Influence of dopamine concentration and surface coverage of Au shell on the optical properties of Au, Ag, and Ag(core)Au(shell) nanoparticles.

Gold (Au), silver (Ag), and Ag(core)Au(shell) nanoparticles (NPs) were explored as optical sensing agents for the sensitive detection of dopamine (DA) neurotransmitters in surface-enhanced Raman scattering (SERS) measurements. In these colloidal systems, dopamine (DA) molecules played as a cross-linker between M NPs (Au or Ag NPs), allowing them to reside in the confined junctions (i.e., hot spots) between aggregated NPs. The progressive addition of DA molecules (from 1 × 10(-6) to 1 × 10(-3) M) consequently decreased a primary absorption peak attributed to the characteristic M NPs and generated a secondary absorption peak at longer wavelengths attributed to heavily aggregated M NPs formed by molecular bridging effects of DA molecules at high concentrations. The aggregation degree of M NPs was also dependent on the surface states of Au and Ag NPs, i.e., DA molecules with positive amine groups induced more aggregations of Au NPs in comparison to Ag NPs with less negative charges. As the final outcome, Au NPs demonstrated higher sensitivity in SERS detection of DA at low concentrations (1 × 10(-7) to 1 × 10(-5) M), whereas Ag NPs exhibited the stronger Raman signals of DA molecules at high concentrations (1 × 10(-4) to 1 × 10(-3) M). Besides, Ag(core)Au(shell) NPs with the lowest surface coverage of Au shell exhibited more sensitive and stronger SERS activity for DA molecules than that of singular Au NPs, probably due to the combined contribution by Ag core with strong SERS intensity and Au shell with high SERS sensitivity.