The pH Dependence of the SERS Spectra of Methyl Yellow in Silver Colloid

Methyl yellow is an important azo dye that may be used as a pH indicator. In this study, the surface-enhanced Raman spectra of methyl yellow in silver colloids with various pH values were investigated. A relative intensity analysis indicated the intensities of the Raman bands changed primarily when the pH was lowered from 4 to 3. The enhancement of surface-enhanced Raman scattering (SERS) signals is determined mainly by surface plasmon resonance, charge-transfer resonance, and their combination. The varying intensities of SERS signals in solutions with different pH levels may result from the change in the contributions of their combined systems, such as the charge of methyl yellow molecules and the adsorption of molecules on the surface of silver.