their limited efficiency, cost-effectiveness, on-site usability, technical skills, and environmental compatibility restrict their use to specific conditions [6]. Thus, researchers have sought alternatives that maintain cost-effectiveness and a sustainable approach to managing environmental pollutants in air, water, and soil. The list of conventional and emerging environmental pollutants is represented in Figure 1.

The advent of nanoscience is a promising and effective solution for addressing all aspects of the environment (i.e., water, air, and soil). Among the broad family of engineered nanomaterials evaluated for environmental applications, silver nanoparticles (AgNPs) have attracted exceptional attention owing to their unique combination of properties that directly address the requirements of environmental monitoring and remediation. For example, the optical properties of AgNPs are primarily governed by localised surface plasmon resonance (LSPR ), which occurs in the 380–450 nm region and depends on the nanoparticles’ size and shape. The unique optical properties of nanosilver were utilised to develop LSPR-based nanosensors that detect different metal ions [7]. A surface coating of ʟ-carnosine on nanosilver enables size tuning and the detection of metal ions such as As³⁺, Cr³⁺, Cd²⁺, and Pb²⁺, while cetyltrimethylammonium bromide (CTAB) coating provides selectivity against Hg²⁺ and Cu²⁺ [8,9]. In addition, CTAB- and ʟ-carnosine-capped AgNPs exhibit catalytic activity in the removal of p-nitrophenol [8,9]. According to the Occupational Safety and Health Administration (OSHA; Permissible Exposure Limits, 29 CFR 1910.1000), arsenic, chromium, cadmium, lead, beryllium, and mercury are the most hazardous metal ions that can cause irreparable health impacts. These occupational thresholds are complemented by environmental exposure standards, including the US EPA National Primary Drinking Water Regulations (e.g., Pb: 0.015 mg/L; As: 0.010 mg/L) and WHO Drinking Water Quality Guidelines, which collectively underscore the necessity for ultrasensitive environmental monitoring tools capable of detecting these ions at trace concentrations in water, air, and soil matrices. Furthermore, nanosilver detected antibiotics, microplastics, bisphenol A, pesticides, bacterial pathogens, per- and polyfluoroalkyl substances (PFASs), and polycyclic aromatic hydrocarbons (PAHs) in environmental samples [10-17]. This indicates that nanosilver acts as a potent nanosensor for detecting various contaminants via different approaches, namely Raman spectroscopy, electrochemistry, colourimetry, and fluorometry [18-21]. In addition, the enhanced antimicrobial activity of AgNPs can be used for water decontamination and is widely employed in membrane filtration technology [22]. AgNPs exhibited broad-spectrum antibacterial activity, encompassing both Gram-positive and Gram-negative bacteria, as well as multidrug-resistant strains [23]. Furthermore, the catalytic activity of AgNPs was utilised to remove synthetic dyes, nitrophenol, and other organic pollutants. Methylene blue (MB), Congo red (CR), 4-nitrophenol, and 4-nitroaniline were degraded into harmless products using AgNPs stabilized by Cestrum nocturnum L. [24]. In a study, chitosan-based poly(chitosan-N-isopropylmethacrylamide-acrylic acid) microgels were used to fabricate AgNPs via borohydride reduction. The study demonstrated the catalytic removal of MB, CR, brilliant blue, rhodamine B (RhB), and methyl orange (MO) with pseudo-first-order reaction kinetics [25]. The addition of AgNPs to a matrix may improve their efficiency in removing various contaminants via adsorption mechanisms, which can be ionic or covalent, depending on the nanocomposite surface chemistry [26]. In a study, Ag-decorated reduced graphene oxide was used as an adsorbent material for removing Nile blue dye, with 94% of the dye being removed within 60 min [27]. Caravaca et al. used magnetic nanoparticles coated with AgNPs for amoxicillin removal from water, where 100% removal was achieved at different nanocomposite concentrations [28]. The removal of PFAS was achieved by formulating nanosilver with activated carbon, where perfluorooctanesulfonate (PFOS) and perfluorooctanoate (PFOA) were adsorbed at concentrations of, respectively, 454.1 and 321.2 mg/g, respectively [29]. Echinochloa pyramidalis-stabilized AgNPs were synthesized for the removal of PAHs with 100% efficiency [30].

AgNPs and AgNP-based nanocomposites provide an efficient system for pollutant removal, with good reusability, as evident from the above studies. However, despite their benefits, the escalating production and utilisation of AgNPs also raise concerns about their safety and the risks associated with environmental applications, in particular regarding the cytotoxicity of Ag⁺ ions [31]. The mechanisms underlying the toxicity of AgNPs towards various organisms, including humans, must be elucidated to inform risk assessment and mitigation strategies [32]. The AgNPs exhibit toxicity towards aquatic organisms, including bacteria, algae, invertebrates, and fish, with reported adverse effects such as growth inhibition, reproductive impairment, and mortality [32,33]. Moreover, various factors influence the fate and transport of AgNPs in environmental samples, including particle size, surface composition, state stability, and organic matter, which ultimately generate bioavailability and toxicity concerns [34]. The regulatory framework governing AgNP environmental applications remains fragmented across jurisdictions; while OECD, ISO, EU, and US EPA guidelines address nanoparticle characterisation and safety assessment, specific maximum contaminant levels or environmental quality standards for engineered AgNPs are largely absent.

Several reviews have addressed individual aspects of the environmental use of AgNPs, including antimicrobial applications, heavy-metal sensing, and photocatalytic degradation, in isolation. However, no single review has provided an integrated cross-domain treatment spanning sensing, photocatalysis, adsorption, membrane filtration, disinfection, and soil/agricultural applications within a unified mechanistic framework. The present review addresses most of the pollutants (emerging and conventional) by providing a comprehensive assessment of AgNPs and AgNP-based nanocomposites in environmental applications. It incorporates emerging topics including green synthesis reproducibility, nanozyme activity, the environmental fate of silver, and metrological validation of sensing platforms. It also identifies critical standardisation gaps and provides a balanced assessment of practical feasibility relative to regulatory thresholds. Together, these elements define the novelty of this review and distinguish it from prior works in the field.

2 Preparation and characterisation of AgNPs

The environmental application of AgNPs is primarily determined by their functional properties, which are highly dependent on their synthesis and surface functionalization. In addition, the physicochemical properties of differently synthesised AgNPs are characterised using standard techniques before their practical use. Furthermore, surface modification with a suitable ligand or surface capping enables targeted interaction with the analyte via the exposed functional group, thus enabling selective detection. Surface capping enhances the stability of nanoparticles and their functional properties for catalytic/photocatalytic applications (Figure 2). Indeed, differently capped nanoparticles were prepared using suitable preparation methods.

2.1 Preparation methods

The environmental applications of AgNPs are primarily governed by physicochemical parameters obtained through various synthetic approaches, including physical, chemical, and biological methods [35]. Each offers distinct advantages and limitations regarding particle size control, morphology, stability, and cost-effectiveness [35]. Chemical methods are commonly employed to synthesise AgNPs due to their simplicity, scalability, versatility, and ability to control size [36]. Chemical reduction methods are widely used to synthesise AgNPs, utilising reducing agents such as sodium borohydride, citrate, and ascorbic acid to reduce silver ions in solution, followed by the use of capping agents or stabilisers to prevent aggregation [36]. The capping agents can be polymers, surfactants, and small and large biomolecules, providing colloidal nanoregime stability [37]. The size-controlled synthesis of AgNPs capped with ʟ-carnosine was developed for metal ion detection and nitrophenol removal [8]. Another study highlighted capping agent-mediated size-controlled synthesis of nanoparticles for peroxide detection and antimicrobial activity [38]. PVA-AgNPs were the smallest and showed enhanced antibacterial activity and H₂O₂ sensing [38]. In addition, incorporating linker molecules enables selective and sensitive sensing, allowing for a design tailored to a specific application. For example, PVP-capped AgNPs were used to selectively detect Hg²⁺ in the presence of methionine as a linker molecule [39]. Laser ablation, evaporation–condensation, and sputtering fall under physical syntheses and offer the advantage of producing highly pure materials without the use of chemical reagents. However, the techniques lack monodispersed nanoparticle distributions [40].

Green methods employing plant and their parts extracts, microorganisms, or biopolymers as reducing and capping agents have gained increasing attention due to their environmental friendliness and biocompatibility [41,42]. Green synthesis can be considered an alternative to chemical and physical methods, utilising renewable resources and mild reaction conditions to produce AgNPs with tailored properties [43]. Developing new AgNPs via green synthesis has emerged as a rapid, practical approach, yielding fewer toxic nanoparticles with specific properties [44]. Plant extracts contain diverse bioactive compounds, including flavonoids, alkaloids, and polysaccharides, which act as reducing and stabilising agents in the synthesis of AgNPs [45]. It was observed that Acacia raddiana-stabilised AgNPs simultaneously detected Co²⁺, Hg²⁺, Pb²⁺, and Cu²⁺, indicating their role as a multimetal sensor for environmental monitoring [41]. The catalytic potential of Plantago ovata leaf extract-stabilised AgNPs was assessed by measuring MB and CR removal, with reaction rates of 0.056 and 0.166 min⁻¹, respectively. In addition, the antifungal activity against A. alternata and F. oxysporum indicates it as a decontaminating agent [46]. The antimicrobial properties of nanosilver are highly dependent on size, shape, and surface functionalization [47]. A prior characterisation using standard techniques is necessary to understand the physicochemical properties of AgNP [48].

2.2 Characterisation techniques

AgNPs are characterised to understand their physical and chemical properties, which are mainly governed by size, shape, and surface complexity. This understanding was needed before engineering AgNPs for particular applications. AgNPs are characterised using optical spectroscopy, electron microscopy, X-ray diffraction, and dynamic light scattering (DLS) to measure plasmonic absorbance, size, shape, structure, and stability [49]. Optical spectroscopy is a rapid and convenient technique for monitoring the formation of AgNPs via size- and shape-dependent absorption bands arising from LSPR upon light excitation [50]. This study also highlighted that small spherical AgNPs were highly antibacterial against Pseudomonas aeruginosa and Escherichia coli compared to larger and triangular particles [50]. Optical methods are employed to selectively detect metal ions, antibiotics, and pesticides using AgNPs, thereby facilitating environmental monitoring [19]. Other techniques include DLS, which works on the principle of Brownian motion and measures a particle's hydrodynamic size and surface charge in a colloidal solution [49]. DLS measures the size distribution and stability of AgNPs in solution, providing valuable information on their tendency to aggregate or agglomerate [51]. DLS measurements are suitable for isotropic particles and help elucidate the surface properties of AgNPs through their interactions with the surrounding medium. Zeta potential measurements are used to assess the surface charge and stability of silver nanoparticles; higher absolute values (>±30 mV) indicate better colloidal stability and resistance to aggregation [52]. This surface charge also validated the surface functionalization with molecules of interest. However, two separate cuvettes were used for the measurement, and the absorption coefficient (k = 3.99) and refractive index (n = 0.135) of AgNPs need to be added. In addition, a similar principle was employed by the nanoparticle tracking analyser, which calculated the AgNP concentration as particles per millilitre [53]. Microscopic techniques, such as transmission electron microscopy, provide high-resolution images of individual nanoparticles, confirming morphology, size distribution, and elemental composition when coupled with energy-dispersive X-ray analysis [49]. TEM analysis is performed on copper grids with varying mesh sizes and coatings, with carbon/formvar-coated copper grids commonly preferred. Scanning electron microscopy (SEM) provides information on the silver nanoparticles’ surface properties and the aggregation state of dried nanoparticles using carbon or silicon wafers as the primary substrate [49]. The X-ray diffraction pattern reveal that AgNP crystal growth occurs at different facets depending on the NPs [49]. The most common facets include 111, 200, 220, and 311, corresponding to 2θ angles of 38.2°, 44.4°, 64.6°, and 77.5°, respectively [54]. Using the Scherrer equation, these 2θ values yield the nanoparticle’s crystallite size [54]. Furthermore, Fourier-transform infrared (FTIR) spectroscopy provides evidence of successful surface capping or functionalization of the desired molecules on nanoparticles [49]. Surface-enhanced Raman spectroscopy (SERS) using AgNP nanocomposites could selectively detect polystyrene nanoparticles with a detection limit of 14 μg/mL [55]. X-ray photoelectron spectroscopy (XPS) provides information on the elemental composition, surface oxidation states, and electronic configuration of AgNPs [56]. This technique can detect environmental contaminants using AgNPs as the probe. The ICPMS (Inductively Coupled Plasma Mass Spectrometry) and AAS (Atomic Absorption Spectroscopy ) were used to measure unknown metal (Ag) content by plotting linear curves using the known Ag standard [49]. Electrochemical techniques, including voltammetry, potentiometry, and amperometry, were used to monitor metal ions, pharmaceuticals, endocrine disruptors, ammonia, and phenolic compounds using AgNPs [18]. Fluorescence spectroscopy provides information on fluorescence-based detection of environmental pollutants via fluorescence enhancement or quenching mechanisms. A study reported that carbon dot/AgNP-based nanosystems selectively detected Hg²⁺ ions with a detection limit of 2.22 × 10⁻⁸ M via a turn-on mechanism [57]. In addition, the environmental application of AgNPs can be affected by various factors, such as surface composition, size, temperature, pH, concentration, and shape.

2.3 Factors affecting AgNP properties relevant to environmental applications

Nanosilver and its properties are well-suited for various ecological applications and can be engineered by varying synthesis parameters that affect AgNP properties. The application of AgNPs depends on factors such as size/shape, capping, pH, temperature, and NP concentration, which can be optimised through the synthesis approach. The size and shape of AgNPs significantly influence their properties and applications, with smaller particles exhibiting enhanced functional properties due to their higher surface-area-to-volume ratio [58]. A study reported that luteolin tetraphosphate-stabilised nanosilver exhibited size- and shape-dependent antimicrobial activity [59]. The study concluded that 100% bacterial and fungal growth was inhibited, indicating potential in water and air filtration systems [59]. Another study highlighted the role of AgNP size and shape in catalytic MB removal [60]. AgNPs with controlled size of 10 nm and nanocapsular shape showed selective detection of Cu²⁺ ions with LOD of 2.6 × 10⁻⁹ mol/L [61]. Size-tuned AgNPs (1–100 nm) were synthesised using trisodium citrate as a capping agent to assess the impact of nanoparticle size on bacterial strain [62]. The study found that 5 nm AgNPs have a greater effect on killing bacterial strains compared to other sizes [62]. Collectively, these studies indicate that morphology and size significantly influence AgNP performance; however, direct cross-study comparison remains challenging owing to differences in synthesis routes, capping agents, test matrices, and reporting conventions. While general trends (e.g., smaller spherical AgNPs typically exhibiting stronger antimicrobial activity and greater LSPR sensitivity) are well-supported, the magnitude of these effects is system-specific and should not be stated categorically without accounting for confounding experimental variables. Besides size and shape, capping/stabilising agents prevent aggregation and provide colloidal stability to AgNPs in the surrounding medium. Selecting the appropriate capping agent from polymers, surfactants, biomolecules, and green extracts is essential to ensure long-term stability and prevent agglomeration, thereby making it suitable for environmental applications [63]. In addition, surface functionalization or coating with biocompatible polymers or targeting ligands can enhance stability, dispersibility, and biocompatibility for specific ecological applications [64]. For example, differently capped AgNPs (e.g., PEG, EDTA, PVP, or PVA) exhibited NP size variations, ultimately affecting antimicrobial and sensing applications. PVA provided the smallest NP size, yielding enhanced antibacterial activity and H₂O₂ sensing abilities [38]. In another study, Acacia lignin-AgNPs showed catalytic, sensing, and antimicrobial activity [65]. The results confirmed that MB and p-nitrophenol were catalytically removed at rates of 0.00098 and 0.01525 min⁻¹, respectively. Additionally, NPs demonstrated metal ion sensing and antibacterial activity against various strains [65]. The above studies concluded that the capping agent played a prominent role in nanoparticle-mediated applications by providing an interaction site for targeted molecules and by stabilising NPs. The pH-dependent detection of metal ions using PVP-AgNPs was reported, with Pb²⁺ detected at pH 9.6 at a LOD of 14.4 nM [66]. The reaction pH influences stability and dissolution of the AgNPs, which are among the main factors in AgNPs-mediated applications [67]. Particle aggregation is a critical factor influencing the behaviour and toxicity of AgNPs in ecological systems [64]. Surface charge affects the electrostatic interactions between nanoparticles and other charged species in the environment, influencing their aggregation behaviour and mobility. The surface chemistry of silver nanoparticles, including surface coatings or functional groups, can alter their reactivity, stability, and interactions with pollutants and microorganisms [68]. Environmental standardization and stabilization of AgNPs surface charge are essential in biological systems for studying interactions [69]. Response surface methodology is also helpful in optimizing experimental parameters and AgNP preparation by minimising the number of trials and errors [70].

3 Environmental application of silver nanoparticles

Silver nanoparticles possess inherent properties that can be utilised for air, water and soil applications, as shown in Figure 3.

3.1 Water applications

The global demand for clean and safe drinking water continues to increase due to population growth, urbanisation, and industrialisation. Clean water and sanitation are part of Sustainable Development Goal 6 (SDG6), which must be achieved by 2030. Nanosilver has emerged as a promising material for water treatment, offering efficient and cost-effective solutions for removing, monitoring, and disinfecting water sources [71]. Nanosilver’s catalytic and antimicrobial properties make it a highly efficient material in treating water pollutants.

3.1.1 Antimicrobial properties and disinfection. Nanosilver showed broad-spectrum antimicrobial activity (bacteria, viruses, and fungi) and can serve as an effective disinfectant for water applications [7,72]. Various mechanisms, including disruption of cellular membranes, interference with DNA replication, and the generation of reactive oxygen species (ROS), mediate the antimicrobial effect [73]. AgNPs stabilised with Hormophysa triquetra extract inhibited Escherichia coli, Bacillus subtilis, Staphylococcus aureus, Pseudomonas stutzeri, and Pseudomonas fragi in a solvent-dependent manner [74]. AgNPs showed an inhibition zone of 22.5 and 25 mm in chloroform and ethanol, respectively, against P. fragi. In contrast, inhibition of E. coli was achieved in methanol (23.5 mm) [74]. In another study, AgNPs stabilized with Eriobotrya japonica seed extract showed enhanced antibacterial activity compared to penicillin. The author found that E. coli, Klebsiella, and S. mutans showed inhibition zones of 20 and 15 mm upon exposure to 150 mg/mL AgNPs [75]. In addition, AgNPs demonstrated efficacy in inactivating viruses, including adenovirus, hepatitis B, herpes simplex, influenza A, Chikungunya virus, and Zika virus, by disrupting their protein coats and interfering with their replication mechanisms [76]. Waterborne viral infections include adenovirus, astrovirus, hepatitis A and E viruses, rotavirus, and norovirus [77]. Additionally, human urine excretion can contain polyomaviruses and cytomegalovirus, which can contaminate water and spread through water sources [77]. Nanosilver can kill viruses and make water safe to drink under controlled laboratory conditions, but translating this to potable water treatment would require comprehensive validation in realistic matrices, assessment of Ag⁺ release, and long-term safety evaluation. Nanosilver coated with tannic acid significantly inhibits adenovirus type 3 (Ad3) in HeLa cells, serving as a model system [78]. The inhibition mechanism involves structural and DNA damage to Ad3 [78]. Another study assessed the dose- and size-dependent antiviral effects of AgNPs against feline calicivirus, which shares a similar genetic makeup with human norovirus [79]. The study found that 10 nm AgNPs significantly reduced viral load (6.5 log10 viral titer) at concentrations of 50 and 100 μg/mL. The protein expression analysis confirmed that the capsid protein showed a reduction in signal, indicating that AgNPs with smaller size-selectivity interact with the virus capsid [79]. Furthermore, nanosilver exhibits antifungal properties against waterborne fungi, including Acremonium, Alternaria, Aspergillus, Chaetomium, Fusarium, Mucor, Lichtheimia, Paecilomyces, Penicillium, Phoma, Scopulariopsis, and Trichoderma [80]. Nanosilver stabilised with an extract of Bacillus thuringiensis MAE 6 was assessed for its antifungal activity against four Aspergillus strains (A. niger, A. flavus, A. terreus, and A. fumigatus) at 500 μg/mL [81]. The inhibition zone and MIC values were 16, 26, 20, and 19 mm, and 125, 15.62, 62.5, and 62.5 μg/mL, respectively [81]. Another concern are waterborne protozoa, which have a significant impact on global health. A recent review suggested that Giardia and Cryptosporidium are prominent, whereas others include Cyclospora cayetanensis, Dientamoeba fragilis, Toxoplasma gondii, Blastocystis hominis, Entamoeba histolytica, and Microsporidia or Naegleria fowleri [82]. A study indicated that commercial AgNPs showed anti-Giardia activity in combination with metronidazole (an antiparasitic), but the efficacy was not optimal [83]. Another study using stabilized AgNPs from Astragalus ecbatanus treated Giardia lamblia infection in vitro and in vivo model systems [84]. The study found that AgNPs killed cysts at 200 and 300 μg/mL after 4 and 2 h, respectively, whereas trophozoite reduction was observed at 100, 200, and 300 μg/mL after 4, 2, and 1 h, respectively [84]. The above studies, along with those in Table 1, confirmed the potential of AgNPs as disinfectants for water treatment applications. However, the environmental remediation perspective of AgNPs needs to be discussed to assess their full potential.

3.1.2 Pollutant removal or degradation. The antimicrobial action of AgNPs has been explored for water treatment and purification. Despite this, several pollutants, including metal ions, pesticides, and dyes, are present in water and significantly contribute to water pollution. To obtain purified water, these pollutants must be removed from water sources through adsorption/absorption or catalytic degradation into harmless molecules.

3.1.2.1 Removal via adsorption/absorption. Nanosilver or nanocomposites containing AgNPs are predominantly used for pollutant removal via ionic or covalent interactions. The colloidal nanosilver has a limited adsorption capacity, which was increased by adding other materials. This includes carbon-based nanocomposites (activated carbon, graphene, and nanotubes), polymer-based nanocomposites (chitosan and alginate), and metal oxide-based nanocomposites (iron, zinc, and titanium) [26]. A recent study confirmed the removal of three metals (Zn²⁺, Pb²⁺, and Fe³⁺) from the aqueous phase using AgNPs immobilised on banana leaf powder [96]. The study reported that pH-dependent metal adsorption capacities (190, 244, and 288 mg/g) and removal efficiencies (79, 88, and 91%) were achieved for Zn²⁺, Pb²⁺, and Fe³⁺ within 40 min of contact [96]. Another study used Murcott mandarin-stabilised AgNPs to remove Pb²⁺ ions, achieving an adsorption of 42.7 mg/g in 60 min at pH 5.5 [97]. Similar to metal ions, dye removal was achieved using Shorea robusta leaf biochar-stabilised AgNPs, yielding more than 90% removal of RhB and CR [98]. The mechanism underlying removal was surface complexation via electrostatic interactions in both dyes, with hydrogen bonding occurring only in CR [98]. MB removal was achieved using AgNPs mediated by Salvinia molesta. The maximum adsorption of 121.04 mg/g was obtained at pH 4 and 35 °C. Additionally, the nanosilver composite exhibited an enhanced adsorption capacity against PFAS, an emerging environmental contaminant. The two nanocomposites were prepared using chemically and physically activated carbon from maize tassel [29]. Chemically activated nanocomposites exhibited the highest adsorption capacities for PFOS and PFOA, at 454.1 mg/g and 321.2 mg/g, respectively [29]. It was also observed that nanosilver interacted with microplastics in water and adsorbed onto their surfaces, including polypropylene, polyethene, and polystyrene [99]. However, the removal of microplastics from the nanosilver composite is still inconclusive. Despite that, nanosilver possesses functional properties against various environmental contaminants. A nanocomposite combined with magnetic nanoparticles removed amoxicillin (a common antibiotic) from water sources with 100% efficiency [28]. Dose-dependent removal of the antibiotic (i.e., 10 and 100 mg/L) was achieved with 100 and 500 μL of the nanocomposite at neutral pH within 15 min. The nanocomposite was reused up to three cycles with 93% efficiency, indicating potentail as a safe, cost-effective nanosystem for environmental remediation [28]. A nanocomposite composed of magnetic nanoparticles and AgNPs showed the removal of the drug ibuprofen from water. 93% removal was achieved at neutral pH, room temperature, and a low adsorbent dose (7 mg in 500 μL), along with reusability of up to three generations (89.3%) [100]. From the above discussion it was confirmed that nanosilver alone or in combination showed efficinency in removal of water pollutant via adsorption mechanism.

Table 2 indicates types of adsorbent, advantage, limitations and initial pollutant concentrations employed in most AgNP adsorption studies remain high (typically 10–500 mg·L⁻¹) to demonstrate maximum capacity, far exceeding WHO and US EPA regulatory thresholds for drinking water (Pb: 0.015 mg·L⁻¹ action level; Cd: 0.005 mg·L⁻¹ MCL; As: 0.010 mg·L⁻¹). This concentration gap highlights the necessity for additional validation at environmentally relevant trace levels. Finally, Ag leaching from the adsorbent must be systematically quantified and reported alongside removal data to evaluate secondary contamination risk, long-term stability, and regulatory compliance factors that remain inconsistently addressed in many studies and are essential for scaling AgNP materials beyond laboratory proof-of-concept. In addition to adsorption, nanosilver exhibits strong catalytic potential against various water pollutants.

3.1.2.2 Removal via catalytic degradation. The AgNPs possess inherent catalytic properties in the nanoregime for the degradation of various water pollutants, including dyes, nitrophenols, and pharmaceuticals. Studies have highlighted the role of AgNPs as nanocatalysts in the degradation of toxic dyes, which contribute to persistent water pollution [25]. The nanocatalytic activity of AgNPs stabilised with groundnut oil cake was assessed using orange II dye, achieving 99.3% dye degradation in 120 min [112]. This study suggested the potential of green AgNPs in water treatment applications. Another study degraded 94.57% of turquoise blue (a textile dye) at 125 mg/L using Spondias pinnata leaf extract stabilized AgNPs. Specific parameters, including pH 5, 6 mL of AgNPs, 150 min, and a 50 W UV lamp, were used for efficient dye degradation [113]. Fenton-like oxidation of organic dyes using a nanocomposite based on AgNPs was employed to degrade MO, MB, and RhB. The dye-degradation efficiency of the nanocomposite exceeded 99% in both individual and mixed samples after 30 and 240 min, respectively [114]. The dye-degradation mechanisms are primarily governed by the generation of ROS, which further degrade dyes, with the synergistic effect of both AgNPs and iron oxide nanoparticles [114]. AgNPs stabilized with Terminalia arjuna showed degradation of dyes as well as nitrophenols. High degradation rates of 86.68%, 93.60%, 92.20%, and 88.80% for MO (0.166 min⁻¹), MB (0.138 min⁻¹), CR (0.182 min⁻¹), and 4NP (0.142 min⁻¹), respectively, were achieved within 20 min [115]. This study confirmed that AgNPs can simultaneously degrade multiple dyes and nitrophenols, indicating their potential as a potent nanocatalyst in water treatment. In the list of pollutants, microplastics are emerging as a hazard to living beings. Titanium dioxide nanotubes doped with Ag degraded polyethene microplastics by up to 18% after 90 min under UV-C exposure, compared to undoped nanotubes (17.33%), indicating a synergistic effect of doping. Upon doping, Ag was observed to be dispersed on the nanotube surface, either as Ag or Ag₂O nanoparticles [116]. Another study confirmed similar results on Ag-doped titanium dioxide, where 100% degradation of polyethene (125–150 μM) was achieved at 100 ppm in 120 min of UV exposure [117]. Microplastics, such as polyamide 66, were degraded by Ag-doped TiO₂, with increased efficiency observed with Ag doping [118]. 1.5% Ag/TiO₂ achieved a 58.9% removal, which significantly increased to 100% upon increasing the catalyst dose (3:1) after 4 h of UV-A exposure. The underlying mechanism includes strong light absorption, a lower bandgap energy, and the generation of UV-induced electron–hole pairs [118]. The role of persulfate-conjugated Ag⁺ in removing the carboxylic PFAS perfluorooctanoic acid was demonstrated through Fenton oxidation at 20 °C, where fluorine was released along with other products [119]. A study showed ciprofloxacin removal via photocatalysis using a nanocomposite (Ag/silica), achieving 98% degradation at pH 6.7, 180 min, and a 0.12 g of catalyst [120]. Anthracene and benzene photocatalytic degradation were achieved using Ag-, Cu-, and Ni-based nanocomposites [121]. The maximum degradation was obtained under specific conditions, including 10 μg/mL (nano-composite), 5 (pH), 2 μg/mL (anthracene and benzene), and UV radiation. Freundlich and Langmuir isotherms confirmed PAH degradation, with R² values of 0.9894 and 0.9885 for benzene and anthracene, respectively [121]. These studies confirmed that the nanosilver’s catalytic properties are significantly utilised for water pollutant removal.

3.1.2.2.1 Mechanism of catalysis/photocatalysis. The catalytic and photocatalytic activity of AgNP-based systems for environmental pollutant removal proceeds through well-defined mechanistic pathways that must be explicitly distinguished to interpret performance data. In dark catalytic (reductive) reactions, such as the NaBH₄-mediated reduction of 4-nitrophenol or azo dyes, AgNPs function as electron-relay platforms: Borohydride ions adsorb onto the Ag surface, donating electrons that lower the activation energy barrier for hydride ion transfer to the substrate. This process follows pseudo-first-order kinetics, with smaller particle sizes (higher surface-to-volume ratio) and electron-rich surfaces (modulated by capping agents) yielding higher apparent rate constants [122,123]. In plasmonic photocatalytic systems, AgNPs and Ag–semiconductor composites harness LSPR via two dominant mechanisms. First, hot-electron injection, where LSPR decay produces energetic electrons that inject across the Schottky barrier into the conduction band of the coupled semiconductor (e.g., TiO₂, ZnO, or g-C₃N₄) within ≈150 fs, generating long-lived charge-separated states [124,125]. Second, near-field electromagnetic enhancement, in which the intensified local electric field around AgNPs boosts electron–hole pair generation in adjacent semiconductor domains [126]. Schottky junction formation at the Ag/semiconductor interface creates an internal electric field that suppresses electron–hole recombination, extending carrier lifetime and enabling band-alignment-dependent redox reactions [127]. Reactive oxygen species, primarily •OH, •O₂⁻, h⁺, and ¹O₂, drive oxidative pollutant mineralisation; their identity and relative contributions must be verified experimentally using EPR spin-trapping (DMPO adducts for •OH/•O₂⁻) and selective scavenger assays rather than inferred solely from degradation percentages [128,129]. Finally, the apparent quantum efficiency should be reported normalised to incident photon flux to allow meaningful cross-study comparisons and to quantify true photonic utilisation [130]. These mechanistic insights, grounded in ultrafast spectroscopy, EPR, and scavenger studies, reveal why AgNP composites often outperform pristine semiconductors and enable rational design of next-generation environmental catalysts. However, various parameters, such as pH, temperature, catalyst dose, and pollutant concentration, are important in designing nanoparticle-based catalytic applications. Importantly, many high-performance examples cited above involve Ag-containing composites (Ag/TiO₂, Ag/ZnO, Ag-Fe₃O₄, Ag/silica) rather than pristine AgNPs (Table 3). The enhanced performance of these systems should be attributed to the composite architecture (Schottky junction, increased surface area, and synergistic ROS generation) rather than to AgNPs alone.

3.1.3 Membrane technologies incorporating AgNPs. The advent of technological advancements aimed at providing safe water for individuals led to the incorporation of AgNPs into membrane technology, enabling sustained Ag release to remove pathogens and biofouling microorganisms in a continuous-flow system [22]. Nanosilver-incorporated aminated polyethersulfone was designed to impart antibacterial properties to membrane technology for water treatment applications [140]. The incorporated AgNPs exhibited enhanced antibacterial activity, resulting in sustained Ag release over 25 days, indicating improved membrane lifespan [140]. Another study assessed water permeability, antifouling performance, salt rejection, and dye removal using a nanofiltration membrane of green AgNPs with polyethersulfone [141]. The membrane having AgNPs (0.75 wt %) demonstrates water permeability (36 L/m²·h⁻¹·bar⁻¹), salt rejection (NaCl (32 to 57%), MgSO₄ (26 to 67%), and CaCl₂ (27 to 41%)) and lowest fouling along with high flux recovery. The surface property, high porosity, and antimicrobial action of AgNPs have added to the advancement of membrane technology [141]. A study developed a nanocatalytic membrane to remove dyes (MO, MB, and RhB) and achieve antifouling. Dye degradation of 99% and 90% was achieved after the first and fifth cycles, respectively, indicating the efficiency of the engineered membrane [114]. Another study developed a photocatalytic membrane using AgNPs, graphene oxide, carbon nitride, and mixed cellulose ester as substrate. This nanosystem degrades more than 89% of dye even after the 25th cycle and increases flux (3–12 times) compared to the layer-by-layer approach [142]. A hydrophilic PVC-based nanofiber membrane was developed by adding PEG, chitosan NPs, and AgNPs. PEG provides hydrophilicity to the membrane, and AgNPs provide antibacterial properties. The results demonstrate that adding 0.5% AgNPs kills Colitinja and E. coli bacteria, while 0.5% chitosan NPs are effective against E. coli [143]. A membrane consisting of AgCl and COOH-MWCNT was used for ultrafiltration of water. An increased flux of 150% (520 L·m²·h⁻¹·bar⁻¹) was achieved with water. The membrane exhibited enhanced photocatalytic activity under UV, with a flux of 98.1% and an inhibition of 99% for E. Coli [144]. A membrane consisting of AgNPs, zirconium-doped silicon (Ag/ZrO₂-SiO₂) killed 98.86% and 97.42% of E. coli and S. aureus, respectively [145]. The membrane exhibited a higher rejection rate of 96.74% and flux of 331.36 L·m²·h⁻¹ for reactive black KN-B (50 mg/L) at 2 bar (25 °C). In addition, the antifouling effect was confirmed by a flux recovery of 91.18% after three cycles [145]. The significant improvement in membrane technology for safe water is due to AgNPs’ multifunctional properties and colloidal/gel stability. The water treatment approach has undergone a substantial revolution with the emergence of nanotechnology. Furthermore, the application of AgNPs in air and soil pollution must be addressed.

3.2 Air applications

Air pollution poses a significant threat to human health and the environment, necessitating the development of efficient and sustainable air purification technologies. The primary air pollutants include particulate matter (PM), gases (CO, NO₂, SO₂), metals (Pb), PAHs, volatile organic compounds (VOCs), and dioxins [146]. Nanosilver has emerged as a promising material for air purification and atmospheric remediation owing to its strong photocatalytic, antimicrobial and adsorbing properties. Nanosilver is often used in air filters and coatings to remove VOCs, PAHs, PM, and other contaminants.

3.2.1 Air purification and indoor air quality. Incorporating AgNPs into air filters enhances their ability to remove airborne pollutants, such as particulate matter, aerosols and microorganisms, improving air quality and protecting public health [147]. Graphene oxide AgNPs impregnated on polyacrylonitrile nanofibers were designed for dual-functional PPE filters. The membrane exhibited effective antibacterial activity against S. aureus and GFP-expressing E. coli. In addition, PM with a size of 2.7 μm was removed, indicating the potential of AgNPs for air filtration technology [147]. Another study used green (Ficus elastica) synthesised AgNPs for the adsorption of gases (SO₂ and NO₂). The study found that 98% of SO₂ removal was achieved with pseudo-second-order reaction kinetics [148]. A study utilised AgNP-incorporated cotton fabrics for removing bioaerosols from air conditioners [149]. The results showed a reduction in CFU when the filter was modified with AgNPs, achieving 76% and 96% microbial inhibition and retention, respectively [149]. A study demonstrates that AgNP-coated HEPA filters have antibacterial efficiency against E. coli and S. epidermidis upon exposure to dust particles. The result showed that with AgNPs the antibacterial effect increases even in the presence of some dust particles [150]. It was reported that particulate respirators lack antibacterial properties, despite being used to filter fine airborne particles. The reported study developed AgNP-incorporated particulate respirators for dual functionality (i.e., antibacterial and filtration). The result confirmed the inhibition of two bacterial strains, S. aureus and P. aeruginosa [151]. The functional nanocomposite filter membrane was developed using AgNPs, TiO₂, GO and polytetrafluoroethylene (PTFE) to remove PM and for antibacterial activity. The membrane removed 99.25% of PM2.5 and inhibited 98.7% of E. coli growth. The membrane showed 95% removal efficiency after five cycles [152]. A recent study developed air filter paper using hardwood pulp and glass fibre, incorporating AgNPs to inhibit the growth of E. coli and S. aureus, with inhibition zones of 1.52 and 2.04 mm, respectively [153]. AgNPs exhibit antimicrobial properties and reduce foul-smelling pollutants, helping maintain air quality. The AgNPs–cellulose derivative-impregnated PP fibres were used to correct air odour by removing sulfur compounds [154]. The removal of foul-smelling pollutants, including hydrogen sulfide (H₂S) and ethanethiol (C₂H₅SH), was achieved at 95% for H₂S (50 ppm) and C₂H₅SH (25 ppm) [154]. An efficient nanosystem was developed using polyamide-6 electrospun nanofibers attached to AgNPs via hydrogen bonding for antibacterial, antiviral and PM removal [155]. The study reported the removal of PM2.5 (99.99%), a low-pressure drop (31 Pa), and the removal of aerosol pollutants (SO_x, NO_x, methylbenzene, and ʟ-nicotine). The nanosystem inhibits E. coli, S. aureus, and Porcine deltacoronavirus with no significant toxicity. This indicates the functional properties of nanosilver in air filtration technology [155]. An Ag/Cu-doped TiO₂ nanosystem was developed for photocatalytic removal of air pollutants. This nanosystem is effective against both Gram-positive and Gram-negative bacterial strains when exposed to UV-A [156]. AgNPs incorporated in PVDF nanofiber membranes were synthesised for improved air purification, achieving efficiencies of 99.95–99.97% and a pressure of 137.5 Pa. Additionally, 99.6% inhibition of S. aureus and E. coli was achieved after exposure to sunlight for two weeks [157]. Chitosan-stabilised AgNPs incorporated in silica hydrogel beads were prepared for an air filter with antibacterial properties. In a bacterial medium, the nanosystem inhibited S. aureus and E. coli, whereas in an air filter, it inhibited B. subtilis [158]. A study developed hybrid nanofibrous membranes consisting of CS/AgNPs/PVA/CA for a highly efficient air filtration system. The results were 99.78% (PM2.5) filtration efficiency, low-pressure drop (61.15 Pa), and enhanced zone of inhibition (100–141%) [159]. A side-by-side nanofiber was designed with ZnO on one and AgNPs on the other side for air pollutant removal application. The nanofiber exhibited filtration efficiency (80%), dye degradation (97% after 140 min), and antibacterial activity [160]. Carbon fibre cloths loaded with AgNPs/TiO₂ were developed for catalytic removal of NO, a common air pollutant and carcinogen. It was reported that 3.70 wt % AgNPs in the nanocomposite showed the highest removal of NO, with minimum and maximum efficiencies of 80 and 95%, respectively. In addition, the generation of NO₂ occurred at modest levels [161]. AgNP-incorporated PP membranes selectively removed 97% of particles with a diameter of 0.3 μm, which includes PM1.0 and PM2.5. The membrane exhibited broad-spectrum antibacterial activity, with a maximum inhibition zone [162].

3.2.2 Removal of VOCs. Volatile organic compounds are major air pollutants that contribute to smog formation and pose risks to human health. These are ubiquitous air pollutants emitted by various sources, including industrial processes, transportation, and consumer products, and pose significant health and environmental risks. AgNPs exhibited light-induced photocatalytic air purification and pollutant degradation [163]. The AgNPs act as electron sinks, promoting charge separation and reducing electron–hole recombination, enhancing photocatalytic efficiency. AgNPs-modified photocatalysts can effectively degrade various air pollutants under UV or visible light irradiation, including VOCs, nitrogen oxides, and PM [164]. AgNPs can be combined with semiconductor photocatalysts, such as titanium dioxide, caesium oxide, or zinc oxide, to enhance their photocatalytic activity and improve the degradation of air pollutants [165,166]. A review article highlighted the role of Ag–CeO₂ in catalytic removal of CO, soot, and VOCs via multiple mechanisms [167]. A recent study demonstrated the oxidation of acetone and ozone using an Ag-modified (CeO₂–Al₂O₃) nanocomposite, achieving oxidation rates of 96% and 98%, respectively. Additionally, the reaction selectivity exceeded 97%. The relative humidity enhanced gas-phase photolysis in the presence of UV [168]. Bifunctional zeolite-Ag catalysts (ZSM-5||Ag/γ-Al₂O₃) showed 100% conversion of HCHO at 55 °C, where 32% conversion was achieved at ZSM-5||Ag/SiO₂ [169]. A study explored the potential of diesel particulate matter (DPM) oxidation, a hazardous environmental contaminant [166]. A study examined the possibility of Ag-supported different metal oxides (Al₂O₃, TiO₂, ZnO, and CeO₂) as a nanocatalyst for DPM. The Ag/Al₂O₃ nanocatalyst showed enhanced catalysis and reduced ignition temperature by >50 °C compared to Ag/ZnO [166]. A photodynamic nanoconstruct (Ag/UiO-66) was developed for the adsorption of the VOC styrene. An Ag-dependent nanoconstruct exhibited enhanced adsorption and desorption even after five reusability cycles [166]. A nanocomposite containing AgNPs demonstrated toluene removal. The 80Ag-CeO₂@CNWs/CF demonstrated enhanced catalysis with 10%, 50%, and 90% conversion rates at 222, 240, and 256 °C, respectively. The nanocatalyst exhibited good stability and water resistance, indicating its long-term stability and reusability [170]. Ethylbenzene is a VOC widely used in various industrial applications. The oxidation of VOC was achieved using an AgNPs/ca nanocatalyst synthesised with different Ag/Ca molar ratios, resulting in 90% conversion at varying temperatures, depending on the ratios, with an increase in relative humidity from 0% to 40% [171]. The air filter comprises nanosilver and TiO₂ for photocatalytic reduction and antimicrobial activity against VOCs, bacteria, and fungi [172]. The VOC degradation was estimated in a 10 m³ area and found to be 91.6%, 80%, 70.1%, and 43% for butanol, acetone, diethyl ether, and benzene removal, respectively, after 55, 100, 120, and 150 min. In addition, 99% of bacteria and fungi were killed after the airflow [172]. A study described AgNPs on graphite carbon nitride (Ag/g-C₃N₄) for photocatalytic activity for air purification. This system removed various VOCs (formaldehyde, acetaldehyde, ethylene, benzene, toluene, and p-xylene) and killed E. coli, S. aureus, and C. albicans [173]. These studies have highlighted the role of AgNPs as an efficient material for the remediation of air pollutants. The application of AgNP-based nanosystems in soil pollution management requires attention.

3.3 Soil remediation and agricultural applications

Soil contamination by heavy metals, pesticides, and other pollutants poses a significant threat to both agricultural productivity and environmental sustainability. Nanosilver has shown potential for soil remediation and agrarian applications, offering unique properties and functionalities that enable the removal of contaminants from the soil and enhance plant growth.

3.3.1 Remediation of soil contaminants. Nanosilver can bind metals in the soil, such as lead, cadmium, and arsenic, reducing their bioavailability and preventing plant uptake [174]. AgNPs promote the removal of organic pollutants, such as pesticides and herbicides, in the soil through catalytic or redox reactions [174,175]. Fipronil is a broad-spectrum insecticide associated with carcinogenic, neurotoxic, and endocrine-disrupting effects that affect humans via plant uptake [176]. A study hypothesized that adding AgNPs to phytoremediation would significantly enhance fipronil removal. Different formulations of AgNPs with Brassica, Ipomoea, Camellia, and Plantago reduced fipronil at 95.45%, 90.15%, 63.65%, and 63.48% (in water) and 68.8%, 54.64%, 43.75%, and 30.99% (in soil), respectively. The combined effect of AgNPs produces fipronil metabolites in Plantago roots and leaves, while a reduction of the translocation of fipronil was observed. This study has highlighted the potential of AgNP-mediated phytoremediation of insecticides [176]. Pesticides (methyl parathion) and herbicides (pendimethalin and trifluralin) were removed via catalytic reduction using ZnO nanostars doped with Ag and Pd. The results showed that Ag and Pd doping improved catalytic reduction, with Pd-doped nanostars performing better than Ag-doped nanostars [177]. AgNPs were synthesised in a mixture of Orthosiphon stamineus leave extract and an ionic liquid using an electrochemical method; they degraded the herbicide 2,4-dichlorophenoxyacetic acid. The herbicide degradation was 65.61% while as per RSM, 97.8% removal was achieved at pH (3.24), catalyst (0.009 g/L) and herbicide (8.15 mg/L) [178]. Activated carbon has good adsorption capacity due to its porous structure. Therefore, AgNPs decorated with activated carbon were synthesised to remove glyphosate. The adsorption efficiency with AgNPs was 149.25 mg/g, compared to 42.92 mg/g for activated carbon only [179]. Additionally, metal ions were removed. The membrane selectively removed Cd and Pb after 40 and 60 min, with adsorption capacities of 625 and 370.37 mg/g at neutral pH [180]. Although this methodology was initially developed for an aqueous environment, it can be optimised for use with future technology on soil samples. Furthermore, antibiotic-resistant bacteria and genes, as well as emerging soil pollutants, significantly impact ecodiversity [181]. A study prepared silver–lignin NPs to inhibit drug-resistant bacterial strains (S. aureus, P. aeruginosa, S. epidermidis, K. pneumoniae, and A. baumannii) without toxicity on THP-1 cells. The NPs upregulated selectively genes responsible for membrane efflux in P. aeruginosa [182]. The AgNPs significantly reduce the metal content of As, Cr, Pb, Mn, and Cu by 75%, 69%, 62%, 86%, and 76%, respectively, in Zea mays plants. The AgNP concentration-dependent accumulation reduces metal ion levels and improves plant shoot, root, and vigour index. The AgNPs increased antioxidant activity, carotenoids, chlorophyll a and b, and reduced malondialdehyde in the plants, indicating the potential of nanosilver-mediated plant growth and bioremediation [183]. The other study focused on the stabilisation of Pb ions using nanocomposites (AgNPs–TiO₂ NPs) in cowpea (Vigna unguiculata (L) Walp). The nanocomposite significantly reduced the malondialdehyde content (29%), increased carotenoids (88%), and enhanced antioxidant activity. In addition, 57% of Pb was immobilised, restricting its translocation from soil to shoots [184]. Green (cocoa pod)-synthesised AgNPs were evaluated for the immobilisation of Cd and Pb in the plant (Moringa oleifera). The results showed that improved plant physiological and biochemical parameters were obtained compared to the control, indicating that AgNPs mediated plant growth and metal ion removal [185].

3.3.2 Agricultural applications and ecotoxicological considerations. Nanosilver is widely used in plant disease management, growth promotion, and soil improvement. AgNP-based formulations can be applied to plant surfaces to control fungal and bacterial diseases, reducing the need for synthetic fungicides and bactericides. AgNPs can disrupt the cell membranes of plant pathogens, inhibit their growth, and prevent disease outbreaks in agricultural fields [186]. AgNPs can be used as seed coatings or foliar sprays to protect plants from diseases and pests, reducing crop losses and improving agricultural productivity. Another study validated the role of foliar AgNPs as phytoremediation agents in removing metal ions. The impact of AgNPs on the growth of Abelmoschus esculentus plants grown in soil from a gold mine was evaluated. There was a significant reduction of heavy metal ions and other contaminants by 60% and 44%, respectively [187]. Spraying AgNPs onto plants infected with Tomato mosaic virus and Potato virus Y significantly reduced viral load at 50 ppm. The TEM analysis confirmed the surface attachment of AgNPs to the virus coat, along with the induction of photosynthesis, total soluble protein, peroxidase, and polyphenol oxidase activity [188]. Duckweed (Lemna minor)-stabilised AgNPs were synthesised and evaluated regarding insecticidal activity. The green AgNPs exhibited insecticidal activity against Sitophilus oryzae (IC₅₀ 6.08 µg/mL) and Tribolium castaneum (IC₅₀ 7.03 µg/mL) along with significant wheat seed germination during one year of storage [189]. This suggests that AgNPs can serve as a protective seed-coating material to promote growth and sustainability. AgNPs have emerged as a promising tool for immobilising heavy metals and facilitating the degradation of organic pollutants in contaminated soils. A study investigated the impact of nanosilver combined with growth-promoting rhizobacteria on plant growth and metabolism in plants irrigated with municipal wastewater. This study confirmed that treating wastewater with nanosilver and rhizobacteria enhanced plant growth and increased the bioremediation potential against toxic metal ions [190]. Azadirachta indica-stabilised AgNPs inhibited the growth of phytopathogens with efficiencies of 92%, 89%, and 69% for Penicillium sp., Fusarium sp., and Aspergillus sp., respectively. Exposure to Ralstonia solanacearum resulted in cellular damage, bulging, pit formation, and nucleic acid discharge, ranging from 8% to 37%. This indicates that AgNPs can be used as agrochemicals [191]. Rice crop is hampered by bacterial leaf-blight disease caused by Xanthomonas oryzae pv. oryzae (Xoo). AgNPs stabilised with Bacillus cereus (SZT1) exhibited an inhibition zone of 24.21 ± 1.01 mm against Xoo [192]. Upon modulating the surface chemistry with chitosan, the MIC and MBC were 2.5 and 20 μg/mL, respectively [193]. Green AgNPs can aid in the germination of aged rice seeds. The result showed that priming aged rice seeds with AgNPs enhanced germination efficiency and seedling vigour. ROS and upregulated aquaporins enhance seed germination and starch metabolism [194]. Several studies have highlighted the role of AgNPs in enhancing plant seed germination and promoting plant growth [195]. However, promotion of AgNPs as agrochemicals, seed treatments, or plant growth promoters must be balanced with rigorous ecotoxicological assessment. At elevated concentrations, AgNPs exhibit phytotoxicity, manifesting as inhibited seed germination, reduced root elongation, and chlorophyll degradation in crop species [196]. Sub-lethal concentrations disrupt soil microbial communities, particularly nitrogen-fixing bacteria (Rhizobium sp.), nitrifiers (Nitrosomonas sp.), and decomposers, which are critical for organic matter cycling [197]. Earthworm and soil invertebrate toxicity has been documented at concentrations exceeding 100 mg/kg soil dry weight [198]. Bioaccumulation of silver in plant tissues and its potential transfer through the food chain raise additional concerns regarding human dietary exposure from AgNP-treated crops [196]. The regulatory status of AgNPs as agrochemicals remains ambiguous in most jurisdictions; neither the US EPA nor the European Chemicals Agency (ECHA) has established maximum residue limits specifically for silver nanoparticles in food or soil [31,199]. Any agronomic application of AgNPs should therefore be preceded by a comprehensive ecotoxicological risk assessment under realistic soil and crop conditions, and the results should be reported alongside efficacy data to ensure a balanced evaluation.

4 Environmental monitoring

Environmental monitoring is crucial for determining pollution levels, identifying contamination sources, and assessing the effectiveness of remediation efforts. Nanosilver, with its unique optical properties, makes it a promising sensing element for environmental monitoring applications, offering high sensitivity, selectivity, and stability for detecting various pollutants and contaminants in water, air, and soil [200]. The strong optical properties of AgNP-based nanosensors enable the monitoring of environmental pollutants, such as metal ions, pesticides, and microplastics [18,201]. The target specificity of nanosilver can be modulated by the NP's physical and chemical properties [200]. The detection methods are based on electrometric, spectrometric, fluorometric, and SERS techniques (Figure 4).

4.1 UV–vis spectrometric/colourimetric

Spectrophotometric sensors developed using AgNPs operate on the principle of LSPR. The LSPR of AgNPs changes in response to interactions with their surroundings. This leads to a visible shift in the colour of NPs. A study using citrate-AgNPs detected eight metal ions depending on their plasmonic shift. A new peak emerged with Zn²⁺, Cu²⁺, Cd²⁺, Fe²⁺, Mn²⁺, and Hg²⁺, whereas Ni²⁺ and Pb²⁺ induced a redshift in SPR [202]. Another study reported size-dependent metal detection using ʟ-carnosine-AgNPs, in which As³⁺, Cr³⁺, Cd²⁺, and Pb²⁺ were detected via a colourimetric/spectrophotometric approach [8]. The LODs for metal detection were 1.2, 0.63, 2.8, and 4.7 μM for Cd²⁺, Pb²⁺, As³⁺, and Cr³⁺, respectively [8]. Also, CTAB-AgNPs were able to detect Hg²⁺ and Cu²⁺ using spectrophotometric methods [9]. In addition to metals, 3-glycidoxypropyltrimethoxysilane-capped AgNPs were used to detect thiram (a fungicide) via LSPR changes. The prepared AgNPs measured thiram in various samples, including river and tap water, fruit and vegetables, and soil, with a LOD of 213 nM [203]. AgNPs functionalized with α-cyclodextrin showed colourimetric detection of chlorpyrifos by a redshift from 410 to 570 nm in SPR. Nanosilver detected chlorpyrifos in fruit and vegetables at concentrations of 4 and 13 ng/mL (LOD and LOQ, respectively) [204]. Casein-functionalized AgNPs were used to detect streptomycin via spectrophotometric measurement at 405 and 520 nm, accompanied by a visual colour change from yellow to orange. The methods showed LODs of, respectively, 98 and 56 nM and linearities of, respectively, 200–650 and 100–700 nM in the tap water and dairy samples [205]. Querecetagetin-AgNPs detected amoxicillin, where a redshift was observed. The method exhibited a LOD of 4.47 μM with a regression coefficient of 0.993 [206]. PVA-AgNPs detected ampicillin in milk and water using a simple colourimetric approach. A redshift from 402 to 550 nm was observed upon adding antibiotics to AgNPs, with LOD and LOQ of 6.2 and 18.8 μM, respectively [12]. The visual detection of mercaptan gases, including methyl mercaptan (CH₃SH), ethyl mercaptan (C₂H₅SH), and butanethiol (C₄H₅SH), was performed using (dodecanethiol, oleic acid, and oleyl amine)-functionalized AgNPs immobilized on polyvinylidene fluoride substrates. LODs of 80, 28, and 109 ppb were obtained for CH₃SH, C₂H₅SH, and C₄H₅SH, respectively, after 10 min exposure [207]. Another study showed that visual detection of formaldehyde, acetaldehyde, propionaldehyde, glutaric dialdehyde, and hydroxy formaldehyde was achieved using AgNP-decorated paper with LODs of 9 ppb, 3.1 ppm, 3.5 ppm, 23.8 ppb and 71.5 ppb, respectively [208].

4.2 Surface-enhanced Raman spectroscopy

Surface-enhanced Raman spectroscopy (SERS)-based detection is a highly sensitive method for identifying and analysing compounds, including single molecules. It enables the identification and measurement of trace analytes by amplifying the Raman scattering signal of molecules adsorbed onto rough metallic surfaces or nanostructures. SERS-based detection of six metal ions was achieved using pectin-functionalized AgNPs [209]. 4-Mercaptobenzoic acid-functionalized AgNPs were used to detect metal ions, with selective detection of Cu²⁺ at a LOD of 2.5 × 10^–7 M and linearity over 10^–3 to 10^–7 M. In addition, Fe³⁺, Co²⁺, Mn²⁺ and Pb²⁺ were detected, indicating the potential for multimetal detection using a single nanoprobe [210]. Flower-like AgNPs were used to detect pesticides in green tea via SERS. The pesticides, namely acetamiprid, methomyl, and 2,4-dichlorophenoxyacetic acid, were detected with LODs of 1.88 × 10⁻⁴, 5.58 × 10⁻⁴, and 4.72 × 10⁻³ µg/mL, respectively. The SERS-dependent monitoring system is highly sensitive and specific [211]. A AgNP/TiO₂ honeycomb structure was used to monitor micro- and nano-polystyrene plastics in different water sources via SERS. The nanoprobe selectively detected microplastics in water samples from tap, lake, soil, and sea with detection limits of 100, 100, 100, and 250 μg/mL, respectively [212]. Nickel oxide nanosheets decorated with AgNPs served as SERS substrates for detecting various microplastics, including polyethene (PE), polypropylene (PP), and polystyrene (PS). The method exhibited high sensitivity for PS, PE, and PP, with detection limits of 5, 25, and 25 μg/mL, respectively. Notably, PS was detected in water, milk, and liquor with sensitivities of 25, 25, and 50 μg/mL, respectively [213]. An AgNPs/Au@AgNRs sandwich structure was used as a substrate for SERS-based detection of various PFAS, namely, perfluorooctanoic acid (PFOA), perfluorohexanoic acid (PFHxA), and potassium perfluorobutanesulfonate (PPFBS) [214]. The SERS substrate GO@PVP@Ag detected the PAH phenanthrene, with a LOD of 10⁻⁸ g/L [215]. PAH detection with a LOD of 0.5 nM was achieved using SERS enhancement in Fe₃O₄@GO@Ag [216]. Oxytetracycline was detected using a glucose-modified AgNPs SERS sensor. The methods worked perfectly on honey samples with a detection limit of 5–20 ppb [217]. Isepamicin detection was performed using BSA-stabilised AgNPs modified with α-Fe₂O₃. The method exhibits a good linear range (20–2000 nmol/L), a LOD of 16.58 nmol/L, and a recovery of 96.29–104.12% [218]. AgNPs decorated SiO₂ were used to detect VOCs, and their detection limits for toluene, benzene, chloroform, and acetone were 68, 56, 129, and 161 ppm, respectively [219].

4.3 Electrochemical sensors

An electrochemical sensor is a device that converts the chemical response into a signal detectable by modern electrical instruments. The sensors use amperometric, potentiometric, voltammetric, and conductometric measurements [220]. Incorporating AgNPs into electrochemical sensors can be achieved through various strategies, including electrode surface modification, signal amplification, and catalytic enhancement of redox reactions. Folic acid-modified AgNPs were used to detect Hg²⁺ ions using electrochemical methods via cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS). Differential pulse voltammetry (DPV) was used to quantitatively estimate Hg²⁺ at 8.43 μM under optimised reaction parameters [221]. Electrochemical detection of the pesticide imidacloprid was achieved using a nanocomposite (Ag@Meso-C/Hematite Ore)-modified GCE. Amperometric and linear sweep voltammetric techniques were employed for quantitative analysis [222]. SDS-stabilised AgNPs were used for electrochemical detection of clothianidin, with a reduction peak at −300 mV vs Ag/AgCl and a LOD of 2.4 nM [223]. A voltammetric sensor for ciprofloxacin was developed using green (Camellia sinensis) synthesized AgNPs attached with carbon black. The two linear ranges were 3.1–24.8 µmol/L and 36.9–130.3 µmol/L, and the LOQ was 0.48 µmol/L [224]. Nanosilver deposited on carbon paste detected metronidazole using EIS. Linearity was observed within the range 10⁻⁶ to 10⁻³ M, along with LOD and LOQ values of 2.06 × 10⁻⁷ and 6.88 × 10⁻⁷ M, respectively [225]. The nanosensor (MWCNs-poly(2-aminothiophenol)@AgNPs) exhibited sensitivity in detecting divalent metal ions (Pb²⁺ and Cd²⁺) in food samples. The method showed good linearities (0.5–60 nmol/L and 8–50 nmol/L) and LODs (0.125 and 1.47 nmol/L) for Pb²⁺ and Cd²⁺, respectively [226]. Electrochemical detection of PVC microplastics was performed using a nanocomposite (AgNPs/GO/MWCNT) in both seawater and soft water. The Au-modified electrode showed −0.30 V vs Ag/AgCl with linearity (1–5 mg/mL) and a LOD of 0.79 mg/mL [227]. PFOS detection was performed using MXene-AgNP, with LOD and LOQ of 33 and 99 ppq, respectively, in the linear range of 50 ppb−1.6 ppt [15]. An electrochemical sensor based on carbon-dot-stabilised AgNPs was used to detect PAHs, including anthracene and naphthalene. The sensor exhibits enhanced sensitivity for anthracene (250 nM–1.15 mM) and naphthalene (500 nM–842 μM), with LODs of 112 and 383 nM, respectively [228].

4.4 Fluorometric sensors

Fluorometric sensing based on nanosilver has emerged as a powerful technique for sensitive, selective detection of environmental contaminants. The underlying principles are fluorescence quenching or fluorescence enhancement. For example, PVP-AgNPs detected Hg²⁺ using OPD as a substrate and AgNPs exhibiting nanozyme activity (i.e., enzyme-mimicking catalytic behaviour arising from surface-mediated electron transfer, not from a proteinaceous active site). The method found metal concentration-dependent fluorescence quenching of nanozymes in the 20–2000 nM range and a LOD of 8.9 nM [229]. The four kinds of polymer-templated silver nanoclusters discriminated among seven different metal ions. This sensor had a linear range of 15–800 μM at a pH of 7 [230]. The quantitative estimation of ceftriaxone and cefepime was performed by quenching AgNP fluorescence in a dose-dependent manner. The method works at 1–10 and 0.9–9 µg/mL with LODs of 0.178 and 0.145 µg/mL for ceftriaxone and cefepime, respectively [231]. Oxytetracycline and erythromycin were detected using fluorescence quenching with LODs of 2.714 and 3.306 nM, respectively, using graphene carbon dots modified with Ag as a nanoprobe [232]. The fluorescence-enhancing approach was utilised to develop a fenitrothion sensor using MOF embedded with methionine AgNPs (Meth-AgNPs@Fe-BTC). Under optimised conditions, the obtained linearity and LOD were 2–95 nM and 1.9 nM, respectively [233]. In another study, Zr-MOF embedded AgNPs (AgNPs@PCN-224) were used to detect fenitrothion with a LOD of 0.037 ng/mL and linearity (0.1–500 ng/mL) [234].

4.5 Gas sensors

Gas sensors are electronic devices that measure the presence and concentration of specific gases in the air or other environments. It converts the chemical interaction between the sensor material and the target gas into an electrical signal. This signal enables the identification and quantification of the gas. AgNPs can be used for gas-sensing monitoring of various gases, either alone or as components of composite materials. AgNPs and AuNP-modified ZnO were used to detect ammonia gas at room temperature (RT). A surface-modified nanosensor exhibited enhanced detection with LODs of 0.495 and 0.405 ppm for AuZn and AgZn, respectively [235]. In another study, NO₂ detection was achieved using AgNP-doped ZnO nanorods. The response rate increased to 434.3% at 250 °C and 1 ppm, while the detection limit reached 1 ppb [236]. A resistive gas sensor was developed using a nanocomposite (ZIF-8/MWCNT/AgNPs) to detect VOCs (methanol, ethanol, acetone, acetonitrile, and n-hexane) at RT [237]. A chemiresistive H₂S sensor was formulated using AgNPs-decorated graphene, achieving detection below 6 ppb after 6 min [238]. TiO₂/AgNPs heteronanostructures detected CO₂ [239]. Silver nanoparticle-based sensors can monitor environmental conditions in real time, providing valuable data for informed decision-making and risk assessment. Table 4 represents the overall sensing ability of AgNPs and their composites for environmental pollutant monitoring reported in the literature.

4.6. Analytical validation and metrological considerations

A rigorous analytical validation is a prerequisite for translating AgNP-based sensing performance from laboratory settings to practical implementation. LOD and linear range were almost universally reported, whereas comprehensive metrological characterization remains inconsistent across the literature. Accuracy, expressed as percentage recovery in spiked real environmental samples, typically ranges from 95 to 120%. For example, the GO-BMIMPF₆-AgNPs@PVC-o-NPOE impedimetric sensor for malathion achieved recoveries of 95–105% in spiked aqueous matrices with excellent agreement to reference methods [254]. Similarly, the Ag@MIPs-CaF₂ core–shell platform for pesticides delivered 84–104% recovery in spiked water and food samples, while the rGO/AgNPs hybrid nanomaterial for the detection of pharmaceutical residues reported 84–114% recovery in real samples [255,256]. Precision, expressed as relative standard deviation (RSD), is reported in only a minority of studies, but it consistently falls within the 1–6.9% range in well-characterised platforms. The GO-BMIMPF₆-AgNPs@PVC-o-NPOE sensor showed intra-day RSD of 1.05–2.04% and inter-day RSD of 1.48% across replicate measurements, while the Ag@MIPs-CaF₂ substrate exhibited <4.1% signal uniformity [254]. Matrix effects remain a major challenge in complex environmental samples (e.g., humic acids, competing ions, surfactants), often leading to nonspecific LSPR shifts or electrode fouling. Successful mitigation strategies documented in the literature include standard-addition protocols, matrix-matched calibration, and controlled sample dilution, which enabled the high recoveries cited above. Sensor-to-sensor and batch-to-batch reproducibility is rarely systematically evaluated; most studies report only intra-electrode RSD, underestimating the variability introduced during AgNP synthesis. Storage stability and shelf-life are infrequently quantified; the GO-BMIMPF₆-AgNPs@PVC-o-NPOE platform retained functionality for 30 days under refrigeration with only 15.61% drift, and the Ag@MIPs-CaF₂ composite maintained 95.5% of its initial intensity after 28 days at room temperature [254,255]. Long-term stability data (>3 months) remains limited for most platforms. These meteorological gaps hinder regulatory acceptance and field deployment. Table 4 provides a study-by-study assessment of each sensor’s performance against common environmental pollutants, including advantages and disadvantages.

5 AgNPs versus alternative nanomaterials: a critical comparison

The position of AgNPs within the broader nanomaterials landscape requires direct, quantitative comparison with alternative materials across each application domain. In photocatalysis, pristine anatase TiO₂ (bandgap 3.2 eV) is active exclusively under UV irradiation (λ < 385 nm). Ag/TiO₂ composites reduce the bandgap to 2.49–2.84 eV through LSPR-driven hot-electron injection and plasmonic near-field enhancement, extending activity into the visible range and increasing apparent rate constants two- to fivefold (or higher) under solar or visible-light irradiation. For example, TiO₂/Ag (9.3%) achieved k_app = 2.451 × 10⁻² min⁻¹ for MB degradation under visible light (>400 nm), compared with much lower values for pristine TiO₂ [260]. ZnO prepared by facile precipitation (ZnO-400) already matches or exceeds TiO₂ (P25) in specific photoactivity for methyl orange and phenol degradation under UV irradiation [261]. Photodeposition of Ag further boosts performance, and the resulting Ag/ZnO-400-R composite achieved significantly higher degradation rates for methyl orange. It can be reused for at least 18 cycles without loss of activity, demonstrating the electron trapping role of metallic Ag nanoparticles [261]. Graphitic carbon nitride (g-C₃N₄, bandgap ≈ 2.7 eV) is an intrinsic visible-light photocatalyst whose activity is further enhanced by Ag decoration, achieving a 2.5-fold higher degradation rate (k_app = 0.01576 min⁻¹ vs 0.00626 min⁻¹) and 80% vs 48% removal of malachite green after 100 min under UV irradiation compared with pristine g-C₃N₄. The improvement arises from metallic Ag nanoparticles (≈6 nm) acting as electron traps, thereby suppressing electron–hole recombination [262]. In sensing applications, AuNPs and AgNPs are the principal LSPR-active materials. Spherical AgNPs exhibit 5–10-fold higher LSPR extinction coefficients than AuNPs of equivalent size, with LSPR peaks at 390–500 nm (optimally overlapping with the spectral response of sensitive photodetectors), making them preferable for colourimetric and SERS platforms. Direct head-to-head comparison of AgNP vs AuNP-based cobalt-porphyrin-DNA genosensors shows nearly identical performance: Both achieve LODs of 5.0 × 10⁻¹⁸ M (AgNP) and 3.8 × 10⁻¹⁸ M (AuNP) with the same linear range (5 × 10⁻¹⁷–1 × 10⁻¹⁶ M), detecting only ≈30 and ≈23 target DNA molecules, respectively. The redox signal is dominated by the cobalt porphyrin (not the NP), and both exhibit excellent selectivity [263]. The AgNP system offers superior DNA economy (lower probe loading) and lower material cost while maintaining equivalent signal-off behaviour, whereas AuNPs offer an easy-to-set-up system for laboratory testing [263]. A study confirmed that AgNPs outperform AuNPs as reporter labels in electrochemical point-of-care immunoassays [264]. Critically, AgNP detection requires only a one-step potential sequence in the presence of chloride (no corrosive HCl addition), enabling true user-friendly operation, whereas AuNP assays require an additional acid step, which is incompatible with point-of-care formats. AgNP–antibody conjugates furthermore exhibit excellent storage stability (≥4 weeks) and sharper stripping peaks at lower potentials [264]. For electrochemical sensing, graphene and carbon-nanotube platforms provide higher electroactive surface areas and faster heterogeneous electron-transfer kinetics than AgNP-modified electrodes, but lack the dual catalytic and optical functionality that renders AgNPs uniquely versatile [265,266]. However, designing AgNP-based adsorbent materials for water pollution control applications is challenging. For a more robust critical appraisal, the reported maximum removal/adsorption capacities must be contextualised against both conventional benchmarks and emerging high-performance materials. Conventionally, activated carbon is considered a better material for achieving 50% removal of methyl orange (2 mg/L, pH 7), whereas the addition of AgNPs significantly improved the adsorption rate to 72.5%, indicating improved efficiency and recyclability up to ten cycles [267]. Ding et al. reported that nanosilver-loaded activated carbon derived from waste rice noodles achieved a maximum adsorption capacity (q_max) of 97.07 mg·g⁻¹ for Cr(VI), markedly higher than the 65.43 mg·g⁻¹ obtained with the unloaded activated carbon under identical conditions (pH 2, room temperature) [268]. Biochar is conventionally used as an adsorbent. For instance, the AgNP/biochar nanocomposite and biochar showed strong adsorption, removing 98.18 ± 1.46% and 87.72 ± 1.58% of Cr(VI) at 10 ppm/10 mL after 240 min, respectively. In addition, the nanocomposite removed 90.10 ± 1.31% of nigrosine at 10 mg/L after 60 min, while the study lacked a biochar assessment [269]. Similarly, for graphene oxide, Vicente-Martínez et al. showed that GO functionalized with AgNPs (GO@AgNPs) achieved 100% phosphate removal instantaneously at a lower adsorbent dose, compared to only 75% after 9 min with a higher dose of pristine GO [270]. For metal–organic frameworks, Mirzajani et al. found that AgNPs encapsulated in ZIF-8 MOF delivered q_max values of 76.34 mg·g⁻¹ for Cd²⁺ and 79.36 mg·g⁻¹ for Cu²⁺ in real water matrices, providing added antibacterial functionality absent in pristine MOFs [101]. Direct comparisons for zeolites remain limited, but AgNP decoration of related composites has shown improvements in heavy-metal uptake analogous to those observed for zeolites. Overall, AgNP integration consistently enhances adsorption capacity, kinetics, and reusability, while providing secondary benefits (e.g., antimicrobial or magnetic recovery).

In antimicrobial applications, AgNPs demonstrate clear superiority over AuNPs and other common nanomaterials such as TiO₂ and ZnO through direct mechanistic and quantitative comparisons. AgNPs exert broad-spectrum bactericidal effects primarily via Ag⁺ ion release, ROS generation, membrane disruption, and interaction with thiol groups in proteins and DNA, achieving MIC values in the range of 1.4–5.6 μg/mL and MBC values of 2.8–5.6 μg/mL against both Gram-positive and Gram-negative strains [271]. Aggregated data across hundreds of studies confirm that AgNPs exhibit MIC of 0.11–1200 μg/mL (MBC of 0.22–1500 μg/mL) and demonstrate broader efficacy against Gram-negative bacteria (64% of studies) due to electrostatic attraction and thinner peptidoglycan layers [272]. In comparison, AgNPs produce equivalent antibacterial zones at 4.86 μg/mL, whereas AuNPs require 197 μg/mL to inhibit S. aureus [273]. AgNPs exhibit distinct potency and oxidative-stress mechanisms compared with CuNPs, ZnONPs, and TiO₂NPs. Direct IC₅₀-based growth-inhibition assays across E. coli, B. cereus, and S. epidermidis reveal AgNPs as the most potent against E. coli (IC₅₀ = 7.84 mg/L), outperforming ZnONPs (176.10 mg/L) and CuNPs (180.80 mg/L), while TiO₂-NPs dominate in B. cereus (50.30 mg/L) and CuNPs in S. epidermidis (112 mg/L) [274]. Strain- and NP-dependent alterations in antioxidant defence directly correlate with antibacterial efficacy, positioning AgNPs as highly effective via rapid ROS overload in Gram-negative species, whereas ZnO/TiO₂NPs require higher concentrations and light-dependent mechanisms for comparable activity [274]. Overall, AgNPs combine the lowest effective concentrations, light-independent action, and broadest spectrum, positioning them as the preferred antimicrobial nanomaterial when rapid, non-photocatalytic bacterial inactivation is required [271-273]. In summary, AgNPs excel in niche multifunctional roles requiring simultaneous optical, catalytic, antimicrobial, or plasmonic properties, while TiO₂/ZnO dominate UV photocatalysis, AuNPs offer stability in sensing, graphene/CNT provide superior electrochemistry, Fe₃O₄ enables magnetic recovery, and MOFs/biochar deliver high-capacity adsorption. Integration of different functional aspects using different materials to form an Ag nanocomposite may provide additional advantages over conventional benchmark systems.

6 Challenges

Nanosilver has advanced environmental science applications. Despite their widespread advantages, specific challenges exist which can limit the use of AgNPs or demand a more sustainable material (Figure 5).

6.1 Standardisation, reproducibility, and inter-laboratory challenges

Standardisation and reproducibility represent pervasive, underaddressed challenges across all AgNP-based environmental applications. Batch-to-batch variability in AgNP synthesis arises from subtle differences in reducing agent concentration, mixing rate, temperature, and nucleation kinetics, producing size distributions, zeta potentials, and LSPR peak positions that can vary significantly between preparations nominally following the same protocol. This variability directly propagates into functional performance, causing irreproducibility in sensing LODs, catalytic rate constants, and antimicrobial inhibition zones. Among all the challenges, the synthesis of desired AgNPs for particular applications is the primary concern; for example, small NPs are more efficient antibacterial agents and can be used in filtration membranes than larger ones [47]. Thus, achieving a small size is paramount for antibacterial applications in water, air, and soil treatment. Similarly, optically active AgNPs can be used to monitor and remediate environmental contaminants. The optical properties can be controlled via various synthetic methods. While environmentally preferable, green synthesis methods using plant extracts or microorganisms often suffer from poor reproducibility and limited control over nanoparticle characteristics. Using chemical reducing agents presents concerns about toxicity, while physical methods are energy-intensive and expensive. Additionally, maintaining colloidal stability without aggregation over time requires careful surface functionalization, which can alter the particles’ environmental behaviour and effectiveness [63]. Other challenges are reproducibility, synthesis scale-up with controlled properties, and cost-effectiveness.

Furthermore, regulating AgNPs represents unique challenges due to their novel properties, diverse applications, and potential environmental and health impacts. One of the primary challenges is the scarcity of comprehensive data on the toxicity and environmental fate of silver nanoparticles, which hinders thorough risk assessments and the establishment of safe exposure limits [275]. Analytical techniques that accurately measure and characterise AgNPs in complex environmental matrices are needed to monitor their presence and behaviour [275]. Another challenge is the rapid pace of innovation in nanotechnology, which drives continuous development of new silver nanoparticle formulations and applications. Several regulatory agencies have formulated guidelines to provide direction for the use of nanomaterials. International organisations, such as the OECD, formulate guidelines for the physicochemical properties of nanoparticles (test guides 125, 124, and 318) and safety (test guides 412, 413, and 39) [276]. Other agencies include ISO, EU, and US FDA programs for managing nanomaterial characterisation and safety perspectives [276]. Nationally, the Department of Science & Technology (DST) and the Department of Biotechnology (DBT) have developed guidelines for the safe handling of nanomaterials.

Pursuing sustainable synthesis methods for AgNPs is crucial for reducing environmental impact and promoting green chemistry principles. Green synthesis approaches employing plant extracts and bacterial and fungal filtrates offer eco-friendly options for nanoparticle synthesis [277]. The extracts comprise various bioactive compounds, including polyphenols, flavonoids, and terpenoids, which provide dual functionality to nanoparticles [278]. Biosynthesised approaches contain natural ingredients that are less harmful to people and the environment [279]. The morphology and size of the produced nanoparticles are influenced by the extract concentration, reaction temperature, pH, and silver salt concentration [280,281]. Optimising the reaction conditions is crucial for controlling the size, shape, and stability of the synthesised AgNPs. Green synthesis methods offer several advantages, including reduced toxicity, lower energy consumption, and the use of renewable resources, making them attractive for large-scale production of AgNPs [282]. Recovery and recycling of AgNPs from environmental matrices are essential for promoting resource efficiency and minimising ecological contamination [283]. Effective separation techniques, such as filtration, centrifugation, and magnetic separation, are required to isolate AgNPs from complex environmental samples. A study suggested that AgNPs can be adsorbed on microplastics and thus removed [99]. Monitoring AgNPs in ecological samples via cloud point extraction (CPE) combined with ICP-MS was achieved [284]. Eco-design principles can be applied to minimise the environmental impacts of silver nanoparticle-containing products throughout their life cycle [285]. The manufacturing industry needs to adopt a circular approach, where products are designed intentionally for multiple lifecycles [286]. Additionally, optimising AgNP dosage and delivery methods can minimise their release into the environment and reduce potential exposure risks. Implementing proper waste management practices, including collection, treatment, and recycling, is crucial for preventing the release of AgNPs into the environment and minimising their potential impacts on ecosystems and human health. Further research is needed to develop effective and scalable technologies for removing AgNPs from wastewater and other environmental matrices.

6.2 Environmental fate, transformation, and toxicity of AgNPs

The environmental fate of AgNPs following release is essential for accurate risk assessment and for contextualising the performance data reported throughout this review. The utilisation of AgNPs for environmental applications has raised concerns about their potential ecological release and subsequent ecotoxicological impact through interaction with various organisms [287]. Once released into environmental compartments via wastewater treatment plant (WWTP) effluent, sludge land application, or atmospheric deposition from AgNP-containing consumer products, AgNPs undergo a suite of abiotic and biotic transformation processes that profoundly alter their bioavailability and toxicity. Environmental fate and behaviour of AgNPs depend on multiple factors, including particle size, surface charge, coating, and environmental conditions such as pH, ionic strength, and organic matter content [288,289].

Sulfidation is the reaction of dissolved Ag⁺ with sulfide (S²⁻) under anoxic conditions to form Ag₂S, which represents the dominant long-term silver sink in anaerobic sediments and WWTP sludge and substantially reduces bioavailability [288,289]. At the same time, chlorination precipitation of AgCl in chlorinated drinking water and saline environments lowers the activity of free Ag⁺ ions. Photochemical reduction converts Ag⁺ back to Ag⁰ nanoparticles under UV–vis irradiation in the presence of organic electron donors, creating photochemical cycling and complexation with dissolved natural organic matter, which can both stabilise AgNPs against aggregation and compete with biological ligands for Ag⁺ [288,289]. The relative rates of these processes depend on the receiving environment (freshwater, saltwater, soil, or sediment) and on AgNP surface chemistry. For example, citrate-capped AgNPs were evaluated for stability in various water samples over 180 min, during which tap water was found to significantly induce aggregation in nanoparticles [290]. Exposure to AgNPs can lead to enhanced ROS production and increased mitochondrial permeability, potentially resulting in mitochondrial DNA damage, cellular apoptosis, and pro-inflammatory responses [291]. AgNPs release Ag⁺ at rates governed by particle size, surface area, dissolved oxygen concentration, pH, and coating identity. In antimicrobial studies, both Ag⁺- and particle-dependent mechanisms (direct membrane contact, ROS generation at the particle surface) contribute to toxicity, and their relative importance varies with organism type and exposure medium. In photocatalytic and nanozyme applications, surface-bound electron transfer is primarily particle-dependent, while dissolution contributes to Fenton-type chemistry. In colourimetric sensing, LSPR shifts are particle-dependent, while certain electrochemical responses exploit Ag⁺ activity. Failure to decouple these contributions through appropriate controls such as dialysis membranes, ion-exchange pre-treatment, or Ag⁺ scavengers leads to mechanistic misattribution and complicates cross-study comparison.

Nanosilver bioaccumulation and biomagnification in food chains pose additional concerns, as they can lead to higher exposure levels in top predators and potentially disrupt ecosystem functioning. Studies have shown that AgNPs can accumulate in various organisms, including bacteria, algae, invertebrates, and fish, with potential consequences for their health and reproduction [32]. The accumulation of AgNPs in aquatic organisms can disrupt their physiological processes, leading to inflammation, oxidative stress, and tissue damage [292]. The antimicrobial properties of AgNPs can have unintended consequences for microbial communities, which play essential roles in nutrient cycling, decomposition, and bioremediation. Exposure to AgNPs can alter the composition and function of microbial communities, potentially disrupting ecosystem processes and affecting the health of higher trophic levels [293]. Nanosilver at low concentrations exhibited toxic effects on zooplankton, which are mediated by the gut microbiota. In contrast, at high concentrations, mortality resulted from the combined impact of the extinct gut microbiota and accumulated toxicity [294]. Introducing AgNPs into natural brackish waters resulted in distinct changes in bacterial community composition and structure, as well as reduced richness and diversity; anoxic conditions may attenuate these effects [295]. Some studies have shown adverse effects of AgNPs on freshwater organisms, ranging from producers to secondary consumers, with evidence suggesting that at least part of the nanoparticle toxicity is due to oxidative stress [296]. Even at low concentrations, nanosilver can harm the bacterial community in the human gut, as evidenced by reduced gas production and changes in fatty acid methyl ester profiles [297]. AgNPs can negatively affect nitrogen-fixing, nitrifying, and denitrifying microbes in vitro, potentially disrupting nitrogen cycling in ecosystems [197].

7 Future perspectives and emerging trends

Nanosilver have garnered substantial attention in recent years due to its unique physicochemical properties and diverse applications across various fields, including medicine, agriculture, and environmental science [298,299]. As research and development efforts continue to expand, several promising future perspectives and emerging trends are expected to shape the trajectory of silver nanoparticle technology. Combining AgNPs with other materials to create hybrid or composite materials is an emerging trend. These hybrid materials can exhibit properties and functionalities superior to those of their components, enabling the development of advanced materials with tailored characteristics for specific applications [300]. For instance, incorporating AgNPs into polymer matrices can impart antimicrobial properties to the resulting composite material, making it suitable for medical devices, packaging, and textiles [301,302]. Combining AgNPs with other nanomaterials, such as carbon nanotubes or graphene, can create synergistic effects and enhance their catalytic, electronic, and mechanical properties [303]. These composite materials can be used in energy storage, sensing, and environmental remediation applications. Combining AgNPs with other cutting-edge technologies, such as 3D printing, microfluidics, and artificial intelligence, is an emerging trend with tremendous innovation potential [304]. Integrating AgNPs into 3D printing processes enables the fabrication of customised devices and structures with antimicrobial properties for medical implants, tissue engineering scaffolds, and environmental sensors [305]. Combining AgNPs with microfluidic devices can create highly sensitive and selective sensors for detecting environmental contaminants, pathogens, and biomarkers [306]. Furthermore, the use of artificial intelligence and machine learning algorithms can optimise the synthesis, characterisation, and application of silver nanoparticles, thereby accelerating the discovery of novel materials and applications [307]. Looking ahead, the field of environmental applications of AgNPs is poised for continued growth and innovation, driven by ongoing research and technological advancements. Further research is needed to understand the long-term environmental impacts of AgNPs fully and to develop strategies to mitigate potential risks. Advances in nanotechnology and materials science will lead to the development of novel silver nanoparticle-based materials with enhanced properties and functionalities for environmental applications. The use of AgNPs in ecological remediation, such as water and soil treatment, is expected to increase as more effective and sustainable technologies are developed. Moreover, the development of nanosensors, which enable in situ, real-time tracking of pollutants, is anticipated to grow rapidly in the coming years. Integrating AgNPs with other nanomaterials and technologies, such as nanocomposites and nanodevices, will create new opportunities for addressing complex environmental challenges.

Conclusion

This comprehensive review has examined the multifaceted environmental applications of AgNPs across various domains. Our analysis reveals that AgNPs have emerged as versatile tools for environmental remediation and protection due to their unique physicochemical properties. The synthesis methods have evolved significantly, with green synthesis approaches gaining prominence for their reduced environmental impact. AgNPs have demonstrated exceptional capabilities in eliminating pathogens, removing heavy metals, and catalysing the degradation of persistent pollutants in water treatment. Their incorporation into membrane technologies has addressed longstanding challenges such as biofouling. Similarly, AgNPs have proven effective in air purification against airborne contaminants, VOCs, and odour-causing compounds. The agricultural applications of AgNPs offer promising alternatives to conventional pesticides while enhancing plant disease resistance. Furthermore, AgNP-based sensing technologies have demonstrated significant advances in sensitivity, selectivity, and real-time detection capabilities at the laboratory scale. However, widespread field deployment remains constrained by long-term stability limitations, matrix interference in complex real samples, potential ecotoxicological risks of released silver, batch-to-batch synthesis variability, and the absence of a coherent regulatory framework governing AgNP use in environmental applications. The limited application of AgNPs has been hindered by their ecotoxicity, challenges in synthesis, and regulatory concerns. The principles of green chemistry, safe-by-design approaches, and the recycling and reuse of materials provide a framework for overcoming the challenges posed by AgNPs. Furthermore, combining AgNPs with emerging technologies such as artificial intelligence and advanced materials science offers unprecedented opportunities to develop innovative environmental solutions. Ultimately, interdisciplinary collaboration among materials scientists, environmental engineers, ecotoxicologists, and regulatory experts is essential to advancing this field comprehensively.