Unobstructed electron transfer on porous polyelectrolyte nanostructures and its characterization by electrochemical surface plasmon resonance

Thin organic films with desirable redox properties have long been sought in biosensor research. We report here the development of a polymer thin film interface with well-defined hierarchical nanostructure and electrochemical behavior, and its characterization by electrochemical surface plasmon resonance (ESPR) spectroscopy. The nano-architecture build-up is monitored in real time with SPR, while the redox response is characterized by cyclic voltammetry in the same flow cell. The multilayer assembly is built on a self-assembled monolayer (SAM) of 1:1 (molar ratio) 11-ferrocenyl-1-undecanethiolate (FUT) and mercaptoundecanoic acid (MUA), and constructed using a layer-by-layer deposition of cationic poly(allylamine hydrochloride) (PAH) and anionic poly(sodium 4-styrenesulfonate) (PSS). Electron transfer (ET) on the mixed surface and the effect of the layer structures on ET are systematically studied. Under careful control, multiple layers can be deposited onto the 1:1 FUT/MUA SAM that presents unobstructed redox chemistry, indicating a highly ordered, extensively porous structure obtained under this condition. The use of SPR to trace the minute change during the electrochemical process offers neat characterization of local environment at the interface, in particular double layer region, allowing for better control over the redox functionality of the multilayers. The 1:1 SAM has a surface coverage of 4.1 ± 0.3 x 10-10 mol cm-2 for ferrocene molecules and demonstrates unperturbed electrochemistry activity even in the presence of a 13 nm polymer film adhered to the electrode surface. This thin layer possesses some desirable properties similar to those on a SAM while presenting ∼15 nm exceedingly porous structure for high loading capacity. The high porosity allows perchlorate to freely partition into the film, leading to high current density that is useful for sensitive electrochemical measurements.