Biosynthesis of Silver Nanoparticles by Aspergillus foetidus: Optimization of Physicochemical Parameters

Optimal conditions for the biosynthesis of silver nanoparticles (AgNPs) by fungi Aspergillus foetidus were investigated. Condition of 0.2% salinity, 4 mM of AgNO₃, pH 9, 10% diluted concentration of cell filtrate and incubation time of 72 h at 30 °C were found to be optimum for the biosynthesis of silver nanoparticles. The activity of nitrate reductase attained optimum level at 0.2% salinity, pH 9 and 30 °C. Activities of nitrate reductase in fugal cell filtrate were 0.5116 and 0.1711 μmol/ml/min, respectively, before and after the synthesis of AgNPs. The protein contents were 407.4 and 207.3 μg/ml respectively before and after the synthesis of AgNPs, and both the nitrate reductase activity and the protein content decreased after the biosynthesis of AgNPs, indicating a possible role for this enzyme in the biosynthesis of AgNPs. The rate of conversion of Ag + to Ag⁰ and the concentration of the synthesized Ag⁰ were found to be 93.39% and 158.65 μg/ml, respectively. The minimum inhibitory concentration (MIC) of synthesized AgNPs was determined against selected plant pathogenic fungi of Aspergillus spp. and F. oxysporum, and MIC value of AgNPs in A. flavus was lowest 1.6 μg/ml.