title: 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.
authors.affiliation: Department of Chemistry, Seoul National University, Seoul 151-742, Korea. kwankim@snu.ac.kr
authors.forename: Kwan
authors.initials: K
authors.lastname: Kim
authors.forename: Dongha
authors.initials: D
authors.lastname: Shin
authors.forename: Kyung Lock
authors.initials: KL
authors.lastname: Kim
authors.forename: Kuan Soo
authors.initials: KS
authors.lastname: Shin
mesh_terms: D006046:Gold / Q000737:chemistry*; D053768:Metal Nanoparticles / Q000737:chemistry*; D010316:Particle Size; D013059:Spectrum Analysis, Raman; D013438:Sulfhydryl Compounds / Q000737:chemistry*; D013499:Surface Properties
chemical_list: D013438:Sulfhydryl Compounds; C568766:p,p'-dimercaptoazobenzene; D006046:Gold