Electrochemically roughened silver surface versus fractal leaf-shaped silver crystals for surface-enhanced Raman scattering investigation of polypyrrole

摘要

In surface-enhanced Raman scattering (SERS) analyses, the surface condition of the metal substrate plays a decisive role in the quality of the recorded Raman spectra. In this contribution, we will examine the SERS spectra of polypyrrole (PPy) in contact with two types of silver substrates prepared according to two different protocols: (i) electrochemical, leading to Ag surface with coral-like structure, and (ii) photochemical, giving fractal leaf-shaped Ag clusters. On the one hand, PPy films were electrochemically synthesized on massive silver electrodes previously SERS-activated according to the well-known oxidation-reduction cycle (ORC) procedure. The Raman spectra of the obtained samples exhibit significant SERS effect with high signal-to-noise ratio in comparison with those recorded in the same conditions on platinum. This made it possible to perform an ex situ rovibrational characterization of PPy, to determine the orientation and anchoring mode of the polymer chains in the coating/electrode interfacial region, and to monitor, in situ, some structural changes during the electrochemical doping-undoping process. On the other hand, to further improve the SERS enhancement factor, a second photochemical approach has been developed to prepare PPy chains embedded in a fractal structure made of leaf-shaped silver crystals. Here, the size and growth mode of the silver clusters give rise to a larger number of hot spots that are responsible for the huge Raman intensity enhancement.