Plant-Mediated Green Synthesis of Nanomaterials for Sustainable Energy Applications

Abstract

Nanoparticles synthesized via physical or chemical methods can pose significant environmental risks due to their inherent toxicity. Consequently, the green synthesis of nanoparticles using plant extracts has emerged as a safer and more sustainable alternative. This paper focuses on the plant-mediated synthesis of various nanoscale metal particles (Ag, Au, Cu), metal oxide nanoparticles (ZnO, TiO₂, Fe₃O₄), and carbon-based nanomaterials, including carbon nanotubes, quantum dots, and graphene. Plant phytochemicals, including polysaccharides, flavonoids, enzymes, reducing sugars, steroids, proteins, terpenoids, and amino acids, play a crucial role in the stabilization, capping, and reduction processes during nanoparticle biosynthesis. Factors such as pH, extract concentration, reaction temperature, metal salt concentration, contact time, and the ratio of plant extract to metal salt are discussed. Beyond biomedical and catalytic applications, this paper highlights the significance of green nanoparticles for the production of biofuels. Despite significant advancements, plant-mediated green nanotechnology requires further development before it can be fully established as a climate-resilient and sustainable solution for meeting the energy demands of a growing global population. To address the pressing issue of fuel depletion, sustained research efforts and careful long-term implementation are required.

Highlights

1. Introduction

The domain of nanotechnology has proven to be one of the most rapidly evolving and dynamic areas of research [1]. This scientific domain primarily focuses on the synthesis, development, structural characterization, and applications of nanomaterials, which typically range in size from 1 to 100 nm. Nanomaterials are generally classified into four categories based on their dimensionality. Zero-dimensional (0D) nanomaterials include those in which all dimensions fall within the nanoscale range, such as fullerenes, nanoparticles, and quantum dots. One-dimensional (1D) nanomaterials include those in which only one dimension extends beyond the nanoscale range, such as nanorods, nanohorns, nanotubes, nanowires, and nanofibers. Two-dimensional (2D) nanomaterials are nanostructures in which two dimensions extend beyond the nanoscale range, including nanolayers, nanosheets, and nanofilms. Three-dimensional (3D) nanomaterials are those in which all dimensions extend beyond the nanoscale range, such as bulk powders and arrays of nanowires [2]. Supplementary Table S1 summarizes this dimensional classification along with representative examples and their key structural characteristics [3-6].

Over the past decade, nanomaterials, particularly nanoparticles, have been extensively explored due to their unique properties [7]. Nanoparticles exhibit diverse properties, including unique biological interactions, mechanical stability, steric stabilization, thermal and electrical conductivity, and altered fusion temperature, compared with their bulk counterparts, owing to their small size and increased surface area per unit mass [8]. The synthesis of nanoparticles has attracted significant interest among researchers due to their wide-ranging applications in medicine, photonics, biotechnology, biofuel production, cosmetics, electronics, drug and gene delivery, catalysis, nonlinear optical devices, and the chemical industry [9,10].

The literature reports a variety of chemical, physical, and biological approaches for the synthesis of different categories of nanoscale particles. Figure 1 illustrates the major nanoparticle synthesis methods, highlighting the biological routes through plants and microbes [1]. Nanoparticle fabrication through physical methods typically follows a top-down approach, in which bulk materials are broken down into nanoscale structures. These methods include laser ablation, inert gas condensation (IGC), laser pyrolysis, and electrospinning [11], whereas chemical methods (e.g., etching, chemical vapor deposition, and the sol-gel method) and biological (green) synthesis methods employ a bottom-up approach involving the assembly of atoms or molecules into nanomaterials [12]. However, physical and chemical methods may contribute to environmental toxicity and related problems. They are accompanied by several limitations such as prolonged synthesis time, high production cost and complex purification steps [13]. Some physical methods (e.g., tube furnace processes) require substantial energy input to maintain high operating temperatures, which can increase processing time because thermal stability must be achieved. In contrast, chemical techniques often involve hazardous reducing agents such as borohydride, hydrazine hydrate, and formaldehyde, which restrict their applicability in pharmaceutical and clinical fields [14]. In contrast, green synthesis approaches offer a more sustainable and environmentally friendly alternative. These methods minimize the use of toxic chemicals, reduce energy consumption, and enhance cost-effectiveness compared to conventional techniques [1, 10, 13-15].

Researchers have recognized the urgent need to minimize the use of harmful chemicals and reduce energy consumption, shifting toward biological methods for nanoparticle synthesis that utilize natural reducing, capping, and stabilizing agents. Numerous biological agents, including microbes (molds, unicellular fungi, algae, and bacteria), agricultural wastes, and plants (leaves, stems, fruits, roots, flowers, bark, and seeds), have been extensively investigated for facilitating environmentally friendly synthesis [16]. Several studies suggest that microbes have a short growth period and are relatively easy to manage, making them suitable for the synthesis of specific nanoparticles [17]. However, plant extracts are considered more efficient, as scaling up microbial preparations is challenging and can reduce the production rate [10,18]. Plants are capable of reducing metal ions significantly faster than microbes [18]. It is widely recognized that plant-derived nanoparticles limit the use of hazardous and toxic residues compared to chemically synthesized nanoparticles, and can be safely applied in agriculture, food science and technology, public health, and biomedical engineering. Various biomolecules present in plants, such as coenzymes, carbohydrates, phenolic acids, amino acids, alkaloids, gums, flavonoids, polysaccharides, steroids, vitamins, tannins, proteins, fats, terpenoids, and alcohols, possess reducing capabilities and can convert inorganic metal ions into metal nanoparticles [10].

This paper specifically highlights the use of plants as a sustainable source for the production of various metal (Au, Ag) and metal oxide nanoparticles (ZnO, TiO₂) through environmentally friendly green synthesis, as well as their significant potential in photocatalysis and biofuel production [8]. These plant-derived nanoparticles have shown promising roles in biofuel systems, particularly as catalysts in transesterification and biomass-conversion reactions and as enhancers of microalgal lipid accumulation and biodiesel yield [19,20]. This paper also discusses the advantages of the green synthesis of nanoparticles over conventional chemical and physical methods.

2. Synthesis of NPs from Plants

2.1. Role of Plant Metabolites in Nanoparticle Synthesis

Crude plant extracts contain a wide variety of primary and secondary metabolites. Primary metabolites include carbohydrates, proteins, fatty acids, nucleic acids, and amino acids, which play essential roles in plant growth and development. In contrast, secondary metabolites are produced by plants only under specific stress conditions. These include phenolic acids, flavonoids, alkaloids, terpenoids, alcohols, antioxidants, organic acids (e.g., oxalic acid, ascorbic acid, and tartaric acid), and quinones [21]. Although secondary metabolites do not directly influence plant growth or development, they are ecologically important and have demonstrated significant potential in medicinal applications, including both traditional and modern medicine. Owing to the wide variety of metabolites present in plants, it is challenging to identify a specific compound responsible for the formation of metallic nanoparticles. Hence, various metabolites, including flavonoids, phenolic acids, terpenoids, and proteins, have been reported to facilitate the synthesis of nanoparticles due to their reducing and stabilizing capabilities throughout the biosynthesis process [22]. These metabolites mediate the reduction of metal ions, resulting in nanoparticle formation through a one-step, environmentally friendly synthesis method [23,24]. Figure 2 demonstrates the typical mechanism of silver nanoparticle formation involving reduction and capping by plant-derived metabolites [25]. Several metabolites act as precursors during the green synthesis of plant-mediated nanoparticles. They offer numerous advantages over chemical methods, making them suitable for applications in medicinal and pharmaceutical fields. Additionally, they are cost-effective and readily available. Biomolecules that serve as reducing and stabilizing agents include sugars, ketones, aldehydes, and carboxylic acids [26].

2.1.1. Phenolics

Phenolic acids belong to the polyphenol family, the most abundant class of secondary metabolites produced by plants. These compounds are biosynthesized via either the polyketide acetate/malonate pathway, which generates simple phenols, or the shikimate/phenylpropanoid pathway, which produces phenylpropanoids.

Gopinath et al. synthesized silver, gold, and silver–gold bimetallic nanoparticles using leaf extracts of Gloriosa superba. These nanoparticles were characterized using X-ray diffraction (XRD), Fourier-transform infrared (FTIR) spectroscopy, ultraviolet–visible (UV-Vis) spectroscopy, atomic force microscopy (AFM), and transmission electron microscopy (TEM). They reported that sugar-derived compounds, such as glycosides and hydrophilic tannins present in the leaves, facilitated the bioreduction of AgNO₃, HAuCl₄·3H₂O, and their 1:1 mixture into silver, gold, and silver–gold bimetallic nanoparticles. This process could be visually monitored by the color changes, from colorless to brown, yellowish white to reddish brown, and yellow to dark red, indicating the successful formation of silver, silver gold bimetallic, and gold nanoparticles [27]. Hemlata et al. (2020) used the aqueous leaf extract of Cucumis prophetarum for the green synthesis of silver-based nanostructures. Further analysis of the bioactive compounds in the leaf extract revealed the presence of triterpenoids, phenols, tannins, and saponins, which played a key role in reducing and capping the nanoparticles to maintain their stability and prevent aggregation [28]. Numerous phenolic acids, such as caffeic acid and ellagic acid, act as reducing agents to facilitate the formation of metal nanoparticles.

The molecular framework of phenolics includes a phenolic ring and, in some cases, a carboxylic acid group. The aromatic ring plays a key role in antioxidant activity and metal chelation [10]. Wang et al. (2014) demonstrated that polyphenols, in combination with charged iron particles, produce complex nanoparticles with a size range of 50–90 nm. They utilized leaf extracts from three plants in their study, namely Melaleuca nesophila, Rosmarinus officinalis, and Eucalyptus tereticornis [29].

2.1.2. Flavonoids

Flavonoids, a class of secondary metabolites, are considered primary reducing agents in the green synthesis of metal nanoparticles. To date, 7000 compounds from this family have been reported. They have low molecular weight and, structurally, consist of 15 carbon atoms with two aromatic rings (A and B) connected by a three-carbon bridge (C). They can be found as flavan-3-ols, flavones, flavanols, isoflavones, flavanones, and anthocyanidins. In plants, they are present as glycosides or methylated derivatives [22].

Flavonoids are biosynthesized via the phenylpropanoid metabolic pathway, which begins with the conversion of phenylalanine into 4-coumaroyl-CoA. The enzyme chalcone synthase catalyzes the formation of chalcone scaffolds from 4-coumaroyl-CoA, which serves as the backbone for all flavonoids [30]. Various flavonoid subclasses arise through the action of different enzymes, including hydroxylases, reductases, Fe²⁺/2-oxoglutarate-dependent dioxygenases, and isomerases, which modify the structural framework of flavonoids [31].

Various studies have been reported in the literature regarding their ability to act as reductants, which is mainly due to their ability to donate electrons and ‘H’ atoms [10]. Sahu et al. (2016) demonstrated the production of silver nanoparticles (AgNPs) using a range of flavonoids, including hesperidin, naringin, and diosmin, to reduce Ag⁺ ions. Additionally, these flavonoids were also found to function as capping agents, thereby influencing the antimicrobial effects of AgNPs [32]. Bose and Chatterjee (2016) reduced Ag⁺ ions to silver nanoparticles using leaf decoction from Psidium guajava (guava plant). The authors suggested that quercetin (a flavonoid) reduces Ag⁺ to Ag⁰ by liberating Hydrogen and is oxidized to form a quinone. The hydroxyl and oxo functional groups of quercetin molecules subsequently act as capping agents, stabilizing the silver nanoparticles [33]. Melkamu and Bitew (2021) reported the eco-friendly synthesis of silver nanoparticles utilizing the leaf extract from the Hagenia abyssinica plant. The phytochemical evaluation of the leaf extract indicated the presence of compounds such as anthraquinones, saponins, tannins, alkaloids, phenols, and flavonoids, which contributed to the reduction, capping, and stabilization of nanoparticles throughout this process. The interaction of flavonoids with metal ions through carbonyl functional groups releases hydrogen, converting the enol form of flavonoids into the keto form, which facilitates the formation of Ag⁰ [34].

2.1.3. Terpenoids

Terpenoids are commonly obtained from the essential oils of plants, with approximately 90% of these oils consisting of terpenoids, which are oxygenated terpenes [22].

The typical formula for terpenoids is (C5H8) n, with ‘n’ indicating the number of isoprene units in the compound. On the basis of number of n, terpenoids can be classified into: hemiterpene (n = 1, C5), monoterpene (n = 2, C10), sesquiterpene (n = 3, C15), diterpene (n = 4, C20), sesterpene (n = 5, C25), triterpene (n = 6, C30), tetraterpene (n = 8, C40) and polyterpene [35].

The biosynthesis of terpenoids mainly occurs through two pathways: the mevalonate (MVA) pathway ... and the 2C-methyl-D-erythritol-4-phosphate (MEP) pathway, which takes place in the plastid. Both these pathways lead to the production of isopentenyl pyrophosphate (IPP) and dimethylallyl pyrophosphate (DMAPP). DMAPP and IPP serve as building blocks for the synthesis of geranyl pyrophosphate (GPP), a 10-carbon molecule that gives rise to monoterpenes. On further addition of IPP units, a 15-carbon molecule, Farnesyl pyrophosphate (FPP), and geranylgeranyl pyrophosphate (GGPP), a 20-carbon molecule, are produced, which give rise to sesquiterpenes and diterpenes, respectively [36]. These sesquiterpenes and monoterpenes are recognized as playing a crucial role in the production of silver nanoparticles [37]. They act as surface-active agents to stabilize and reduce nanoparticles; however, their exact mechanism remains unclear. Shankar et al. reduced silver ions into nanoparticles by using geranium leaves extract to demonstrate the role of terpenoids in the biosynthesis of nanoparticles [38]. Pungle et al. (2022) demonstrated the eco-friendly synthesis of silver nanoparticles using aqueous leaf extracts of Tridax procumbens. FTIR analysis of the leaf solution showed the presence of amines and hydroxyl groups in combination with fluoro compounds. Upon further analysis by HRLC-MS, the major capping agent was found to be fusinopril, and reducing agents were found to be peptides, terpenoids, polyphenols, and alkaloids [39]. According to Ahmed et al. (2010), eugenol plays a key role in the bioreduction process during the synthesis of silver and gold nanoparticles from extracts of C. zeylanicum [40].

Scientists have suggested that the phenolic OH bond of eugenol undergoes homolytic cleavage in the presence of sunlight, releasing H⁺ ions that reduce Ag⁺ to Ag⁰ [41].

2.1.4. Proteins

Proteins act as reducing agents by donating electrons to metallic ions during the plant-mediated green synthesis of nanoscale particles. Mukherjee et al. performed SDS-PAGE analysis of leaf extracts of Olax scandens and the concentrated supernatant of silver nanoparticles derived from this extract. The results indicated the presence of low-molecular-weight proteins in the leaf extracts; however, no such proteins were detected in the newly synthesized nanoparticles. This suggests the utilization of low-molecular-weight proteins during nanoparticle biosynthesis [42]. Shukla et al. (2008) demonstrated the production of gold nanoparticles derived from soybeans by directly mixing soybeans with sodium tetrachloroaurate (NaAuCl₄) in an aqueous medium. To investigate the role of soybean proteins in the reduction of sodium tetrachloroaurate (NaAuCl₄), the proteins were separated into low-molecular-mass (<5 kDa) and high-molecular-mass (>5 kDa) fractions using Centricon Plus-20 centrifugal filtration devices. Both protein fractions were then separately mixed with sodium tetrachloroaurate (NaAuCl₄) in an aqueous medium, and each fraction successfully facilitated the formation of gold nanoparticles, thereby confirming their role as key reducing agents in the process [43].

Tavaf et al. (2015) effectively utilized reduced glycated adducts of the whole casein fraction as both reducing and stabilizing agents in the synthesis of silver nanoparticles [44]. Using FTIR spectroscopy, a variety of C=O groups present in proteins have been proven to act as capping ligands. They associate with the nanoparticle surface, contributing to colloidal stability. Additionally, proteins and amino acids containing exposed disulfide bonds and thiol groups act as both reducing and stabilizing agents during the biosynthesis of nanoscale particles [41]. Bhattacharjee et al. (2005) generated gold nanoparticles using a tripeptide sequence featuring an exposed N-terminal group [45].

3. Mechanism of Plant-Derived Nanoparticles

The application of green chemistry in nanoparticle synthesis has been extensively explored in recent years. This green synthesis relies on a bottom-up approach, in which nanoparticles are assembled from smaller precursor entities. Unlike conventional synthesis methods, plant-mediated synthesis typically requires only an aqueous extract, ambient temperature, and atmospheric pressure, thereby minimizing energy consumption [46]. However, careful consideration of the solvent type is essential, with water and ethanol being the most commonly employed due to their effectiveness in extracting phytochemicals and maintaining their stability, as well as their low environmental impact [47,48]. Lee et al. (2024) demonstrated that ethanol and water extracts effectively recover antioxidant and phenolic compounds, offering lower safety risks compared with more toxic organic solvents such as methanol [49]. Nortjie et al. (2022) further emphasized that an ideal extraction solvent should possess low toxicity, suitable volatility, strong solubility for target compounds, and the ability to evaporate readily at mild temperatures. Accordingly, the selection of solvents such as water or ethanol, which meet these criteria and present minimal environmental and health hazards, is preferred in green synthesis approaches [47]. Compared with conventional chemical nanoparticle syntheses that often employ strong reductants (e.g., NaBH4, hydrazine) and hazardous surfactants/solvents, the plant-based (green) approach generally reduces the use of highly toxic reagents and associated toxic by-products [50,51].

The essential tools, processes, and materials used in nanoparticle synthesis encompass reductants, stabilizers, solvents, metal precursors, nucleation, growth, aggregation, stabilization, and characterization [1]. Plant-based nanoparticle synthesis has emerged as a more favorable approach for large-scale industrial applications due to its minimal environmental impact and economic advantages, as it is relatively cost-effective to scale up [23]. A systematic approach is required for plant-mediated nanoparticle synthesis, which can be described as follows:

• 1)

  The plant should be taxonomically identified, and the specific plant material should be carefully selected.

• 2)

  The extraction process is carried out using appropriate solvent(s), followed by filtration to minimize the risk of introducing impurities.

• 3)

  A metal salt solution is used as a nanoparticle precursor and is mixed with the plant extract.

• 4)

  Controlling physical factors (such as pH, temperature, phytochemical concentration, and metal salt concentration) is necessary to ensure a successful reaction.

• 5)

  The nanoparticle formation is signified by a change in the solution’s color.

• 6)

  Centrifugation is then carried out to separate the nanoparticles from the solution and remove any remaining impurities.

• 7)

  Finally, the synthesized nanoparticles are characterized using a range of analytical techniques to determine their size, shape, composition, and physicochemical properties [10].

The eco-friendly synthesis of plant-derived nanoparticles can be summarized in three primary stages: (1) the activation phase, (2) the growth phase, and (3) the termination phase:

• The Activation Phase

Plant metabolites play a crucial role in reducing metal ions from their respective salt precursors during the activation phase. Researchers have reported that each metal exhibits different reductive capabilities, which directly influence the reduction of metal ions or their precursors during the green synthesis process. In addition, the oxidation state of metal ions changes from monovalent or divalent forms to the zero-valent state, and the reduced metal atoms subsequently undergo nucleation [52]. It represents the initial stage of nanoparticle formation in plant-mediated green synthesis. Nucleation leads to the formation of clusters, which are referred to as nuclei in wet-chemical nanoparticle synthesis. The radius of these clusters corresponds to the minimum size at which a particle can remain stable in the solution without redissolving into it. This process is essential for the formation of stable clusters, which, upon growth, give rise to nanoparticles [53].

• The Growth Phase

The growth phase follows a bottom-up approach and primarily depends on the interaction between stable nuclei and surfactants. The length and chemical nature of the surfactant should also be taken into consideration. Several studies suggest that surfactant–solvent interactions can influence the size and structural characteristics of nanoparticles. Additionally, it was experimentally demonstrated that stable nuclei are observed at low surfactant concentrations. In contrast, nucleation and growth processes are inhibited in the presence of an excess of a strongly binding surfactant [53].

As the growth phase progresses, smaller nanoparticles aggregate to form nanotubes, nanotriangles, nanoprisms, and various other irregular morphologies until thermodynamic stability is reached. This thermodynamic process is known as Ostwald ripening [54].

• The Termination Phase

When the nanoparticles acquire the most energetically favorable conformation and attain a constant size and shape, the process terminates. The lower the Gibbs free energy of a nanoparticle, the greater its stability. This phase is highly dependent on plant extracts, which act to stabilize the nanoparticles. Based on FTIR analysis, certain plant functional moieties, such as —C—O—C—, —C=C—, and —C=O, were observed to act as protective stabilizers, thereby aiding nanoparticle stabilization and preventing further changes in their morphology [54]. The schematic in Figure 3 illustrates a typical synthesis procedure using zinc acetate and strawberry extract, highlighting the role of phytochemicals in the reduction process, Ostwald ripening, and stabilization stages under controlled temperature and stirring conditions.

4. Factors Affecting Plant-Mediated Synthesis of Nanoparticles

5. An Overview of the Different Types of Plant-Derived NPs

5.1. Metal-Based Nanoparticles

Nanoparticles can be categorized as organic, inorganic, and carbon-based types based on their chemical nature and structural characteristics. Organic nanoparticles include micelles, liposomes, and dendrimers, whereas inorganic nanoparticles, which lack carbon, include metal nanoparticles composed of pure metals [80]. Figure 4 illustrates this classification. Metal nanoparticles have recently gained significant attention due to their wide range of applications, spanning pharmaceutical fields to various cosmetic products used in daily life [81]. Metallic nanoparticles can be generated in large-scale industries by employing a downstream process optimization. Various metallic nanoparticles are derived from different metals, including silver, gold, platinum, palladium, zinc, copper, titanium, magnesium, and others. The unique properties of metallic nanoparticles, such as distinctive optical characteristics, high stability, and quantum size effects, enable their application across diverse fields, including the food industry, agriculture and crop production, cosmetics, the pharmaceutical sector, and wastewater treatment. Based on the number of metals in their composition, they are categorized as monometallic, bimetallic, trimetallic, and polymetallic nanoparticles. As their names suggest, monometallic nanoparticles are composed of a single metal, bimetallic nanoparticles consist of two metals, trimetallic nanoparticles contain three metals, and polymetallic nanoparticles comprise more than three metals [46]. An overview of selected studies on plant-mediated synthesis of metal-based nanoparticles, along with their key characteristics and applications, is presented in Supplementary Table S3 [58, 75, 82-105].

5.1.1. Monometallic Nanoparticles

The synthesis of monometallic nanoparticles has garnered significant interest in recent years due to their ease of preparation and wide range of applications across various fields. Monometallic nanoparticles can be further categorized into metallic, transition, and magnetic nanoparticles on the basis of the type of metals present in them. The synthesis of silver and gold nanoparticles is widely recognized as the most prevalent choice among researchers [46]. Various monometallic nanoparticles prepared in recent years using metals such as platinum, silver, copper, palladium, and gold are described below:

5.1.1.1. Silver Nanoparticles

In ancient times, various forms of silver were used as food preservatives, and the use of silver utensils was common due to their antimicrobial properties. Silver nanoparticles have attracted significant attention over the past one to two decades due to their pronounced cytotoxic activity against various pathogenic microorganisms, including bacteria, protozoa, and fungi [106]. The unique characteristics of silver nanoparticles include high catalytic and antimicrobial activity, high electrical and thermal conductivity, and chemical stability [107]. According to Larue et al. (2014), silver nanoparticles were among the most widely produced nanoparticles globally, with an estimated annual production of approximately 500 tons [108]. Numerous researchers have successfully synthesized silver nanoparticles using a wide range of plant components—including leaves, fruits, stems, roots, flowers, and bark—owing to the presence of diverse phytochemicals within these materials. Khosravi et al. (2023) demonstrated that silver nanoparticles synthesized by the green route using Malva sylvestris extract effectively suppressed egg hatching and enhanced juvenile mortality of the root-knot nematode Meloidogyne javanica under both laboratory and greenhouse conditions, highlighting their promise as an environmentally friendly nematicidal agent [109]. Onitsuka et al. (2019) reported the synthesis of silver nanoparticles using black and green tea infusions prepared from the leaves of Camellia sinensis. Transmission electron microscopy (TEM) analysis revealed that nanoparticles synthesized using green tea extracts had an average size of 18.9 ± 4.4 nm, whereas those produced using black tea extracts exhibited a larger average particle size of 34.4 ± 9.6 nm. The nanoparticles exhibited a spherical morphology and were relatively stable. The nanoparticle colloidal solution was used to dye cotton fabrics, providing antimicrobial activity against K. pneumoniae and S. aureus [110].

Findings from recent work utilizing Hyoscyamus muticus L. extract for the green synthesis of silver nanoparticles demonstrated strong nematicidal activity against Meloidogyne javanica while maintaining biocompatibility with host plants. The biogenic AgNPs effectively reduced egg hatching and juvenile survival rates, thereby enhancing the plant’s natural defense mechanisms without exhibiting any observable phytotoxic effects. These results emphasize that green-synthesized nanoparticles, owing to their eco-friendly origin and low toxicity, hold considerable promise as replacements for hazardous synthetic agents [111]. In another study, leaves of Solanum nigrum were used to synthesize silver nanoparticles by stirring the Solanum nigrum leaf extract with silver nitrate for 10 min, as reported by Vigilvani et al. (2019). Spherical, small-sized silver nanoparticles were obtained (1.74–6.96 nm), exhibiting pronounced antimicrobial and catalytic activity [112]. Silver nanoparticles with an average size of 30 nm were synthesized using a leaf-derived solution of Ligustrum lucidum. As reported by Huang et al. (2020), these nanoparticles exhibited antifungal activity against Setosphaeria turcica, with a half-maximal inhibitory concentration (IC50) of 170.20 µg/mL. Moreover, the silver nanoparticles exhibited enhanced antifungal activity when combined with epoxiconazole in 9:1 and 8:2 ratios [113].

Bekhradian et al. (2024) demonstrated the potential of Malva sylvestris extract as a biogenic reducing and stabilizing agent for the synthesis of silver and silver oxide nanostructures. The phytochemical components of the extract, including phenolics and flavonoids, play a crucial role in reducing Ag⁺ ions and stabilizing (capping) the resulting nanoparticles. These biosynthesized Ag/Ag₂O nanostructures exhibited notable antimicrobial and antioxidant activities, confirming the synergistic effect of the plant-derived biomolecules and the metallic core [114]. Qais et al. (2020) demonstrated the synthesis of silver nanoparticles using seed extract of Carum copticum under constant stirring at ambient temperature. The resulting nanostructures predominantly exhibited a spheroidal morphology with varying sizes and demonstrated antivirulence and antibiofilm properties [115]. The production of silver nanoparticles was also documented by Odeniyi et al. (2019), using aqueous and methanolic fruit extracts of Nauclea latifolia. The particles were irregularly shaped with an average size of 10 nm, as analyzed by SEM. The resulting nanoparticles exhibited a broad range of applications; however, those synthesized using aqueous fruit extract were found to be more efficient than those prepared using methanolic fruit extract. They showed antibacterial properties against most of the pathogens they were tested for (C. Freundii, Staphylococcus sp., S. aureus, Escherichia coli, and Klebsiella sp.), antifungal properties (Rhizopus sp. and Candida albicans). The nanoparticles were also used to formulate cold creams, which were found to be aesthetically appealing and satisfactory, and they demonstrated antimicrobial and antifungal activity against several tested pathogens [116]. According to Rajabi et al. (2016), plant extracts such as those from Suaeda Acuminata provide bioreductants and stabilizing compounds, enabling green synthesis of silver nanostructures without surfactants or templates, thereby lowering cost and complexity [117].

5.1.1.2. Gold Nanoparticles

Gold has long been regarded as a symbol of wealth and prosperity. Its use in jewelry, ornaments, royal palaces, and crowns is well documented throughout history. In India, gold ash was also used as an Ayurvedic medicine in ancient times. Although bulk gold is largely inert, it exhibits unique properties at the nanoscale. Gold nanoparticles have attracted significant attention over the past few decades despite their relatively high cost, owing to their wide range of industrial applications. These include minimal to no toxicity toward human cells, good solubility and stability, catalytic activity, ease of one-step green synthesis, ability to conjugate with proteins, as well as applications in disease diagnostics, gene expression studies, and drug delivery [46]. Elia et al. (2014) reported the fabrication of gold nanoparticles using fruit extract of Punica granatum and leaf extracts of Pelargonium graveolens, Lippia citriodora, and Salvia officinalis. The nanoparticles demonstrated excellent biocompatibility and stability [72]. According to Patil et al. (2023), fruit extract of Capsicum annuum can be used in the production of gold nanoparticles. This synthesis was carried out at 90 °C with vigorous stirring for several minutes. The transition in mixture color from pale yellow to deep violet signified the formation of gold nanoparticles. The obtained nanoparticles exhibited a spherical shape, with sizes ranging from 20 to 30 nm and an average diameter of 25.92 nm. The nanoparticles exhibited dose-dependent antioxidant activity against DPPH (86.0% at 100 µg/mL), anti-inflammatory activity (IC50 = 619.4 µg/mL) within a concentration range of 100–1200 µg/mL, and anti-angiogenic effects at 100 µg/mL [118].

Fouda et al. (2022) synthesized gold nanoparticles with crystalline, spherical morphology using an HAuCl₄ solution and an aqueous extract of Zingiber officinale rhizome. The nanoparticles exhibited a size distribution ranging from 5 to 53 nm, with an average size of 15.11 ± 8.5 nm. The synthesis was carried out at pH 8 and 60 °C for 60 min. The effective synthesis of gold nanoparticles was indicated by a color shift in the solution from pale yellow to purple. The synthesized nanoparticles showed dose-dependent antimicrobial activity towards Bacillus subtilis, Escherichia coli, Aspergillus brasiliensis, Pseudomonas aeruginosa, Staphylococcus aureus, Candida albicans, and Pseudomonas aeruginosa, with the highest zone of inhibition, ZOI, at a concentration of 200 µg/mL. The antioxidant activity of the gold nanoparticles against DPPH was expressed as an IC₅₀ value of approximately 16 µg/mL, indicating a slightly lower effect compared with ascorbic acid, which served as the standard and showed an IC₅₀ value of 12.8 µg/mL. Additionally, the cytotoxic effects of the gold nanoparticles on adenocarcinoma (MCF-7) and hepatocellular carcinoma (HepG2) cell lines were recorded with IC₅₀ values of 288.23 ± 31.39 µg/mL and 131.9 ± 9.34 µg/mL, respectively, while showing negligible toxicity toward normal oral epithelial cells (IC₅₀ = 487.612 ± 3.53 µg/mL) [119]. In another work by Ahmad et al. (2024) reported the synthesis of gold nanoparticles using crude extract of Aconitum violaceum under constant stirring at pH 9 for 50 min. Following SEM and TEM analysis, the particles exhibited spherical morphology with sizes below 100 nm, as well as triangular morphology with sizes approaching 100 nm. The nanoparticles showed potent antibacterial, antioxidant, and photocatalytic activity [120].

5.1.1.3. Copper Nanoparticles

Copper has long been used in medical and dental fields due to its antimicrobial, anti-inflammatory, and low-toxicity properties [121]. In the literature, extensive data are available on the plant-mediated biosynthesis of copper nanoparticles and their wide-ranging applications. Copper nanoparticles are obtained by reducing Cu²⁺ ions from a copper salt solution to the Cu⁰ form. Bioactive compounds present in plant extracts contribute to the stability of the nanoparticles [122]. The size and morphology of copper nanostructures are largely influenced by the concentration of the precursor salt, as demonstrated by Nagar and Devra (2018) during the synthesis of copper nanoparticles using leaf extracts of Azadirachta indica [71]. Mehdizadeh et al. (2020) reported the synthesis of copper nanoparticles using cellulosic walnut shell as a base material. The process was initially carried out at 50 °C for a duration of 2 h. In the subsequent step, the reduction of Cu²⁺ ions was achieved by adding an ethanol solution of NaBH4 and stirring for another 3 h. The nanoparticles were found to be thermally stable up to 200 °C. TEM analysis revealed three distinct types of nanoparticles: CuNPs with sizes ranging from 15–22 nm, CuNPs with sizes ranging from 60–80 nm, and unsupported CuNPs. The nanoparticles exhibited antimicrobial properties against Escherichia coli, Listeria monocytogenes, and Staphylococcus aureus, antioxidant properties for DPPH, and cytotoxic properties for K562 (erythroleukemia) cell line with an IC₅₀ value of 25.24 ± 5 µg/mL in a dose-dependent manner [123].

Mali et al. (2020) synthesized spherical copper nanoparticles from a CuSO₄·5H₂O solution and an aqueous leaf extract of Celastrus paniculatus. The reaction pH was adjusted to 7.0. The obtained particles were 2–10 nm in size, with a mean diameter of 5 nm. The nanoparticles exhibited good photocatalytic properties and antifungal activity against Fusarium oxysporum. At copper nanoparticle concentrations of 0.24%, 0.18%, and 0.12%, mycelial growth inhibition was recorded as 76.29 ± 1.52%, 73.70 ± 1.52%, and 59.25 ± 0.57%, respectively [124]. According to Amjad et al. in 2021, the copper nanoparticles can be synthesized from the leaf extract of Fortunella margarita and CuSO₄.5H₂O solution at 70 °C and 5.5 pH for 30 min. The color change from dark green to bluish green indicated the successful formation of nanoparticles. The nanoparticles appeared spherical but were agglomerated, exhibiting sizes ranging from 51.26 to 56.66 nm under SEM analysis [63]. Sharma et al. (2018) synthesized copper nanoparticles using leaf extracts of Tinospora cordifolia and a CuCl₂ solution at 85 °C for 20 min. The particles exhibited a spherical morphology with sizes ranging from 50 to 130 nm. The synthesized nanoparticles showed excellent catalytic degradation towards reactive dye, eosin yellowish, safranin, and direct dye [125]. Mali et al. (2020) utilized secondary metabolites from leaf extract of Celastrus paniculatus to synthesize copper nanoparticles. SEM and TEM images revealed that the particles exhibited a spherical morphology with a size range of 2–10 nm. Plant-synthesized copper nanoparticles demonstrate effective catalytic degradation of methylene blue (MB) and notable antifungal activity against Fusarium oxysporum, exhibiting 76.29 ± 1.52% mycelial growth inhibition at a concentration of 0.24% copper nanoparticles [124].

5.1.1.4. Palladium Nanoparticles

Palladium nanoparticles have a broad applicability in bioscience, biomedicine, and pharmacy. These nanoparticles have attracted considerable attention due to their high surface-area-to-volume ratio, which enables them to function as highly efficient and selective catalysts, depending on their morphology, as reported by Cheong et al. [96]. A variety of plant-derived phytochemicals facilitate the green synthesis of palladium nanoparticles by reducing Pd (II) ions present in palladium salts [1]. Petla et al. synthesized palladium nanoparticles using leaf extracts of Glycine max. The leaf extract contains various polyols, terpenoids, and reducing sugars, which act as reducing agents for the synthesis of palladium nanoparticles. The resulting nanoparticles exhibited a face-centered cubic (FCC) crystalline structure with a size of 15 nm [96]. Vinodhini et al. (2022) reported the plant-mediated synthesis of palladium nanoparticles using leaf extracts of Allium fistulosum, Basella alba, and Tabernaemontana divaricata, with particle sizes of 500 nm, 2 µm, and 2 µm, respectively. The obtained nanoparticles exhibited a spherical morphology. Nanoparticle formation was indicated by a color change from mild brown to dark brown in T. divaricata and A. fistulosum, while a transition from pale brown to green was observed in the case of Basella alba. The nanoparticles were further evaluated for their biomedical application. They exhibited highly effective antibacterial activity against Bacillus cereus, Bacillus subtilis, Escherichia coli, and Staphylococcus aureus, with the highest antibacterial effect observed at a concentration of 100 µL. They also demonstrated antifungal activity against Aspergillus flavus, Penicillium sp., and Candida albicans, with particle sizes of 150, 100, and 50 nm, respectively. Palladium nanoparticles synthesized from Basella alba also exhibited the highest antioxidant activity at 1000 µg concentration as compared to others. The nanoparticles also showed dose-dependent anti-diabetic properties by repressing alpha-amylase enzyme.

Additionally, the nanoparticles exhibited good photocatalytic activity [99]. Sattar et al. (2024) utilized root extract of Curcuma longa for the biosynthesis of palladium nanoparticles. The reaction mixture was continuously stirred at 70 °C for 3 h. The generated nanoparticles were characterized as spherical, with sizes ranging from 50 to 150 nm. Palladium nanoparticles showed dose-dependent antioxidant activity, antifungal activity towards Candida sp., with the highest zone of inhibition reported as 17 ± 0.28 mm at 130 µg/mL. Similar results were observed when evaluating their antibacterial efficiency against Staphylococcus aureus and Escherichia coli, with maximum inhibition zones of 17 ± 0.28 mm and 27 ± 0.22 mm, respectively, at a concentration of 130 µg/mL. The palladium nanoparticles also inhibited the MCF-7 breast cancer cell line in a concentration-dependent manner, with the highest inhibition observed at 130 µg/mL (26.53% ± 2.45) compared with untreated control cells [126]. Bathula et al. (2020) used leaf extracts of Coleus amboinicus to derive palladium nanoparticles ultrasonically. The transformation of the precursor solution from brown to black indicated the successful formation of palladium nanoparticles. Further analysis using HRTEM and TEM confirmed that the nanoparticles possessed a spherical, crystalline structure with sizes ranging from 16 to 23 nm and an average diameter of 20 nm. The palladium nanoparticles were found to serve as highly efficient catalysts in ultrasound-assisted Suzuki–Miyaura coupling reactions [127].

5.1.1.5. Platinum Nanoparticles

Platinum nanoparticles have been extensively studied, particularly for their applications in the biomedical field. Patel et al. investigated the potential of platinum nanoparticles in nanomedicine and for their antibacterial properties [128]. In another study, Sahin et al. (2016) investigated Punica granatum-mediated platinum nanoparticles and evaluated their cytotoxic effects against the human breast cancer cell line MCF-7 using the MTT assay. Platinum nanoparticles showed a dose-dependent inhibition in the proliferation of MCF7 cells by inducing cell apoptosis during the G0/G1 phase [129]. Zhang et al. (2022) performed the green synthesis of platinum nanoparticles utilizing the floral extract of Nymphaea tetragona, in the H₂PtCl₆/N.tetragona floral extract ratio of 1:4 and 1:1, and confirmed the average particle size of formed nanoparticles as 2.01 ± 0.80 nm and 4.04 ± 1.31 nm, respectively. The authors evaluated the use of platinum nanoparticles in anti-aging and skin lightening. Upon culturing the human melanogenic cell line (A375) with platinum nanoparticles, it was revealed that both types of nanoparticles exhibit dose-dependent inhibition of melanin production by inhibiting the tyrosinase enzyme. They also inhibited UVB-induced melanin synthesis. In studies on the human skin fibroblast cell line (HFF-1), the nanoparticles were confirmed to exhibit anti-aging activity by enhancing collagen I production through activation of the underlying molecular mechanisms, including the TGF-β and Smad signaling pathways responsible for collagen I synthesis [105]. In another study, Eltaweil et al. (2021) prepared platinum nanoparticles using leaf extracts of Atriplex halimus. The reaction was carried out at pH 9.6 and 95 °C under constant stirring for approximately one hour, after which a color change from dark yellow to black was observed. The prepared nanoparticles exhibited particle sizes ranging from 1 to 3 nm and predominantly displayed a spherical morphology. The nanoparticles exhibited excellent catalytic, antioxidant, and antibacterial activity [100]. In a similar study conducted by Coman et al. (2024), platinum nanoparticles were synthesized using bark extracts of Quercus petraea, Quercus frainetto, and Quercus dalechampii, yielding nanoparticles with average diameters of 41 ± 5.3 nm, 41.6 ± 4.4 nm, and 58.5 ± 7.6 nm, respectively. The synthesized nanoparticles had a spherical morphology and showed superb antioxidant properties, with Nanoparticles synthesized from Q. petraea having the maximum DPPH (539 mg GAE/g dw), FRAP (783 mg GAE/g dw), and CUPRAC (573 mg GAE/g dw) free radical scavenging activity, while nanoparticles synthesized from Q. dalechampii showed the highest ABTS free radical scavenging activity (605 mg GAE/g dw). The nanoparticles also exhibited potent antimicrobial activity against certain bacteria and fungi, particularly Escherichia coli, Enterococcus faecalis, Candida krusei, and Candida auris [64].

5.1.2. Bimetallic Nanoparticles

The bimetallic nanoparticles resulted because of the discovery of hybrid nanoparticle synthesis in which two distinct metals are combined in a reaction vessel under carefully controlled conditions [130]. They can be classified into two types: (i) those in which both metals are homogeneously mixed within the same particle, and (ii) core–shell structures, in which one metal forms a core that is encapsulated by another metal forming the shell. Figure 5 illustrates the green synthesis of Ag–Cu bimetallic nanoparticles using Salvia officinalis leaf extract, where phytochemicals act as reducing and capping agents during nanoparticle formation [131]. These nanoparticles are considered particularly significant as they overcome the limitations of their corresponding monometallic counterparts and enhance their overall properties [46]. Bimetallic nanoparticles exhibit distinct physical properties, including high surface area, quantum effects, enhanced mobility, and improved thermal, catalytic, magnetic, mechanical, chemical, and optical characteristics, making them superior to their monometallic counterparts [130]. As reported by Fahmy et al. (2021), platinum–palladium (Pt–Pd) bimetallic nanoparticles synthesized using seed extract of Peganum harmala at 60 °C for 24 h were more functionally efficient than their individual platinum and palladium counterparts. In the FRAP assay, platinum–palladium nanoparticles (843 ± 60 µM TE/mg) exhibited approximately four times higher antioxidant activity than the harmala alkaloid fraction (33.91 ± 5.82 µM TE/mg), as well as greater activity than platinum nanoparticles (277.3 ± 13.5 µM TE/mg) and palladium nanoparticles (167.6 ± 4.8 µM TE/mg). Upon evaluation of their cytotoxic activity against human breast cancer (MCF-7) and lung cancer (A549) cell lines, it was observed that platinum–palladium nanoparticles exhibited significantly higher cytotoxicity, with IC₅₀ values of 3.6 and 8.8 µg/mL, respectively. In comparison, platinum nanoparticles showed IC₅₀ values of 6.7 and 10.9 µg/mL, while palladium nanoparticles demonstrated comparatively lower cytotoxic effects, with IC₅₀ values of 10.8 and 31 µg/mL, respectively [84].

A study by Ashishie et al. (2018) documented the eco-friendly synthesis of Ag–Cu bimetallic nanoparticles using an aqueous infusion of Kigelia africana fruits under reflux at 120 °C for 6 h. The resulting nanoparticles were crystalline in nature, with an average size of 10 nm, and demonstrated greater effectiveness against both Gram-positive and Gram-negative bacteria compared with the antibiotics tested, including Augmentin, Meropenem, Ofloxacin, Ciprofloxacin, and Racinef [132]. Moringa oleifera leaf extract was used to synthesize Ni/Fe₃O₄ MNPs at 70 °C by Prasad et al., 2017. The nanoparticles exhibited a spherical morphology, with diameters ranging from 16 to 20 nm. Ni/Fe₃O₄ nanoparticles functioned as a magnet adsorbent for the removal of Malachite green dye from an aqueous solution [133]. Luo et al. (2016) also documented the fabrication of Fe/Pd bimetallic nanoparticles using an aqueous extract of grape leaves. The process was carried out at ambient temperature for 30 min. The synthesized nanoparticles exhibited a spherical shape with sizes ranging from 20–100 nm [134]. A study by Mamatha et al. (2020) reported the fabrication of Ag-Cu nanoparticles on cotton fabrics utilizing aloe vera leaf extract by constant stirring for 24 h. These nanoparticle-incorporated cotton fabrics were used in medical applications, particularly as wound dressings [135].

5.1.3. Trimetallic Nanoparticles

Trimetallic nanoparticles are synthesized by combining three different metals or metal oxides. They typically exhibit a “triple core–shell architecture,” in which one element forms the core, a second element constitutes an intermediate layer surrounding the core, and a third element forms the outer shell [136]. Figure 6 illustrates the eco-friendly synthesis of Cu–Co–Ni trimetallic nanoparticles using Origanum vulgare extract, emphasizing the bio-reduction and stabilization processes mediated by plant-derived phytochemicals [137]. Polymetallic or trimetallic nanoparticles have attracted significant attention for many years due to their enhanced biological properties compared with single-metal and bimetallic nanoparticles [138]. Compared with mono- or bimetallic nanoparticles, they exhibit enhanced stability, greater morphological diversity, and improved catalytic and antibacterial properties, as well as other therapeutic activities [139]. Their optical, magnetic, electronic, and catalytic properties arise from the synergistic effects of the constituent metals, thereby broadening their applications to catalysis, antimicrobial and antibiofilm activity, biomedicine, and sensing. Trimetallic nanoparticles have also been incorporated into food packaging, as reported in several studies [140]. To date, plant-mediated fabrication of trimetallic nanoparticles has been scarcely explored, as highlighted in the literature. A recent study by Farooq et al. (2025) highlighted the effectiveness of green-synthesized Co–Fe–ZnS nanocomposites prepared using plant extract of Avena fatua. The nanocomposites demonstrated remarkable multifunctional activity, including photocatalytic degradation of amoxicillin, hydrogen generation, and CO₂ reduction. The study further optimized reaction conditions through response surface methodology (RSM), confirming the material’s potential for sustainable and scalable photocatalytic applications. Such findings emphasize how combining multiple metal ions within a single nanosystem can significantly improve photocatalytic performance while adhering to environmentally friendly synthesis principles [141].

Vaseghi et al. (2018) synthesized Cu–Cr–Ni oxide trimetallic nanoparticles using aqueous leaf infusions of Eryngium campestre and Froriepia subpinnata as both reducing and capping agents. The process was carried out for 3 min at 34 °C with the addition of precursor salts (Cr(NO₃)₃·9H₂O, Ni(NO₃)₂·6H₂O, and CuSO₄·5H₂O) along with E. campestre leaf extract, while for F. subpinnata leaf extract, the temperature was maintained at 40 °C. The transformation from transparent brown to colloidal brown confirmed the synthesis of trimetallic oxide nanoparticles. FESEM analysis revealed that the nanoparticles exhibited a nanoplates morphology, with average thicknesses of 14.15 nm, 10.96 nm, and 18.73 nm for the mixed leaf extract, F. subpinnata, and E. campestre, respectively. The nanoparticles exhibited strong antibacterial properties against Staphylococcus aureus and Escherichia coli [142].

Kannaiyan et al. (2021) documented the fabrication of trimetallic oxide Ni–Cr–Cu nanoparticles using leaf extract of Coriandrum sativum. Cr(NO₃)₃·9H₂O, Ni(NO₃)₂·6H₂O, and CuSO₄·5H₂O were used as precursor salts. The process was carried out for 45 min in a water bath maintained at 40 °C. The synthesized nanoparticles exhibited dose-dependent antibacterial and antifungal activity, with the highest zone of inhibition (ZOI) observed at 1000 µg/mL, measuring 27 mm, 24 mm, 23 mm, and 15 mm against E. coli, Staphylococcus aureus, A. flavus, and Penicillium sp., respectively [143].

In a similar study, Hussein et al. (2022) investigated the fabrication of Ru/Ag/Pd nanoparticles using an aqueous extract of garlic tunicate leaves. The obtained nanoparticles exhibited a spherical morphology, with sizes ranging from 50 to 90 nm. The nanoparticles exhibited significant antimicrobial activity against all tested microorganisms, including B. cereus, A. niger, C. glabrata, A. flavus, E. coli, and C. albicans. Notably, their efficacy was higher than that of individual palladium, silver, and ruthenium nanoparticles. The nanoparticles also exhibited cytotoxic activity against K562, HepG2, and Caco-2 cell lines, with IC₅₀ values of 35.87 ± 2.0, 68.8 ± 3.9, and 47.35 ± 2.7 µg/mL, respectively. These values were further enhanced upon UV exposure, decreasing to 28.32 ± 1.6, 46.77 ± 2.6, and 9.32 ± 0.52 µg/mL, respectively [144]. Radhakrishnan and Subramaniam prepared Cu-Ag-Ru trimetallic nanoparticles using Andographis paniculata leaf extract. The reaction was carried out at 70 °C for 1 h, using 0.1 N copper sulfate, silver nitrate, and ruthenium chloride as precursor solutions. The TEM image showed that the synthesized nanoparticles were spherical, exhibited an agglomerated structure, and had a mean size of 22 nm. The nanoparticles exhibited exceptional photocatalytic activity and strong antibacterial activity against E. coli, S. pyogenes, S. aureus, and K. pneumoniae, with the largest zone of inhibition observed at 75 µL. They also demonstrated anticancer activity against human breast cancer cell lines, as well as notable antioxidant properties [145]. Mohammed and Idrees (2024) reported the synthesis of Co–Zn–Ni trimetallic oxide nanoparticles using leaf extracts of Cicer arietinum. The resulting nanoparticles exhibited a spherical morphology with an average diameter of 25.72 nm. Furthermore, they demonstrated stronger anti-biofilm activity against Staphylococcus aureus compared to the commonly used antibiotic ciprofloxacin [140].

5.2. Metal Oxide-Based Nanoparticles

Metal oxide nanoparticles represent a distinct class of inorganic nanomaterials. A range of metal oxide nanoparticles, including copper oxide, zinc oxide, cerium oxide, cobalt oxide, titanium dioxide, calcium oxide, silicon oxide, manganese oxide, iron oxide nanoparticles, etc., have been cited in scholarly literature. To date, their applications in renewable energy conversion, chemical reactions, sensing systems, data storage technologies, and electronics have been extensively studied [146]. Recent studies have demonstrated the green synthesis of metal oxide nanoparticles, such as γ-Al₂O₃, using plant extracts like Oxystelma esculentum, which exhibit excellent photocatalytic efficiency for both pollutant degradation and CO₂ photoreduction. The biogenic approach not only reduces reliance on hazardous chemical precursors but also enhances surface reactivity and stability, thereby highlighting the potential of such nanomaterials for applications in environmental remediation and renewable energy [147]. A similar study by Farooq et al. (2025) investigated the green synthesis of cerium oxide (CeO₂) nanoparticles, highlighting their remarkable photocatalytic activity and potential for environmental remediation. They synthesized CeO₂ nanoparticles using Ammi visnaga plant extract and achieved 96.19% degradation of methylene blue within 1 h, along with a CO₂ photoreduction efficiency of 109.27 μmol·g⁻¹·h⁻¹ under optimized conditions [148]. Although plant-mediated fabrication of metal oxide nanoparticles is more complex than that of metal-based nanoparticles, it often requires high-temperature treatment to obtain nanoparticles from metal oxides, as these compounds tend to be less stable. Therefore, to remove oxygen from metal oxides, treatments such as annealing or calcination are employed, typically requiring temperatures above 300 °C [149]. The primary purpose of their synthesis is to enhance the properties of the corresponding metal nanoparticles [80]. Metal oxide nanoparticles exhibit a wide range of unique optical, magnetic, and electrical properties, which has led to their extensive exploration for applications across various fields [146]. Representative studies on plant-mediated synthesis of metal oxide nanoparticles are summarized in Supplementary Table S4 [52, 65-67, 79, 104, 150-180].

5.2.1. Copper Oxide Nanoparticles

Due to the magnetic, optical, and electrical properties of copper oxide, there has been accelerated growth in the synthesis of its nanoparticles, which exhibit a broad range of applications. In the existing literature, copper oxide nanoparticles derived from various plants have undergone exploration for their antimicrobial efficacy, photocatalytic activity, and cytotoxic properties [150]. Alhalili Z (2022) reported the fabrication of copper oxide nanoparticles using a leaf extract of Eucalyptus globulus as a reducing and stabilizing agent. The synthesized nanoparticles exhibited a spherical morphology with an average diameter of 88 nm. Furthermore, their potential as an adsorbent was evaluated by employing copper oxide nanoparticles for the removal of methyl orange from aqueous environments. Their study indicated a maximum adsorption capacity of 92.2 mg/g under optimized conditions, including an adsorbent dosage of 0.045 g, pH 6, and a temperature of 25 °C [150]. In a study conducted by Nasrollahzadeh et al. (2015), copper oxide nanoparticles were synthesized using flower extracts of Anthemis nobilis and subsequently employed as catalysts in the aldehyde–amine–alkyne (A3) coupling reaction [181].

According to Andualem et al. (2020), copper oxide nanoparticles were synthesized using Catha edulis extracts, which acted as both stabilizing and reducing agents, with copper nitrate trihydrate serving as the precursor. The reaction mixture was stirred constantly by keeping the temperature at 80 °C until the solution’s color shifted from deep green to deep brown. The biosynthesized nanoparticles were observed to exhibit a spherical morphology, as confirmed by SEM and TEM analyses. The authors evaluated the antibacterial activity of the synthesized nanoparticles and reported that those prepared using a leaf extract-to-precursor salt ratio of 1:2 exhibited the highest antibacterial efficacy, with zones of inhibition (ZOI) of 29 ± 0.03 mm, 32 ± 0.02 mm, 24 ± 0.02 mm, and 22 ± 0.01 mm against Klebsiella pneumoniae, Escherichia coli, Streptococcus pyogenes, and Staphylococcus aureus, respectively [182]. Similarly, a study conducted by Abosede et al. (2022) utilized leaf extract from Justicia carnea for the synthesis of copper oxide nanoparticles, using copper sulfate pentahydrate as the precursor salt. The formation of a brown precipitate after 40 min confirmed the successful synthesis of nanoparticles [183]. Afzal et al. (2022) documented the synthesis of copper oxide nanoparticles using leaf extracts of Aerva javanica and copper sulfate pentahydrate. The process was carried out at pH 11 and maintained at 60 °C for 2 h under constant stirring. The nanoparticles showed an agglomerated structure and had promising potential as antifungal and antibacterial agents [184]. Sukumar et al. (2020) discovered that the seed extract of Caesalpinia bonducella can serve as an effective agent for synthesizing copper oxide nanoparticles. The antibacterial potential of the obtained nanoparticles was assessed against Staphylococcus aureus and Aeromonas [185]. Atri et al. (2023) observed the octahedral crystal clusters of copper oxide nanoparticles synthesized using the aerial parts of Ephedra alata. The reaction was carried out at 90 °C for 2 h. The obtained nanoparticles also exhibited photocatalytic activity and antimicrobial potential against C. albicans (ZOI = 16.2 mm), S. cerevisiae (ZOI = 18.4 mm), B. Subtilis (ZOI = 16 mm), and S. aureus (ZOI = 20.4 mm). Additionally, biosynthesized copper nanoparticles were found to be much more efficient than chemically synthesized ones [186].

5.2.2. Zinc Oxide Nanoparticles

Zinc oxide, an n-type semiconductor, possesses a high exciton binding energy and a wide band gap, making it suitable for applications in photocatalysis, light-emitting diodes, electrochemical systems, sensors, and solar cells [149]. Plant-extracted Zinc oxide nanoparticles have found widespread applications in the pharmaceutical field owing to their antimicrobial, antilarvicidal, anti-inflammatory, and antitumor properties, as well as their potential in drug delivery and imaging. Their low toxicity and cost-effectiveness have also garnered a considerable amount of attention from the researchers [187]. Manojkumar et al. (2023) reported a hexagonal wurtzite structure for zinc oxide nanoparticles synthesized using leaf extract of Brassica oleracea var. botrytis, with an average particle size of 52 nm. The nanoparticles displayed good antimicrobial potential against Escherichia coli (inhibition zone of 9 mm), Klebsiella pneumonia (inhibition zone of 13 mm), Staphylococcus aureus (inhibition zone of 13 mm) and Bacillus subtilis (inhibition zone of 16 mm) respectively, effective photocatalytic breakdown of methylene blue and larvicidal activity against the larva of Culex quinquefasciatus with the LC50 and LC90 value being 76.03 and 190.03 ppm, respectively [188]. Idris et al. (2024) successfully synthesized zinc oxide nanoparticles using leaf extract of Mariposa christia vespertilioni. The nanoparticles exhibited an agglomerated and irregular morphology, with particle sizes ranging from 38 to 65 nm. The nanoparticles demonstrated excellent potential as anode materials for sodium-ion batteries (SIBs) [189]. Al-Radadi et al. (2022) synthesized zinc oxide nanoparticles using underground stem extract of Zingiber officinale. The reaction was carried out at 60 °C, 11 pH, 60 min of incubation time, and 2.0 mg of metal salt concentration. The authors also revealed that the obtained nanoparticles improved the antibacterial potential of various antibiotics like Ciprofloxacin, Imipenem, Vancomycin, and Amoxicillin-Clavulanic acid against Escherichia coli and Staphylococcus aureus. The nanoparticles were also found to exhibit hemo-compatible, antioxidant, and anti-inflammatory properties. Additionally, the nanoparticles exhibited anti-diabetic properties by inhibiting alpha-amylase and anti-Alzheimer’s properties by inhibiting acetylcholinesterase (AChE) and butyrylcholinesterase (BChE) [91].

Al-Awadh et al. (2022) reported the synthesis of spherical zinc oxide nanoparticles with an average particle size of 66.47 nm using leaf extract of Raphanus sativus and zinc acetate dihydrate (Zn(O₂CCH₃)₂·(H₂O)₂) as the precursor salt. The synthesis of nanoparticles was found to be highly efficient at 12 pH and 80 °C and showed good cytotoxicity in inhibiting human breast cancer cells [66]. Karimzadeh et al. (2020) reported the synthesis of spherical ZnO nanoparticles with particle sizes ranging from 20 to 80 nm, using aerial parts of Acantholimon serotinum at 60 °C and pH 8 for 6 h. The development of a yellowish-brown coloration indicates the successful formation of nanoparticles. The nanoparticles were further investigated for their anti-cancerous activity against HEK-293 (IC50: 60 µg/mL), MDA-MB-231 (IC50: 24 µg/mL), SH-SY5Y (IC50: 42 µg/mL), and Caco-2 (IC50: 61 µg/mL) [190]. Sana et al. (2020) reported the synthesis of ZnO nanoparticles using leaf extract of Crotalaria verrucosa. The biosynthesized nanoparticles exhibited a hexagonal structure with particle sizes ranging from 16 to 38 nm. They also demonstrated promising antimicrobial activity against Klebsiella pneumoniae, Proteus vulgaris, Staphylococcus aureus, and Escherichia coli, as well as anticancer potential against DU145 and HeLa cell lines [191].

5.2.3. Iron Oxide Nanoparticles

Iron, one of the most abundant elements in the Earth’s crust, has attracted significant interest for the synthesis of nanoparticles. Its oxides exist in four crystalline forms: α-Fe₂O₃, β-Fe₂O₃, γ-Fe₂O₃, and ε-Fe₂O₃. Among these, α-Fe₂O₃, β-Fe₂O₃, and γ-Fe₂O₃ are commonly referred to as hematite, magnetite, and maghemite, respectively. Among these structures, α-Fe₂O₃ (hematite) is the most thermodynamically stable and exhibits desirable properties, including low toxicity, chemical inertness, and biocompatibility. These characteristics have motivated extensive research into its nanoparticle synthesis and its applications across various fields [192]. To date, many methods have been employed to derive α-Fe2O3 nanoparticles, including Sol-gel reactions, co-precipitation, microwave processing, and microemulsion. However, green synthesis is currently being preferred over them. Aida et al. (2023) reported the synthesis of hematite (α-Fe₂O₃) nanoparticles using hibiscus floral extract and FeCl₃ solution, resulting in nanoparticles with an average size of 20 nm [193]. Earlier, Miri et al. (2019) described the fabrication of α- Fe₂O₃ nanoparticles using a bark extract of Salvadora persica. The reaction was carried out at 70 °C for 2 h, with the pH adjusted to 7 and 11. At pH 7, calcination at 350 °C led to the predominance of maghemite (γ-Fe₂O₃) nanoparticles, whereas calcination at 450 °C favored the formation of hematite (α-Fe₂O₃) nanoparticles. The hematite phase exhibited a spherical morphology with particle sizes ranging from 15 to 20 nm. The study also revealed the superparamagnetic properties of the biosynthesized nanoparticles [161]. Desalegn et al. (2019) reported the synthesis of iron nanoparticles using peel extract of Mangifera indica [194].

Rostamizadeh et al. (2020) reported the synthesis of Fe₂O₃ nanoparticles using Cornus mas L. fruit extract. The nanoparticles were spherical in morphology, with sizes ranging from 20 to 40 nm. The study indicated the potential of Fe₂O₃ nanoparticles for producing nanofertilizers [195]. Jamzad et al. (2020) also successfully produced Fe₂O₃ nanoparticles using the leaves extract of Laurus nobilis L. The nanoparticles exhibited a predominantly spherical morphology with partial hexagonal features, and an average size of 8.03 ± 8.99 nm. They also demonstrated significant antibacterial and antifungal activity [196]. Buarki et al. (2022) evaluated the antibacterial inhibition efficacy of nearly spherical Fe₂O₃ nanoparticles synthesized using Hibiscus rosa-sinensis flower extract and FeCl₂·4H₂O in a 2:1 ratio. The nanoparticles were tested against various bacterial strains, including Escherichia coli, Klebsiella pneumoniae, Pseudomonas aeruginosa, and Staphylococcus aureus. The nanoparticles exhibited an average diameter of 51 nm. However, nanoparticles synthesized using a 1:1 ratio of flower extract and precursor salt exhibited better antibacterial activity, with inhibition zones of 5 mm (Escherichia coli), 6 mm (Klebsiella pneumoniae), 4 mm (Pseudomonas aeruginosa), and 3 mm (Staphylococcus aureus), respectively [156]. Shabbir et al. (2023) successfully synthesized uniformly spherical Fe₂O₃ nanoparticles using Madhuca indica leaf extract and FeSO₄ solution at 37 °C for 24 h. The nanoparticles exhibited a size range of 50 to 60 nm. The synthesized nanoparticles exhibited effective free radical scavenging activity at a concentration of 1000 µg/mL, along with notable anti-inflammatory and anti-diabetic effects via inhibition of the α-amylase enzyme in a concentration-dependent manner (IC₅₀: 73.3% at 1000 µL/mL) [155]. Lakshminarayanan et al. (2021) conducted a study using leaf extract of Bauhinia tomentosa for the synthesis of cluster-like Fe₂O₃ nanoparticles with an average diameter of 70 nm. The synthesized nanoparticles were subsequently employed in the production of 1,3-diolein [157].

5.2.4. Titanium Dioxide Nanoparticles

Titanium dioxide, a white inorganic compound, has been extensively used in water purification due to its catalytic properties. Titania (TiO₂) nanoparticles are well known for their optical and dielectric properties, chemical stability, non-toxicity, and strong oxidative capability. Titania exists in three polymorphic forms, namely anatase, rutile, and brookite. Among these, anatase is more commonly preferred in green synthesis due to its relatively higher stability and enhanced photocatalytic activity [197]. Bopape et al. (2023) prepared titanium dioxide nanoparticles using different concentrations of Commelina benghalensis plant extract, 10–30 g, and reported that on increasing the plant extract’s concentration, much smoother and distinct spherical nanoparticles were observed with a size range of 30–200 nm. They also noted their photodegradation efficiency for MB (65%) and SSX (82.4%) [167]. Rathi et al. (2023) reported the synthesis of spherical (15–28 nm) TiO₂ nanoparticles using flower extract of Caesalpinia pulcherrima, irregular and near-spherical TiO₂ nanoparticles using peel extract of Manihot esculenta, and agglomerated quasi-spherical TiO₂ nanoparticles using leaf extract of Nervilia aragona. These nanoparticles were effectively applied as catalysts for the degradation of methylene blue and also exhibited antimicrobial activity against Pseudomonas aeruginosa, Escherichia coli, Candida albicans, and Staphylococcus aureus [198]. Mbenga et al. (2023) recently reported the synthesis of TiO₂ nanoparticles using leaf extract of Tulbaghia violacea. The resulting nanoparticles exhibited rectangular and irregular morphologies and demonstrated moderate antioxidant and cytotoxic activities against HEK 293 and HeLa cell lines [168].

Sethy et al. (2020) synthesized titanium dioxide nanoparticles using leaf extract of Syzygium cumini for stabilization and capping at room temperature over 8 h. The resulting nanoparticles were employed as catalysts for the photocatalytic removal of lead (Pb) from water bodies in industrial areas [199]. Aravind et al. (2021) reported that titanium dioxide nanoparticles synthesized using jasmine flower extracts exhibited superior photodegradation efficiency of methylene blue (92%) compared to titanium dioxide nanoparticles prepared via the hydrothermal (chemical) method [200]. Balaraman et al. (2022) reported the plant-mediated synthesis of titanium dioxide nanoparticles using an aqueous extract of Sargassum myriocystum, yielding nanoparticles with cubic, square, and spherical morphologies and a particle size distribution of 50–90 nm. These nanoparticles exhibited effective antibiofilm activity against Proteus vulgaris, Staphylococcus epidermidis, Staphylococcus aureus, Escherichia coli, Pseudomonas aeruginosa, and Klebsiella pneumoniae, as well as larvicidal activity against Aedes aegypti and Culex quinquefasciatus, and photocatalytic degradation of methylene blue [201]. Mohany et al. (2023) synthesized spherical titanium dioxide nanoparticles using plant extract of Solanum surattense, with particle sizes ranging from 10 to 80 nm. The color change of the reaction mixture from light green to whitish-brown after continuous stirring at ambient temperature for 24 h confirmed the formation of nanoparticles. These nanoparticles were found to be effective in reducing seizure duration, as demonstrated in maximal electroshock-induced epilepsy (MES) and pentylenetetrazole (PTZ) models. Additionally, they exhibited significant cytotoxic activity against the THP-1 cell line at a concentration of 80 µg/mL [173].

5.2.5. Magnesium Oxide Nanoparticles

Magnesium oxide nanoparticles are amongst the most widely studied metal oxide nanoparticles because of their high stability, low cost, and wide band gap. To date, they have been employed in a wide range of applications, including water purification, fuel additives, sensing, adsorption, and antimicrobial applications [180]. Magnesium oxide (MgO) nanoparticles have also been extensively utilized in biomedical applications, including the treatment of stomach ailments, incorporation into blood collection devices, relief of heartburn, capsule coatings, and bone regeneration, among others [202]. Muhaymin et al. (2024) utilized fruit extract of Hyphaene thebaica for the synthesis of magnesium oxide nanoparticles with a spherical morphology and an average diameter of 32.6 ± 5 nm. The nanoparticles showcased remarkable photocatalytic activity of methylene blue dye (98% after 110 min) [180]. Younis et al. (2021) synthesized MgO nanoparticles by employing flower extracts of Rosa floribunda charisma and obtained polyhedral-shaped particles of 35.25–55.14 nm size range as confirmed by HR-TEM images. The synthesized nanoparticles exhibited antioxidant activity against xanthine oxidase, hydroxyl, nitric oxide, and superoxide radicals, with IC₅₀ values of 15.9 µg/mL, 31.9 µg/mL, 52.9 µg/mL, and 26.2 µg/mL, respectively, using ascorbic acid as the control. The nanoparticles also exhibited anti-aging activity in dose-dependent manner by inhibition of enzymes like tyrosinase (IC₅₀: 158.6 ± 5.20 µg/mL), hyaluronidase (IC₅₀: 191.4 ± 5.68 µg/mL), elastase (IC₅₀: 82.5 ± 2.93 µg/mL), hyaluronidase (IC₅₀: 191.4 ± 5.68 µg/mL) and collagenase (IC₅₀: 58.7 ± 1.66 µg/mL). Moreover, the nanoparticles were found to be antibacterial against three skin pathogens, P. aeruginosa (MIC: 31.25 µg/mL), Streptococcus pyogenes (MIC: 7.81 µg/mL), and Staphylococcus epidermis (MIC: 15.63 µg/mL). Additionally, the nanoparticles exhibited anti-biofilm activity against the aforementioned bacterial strains, with minimum inhibitory concentrations for biofilm formation of 7.81, 1.95, and 1.95 µg/mL, respectively [175].

Hirphaye et al. (2023) demonstrated the synthesis of magnesium oxide (MgO) nanoparticles using female flowers of Hagenia abyssinica. The process was conducted at pH 12 and 60 °C for 120 min. SEM analysis revealed that the biogenic nanoparticles were spherical in shape, with particle sizes ranging from 10 to 40 nm. Authors also discussed how different parameters such as duration of reaction, concentration of precursor salt, temperature, volume of plant extract, and pH play an important role in the biogenic formation of MgO nanoparticles and showcased the antimicrobial potential of the obtained nanoparticles against Escherichia coli and Staphylococcus aureus with their zone of inhibition recorded to be 15 ± 0 mm and 27 ± 0.28 mm, respectively [67]. Abinaya et al. (2023) described the synthesis of magnesium oxide nanoparticles using leaf extract of Psidium guajava, which exhibited a spherical morphology and an average particle size of 85.8 nm. The nanoparticles were evaluated for their larvicidal and antibacterial activities [177]. Likewise, Dinga et al. (2022) reported the synthesis of magnesium oxide nanoparticles using seed extract of Melia azedarach at varying extract volumes of 20, 10, and 5 mL. The authors observed that, although all nanoparticles were spherical in shape, their edges became more defined with increasing seed extract volume, while particle size decreased from 73.29 nm to 62.4 nm and 44.29 nm with the use of 5, 10, and 20 mL of seed extract, respectively. Moreover, these nanoparticles were found to be effective as larvicidal and antioxidant agents [174].

5.3. Carbon-Based Nanoparticles

The emergence of carbon nanoparticles has gained significant interest due to their extensive applications in modern technologies, including water purification, energy storage materials, catalysis, electronics, biofuels, and bioimaging. Carbon nanoparticles comprise a diverse class of nanomaterials, including nanodiamonds (NDs), graphene and its derivatives, carbon nanotubes (CNTs), carbon nanohorns (CNHs), carbon nanofibers (CNFs), carbon-based quantum dots (CQDs), graphene oxide, and fullerenes [203,204]. Carbon nanotubes are classified into single-walled and multi-walled types based on the number of graphene layers in their structure. Their dimensions and morphology can be tailored by modifying synthesis conditions, such as reaction gases, temperature, precursors, and catalysts, during biosynthesis. With the increasing popularity of carbon nanotubes in biomedical applications, green synthesis has become essential for their cost-effective large-scale production. This one-step synthesis approach eliminates the need for conventional chemical catalysts, which are often limited or hazardous, and instead utilizes plant extracts as eco-friendly alternatives. Quantum dots (QDs) exhibit high luminescence, strong photostability, low toxicity, chemical inertness, and good biocompatibility compared to other carbon nanomaterials, giving them a significant advantage in biomedical applications [205].

Carbon quantum dots can function simultaneously as nanocarriers and imaging agents in drug delivery systems, as reported by Ross (2020) [206]. Nanodiamonds were first discovered incidentally during detonation processes. Their core lattice exhibits sp³ hybridization, while sp² hybridization is typically present at the surface. Due to their low toxicity and fluorescence-emitting properties, nanodiamonds are considered highly suitable for applications in diagnostics and bioimaging. Nanodiamonds include nanocrystalline diamonds (NCD) with a particle size measuring below 100 nm and ultrananocrystalline diamond (UNCD) with a particle size measuring below 10 nm [205]. Liu et al. administered nanodiamonds to a preadipocyte fibroblast cell line (3T3-L1) and a lung adenocarcinoma epithelial cell line (A549). Cell imaging studies have demonstrated that nanodiamonds do not induce cytotoxic effects, even with prolonged exposure, and do not interfere with essential cellular processes such as cell division, differentiation, or gene expression. These properties make them highly suitable for tracking and identifying both stem cells and cancer cells [207]. In a related study by Perevedentseva et al., the uptake of nanodiamonds was investigated in both non-cancerous and cancerous cell lines, including HFL1 fibroblast-like human fetal lung cells, BEAS-2B non-tumorigenic human bronchial epithelial cells, and A549 lung adenocarcinoma cells, and subsequently evaluated [208]. However, the green synthesis of nanodiamonds remains highly limited to date. In contrast, graphene, which possesses a honeycomb-like structure, has attracted considerable interest in green chemistry, as several studies have reported its synthesis using biological agents, as described by Kim et al., Ruan et al., and Gurunathan et al. [209-211].

Graphene nanomaterials, owing to the presence of carboxyl (–COOH) functional groups on their surface, can form strong interactions with nucleic acid bases, enabling their effective use as biosensors for DNA and RNA. Their potential applications as nanocarriers for anticancer drugs have also been extensively explored [205]. Qu et al. (2016) described the green fabrication of carbon nanotubes using Brassica juncea extracts. Further analysis revealed that the carbon nanotubes possess a hollow core, with an outer diameter of approximately 80 nm [212]. Recently, Singh et al. (2024) synthesized carbon nanoparticles utilizing different plants, including Azadirachta indica, Pisidium guajava, Mangifera indica, Bergera koenigii, and Syzygium cumini leaves extract. Based on scanning electron microscopy (SEM) analysis, all nanoparticles were observed to exhibit a quasi-spherical morphology with varying particle sizes. The average size distribution was around 23.5 nm, 32 nm, 33 nm, 42.5 nm, and 37 nm for nanoparticles synthesized from Syzygium cumini, Mangifera indica, Azadirachta indica, Bergera koenigii, and Pisidium guajava, and Holy basil, respectively. The obtained nanoparticles exhibited remarkable antioxidant properties [213].

Tripathi et al. (2017) observed the hollow rectangular cross-section carbon nanobelts and single-walled nanotubes by utilizing several plants like Cynodon dactylon, Rosa, Azadirachta indica, and Juglans regia leaf extracts. The nanomaterials were further characterized by using HR-TEM, Raman, and IR spectroscopy [214]. However, green synthesis routes for carbon nanomaterials often require high temperatures and prolonged reaction times. To address this high energy consumption limitation, Arizaga et al. (2022) fabricated carbon nanomaterials using tomato extracts. Tomato pulp was mixed with aluminum nitrate nonahydrate (Al(NO₃)₃·9H₂O) and magnesium nitrate hexahydrate (Mg(NO₃)₂·6H₂O) at pH 8.5, followed by calcination at 60 °C. FESEM analysis confirmed a semi-spherical morphology of the nanoparticles, which were subsequently investigated for their fluorescent response toward metal cations and photothermal activity [215]. Nadeem et al. (2022) synthesized spherical carbon nanoparticles using linseed, olive, and mustard seed oils, yielding particle sizes of 57 nm, 24 nm, and 18 nm, respectively. The nanoparticles produced were assessed for their antibacterial efficacy against Staphylococcus aureus, Proteus refrigere, Streptococcus haemolyticus, and Pseudomonas aeruginosa and showed excellent results [216]. Damera et al. (2020) described the production of carbon nanoparticles utilizing Eucalyptus twigs and ammonium chloride. The nanoparticles were multi-emissive and referred to as light blue, blue, green, and red carbon nanoparticles. Light blue carbon nanoparticles exhibited remarkable sensing capabilities toward Brilliant Blue (BB), a toxic synthetic food colorant [217].

6. Application in Energy

7. Conclusions and Future Prospects

List of Abbreviations

Author Contributions

Conflicts of Interest

Funding

Acknowledgements

AI-Declaration

Supplementary Materials

References

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