INTERACTIONS BETWEEN ORGANIZED, SURFACE-CONFINED MONOLAYERS AND LIQUID-PHASE PROBE MOLECULES .3. FUNDAMENTAL-ASPECTS OF THE BINDING INTERACTION BETWEEN CHARGED PROBE MOLECULES AND ORGANOMERCAPTAN MONOLAYERS

We report that Au electrodes modified with monolayers of functionalized organomercaptan derivatives show a pH-dependent electrostatic binding interaction with oppositely charged probe molecules. The extent of binding is discussed in terms of the position of the charge on the organic surface, the magnitude and distribution of the charge on the probe, the probe concentration, the degree of surface protonation, and intermolecular interactions between the organomercaptan monolayer and the electrolyte ions. Specifically, we examined the interaction between a series of sulfonated anthraquinone probe molecules, anthraquinone-2,6-disulfonic acid (2,6-AQDS(2-)), anthraquinone-1,5-disulfonic acid, and anthraquinone-2-sulfonic acid, and surface-confined isomers of mercaptopyridine. 4-Mercaptopyridine and 2-mercaptopyridine monolayers dramatically alter the molecular adsorption characteristics of Au surfaces. For example, at low pH, when the surface-confined mercaptans are protonated, they bind sulfonated anthraquinones, but at high pH, when the monolayer is deprotonated and electrically neutral, no binding occurs. Quinones bound to the mercaptopyridine surfaces can be released by changing the pH of the phosphate buffer from acidic to neutral pH. We also find that probe molecules having multiple binding sites interact most strongly with the 4-mercaptopyridine surface, that the surface concentration of adsorbed 2,6-AQDS(2-) increases as its bulk-phase concentration increases, and that increasing the concentration of electrolyte decreases the surface concentration of adsorbed 2,6-AQDS(2-).