Electrochemical titration of carboxylic acid terminated SAMs on evaporated gold: Understanding the ferricyanide electrochemistry at the electrode surface

Ordered 11-mercaptoundecanoic acid (MUA) structures formed from self-assembly on evaporated gold electrodes were probed with potassium ferricyanide through electrochemical titration. The ferricyanide response was limited at these highly blocking films at low salt concentrations necessitating addition of 1 M KCl electrolyte. Voltammetry results strongly support the penetration of ferricyanide probe molecules into the SAM films for electron transfer. Recent models for electrochemical titration of SAMs, and interactions in weak polyelectrolytes are evoked for data analysis. The pKa determined using the electrochemical titration model was 5.6 ± 0.2 for 50% MUA/50% 6-mercaptohexanol monolayers and 5.8 ± 0.2 for the 100% MUA films. Utilizing a statistical mechanical model for weak polyelectrolytes at the water–solid interface (1 − pK) and a pK   of 5.7, an intermolecular interaction component (βE¯) of 2 was obtained for the 100% COOH films and βE¯ of 1 for the 50% COOH films, which suggests a more deprotonated 50% MUA film with less intermolecular interactions compared to pure MUA films.

► Surface pKa characterization of pure and mixed carboxylic acid SAMs. ► Solution ionic strength and Au substrate play major roles in probe molecule signal. ► Electrochemical titration is effective for pKa determination of less ordered SAMs. ► The ferricyanide probe molecule penetrates the SAM films for electron transfer. ► Highly ordered SAMs produce blocking films preventing ferricyanide penetration.