Surface-enhanced Raman scattering of 4,4′-dimercaptoazobenzene trapped in Au nanogaps

Abstract

The surface-enhanced Raman scattering (SERS) of 4,4'-dimercaptoazobenzene (4,4'-DMAB), an alpha, omega-dithiol possessing also an azo moiety, has seen a surge of interest recently, since 4,4'-DMAB might be able to form from 4-aminobenzenethiol (4-ABT) via a surface-induced photoreaction. An understanding of the intrinsic SERS characteristics of 4,4'-DMAB is thus very important to evaluate the possibility of such a photoreaction. We found in this work that 4,4'-DMAB should adsorb on a flame-annealed Au substrate via one of its two thiol groups such that Au nanoparticles could adsorb further on the pendent thiol group, forming a SERS hot site. The most distinctive feature in the SERS of 4,4'-DMAB was the appearance of a(g) bands, which were quite similar to the b(2)-type bands occurring in the SERS of 4-ABT. In an electrochemical environment, the a(g) bands of 4,4'-DMAB at 1431, 1387, and 1138 cm(-1) became weakened at lower potentials, completely disappearing at -1.0 V, but the bands were restored upon increasing the electrode potential, implying that neither electro- nor photo-chemical reaction to break the azo group took place, in agreement with data from a cyclic voltammogram. The appearance and disappearance of these a(g) bands are thus concluded to be associated with the charge transfer phenomenon: 4,4'-DMAB must then be one of a unique group of compounds exhibiting chemical enhancement when subjected to a SERS environment.

Direct evidence for conversion of p‐aminothiophenol to p,p'‐dimercaptoazobenzene by in situ reduplicative surface‐enhanced Raman scattering measurements

“…Previous studies found that the SERS spectrum of PATP could change under effects of pH of solution and power of excitation . In this work, we re‐investigated the effects of solution pH value and excitation powe on the SERS spectra PATP and DMAB.…”

“…However, the opposite results were gotten in previous literatures, i.e. the b 2 mode bands of PATP SERS spectra vary drastically with the measurement conditions …”

“…Especially to the effect of solution pH, it is found that the SERS spectra of DMAB were not subjected to change upon variation of pH. But the SERS spectra of PATP varied in different pHs, and the pH dependence was reversible . Thus, they concluded that the appearance and disappearance of the b 2 modes had nothing to do with formation of DMAB, but associated with the chemical enhancement in the SERS of PATP.…”

“…But it is hard to conclude whether the selective chemical enhancement or the formation of DMAB is the genuine origin of the b 2 modes according to the presented dependency relation between the variety law of SERS spectra and the measurement conditions. The two opponents draw the different conclusions on the similar phenomena . It had been found that in previous studies, b 2 modes of PATP were obtained easier in a basic solution than in an acidic solution, and the SERS spectrum was of reversibility in a reversible measurement system .…”

Mechanistic understanding of surface plasmon assisted catalysis on a single particle: cyclic redox of 4-aminothiophenol

“…As shown in Fig. 2 a and b, conversion of 4ATP into DMAB can be clearly distinguished from the new Raman bands that appear at 1140 cm −1 due to β (CH), and 1390 and 1432 cm −1 due to ν (N = N)18. The reaction will be terminated at a point when no obvious intensity changes of the DMAB Raman bands are observed.…”