pH-switched electrochemistry of pyrroloquinoline quinone at Au electrodes modified by functionalized monolayers

Gold electrodes were modified with thiolate monolayers that included dissociative groups of different pK_a values: cystamine, 4-aminothiophenol and 4-pyridyl disulfide. The electrochemistry of the redox probe pyrroloquinoline quinone (PQQ) was examined in a broad pH region using the different monolayer-modified electrodes. The redox probe PQQ reveals reversible electrochemical properties only in the presence of positively-charged monolayer-modified electrodes, owing to its electrostatic attraction to the monolayer|electrode interface. For the different monolayer-modified electrodes exhibiting different pK_a values, the transition from a reversible to an irreversible electrochemical process for PQQ was observed at different pH values. The electrochemical response of the modified-electrode system can be reversibly switched between ON and OFF states by controlling the pH of the assemblies in a narrow range. The transduced amperometric signals at the different pH environments were amplified by the PQQ-electrocatalyzed oxidation of NADPH in the presence of Ca²⁺. The electrode systems represent novel examples of pH-controlled electrochemical ON-OFF systems.