Optical Properties of Dopamine Molecules with Silver Nanoparticles as Surface-Enhanced Raman Scattering (SERS) Substrates at Different pH Conditions

Abstract

Silver nanoparticles (Ag NPs) prepared by the citrate reduction method were examined as surface-enhanced Raman scattering (SERS) substrates in the detection of dopamine (DA) molecules at different hydrogen ion concentrations. The aggregation of Ag NPs was influenced by the cross-linking effect of DA molecules as the function of pH of colloidal solution. Somewhat clustering of Ag NPs in a limited pH range (pH 7-9) exhibited the strong red-shift of absorption peak and maximal SERS activity to DA molecules, highlighting the importance of strong electrostatic adsorption and cross-linking effect that allowed DA molecules to reside in the junctions (hot spots) between aggregated Ag NPs. Furthermore, Ag NPs with DA molecules at strongly basic condition (>pH 9) exhibited the relatively high SERS activity as compared to negligible SERS activity at acidic condition (<= pH 5), indicating the important role of oxidized surface of silver NPs which can interact with hydroxyl groups of DA molecules.