Polymer Brush-Modified Electrode with Switchable and Tunable Redox Activity for Bioelectronic Applications

Abstract

A new signal-responsive interface with switchable/tunable redox properties based on a pH-responding polymer brush was studied. Poly(4-vinyl pyridine), P4VP, functionalized with Os-complex redox units was grafted to an indium tin oxide (ITO) conductive support in the form of a polymer brush. The modified electrode surface was responsive to the changes of the pH value of the electrolyte solution: at acidic pH = 4.0 the redox-polymer film demonstrated the reversible electrochemical process, E° = 0.29 V (vs Ag/AgCl), while at neutral pH > 6, the polymer was not electrochemically active. The reversible transformation between the active and the inactive state originated from the structural changes of the polymer support. The protonation of the pyridine units of the polymer backbone at the acidic pH resulted in the swelling of the polymer brush allowing quasi-diffusional translocation of the flexible polymer chains, thus providing direct contact of the polymer-bound redox units and the conducting electrode support. The uncharged polymer formed at the neutral pH values existed in the shrunk state, when the mobility of the polymer chains was restricted and the polymer-bound redox units were not electrically accessible from the conducting support, thus resulting in the nonactive state of the modified electrode. The reversible changes of the electrochemical activity of the modified electrode and the respective structural changes of the polymer-brush were characterized in details by electrochemistry, AFM, and ellipsometry. The stepwise changes of the pH value between 3.0 and 7.0 resulted in the reversible switching on and off of the electrode redox activity, respectively. The redox activity of the modified electrode was also tunable upon precise titration of the electrolyte solution between pH 3.0 and 7.0 demonstrating a titration-like curve for the amount of the redox-active group because of the smooth transition between the swollen and the shrunk states. The primary electrochemical activity of the modified electrode was coupled with a biocatalytic oxidation of glucose in the presence of soluble glucose oxidase (GOx), thus allowing reversible activation of the bioelectrocatalytic process. The modified electrode with the pH-controlled switchable/tunable redox activity was proposed as a “smart” interface for a new generation of electrochemical biosensors and biofuel cells with a signal-controlled activity.