Surface-enhanced Raman scattering (SERS) is a powerful technique for studying the adsorption behavior of molecules on substrate . In SERS studies, Ag electrodes, Ag islands, Ag deposited films, and Ag colloids are used as substrates . Among them, Ag colloids reduced by borohydride and citrate are the most commonly methods used in many researches due to its better enhancement effect and particle stability.
Azo-dye molecules, characterized by electron-donating and electron-accepting groups connected by a conjugated p-system, usually show large nonlinear optical responses, which are necessary for optical devices in telecommunications and signal processing applications . Moreover, the Azo-dye molecules have been found with wide applicability in reversible optical data storage . The storage process utilizes the light-induced transcis-trans isomerization of the Azo-dye molecules thereby utilizing the local variation of the refractive index of the medium. Methyl Orange (MO) is one of the Azo-dye molecules and was studied by many methods, such as liquid chromatography spectrometry and mass spectrometry . However, to our knowledge, no one investigated it with SERS. The main reason was that by using the reduction method to gain the Ag colloids, the borate or citrate ions would be introduced into the colloidal system, which may lead to competition with the MO . So one of the key points in wild SERS applications is finding a method of acquiring a highly SERS active substrate .
To avoid the chemical competition, Ag colloids were prepared by laser ablation form solid Ag, where ablation had the advantage of generating chemically clean colloidal surfaces . However, it was difficult and expensive to gain the Ag colloids using laser ablation method.
In this paper, an easy method, electrolysis method, was utilized to obtain chemically clean Ag colloids. The UV-vis spectrum and the transmission electron microscopy (TEM) image were employed to characterize the Ag particles. The SERS spectra of MO in the Ag colloids were investigated and a calculation was performed. The calculated result was compared with the experimental ones.
Experimental
2.1. Colloid preparation
Two Ag rods (purity !99.5%) were used as anode and cathode, respectively. The diameter of the Ag rods was 0.5 cm. Mixture aqueous solution of polyvinyl alcohol (PVA) and AgNO 3 were used as electrolyte. A 7-V direct current was applied on Ag rods for 1 h. Ag colloids with shallow yellow color were obtained as a