Optimization of Enterobacter cloacae mediated synthesis of extracellular silver nanoparticles by response surface methodology and their characterization

Synthesis of silver nanoparticles (AgNPs) with desired physicochemical properties using a cost-effective, energy, and time efficient way is vital. In this study, we have optimized the synthesis of AgNPs through protein containing cell-free supernatant of Enterobacter cloacae (SMP1). 2³ full factorial central composite design (CCD) of the response surface methodology (RSM) was used for the process optimization. Individual and mutual interaction effects of the experimental variables including temperature, pH, inoculum size, and metal concentration were investigated on the synthesis of AgNPs. Results showed the involvement of 15 kDa protein in the synthesis of AgNPs. The optimized values of pH, temperature, cell-free culture supernatant (CFCS), and metal concentration were 10, 37 °C, 10% (v/v) and 2 mM, respectively. Characterization of AgNPs using UV-Vis spectroscopy reveals surface plasmon resonance peak at 450 nm. AFM and TEM micrographs revealed oval and spherical shaped AgNPs with the size of spherical (15–44 nm) and oval (20–46 nm wide and 240–325 nm long) and zeta potential of −41.3 mV under different experimental conditions. The EDS, X-ray elemental mapping, and FTIR reviled the presence of metallic AgNPs and signatures for nitrogen, carbon, oxygen, sulfur, and functional groups of different biomolecules.