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NEPEDS College of Pharmaceutical Sciences, Gandhinagar, Tetelia, Dist- Kamrup Metro, Assam, 782403, India.
Background: The ecological and economic impacts of traditional nanomaterial manufacturing technologies are profound, and are mainly associated with the use of toxic substances, energy-demanding processes, and the production of toxic wastes. Such restrictions have continued to fuel the international quest to find viable, yet affordable substitutes. Green synthesis, as a concept, has become a plausible answer, and agricultural wastes such as fruit peels, lignocellulosic wastes and other wastes that are left behind in the plantation have become a ready and renewable source of raw materials. This practice is consistent with the greater concepts of the Circual Economy that supports the valorization of waste and resource efficiency. Aim and Purpose: The purpose of this review is to record and discuss how the use of agricultural biowastes as the main feedstock to synthesize silver nanoparticles (AgNPs) greenly has been changed, compared to traditional methods that used plant extracts as the starting material. In particular, it assesses green extraction technologies, explains the bioreduction mechanisms, and critically reviews the standardized life cycle assessment related to the production of biogenic AgNPs. Discussion: Active phytoconstituent of agricultural wastes such as phenolics, flavonoids and lignin are effective natural reducing and capping agents, which help to form stable, biocompatible AgNPs with well-defined morphologies. These biogenic nanoparticles exhibit excellent multi-targeted antibacterial and antibiofilm effects against multidrug-resistant (MDR) pathogens, which indicates their therapeutic importance. In addition to biomedical uses, waste-derived AgNPs have significant potential in environmental purification, such as improved wastewater treatment, and nano-agriculture, e.g., seed nano-priming or nano-pesticides.Although having these techno-economic benefits, there are a few obstacles to large-scale commercialization such as intrinsic variability of biowaste composition, absence of a standard in synthesis, and unaddressed ecotoxicity issues. Conclusion: Waste-to-nano-wealth paradigm is a scientifically promising and environmentally friendly approach to sustainable nanomaterial production. The way forward to maximize its potential is to focus on safe-by-design frameworks, optimized and scalable extraction protocols and global regulatory harmonization. The gap between innovativeness and scalability of green AgNPs in the laboratory and industrial setting will be important in ensuring that green AgNPs are translated into real-world solutions.
1.1. The Global Agricultural Waste Crisis and the Circular Economy
High energy demand, chemical toxicity, and low biocompatibility are just a few of the major environmental concerns linked with traditional physical and chemical-based techniques for producing industrial nanomaterials [1]. Recent investigations have strongly supported the "green synthesis" of nanomaterials using abundant plant-based materials and agricultural wastes to overcome these hurdles [1]. There is an urgent need to develop techniques in agriculture that are more sustainable, given the accelerated rate at which the world's population is increasing, which demands an increase in the production of crops to meet the demands of food security [2]. Instead of using toxic artificial chemicals, this novel and eco-friendly method uses plant extracts, agricultural materials, and bio-waste materials such as pistachio shell waste and tomato flower waste, which are natural reducing and stabilizing agents [1]. This method is also beneficial in terms of integrating the latest innovations in nanotechnology with waste management techniques. This method uses agricultural waste to synthesize useful nanoparticles [1]. Moreover, the synthesized bionanomaterials are reusable in the agricultural sector in the form of eco-friendly nano-fertilizers and nano-pesticides. This method is useful for the treatment of nematodes and plant infections that cause harm to agricultural crops. Finally, the bio-based synthesis of useful nanomaterials using agricultural materials is beneficial in terms of the principles of the circular economy, which reduce environmental degradation, reduce the cost of managing agricultural crops, and promote global ecological sustainability [1, 2].
Figure 1: The "waste-to-nano-wealth" circular economy paradigm for the sustainable synthesis of silver nanoparticles (AgNPs).
1.2. Green Nanotechnology: Transitioning from Plant Extracts to Agro-Waste
Green nanotechnology is a sustainable alternative to traditional methods of synthesis, replacing harmful chemicals with biological reducing agents [3]. The phytochemical properties found in natural, untainted sources such as leaves, seeds, and roots are excellent reducing and capping agents, and this method was first utilized [3]. But a new era known as "sustainable green synthesis" has shifted the emphasis from primary plant extracts to the value adding of agricultural and food sector wastes [3]. This novel approach turns cheap, plentiful agro-industrial byproducts into useful precursors for the synthesis of nanoparticles [3]. Fruit and vegetable peels, cereal straws, sugarcane bagasse, rice husks, and leftover coffee grounds are a few examples of these resources [3, 4]. The bioactive compounds present in these agricultural residues, which include proteins, lignin, flavonoids, and phenolic acids, are effective in the reduction of silver ions to nanoparticles. This process is effective in solving the sustainability problem since it utilizes the waste biomass to produce useful nanoparticles without the need for expensive synthetic chemicals. Moreover, promoting resource productivity and reducing environmental impacts associated with waste management through the use of agro-waste in producing nanoparticles is in line with the bio-economy concept of circularity. [3, 4].
1.3. Scope and Significance of the Review
The increasing demand for the use of sustainable and non-toxic nanomaterials has created a sense of interest in the development of the eco-friendly synthesis of silver nanoparticles rather than the traditional and toxic methods [5]. This review article highlights the various emerging trends in the eco-friendly synthesis of silver nanoparticles with special reference to the use of bio-entities and agricultural wastes as reducing and stabilizing agents [5,6]. The scope of the present papers includes an in-depth study of the parameters affecting nanoparticle synthesis, the importance of characterisation, and the mechanisms of biological reduction [5]. Additionally, it offers a comparative evaluation of biogenic AgNPs in a variety of applications, from sustainable agriculture methods to biological advancements like antibacterial and anticancer treatments [5]. In particular, it highlights the agricultural importance of AgNPs in enhancing postharvest quality, maintaining nutritional content, and successfully reducing plant diseases that harm output, such as green mold [6]. Finally, the review provides a critical evaluation of the viability of agricultural resources in the manufacturing of high value bionanomaterials, including the current challenges facing the use of these products due to safety, regulatory, and economic issues [6].
2. Classification of Agricultural By-products as Green Precursors
2.1. Fruit &Vegetable Peels/Pomace (e.g., Bergamot pomace, Calamondin peel, Cashew apple residue)
Recovery of bioactive compounds which are natural reducing and stabilizing agents in green nanotechnology can be performed in a very sustainable and profitable manner by valorizing the wastes of the processing of fruits and vegetables, especially the peels and pomace of the fruit and vegetables. For example, during the industrial processing of the fruit of the calamondin tree (Citrofortunella microcarpa), significant amounts of by-products like peels, seeds, and pulp are obtained. These by-products are rich in dietary fibers like pectin and essential oil. Although the peels are rich in phenolic compounds and flavonoids like hesperidin, naringin, and diosmin, the essential oil of the peels of the calamondin fruit is mainly composed of D-limonene and other volatile compounds like $\alpha$-pinene and linalool [7]. Apart from the fact that these bioactive compounds are abundant and can serve as potent reducing agents in the production of metal nanoparticles with inherent antioxidant, antibacterial, anti-diabetic, and anti-inflammatory properties [7]. Similar to this, enormous volumes of solid trash known as bergamot pomace—which makes about 50–65% of the fruit weight—are produced during the commercial extraction of bergamot essential oil [8]. The high concentration of certain flavanones, such as neoeriocitrin, naringin, and neohesperidin, as well as complex glycosyl-flavones, make this residual biomass an exceptional source of bioactive compounds [8]. According to recent research, the maximal recovery of these powerful polyphenols can be achieved by using green extraction technologies, such as ultrasound-assisted techniques using environmentally friendly hydroalcoholic solvents [8]. These bergamot pomace extracts' strong antioxidant potential has been successfully used to fortify plant-based vegetable fats, greatly improving the oxidative stability and prolonging the shelf life of baked goods like biscuits, so successfully substituting artificial preservatives [9]. Bergamot pomace's extremely reactive antioxidant properties make it a perfect biological precursor for quick metal ion reduction in the context of green nanotechnology [8, 9]. Additionally, the production of cashew (Anacardium occidentale) yields massive amounts of cashew apple pseudofruit, up to 90% of the crop's total weight, which is typically thrown out in the field because it is extremely perishable [10]. Significant amounts of ascorbic acid, carotenoids, and complex polyphenols that provide an exceptionally potent antioxidant capability can be found in this underused residue [10]. A unique natural protein-polysaccharide complex has also been effectively isolated from leftover cashew apple bagasse thanks to recent developments using ultrasound-assisted extraction techniques [11]. This special macromolecular complex is mostly made up of arabinogalactan-proteins, which are rich in galactose and arabinose and have remarkable functional and rheological characteristics that are very similar to those of commercial gum Arabic [11]. Cashew apple residues have the potential to be excellent capping agents to avoid nanoparticle agglomeration due to their strong emulsification, high interfacial tension decrease, superior water retention, and stability characteristics [11]. At the end of it all, it is worth pointing out that the varied structural and chemical properties of all these agricultural leftovers underscore their enormous potential as highly effective yet inexpensive "bionanofactories." Indeed, scientists can readily facilitate the green synthesis of highly valuable bionanomaterials in a perfectly sustainable and environmentally friendly manner simply by leveraging the reusable complex protein-polysaccharide complexes of cashew apple bagasse, along with the flavonoid-containing skins of calamondin and bergamot. By converting environmentally harmful waste materials from nanoparticle synthesis into highly valuable resources with enormous potential for greatly enhanced nanoparticle stability, biocompatibility, and therapeutic activity, this approach is perfectly in harmony with the principles of the circular bio-economy.
2.2. Lignocellulosic Wastes: Straws, Rice Husks, and Sugarcane Bagasse
For the production of new materials, lignocellulose biomass, particularly the abundant agricultural residues available in the form of straws, rice husk, and sugarcane bagasse, is considered to be an extremely accessible and green source material [12]. This agricultural waste is composed of a hard complex matrix primarily composed of cellulose, hemicellulose, and lignin. This matrix is generally required to be fractionated or subjected to pretreatment in order to extract the biogenic precursors [12]. One of the significant by-products of sugarcane processing at a global scale, sugarcane bagasse is an “amazingly inexpensive source of biomass” [13]. It is an “excellent source of cellulosic microfibrils/nanofibers” since it contains 40-50% cellulose, 25-35% hemicellulose, and 20-30% lignin [13]. Also, the ash that is produced from the thermal digestion of sugarcane bagasse is highly rich in silica [14]. This ash can be sustainably transformed into highly functional silica and silicon nanoparticles using green synthesis, thus creating a non-toxic, energy-efficient alternative to conventional chemical synthesis [14]. Similarly, rice production on a global scale generates huge volumes of agricultural waste, mostly in the form of rice husk and straw [15]. Rice straw is a perfect precursor for extracting functional cellulose particles and creating biodegradable materials because of its large lignocellulosic matrix, which contains 32–47% cellulose and 19–31% hemicellulose [15]. Rice husks and rice straw are well known for their exceptionally high silica content, which goes beyond cellulose extraction; rice husk ash, in example, contains 86% to 97% silica [14]. As a result, these residues serve as excellent biological sources for the production of very reactive nanomaterials [14]. Critical worldwide waste management issues are directly resolved by the sustainable valorization of these complex agricultural wastes [13]. Industries can greatly reduce environmental pollution by converting heavy lignocellulosic residues into biogenic precursors, such as the complex fiber networks of sugarcane bagasse or the silica-dense matrix of rice husks and straws [15]. In the end, taking advantage of these inflexible waste structures opens the door to the scalable manufacturing of valuable bionanomaterials, which is highly consistent with the core ideas of a circular bioeconomy [12, 13].
2.3. Forest and Plantation Residues (e.g., Spent Coffea robusta grounds, Tea waste)
Forests and agricultural plantations are at the vanguard of the circular bioeconomy because they constitute a vast and sustainable reservoir of biogenic leftovers that can be easily converted into high-value functional materials [16]. An underutilized abundance of phytochemicals can be found in plantation by-products, such as those obtained from the extensive cultivation of rubber plants (Hevea brasiliensis) [16]. In particular, rubber seeds that have been rejected are incredibly rich in polyphenols and flavonoids, which can be effectively recovered employing cutting-edge green extraction techniques like microwave and ultrasonic assistance [16]. These woody plant materials and plantation residues are perfect, inexpensive precursors for the environmentally friendly biosynthesis of nanomaterials in the quickly developing field of green nanotechnology [17]. Strong secondary metabolites including phenolics, alkaloids, and terpenes are abundant in a variety of forest tree parts, from leaves, bark, and seeds to extracted latex and gums [17]. By acting as extremely potent natural reducing and stabilizing agents, these intrinsic biomolecules enable the quick creation of nanoparticles without the need of dangerous synthetic chemicals [17].
Large amounts of residual biomass, such as understory trees, trimmed branches, and bark from species like Scots pine and alder, are also produced by regular forest management and logging activities [18]. These logging residues are a powerful source of physiologically active extractives, like proanthocyanidins, which can provide engineered materials powerful inherent qualities [18]. In order to increase agricultural production, the post-extraction lignocellulosic biomass is upcycled into multipurpose organo-mineral fertilizers as part of a zero-waste cascade method that goes beyond simple extraction [18]. Industries can effectively create stable, highly functioning bionanomaterials while reducing waste disposal problems and promoting global sustainable development by repurposing the structural and chemical complexity of plantation and forest leftovers [17, 18].
3. Mechanistic Insights into Waste-Mediated Synthesis
3.1. Green Extraction Techniques for Active Phyto-metabolites
A crucial first step in the green synthesis of nanomaterials is the recovery of active phyto metabolites from plant biomass, which calls for extremely effective and environmentally friendly extraction methods [19]. This deliberate shift towards cutting-edge technologies such as UAE has been motivated by the fact that conventional techniques often require longer processing times and considerable amounts of solvents [19]. The use of high-frequency sound waves in UAE creates acoustic cavitation, which effectively breaks down rigid cell walls in plants and enhances the rate of penetration of solvents into the cell matrix [19]. The process has been shown to accelerate the rate of extraction of crucial phytochemicals, such as flavonoids, saponins, and alkaloids, without the use of harmful solvents. For instance, in the case where aqueous methanol is employed as a solvent in the application of UAE in the extraction of leaves from the Ligustrum lucidum plant, it has been shown to be effective in removing crucial functional groups such as hydroxyl and carbonyl groups, which are crucial in metal ion reduction and stabilization [19]. Moreover, UAE was efficiently used for the treatment of cinnamon bark powder in pure distilled water to form an extract that was rich in phenolic compounds with only a short sonication time [20]. UAE is an energy-efficient process that generates a strong reduction potential, which is needed for the rapid production of stable nanoparticles, as well as maintaining the structural integrity of thermosensitive antioxidants [20].
Figure 2. Mechanistic pathways of bioreduction and stabilization in the green synthesis of silver nanoparticles (AgNPs).
Strictly controlling the operational parameters is necessary for the maximization of the extraction efficiency of these useful secondary metabolites [21]. Advanced statistical tools, such as the Box Behnken design, have shown that temperature, contact time, and solvent pH are significant factors for the maximization of the total yield of polyphenols [21]. Research on green tea leaves indicates that there is significant enhancement in the polyphenol extract using hydroalcoholic solutions, i.e., ethanol/water solutions with moderately acidic or neutral pH levels, and at high temperatures up to 80 °C, rather than using pure water [21]. Researchers are always comparing UAE with other traditional extraction methods, as, despite the fact that UAE has shown promising time efficiency, the yield of UAE is highly influenced by the solvent system and time of extraction [22]. Extensive comparative analysis indicates that in using optimally balanced polar solvents, such as methanol/water, continuous hot solvent percolation using a Soxhlet apparatus can occasionally extract a higher concentration of bioactive compounds, e.g., from Ziziphus spina-christi leaves [22]. The long process of solvent recycling effectively breaks the hydrogen bonds and frees the securely bound phenolics that might not be extracted even with the application of continuous temperature stress in the Soxhlet extraction process [22]. On the other hand, in order to achieve similar yields, typical cold maceration requires longer times of soaking (up to 72 hours) [22]. In order to effectively isolate the desired phyto-metabolites, selection of the best green extraction method ultimately requires balancing processing time, thermal constraints, and particular solvent polarity [21,22].
3.2. The Role of Lignin, Cellulose, and Hemicellulose in Bioreduction
Lignocellulosic biomass, which is mainly made of a stiff matrix of cellulose, hemicellulose, and lignin, is an abundant, green, and renewable source of biological precursors of advanced green nanotechnology [23]. Each of the macromolecular components of the complex matrix plays an unique and crucial role in the stabilization of freshly synthesized nanoparticles and in the bioreduction of metal ions [23]. The most common linear homopolymer is cellulose, which is constructed of β(1-4)-linked D-glucose residues [23]. Cellulose provides superior structural support and functions as an important stabilizing agent during the bioreduction process despite the fact that it is not predominantly functioning as a reducing agent [23]. Targeted chemical treatments can be used effectively to separate pure flexible cellulose nanofibers from agricultural waste materials [24]. These highly hydrophilic cellulose networks form strong three-dimensional flexible structures that can be used to effectively bind and stabilize biogenic nanoparticles during the green synthesis of nanomaterials [24]. Strong nanocomposites with uniformly distributed bio-reduced silver nanoparticles can be synthesized by hybridizing these cellulose nanofibers with other materials, e.g., graphene oxide [24]. It maximizes the surface reactivity, structural stability, and antimicrobial activity of the nanoparticles while preventing their aggregation. On the other hand, a very direct and active chemical function in the synthesis process is played by lignin, the second most prevalent terrestrial biopolymer [25]. Methoxylated and hydroxylated phenylpropanoid units make up its intricate, highly branching, three-dimensional aromatic network [25]. Lignin has extraordinary natural antioxidant and electron-donating properties due to the very high concentration of phenolic hydroxyl, aliphatic hydroxyl, and carboxyl functional groups dispersed along its polymeric backbone [25]. These abundant free phenolic groups function as potent biological reducers, quickly transforming metal ions into extremely stable metal nanoparticles through basic chemical processes as single-electron transfer and hydrogen atom transfer [25]. Concurrently, the lignin macromolecule's complicated steric bulk acts as a very powerful capping agent to stop the nanoparticles from clumping together during the synthesis procedure [25]. Lastly, hemicellulose forms a closely related supporting carbohydrate network inside the lignocellulosic structure, whereas cellulose serves as the main structural stabilizer and lignin is a strong reducing agent [25]. Hemicellulose's essential synergistic interactions with lignin and cellulose within raw biomass collectively contribute to the highly favorable intrinsic reducing environment when crude plant extracts are used, despite the fact that it is often removed to isolate pure cellulose nanofibers for composite fabrication [25]. In the end, utilizing these natural macromolecules' synergistic chemical reactivities offers a productive, environmentally responsible route for the production of sustainable nanoparticles [24, 25].
3.3 Phenolic and Flavonoid Capping: Guaranteeing In Vivo Stability and Regulated Release
Plant-derived secondary metabolites, such as phenolics/flavonoids, have been reported to be excellent reducing/capping agents in the biogenic synthesis of metal nanoparticles, which form a protective layer around newly synthesized nanoparticles, thereby stabilizing them [26]. For instance, leaf extracts of Gardenia thailandica, which contain excellent amounts of quercetin 3-rutinoside and chlorogenic acid, have been reported to be excellent capping agents for stable silver nanoparticles with inherent antibacterial and antioxidant properties [26]. It has been widely acknowledged that this capping mechanism has been crucial in ensuring that nanoparticles do not agglomerate, thereby ensuring their stability in solution. Moreover, extremely stable nanoparticles have been synthesized using the aqueous extract of Cynara cardunculus, where the flavonoid capping has been reported to enhance their potent antifungal activity against multidrug-resistant isolates, including Candida auris through cell cycle arrest and mitochondrial damage, thereby highlighting their potent antifungal activity. [28]. The in vivo security of biogenic materials, apart from ensuring structural integrity, also requires a thorough evaluation of acute and subacute toxicities, similar to what has been recommended for new oral probiotics [29]. Phytocapping, through controlled release and regulated contact with biological membranes, plays a vital part in ensuring in vivo stability and overall biocompatibility, following a security evaluation. Active food coatings, depending on these nanoparticles, use this controlled release to prolong shelf life significantly, along with providing a broad spectrum of protection from infections and diseases [30]. The therapeutic stability of flavonoid-capped AgNPs has also been exceptional when tested in in vivo models of biomedical applications, such as a rat model with Staphylococcus aureus-infected wounds, effectively triggering epidermal regeneration and reducing inflammatory cell infiltration [26]. The therapeutic stability and overall biogenic materials are also extremely attractive for use in sophisticated tissue-engineered scaffolds, requiring strict physiological integration to facilitate intricate tissue reconstruction and prevent scar contraction, owing to the remarkable biocompatibility provided by natural capping agents [31]. These green-synthesised nanomaterials are ultimately transformed into safe, highly effective platforms for cutting-edge in vivo applications by the synergistic combination of phenolic caps and controlled release [26, 31].
3.4. Influence of Agro-waste Chemical Composition on AgNP Morphology and SPR
Functional nanomaterials' morphological and optical characteristics are largely determined by their precursors and chemical production environment. To effectively seal cracks and restore mechanical durability, the integration of autonomous healing agents in large-scale structural matrices, such self-healing concrete, requires exact chemical compatibility [32]. Similar to this, changing the chemical makeup of sophisticated photocatalytic systems—for example, by doping calcium titanate ceramics with lanthanum—directly controls structural phase changes and greatly accelerates the breakdown of organic dyes like Rhodamine B [33]. The chemical makeup of biological extracts serves as a potent structural direction in the environmentally friendly creation of nanoparticles. For example, the aqueous extract of Petasites hybridus rhizomes successfully regulates the uniform dispersion and spherical shape of multi component metallic nanocomposites, resulting in average particle sizes of 33 nm [34]. More precisely, the surface plasmon resonance (SPR) and particle size of silver nanoparticles (AgNPs) are significantly influenced by secondary metabolites originating from plants. Citrus paradisi peel extract, which has a clear SPR band absorption at 430 nm, is rich in flavonoids, hesperidin, and naringin, and can efficiently mediate the reduction and stabilization of chitosan-silver nanocomposites [35]. In addition, the concentration of these bioactive phytochemicals can greatly influence the optical properties of the nanocomposites. For instance, the use of leaf extracts from Azadirachta indica, which are rich in phenolics, alkaloids, and fatty acids, has been shown to produce strong SPR band absorbance at 400 nm when the concentration is high, while the absorbance shifts to 600 nm when the concentration is low, while maintaining the average size below 100 nm [36]. Finally, in order to obtain AgNPs with an average diameter of 38.41 nm and SPR at 420 nm, agricultural products or by-products, e.g., the fruit pericarp of Sandoricum koetjape, which is known for its high tannin and betacyanin content, could be used as effective reducing and capping agents [37]. Ultimately, using the chemical composition of these plant-based materials provides an environmentally friendly approach that is tunable for optimizing AgNP shape and SPR for numerous valuable purposes.
4. The "Nano-Wealth": High-Value Applications
4.1. Next-Generation Antimicrobial Agents (Targeting multidrug-resistant foodborne pathogens & biofilms)
The existing state of affairs in both the medical and agricultural fields faces a big challenge in the form of the emerging global problem of antibiotic resistance, for which new and creative therapeutic strategies need to be developed urgently [38]. The strains of multidrug-resistant pathogens have become common because of the uncontrolled use of conventional antibiotics, especially in animal farming and extensive agricultural operations [38]. The challenge to global infection control is compounded by this pervasive chemical pressure, which not only increases the escalation of bacterial resistance but also accelerates the escalation of resistance in many biological networks, e.g., different mosquito vectors [40]. Green nanotechnology has synthesized biogenic silver nanoparticles, which are very potent next-generation antibacterials that bypass the conventional biological resistance mechanisms that these tenacious pathogens have developed to address this dilemma [41]. Biogenic silver nanoparticles exhibit potent bactericidal properties through simultaneous physiological pathways, whereas conventional antibiotics generally affect the microorganism through a singular pathway [41]. When silver nanoparticles are introduced into the bacterial environment, they show high adherence potential to the bacterial wall, which adversely affects the membrane integrity and permeability of the bacterial membrane [42]. When the nanoparticles are internalized into the host cell, they rapidly induce oxidative stress through the production of reactive oxygen species [42].
Figure 3: Multi-targeted antimicrobial and anti-biofilm mechanisms of biogenic silver nanoparticles (AgNPs).
This is because oxidative stress is extremely high, leading to irreversible damage in the bacteria’s DNA, denaturation of structural components, and gradual destruction of key cellular enzymatic mechanisms, which ultimately leads to rapid apoptosis [41]. However, biogenic silver nanoparticles have been proved to be highly effective in dealing with persistent foodborne pathogens through these synergistic, multi-targeted mechanisms [43]. For example, Cymbopogon citratus, or lemongrass, is used in the phytosynthesis of silver nanoparticles that have been proved to have significant synergistic antibacterial activity against complex foodborne pathogens [43]. The phytochemicals in the lemongrass extract act as strong stabilizing caps in the bio-reduction of silver ions in an environmentally friendly manner [43]. Their remarkable efficacy as safe, natural preservatives created especially for reducing foodborne disease outbreaks and raising international food safety standards is demonstrated by their natural capping, which greatly improves biocompatibility [43]. In order to effectively address the current antibiotic resistance challenge, it is necessary to attack complex bacterial biofilms in addition to solitary, planktonic germs [44]. Biofilms, which are extremely complex, self-produced protective matrices made of structural exopolysaccharides, functional proteins, and extracellular DNA, are often formed by both aggressive foodborne and clinical pathogens [44]. This architectural barrier greatly reduces the penetration of traditional prescription antibiotics and also protects the confined bacterial colonies from harmful environmental stressors [44]. For thorough infection control, it is therefore imperative to carefully remove these structural impediments [44]. By specifically targeting and interfering with bacterial quorum sensing, green-synthesized nanomaterials serve as powerful antibiofilm and antivirulence agents [44]. The highly sophisticated cell-to-cell chemical communication network known as quorum sensing is in charge of managing population density adaption, controlling the collective production of harmful virulence factors, and coordinating the creation of first biofilms [44].
Recent uses of silver nanoparticles that are precisely phytosynthesized show impressive practical performance in totally destroying these resilient biofilm formations without causing additional evolutionary resistance [39]. Significant anti-quorum sensing action against virulent Gram-negative infections is demonstrated by silver nanoparticles that are effectively mediated by the aqueous leaf extract of Azadirachta indica (neem), which have a crystalline form with an average diameter between 20 and 43 nm [39]. These particular biogenic nanoparticles intentionally cause a significant 75% to 80% decrease in total structural biofilm mass in a highly dose-dependent manner and effectively prevent the pathogenic generation of essential virulence pigments [39]. In a similar vein, sophisticated biogenic nano-formulations have demonstrated remarkable efficacy against infamously stubborn biofilm formers like Pseudomonas aeruginosa [45]. These highly specific and tailored biogenic formulations have shown the capacity to effectively induce over 90% mature biofilm dispersal and dynamically attain up to 96% comprehensive biofilm inhibition [45]. This is achieved through the vigorous inhibition of the targeted manufacture of crucial extracellular matrices, such as the complex manufacture of functional rhamnolipids and the hard structural matrices of exopolysaccharides, respectively [45]. Additionally, complex molecular profiling is also identifying the exact genetic mechanisms that are responsible for this comprehensive eradication of biofilms. For example, through the specific downregulation of the genetic expression of crucial quorum sensing-associated regulatory genes, nanoscale silver particles, which are synthesized through the utilization of the unique biochemical reduction potential of Lantana camara leaf extracts, severely undermines the general integrity of biofilms. Specifically, for pathogenic strains of Pseudomonas aeruginosa, these green-manufactured nanoparticles vigorously constrain the crucial lasI and pqsA functional genetic pathways, respectively [46]. The use of biogenic silver nanoparticles is a novel approach to vigorously inhibit multidrug-resistant foodborne pathogens and persistent biofilm infections through the vigorous disarming of these complex biological communication networks, halting targeted transcription of crucial virulence genes, and methodically degrading highly protective matrices of exopolysaccharides.
4.2. Environmental Remediation (Catalytic degradation and biochar-supported wastewater treatment)
The prompt application of cutting-edge environmental restoration techniques and sustainable biological materials is crucial to successfully reducing extensive ecological effects. The extensive use of conventional synthetic plastics in contemporary agriculture and soil management settings results in severe, persistent polluting wastes that continuously deteriorate delicate terrestrial ecosystems [47]. By cleverly recycling copious agricultural waste into highly practical, environmentally acceptable alternatives, these extremely toxic conventional materials can be effectively substituted [47]. For example, strong, high-performing agri biodegradable mulch films are effectively produced by combining synthetic polyvinyl alcohol with natural, renewable lignin that is chemically removed from abandoned empty fruit bunches [47]. As they naturally break down over time, these cutting-edge sustainable bio-materials actively stop persistent soil contamination and enhance the organic profile of the terrestrial environment, totally eradicating the long-term buildup of dangerous microplastics in agricultural soils [47]. In order to effectively treat complicated toxic water contamination in aquatic systems, highly focused chemical removal procedures leveraging cutting-edge nanoscale extraction technologies are needed. In advanced wastewater treatment scenarios, highly porous, precisely designed nanomaterials, particularly metal-organic structures like zeolitic imidazolate frameworks (e.g., ZIF-8), exhibit remarkable chemical extraction performance [48]. These sophisticated nanomaterials function as highly effective visual self-reporting sensors and simultaneously extract persistent aquatic contaminants by deliberately encasing specific chemical guest molecules, such rhodamine B, directly into their crystalline structural framework [48]. In highly contaminated industrial and municipal water sources, they exhibit exceptional absorption performance in the selective removal of dangerous, extremely hydrophobic endocrine-disrupting compounds including bisphenol A and 4-nonylphenol [48]. However, the resolution of these complex environmental cleanup problems demands the continuous improvement of methodologies, exact technology implementation, and the rigid adherence to optimum operating procedures. The steep learning curve, practical training, and precise accuracy required to successfully minimize severe physiological complications of highly advanced and minimally invasive medical procedures, such as percutaneous endoscopic lumbar discectomy, are remarkably similar to this absolute requirement of technical precision and continuous improvement of operating procedures [49]. Recent large-scale ecological investigations have revealed that significant aquatic ecosystems, especially major national water reservoirs, are increasingly suffering from severe ecological impairments, some of which are irreversible [50]. This has further emphasized the need for such precise and highly effective environmental interventions. Excessive agricultural nutrients, ongoing heavy metal contamination, and very harmful sedimentation processes are seriously deteriorating these vital water reservoirs [50]. Moreover, the worldwide health concern is greatly exacerbated by the uncontrolled atmospheric proliferation of ambient air pollution. Human populations are greatly affected by the systemic physiological and psychological effects of fine particulate matter and toxic nitrogen dioxide; recent exhaustive epidemiological research has established a direct correlation between acute exposure to these particular air pollutants and the instantaneous, transient increases in daily hospital admissions for acute mental health disorders, such as generalized anxiety disorders [51]. In the end, using cutting-edge nanoscale extraction frameworks and aggressively utilizing sustainable agricultural by-products as a solution provides a very resilient, multifaceted technological solution to methodically reduce this ever-increasing burden of environmental pollution worldwide.
4.3. Nano-Agriculture (Seed nano-priming, nano-pesticides, and crop disease management)
The integration of nanotechnology in agriculture, otherwise known as nano-agriculture, is rapidly changing the current state of farming through the development of very advanced long-term solutions for improving crop hardiness, enhancing seedling establishment, and managing major plant diseases. Initially, the development of this revolutionary industry focused on replacing harmful traditional agrochemicals with bio-derived ones in order to proactively prevent serious environmental threats and combat the development of resistance in pests. For example, highly targeted nano-pesticides have been successfully synthesized by precisely encapsulating natural citrus peel essential oils in advanced polyethylene glycol nanoparticles [52]. This powerful, environmentally safe combination can greatly increase lethal ingestion toxicity against damaging invasive agricultural pests such as the tomato borer (Tuta absoluta), while reducing visual, accidental phytotoxicity to sensitive host plants [52]. After these early successes in pest management, nano-agriculture shifted its focus to directly control the internal physiological processes of the plant using advanced pre-sowing seed treatments. Intensive biochemical analyses have convincingly proved that pre-treatment of seeds with some metal oxide nanoparticles, such as aluminum oxide nanoparticles (Al2O3), can induce significant physiological and metabolic changes [53]. This specific nanoscale treatment enhances general vegetative plant growth, controls the levels of significant photosynthetic pigments, and significantly raises the activity of protective antioxidant enzymes in cultivated plants such as the Egyptian Roselle [53]. However, the practical efficacy of these technologically advanced nano-agricultural techniques is heavily dependent on the achievement of sufficient nanoparticle dispersion in complex heterogeneous natural environments. The use of specific polymer and organic coating materials on metal/metal oxide nanoparticles is of significant importance in avoiding agglomeration in diverse, mineral-rich agricultural soils [54]. The coating materials precisely control the dissolution rates of the nanoparticles and achieve the target zone of plant root growth for maximum cellular uptake and biological efficiency [54]. Through the successful mitigation of these key targeted delivery challenges, the viability of advanced nano-priming has been established as a forward-thinking approach for ensuring sustainable agriculture and actively defending vulnerable crops against a variety of severe abiotic environmental stresses [55].Notwithstanding these significant improvements in function, the traditional chemical-based manufacture of functional nanoparticles often leads to unforeseen phytotoxicity, cytotoxicity, and genotoxicity, which can take the form of very restricted plant growth and hindered root development. It is, therefore, obvious that a major paradigm shift in the world of science was witnessed in the creation of "green nano-priming," which is considered to be an extremely "environmentally conscious" method, relying on the inherent biochemical reduction potential of plant extracts [56]. The biological method is considered to be innovative, as it is capable of creating biocompatible nanoparticles with the help of non-toxic compounds, fulfilling the requirements of agricultural growth, characterized by high ecological standards [56]. Recent experiments conducted in the field have shown significant improvement in crop stress tolerance through the application of the "green synthesis technology." For example, "eco-friendly" seed nano-priming using biogenic zinc oxide (ZnO) and silicon dioxide (SiO2) nanoparticles was found to enhance the initial germination rate of seeds and plant vigor while effectively boosting the plant's internal antioxidant defense system against extreme conditions of drought [57]. Moreover, ongoing scientific developments in biological nanomaterials have resulted in the promising discovery of completely new, non-metal-based active agents for widespread agricultural application. New biological applications involving unique fluorescent carbon dots, derived from Spirulina algae, as a nano-priming tool have shown remarkable breakthroughs [58]. These highly active biological-derived nanoparticles have shown enhanced germination kinetics of important agricultural staples such as rice (Oryza sativa), improving crop establishment and early-stage metabolic processes [58]. Finally, the strategic application of these highly advanced and environment-friendly techniques of seed nano-priming along with the intelligent application of targeted nano-pesticides provides a multi-dimensional and highly effective approach to crop diseases, thereby ensuring food security on a global scale in the face of the rising threats of climate change. [59].
5. Critical Evaluation of Sustainability and Commercial Viability
5.1. Scalability of Agro-Waste Synthesis vs. Traditional Methods
The evaluation of the commercial viability of silver nanoparticles (AgNPs) requires a detailed comparison of traditional synthesis routes and newly developing biological techniques. Although green synthesis is considered a more sustainable option, life cycle analyses have shown that biological synthesis is not always more sustainable than chemical synthesis, often due to surprisingly large water or energy demands in the initial extraction stage [60]. To successfully reduce these hidden environmental footprints, the strategic reutilization of agricultural wastes, such as vegetable peels, acts as a highly efficient natural bionanofactory [61]. This waste-based strategy eliminates the requirement for harmful chemical solvents, minimizes precursor costs, and aligns with green synthesis strategies for silver nanoparticles using agricultural wastes as precursors [61].In contrast, the conventional chemical and physical synthesis methods are invariably energy-intensive, require complex purification techniques, and involve the use of hazardous reactive chemicals, which hinder their commercialization and environmental acceptability [62]. Finally, the move towards the application of agro-waste-mediated synthesis represents an environmentally sustainable and economically viable option compared to the conventional approaches, as long as the initial biological extraction processes are rigorously optimized to ensure the avoidance of excessive resource depletion during the scale-up processes [62].
5.2. Techno-Economic Analysis of Waste-to-Nanomaterial Processes
The significant investment costs involved in the process of conventional synthesis methods are often a major impediment for the wider acceptance of nanotechnology. For example, in chemical reduction methods costly chemicals are required, high-temperature conditions along with special equipment such as a probe sonicator are needed, which increases the overall investment costs significantly [63, 64]. In order to overcome these economic constraints, current techno-economic studies emphasize the prospects of using agricultural and biological wastes to set up sustainable bionanofactories [63]. Green synthesis techniques using particular plant extracts (e.g., BX3) can reduce the cost of silver nanoparticles to as low as $13,000/kg, making them about fifteen times cheaper than the standard sodium borohydride reduction [65]. Similarly, innovative production techniques, including the production of effervescent tablets using simple reagents, eliminate the requirement for electricity and the use of costly homogenizers, which can lead to significant cost savings in terms of project costs [64]. However, the cost-effectiveness of the green method is greatly dependent on the biological precursor used. For example, the use of green tea as the precursor demands an excessive amount of silver nitrate, thus increasing the cost [65]. Finally, the use of cost-effective precursors derived from waste is important in the commercialization of nanostructured materials [65].
5.3. Life Cycle Assessment (LCA): Quantifying the Environmental Footprint
Life Cycle Assessment (LCA) is an essential, methodical approach to quantify the entire environmental footprint of the production process of nanomaterials and their integration into commercial products [66]. In order to prospectively assess the precise environmental implications of the diverse applications, especially the widespread use of nanosilver-based textile products, researchers primarily make use of specialist LCA databases, especially Ecoinvent, and sophisticated software tools [66]. The detailed analysis through the entire life cycle of the nanomaterials unequivocally proves that these innovative techniques are not inherently environmentally friendly, although the plant-mediated or biological methods are generally and extensively promoted as inherently "green" alternatives to the traditional chemical production methods [67]. The detailed comparison analysis clearly proves that the biosynthesis of noble metal nanoparticles, especially gold and silver, is generally and extensively misnomered as "green" unless confirmed through detailed, cradle-to-nanoparticles life cycle analysis [67]. These detailed analyses are extremely important, especially because they actively avoid the unintentional "shift" of environmental/toxicological consequences to other, less obvious phases of the nanomaterial's life cycle, and they comprehensively take into account all the input factors, hidden energy requirements, and outcome uncertainties. [67].
Life Cycle Assessment (LCA): Quantifying the Footprint on the Environment Advanced comparative life cycle analyses can be employed for comparing and contrasting significantly different approaches to various manufacturing processes, thus enabling an accurate quantification and deconvolution of the complex impact of such different approaches on the environment [68]. For instance, recent comparative and contrastive life cycle analyses of significantly different approaches to the synthesis of titanium dioxide nanoparticles, including green biosynthesis approaches, have utilized hybrid sustainability metrics for quantifying the impact of such different approaches to nanoparticle synthesis on the environment [68]. Such accurate evaluations enable a holistic assessment of the actual impact on the environment by carefully studying various key factors, such as direct energy consumption, constant waste generation, and specific toxicity of all chemical and biological precursors [68]. It is, therefore, imperative for strict and standardized approaches to life cycle assessment to be implemented in order to ensure that future developments in green nanotechnology have a tangible impact on reducing the impact of human activities on the environment instead of merely replacing one hidden risk with another. [67, 68].
6. Current Bottlenecks and Future Perspectives
6.1. Standardization Challenges of Highly Variable Biowaste
Silver nanoparticles can also be synthesized through biological methods by the use of microorganisms, plant extracts, and biomolecules as green reducing agents [69]. Despite the obvious green advantages associated with the process, there are still barriers to the standardization of the synthesis methods [70]. The intrinsic variability of the biological materials, including the specific plant species, seasonal growth conditions, and heterogeneous chemical composition of the extracted biowaste, leads to large inconsistencies in the size, morphology, and biological activity of the formed nanoparticles [70]. Moreover, differences in extraction techniques and variable kinetic interaction conditions during synthesis hinder the establishment of widely accepted synthesis protocols [69]. The absence of accepted standards has a direct impact on reproducibility and scalability of synthesis for industrial-scale production [70]. In response to the variability of biowaste, statistical optimization techniques such as experimental design have been successfully used to optimize synthesis conditions for consistent nanoparticle quality [70].
6.2. Ecotoxicity, Pharmacokinetics, and Safe-by-Design Approaches
From a pharmacokinetic point of view, biogenic silver nanoparticles (AgNPs) have shown promise for use in therapy, such as improving the pharmacokinetics of heavy metals by reducing blood lead levels in vivo by significant amounts [71]. However, for large-scale use, it is essential to employ Safe by Design (SbD) principles to reduce environmental and health risks. Sophisticated multi-criteria decision analysis techniques, for example, can help optimize green synthesis routes for AgNPs, reducing environmental impact in terms of climate change, human toxicity, and improving antibacterial activity [72]. Knowledge of the pharmacokinetics of nanoparticles is important for safety assessment because the distribution and clearance of nanoparticles are size-dependent. Nanoparticles with diameters less than 20 nm can be rapidly cleared via the kidney, while larger nanoparticles can accumulate in the liver and spleen, facilitating hepatic clearance [73]. Considering that long-term exposure to silver nanoparticles (AgNPs) poses substantial hazards, including genotoxicity, hepatotoxicity, and neurotoxicity, Safe-by-Design (SbD) strategies and in vivo safety profiling are essential for the safe application of nanoparticles.
6.3. Regulatory Frameworks (e.g., EFSA, FAO, EPA guidelines) for Biogenic Nanomaterials
Establishing strong and robust regulations is a key requirement for the clinical and industrial application of biogenic silver nanoparticles and existing regulations have to be updated to overcome the challenges faced by green synthesis [74]. Though chemical synthesis of nanoparticles is a standard procedure, biogenic nanoparticles have faced major problems regarding batch-to-batch reproducibility and uniformity and decision-making strategies are required for the sustainability and functional control of the production process [74]. Moreover, the implementation of "greener" practices is vital for enhanced resilience in the long term, in addition to meeting global sustainability targets [75]. Even though these nanoparticles hold considerable therapeutic potential for treating infected wounds caused by biofilms, the translation of these materials in a clinical context is hindered by a significant "valley of death" between lab-scale data and the stringent demands of a regulated environment [76]. Overcoming this "valley of death" necessitates the development of unified regulations that address sustainability targets in addition to exhaustive safety evaluations, thus promoting the safe and effective translation of these materials from the lab to the market [75, 76].
CONCLUSION
Biogenic silver nanoparticles (AgNPs) derived from agricultural and forestry waste materials have come to be regarded as revolutionary entities with considerable biological and agricultural implications. Within the context of the health care sector, the capping of biogenic AgNPs with phyto-metabolites has been recognized as having powerful and multifaceted capabilities for interfering with persistent bacterial biofilms and fighting the rising global pandemic of multidrug-resistant pathogens. Green nanotechnology is catalyzing a paradigm shift in agriculture due to the role of eco-friendly nano-pesticides and advanced seed nano-priming techniques in developing crops with stress tolerance capacity, effectively managing severe diseases in plants and offering long-term environmental remediation solutions including advanced wastewater treatments. Despite the high-value application potential, the commercial viability of waste-mediated synthesis must be optimized. Techno-economic analyses have suggested that the use of biowaste-based precursors can significantly reduce the cost of synthesis compared to traditional synthetic routes; however, the inherent chemical complexity of plant-based materials is associated with significant standardization issues and batch-to-batch consistency issues. In addition, detailed Life Cycle Assessments show that true industrial scale-up depends on the improvement of the initial stages of extraction to reduce hidden energy and water footprints, making the biological pathway truly sustainable. In closing the significant gap between innovation and application, the strict application of the guidelines for the application of Safe-by-Design (SbD) and the development of regulatory standards are crucial. Lastly, the move from the production of hazardous chemicals to the application of "bionanofactories" through agro-waste is not just the application of an alternative synthesis route. It is the defining future of the circular economy, masterfully converting the ecological liabilities of the world into nano-wealth for the health and resilience of the environment and human populations.
ACKNOWLEDGEMENTS
The authors gratefully acknowledge their institute for academic support and research encouragement. The constructive comments and discussions from colleagues greatly helped improve the quality of this review article.
Competing interests
The authors declare no competing financial interests or personal relationships that could influence the work reported in this manuscript
Authors’ Contributions
Sasanka Hazarika designed the study, performed the statistical analysis, wrote the protocol, Rupjyoti Kalita wrote the first draft of the manuscript. Sushmita Chamuah and Nirban Barman managed the analyses of the study. Emlang Sayoo managed the literature searches.
REFERENCES
Sasanka Hazarika* Rupjyoti Kalita, Sushmita Chamuah, Nirban Barman, Emlang Sayoo, From Waste to Nano-Wealth: Exploiting Agricultural By-products for the Sustainable Synthesis of Silver Nanoparticles, Int. J. of Pharm. Sci., 2026, Vol 4, Issue 5, 204-644. https://doi.org/10.5281/zenodo.19981075
10.5281/zenodo.19981075