Electrochemical sensors based on porous nanocomposite films with weak polyelectrolyte-stabilized gold nanoparticles

Porous hybrid multilayer films composed of cationic poly(allylamine hydrochloride) (PAH)- and anionic poly(acrylic acid) (PAA)-stabilized gold nanoparticles (Au_NPs) (i.e., PAH-Au_NPs and PAA-Au _NPs) were prepared on indium tin oxide (ITO) electrodes using pH-controlled layer-by-layer (LbL) assembly method with subsequent acid treatment. The exponential growth of Au_NP deposition layers was caused by the "in-and-out" diffusion of PAH and PAA chains not bound to Au_NPs. Immersion of the films in an acidic solution (pH 2.4) converted the nonporous films to porous films via the disruption of ionic bonds and the rearrangement of free PE chains. In this case, the pH-induced porous films showed high electrochemical activity. Nonporous/dense films were found to prevent direct contact between probe molecules in solution and the catalytic components immobilized on an electrode. Electrodes coated with porous films, however, exhibited higher electrocatalytic activity toward nitric oxide oxidation compared with electrodes coated with nonporous films, despite the same levels of Au_NP loading. This work demonstrates that structural transformations via a facile pH treatment can significantly improve electrode sensitivity without the aid of porous supports or additional catalytic components.