Does Nitrate Reductase Play a Role in Silver Nanoparticle Synthesis? Evidence for NADPH as the Sole Reducing Agent

Abstract

A leading hypothesis for silver nanoparticle biosynthesis suggests that the enzyme nitrate reductase plays a key role in the process, assisted by the coenzyme nicotinamide adenine dinucleotide phosphate (NADPH). Here, we show that NADPH alone can act as the reducing agent of silver nitrate salt to form silver nanoparticles. In fact, the addition of nitrate reductase during the synthesis causes a decrease in the reaction rate and broadening of the size distribution of the particles. The NADPH-synthesized particles were monodispersed with a mean radius of 5.1 ± 1.0 nm, as indicated by transmission electron microscopy (TEM) imaging and supported further by in situ single entity electrochemistry. The bottom-up molecular approach enabled facile purification of the nanoparticles, and the minimal components allowed the determination of the kinetic parameters of the reaction. The kinetics of the biomolecular synthesis is shown to be a pseudo-first-order reaction with respect to NADPH with a rate constant of 6.6 × 10–3 s–1 at 25 °C. Finally, using 1 mM NADPH, the reaction was 90% completed within 7 minutes when run at 80 °C. These mechanistic insights provide a deeper understanding toward the future realization of environmentally friendly, rapid, and low-cost biomolecular synthesis agents for monodispersed nanoparticles.