Modification of a Silver Substrate for Advanced Spectro-Electrochemical Applications of SERR Spectroscopy

A substrate for surface-enhanced resonance Raman spectroscopy (SERRS) in the near-ultraviolet (UV) range is presented, extending the potential window for electrochemical applications. Silver nanoparticles were synthesized exhibiting a localized surface plasmon resonance at the excitation wavelength and adsorbed onto a template-stripped silver substrate, whereby the number of particles per unit area was controlled by the adsorption time. Any attempt to employ spectro-electrochemistry on these surfaces, however, was hampered by the anodic dissolution of silver at potentials higher than 300 mV vs. standard hydrogen electrode (SHE). In order to extend the potential window for electrochemistry and still being able to use the resonance effect from silver nanoparticles, a 5-nm thick gold layer was sputtered on top of the Ag/AgNPs substrate. Cyclic voltammetry measurements of cytochrome c (cc) were carried out showing that the electrochemical behavior of gold can extend the potential range of the composite surface significantly. Furthermore, a potentiostatic titration of cc on this substrate by SERRS demonstrated that the resonance Raman effect of silver nanoparticles with the Soret band of the heme had been maintained in the presence of the gold adlayer. The positions of the plasmon resonances measured by reflection spectroscopy method were confirmed by finite-difference time-domain simulations. Gold is the optimal substrate for electrochemistry, whereas silver is the optimal material for plasmonic applications. Combining both metals gives us a surface with good performance for electrochemical applications as well as an enhancement effect sufficient to study redox-active biomacromolecules such as cc.