Gold surface cleaning by etching polishing: Optimization of polycrystalline film topography and surface functionality for biosensing

Abstract

Modern bio-chemical sensors rely on functional interfacial architectures with well-defined structural nano-motifs over a physical transducer. Gold-coated interfaces are of particular interest for their desirable chemical (functionalization) and optical (plasmonic) properties. Here we investigate the cleaning and polishing of polycrystalline gold films in preparation of advanced surface functionalization. We focus on soft wet chemical etching to decrease the small-scale roughness commonly observed after evaporation or sputtering of gold. We show that optimized surfaces are obtained by etching in solutions of hydrochloric acid and hydrogen peroxide. We systematically quantify the films wettability, surface nano-topography, UV-VIS spectrum and the electrochemical and Surface Plasmon Resonance (SPR) changes throughout the etching process. Optimal results are obtained by etching with a HCl(37%):H2O2(30%):H2O mixture, with a volume ratio of reagents 3:3:94 during 15-20 minutes at room temperature for the main step. This reduces by a factor two the root-mean-square roughness, removes contaminants, increases hydrophilicity and modifies the gold surface by Au(Cl)x complexes. Significantly, the resulting the surface is hydrophilic enough to prevent globular proteins such as HSA to unfold upon deposition at concentrations more than ~1 mg/mL. Our protocol offers a simple, reliable and rapid method for the preparation of gold surface in view of further functionalization including the binding of receptor layers and various micro- and nanostructures required in chemical and biochemical sensing.