Study of electrode potential effect on acid-base behavior of ω-functionalized self-assembled monolayers using Fourier transform surface-enhanced Raman scattering spectroscopy

Interfacial proton transfer reactions of 2-aminoethanethiol (2AT) self-assembled monolayers (SAMs) at different fixed potentials were studied using the Fourier transform surface-enhanced Raman scattering (FT-SERS) titration method. ClO4- was used as a counter anion of the protonated omega-amino group of the monolayers to probe the extent of protonation with change of solution pH. At aqueous buffer/SAMs-modified gold interfaces, the electrochemical in situ FT-SERS spectra were measured at different electrode potentials. Under potentials of 0, 0.1 and 0.2 V vs Ag vertical bar AgCl (saturated KCl), the surface pK(a) values determined for the 2AT SAMs modified on gold were 5.0 +/- 0.2, 4.2 +/- 0.2 and 3.4 +/- 0.2, respectively, at 0.25 M ionic strength. The surface pK(a) shift direction follows the polarity of the applied potential. This indicates that the surface pK(a) is controllable within this potential range by the electrode potential. Concerning the origin of the surface pK(a) shift to the acidic side with positively increased potential, a preliminary explanation based on analysis with local pH at the interface and a discussion about the effect of the double layer on the surface pK(a) shift of the 2AT SAMs by the electrode potential are given. Copyright (c) 2005 John Wiley I Sons, Ltd.