Metal oxide nanoparticles have wide applications, primarily in the technology field, including their use as a semiconductor, electroluminescent or thermoelectric material, but they are also used in biomedical applications as drug delivery systems for treatment and diagnosis and in environmental decontamination applications [1,2]. The classical methods for obtaining metal oxide nanoparticles are based on chemical and physical techniques that employ hazardous and expensive chemicals with high energy input and a negative effect on the environment . The production of metal oxide nanoparticles via biogenic synthesis has received increasing attention recently because it is a novel process for the development of engineered materials . The biogenic synthesis of nanomaterials by different organisms offers a reliable, low-cost and environment friendly alternative approach compared with classical chemical and/or physical methods [3,4,5,6,7,8]. The biogenic synthesis of metallic nanoparticles leads to the formation of capped nanostructures with proteins/biomolecules from the organism during the biosynthesis. These capping agents prevent nanoparticle aggregation and likely play an important role in the stabilization of the nanosystem. The presence of capping agents may improve the biocompatibility of biogenic nanomaterials [3,4,5,6,7,8]. Figure 1 shows a schematic representation of the simplicity of biogenic synthesis of metal oxide nanoparticles along with the advantages and disadvantages of green processes.
Figure 1. Schematic representation of the biogenic synthesis of metal oxide nanoparticles and its advantages and disadvantages.
As highlighted in Figure 1, biogenic methods to obtain metal oxide nanoparticles are performed at room conditions, in a simple and cost effective manner and with no contamination to the environment. However, the main disadvantages are the limitations related to the scaling up the syntheses processes. In addition, the reproducibility of the biogenic processes needs to be improved, and in most of the cases, the mechanisms of nanoparticle formation are not completely elucidated [3,4,5,6,7,8].
The increasing production and use of metal oxide nanoparticles in numerous applications leads to adverse effects on health . Several studies have demonstrated nanoparticle toxicity and increased cytotoxic potential of these materials . However, a better understanding of the biological mechanisms of cytotoxicity and/or genotoxicity is necessary . Silver nanoparticles are the most studied metallic nanoparticles but their cytotoxicity and genotoxicity are not fully understood [10,12,13,14,15]. The toxicity of more complex nanostructures, such as graphene and carbon nanotubes, is also uncertain .
This review describes the biogenic synthesis of important metal oxide nanoparticles and their cytotoxicity in vivo and in vitro. The safety implications and environment effects of these nanoparticles are also discussed.
2. Biogenic Synthesis of Metal Oxide Nanoparticles
This section describes the biogenic routes (green approaches) to synthesize different metal oxide nanoparticles. These particles are important for technological, biomedical and environmental applications.
2.1. Bismuth Trioxide (Bi2O3) Nanocrystals
Bi2O3 nanocrystals are an optoelectronic material. This metal oxide has attracted a great deal of attention as a semiconductor that is sensitive to visible light and has superior photocatalytic activity for environmental purposes, such as water treatment . The traditional methods used to obtain Bi2O3 require the addition of organic/toxic solvents and high temperatures [17,18]. Uddin et al.  reported the room temperature biosynthesis of monodisperse Bi2O3 nanoparticles (5–10 nm) by Fusarium oxysporum as an alternative to conventional chemical methods. An important advantage of this ecofriendly biosynthesis is the formation of Bi2O3 nanoparticles with a protein layer, in contrast to the delicate surface coating that is obtained by using the conventional chemical methods, which are not capable of providing thermal stability or avoiding the agglomeration of nanoparticles.
2.2. Cobalt Oxide (Co3O4) Nanocrystals
Co3O4 nanomaterials possess desirable optical, magnetic and electrochemical properties and have been used as a super capacitor in energy storage devices. The classical methods of synthesis are solvothermal and thermal decomposition and the use of templates [20,21,22]. These synthetic routes are costly, time-consuming and toxic.
The microbial synthesis of Co3O4 nanoparticles using the marine bacterium Brevibacterium casei, was described by Kumar et al. . This was likely the first study in which the quantitative and qualitative analyses that were conducted during the biogenic synthesis indicated the sensitivity of the micromechanical properties of cells to the surrounding toxic environment. Transmission electron microscopy (TEM) of the as-synthesized nanoparticles revealed the quasi-spherical morphology of the particles with an average size of 6 nm. The protein coating on the biogenic Co3O4 nanoparticles reduced agglomeration and conserved the identity of the isolated nanoparticles .
2.3. Copper Oxide (CuO, Cu2O) Nanoparticles
Copper and copper oxide nanoparticles are used in optical and electronics applications and are a promising antimicrobial agent [5,24]. Several researchers have described the biogenic synthesis of copper based nanoparticles for a variety of applications. Hasan et al.  demonstrated that Serratia sp. produces an intracellular mixture of metallic copper and different copper oxides. Copper oxide (Cu2O) nanoparticles (10–20 nm) were synthesized at room temperature using the baker’s yeast Saccharomyces cerevisiae . The proposed mechanism is based on the partial gaseous hydrogen pressure of the reduction potential of metallic ions, which indicates the dependence of membrane bound oxido-reductases .
Usha et al.  reported the synthesis of copper oxide by Streptomyces sp. for antimicrobial applications in textiles. Copper oxide nanoparticles (100–150 nm) were obtained in solution by the reduction of copper sulfate by the reductase enzymes of the microorganism. The authors demonstrated the antibacterial (against Escherichia coli (E. coli) and Staphylococcus aureus (S. aureus)) and antifungal (against Aspergillus niger) efficacies of nanoparticle-coated fabrics. Scanning electron microscopy (SEM) revealed nanoparticles embedded on the treated fabric textile. The durability of the finished fabric was evaluated . Singh et al.  reported the biological synthesis (E. coli) of copper oxide nanoparticles with different sizes (10–40 nm, plus aggregates) and shapes (quasi-spherical). The results indicated the presence of a mixture of Cu2O and CuO phases. The proteins secreted by E. coli, with molecular weights ranging from 22 to 52 KDa, were attributed to reduced copper ions and stabilized the nanoparticle suspension .
Fungi can also synthesize metallic oxide nanoparticles. The biogenic synthesis of copper oxides was performed using Penicillium aurantiogriseum, P. citrinum and P. waksmanii isolated from soil . The authors investigated the effects of experimental parameters (pH and salt concentration) on the size of biogenic nanoparticles. SEM indicated a spherical shape of the nanoparticles . Another green synthesis of Cu2O used Tridax procumbens leaf extract . The resulting Cu2O nanoparticles were coated with polyaniline by a chemical polymerization technique. Hexagonal and cubic nanoparticles with rough surfaces were observed by SEM. The antibacterial effect of the Cu2O nanoparticles was evaluated against E. coli. A 65% inhibition of bacterial growth was observed upon the incubation of E. coli with 20 µg/cm3 of nanoparticles. A 100% inhibition was found for Cu2O concentrations in the range of 50–60 µg/cm3 . Sangeetha et al.  produced mono-dispersed, versatile and highly stable CuO nanoparticles from Aloe vera extract. This method is both ecofriendly and inexpensive, and it produced spherical CuO nanoparticles with a size range of 15–30 nm .
2.4. Iron Oxide (Fe2O3, Fe3O4) Magnetic Nanoparticles
Magnetic iron oxide nanoparticles show potential in several biomedical applications, including drug delivery, hyperthermia and nuclear magnetic resonance imaging [2,32,33]. In addition to the classical chemical methods of synthesis, there is an increasing interest in the use of biogenic techniques to obtain iron oxide nanoparticles .
In the presence of anionic iron complexes, and under aerobic conditions, Actinobacter spp. yielded two new proteins that synthesize magnetite nanoparticles. The biotransformation of ferri-/ferrocyanide complexes into magnetite was dependent on the proteins secreted by this bacterium . Incubating Actinobacter spp. with a ferricyanide/ferrocyanide mixture for 24 or 48 h resulted in quasi-spherical magnetite nanoparticles (10–40 nm) and cubic nanoparticles (50–150 nm), respectively. The nanoparticles were stable in aqueous solutions for several weeks because of the biomolecules secreted by the bacterium and were superparamagnetic at room temperature . The mycelia of acidophillic fungi, Verticillium sp. and Fusarium oxysporum, extracellularly form magnetite when they are exposed to an aqueous solution of K3[Fe(CN)6] and K4[Fe(CN)6] .
Shewanella strain HN-41, a dissimilatory iron-reducing bacterium, forms iron oxide, with formate, pyruvate or lactate as an electron donor, through the reduction of Fe(III)-oxyhydroxide, akaganeite (β-FeOOH) . DNA-binding protein from the starved cells of the bacterium Listeria innocua, LiDps, and its triple-mutant lacking the catalytic ferroxidase centre LiDps-tm produced nanomagnets at the interface between molecular clusters and traditional magnetic nanoparticles in the presence of a ferroxidase center . Yaaghoobi et al.  reported the biogenic production of magnetic iron oxide nanoparticles (≤104 nm) from Acinetobacter radioresistens. The authors compared the toxicity of biogenic and commercial iron oxide nanoparticles on red blood cells by evaluating hemagglutination, hemolysis and morphological changes. Severe hemagglutination was observed for commercial nanoparticles in a concentration-dependent manner from a concentration of 50 µg/mL. Toxic effects and morphological changes in the peripheral blood cells were not observed from bacterial synthesized magnetic iron oxide nanoparticles . Biogenic ferrihydrite (Fe2O3nH2O) nanoparticles that were synthesized by the bacteria Klebsiella oxytoca demonstrated composites in which amorphous or crystalline nanomaterials were observed with organic molecules [39,40,41]. Dissimilatory Fe(III)-reducing bacteria, such as Geobacter metallireducens and Shewanella putrifaciens, produce magnetite (nanocrystals) as a by-product of their metabolism in a growth medium . Byrne et al.  described the production of Fe3O4 nanoparticles by Geobacter sulphurreducens by modulating the total biomass used at the start of the synthesis. The authors observed that smaller particle sizes and narrower size distributions were achieved with higher concentrations of bacteria. This finding indicated that adjusting experimental parameters in the microbial synthesis of nanoparticles affects the physical, chemical and morphological properties of biogenic nanomaterials. Nanosized biogenic magnetite nanoparticles (10.0 ± 4.0 nm in diameter) were synthesized by the dissimilatory iron-reducing bacterium, Shewanella sp., for heterogeneous catalysis in ozonation . Iron oxide nanoparticles were produced by tannins, a natural and non-toxic polyphenolic compound extracted from plants [45,46]. Herrera-Becerra et al.  described the biogenic synthesis of magnetic hematite (Fe2O3) nanoparticles with a size less than 10 nm and pH 10 using tannins. Phenolic compounds, acting as capping agents, improve stabilization of the colloidal suspension and avoid nanoparticle aggregation.
2.5. Antimony Oxide (Sb2O3) Nanoparticles
As an inorganic semiconductor compound, antimony (III) oxide (Sb2O3) has several applications in technology and in chemical catalysis . Jha et al. [48,49] reported the low-cost reproducible biosynthesis of Sb2O3 nanoparticles at room temperature in the presence of baker’s yeast (S. cerevisiae). Different characterization techniques revealed the formation of Sb2O3 nanoparticles in a face-centered cubic unit cell structure, with an average size of 3–12 nm .
2.6. Silica (SiO2) Nanoparticles
Silica nanoparticles are important nanomaterials in biomedical applications such as nanocarriers for drug delivery systems [50,51]. Silica nanoparticles are widely used in industry, biomedical engineering and cosmetics .
In the presence an aqueous solutions of K2SiF6 (pH 3.1), mycelia of Fusarium oxysporum led to the formation of silica nanoparticles that ranged in diameter from 5 to 15 nm with an average size of 9.8 ± 0.2 nm . The authors demonstrated that the fungus Fusarium oxysporum secretes proteins that extracellularly hydrolyze SiF62−, yielding silica nanoparticles at room temperature . Actinobacter sp. cells were harvested and washed with water under sterile conditions and resuspended in an aqueous solution of K2SiF6. They formed quasi-spherical silicon/silica (Si/SiO2) nanoparticles with an average size of 10 nm . The cytotoxicity of the Si/SiO2 nanocomposites towards human skin cells was evaluated because silica nanoparticles are used in applications that require direct skin contact . The results demonstrated that the particles are not toxic to human skin cells .
2.7. Titanium Dioxide (TiO2) Nanoparticles
TiO2 nanoparticles have important environmental, technological and biomedical applications [51,55]. Jha and Prasad  reported the reproducible room temperature biosynthesis of TiO2 nanoparticles (10–70 in size) by Lactobacillus sp. that were obtained from yogurt and probiotic tablets. In the presence of suitable carbon and nitrogen sources, lactobacillus or yeast cells interact with a TiO(OH)2 solution to produce TiO2 nanoparticles (8–35 nm) with few aggregates . Lactobacilli have a negative electrokinetic potential, which is suitable for the attraction of cations, a step that is required for the biosynthesis of metallic nanoparticles.
2.8. Uraninite (UO2) Nanoparticles
Nanoparticles of UO2 are important for nuclear applications. The reduction of soluble uranium salts by microbial agents represents an important part of the geochemical cycle of this metal and highlights a mechanism for the bioremediation of uranium contamination [58,59]. Dissimilatory metal- and sulfate-reducing bacteria, such as Desulfovibrio desulfuricans, results in the precipitation of biogenic UO2 (bio-UO2) [58,59,60]. Biogenic uraninite was anaerobically produced by Shewanella oneidensis strain MR-1, at pH 6.3 [UO2(CO3)22−] and 8.0 [UO2(CO3)34−] . Shewanella putrefaciens