Biomimetic Mineralization and Assembly Route To Synthesize Au/ZnO Nanostructures for the Oxidation of NADH

Polysaccharides are a promising and cost-effective alternative to polypeptides for biomimetic mineralization. This study presents a biomimetic approach for synthesizing flower-like ZnO nanostructures using starch as both a structure-directing and mineralizing agent. These structures are functionalized with gold (Au) nanoparticles using two methods: a polyamine-mediated layer-by-layer (LbL) assembly of citrate-capped gold nanoparticles (Au-Cit) and an in situ reduction of gold salt on the ZnO surface. The Au/ZnO-5-in situ-8 catalyst demonstrates superior performance in NADH oxidation (73.7% conversion vs 27.9% for Au/ZnO-5-LbL), attributed to smaller size, better dispersion, and favorable surface charge of Au nanoparticles. Most importantly, the greater surface coverage of Au nanoparticles on the ZnO support plays a crucial role in determining the catalytic activity. The study demonstrates that the Au/ZnO-5-in situ-3 catalyst is effective for both biotransformation and electrocatalytic oxidation of NADH. In the enzymatic reaction, the enzyme aldehyde dehydrogenase (ALDH) oxidizes formaldehyde into formic acid using NAD⁺, which is generated catalytically by Au/ZnO-5-in situ-3. In electrocatalysis, Au/ZnO-5-in situ-3 outperforms Au/ZnO-5-LbL due to its positive surface charge, which enables efficient electrostatic binding of NADH, thereby lowering the oxidation potential and increasing the oxidation current. This easily adaptable synthetic strategy for devising hybrid catalysts with controlled nanoparticle size, dispersion, and surface coverage represents a pivotal advancement for developing optical and electronic sensory devices.