In situ monitoring of diffuse double layer structure changes of electrochemically addressable self-assembled monolayers with an atomic force microscope

An alkyl disulfide containing two sulfonate-substituted ferrocenes, 7,8-dit hiatetradecane-1,14-di(aminocarbonyl)-bis(1'-ferrocene-l-sulfonic acid), wa s synthesized and used to form electroactive self-assembled monolayers on g old electrodes. Atomic force microscope (AFM) force curves were employed to measure in situ the change in surface charge as the ferrocene groups were oxidized to compensate for the negative charges on the sulfonate groups. Th e electrode surface charge density was calculated from the surface coverage measured by electrochemical oxidation of the ferrocene groups, while the d iffuse double layer charges were obtained from theoretical fits of the forc e data to solutions of the complete nonlinear Poisson-Boltzmann (PB) equati on with knowledge of the silica probe surface potential. A significant diff erence between the AFM measured (i.e., effective) surface charge and the su rface charge calculated from electrochemical measurements was found. This d ifference is attributed to a layer of counterions near the surface that scr eens a large fraction of the surface charge (variously described as "ion co ndensation" or failure of the nonlinear PB theory). The experimental result s also showed that the extent of this ion screening (similar to 97%) was re latively constant and independent of the total electrode surface charge.