Biosynthesis of metal nanoparticles: Bioreduction and biomineralization

2.1 By plants

It has been found that a large number of plants such as acanthopanax, senvy, and ilex can realize the phytoremediation of heavy metals from seriously polluted soil [26,27,28,29]. Plants achieved detoxification by reducing metal ions and promoting the agglomeration of nanoparticles. Plants could gather metal elements, provide a framework for nucleation and growth, and generate metal nanoparticles with different sizes and morphologies [26,29]. It has been reported that AgNPs can be biosynthesized using different plant extracts, such as Aesculus hippocastanum extracts [30] and Lythrum salicaria extracts [31]. The characteristics of AgNPs were verified using UV-visible spectroscopy, Fourier-transform infrared spectroscopy, X-ray diffraction (XRD), zeta potential, and scanning electron microscopy (SEM). The results indicated that the biosynthesized AgNPs retained their normal physicochemical properties with nanoscale spherical morphology and stable negative charge in the UV wavelength range of 420–470 nm. In addition, they indicated that AgNPs biosynthesized by plants significantly inhibited the growth of bacteria and fungi at 0.15–1.25 mg/mL. Biosynthesized AgNPs not only retain antibacterial and antioxidant properties but also possess antioxidant potential and anti-tumor ability at different concentrations. This indicates that plants might be applied for eco-friendly biosynthesis of AgNPs with possible antibacterial and antineoplastic applications. Similarly, Boomi et al. also synthesized AuNPs from the extract of Acalypha indica with surface plasmon resonance features and crystalline behavior. SEM revealed that the particles had spherical or rod-like nanostructures with a size of 20 nm. Their antibacterial properties were also verified against Staphylococcus epidermidis and Escherichia coli strains [32]. The preparation of alloy nanoparticles can also be achieved by plants. Nasrollahzadeh et al. found that CuNPs could form alloy nanocomposites through green synthesis of different leaf extracts using MgO or sodium borosilicate glass [33,34].

Biosynthesized metal nanoparticles by plants could not only retain the original properties and functions but also enhance biocompatibility and realize the transformation of different shapes. Bhaskaran et al. applied Medicago sativa to the reduction process to prepare AuNPs with various shapes of isodiametric spheres, exotic tetrahedrons, pentagons, or pentagonal prisms. The nanoparticles in the study were less cytotoxic to normal human cells but more cytotoxic to the cancer cell line 4T1 compared to the citrate-synthesized nanoparticles. The results suggest that biosynthetic particles have a wide range of biological applications [35]. Further, studies found that plants had strong reduction potentials as they could produce a variety of secondary metabolites, which were important sources for metal nanoparticle biosynthesis. Several functional groups such as linalool, eugenol, and geraniol in Ocimum basilicum were involved in the bioreduction of AgNPs, and their interactions were also proved. The obtained nanoparticles possessed the desired characteristics with the variation of synthetic parameters (Figure 2a) [36]. In the synthesis of Cu nanoparticles (CuNPs), the extracts of Vaccinium myrtillus L. also had a reducing role. The polyphenol in the V. myrtillus L. was proved to actively participate in the reduction of Cu²⁺ from CuCl₂, Cu(CH₃COO)₂, and Cu(NO₃)₂, resulting in the formation of CuNPs [37]. It was also been reported that epigallocatechin gallate in green tea was also responsible for the bioreduction of AuNP and Mn nanoparticles, which played a role in tumor treatments [38,39]. For the treatment of hyperglycemia, Kumari et al. found that apple polysaccharides can also be used to biosynthesize AuNP [40]. Hence, the biosynthesis of metal nanoparticles using plants and plant extracts was an economical and environmentally friendly method, in which various biomolecules such as amino acids, peptides, proteins, collagens, enzymes, vitamins, and polysaccharides acted as reductants and capping agents for metal ion reduction reactions [41].

Figure 2

(a) Schematic representation of the possible mechanism of formation of bio-reduced AgNPs in Ocimum basilicum. Reproduced with permission from ref. [36], copyright 2019 Elsevier. (b) Preparation of bio-reduced AgNPs in Nostoc muscorum NCCU 442. Reproduced with permission from ref. [44], copyright 2021 Elsevier. (c) Schematic representation of the possible mechanism of action of the capped silver nanoparticles against S. sclerotiorum. Reproduced with permission from ref. [52], copyright 2021 BMC.

2.2 By microorganisms

Similar to the biosynthesis by plants, microorganisms including bacteria, fungi, and viruses could also activate and immobilize metal ions with their reductive physiological structures, such as cell walls, cell membranes, cytoplasm, nucleus, capsules, and flagella, and internalized the ions and reduced them to nanoparticles [42]. Microbial biosynthesis of metal nanoparticles was a fast and green method for preparing Au, Ag, Pt, Pd, Fe, Ti, and other metal nanoparticles, which could also control the morphology and size by further optimizing. It provided a green and eco-friendly approach over physical or chemical approaches [43]. For instance, the extraction from Nostoc muscorum NCCU 442 and the exopolysaccharide extracted from probiotic Lactobacillus brevis MSR104 could both serve as reducing and capping agents to biosynthesize AgNPs, whose average size was 30 and 45 nm, respectively (Figure 2b) [44,45]. Shewanella oneidensis MR-1 was applied to reduce palladium ions to Pd nanoparticles (PdNPs) under aerobic conditions, and the size and distribution of PdNPs could also be controlled by adjusting the ratio of the microbial biomass to palladium precursors [46]. Hollow cylinder-shaped tobacco mosaic virus coat protein (TMV disk) was also studied to generate AuNPs, with the advantage of the intrinsic structure to guide the assembly of AuNP lattices with tunable arrangement modes, which provided a convenient method for designing and assembling hierarchically ordered architectures of nanoparticles [47].

It was speculated that some components in microorganisms also played an important role in biosynthesis, controlling the generation of metal nanoparticles. Srinath’s study found that with the cell-free supernatant of Streptomyces sp., quasi-spherical crystalline-shaped FeNPs with an average size ranging from 65.0 to 86.7 nm could be generated. They also displayed strong antioxidant activity and extensive bactericidal effects against pathogens, such as Bacillus subtilis, Staphylococcus aureus, Klebsiella pneumoniae, Shigella flexneri, and E. coli [48]. Moreover, the operon in S. aureus was also found to have a regulatory effect on the biosynthesis of Ni, Co, Zn, Cu, Fe, and other nanoparticles. Further research showed that the active ingredient would be tobacco amine, which was related to the uptake of metal ions. The metal ions that combined with tobacco amine would form a complex, which could be transported into the bacterial cytoplasm with the help of the ABC transporter on the cell wall. Under the action of enzymes in the cytoplasm, the reduction reaction occurred to reduce the metal ions into metal nanoparticles, which increased the uptake of metal ions in bacteria and the biosynthesis efficiency of metal nanoparticles [49]. NADH-dependent enzymes and coenzyme factors produced by F. oxysporum could reduce HAuCl₄ and AgNO₃ to form Au–Ag alloy nanoparticles. By increasing the amount of fungi in the reaction system, the production of NADH was affected, thus promoting the bio-reduction of AuNPs and AgNPs [50]. A 32 kDa protein secreted by Aspergillus flavus was also proved to be a reducing agent to reduce Ag⁺ to AgNPs, and the 35 kDa protein acted as a capping agent and bound to the surface of the AgNPs for stabilization [51]. It could be inferred that proteins and enzymes (hydrolytic enzymes NAGase, β-1,3-glucanase, chitinase, and acid protease), organic molecules (succinic acid, lactic acid, malic acid, and glutathione), the intracellular environment (such as pH), and other biological macromolecules could participate in the bioreduction of metal nanoparticles, and served as capping agents. They played critical roles in controlling the size of nanoparticles and further contributed to the biological activity (Figure 2c) [52].

2.3 By biological ingredients

The biological components in plants and microorganisms were demonstrated to play an important role in the bioreduction of metal nanoparticles; therefore, some biological ingredients were also applied to try to induce biosynthesis, such as the liposomes consisting of phospholipids, lecithin, and cholesterol, as well as peptides composed of various amino acids, nucleic acid, and other polymers [53,54].

2.3.1 Liposomes

As an important biological nanocarrier, liposomes could effectively package and deliver drugs, avoid the influence of the external environment, control drug release, improve biocompatibility, and enhance the relationship with target cells [55,56]. Soto-Cruz et al. synthesized AuNPs using the liposome mixture of phospholipids, phosphatidylcholine, and phosphoglycerol as a template phase. The negatively charged liposome mixture served as the reducing agent and stabilizer during the synthesis, resulting in a green-colored dispersion containing a high purity, well-defined, negatively charged Au nanotriangles with an edge length of 60–80 nm and an average thickness of 7.8 ± 0.1 nm [57]. Liposome mediation could also achieve the bioreduction of degradable AuNPs of a honeycomb shape. By adjusting and optimizing the ratio of Au ions and liposomes, the interventional photothermal therapy and radiotherapy could be regulated, which successfully increased the targeted aggregation of nanoparticles at the tumor sites and mediated the interventional therapy to achieve deep killing of tumors [58].

To further enhance the biocompatibility of the metal nanoparticles, liposomes were not only used for bioreduction but also coatings to encapsulate nanoparticles. A two-layer nanostructure simulating phospholipid was designed, using the first layer to bio-generate AuNPs and the second liposomal layer to camouflage the AuNPs, thereby improving the biocompatibility, excretory properties, and in vivo stability (Figure 3a) [59]. A programmable liposome through encoding reducing agents was also designed to serve as the template for the self-generation of hybrid metal nanoparticles (Au–Pt, Au–Pd, and Au–Ag). This indicated that the lipid bilayer could not only deal with poor selectivity and low yield of traditional synthesis but also enhance the stability and cellular endocytosis efficiency (Figure 3b) [60].

Figure 3

(a) Schematic illustration of the experimental procedure of LAL for photothermal therapy and ⁶⁴Cu labeled LAL (⁶⁴Cu-LAL) for in vivo imaging. Reproduced with permission from ref. [59], copyright 2021 BMC. (b) Programmable and facile synthesis of various liposome/metal hybrid nanoparticles. Reproduced from ref. [60] with permission, copyright 2016 AAAS. (c) Leveraging peptide sequence modification to promote the assembly of chiral helical gold nanoparticle superstructures. Reproduced from ref. [69] with permission, copyright 2021 ACS Publications. (d) The model of sRNAs influencing the AgNP biosynthesis using D. radiodurans. Reproduced with permission from ref. [70], copyright 2021 ACS Publications. (e) Illustration of the fabrication of the CG/PDA@Ag hydrogel and its application as a photothermal antibacterial platform for wound dressing. Reproduced with permission from ref. [71], copyright 2022 Wiley.

2.4 Cells

Cells are basic units of the biological structure and function. They exhibit complex biological function, finely regulated internal environment, and unique metabolic characteristics, according to their compositions of various amino acids, peptides, proteins, enzymes, nucleic acids, and other organics. As the structure, composition, secretion, and function of cells have been intensively studied, it was speculated that cells could also reduce metal ions to form metal nanoparticles [74–76].

In our previous studies, we tried the biosynthesis of AuNPs in various cells such as tumor cells, fibroblast cells, and immune cells. The results demonstrated that the studied cells could all achieve biosynthesis with different efficiencies. We thus developed a cell reactor based on colon cancer cells (MC38) to obtain the biosynthesized AuNPs. The intracellular biosynthesis and exocytosis process of MC38-mediated AuNPs could be affected by regulating cellular metabolite levels and other factors, such as glutathione and reactive oxygen species (ROS), autophagy, and UV irradiation. The obtained particles greatly improved the radiation-induced DNA damage and ROS generation and promoted cell apoptosis and necrosis. This demonstrated that for the bio-generated AuNPs, their original properties on radiosensitization were retained and enhanced, which could be due to the enhanced accumulation, homologous targeting, and transcytosis effect. AuNPs also enhanced the efficacy of local radiation by initiating immunogenic cell death as well as the immune response (a significant increase of CD8a⁺ dendritic cells [DCs]). This radio-sensitization strategy by self-generated metal nanoparticles of tumor cells revealed effective inhibition on the primary tumor and the synergistic effect with immune checkpoint blockade (Figure 4a and b). This bio-inspired synthetic strategy of metal nanoparticles also promoted the development of its therapeutic applications [77]. It was also suspected that cells would pass their original bioinformation to the generated metal nanoparticles. The bioreduction of AuNPs was conducted in murine melanoma B16F10 cells, breast cancer cell 4T1, mouse epithelial fibroblasts L929 cells, and DCs, forming AuNP@B16F10, AuNP@4T1, AuNP@L929, AuNP@BMDC, and AuNP@DC_B16F10. Compared with AuNPs@L929, AuNPs intracellularly generated from tumor cells acquired the tumor antigens of their original cells (Figure 4c), which could thus boost immune response. The AuNPs generated from tumor antigens pulsed DCs could further obtain the antigen-presenting ability. The biological camouflage could enhance the immunological properties, confer biocompatibility and stealth to AuNPs, which finally realized the combination of AuNP-mediated photothermal therapy (PTT) and tumor antigen-based immunotherapy in the treatment of murine melanoma [78].

Figure 4

(a) Schematic illustration of the preparation of Au@MC38 and in vivo radio-sensitization for cancer therapy. Reproduced with permission from ref. [77], copyright 2021 Wiley-VCH. (b) Schematic presentation of possible mechanisms in the intracellular synthesis and exocytosis process of Au@MC38. Reproduced with permission from ref. [77], copyright 2021 Wiley-VCH. (c) Proteomics analysis of nanoparticles of AuNP@B16F10, AuNP@DCL929, and their parent cells. Reproduced with permission from ref. [78], copyright 2019 ACS Publications. (d) The preparation of EVdox@AuNP- and EVdox@AuNP-mediated synergistic photothermal and chemotherapy. Reproduced with permission from ref. [79], copyright 2019 Elsevier.

For life research and disease treatment, a variety of bionic structures mimicking cells have been developed such as cell-derived vesicles, membrane vesicles, and exosomes. It was found that these cell-mimic structures contained bioinformation like intracellular metabolites or exogenous substances. Their development on bioreduction may further extend the application of metal nanoparticles. We successfully generated popcorn-like AuNPs by extracellular vesicles (EVs) extruded from DCs. Taking advantage of the similar composition to cells, the EVs achieved the self-growth of AuNPs surrounding the vesicles and assembled into the popcorn-like nanostructure to finish the biosynthesis. In addition, the formulated nanostructures, consisting of self-grown AuNPs and chemical-loaded EVs, would retain the photothermal transduction from AuNP assemblies and the cytotoxicity of chemicals (Figure 4d). Under external near-infrared irradiation, the nano-system could produce hyperthermia to induce tumor ablation and trigger chemical release, achieving combinatorial chemo-photothermal therapy [79]. Exosomes, proven to be excellent carriers for carrying lipids, proteins, RNAs, and DNAs, also exhibited characteristics favorable for the biosynthesis of metal nanoparticles [80]. As reported, exosomes could mediate the synthesis of metal nanoparticles in an eco-friendly pattern without harsh conditions and toxic chemicals. After the processes of biosynthesis, the dissemination of particles was also performed upon cargo exosomes within the circulatory system [81,82]. The utilization of cell-derived vesicles for bioreducing metal nanoparticles could improve biocompatibility and further endow the particles with biological information and functions, which might broaden the medical application. In general, the presentation above was the processes and mechanisms of the biorecution of metal nanoparticles. The relevant metal elements, reducing the medium and nano-product of classical studies, are summarized in Table 1.

Compared with larger plants and cells, microorganisms and bio-ingredients like enzymes and peptides have significant advantages as cell factories produce metal nanoparticles, and their composition and structure are simpler. The morphology and size of the biosynthesized nanoparticles were more controllable by adjusting the pH and reactant ratio. Tryptone in Saccharomyces cerevisiae was found to affect the synthesis process, which realized the highest yield of Cd–Se QDs at a concentration of 25 g/L [83]. The ATP energy pathway was also found to affect the biosynthesis of Cd–Se QDs in Saccharomyces cerevisiae. The ATP content in the reaction system could significantly affect the accumulation of Se and Cd and then achieve the formation of QDs [84]. While in situ biosynthesis of metal nanoparticles could provide higher biocompatibility, less toxicity, and enhanced targeting ability. In particular, the biosynthesized metal nanoparticles in mammalian cells and EVs can avoid drawbacks such as poor targeting ability, autoimmune responses, hydrophobic nature, and so on, which can extend the biomedical application.

3.1 Biomineralization mediated by acid reaction

The acidic environment, such as HCO 3 − and CO 3 2 − , could mediate a variety of biomineralization of metal-compound nanoparticles in organisms. HCO 3 − and CO 3 2 − could combine with metal ions such as Ca²⁺ to generate the metal-compound nanoparticles (Figure 5a) [88]. The biomineralization of Ca²⁺ was the most common phenomenon in nature. It plays an important role in the development of bones, teeth, shells, and other growth in the body [89]. At first, the biomineralization of Ca²⁺ was intensively observed in fossils, which could be used as a cytoprotective shell. This kind of biomineralized shells aroused the interest of researchers that it could be applied to protect cells from lytic enzymes, nutrient deprivation, osmotic pressure, shear force, and heat, and finally endowed them with good robustness and rigidity, as well as tunable physicochemical properties [90,91].

Figure 5

(a) Precipitation of CaCO₃ in the presence of 14 nm gold NPs functionalized with coronas of two commercially available proteins (protein NPs): α-1-acid glycoprotein and bovine serum albumin. Reproduced with permission from ref. [88], copyright 2019 Nature. (b) Cellulosic hydrogel anchoring carboxylic moiety developed as a superior carrier for nano Fe₃O₄. Reproduced with permission from ref. [97], copyright 2021 Elsevier. (c) Schematic representation of the novel synthetic protocol, characterization of ZnO NPs using the Annona squamosal leaf extract as a reductant for potential synergetic antibacterial and anticancer activity with cytocompatability assessment. Reproduced with permission from ref. [105], copyright 2019 Elsevier. (d) Phytofabrication of G-MgO NP using the floral extract of Calotropis gigantea. Reproduced with permission from ref. [107], copyright 2020 Elsevier.

The biomineralization of Ca²⁺ was further studied and applied to biomedicine and disease treatments. For instance, brown algae were able to accumulate Ca²⁺ to interact with the fluxed HCO 3 − in the reaction medium and finally deposited CaCO₃. After the process of deposition, the biomolecules in algae served as catalysts and stabilizers, and the cellular structure in the nanoscale provided the template, which induced CaCO₃ to generate CaCO₃ NPs with the shape of a spindle and sizes of 1–2 μm in length, as well as 300–500 nm in width. The obtained particles could be applied in the field of antibacterial and anticancer therapy [92]. The mesenchymal stem cell (MSC)-derived matrices also supported the biomineralization of Ca²⁺, which could upregulate the expression of a subset of osteogenic genes (alkaline phosphatase, osteopontin, osteocalcin, osteonectin, osteomodulin, and parathyroid hormone receptor) and decrease the expression of aggrecan, a protein associated with chondrogenesis. It was also reported that regulating the biomineralization of Ca²⁺ on intrafibrillar collagen could improve the nanomechanics and cytocompatibility of collagen matrix, which could be applied as novel biomaterials for bone grafting and tissue-engineering applications [93,94]. Proteins and peptides could also induce the biomineralization of Ca²⁺. The collagen of organic fibrils could interact with the deposited Ca²⁺ and its biostructure could serve as templates to form CaP nanoparticles (CaP NPs) [95,96].

3.2 Biomineralization mediated by oxide precipitation

Oxidation reaction was involved in the precipitation of oxides. For example, metal ions were accumulated in bio-organics and were then passivated by OH⁻ fluxes, finally generating the metal oxide nanoparticles (Figure 5b) [97]. As reported, acidic macromolecules such as hydroxyl-rich proteins were key to calcium carbonate biomineralization [88]. As the magnetotactic bacteria were first found by Richard in 1975, exploiting microorganisms to internalize Fe ions and convert them into magnetic nanoparticles was extensively studied [98]. The magnetotactic bacteria had the ability to absorb Fe²⁺ and Fe³⁺ in the environment and transform them into magnetic nanoparticles, Fe₃O₄ nanoparticles (Fe₃O₄ NPs), without other impurities. The synthesized particles were highly crystalline, monodisperse, bioengineered, and with high magnetism, which could even be comparable to those made by advanced synthetic methods. The biomineralization of magnetic nanoparticles endowed them as candidate materials for broad biomedical applications [99]. Plants could also be applied in the metal oxide precipitation, whose technology was green and produced no harmful chemicals. Fe₃O₄ NPs were biomineralized by the aqueous extracts of brown seaweeds (Petalonia fascia, Colpomenia sinuosa, and Padina pavonica). The analyses showed that after the generation of Fe₃O₄, the biomolecules in seaweeds had dual functions of biomineralization and stabilization to form Fe₃O₄ NPs. The proteins in seaweed extracts were also confirmed to reduce Fe³⁺ through a similar mechanism, and the aromatic compounds could stabilize the biogenic Fe₃O₄ NPs. The obtained Fe₃O₄ NPs might be an alternative and safe bioremediation for wastewaters to reduce the pollution of nitrogen and phosphorus [100]. CuO NPs were also found to be biosynthesized by Bacillus subtilis and Morganela morganii bacteria, whose size was less than 10 nm and the enhanced antibacterial activity against Streptococcus pneumoniae was also confirmed [101,102]. Nabila and Kannabiran reported the application of actinomycetes in the biomineralization of CuO NPs with a mean size of 61.7 nm. XRD and energy dispersive X-ray spectroscopy also showed that the particles were of high purity and stability with a zeta potential of 31.1 mV. The particles also exhibited a strong inhibitory effect on pathogenic bacteria such as S. aureus [103]. There are also various studies on the preparation of CuO NPs through biomineralization, such as brown seaweed (particles with sizes of 5–45 nm) and brassica oleracea (particles with a size of 26 nm), and finally found similar antibacterial properties [104].

Other reactive groups were also found to be involved in the precipitation of oxides. At high temperatures, O₂ could be utilized to biosynthesize ZnO nanoparticles (ZnO NPs) from the Annona squamosa leaf extracts, which is known to attain stable, reliable, and toxic free synthesis. The obtained particles possessed a hexagonal-shaped crystalline structure with diverse phytochemicals and functional groups as well as a size of 20–50 nm. It could enhance the antibiotic capacity and anticancer activity (Figure 5c) [105]. Furfural and hydroxymethyl compounds were also found to be involved in the processes of oxidation-induced biomineralization. Sargassum wightii and Calotropis gigantea extracts were found to transform Mg (NO₃)₂ solution to MgO nanoparticles (MgO NPs) through biomineralization (Figure 5d). Further research showed that MgO NPs exhibited significant antibacterial activity against P. aeruoginosa and anti-proliferation effect on the lung cancer cell line A549 [106,107].

3.3 Biomineralization mediated by sulfide precipitation

There are various sulfides, such as S 2 − , H₂S, sulfur-containing organic compounds, and amino acids, in organisms, which could undergo sulfurization reactions to transform metal ions into metal sulfides, and then biomineralize the metal sulfides into nanoparticles on biomolecules [108]. The biogenic FeS nanoparticles (FeS NPs) could be synthesized by Shewanella by biomineralization. The size of FeS NPs could also be controlled by the releasing rate of S 2 − , which could be a key parameter to combine with Fe ions. After subtle tuning, it was indicated that the gradually increasing releasing rate of S 2 − induced the biomineralization of FeS NPs with sizes between 30 and 90 nm [109]. CdS nanoparticles (CdNPs) could be biosynthesized by Bacillus cereus in high Cd-contaminated soil. Meanwhile, Bacillus cereus could also enrich plant growth promotion bacteria and downregulate the expression of genes related to bacterial motility, membrane transport, carbon, and nitrogen metabolism in the rhizosphere soil, decreasing Cd bioavailability in the soil. The Cd in soil was reabsorbed with the generation of CdS NPs. The biomineralization of CdS NPs provided a feasible method for improving the safety of crops via the inoculation of Bacillus cereus under Cd pollution [110]. ZnS nanoparticles (ZnS NPs) were obtained at >97% yield after a 96 h reaction between desulfovibrio and (CH₃COO)₂Zn at room temperature and atmospheric pressure conditions. The high-resolution transmission electron microscopy and XRD showed ZnS NPs with a granular microstructure with sizes of 5–8 nm. The kinetics and cellular distribution were also determined by measuring the concentrations, which finally indicated that ZnS NPs mostly concentrated in the membrane, followed by the cell wall and cytoplasm. The obtained ZnS NPs could bring impetus to the field of electroluminescence and photovoltaic devices, lasers, and single-electron transistors, as well as in potential biological applications [111]. There are also studies that utilized the mixed sulfate-reducing bacteria, consisting of 25% desulfovibrio, 25% clostridiaceae, 25% proteiniphilumsp, 12.5% geotoga and 12.5% sphaerochaeta to biosynthesize ZnS NPs, obtaining an average size of 6.5 nm and reaching a monthly yield of 35.0–45.0 g/L [112]. Similarly, CuS nanoparticles (CuS NPs) were prepared through the biomineralization by Calotropis gigantean (with sizes of 15–25 nm) [104].

In general, increasing requirements of metal-compound nanoparticles have raised public concerns on the biocompatibility and toxicity on human health and ecological safety, which promoted the investigation of the toxicity mechanism and further explored an ecofriendly synthesis as a potential solution. Hence, the biosynthesis of metal-compound nanoparticles by organisms deeply exhibited the scope for multifaceted biological applications. The processes and mechanisms of the biomineralization of metal nanoparticles were discussed. The relevant ions, metallic element, inducing medium, and nano-product of classical studies are summarized in Table 2.

4.1 Antibacterial properties

Biosynthesized nanoparticles have been widely studied and first applied to clinical anti-infective therapy with their antibacterial properties. AgNPs are typical antibacterial nanomedicine. The biosynthesized AgNPs retained the original antibacterial properties and displayed different therapeutic effects. As reported, Artemisia extracts exhibited the AuNP antibacterial ability against the Gram-negative E. coli BW 25113 and Gram-positive Enterococcus ATCC 9790 [115]. AgNPs synthesized using the culture filtrate extractions of the actinomycete strains exhibited significant antimicrobial activity towards P. aeruginosa and E. cloacae [116]. The potential application of Au–Ag alloy NPs synthesized by E. coli on the photothermal and antibiotic therapy was also demonstrated. In comparison with single AuNPs or AgNPs, the alloy NPs enhanced the antibacterial ability without increasing cytotoxicity, which provided a guarantee for clinical applications [117].

Other metal nanoparticles were also demonstrated to possess excellent antibacterial abilities. After incubation of the Streptomyces extract and ZnCl₂ solution for 72 h, spherical ZnO NPs with average particle sizes of 20–50 nm were obtained, which had antibacterial activity against E. coli and B. subtilis, and revealed a potential method for antisepsis [118]. Moreover, studies have also found that the antibacterial effect and cytotoxicity of ZnO NPs could be changed by artificially regulating the nano-shape, which showed that the rod-shaped nanoparticles performed better than hexagon-shaped ones and had lower cytotoxicity [119]. Similarly, CuO NPs biosynthesized by Streptomyces and Streptomyces pseudogriseolus also exhibited strong antibacterial activity against Gram-positive bacteria, Gram-negative bacteria, unicellular, multicellular fungi, and fungal strains, such as Fusarium oxysporum, Pythium ultimum, Aspergillus niger, and Alternaria alternate. Furthermore, the nanoparticles also showed strong insecticidal activity against Culex pipiens and other mosquitoes, which further broadened the application of metal-compound nanoparticles [120].

4.2 Anticancer

Along with further research on metal nanoparticles, different functions were developed. The medical application of biosynthesized nanoparticles was also extended to oncotherapy, which had already obtained satisfying effects. For instance, antitumor therapy could be promoted by biosynthesized metal nanoparticles by mediating the photothermal or photodynamic conversion, which could damage the tumor cells [121,122]. Besides, the ROS or reactive nitrogen species produced by the biosynthesis processes or the biosynthesized nanoparticles could also induce tumor cell apoptosis and mediate antitumor therapy [123,124]. For instance, the green synthesis of AuNPs by Acai berry and Elderberry extracts exhibited a positive effect in the treatment of prostate and pancreatic cancer. As reductant and stabilizing agents, phytochemicals played a crucial role in the bioreduction of metal nanoparticles, which endowed the AuNPs with selective toxicity toward the tumor cells and was considered a safer and cheaper option [125]. The AuNPs biosynthesized by the Terminalia mantaly (TM) extracts exhibited enhanced and selective cytotoxicity to the colon, breast, and liver tumor cells (Caco-2, MCF-7, and HepG2), which could promote the antitumor treatment [126]. Similarly, the coleus aromaticus-mediated AuNPs also exhibited significant cytotoxicity against human liver cancer cells (HepG2), which could be applied in the production of antibacterial and anticancer agents [127]. Moreover, the biosynthesized nanoparticles could also be combined with hypoxia responding to tumor-targeted drug delivery, which further promoted the effect of photothermal and photodynamic therapy [128,129].

4.3 Others

There are many other applications of the biosynthesized nanoparticles in clinical research. For instance, in the treatment of Cd-induced myocarditis, it has been reported that the application of biosynthesized nano-Se could alleviate the Cd-triggered inflammation response through the NF-kB/IκB pathway and reverse Cd-induced histopathological changes, which performed better than Se-enriched yeast, sodium selenite, and other Se compounds [130]. Moreover, the SeNPs biosynthesized by bacteria, fungi, and plants had numerous biomedical and pharmaceutical applications, whose antioxidant, antimicrobial, antidiabetic, and anticancer effects brought hope to patients with infection, diabetes, and cancer [131]. In the treatment of tuberculosis, the green-synthesized AuNPs (plants were used as reducing and capping agents) were used as drug and gene delivery systems in the medical treatment, benefited from their unique small size, physical resemblance, high biocompatibility, and non-cytotoxicity [132]. Cuminum cyminum, which had been traditionally used for the treatment of tendon injuries, provided the biosynthesized AgNPs with enhanced ability on tendon tissue regeneration. The results showed the particle with high formation and stability, which could be an ideal candidate for therapeutic application toward injured tendon tissue [133]. Fe₃O₄@CuO NPs synthesized with the Euphorbia polygonifolia extract also exhibited excellent catalytic performance in the degradation of antibiotics such as metronidazole, ciprofloxacin, and cephalexin [134]. Moreover, the thermally sensitive ELP-Au nanoparticles, which were generated on thermally sensitive elastin-like polypeptide (ELP), showed high near-infrared light absorption and photothermal effect at elevated temperatures, with which the application of metal nanocluster structures could mediate biological imaging such as photothermal imaging, photoacoustic imaging, X-ray computed tomographic imaging, magnetic resonance, fluorescence imaging, and CT scanning [135,136].