Synthesis of NPs using plant extracts is the easiest route, as these are cost-effective and easily available ^[1][115]. Medicinal plants contain a large number of metabolites and reducing agents, which help in the reduction of many metal ions to NPs. Some plants also contain a significant quantity of metals that are incorporated into NPs during synthesis. Plants do not require much time for the reduction of metal ions compared to fungi and bacteria. NPs of different sizes and morphologies can be achieved in a short duration ^[2][116]. Different parts of plants can be used for green syntheses, such as leaves, flowers, fruits, peels, and stems, owing to the presence of phytochemicals ^[3][117]. Plant leaves and flowers should be subjected to drying at room temperature after washing with tap water or distilled water. After complete drying, they are finely crushed into a smooth powder. Extract can be prepared by mixing a weighted amount of powder in distilled water, whereas complete mixing is achieved by heating and stirring. Afterwards, the cooled solution is filtered, and the filtrate is employed for the synthesis of NPs ^[4][118]. Some plants produce a large amount of hydrogen ions during the glycolysis process, which play a critical role in the reduction and stabilization of NPs ^[5][119]. Furthermore, the nature and type of NPs is the key parameter determining their photocatalytic activity. NPs can be obtained in the form of nanospheres, nanotubes, nanorods, nanofilms, etc.

Studies show that the NPs synthesized from plant extract (such as Convolvulus arvensis) can be efficiently used for the degradation of environmental pollutants such as azo dyes and drugs. These dyes are used as additives in pharmaceuticals for aesthetics and for practical reasons to help patients in distinguishing between different colors of medicine. Thus, silver NPs with well-defined morphology and catalytic activity show the best environmental response ^[6][120]. Leaf extract of Vernonia amygdalina was used to synthesize Nb-doped ZnO NPs by using a green approach to photocatalytically degrade tetracycline (TC), which is an important broad-spectrum antibiotic used to treat infectious diseases. The photocatalytic activity under visible light was shown to be 93.2% in 2 h and only reduced to 6% after five recovery cycles ^[7][121]. Sensitized ZnO NPs synthesized by chemical precipitation method were found to photocatalytically degrade TC to >80% under visible light for 5 h ^[8][122]. Employing a nanocomposite Au_0.1Ag_0.9/TiO₂ in a citric acid membrane resulted in an efficiency of 90% under visible light ^[9][123]. Similarly, extract of hydrophyte species such as Persicaria salicifolia is used for the green synthesis of Ag-doped ZnO supported on a carbon-rich support such as biochar for photocatalytic degradation of TC. The photocatalyst showed reasonable recyclability after six successive cycles of use, with biochar facilitating the regeneration of the NPs ^[10][124]. Carbon quantum dots (CQDs) synthesized from waste of Citrus limetta modified with Au/Ag boosted the surface plasmon resonance effect when decorated on TiO₂ nanofibers. The nanocomposite was able to photocatalytically degrade erythromycin. The photocatalyst exhibited recyclability with reduced photocatalytic activities after several cycles ^[11][125]. Plant-assisted synthesis of ZnO used as a photocatalyst has led to the removal of an antibiotic (ciprofloxacin). ZnO NPs were synthesized using extract obtained from Citrus aurantifolia (Lemon peels). A removal efficiency of 90% was obtained after 160 min. A variety of phytochemicals such as proteins, amines, alkaloids, cellulose, hemicellulose, and other aromatic components are present inside the lemon peel extract, resulting in stabilization of ZnO NPs. These green NPs produce no secondary pollutants. Complete degradation resulted in the formation of water and carbon dioxide ^[12][126]. The degradation of ciprofloxacin has also been reported by employing ZnO NPs synthesized using the chemical precipitation method under UV light, showing only around 50% degradation after 60 min and proving to be less efficient than green synthesized NPs ^[13][127]. Upon exposure to sunlight, excitation of electrons occurs from valence to the conduction band of ZnO NPs, leading to the generation of electrons and holes, which further react with water to generate free radicals. These free radicals are responsible for the breakage of drug structures. However, bandgap modulation in ZnO NPs (3.3 eV) can be accomplished by the addition of certain transition metals (silver, etc.) as a dopant. Narrowing of the bandgap results in shifting of the absorption peaks towards longer wavelengths and low-energy regions. These impurities (dopants) result in the enhancement of photocatalytic activity of ZnO, making it slightly active in response to visible radiation. Similarly, the reduction in the bandgap of titania is also ensured by adding dopants such as nitrogen. These impurities result in the enhancement of optical properties of TiO₂ ^[14][128]. Thus, chemically synthesized γ-Fe₂O₃@ZnO showed improved photocatalytic efficiency for ciprofloxacin, from 11.5% for pure γ-Fe₂O₃ to 92.5% for functionalized NPs that require additional use of chemicals.

Different vitamins contain a variety of reducing and capping agents. Nanorods and nanospheres can be synthesized by using vitamin B2 as a reducing and capping agent. Gold and platinum NPs of specific shape were prepared using riboflavin (vitamin B2) by reacting the salt precursors of gold and platinum with different solvents. Vitamin B2 is an efficient reducing and capping agent as compared to other chemical compounds, such as sodium borohydride (NaBH₄), and appears to be perfect for preparation of NPs ^[15][129]. The morphologies and sizes of NPs can be controlled by varying the type of solvents used; density plays an important role in the formation of self-assembled structures and different shapes. The reaction of metal salt with vitamin B2 results in reduction of the metal salt and simultaneous oxidation and capping of the vitamin. Similarly, uniformly sized NPs can be synthesized using ascorbic acid (vitamin C) as a capping and reducing agent. The interaction of radiation with the collective oscillations of electrons in the conduction band result in extinction properties for Au and Ag NPs. This could be circumvented by efforts to tune size and shape by using a different synthesis method. Hence, Au and Ag NPs have been obtained utilizing vitamin C, resulting in a fast and efficient one-pot synthesis method. The size of synthesized NPs is tuned by pH changes, varying the metal or vitamin C solution. This method also allows for changes in the surface chemistry by using different surfactants and can be employed for a vast array of applications ^[16][130]. Green synthesis of silver NPs has been reported using vitamin-C-enriched Phyllanthus emblica extract. Here, vitamin C is used as a reducing agent, and the synthesis of AgNPs is affected by the reaction time, temperature, and concentration of metal salt and reducing agent in the solution matrix. Under optimum conditions, AgNPs of 41.2 nm are obtained. Ag NPs have enhanced photocatalytic efficiency for the removal of toxic moieties from water bodies ^[17][18][131,132]. Furthermore, although this area of research is in its incipient stage, core shell noble metal NPs (Fe-Cu) with Pt, Au, and Pd shells have been obtained using vitamin C, resulting in different morphologies that can be further explored for different applications, such as catalysts, nanodevices, etc. vitamin C was reported to reduce Fe and Cu NPs resulting in a core shell structure by the addition of Pt, Au, or Pd ^[19][133].

A broad range of biological activities is associated with algae, such as antibacterial and antimicrobial applications ^[20][134]. These are marine species and are extensively used for the synthesis of metal NPs. Chlorella vulgaris (unicellular algae) can be used to synthesize metallic NPs such as gold NPs with particle sizes of 9–20 nm. Different shapes, sizes, and morphologies of metallic NPs can be achieved using various algal species and reaction conditions ^[21][135]. Gold NPs are synthesized using extract from aquatic algae, i.e., Galaxaura elongate, with excellent antibacterial properties ^[22][136]. Chlorella ellipsoidea extract is used for the synthesis of silver NPs with enhanced photocatalytic activities for the removal of water contaminants ^[23][137].

Gold (I) sulfide NPs can be synthesized using cyanobacteria in aqueous solution ^[24][138]. Furthermore, NPs of different sizes, such as gold and silver NPs, ranging from 8 to 12 nm can be obtained using marine algae ^[25][139]. Algae lack proper structure such as stems, roots, and shoot systems, but they possess a variety of pigments that play an important role in the synthesis of NPs ^[26][140]. Particle size is a key parameter in determining the properties of NPs, which, in turn, is affected by several factors, including pH, temperature, solvent, and chemical concentration ^[27][141]. The mechanism of synthesis of nanoparticles using algae is not fully understood, as different algal species react differently with metal precursors. Many species produce inorganic compounds either intra- or extracellularly and have different mechanisms of action to produce NPs ^[28][142]. The intracellular mechanism of the synthesis of NPs involves transportation of metal ions to the microbial cell, where positively charged metal ions are attracted towards the negatively charged cell wall. The metal ions are, in turn, reduced by the enzymes released from the microbial cell wall. The extracellular mechanism involves the secretion of enzymes, which reduce the metal ions for the synthesis of NPs ^[29][143].

Extracellular and intracellular synthesis of NPs using fungi such as Fusarium oxysporum, Colletotrichum sp., Trichothecium sp., A. fumigatus, Coriolus versicolor, Aspergillus niger, Cladosporium cladosporioides, and others are used widely worldwide, owing to several advantages lacking in other microbes, e.g., tolerance to high flow pressure in bioreactors and easy growth ^[30][31][144,145]. A variety of particle sizes can be obtained by using different fungal strains. Aspergillus flavus has been employed to produce monodisperse Ag NPs with particle sizes of 8.92 ± 1.61 nm. The stability of NPs was also found to be high due to the secretion of fungus, making NPs stable for 3 months ^[31][145]. Cultural conditions also impact the synthesis of NPs. Non-agitated culture leads to the production of extracellular metallic NPs, while agitated culture leads to the synthesis of intracellular NPs. It has been observed that the secretion of enzymes and proteins is considerably enhanced in the case of stationary culture as compared to agitated culture conditions ^[32][146].

It has been reported that gold NPs possessing biomedical and anticancerous properties can be synthesized by using fungus extract i.e., Fusarium solani ^[33][147]. A variety of metallic NPs, such as zinc, copper, silver, gold, iron etc., have been synthesized using fungus extract for the photocatalytic degradation of water pollutants ^[34][148]. Fungal extract contains multiple phytochemicals that are useful in the synthesis of nanoparticles such as spherically shaped iron NPs prepared from extract obtained from endophytic fungi Penicillium oxalicum. This extract exhibits expanded surface area for the reduction of metals. These fungus-mediated iron NPs were employed for photocatalysis of colored water contaminants, demonstrating a removal efficiency rate of 99.17%. Furthermore, morphologically controlled synthesis of silver NPs is also achieved by using fungal extract, which can be used for the catalytic degradation of water pollutants due to enhanced catalytic properties ^[35][149]. Fungus-assisted production of a nitrogen-doped Co₃O₄ nanocatalyst was also ensured by using Fusarium oxysporum, which exhibited excellent catalytic efficiency of 87% for toxic moieties ^[36][150].

Green synthesized NPs are formed when metal ions are trapped by microbes, whether at the surface or inside the microbes, which are then reduced in the presence of enzymes and act as electron acceptor moieties for anaerobic respiration. These microbes that include bacteria, viruses, and algae are known to either accumulate these NPs or secrete them to the environment in the presence of stabilizing and capping agents secreted intracellularly and are also known as green nanofactories. Thus, this method is a proven route for bioremediation or synthesis of materials. Many bacteria have been employed in the green synthesis of Au and Ag NPs, such as Bacillus subtilis, Escherichia coli, Rhodobacter capsulatus, Corynebacterium sp., and Lactobacillus sp. CdS NPs have also been synthesized using Escherichia coli, Klebsiella aerogenes, Rhodopseudomonas palustris, and Gluconoacetobacter xylinus ^[30][144]. ZnS NP synthesis was reported using Rhodobacter sphaeroides with a particle size of 8 nm ^[37][151]. CdS with particle sizes of 3.2–44.9 nm were formed from Escherichia coli and Klebsiella pneumoniae ^[38][152]. Copious references on the synthesis of NPs through green nanofactories can be found in ^[39][153]. Biogenic nano-TiO₂ is formed by employing Bacillus subtilis, which produces NPs with a spherical morphology and a size of 10–30 nm. TiO₂ NPs have proven to be a good photocatalyst that act with the help of H₂O₂ ^[40][154]. CuO NPs with an average particle size of 6–7.8 nm were prepared using Cystoseira trinodis extracts from brown alga and showed improved photocatalytic activity ^[41][155]. Bacteria are also employed in the synthesis of well-known ZnO NPs with excellent photocatalytic performance. ZnO nanoflowers can easily be synthesized using B. licheniformis, which is an effective and environmentally friendly approach. Nanoflowers have a large surface area with enhanced photocatalytic activities. These nanoflowers were found to be 40 nm in width and 400 nm in height. Absorption of light by the photocatalyst leads to the generation of free radicals (active species). These active species facilitate the process of degradation of organic contaminants. Hence, these photocatalysts help in bioremediation ^[42][156].