Viologen-functionalized conductive surfaces: Physicochemical and electrochemical characteristics, and stability

Abstract

The surface functionalization of electrically conductive substrates (indium-tin oxide (ITO), gold, free-standing polypyrrole film (PPY)) with a vinyl group containing viologen (vinyl benzyl viologen, VBV) has been carried out. The ITO was first silanized, while for the gold substrate, self-assembled alkanethiol monolayers were first prepared. Both substrates were then subjected to argon plasma pretreatment prior to UV-induced VBV graft polymerization. No pretreatment of the PPY film was necessary before UV-induced VBV graft polymerization. The surface composition and microstructure of the as-functionalized substrates were characterized by X-ray photoelectron spectroscopy (XPS), UV-visible absorption spectroscopy, and atomic force microscopy (AFM). The effects of plasma pretreatment time of the silane-modified ITO substrates and monomer concentration on the VBV graft polymer concentration were investigated. The cyclic voltammetric (CV) response of the as-functionalized ITO substrates was studied in aqueous sodium chloride solution. The CV results support the viologen redox mechanism in which VBV is first reduced to the radical cation during the cathodic scan and subsequently oxidized to the dication during the anodic scan. Good stability of the VBV was observed after repeated CV scans, as well as after being maintained at 100 °C for an extended period of time. This approach of surface functionalization of electrically conductive surfaces with viologen shows great promise for designing electrodes for use in electrochromic and sensing devices.