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Nanomaterials 2020, 10, 1146 5 of 24 produced NPs intracellularly when cultured with agitations. The possible mechanism involved here is the non-agitation condition, but not the agitation condition, enhancing the release of enzymes and proteins [60]. The desired characteristics of NPs from different fungal species can be obtained through the adjustment of some factors like temperature, agitation, light, and culture and synthesis times. These parameters should be maintained during the fungal culture and NPs synthesis to control of the size and shape of NPs [61]. It was also found that differences in pH, temperature, culture medium, biomass quantity and concentration of the metal precursor can be used to determine physicochemical characteristics of NPs [58,61–63]. AgNPs were synthesized by using the filtrate of Rhizopus stolonifer with NPs size of 2.86, 25.89, and 48.43 nm under the temperature regime of 40, 20, and 60 ◦C, respectively, but not at 10 or 80 ◦C. At a very low or high temperature, enzymes and active molecules may be denatured or inactivated which are needed in AgNPs biogenesis [64]. Husseiny et al. reported the biosynthesis of AgNPs using Fusarium oxysporum and reported the effect of substrate concentration and incubation temperature [65]. Most of the AgNPs were smaller at 50 ◦C and, at higher temperature, particle size increased. The amount of biomass played a key role in synthesis or complete reduction of Ag+ to Ag0. The optimum weight of fungal biomass was 11 g for the smallest particle size. For the AgNPs synthesis by F. oxysporum, pH was found to be an important factor and the smallest size particles were obtained at pH 6. Due to the lower pH, protein structure might be affected or denatured and its potential may have been lost; thus, NP size was found to be large [66]. In alkaline conditions, the catalyzing activity of reductase enzyme for the synthesis might be gradually deactivated, and reduced synthesis and increase in size of the particles at higher pH. A similar phenomenon was observed during AgNPs synthesis using Penicillium fellutanum [67]. A seven-day old fungus used as a young culture for 72 h was better than a fifteen-day old culture for s similar incubation time in the case of AgNPs [65]. Yeast cells act as one of the most important agents for bioremediation of heavy metals. Yeasts are easily cultured in low-cost media and capable of removing various heavy metals. Yeasts have the adaptive capacity to extreme environmental conditions like pH, temperature and high concentrated organic and inorganic contaminants. Most of the available studies concern Ascomycota such Saccharomyces cerevisiae, Schizosaccharomyces pombe and Candida sp. Yeasts may have evolved some mechanisms for detoxification such as mobilization, immobilization or transformation of metals [68,69]. The immobilization mechanisms involve biosorption, biotransformation and bioaccumulation of metal ions by living microorganisms [70]. These bioremediation properties of yeasts can be exploited for the green synthesis of NPs to be applied in fields. Saccharomyces cerevisiae was used for biosynthesis of AgNPs by biotransformation. Both the dried and fresh culture S. cerevisiae was used as the biocatalyst. More AgNPs were obtained from freshly cultured yeast than dried culture. The AgNPs were spherical with a size of 2–20 nm in diameter, and 5.4 nm sized particles were mostly found. AgNPs were found inside the cells, within the membrane of cells, attached to the cell membrane, and outside of the yeast cells [11]. A marine yeast Yarrowia lipolytica strain was used for the biosynthesis of AgNPs in a cell associated manner. This study suggested that the brown pigment (melanin) might be the possible reason for biomineralization of metallic ions [71]. Pichia jadinii was used for intracellular synthesis of AuNPs ranging from 1–100 nm. In this study, the growth and cellular activities of P. jadinii were controlled easily to regulate AuNPs size and shape [72]. The green synthesis AgNPs was obtained in an extracellular process by using Candida utilis NCIM 3469 with a size 20–80 nm [73]. In another study, Saccharomy cescerevisiae was capable of synthesizing copper NPs (CuNPs) extracellularly, where more than 70% of the particles were about 10–12 nm [74]. Several studies illustrate that viruses are considered to be a suitable group which serves as a biotemplate for material synthesis at the nanoscale to microscale [75]. Recently, material science researchers were using the viral NPs (VNPs) as templates or scaffolds for the synthesis of novel hybrid nanomaterials [76]. A number of plant viruses were employed as nano-factories because of their special structural integrity, easy manipulation and lower infectivity to human [76,77]. Furthermore,PDF Image | Plant and Microbe-Based Synthesis of Metallic Nanoparticles
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