Electrochemically fabricated gold dendrites with underpotential deposited silver monolayers for a bimetallic SERS-active substrate

Abstract

With electrochemical deposition, cysteine-directed crystalline gold dendrites (Au-Ds) on glassy carbon electrodes were fabricated. The Au-Ds surfaces were further modified with Ag adatoms by underpotential deposition (UPD) for Ag-covered Au-Ds (Ag–Au-Ds). The Ag–Au-Ds possessed a hierarchical architecture comprising trunks, branches, and nanoleaves for a threefold-symmetry, resulting in a high density of sharp tips and edges for hot spots of surface-enhanced Raman scattering (SERS). Prominent SERS was observed with p-nitrothiophenol (p-NTP) adsorption onto either Au-Ds or Ag–Au-Ds, for a best p-NTP detection limit down to 5–10 nM at 785 nm laser excitation. However, at specific 633 nm laser excitation, SERS with p-NTP adsorption on Ag–Au-Ds exhibited a three-fold higher enhancement over that measured for p-NTP adsorbed on unmodified Au-Ds, suggesting an increased chemical SERS enhancement with the Ag–p-NTP bonding. Furthermore, adsorption isotherms of p-NTP with Au-Ds and Ag–Au-Ds adsorption in solution are established from solution p-NTP-concentration dependent SERS; from which, comparable binding constants of p-NTP to Au-Ds and Ag–Au-Ds are extracted.