Nitrate Reductase mediated synthesis of surface passivated nanogold as broad-spectrum antibacterial agent

The green synthesis of gold nanoparticles has attracted tremendous interest owing to their unique physicochemical properties and widespread applications which are primarily size-dependent. The stability, less reaction time, and use of biological resources as novel nanofactories as an alternative to conventional synthesis strategies are the main objectives of green synthesis approaches. However, to attain size-controlled synthesis from the biogenic route is still a challenge. Hence, use of nontoxic stabilizers becomes increasingly essential. Herein, we describe an emerging, simple, nonconventional approach to synthesize stable and size-controlled biogenic nanogold using cell lysate supernatant containing nitrate reductase of Bacillus licheniformis in the presence of Tween 20 and dodecanethiol respectively. The face-centered central composite design used for the optimization of gold nanoparticles (AuNPs) biosynthesis. The maximum AuNPs biosynthesis obtained using the optimized media variables, glucose (2.1 g/L), peptone (14.05 g/L), yeast extract (4.14 g/L), and potassium nitrate (3.91 g/L) was 0.769 a.u. Highly stable monodispersed nanogold of 10.4 +/- 0.6 nm and 12.5 +/- 0.9 nm sizes arranged in ordered self-assembly was obtained. The stability profile and kinetics of bioreduction was evaluated with respect to time, and the involvement of the nitrate reductase enzyme in bioreduction was validated by inhibitor study. The physicochemical properties of biogenic nanoparticles were characterized using multiple spectroscopy and microscopy techniques. The obtained nanogold demonstrated excellent bactericidal property against both gram-negative and gram-positive bacteria in size-dependent manner and thus could find tremendous utility in clinical, biological, and environmental applications as a broad-spectrum antibacterial agent.