Role of Plant Phytochemicals in Reduction, Nucleation and Stabilization of Metallic Nanoparticles in Green Synthesis

The rapid advancement of nanotechnology has profoundly transformed multiple scientific and industrial sectors, including medicine, agriculture, catalysis, and environmental remediation. Metallic nanoparticles (NPs), particularly those composed of silver, gold, zinc, palladium, copper, iron, and titanium, have attracted substantial attention owing to their unique physicochemical properties, such as high surface-area-to-volume ratios, enhanced catalytic activity, tunable optical behaviour, and pronounced antimicrobial and therapeutic efficacy¹. These properties arise primarily from nanoscale dimensions and surface-dominated phenomena that are absent in bulk materials^2,3.

As a result, the controlled synthesis of metallic nanoparticles with defined size, morphology, surface chemistry, and long-term stability remains a central focus of nanomaterials research. Conventional synthesis routes are broadly classified into physical and chemical approaches. Physical methods, including laser ablation, evaporation condensation, and ball milling, require high energy input, sophisticated instrumentation, and substantial capital investment⁴. Chemical synthesis methods, while more scalable and tunable, often rely on hazardous reducing agents such as sodium borohydride, hydrazine, or organic solvents that pose environmental and biological risks.Furthermore, residual toxic reagents adsorbed on nanoparticle surfaces can severely restrict biomedical and agricultural applications^5,6.

These limitations have driven increasing interest in environmentally benign synthesis strategies aligned with the principles of green chemistry. Among biological approaches, plant-mediated synthesis has emerged as one of the most promising alternatives due to its simplicity, low cost, rapid reaction kinetics, and avoidance of pathogenic microorganisms ^7,8.

Plants synthesize a wide range of secondary metabolites commonly referred to as phytochemicals that are not directly required for growth but play critical roles in defence, signalling, and stress adaptation. According to the British Nutrition Foundation classification, these metabolites are broadly grouped into terpenoids, phenolics, nitrogen containing compounds, and sulphur-containing compounds⁹. Importantly, plant extracts contain diverse phytochemicals capable of reducing metal ions and stabilizing nanoparticles under mild conditions without high temperature, pressure, or toxic reagents ¹⁰.

Numerous studies demonstrate that phenolic compounds, flavonoids, carboxylic acids, carbohydrates, terpenoids, alkaloids, and proteins are the primary phytochemicals involved in nanoparticle biosynthesis. Their functional groups including hydroxyl, carbonyl, carboxyl, amine, and thiol moieties enable coordination with metal ions, electron donation, and surface passivation. Notably, these compounds often function simultaneously as reducing agents, nucleating agents, and stabilizers, integrating roles that are otherwise performed by separate reagents in conventional chemical synthesis ^2,11.

Despite extensive reporting of successful plant mediated nanoparticle synthesis,  most studies remain descriptive, focusing on plant species , nanoparticle types ,nanoparticle formation and basic characterization rather than mechanistic roles of phytochemicals ^5,10 .Critical aspects such as reduction kinetics, nucleation pathways, facet selective growth, and longterm stability are often attributed vaguely to “plant extract effects,” without identifying the specific phytochemicals or functional groups responsible ^12,13

This review emphasizes the chemical functions of phytochemicals in reduction, nucleation, growth, and stabilization of metal nanoparticles, providing a predictive framework that transcends botanical classification ^14–16. The tabulated analysis is restricted to studies published between 2023 and 2025 to highlight recent advances in analytical methods and mechanistic understanding^16–18 .While earlier work is cited selectively to provide context and support mechanistic inferences^2,19,20 .The review aims to (i) elucidate how key phytochemical classes mediate nanoparticle formation, (ii) identify recurring mechanistic trends across metals, and (iii) highlight gaps for rational design and scalable green synthesis.

 Phytochemical Classification in Plant Mediated Nanoparticle Synthesis

In plant mediated green synthesis of metallic nanoparticles, phytochemicals are the principal bioactive compounds that reduce metal ions, direct nucleation and growth and stabilize the resulting nanoparticles. Unlike conventional chemical methods that use separate toxic reducing and capping agents, plant extracts serve multifunctional roles, thanks to their diverse metabolite profiles²¹. These metabolites can generally be classified into major groups based on their chemical structure and functional roles in nanoparticle formation polyphenols, flavonoids, terpenoids, alkaloids, sugars and polysaccharides, proteins and organic acids as shown in Table 1-3.

 Polyphenols and Phenolic Acids

Polyphenols are among the most influential phytochemicals in green nanoparticle synthesis. Characterized by multiple phenolic hydroxyl (–OH) groups attached to aromatic rings, polyphenols readily donate electrons to metal ions such as Ag?, Au³?, and Pd²?, facilitating their reduction to zero-valent metals ^10,22 .The oxidation of phenolic –OH groups to quinone or carbonyl derivatives provides the electrons required for metal ion reduction and initiates nanoparticle nucleation.

Phenolic compounds also exhibit strong metal chelating capacity due to the nucleophilic nature of deprotonated hydroxyl and carboxylate groups. In aqueous media, –OH and –COOH moieties can ionize to O? and COO? species, which electrostatically interact with positively charged metal ions, enabling electron transfer and reduction^10,12. The resulting oxidized phenolic derivatives frequently remain adsorbed on nanoparticle surfaces, acting as capping agents that confer colloidal stability.

Phenolic acids such as gallic, caffeic, and tannic acid are repeatedly implicated in mechanistic studies. Their redox potential and metal-reducing efficiency depend on the number and position of hydroxyl and carboxyl groups, which directly influence reduction kinetics and nanoparticle size distribution^11,23

 Flavonoids

Flavonoids constitute a major subclass of polyphenols and are widely implicated in plant mediated nanoparticle synthesis. Structurally defined by a C?–C?–C? framework, flavonoids possess multiple hydroxyl and carbonyl functionalities that enable strong metal chelation and redox activity ^22,24. Flavonoids reduce metal ions through electron donation and hydrogen abstraction mechanisms, often involving keto–enol tautomerism. Coordination typically occurs via hydroxyl groups at C3, C5, or C7 and the carbonyl group at C4, forming stable flavonoid metal complexes that facilitate reduction and controlled nucleation ^19,25.Mechanistic studies demonstrate that flavonoids such as quercetin, orientin, and catechin reduce Ag? and Au³? ions via complex intermediate formation, followed by oxidation to quinone derivatives that remain bound to nanoparticle surfaces, providing stabilization and growth control ^26,27.Alkaline conditions enhance flavonoid mediated reduction by increasing phenolic deprotonation and electron density, accelerating nucleation at ambient temperatures ¹¹.

Terpenoids

Terpenoids, derived from isoprene units, represent a structurally diverse class of secondary metabolites containing hydroxyl, carbonyl, aldehyde, and carboxyl functional groups. These moieties underpin their ability to participate in redox reactions during nanoparticle synthesis ²⁸.Terpenoid rich extracts have been shown to reduce Ag?, Au³?, and Zr?? ions through oxidation of alcohol groups to carbonyl derivatives, accompanied by electron transfer ^29,30. In addition to reduction, terpenoids often act as steric stabilizers due to their bulky and hydrophobic structures, influencing nanoparticle morphology and aggregation behaviour^29,31.

Alkaloids

Alkaloids, though less extensively studied, participate in nanoparticle synthesis through nitrogen lone pair donation and deprotonation driven electron transfer ^15,32. Alkaloid mediated reduction has been reported for silver, palladium, and platinum nanoparticles, although mechanistic understanding remains limited.³²

Sugars and Polysaccharides

Reducing sugars such as glucose and fructose contribute to nanoparticle synthesis via mild redox pathways involving their aldehyde or ketone functional groups. Although their reduction kinetics are slower than those of polyphenols, sugars often act synergistically with stronger reductants in plant extracts ³³. Polysaccharides, including starches and cellulose derivatives, primarily function as stabilizing and capping agents, forming a physical barrier that prevents aggregation and provides colloidal stability. Their high molecular weight and multiple binding sites enable effective surface coverage and enhance longterm dispersion stability ^17,34.

 Proteins, Amino Acids and Other Biomolecules

Proteins, peptides and amino acids present in plant extracts contribute to nanoparticle synthesis through functional groups such as. These groups can bind metal ions and participate in both reduction and capping processes. Amino acids and peptides can complex with metal surfaces, enhancing stability and modulating nanoparticle growth³⁵.

Proteins peptides and amino acids contribute predominantly to nanoparticle stabilization through amine (-NH?), carboxyl (-COOH), and thiol (-SH) side chains .These moieties coordinate strongly with metal surfaces, forming organic coronas that suppress aggregation and improve biocompatibility ¹⁷. Amino acids such as cysteine, containing thiol groups, exhibit particularly strong metal-binding affinity and can promote localized reduction at coordination sites³⁵.

 Organic Acids and Alkaloids

Organic acids, including citric, malic, and tartaric acids, act as metal chelators and mild reducing agents, frequently serving as coreductants alongside polyphenols and flavonoids ³.Alkaloids, though less extensively studied, participate in nanoparticle synthesis through nitrogen lone pair donation and deprotonation driven electron transfer. Alkaloid mediated reduction has been reported for silver, palladium and platinum nanoparticles, although mechanistic understanding remains limited .The presence of these compounds often results in biocompatible nanoparticle surfaces, which is advantageous for biomedical applications^4,22.

 Phytochemicals as Reducing Agents

Plants provide a chemically rich milieu that readily supports the biosynthesis of Ag?, Au³?, Pd²?, and others into zero valent metal atoms (M?). The biosynthetic process generally follows three stages electron driven reduction of Ag? to form small clusters, growth of these clusters into nuclei, and subsequent enlargement into nanoparticles each stage being strongly influenced by the chemical nature and concentration of extract components, as well as reaction conditions (pH, temperature, precursor concentration). Crucially, the ability of a plant extract to generate and stabilize is governed not by taxonomic identity but by the functional groups present in its constituent biomolecules ⁸. A critical assessment of  phytochemical mediated nanoparticle synthesis studies published between 2017 and 2025 demonstrates that phenolic compounds and flavonoids are the most frequently implicated phytochemicals involved in both metal ion reduction and nanoparticle stabilization, irrespective of plant species or metal precursor ³⁶.This convergence across diverse botanical systems indicates that nanoparticle biosynthesis is dictated by shared chemical functionalities rather than taxonomic origin. Such evidence supports a chemistry driven framework for understanding green nanoparticle synthesis, wherein functional groups govern reduction kinetics, nucleation behaviour and colloidal stability.

 Phenolic Compounds and Polyphenols

Phenolic compounds are also recognized for their strong metal chelating capacity, which arises from the high nucleophilicity of their aromatic rings. Functional groups such as hydroxyl (–OH) and carboxyl (–COOH) moieties play a central role in mediating metal ions reduction via electron or hydrogen donation. In aqueous solution phenolic –OH and carboxylate groups can deprotonate to yield nucleophilic O? or COO? species that electrostatically interact with metal ions ⁶. These interactions facilitate electron transfer or hydrogen donation from the phytochemicals to the metal ions, resulting in the reduction of metal ions X? to elemental X? and the subsequent nucleation of X nanoparticles ¹⁰.

Phenolic acids such as gallic acid, caffeic acid and tannic acid, the number and position of hydroxyl and carboxyl groups influence the redox potential and electron donation capacity of phenolic molecules, thereby directing reduction kinetics and nanoparticle characteristics ⁵.The phenolic redox process involving Au³? ions is proposed to proceed via a transient metalpolyphenol complex, followed by oxidative conversion of the phenolic moieties into quinone derivatives. These oxidized quinone species adsorbs onto the nanoparticle surface, conferring long-term colloidal stability by preventing particle aggregation ².Salvia sclarea extract polyphenols showed successful Au³?  reduction and surface binding via hydroxyl/carbonyl groups ³⁷. In a Pd/Au (bimetallic) system polyphenols (OH rich compounds) showed reduction and surface capping ³⁸. Polyphenols assisted (gallic acid type phenolics) ZrO? nanosythesis observed a similar reduction and surface stabilization ⁶.

Ag? while being oxidized to its corresponding quinone form. The resulting oxidized functional groups, particularly carboxyl and carbonyl moieties, coordinate with the silver nanoparticle surface, thereby acting as capping agents that enhance dispersion stability. Reductive electron donation  and surface capping via adsorbed polyphenols was observed in mint leaf extract phenolics (Silver) Ag? nanosynthesis ³⁹.Many other mechanistic processes rely on the of presence of  both phenols and flavonoids in their extracts as shown in table 1. ¹⁶ identified Tannins / Phlorotannins   (Bistortaside, Agrimol A phenolic compounds with multiple OH, C=O on the surface of Embelia laeta extract mediated silver nanoparticles AgNPs with Strong chelation, nucleation control and steric stabilization of the nanoparticles. Additionally secondary redox mediation and surface adsorption by  Anthraquinones / Chromones (Emodin-6,8-dimethyl ether, Isoaloeresin A )Conjugated to C=O, –OH groups was confirmed by UPLC-QTOF-MS.

 In ZnO nanosythesis systems phenolic acids including gallic, chlorogenic, p-coumaric, vanillic acids and rutin participated in electron donation for Zn²? reduction and surface stabilization^18,40.

However, in copper (Cu) based nanosystems, polyphenols initially form coordination complexes with Cu²? ions, enabling their reduction to metallic copper nanoparticles (Cu?). Subsequent oxidative conversion in the presence of atmospheric oxygen yields thermodynamically stable copper oxide (CuO) nanoparticles. The reduction, capping and surface stabilization of Copper (Cu²?) to CuNPs by eucalyptus globulus polyphenols was confirmed by FTIR peaks for –OH, C=O ⁴¹.This pathway underscores the combined chelating, reducing and redox buffering roles of polyphenols in nanoparticle biogenesis ³.Lignin based polyphenolic structures were involved in CuO reduction and surface passivation ⁴² table 1. At a fundamental level, phenol mediated nanoparticle synthesis is governed by the oxidation of hydroxyl (–C–OH) groups to carbonyl (C=O) functionalities, which supplies the electrons required for metal ion reduction (e.g., Ag? to Ag?). The concomitant formation of keto or quinone derivatives represents the oxidative counterpart of this process, providing both mechanistic consistency and surface stabilization across diverse metal systems ^2,20.

Comparative studies employing isolated phytochemicals alongside complex plant extracts provide critical mechanistic insight into plant mediated nanoparticle synthesis. Investigations contrasting pure gallic acid with polar leaf extracts demonstrate that simple phenolic compounds are sufficient to drive metal oxide nanoparticle nucleation and surface stabilization, yielding highly crystalline and homogeneous nanostructures^22,43. In contrast, complex phytochemical mixtures primarily modulate crystal growth, surface area, porosity and acidity through differential thermal decomposition during calcination. These findings suggest that while polyphenols dominate redox mediated nanoparticle formation, coextracted secondary metabolites act as auxiliary structure directing and surface modifying agents rather than primary reductants. The successful formation of zirconia nanoparticles using a single, well defined phenolic compound challenges the prevailing assumption that complex phytochemical mixtures are required for green nanoparticle synthesis²⁷.

 Flavonoids

Structurally, they possess a characteristic C?–C?–C? framework comprising two phenyl rings (A and B) linked by a heterocyclic oxygen-containing ring (C), which may exist in open or closed forms depending on substitution patterns ²⁴.This structural architecture underpins their redox activity and metal binding capacity.

The presence of multiple hydroxyl and carbonyl functionalities enables flavonoids to act as both metal chelators and electron donors. Coordination typically occurs through bidentate or multidentate interactions involving hydroxyl groups at positions C3, C5, or C7 and the carbonyl group at C4, forming stable complexes with transition metal ions such as Ag?, Au³?, Cu²?, Fe³? and Zn²?^{15,29,32,44,45} . These interactions increase metal ion reactivity and facilitate subsequent reduction. The nucleophilic nature of the aromatic rings and delocalized π-electron systems further enhances electron transfer to metal ions, initiating nanoparticle nucleation. Reduction mechanisms often involve keto-enol tautomerism, in which hydrogen atoms or electrons released from hydroxyl groups reduce metal ions to their zero valent state (e.g., Ag? → Ag?, Au³? → Au?)^24,46.

UPLC-QTOF-MS and FTIR analysis identified the presence of (flavonoids (Quercetagetin, 3′-Hydroxypuerarin on the surface of Embelia laeta mediated silver nanoparticles AgNPs through OH, C=O, C–O surface coordination complexation ¹⁶.

In aqueous systems, trivalent gold ions, in particular, form intermediate flavonoid  metal complexes that undergo oxidative transformation, leading to controlled nucleation and growth of gold nanoparticles ¹⁹.  Lupinus Albus extract flavonoids exemplify these mechanistic roles. Orientin, containing multiple phenolic  hydroxyl groups, forms coordination complexes with Ni²? ions; subsequent thermal treatment decomposes the organic matrix, yielding NiO nanoparticles. ⁴⁵. Quercetin, characterized by high reductive potency due to its catechol and carbonyl functionalities, reduces Ag? ions through complex intermediate formation followed by hydrogen abstraction and oxidation to hydrated quinone species²⁵.A generalized mechanistic model for flavonoid mediated nanoparticle synthesis has been proposed using silver as a representative system. In this framework, hydroxyl groups serve as strong ligands and reducing centers, converting Ag? to Ag?. Alkaline conditions, such as the addition of sodium hydroxide, enhance deprotonation of phenolic groups, thereby increasing electron density and accelerating reduction kinetics at ambient temperature (Sherin et al., 2020). Overall, the effectiveness of flavonoids as reducing agents is governed by their functional group density, substitution pattern, and π-electron delocalization, which collectively determine metal-binding affinity, reduction potential, and nanoparticle stabilization efficiency.  

Terpenoids

Terpenoids, also referred to as isoprenoids, constitute a structurally diverse class of plant secondary metabolites derived from isoprene (C?) units. Based on carbon number, they are classified as hemi- (C?), mono- (C??), sesqui- (C??), di- (C??), tri- (C??), tetra- (C??; carotenoids), and polyterpenes (C?, n > 40) ²⁸ .This structural diversity is accompanied by a wide range of oxygenated functional groups, including hydroxyl, carbonyl, aldehyde and carboxyl moieties, which underpins their redox activity in nanoparticle synthesis.

Numerous studies have demonstrated the ability of terpenoid rich plant extracts to mediate the reduction of metal ions to their zero valent states. For instance, terpene fractions isolated from Lantana camara have been shown to directly reduce Ag? ions to metallic silver nanoparticles ⁴⁷. Similarly, Withania coagulans contains polyoxygenated steroidal triterpenoids (withanolides) that participate in silver nanoparticle formation through coordinated metal binding and electron transfer ^48,49

In gold nanoparticle synthesis, terpenoids from Annona squamosa interact with Au³? ions via hydroxyl and carbonyl groups, forming transient complexes that undergo oxidative transformation. The reduction of Au³? to Au? is driven by oxidation of these functional groups, highlighting the role of terpenoids as sacrificial electron donors ⁵⁰. Comparable behavior has been reported for pentacyclic terpenoids such as lupeol and β-sitosterol isolated from Euclea natalensis, which facilitate the formation of zirconium oxide nanoparticles.

In this system, hydroxyl mediated enol -keto tautomerization is proposed to release hydrogen atoms or electrons that drive metal ion reduction, followed by oxide formation upon thermal or oxidative treatment ⁵¹. Classical studies using geranium leaf extracts identified terpenoids particularly eugenol as highly active redox agents capable of reducing both silver and gold ions via direct electron donation¹¹.

Similarly, in iron, zinc oxide and silver nanoparticle synthesis with a Polyphenols, flavonoids and terpenoids mixture terpenoids were identified to act mainly auxiliary in stabilization via surface coordination by OH functional groups^29,52,53. In these studies the oxidation of alcohol, aldehyde groups,π-electron and carbonyl interaction with metal surface providing  steric stabilization were identified by UV-Vis and FTIR carbonyl bands ⁵³.At the mechanistic level, the reductive capacity of terpenoids is primarily attributed to the oxidation of alcohol functionalities to carbonyl groups, with concomitant electron transfer to metal precursors such as Ag?, Au³?, or Fe??¹¹.

Sugars and Carbohydrates

Sugars and carbohydrates, while generally weaker reducing agents than polyphenols or flavonoids, contribute meaningfully to nanoparticle synthesis through mild redox pathways. Reducing sugars such as glucose and fructose contain aldehyde or ketone functionalities capable of donating electrons to metal ions, particularly under alkaline conditions or elevated temperatures. Although their reduction kinetics are comparatively slow, these molecules can initiate metal ion reduction and often act synergistically with stronger phytoreductants present in plant extracts. In systems involving Ag?, Zn²? and  ternary Ag?O/CuO/ZnO glucose, fructose, dextrans  primarily stabilize particles and prevent aggregation  confirmed by FTIR (Aldehyde, ketone, –OH), UV-Vis and high negative zeta potential enhancing colloidal stability via surface binding of organic groups ^46,54 . The capping and  dispersion enhancement of Saponins and glycosides (–OH, glycosidic bonds) through stable SPR peaks were identified on the surface of Embelia laeta mediated silver nanoparticles AgNPs ⁵⁵.

Proteins and Amino Acids

Proteins and amino acids present in plant extracts contribute to nanoparticle formation primarily through their diverse functional groups, including amine (NH?), thiol (SH), carboxyl (COOH), and carbonyl (C=O) moieties. These groups can interact with metal ions via coordination and, in some cases, facilitate electron transfer leading to reduction. Amino acids containing thiol groups, such as cysteine, exhibit particularly strong metal binding affinity, promoting nucleation through localized reduction at coordination sites^17,35.

 In a (Neem leaf extract) consisting of phenolics, flavonoids and proteins involving silver nanoparticle synthesis .analytical studies proved that reduction , nucleation and capping is driven by hydroxyl and carbonyl containing phytochemicals while  stabilization ( surface bound functional groups (phenols, amides proteins, alcohols) ¹⁷.

Metal chelation, electrostatic stabilization driven by peptides and nitrogen containing metabolites (Lyciumin A, Indolylmethyl glucosinolate on the surface of Embelia laeta mediated silver nanoparticles AgNPs were identified through (NH, C–N, –S–) via UPLC-QTOF-MS, and FTIR. This further validates the statement that peptides and amino acids contribute predominantly to nanoparticle stabilization through amine (-NH?), carboxyl (-COOH), and thiol (-SH) functional groups⁵⁵. These moieties coordinate strongly with metal surfaces, forming organic coronas that suppress aggregation and improve biocompatibility ¹⁷. Amino acids such as cysteine, containing thiol groups, exhibit particularly strong metal binding affinity and can promote localized reduction at coordination sites³⁵ .

Other Small Biomolecules: Organic Acids and Alkaloids

Organic acids such as citric, malic and tartaric acids contribute to nanoparticle biosynthesis through metal chelation and mild reduction mechanisms. Their multiple carboxyl and hydroxyl groups enable coordination with metal ions and facilitate electron transfer, particularly in conjunction with stronger reductants such as polyphenols and flavonoids. These acids frequently act as coreductants and stabilizers, influencing particle size and dispersion. In systems involving Zn²?, Ag? nanosynthesis by citric, tartaric, malic acids ,FTIR carboxylate shifts (–COOH, –OH) confirmed the mechanism of metal ion chelation, electron donation, influence on nucleation, particle growth stabilization via surface adsorption ⁵⁶.The presence of fatty acids and Fatty alcohols (Nonadecanoic acid, Heptadecadienetriol) confirmed by the presence of (–COOH, –OH) functional groups on the surface of Embelia laeta mediated silver nanoparticles (AgNPs) via UPLC-QTOF-MS confirms their Surface modification and aggregation suppression ability ⁵⁵.

Alkaloids

Earlier studies indicated limited mechanistic understanding of alkaloid mediated nanoparticle synthesis with relatively few studies addressing reaction pathways, intermediate species, or structure activity relationships ^57–59.

In green nanoparticle synthesis ,alkaloids being nitrogen containing secondary metabolites characterised by heterocyclic nitrogen atoms their role is primarily attributed to metal ion chelation, controlled nucleation and surface stabilization, rather than strong reducing capacity. Spectroscopic and compositional analyses from representative plant systems suggest that heterocyclic nitrogen atoms, hydroxyl substitutions, and conjugated π-electron systems enable alkaloids to coordinate metal ions and stabilise intermediate metal complexes during nanoparticle formation ^15,60,61.

Mechanistically, alkaloids can interact with metal ions through lone pair donation from nitrogen atoms, forming transient metal ligand complexes that facilitate nucleation and limit uncontrolled particle growth. In some systems, weak redox activity associated with phenolic or hydroxylated alkaloids contributes to partial metal ion reduction, while structural rearrangements and deprotonation enhance surface binding and passivation. Chelation driven mechanisms have also been proposed in alkaloid rich extracts where nanoparticle formation proceeds via metal hydroxide intermediates, highlighting a role in hydrolysis-assisted nucleation pathways ^31,62.

Evidence across a wide range of metal and metal oxide systems indicates that alkaloids play a consistent, though often auxiliary, role in phytochemical-mediated nanoparticle synthesis by contributing to metal ion coordination, controlled reduction, and surface stabilization .

In cobalt oxide nanoparticle synthesis, alkaloids present alongside flavonoids and phenolics facilitate cobalt salt reduction by forming transient metal ligand complexes, while their heterocyclic nitrogen atoms promote surface adsorption and post-reduction stabilization of CoO nanoparticles⁸. . In zinc oxide nanoparticle biosynthesis, alkaloids act in concert with terpenoids, flavonoids, tannins, and plant gums, where nitrogen-containing heterocycles assist in Zn²? complexation and nucleation control, while bulky organic matrices enhance steric stabilization and suppress aggregation of ZnO nanoparticles ³¹. Similarly, in copper oxide nanoparticle formation, alkaloids synergize with polyphenolic compounds to facilitate Cu²? reduction and subsequent surface capping. While flavonoids and tannins provide primary redox activity, alkaloids contribute to surface passivation through coordination interactions that limit particle growth and enhance colloidal stability ³².In nickel oxide systems, alkaloids such as berberine have been shown to participate directly in both reduction and stabilization, where their conjugated ring structures and protonatable nitrogen atoms enable electron donation to Ni²? ions and persistent adsorption onto NiO surfaces, resulting in stabilized nanoparticles with reduced aggregation tendencies ⁴⁵.In polyherbal systems used for Ag/ZnO nanocomposites, alkaloids act synergistically with phenolics and flavonoids, where weak redox activity supports partial Ag? and Zn²? reduction, and nitrogen-containing functional groups contribute to surface passivation by coordinating exposed metal sites ¹⁵.More complex ternary systems such as Ag?O/CuO/ZnO further highlight the stabilizing dominance of alkaloids. Here, alkaloids participate primarily in nucleation control and capping, binding to nascent metal oxide surfaces through lone pair donation and electrostatic interactions, thereby suppressing aggregation and enabling stable nanocomposite formation in conjunction with tannins, saponins, and carbohydrates ¹³. Comparable mechanistic roles are observed in cobalt oxide, Ag/ZnO, and ternary oxide nanocomposites, where alkaloids primarily influence nucleation density, surface chemistry, and post reduction stabilization, rather than acting as dominant reducing agents^8,13,15.

Collectively, available studies indicate that alkaloids function mainly as secondary reductants and surface stabilizing agents, with their strongest influence occurring during nucleation and post reduction stabilization of nanoparticles across various metal oxide systems. Compared with polyphenols and flavonoids, alkaloids exhibit weaker reducing power and greater structural diversity, suggesting that their primary role lies in metal ion coordination, controlled nucleation, and surface passivation. The limited mechanistic resolution of alkaloid mediated pathways highlights the need for advanced spectroscopic and kinetic studies to clarify structure function relationships^31,32.

Cross Study Trends in Plant Mediated Nanoparticle Synthesis

The repeated involvement of phenolics, flavonoids, terpenoids, carbohydrates, proteins, and alkaloids across a wide range of plant mediated nanoparticle synthesis studies provides a strong rationale for organizing this review according to phytochemical classes rather than botanical sources. Such a mechanistic classification enables clearer interpretation of structure function relationships, highlights shared redox and coordination pathways, and facilitates comparison across metal systems²⁶. This approach not only reduces redundancy but also enhances the predictive value of green synthesis strategies. A comparative evaluation of plant mediated nanoparticle synthesis studies published between 2017 and 2022 reveals a striking convergence in the classes of phytochemicals responsible for metal ion reduction and nanoparticle stabilization¹¹.. Despite the extensive botanical diversity of plant species employed, the same groups of biomolecules namely phenolic compounds, flavonoids, terpenoids, carboxylic acids, carbohydrates, alkaloids, and proteins are repeatedly implicated across independent reports^23,44,62,63. This consistency strongly indicates that nanoparticle formation is governed primarily by shared chemical functionalities rather than plant taxonomy. The recurrence of identical functional groups across unrelated plant systems provides compelling evidence for a conserved, chemistry driven mechanism underlying green nanoparticle synthesis

Unified Mechanism of Phytochemical Mediated Nanoparticle Formation

Across plant mediated nanoparticle synthesis systems, functional group chemistry emerges as the dominant determinant of reduction efficiency and nanoparticle stability. Hydroxyl (–OH), carbonyl (C=O), carboxyl (–COOH), amine (–NH?), and thiol (–SH) groups recurrently serve as active sites for metal ion coordination and electron transfer ²³. These moieties facilitate redox transformations by donating electrons or hydrogen atoms, reducing metal precursors such as Ag?, Au³?, Cu²?, Fe³?, and Zn²? to their zero valent or oxide forms. Simultaneously, the oxidized biomolecules adsorb onto nanoparticle surfaces, acting as intrinsic capping agents that prevent aggregation and promote colloidal stability¹⁰. The repeated involvement of these functional groups across diverse plant extracts underscores a universal structure function relationship that transcends species specific biochemical complexity. Understanding these mechanisms is critical for rational design of plant based synthesis protocols, enabling control over reaction kinetics, nanoparticle size, shape and functional properties, which are essential for targeted applications in medicine, catalysis, sensing and agriculture^26,35,36.

To consolidate these mechanistic insights, Table 1 summarizes representative examples of plant mediated nanoparticle synthesis reported between 2021 and 2025, organized according to metal type, dominant phytochemical class and primary functional role (reduction, stabilization, or dual functionality). This synthesis oriented presentation highlights cross system trends and reinforces the central role of phytochemical functionality in governing nanoparticle formation, growth and stability.

Table 1. Plant mediated synthesis of metal and metal oxide nanoparticles reported between 2023 and 2025, highlighting dominant phytochemical classes and their mechanistic roles.

While Table 1 summarizes dominant phytochemical classes implicated in plant mediated nanoparticle synthesis across recent studies .Table 2 provides a compound resolved mechanistic case study, illustrating how chemically distinct phytochemicals within a single extract cooperatively drive reduction, nucleation and stabilization processes.

Although all compounds were derived from a single Embelia laeta extract ⁵⁵ they were categorized according to their dominant mechanistic roles in nanoparticle synthesis. This functional classification highlights the synergistic contribution of chemically distinct phytochemicals during reduction, nucleation and stabilization processes rather than emphasizing botanical origin

Table 2. Phytochemicals identified on the surface of Embelia laeta mediated silver nanoparticles AgNPs and their proposed mechanistic roles.

UPLC-QTOF-MS- Ultra-Performance Liquid Chromatography-Quadrupole Time-of-Flight Mass Spectrometry. 

Metal Specific Trends in Phytochemical Mediated Nanoparticle Formation

Analysis of the literature further reveals metal specific preferences in phytochemical involvement, reflecting differences in redox potential, coordination chemistry and surface energetics of metal precursors. Silver nanoparticle synthesis is most frequently associated with phenolic compounds, flavonoids and carbohydrates, whose hydroxyl- and carbonyl rich structures efficiently reduce Ag? ions while simultaneously capping nascent particles ^{1,9,16,20,22,39,44,49,57,63,67,68}.

Zinc oxide nanoparticle formation commonly involves phenolics, organic acids and terpenoids, which facilitate controlled hydrolysis and oxide growth ^{15,18,27,31,35,40,66,66}.Gold nanoparticle synthesis is predominantly mediated by flavonoids, terpenoids and proteins, reflecting the strong affinity of Au³? ions for π-electron systems and heteroatom containing ligands^{14,19,37,38,50,62,63,69} .In contrast, copper ^7,32,41,42and iron^29,53 based nanoparticles are often stabilized by phenolic compounds and proteins, which provide robust surface passivation and prevent oxidative aggregation. These trends highlight that while phytochemical functionality is broadly conserved, its relative contribution is modulated by metal specific coordination and redox requirements.

Table 3 Phytochemical functional groups involved in reduction, capping, and stabilization of plant mediated Nano synthesis

4. Phytochemical Control of Nucleation, Growth and Stabilization in Plant Mediated Nanoparticle Synthesis

Following the initial reduction of metal ions into zero valent atoms, nanoparticle formation proceeds through nucleation and growth stages that ultimately define particle size distribution, morphology, crystallinity, and colloidal stability. In plant-mediated synthesis, these stages are not passive but are actively regulated by phytochemicals present in the extract, whose functional groups interact dynamically with metal atoms, clusters, and developing nanoparticle surfaces (Table 3).

 Role of Phytochemicals in Nucleation

Nucleation begins when reduced metal atoms reach a supersaturation threshold and condense into thermodynamically stable nuclei. The frequency and uniformity of nucleation events are strongly influenced by phytochemicals capable of chelating metal atoms or binding to nascent clusters. Polyphenols and flavonoids, enriched with hydroxyl and carbonyl groups, readily adsorb onto newly formed nuclei, reducing surface energy and promoting the formation of a larger number of smaller nuclei. This mechanism typically results in nanoparticles with reduced mean size and narrower size distributions, as consistently reported for silver, gold, zinc oxide, and copper-based systems (Table 1) ^11,14.

Compound resolved evidence from Embelia laeta extract (Table 2) further demonstrates that chemically distinct phytochemicals within a single extract act cooperatively during nucleation. Strong chelators such as flavonoids and tannins primarily drive Ag? reduction and nucleus formation, while auxiliary metabolites modulate early-stage surface stabilization ¹⁶. This functional differentiation underscores that nanoparticle nucleation is governed more by phytochemical composition and functional group availability than by botanical origin alone.

 Phytochemical Regulation of Growth and Morphology

Once nuclei are formed, subsequent particle growth is dictated by the selective adsorption of phytochemicals onto specific crystallographic facets. Flavonoids and tannins exhibit preferential binding to particular crystal planes via π–π interactions, hydrogen bonding, and metal ligand coordination, thereby altering relative growth rates along different axes. This facet-selective adsorption explains the frequent observation of anisotropic morphologies such as rods, plates, or cubes in plant-mediated systems, in contrast to the predominantly spherical particles produced by uncontrolled chemical routes^17,39,41.

Terpenoids and organic acids further influence growth kinetics by moderating metal atom diffusion and inhibiting Ostwald ripening. Their adsorption onto growing nanoparticles limits atom migration from smaller to larger particles, preserving smaller sizes and reducing polydispersity (Table 3) ^28,30,49,52.These mechanisms are particularly evident in ZnO, ZrO?, and CuO systems summarized in Table 1, where growth regulation is closely linked to the redox and coordination chemistry of the metal precursor.

The relative concentration and ratio of phytochemical classes play a decisive role in growth dynamics. High concentrations of strong chelators promote rapid, frequent nucleation, whereas larger or weaker binding molecules such as polysaccharides favor slower, diffusion controlled growth. This interplay explains why similar reaction conditions can yield markedly different nanoparticle characteristics when extract composition varies ⁴⁶.

 Phytochemicals as Stabilizing and Capping Agents

Following nucleation and growth, newly formed nanoparticles possess high surface energy and are inherently prone to aggregation. In green synthesis, stabilization is achieved through the adsorption of phytochemicals that form an organic capping layer, imparting steric and electrostatic repulsion. Smaller phytochemicals such as flavonoids and phenolic acids contribute both to late stage reduction and to surface stabilization by forming metal–ligand complexes on nanoparticle surfaces (Table 3). Their aromatic systems engage in π–π interactions with metal clusters, suppressing aggregation and maintaining colloidal stability^22,71.

Phenolic compounds consistently emerge as universal capping agents across multiple metal systems including Ag, Au, Cu, and Zn highlighting a conserved stabilization pathway in phytochemical-mediated nanoparticle synthesis (Table 1). Coordination of ionized hydroxyl and carboxylate groups with surface atoms increases negative surface charge, enhancing electrostatic repulsion and improving long-term dispersion stability, as reflected by high negative zeta potential values⁹.

Large biomolecules such as proteins, polysaccharides and tannins predominantly provide steric stabilization. These macromolecules bind through multiple functional groups including amines, carboxylates, thiols, and hydroxyls forming a physical corona around the metal core that prevents close particle particle contact (Table 3). Evidence from compound resolved studies (Table 2) confirms that peptides and nitrogen containing metabolites primarily act as post-reduction stabilizers rather than dominant reducing agents ^12,55.

Proteins, in particular, play a central role in stabilization across diverse systems. Following metal ion reduction, proteins rapidly adsorb onto nanoparticle surfaces, forming a stabilizing organic corona that modulates surface charge, enhances biocompatibility, and governs colloidal behavior. Empirical studies using Camellia sinensis, Carissa carandas, and Moringa oleifera extracts demonstrate that proteins and amino acids contribute mainly to capping and dispersion stability rather than direct metal ion reduction ^11,17.

Implications for Nanoparticle Design

Capping efficacy directly influences nanoparticle shelf life, surface reactivity, and biological interactions. For example, ZnO nanoparticles synthesized using Ocimum tenuiflorum extracts exhibited superior dispersibility and reduced surface defects compared with chemically synthesized counterparts, attributable to flavonoid and polyphenol mediated capping (Table 1).The nature of the capping layer also determines zeta potential and governs nanoparticle biomembrane interactions, which are critical for antimicrobial and drug delivery applications.

Collectively, the integration of Tables 1–3 demonstrates that nanoparticle properties arise from the synergistic and stage specific roles of phytochemicals in reduction, nucleation, growth and stabilization. Systematic identification and quantification of these capping molecules are therefore essential for rationally designing plant mediated nanoparticles with predictable physicochemical and biological performance^9,72.

6. Influence of Reaction Parameters on Phytochemical Activity

The interaction between phytochemicals and metal ions, and the resultant nanoparticle features, are strongly  modulated by reaction parameters such as pH, temperature, extract concentration, and metal precursor concentration. These physicochemical conditions influence the redox activity, ionization state, molecular interactions and structural stability of phytochemicals, which in turn determine the yield, size distribution, morphology and stability of the synthesized nanoparticles⁷³.

Influence of pH on Phytochemical Activity in Nanoparticle Synthesis

The pH of the reaction medium is a decisive factor governing the activity of phytochemicals during green synthesis of nanoparticles (NPs). Phytochemicals such as flavonoids, phenolic acids, alkaloids, terpenoids, and proteins serve dual roles as reducing and stabilizing agents. Their ionization state, reactivity, and binding affinity to metal ions are strongly pH-dependent, which in turn affects nucleation, growth, stability, and functional performance of nanoparticles ^11,74.

At acidic pH (≤ 3), phytochemicals tend to exist in a protonated state, which suppresses their electron-donating ability. This reduces their efficiency in reducing metal ions (e.g., Au³?, Ag?) into their zerovalent forms ²². However, strong protonation can enhance intermolecular hydrogen bonding and reduce electrostatic repulsion, contributing to temporary stabilization of dispersed particles. Despite this, the limited nucleation centers formed under highly acidic conditions often result in fewer but larger nanoparticles, primarily due to aggregation and uncontrolled growth ¹⁰.

In the moderately acidic range (pH 3–6), nanoparticle systems typically exhibit reduced stability. At this pH range, phytochemicals are partially ionized, leading to insufficient electrostatic repulsion between particles. Consequently, van der Waals attractive forces dominate, promoting aggregation. This aggregation reduces colloidal stability and often leads to polydisperse nanoparticle populations.Additionally, many phenolic compounds exhibit diminished redox potential in this pH range, further limiting efficient nanoparticle formation⁷⁵

At or near neutral pH (around pH 7), phytochemicals achieve an optimal balance between protonation and deprotonation. This enhances their reducing capacity and surface-binding ability, facilitating efficient nucleation and controlled growth of nanoparticles¹². At this pH, many systems approach their isoelectric point, where the net surface charge is minimal; however, sufficient steric stabilization from phytochemical capping agents can still prevent aggregation. Empirical studies frequently report that nanoparticles synthesized at neutral pH exhibit uniform size, better dispersion, and enhanced catalytic activity, such as in the reduction of 4-nitrophenol ^3,32,46

Under alkaline conditions (pH > 7), phytochemicals undergo deprotonation, increasing their electron density and significantly enhancing their reducing power. This leads to rapid formation of a large number of nucleation centers, resulting in smaller and more uniformly distributed nanoparticles ⁶⁰. Additionally, increased negative surface charge due to deprotonated functional groups (e.g., –OH, –COO?) generates strong electrostatic repulsion between particles, thereby improving colloidal stability and preventing aggregation. However, excessively high pH may lead to overly rapid nucleation, causing irregular morphology or incomplete capping^5,56. Representative findings: gold and silver systems showed different SPR behavior and particle sizes across pH ranges (e.g., smaller AuNPs at pH 3–4 vs larger at pH 2⁴⁹ stable AuNPs at pH 11, 18–30 nm) [41,45⁷⁶ Garcinia mangostana gave smaller AgNPs as pH increased from 4 to 7 (32.7 → 7.12 nm) ⁷⁷; Tragopogon collinus produced smaller, homogeneous AgNPs at pH 10 ⁷⁸. Optimal pH therefore depends on the extract chemistry and target metal.Careful optimization of pH is therefore essential to tailor nanoparticle size, morphology, and functional properties. In phytochemical-mediated synthesis, neutral to mildly alkaline conditions are generally preferred for achieving stable, small-sized, and functionally active nanoparticles ^14,76.

Influence of Temperature on Phytochemical Activity in Nanoparticle Synthesis

Temperature is a critical physicochemical parameter that significantly influences the activity of phytochemicals during nanoparticle (NP) synthesis. In comparison to physical methods (>350 °C) and chemical methods (<350 °C), green synthesis typically operates under milder conditions, usually below 100 °C, which helps preserve the structural integrity and functionality of bioactive compounds ⁷⁹. Temperature directly affects reaction kinetics, nucleation rate, growth dynamics, and ultimately the physicochemical properties of the synthesized nanoparticles.

At lower temperatures (around 20–30 °C), the kinetic energy of the system is relatively low, which slows down the reduction of metal ions by phytochemicals. As a result, fewer nucleation centers are formed, leading to the production of larger and often anisotropic nanoparticles due to prolonged growth phases⁷⁷. For example, phytochemical-mediated synthesis using plant extracts at lower temperatures has been shown to yield triangular or irregularly shaped nanoparticles, reflecting controlled but slow reduction processes ⁷³.

At moderate temperatures (30–60 °C), phytochemicals exhibit enhanced reducing capacity due to increased molecular motion and collision frequency. This promotes a balance between nucleation and growth, resulting in more uniform and improved dispersion of nanoparticles. At this range, phytochemicals such as polyphenols and flavonoids maintain structural stability while efficiently reducing metal ions, leading to controlled synthesis of spherical or mixed morphology nanoparticles ^75,80.

At elevated temperatures (60–100 °C), the rate of reduction of metal ions increases significantly due to enhanced activation energy and faster electron transfer from phytochemicals ^25,39,80. This leads to the rapid formation of a large number of nucleation centers, producing smaller, uniform, and stable nanoparticles. Increased also enhances the solubility and diffusion of reactants, improving yield and reaction completeness. Empirical observations indicate that higher temperatures correspond to increased optical absorbance, reflecting higher nanoparticle concentration⁷⁸.

However, excessively high temperatures (>100 °C) can adversely affect phytochemical activity. Many bioactive compounds, particularly phenolics and proteins, are thermolabile and may undergo degradation or denaturation at elevated temperatures⁸¹. This compromises their dual role as reducing and stabilizing agents, potentially leading to particle aggregation, irregular morphology, or incomplete reduction of metal ions ¹¹. Therefore, maintaining temperature within an optimal range is essential to preserve phytochemical functionality and ensure efficient nanoparticle synthesis.

Temperature governs the efficiency and mechanism of phytochemical activity in nanoparticle synthesis. Mild to moderately elevated temperatures (typically 30–100 °C) are optimal for achieving rapid reduction, controlled nucleation, and stable nanoparticle formation, while preserving the integrity of plant-derived bioactive compounds ^74,75,79

Extract concentration

Plant extract concentration is a key determinant in phytochemical-mediated nanoparticle synthesis, as it directly influences particle size, morphology, yield, and stability by regulating the availability of bioactive compounds that act as reducing and stabilizing agents. Generally, increasing extract concentration enhances the availability of phytochemicals such as flavonoids, phenolics, and proteins, thereby accelerating the reduction of metal ions and increasing nucleation rates.This often leads to the formation of smaller and more stable nanoparticles at optimal concentrations, although beyond a certain threshold, excessive extract can promote particle growth and aggregation due to overlapping nucleation and growth processes ^29,73.For instance, in the synthesis of gold nanoparticles using Ginkgo biloba leaf extract, increasing the extract volume from 2 to 8 mL resulted in a blue shift in the surface plasmon resonance (SPR) peak, indicating a reduction in particle size⁸². Similarly, increasing Anogeissus latifolia gum concentration from 0.1% to 0.5% enhanced the reduction rate of palladium ions and favored the formation of smaller nanoparticles, with optimal synthesis observed at 1 mmol/L PdCl? ⁸³. In another example, higher concentrations of T. collinus leaf extract increased the availability of secondary metabolites, producing smaller and more stable nanoparticles, while Hibiscus sabdariffa extract concentration influenced the size of ZnO nanoparticles⁸⁴.

The concentration of plant extract also plays a crucial role in determining nanoparticle morphology. For example, in Aloe vera-mediated synthesis, low extract concentrations produced larger, triangular-shaped gold nanoparticles, whereas higher concentrations resulted in smaller, spherical particles. Similarly, variations in extract concentration have been shown to alter nanoparticle shape in multiple plant systems⁸⁵. Optimal concentrations are also necessary for achieving stable nanoparticles, as demonstrated by red carrot extract, where a 5% concentration produced stable ruby-red gold nanoparticles⁷⁹. However, the relationship between extract concentration and nanoparticle size is not always linear. While moderate increases in extract concentration typically promote rapid nucleation and smaller particle formation, excessive concentrations may accelerate reduction to the extent that particle growth mechanisms such as Ostwald ripening and coalescence dominate, resulting in larger nanoparticles⁵². This phenomenon is illustrated in studies using Gomphrena globosa, where low extract concentrations produced silver nanoparticles with an average size of 15.64 nm, while higher concentrations increased the size to 22.16 nm⁸⁶. Thus, plant extract concentration must be carefully optimized to balance nucleation and growth processes. Overall, these studies demonstrate that extract concentration controls nanoparticle synthesis by modulating reduction kinetics, nucleation density and stabilization efficiency, making it a critical parameter for tailoring nanoparticle characteristics in green synthesis approaches ^73,87

Influence of Reactant/Metal Ion Concentration on Phytochemical Activity in Nanoparticle Synthesis

The concentration of metal ions (precursors) in the reaction system is a critical parameter that directly influences phytochemical activity during green synthesis of nanoparticles (NPs). Phytochemicals present in plant extracts act as reducing, capping, and stabilizing agents, and their effectiveness is strongly dependent on the ratio between available metal ions and bioactive compounds ⁸⁸

At low metal ion concentrations, the availability of ions for reduction is limited, which results in fewer nucleation centers. Under such conditions, phytochemicals can efficiently reduce and stabilize the available ions, often producing small, well-dispersed and monodisperse nanoparticles due to sufficient capping by plant metabolites. However, excessively low concentrations may lead to low yield and incomplete nanoparticle formation, as nucleation remains insufficient ²²

At optimal concentrations around 1 mM for Ag? systems), a balance is achieved between metal ion availability and phytochemical reducing capacity. This promotes controlled nucleation and growth, resulting in nanoparticles with uniform size,morphology, and high stability. In such conditions, phytochemicals efficiently cap the nanoparticle surface, preventing aggregation and ensuring reproducibility¹⁵. Experimental observations have shown that appropriate precursor-to-extract ratios enhance both reduction efficiency and nanoparticle yield².

At high metal ion concentrations, the reduction capacity of phytochemicals becomes insufficient relative to the excess ions present. This leads to rapid and uncontrolled nucleation, followed by aggregation due to inadequate capping of newly formed nanoparticles ¹⁶.Consequently, larger, polydisperse, and unstable nanoparticles are formed. For instance, increasing silver ion concentration in plant-mediated synthesis has been shown to increase particle size significantly and promote aggregation ⁸⁹. Similarly, excess reducing agents (e.g., NaBH? in chemical systems) can disrupt controlled growth, resulting in irregular or network-like nanostructures rather than discrete particles⁹⁰.

The type and chemical nature of the precursor also influence phytochemical activity. Different metal salts such as chlorides, nitrates, acetates exhibit varying reduction potentials and interaction affinities with phytochemicals, thereby affecting reduction kinetics and nanoparticle characteristics ⁵⁶. Moreover, the extract-to-metal ion ratio plays a crucial role; higher concentrations of phytochemicals relative to metal ions improve stabilization and prevent aggregation, while insufficient phytochemical content leads to incomplete reduction and instability ⁴.

Precursor concentration affects available nuclei and the demand on phytochemical reducers/cappers, low concentrations may limit yield, moderate concentrations permit controlled nucleation and monodispersity. For instance, Ag? ≈1 mM often cited as suitable), and high concentrations overwhelm capping capacity, causing rapid nucleation followed by aggregation and larger, polydisperse particles. (Coleus aromaticus: Ag? ≤1 mM gave efficient NPs; higher led to aggregation)⁹¹. The chemical form of the precursor also matters (chlorides vs nitrates vs acetates) because salt identity influences reduction potential and coordination with biomolecules. Maintaining an appropriate precursor: extract ratio is therefore critical to avoid partial reduction or unstable precipitates.

Therefore, achieving an optimal balance between precursor concentration and phytochemical availability is essential for producing nanoparticles with desired size, morphology, stability, and functional performance. Controlled concentrations favor efficient reduction, proper capping, and formation of stable, monodisperse nanoparticles ⁷³Reaction Time on Phytochemical Activity in Nanoparticle Synthesis

Reaction time is a fundamental kinetic parameter that governs the progression of phytochemical-mediated nanoparticle (NP) synthesis. It directly influences nucleation, growth, maturation, and stabilization processes, thereby determining the  physicochemical properties of nanoparticles, including size, yield, morphology, and stability (Ahmed et al., 2016; Singh et al., 2016).

Effect of Reaction Time on Particle Size

Reaction time plays a dual role in determining nanoparticle size. During the initial phase, rapid reduction of metal ions by phytochemicals leads to the formation of numerous nucleation centers, often producing small-sized nanoparticles⁷⁵.For instance, rapid synthesis observed within minutes in plant-mediated systems results in nanoclusters with sizes in the lower nanometer range.

However, with prolonged reaction time, particle growth becomes dominant over nucleation, leading to increased particle size due to mechanisms such as Ostwald ripening and coalescence ³⁷. Empirical evidence shows that extending reaction time (e.g., from 30 minutes to several hours) can increase nanoparticle size significantly, such as the growth of silver nanoparticles from ~10 nm to 35 nm ⁷⁹. Thus, shorter reaction times favor smaller particles, whereas longer durations promote particle enlargement.

Effect on Nanoparticle Yield

Time also controls yield up to the point of reagent depletion; for instance, Ananas comosus extracts produced detectable Ag nanoclusters within 2 min and increasing absorbance over time indicated continued particle formation , Garcinia indica showed an optimal incubation at 24 h for AgNPs whereas AuNPs formed in under 2 h in the same system ⁹²and Azadirachta indica yielded 10–35 nm AgNPs when reaction time extended from 30 min to 4 h  Storage time likewise affects stability: some suspensions remain stable for weeks (limited by capping availability), while others aggregate on prolonged standing⁶⁸

 As time progresses, more metal ions are reduced into their zerovalent state, resulting in an increase in nanoparticle concentration. This is often evidenced by the gradual increase in UV–Vis absorbance intensity over time⁸⁰.

For example, systems using plant extracts such as Ananas comosus and Garcinia indica demonstrate a steady increase in nanoparticle formation with time, with optimal yields often achieved within specific durations (e.g., 1–24 hours depending on the system).^79,93However, beyond the optimal reaction time, the yield may plateau due to depletion of metal ions or phytochemicals.

Effect on Morphology (Shape and Structure)

Reaction time significantly influences nanoparticle morphology. During early stages, rapid nucleation can result in anisotropic or irregular shapes due to uncontrolled growth dynamics. As the reaction proceeds, these structures may evolve into more thermodynamically stable morphologies, such as spherical nanoparticles ⁷³.

Extended reaction times may also induce shape transformations, where nanoparticles undergo restructuring to minimize surface energy. However, excessive durations can lead to polydispersity and irregular morphologies, as continuous growth and aggregation disrupt uniformity ^82,93. For instance, different incubation times have been shown to produce spherical nanoparticles at shorter durations and larger, less defined structures at extended times⁷³.

. Effect on Stability of Nanoparticles

Stability is highly dependent on the balance between nanoparticle growth and phytochemical capping over time. At optimal reaction times, phytochemicals effectively stabilize nanoparticles by forming a protective layer, resulting in stable colloidal suspensions that can persist for weeks³².

However, prolonged reaction or storage time can negatively impact stability. Over time, depletion of capping agents or changes in the chemical environment may lead to aggregation, sedimentation, or even particle shrinkage⁷³. Additionally, continued interaction between nanoparticles and residual phytochemicals may alter surface properties, affecting long-term stability.

 Kinetic Considerations and Optimal Time Window

The rate of nanoparticle biosynthesis reflects the balance between nucleation and growth processes. A fast reaction rate typically produces a large number of nuclei but may limit overall yield, whereas slower reactions allow for gradual growth and increased particle⁷³. Therefore, identifying an optimal reaction time window is crucial to achieving desired nanoparticle characteristics without compromising stability or uniformity.

In practice, these parameters are interdependent: raising temperature can compensate for lower extract concentration to some extent by increasing reduction kinetics, but the same temperature may accelerate phytochemical degradation and change optimal pH; similarly, increasing metal concentration without adding capping capacity or shortening reaction time often increases polydispersity^80,93. Therefore empirical optimization for each plant-metal pair is essential starts by screening extract concentration, precursor concentration, temperature, pH and short reaction times (minutes to hours), monitor UV–Vis and TEM for size/morphology changes, then refine toward the window that yields the desired size, shape, yield, and colloidal stability observed in the cited studies (e.g. Annona squamosa L ⁵⁰. Embelia laeta¹⁶ , Psidium guajava (Linn.)²⁹, Salvadora persica⁵³.

7. Characterization Techniques and Insights

Comprehensive characterization of nanoparticles is essential to determine their physicochemical properties, including particle size, morphology, crystallinity, and surface chemistryas shown in Table. A combination of spectroscopic, microscopic, and analytical techniques is commonly employed to obtain detailed structural and functional information. For instance, ultraviolet–visible spectroscopy is widely used to monitor nanoparticle formation through surface plasmon resonance, while electron microscopy techniques such as transmission electron microscopy and scanning electron microscopy provide direct visualization of particle morphology. In addition, techniques such as dynamic light scattering and zeta potential analysis are used to evaluate particle size distribution and colloidal stability. Structural and chemical characterization is further achieved through X-ray diffraction and Fourier transform infrared spectroscopy, which reveal crystallinity and surface functional groups involved in nanoparticle stabilization⁷³. Collectively, these techniques not only verify nanoparticle formation but also help dissect the chemical interactions between phytochemicals and metal surfaces essential for mechanistic understanding.

Table: 4 Common Techniques for Nanoparticle Characterization