| The structure of azo dyes has attracted considerable attention in recent times, since they represent the largest class of dyes used in industry, and also has extensive applicability in analytical chemistry as acid-base, redox and metallochromic indictors. Moreover, azo dyes find wide applicability in reversible optical data storage. The storage process utilizes the light induced trans-cis-trans isomerization of the azo dye, thereby utilizing the local variation of the refractive index of the medium. The photoinduced changes in polarity and molecular structure have led to widespread application of azobenzene derivatives as reversible molecular switch devices. 4-Dimethylaminoazobenzene (DAB) is an important azo dye, which is used to the production of color polishes, wax products, polystyrene, and soap. It is also used as a chemical indicator for free hydrogen chloride (HC1) in juice, as a spot test identification of peroxidized fats, and as a PH indicator. There were many literature on the studies of the structure of DAB by means of various spectroscopic and photochemical methods including UV-Vis absorption, infrared (IR), and Raman. While very few reports on the SERS of DAB appeared. Surface-enhanced Raman scattering (SERS) is widely used as a highly powerful analytical tools to investigate molecular structure and interaction between molecules and metal surface resulting in strongly increased Raman signals from molecules which have been attached to nanometer sized metallic structures. The SERS spectra of DAB adsorbed on the same substrate in different environment and on different substrate were investigated in this paper. The objective of this work was to obtain information about the spectral properties of DAB and use these information to gain an overall picture of the nature of the adsorption sites on these metal surfaces.This paper includes five sections as following:In the first section, the discovery, characteristics, mechanisms, and recent developments of SERS are simply summarizedIn the second section, the SERS spectra of DAB molecules adsorbed on silver sol, silver mirror, and copper were investigated. The analysis of spectra shows that DAB molecules adsorbed on the metal surfaces through the Dimethylamino group, but the adsorptions of DAB molecules were different in varied environment. For instance, the structure of DAB in silver sol is a stable cis form, while that is an unstable trans form on silver mirror, which could be due to the difference of interaction between the solvent molecules and DAB molecules. The difference of SERS spectra of DAB adsorbed on silver mirror and copper could be due to the difference of apparent enhanced factors of them.In third section, the SERS spectra of DAB in silver sol with varied PH values were obtained. When the PH of the solution was lower than 4, the structure of DAB changes from an azo type to a quinoid type. This change was studied by SERS and the reversibility of transformation was found. In addition, the interactions between DAB molecules and silver particles in varied PH values were different, which is mainly due to the N atom in Dimethylamino group positively charged.In the forth section, the SERS spectra of single DAB molecule were recorded. SERS spectrum of single DAB molecule under normal temperature and condition was reported in this section by a simple method, in which the sample was prepared by mixing 4-dimethylaminoazobenzene with silver colloid and SERS spectra were directly recorded from the sample droplet on the aluminum foil. Blinking of the SERS spectra and the Poisson statistical distribution of the Raman signal were observed. Moreover spectral diffusion of Raman shift with time was reported by the first time according to our knowledge, which could be due to the difference of interaction between varied single molecules and varied silver particles surface, for instance, due to the differentadsorption states of DAB molecules on the silver surface, which should be a powerful evidence for single molecule detection by SERS.
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