Study on self-assembled monolayers of functionalized azobenzene thiols on gold: XPS, electrochemical properties, and surface-enhanced Raman spectroscopy

Self-assembled monolayers (SAMs) of 4-acetamino-4′-(4-mercaptobutoxy)azobenzene (CH₃CONH-ph-N=N-ph-O(CH₂)₄SH, abbr. aaAzoC4SH) and 4-mercaptobutoxy azobenzene (ph-N=N-ph-O(CH₂)₄SH, abbr. AzoC4SH) on a gold surface have been studied by X-ray photoelectron spectroscopy (XPS), FT Raman spectroscopy, and electrochemistry. A surface-enhanced Raman scattering (SERS)-active system with a "sandwiched" structure of Ag|R-Azo-C4S-|Au was conveniently obtained by the method of Tollen's test. The relationship between the SERS effect and the structural nature of the system indicates that the enhancement correlates to both the silver islands above and the gold substrate underneath. The redox behaviors of the self-assembly on gold electrodes showed that the SAMs of the two compounds exhibit well-behaved voltammetric responses in a Britton-Robinson buffer corresponding to the irreversible two-electron, two-proton reduction-oxidation of azobenzene. The apparent electron-transfer rate kinetics is very sluggish, and the rate constant k_app of aaAzoC4SH/Au (1.34 × 10^-6 s^-1) is lower than that of AzoC4SH/Au (1.63 × 10-⁴ s^-1), which may be attributed to the different spatial restriction of close-packing structures on the conformational change accompanied by electron and proton transfer in the SAMs.