Redox Surfactants Are Chemical Probes of Electrode Surface Functionalization Derived from Disulfide Immobilization on Gold

The use of selected disulfide reagents to immobilize various functional groups at submonolayer to near monolayer coverages onto gold electrodes is described. The influence of this surface functionalization on the adsorptive behavior of a redox surfactant derived from ferrocene is used to infer the coverage of the disulfide reagents through the use of implicit isotherms. The following changes in adsorption of the cationic redox surfactant relative to adsorption on a bare gold surface are observed for the three types of surface functionalization investigated: Immobilization of hydroxyl groups depresses adsorption due to increased hydrophilicity of the surface; immobilization of ethyl groups changes the orientation of the adsorbed surfactants at saturation coverage from perpendicular (i.e. with the alkyl chains roughly normal to the surface) to parallel (i.e. with the alkyl chains lying flat on the surface), due to favorable hydrophobic interactions between the ethyl groups and the alkyl chains of the redox surfactants; immobilization of sulfonate groups enhances adsorption (especially at intermediate sulfonate coverages) due to favorable electrostatic interactions between the cationic redox surfactant and the sulfonate groups. The electrochemical quartz crystal microbalance method is used to monitor redox-induced adsorption and desorption events for the redox surfactants. For the sulfonate case, the combination of information available from the electrochemical and microgravimetric methods reveals strong adsorption of the dicationic form of the redox surfactant as well as the monocationic form, contrary to the findings on bare gold, the hydroxylated, and the ethylated surfaces. In addition, the presence of competitive adsorption between the redox surfactant and the sulfonated disulfide is demonstrated. Adsorption isotherms for the redox surfactant on some of the functionalized surfaces are presented. The generality of the use of redox surfactants as chemical probes of surface functionalization is discussed.