Competition of ion-pair during the transition from hydrogen bonding to electrostatic interaction on self-assembled monolayer

The competition of intermolecular interactions occurring on the cellular surface plays a vital role in governing biological recognition processes. Herein, the competition between ion-pair and electrostatic interaction on the surface of a reliable artificial membrane, 3-mercaptopropionic acid self-assembled monolayer (3-MPA SAM), was investigated using scanning electrochemical microscopy (secm). The electroactive agent ferrocenemethanol (FcCH₂OH) with one hydroxyl group is adsorbed by terminal carboxyl groups of 3-MPA SAM via hydrogen bonding. Once FcCH₂OH was oxidized into ferroceniummethanol (Fc⁺CH₂OH) by applying an oxidative potential, the electrostatic interaction between Fc⁺CH₂OH and the deprotonated carboxyl group of 3-MPA became the dominant intermolecular force in the reaction system accompanied by the breaking of hydrogen bonding. With the addition of perchlorate anion (ClO₄⁻), noticeable competition between the ion-pair and electrostatic interaction was observed owing to the spontaneous pairing of Fc⁺CH₂OH and ClO₄⁻. The competitive strength of ion-pair could be enhanced by increasing ClO₄⁻ concentration or solution acidity. Compared with the other six anions, ClO₄⁻ has been proved to exhibit more effective competition with the electrostatic interaction, which might attribute to the differences in hydration energy and the pairing capacities with Fc⁺CH₂OH. This work may contribute to comprehending the biological recognition on the cellular surface and deserve further investigation.