Exploring the mechanism for mycobiosynthesis of silver nanoparticles from Aspergillus spp. and optimisation of synthesis parameters

The biosynthesis of silver nanoparticles from Aspergillus flavus (A. flavus) was observed by the brown colour formation of fungal filtrate because of the reduction of silver ions to nano form. The UV scan of the filtrate revealed absorption peaks at 430 and 360 nm, which correspond to silver nanoparticles and inorganic phosphate, respectively. The simultaneous increase in the absorption of both peaks, at each successive recording revealed that the synthesis of nanoparticles is accompanied with the release of inorganic phosphate. The mechanism could possibly involve inactive phosphorylated nitrate reductase, which converts into the active dephosphorylated form during its contact with silver ions by releasing inorganic phosphate. The enzyme may then reduce Ag⁺ ions to silver nanoparticles (Ag⁰). Inorganic phosphate production was noticed only in A. flavus, A. niger and A. terreus as no peak at 360 nm was observed in filtrates of Fusarium oxysporum and Trichoderma reesei. Powder-X-ray diffractometry analysis revealed four peaks for 2θ values which correspond to a face-centred cubic crystal structure. The Fourier transform infrared spectrum indicated the presence of various functional groups. Transmission electron microscope analysis showed nanoparticle sizes of between 3 and 25 nm. Atomic force microscope images showed different shapes of nanoparticles at various concentrations of substrate. Optimum conditions for the biogenesis of silver nanoparticles were found to be a temperature of 30°C, pH 5.0, 0.5% NaCl and 1.5 mM AgNO₃.