Probing electrocatalytic and bioelectrocatalytic processes by contact angle measurements

Electroswitchable wetting of electrode surfaces modified with redox-active monolayers and thin films is described. Electrocatalytic and bioelectrocatalytic processes that are activated by the redox-active interfaces associated with electrodes control the hydrophobic/hydrophilic properties of the surfaces, thus allowing the probing of the chemical transformations by static contact angle measurements. A Prussian Blue film associated with an ITO electrode undergoes redox transformations between the hydrophilic reduced state, PB4-, the hydrophobic semioxidized state, PB0, and the hydrophilic fully oxidized state, PB3+. Contact angle measurements follow the reversible switching of the film between the three states. The oxidized state, PB3+, electrocatalyzes the oxidation of NADH, and thus, the ratio of PB3+/PB0 on the film interface upon the electrochemical oxidation of NADH is controlled by the cofactor concentration. This enables following the electrocatalyzed oxidation of NADH by static contact angle measurements. Similarly, the hydrophobic/hydrophilic properties of a naphthoquinone-functionalized polyethylenimine film are reversibly electroswitched by the reduction and oxidation of the film. In the reduced state of the film the naphthohydroquinone units catalyze the reduction of O-2, thus leading to a hydrophobic film that originates from the high naphthoquinone/naphthohydroquinone ratio associated with the film. The hydrophobic/hydrophilic properties of an An electrode modified with a ferrocene monolayer are electroswitched between a hydrophilic state in the presence of the ferrocenylium (Fc(+)) oxidized monolayer and a hydrophobic state in the presence of the ferrocene (Fc) monolayer configuration. The ferrocenylium monolayer activates the bioelectrocatalyzed oxidation of glucose in the presence of glucose oxidase. The bioelectrocatalyzed oxidation of glucose leads to the control of the Fc(+)/Fc ratio associated with the monolayer by the glucose concentration in the system. This enables following the bioelectrocatalytic oxidation of glucose by static contact angle measurements.