The small size of nanoparticles leads to a large surface area relative to volume, which enhances reactivity and interaction with other substances. This makes nanoparticles highly effective in catalysis and adsorption processes (17,18).

At the nanoscale, quantum mechanics begins to dominate material behaviour, leading to unique optical, electronic and magnetic properties that are not present in larger particles (19).

Nanoparticles possess higher surface energy, which can affect their stability and reactivity. As a result, they tend to agglomerate to reduce surface energy, but can be stabilised through surface treatments or coatings (20).

Some nanoparticles, such as CNTs, exhibit exceptional mechanical properties, such as high tensile strength and flexibility (21).

Nanoparticles can exhibit unique optical properties due to surface plasmon resonance, particularly in metals such as gold and silver, rendering them useful in imaging, sensing and photothermal applications (19).

Major developments in the field of MetNPs include a transition from early physical and chemical synthesis methods to modern green, biogenic methods that provide sustainable and eco-friendly production (Table II). Advances in nanoscale characterisation and mechanistic understanding have enabled greater control over particle size, shape and surface chemistry. These breakthroughs have led to rapid growth in applications such as biomedicine, catalysis, sensing, and environmental remediation, with recent advances focusing on the synthesis of hybrid nanocomposites and nanozymes.

MetNPs are metallic particles ranging from 1 to 100 nanometres in size and exhibit unique physical, chemical and optical properties that differ significantly from their bulk counterparts. These properties arise from their high surface-area-to-volume ratio and quantum effects, making them highly versatile across various fields (22). The size, shape and surface characteristics of MetNPs can be tailored during synthesis, influencing reactivity, catalytic activity, and optical behaviour (23). Common types include gold nanoparticles, known for their biocompatibility and use in medical diagnostics and drug delivery; silver nanoparticles, widely used for antimicrobial properties; and iron nanoparticles, effective in environmental remediation. Platinum and copper nanoparticles are valued for their catalytic and electrical properties, respectively. The applications of MetNPs span diverse areas. In environmental remediation, they are used for heavy metal removal, the degradation of organic pollutants and the catalytic breakdown of contaminants (24). In medicine, they serve as drug delivery agents, imaging tools and antimicrobial agents. Their role in energy applications includes use in fuel cells and batteries, while in sensors and diagnostics they enhance disease detection and environmental monitoring (25). MetNPs are synthesised using chemical, physical and biological methods, including chemical reduction of metal salts, laser ablation and green synthesis using plant extracts or microorganisms. However, stability issues, potential toxicity and scalability concerns need to be addressed to fully harness their potential. Despite these challenges, MetNPs represent a promising avenue for innovation across multiple industries.

MONPs are a class of nanomaterials consisting of metal cations bonded to oxygen anions. These nanoparticles exhibit exceptional properties, including high stability, catalytic activity, and tuneable electronic, magnetic and optical characteristics. Their unique properties are attributed to their small size, high surface-area-to-volume ratio, and quantum confinement effects, rendering them highly desirable in various scientific and industrial applications. MONPs are synthesised from a wide variety of metals, with some of the most common types being TiO₂, ZnO, iron oxide (Fe₂O₃/Fe₃O₄), cerium oxide (CeO₂) and aluminium oxide (Al₂O₃). TiO₂ is widely used for its photocatalytic activity in environmental remediation and self-cleaning surfaces (26). ZnO is prized for its antimicrobial properties and applications in sunscreens and sensors. Iron oxide nanoparticles, particularly magnetite (Fe₃O₄), are utilised in biomedical applications, such as magnetic resonance imaging (MRI) and targeted drug delivery. CeO₂ is known for its role as a redox catalyst, particularly in automotive catalytic converters and as an antioxidant in biological systems (27). The applications of MONPs span a range of fields. In environmental science, they are employed for water and air purification, as well as for the degradation of organic pollutants through photocatalytic processes (28,29). In the medical field, they play a role in drug delivery, imaging, and antimicrobial treatments (30). MONPs are also extensively used in energy storage and conversion devices, such as lithium-ion batteries, supercapacitors, and fuel cells (31). Their catalytic properties make them essential in industrial chemical processes, including the production of fertilisers and the refinement of fuels. Several synthesis methods are used to produce MONPs, including sol-gel processes, hydrothermal methods, chemical vapor deposition, and green synthesis using biological agents (32). These methods allow control over the size, shape and surface characteristics of the nanoparticles, which are critical for optimising their performance in specific applications. Despite their advantages, MONPs pose challenges, such as potential toxicity, environmental persistence and the need for cost-effective and scalable production methods (33). Addressing these challenges is essential for their safe and sustainable use across various sectors, highlighting their potential to revolutionise industries ranging from healthcare to environmental management.

CBNPs are nanomaterials composed primarily of carbon atoms, engineered in various structures and dimensions. These nanoparticles exhibit notable mechanical, electrical, thermal and chemical properties, rendering them a cornerstone of nanotechnology research and applications. Their versatility stems from the unique bonding nature of carbon, which allows them to form various allotropes and hybrid structures (34). Key types of CBNPs include CNTs, fullerenes, graphene, graphene oxide, carbon dots (CDs) and nanodiamonds. CNTs are cylindrical structures with extraordinary tensile strength, electrical conductivity, and thermal stability, making them valuable in electronics, materials reinforcement, and energy storage (21). Fullerenes, spherical carbon molecules, are known for their electron-accepting capabilities and are used in drug delivery, solar cells and antioxidants. Graphene is a two-dimensional sheet of carbon atoms arranged in a hexagonal lattice, renowned for its high electrical and thermal conductivity, flexibility, and strength, leading to applications in flexible electronics, sensors and advanced composites (34). CDs are fluorescent nanoparticles with tuneable optical properties, commonly used in bioimaging and sensing. Nanodiamonds exhibit exceptional hardness and biocompatibility, making them ideal for polishing, drug delivery, and imaging (35). The applications of CBNPs span multiple disciplines. In medicine, they serve as drug delivery agents, bioimaging tools and antimicrobial materials. In electronics, they are employed in transistors, flexible displays and conductive coatings. Their thermal properties render them useful in heat management systems, while their mechanical strength is exploited in the development of advanced composites. Environmental applications include water purification, pollutant adsorption and catalysis for pollutant degradation. Additionally, their role in energy storage and conversion, such as in batteries, supercapacitors, and fuel cells, highlights their importance in renewable energy technologies. CBNPs are synthesised through various techniques, including chemical vapor deposition, laser ablation, arc discharge and green synthesis using biological agents. These methods allow control over their size, structure and functionalisation, which are critical for tailoring them to specific applications. However, challenges, such as production scalability, cost, potential toxicity and environmental impact need to be addressed. Despite these obstacles, CBNPs represent a transformative class of nanomaterials, with the potential to drive advancements in science, technology and industry (29,33-35).

PNPs are nano-sized particles composed of polymers, typically ranging in size from 1 to 1,000 nanometres. These nanoparticles are highly versatile and can be engineered for a wide range of applications due to their biocompatibility, chemical tunability, and ability to encapsulate and protect active agents. Their structure often includes a polymeric core and shell, which can be functionalised to enhance stability, target specificity and controlled-release properties. PNPs are categorised into different types based on their structure. Nanospheres are solid matrix-like particles in which the active compound is dispersed or adsorbed throughout the polymer matrix. Nanocapsules, on the other hand, are vesicular systems in which the active agent is enclosed within a polymeric shell (36,37). The polymers used in PNP synthesis can be natural (e.g., chitosan, gelatin and alginate) or synthetic [e.g., polylactic acid, polyglycolic acid and poly(lactic-co-glycolic acid)] and are selected based on the intended application and required properties (38). The applications of PNPs are vast, with their most prominent role in the biomedical field. They are widely used as drug delivery systems, enabling the encapsulation of therapeutic agents for enhanced solubility, stability and targeted delivery. Their ability to release drugs in a controlled manner reduces side-effects and improves therapeutic efficacy. In cancer therapy, PNPs can be designed to deliver chemotherapeutics directly to tumour cells, minimising damage to healthy tissues (36,39). They are also used in gene therapy, vaccine delivery and tissue engineering. Beyond medicine, PNPs are employed in the food industry for the controlled release of nutrients and additives, in agriculture for the delivery of pesticides and fertilisers, and in environmental applications for pollutant removal. Synthesis methods for PNPs include emulsion techniques (single or double emulsion), precipitation, solvent evaporation, and ionic gelation, among others. These methods allow the precise control over particle size, shape and surface properties. The functionalisation of the nanoparticle surface with ligands or coatings further enhances their ability to target specific cells or tissues, a feature particularly valuable in medical applications. Despite their advantages, challenges remain in the use of PNPs, such as scaling up production, ensuring uniformity, and addressing potential environmental and biological toxicity. Advances in polymer chemistry and nanoparticle engineering continue to overcome these hurdles, rendering PNPs a key player in modern nanotechnology and materials science. Their adaptability and functionality promise significant contributions across diverse industries, from healthcare to environmental sustainability (37,39-42).

LNPs are nanoscale structures composed of lipids, typically ranging in size from 10 to 1,000 nanometres. They have garnered significant attention due to their biocompatibility, ability to encapsulate hydrophilic and hydrophobic molecules and potential for targeted delivery. Their unique properties render them indispensable in drug delivery systems, particularly for delivering challenging therapeutic agents such as nucleic acids and poorly soluble drugs. The structure of LNPs generally includes a lipid bilayer or a core-shell configuration, in which a hydrophobic core is surrounded by a lipid shell (43-45). Common types of LNPs include liposomes, solid lipid nanoparticles (SLNs) and nanostructured lipid carriers (NLCs). Liposomes are spherical vesicles with one or more lipid bilayers, capable of encapsulating hydrophilic drugs in their aqueous core and hydrophobic drugs within their lipid bilayer (46). SLNs are composed of a solid lipid core stabilised by surfactants, providing a highly stable system for drug encapsulation. NLCs are an advanced version of SLNs, incorporating both solid and liquid lipids to improve drug-loading and release characteristics. LNPs are widely used in the pharmaceutical and biomedical fields. They play a crucial role in the delivery of small molecules, peptides, proteins and nucleic acids, providing protection from enzymatic degradation and enhancing cellular uptake. LNPs have revolutionised the field of gene therapy and mRNA vaccines, as demonstrated by their use in COVID-19 mRNA vaccines (43,46-48). In cancer therapy, they enable targeted delivery of chemotherapeutics to tumour cells while minimising systemic toxicity. Beyond healthcare, LNPs are applied in cosmetics for enhanced skin delivery, in agriculture for the delivery of pesticides and fertilisers, and in food technology for encapsulating nutrients and bioactives. The synthesis of LNPs involves methods, such as thin-film hydration, microfluidics, high-pressure homogenisation and solvent evaporation (38). These techniques allow for the precise control over particle size, surface charge and encapsulation efficiency. Surface functionalisation with ligands, such as antibodies or peptides, enables targeted delivery to specific cells or tissues, enhancing therapeutic efficacy. While LNPs offer numerous advantages, challenges persist, including stability issues, scalability for industrial production and potential immunogenicity (49,50). Addressing these challenges through advances in lipid chemistry and nanotechnology will further enhance their efficacy and broaden their applications. LNPs continue to be a cornerstone of modern nanomedicine and materials science, with the potential to address unmet needs in healthcare, agriculture and beyond.

Semiconductor nanoparticles, commonly known as QDs, are nanometre-scale crystals typically ranging from 2 to 10 nanometres in size. These nanoparticles exhibit unique quantum mechanical properties, particularly quantum confinement, which arises when the particle size is smaller than the exciton Bohr radius. This confinement results in discrete energy levels and size-dependent optical and electronic properties, rendering QDs highly versatile in various applications. QDs are composed of semiconductor materials such as cadmium selenide (CdSe), cadmium telluride (CdTe), indium phosphide (InP) and zinc sulfide (ZnS). Their most notable property is their size-tuneable fluorescence; by varying the particle size, QDs can emit light across the visible to infrared spectrum (9). This property, combined with their high photostability, broad absorption spectra, and narrow emission peaks, renders QDs invaluable for imaging and optoelectronic applications. The applications of QDs span multiple fields. In biomedicine, they are widely used as fluorescent probes for bioimaging and biosensing due to their brightness and resistance to photobleaching. QDs are instrumental in the development of advanced diagnostics, enabling high-resolution imaging of cellular and molecular processes. In optoelectronics, QDs are employed in QD light-emitting diodes (QD-LEDs), offering vibrant displays with superior colour accuracy and energy efficiency (9,35). They are also used in solar cells to improve light absorption and energy conversion efficiency, as well as in lasers and photodetectors. In environmental science, QDs are used in sensors for detecting pollutants and toxins, as well as in photocatalysis for water and air purification. QDs are typically synthesised using methods such as colloidal synthesis, chemical vapour deposition, molecular beam epitaxy, and microemulsion techniques. These methods allow precise control over particle size, composition, and surface properties, which are crucial for tuning their optical and electronic characteristics. Surface modification and functionalisation of QDs are often employed to improve their stability, biocompatibility, and specificity for targeted applications (51,52). Despite their advantages, the widespread use of QDs faces challenges. A number of QDs are made from heavy metals, such as cadmium, raising concerns about environmental toxicity and biocompatibility. Research is ongoing to develop eco-friendly and non-toxic alternatives, such as graphene and silicon QDs. Additionally, achieving cost-effective and scalable production remains a focus of current efforts. QDs represent a transformative class of materials with immense potential to impact science and technology. Their unique properties and adaptability make them a cornerstone of advancements in imaging, electronics, renewable energy and environmental applications (9,19).

Ceramic nanoparticles are nanoscale materials made from inorganic, non-metallic solids, such as oxides, carbides, nitrides, or silicates. Typically ranging in size from 1 to 100 nanometres, these particles are characterised by their high stability, hardness, chemical inertness, and resistance to heat and corrosion. Their properties can be finely tuned by controlling their size, composition and morphology, rendering them highly versatile in various scientific and industrial applications (53). Common types of ceramic nanoparticles include TiO₂, Al₂O₃, silicon dioxide (SiO₂), zirconium dioxide (ZrO₂) and magnesium oxide (MgO). These materials exhibit diverse properties, such as photocatalytic activity, thermal conductivity, and electrical insulation. For instance, TiO₂ is widely used for its photocatalytic and UV-blocking properties, while SiO₂ finds applications in reinforcement, electronics and drug delivery (54,55). Ceramic nanoparticles are utilised in a broad range of fields. In medicine, they serve as carriers for drug delivery, imaging agents in diagnostics, and materials for bone and dental tissue engineering. Their biocompatibility and ability to be functionalised make them suitable for these applications. In environmental science, they are employed for water purification, pollutant adsorption, and photocatalysis for the degradation of organic contaminants. In electronics, ceramic nanoparticles enhance the properties of devices such as capacitors, sensors, and thermal insulators. In industrial applications, they are used as additives to improve the mechanical strength, heat resistance, and durability of materials such as coatings, paints, and polymers (56,57). Furthermore, they are integral to the development of advanced ceramics for high-performance applications in the aerospace, automotive and energy industries. Synthesis methods for ceramic nanoparticles include sol-gel processes, hydrothermal synthesis, flame spray pyrolysis and mechanical milling. These methods enable precise control over particle size, morphology, and composition. Surface functionalisation techniques are often employed to enhance their dispersibility and compatibility with specific environments or applications. Despite their advantages, ceramic nanoparticles face challenges, such as aggregation, which can reduce their effectiveness, and concerns regarding their environmental and biological impacts. Addressing these challenges through improved synthesis techniques and comprehensive toxicity assessments is essential for their sustainable and safe use. Ceramic nanoparticles represent a critical class of materials with wide-ranging applications due to their unique physical and chemical properties. As research and technology advance, they continue to play a crucial role in addressing challenges across healthcare, environmental management, energy storage and materials engineering (53,58,59)

MNPs are nanoscale particles that exhibit magnetic properties due to their composition of magnetic materials such as iron oxide (magnetite Fe₃O₄ or maghemite γ-Fe₃O₄), cobalt, or nickel. These particles display unique characteristics, including superparamagnetism, whereby they exhibit strong magnetisation only in the presence of an external magnetic field and lose their magnetisation when the field is removed. This property, along with their high surface-area-to-volume ratio, makes them highly versatile for various applications. MNPs are often engineered with a core-shell structure, in which the core consists of a magnetic material and the shell is coated with substances such as silica, polymers, or carbon to enhance stability, biocompatibility, and dispersibility. These features render MNPs particularly suitable for biomedical, environmental and industrial applications (25). In the biomedical field, MNPs are widely used for drug delivery, where they function as carriers to transport drugs to specific sites in the body, guided by an external magnetic field. They are also employed in hyperthermia therapy, in which they generate localised heat under an alternating magnetic field to destroy tumour cells, and as contrast agents in MRI to improve imaging quality (60). Additionally, MNPs play a role in biosensing, aiding in the detection of biomolecules in diagnostic assays. In environmental science, MNPs are effective in water treatment, as they can remove heavy metals, organic pollutants and oil spills. They are also used as catalysts for pollutant degradation. In technology, MNPs are integral to high-density data storage and spintronics due to their exceptional magnetic properties. In industrial applications, they are used in magnetic fluids for seals, damping systems, and actuators, as well as in catalysis for chemical synthesis (61). MNPs are synthesised using various methods, such as co-precipitation, thermal decomposition, sol-gel synthesis and hydrothermal techniques, which allow precise control over their size, composition and surface properties. The functionalisation of their surface is often performed to attach specific ligands, polymers, or biomolecules, enabling targeted applications. Despite their advantages, challenges such as particle aggregation, long-term stability, scalability of production and potential toxicity remain. Addressing these challenges through advances in synthesis techniques, surface modifications and comprehensive safety assessments is crucial. Magnetic nanoparticles represent a transformative class of materials with diverse applications in medicine, environmental management, and technology, driven by their unique ability to respond to external magnetic fields (25,60,62).

Core-shell nanoparticles are a class of nanostructures comprising a core material surrounded by a shell of a different composition, typically designed to combine or enhance the properties of both the core and the shell materials. The core and shell can be made from various materials, such as metals, oxides, polymers, or carbon, providing versatility in tailoring their physical, chemical and optical properties for specific applications. The core-shell structure provides several advantages, including improved stability, enhanced functionality and controlled interaction with the surrounding environment. The core is typically selected for its unique intrinsic properties, such as magnetic, optical, or catalytic characteristics, while the shell is engineered to enhance biocompatibility, prevent core oxidation, or introduce new functionalities (63). For example, metallic cores such as gold or silver are often combined with silica or polymer shells to enhance stability and enable surface functionalisation. Similarly, magnetic cores, such as iron oxide, are coated with inert shells to reduce toxicity and improve dispersibility. Core-shell nanoparticles are widely utilised across various fields. In medicine, they are employed in drug delivery systems, imaging and diagnostics. For instance, nanoparticles with a magnetic core and a biocompatible polymer shell can deliver drugs to targeted sites under the influence of an external magnetic field while minimising off-target effects (64). In environmental applications, core-shell structures are used in pollutant adsorption, water purification and photocatalysis. In energy and electronics, they play a critical role in solar cells, fuel cells, and advanced sensors by enhancing efficiency and stability. Catalysis is another key area where core-shell nanoparticles are utilised, with the shell often serving to protect the catalytic core while allowing reactant access. Core-shell nanoparticles are synthesised using various techniques, such as co-precipitation, sol-gel processes, layer-by-layer assembly and chemical vapour deposition. These methods enable precise control over the size, morphology and composition of the particles (61,65). Surface functionalisation is often employed to introduce specific ligands or coatings that enhance compatibility with biological systems or improve stability in various environments. Despite their numerous advantages, challenges, such as scalability of production, potential environmental and biological impacts, and cost-effective synthesis need to be addressed (64,66,67). Ongoing research is focused on optimising fabrication techniques, exploring sustainable materials and ensuring safe use (68). Core-shell nanoparticles represent a versatile and innovative class of materials with significant potential to advance technology and address critical challenges in healthcare, energy and environmental sustainability.

Dendrimers are highly branched, three-dimensional macromolecules with a well-defined, tree-like structure. These nanoscale materials, typically ranging from 1 to 10 nanometres in size, are synthesised in a controlled, stepwise manner to achieve uniformity in size, shape and surface functionality. The unique architecture of dendrimers consists of three main components: A central core, repetitive branching units, and peripheral functional groups. This structure endows dendrimers with a high degree of molecular precision, large surface area, and tuneable properties, making them highly versatile in a variety of applications (67). The interior of dendrimers features void spaces that can encapsulate guest molecules, while the terminal groups on their surface can be functionalised to interact with specific targets. The high density of functional groups on the surface allows for multivalency, which is particularly valuable in applications requiring targeted interactions, such as drug delivery, gene therapy and diagnostics. Dendrimers are classified by generations, with each successive generation adding more branching layers and increasing size and functionality (69). In biomedical applications, dendrimers have shown promise as drug delivery vehicles due to their ability to encapsulate drugs within their internal cavities or conjugate drugs to their surface functional groups. This dual functionality enables controlled drug release, improved solubility and targeted delivery to specific cells or tissues, reducing side-effects. They are also used in gene therapy as carriers for nucleic acids, enhancing stability and delivery efficiency. Additionally, dendrimers are employed in diagnostics as imaging agents and biosensors, benefiting from their multivalency and high surface reactivity (67,70). In environmental science, dendrimers are utilised for water purification, pollutant removal and as catalysts or photocatalysts for environmental remediation. Their functional groups can be tailored to bind specific contaminants, enabling selective and efficient removal. In materials science, dendrimers contribute to the development of advanced coatings, adhesives and nanocomposites, providing improved mechanical, thermal and optical properties. They are also employed in electronics and energy applications, such as in the fabrication of LEDs and fuel cells. Dendrimers are synthesised using two main approaches: Divergent synthesis, where branching units are added outward from the core, and convergent synthesis, where dendrons (branching segments) are built separately and then attached to the core. Both methods provide precise control over structure and functionality, although they differ in scalability and complexity (71). Despite their advantages, dendrimers face challenges such as high production costs, potential cytotoxicity, and complex synthesis processes. Advances in synthetic methodologies, the development of biocompatible dendrimers, and cost-effective production techniques are crucial for their broader adoption. Dendrimers represent a cutting-edge class of nanomaterials with immense potential to transform fields such as medicine, environmental science and materials engineering due to their unique architecture and versatile properties (36,67,43,44).

Despite the advantages of MetNPs, conventional methods of synthesis often pose significant environmental and health risks. Traditional chemical synthesis typically involves toxic reagents, hazardous solvents, and high energy inputs, leading to harmful by-products that can impact both human health and ecosystems. A number of nanoparticle synthesis methods (e.g., chemical reduction, pyrolysis and chemical vapour deposition) rely on toxic chemicals, high energy inputs and non-renewable resources. These processes can generate hazardous waste, consume large amounts of energy and can lead to the release of pollutants, including toxic solvents used in chemical reactions and by-products that are harmful to ecosystems. Energy consumption can also lead to increased carbon emissions. Some conventional synthesis methods use hazardous precursors and produce toxic by-products. Different synthesis methods of nanoparticles are illustrated in Fig. 1.

The green synthesis of nanoparticles using plant extracts has emerged as an eco-friendly and sustainable approach to nanomaterial production. This method leverages the rich diversity of phytochemicals present in plants, such as flavonoids, alkaloids, terpenoids, phenols and enzymes, which act as natural reducing and stabilising agents. When plant extracts are exposed to metal ions, these biomolecules facilitate the reduction of the ions to their corresponding nanoparticles and stabilise them to prevent aggregation (3,72,77). This process is simple, cost-effective and scalable, making it an attractive alternative to conventional chemical methods that often involve toxic reagents and harsh conditions. Plant-mediated synthesis is versatile and has been successfully employed to produce a wide range of nanoparticles, including gold, silver, copper and zinc oxide, with controlled size and morphology. Extracts from plants such as Azadirachta indica (neem) (8), Moringa oleifera, Aloe vera (78) and Ocimum sanctum (holy basil) have been extensively studied for their efficacy in nanoparticle synthesis (79,80). The resulting nanoparticles often exhibit enhanced biocompatibility and bioactivity, rendering them suitable for applications in medicine, environmental remediation, and agriculture. For instance, silver nanoparticles synthesised using plant extracts have demonstrated potent antimicrobial activity, while gold nanoparticles have shown promise in drug delivery and cancer therapy (3,8). This green synthesis approach aligns with the principles of green chemistry by minimising waste, reducing energy consumption and avoiding hazardous chemicals. Despite its advantages, challenges such as achieving reproducibility, controlling particle size and shape, and scaling up the synthesis process remain (3,72,77,79,80).

The green synthesis of nanoparticles using microorganisms is an innovative and eco-friendly approach that leverages the natural capabilities of bacteria, fungi and algae to produce nanoparticles. Microorganisms are particularly effective in nanoparticle synthesis because they possess enzymes and metabolites that can reduce metal ions to their nanoparticle forms, while also stabilising and shaping the nanoparticles during the process. This biogenic method of synthesis is beneficial for producing nanoparticles without the need for toxic chemicals, thus ensuring a more sustainable and environmentally friendly production process. Bacteria such as Escherichia coli, Pseudomonas aeruginosa and Bacillus subtilis are commonly used in the biosynthesis of MetNPs, including silver, gold, and copper. These bacteria can secrete enzymes, such as nitrate reductase and cytochrome, which play crucial roles in the reduction of metal ions. Fungi, particularly species such as Aspergillus and Fusarium, are also widely utilised for their ability to synthesise nanoparticles extracellularly. The metabolites and proteins produced by fungi not only reduce metal ions, but also function as stabilising agents, controlling the size and shape of the nanoparticles (81,82). Microalgae, such as Chlorella and Spirulina, are used to produce nanoparticles such as silver and selenium due to their high metal ion absorption capabilities. The microbial synthesis of nanoparticles provides numerous advantages, including the ability to precisely control the size, shape, and surface properties of nanoparticles, which are crucial for their functionality in various applications (83). These nanoparticles are often highly biocompatible, making them suitable for biomedical applications such as drug delivery, cancer therapy and biosensing, and find applications in environmental remediation, where they can be used for water purification, pollutant degradation, and heavy metal removal. Despite its advantages, microbial synthesis of nanoparticles does face challenges, such as scalability for industrial production, the variability of microbial strains, and the need for optimisation of culture conditions. Additionally, ensuring safety and minimising the toxicity of biogenic nanoparticles is an ongoing area of research. However, the use of microorganisms in green nanoparticle synthesis represents a promising, sustainable alternative to traditional chemical methods, offering both environmental and economic benefits in nanotechnology (32,83).

The green synthesis of nanoparticles using algae and enzymes is an emerging and eco-friendly approach that combines the unique capabilities of biological systems to produce nanoparticles in a sustainable manner. Algae, both microalgae and macroalgae, are increasingly being explored for nanoparticle synthesis due to their high metal ion absorption capacity and ability to secrete biomolecules, including proteins, polysaccharides and secondary metabolites, which serve as reducing and stabilising agents. Enzymes, on the other hand, are natural catalysts that facilitate the reduction of metal ions to nanoparticles, providing precise control over the size and shape of the particles. This approach avoids the use of toxic chemicals, making it more environmentally friendly compared with conventional chemical synthesis methods. Algae such as Chlorella, Spirulina, Dunaliella and Phaeodactylum have been used for synthesising various MetNPs such as silver, gold and copper (61). These algae possess the ability to accumulate and reduce metal ions through their biochemical pathways, producing nanoparticles that are often biocompatible and environmentally safe. The synthesis process involves the exposure of algae to metal salt solutions, where enzymes and other cellular components facilitate the reduction of metal ions to their nanoparticle forms. The biogenic nanoparticles formed are stabilised by the natural biomolecules secreted by the algae, which also help control their size, shape and surface properties. Enzymes play a crucial role in the green synthesis of nanoparticles due to their high specificity and catalytic efficiency (84). Enzymes such as nitrate reductase, dehydrogenases and laccase are often used to reduce metal ions to their nanoparticle forms and are capable of selectively reducing metal ions, allowing for the controlled synthesis of nanoparticles with well-defined characteristics. In addition to their reducing ability, enzymes can stabilise the nanoparticles by coating them with functional groups that enhance their solubility and biocompatibility. The use of algae and enzymes in nanoparticle synthesis provides several advantages. This method is cost-effective, environmentally friendly and scalable, with algae being abundant and easy to culture. The nanoparticles produced are often highly biocompatible, making them ideal for applications in medicine, such as drug delivery, imaging and antimicrobial treatments. In environmental applications, algae-based nanoparticles are used for water purification, pollutant removal and the remediation of heavy metals (85). Additionally, enzyme-mediated nanoparticle synthesis is employed in catalysis and the development of advanced materials. Despite the benefits, challenges remain in optimising the synthesis process, such as improving the reproducibility of nanoparticle production, controlling particle size distribution and scaling up for industrial applications (86). Furthermore, research is ongoing to better understand the mechanisms involved in algae-mediated and enzyme-mediated synthesis to enhance efficiency and ensure the safety of the biogenic nanoparticles (87). Overall, the green synthesis of nanoparticles using algae and enzymes is a promising and sustainable method that holds great potential for applications in diverse fields, from environmental protection to biomedical engineering.

The biological agents reduce the metal ions (e.g., Ag⁺, Au³⁺, Cu²⁺) to their corresponding nanoparticles. The reduction of metal ions is a central mechanism in the green synthesis of nanoparticles, where metal ions are reduced to their elemental or nanoparticulate state with the aid of biological agents, such as plant extracts, microorganisms and enzymes. In this process, metal ions, often in their higher oxidation states (e.g., Ag⁺, Au³⁺, Cu²⁺), are converted into their zero-oxidation state (e.g., Ag⁰, Au⁰, Cu⁰) through electron transfer facilitated by natural reducing agents (18). Phytochemicals in plant extracts, such as polyphenols, flavonoids and terpenoids, function as electron donors, reducing metal ions while being oxidised themselves. Similarly, microorganisms, including bacteria and fungi, produce enzymes such as nitrate reductase and laccase that catalyse the reduction of metal ions. These enzymes typically transfer electrons from biological substrates to metal ions, promoting nanoparticle formation. The reduction of metal ions can also be accelerated by light in certain cases, a process known as photoreduction, where light energy induces the production of reactive oxygen species that aid in the reduction. This reduction results in the formation of nanoparticles that are stabilised by the biomolecules present in the biological agents. These biogenic nanoparticles often exhibit enhanced biocompatibility and tuneable properties, making them ideal for applications in medicine, environmental science, and materials engineering (18,88).

Biomolecules from the biological agents stabilise the synthesised nanoparticles, preventing agglomeration and controlling size and shape. In the green synthesis of nanoparticles, stabilisation is a crucial mechanism that ensures the nanoparticles retain their size, shape, and functionality without agglomerating or degrading over time. Once metal ions are reduced to form nanoparticles, the biological agents involved in the synthesis, such as plant extracts, microorganisms, or enzymes, play an essential role in stabilising the nanoparticles (89). This stabilisation is typically achieved through the adsorption of biomolecules onto the surface of the nanoparticles. Phytochemicals such as polyphenols, flavonoids and proteins, which are naturally present in plants, provide functional groups such as hydroxyl, carboxyl and amino groups that form a protective layer around the nanoparticles. This surface coating prevents the nanoparticles from aggregating by hindering interactions between particles through electrostatic repulsion or steric hindrance (90). In microbial systems, proteins, enzymes and polysaccharides secreted by bacteria, fungi and algae can serve as capping agents that not only stabilise the nanoparticles but also impart specific properties, such as increased biocompatibility or targeted functionality. These biomolecules function by binding to the surface of the nanoparticles, reducing surface energy and preventing the particles from clumping together. Furthermore, the size and shape of the nanoparticles can be controlled by adjusting the concentration and type of stabilising agents used, which is essential for tailoring the properties of the nanoparticles for specific applications. This stabilisation mechanism also helps in maintaining the dispersion of nanoparticles in solution, preventing agglomeration and enhancing their stability under varying environmental conditions. The biocompatible nature of the stabilising agents makes these nanoparticles suitable for applications in fields such as medicine, environmental remediation and sensor technology, where prolonged stability and safe interaction with biological systems are paramount (3,72,80,91).

The synthesised nanoparticles are purified and characterised using techniques, such as scanning electron microscopy (SEM), UV-visible spectroscopy, X-ray diffraction (XRD) and transmission electron microscopy (TEM). In the green synthesis of nanoparticles, purification and characterisation are essential steps that ensure the quality, size, shape and functionality of the synthesised nanoparticles. After the nanoparticles are formed through the reduction of metal ions by biological agents (such as plant extracts, microorganisms, or enzymes), they must be purified to remove any residual reactants, by-products, or unreacted metal ions. This purification is typically achieved through methods, such as centrifugation, filtration, dialysis, or gel electrophoresis, which help separate the nanoparticles from unwanted materials. These methods rely on differences in particle size, charge, or solubility to isolate the nanoparticles in their purest form, ensuring that they are free from contaminants that could affect their properties or hinder their intended applications (6,80,92,93).

Once purified, the nanoparticles are characterised to determine their physical, chemical and structural properties. Characterisation techniques provide insight into the size, shape, morphology, surface charge and composition of the nanoparticles, which are critical for assessing their suitability for specific applications. Common techniques used in the characterisation of green-synthesised nanoparticles include the following:

i) TEM and SEM. These techniques provide detailed images of the size, shape and surface morphology of the nanoparticles, enabling researchers to assess their uniformity and dispersion (6).

ii) XRD. XRD aids in the determination of the crystalline structure of the nanoparticles and can provide information about their phase composition (80).

iii) UV-Vis spectroscopy. UV-Vis spectroscopy is used to monitor the optical properties of nanoparticles, such as their absorption spectra, which are influenced by particle size and shape. It can also confirm the formation of nanoparticles by detecting the characteristic plasmon resonance peaks (92).

iv) Dynamic light scattering (DLS). DLS measures the size distribution and zeta potential of nanoparticles, providing information on their stability and surface charge (93).

v) Fourier transform infrared spectroscopy (FTIR). FTIR is used to analyse the functional groups present on the surface of the nanoparticles, which helps identify the biomolecules responsible for stabilisation and capping (94).

vi) Energy-dispersive X-ray spectroscopy (EDX). EDX, often coupled with SEM or TEM, provides elemental analysis of the nanoparticles, confirming their composition and verifying the successful incorporation of metal ions (95).

Through purification and characterisation, researchers can ensure that the green-synthesised nanoparticles possess the desired properties, are free of contaminants, and are suitable for various applications in medicine, environmental remediation, catalysis and materials science. These steps are crucial in ensuring that the nanoparticles meet the necessary quality standards for their intended uses.