The surface Raman spectroscopy of polymer-modified electrodes

Abstract

Applications of surface-enhanced Raman spectroscopies and the quartz crystal microbalance to electrode interfaces, especially polymer-modified electrodes, are investigated. The quartz crystal microbalance is used to investigate the adsorption of pyridine at electrochemically roughened silver electrodes from aqueous chloride electrolyte, using conditions comparable to those used in SERS studies. During roughening in aqueous chloride solution, delays seen between the onset of oxidation and reduction at the Ag surface and the commencement of frequency decrease and increase, respectively, are explained by release of trapped species upon initial formation of the AgCl layer and then re-trapping upon reduction. At anodic limits of +0.25 V and above, the reduction proceeds in two steps representing the reduction of two separate silver chloride phases. Below the silver oxidation potential corrosion inhibition of the silver surface by pyridine leads to a lower hysteresis between cathodic and anodic sweeps in the frequency vs potential plots when the heterocycle is present in solution. The most intense pyridine SERS spectra occur at potentials where the frequency of the quartz crystal is highest, suggesting that adsorption of the organic species leads to break-down of long-range viscous coupling of solvent to the surface via hydrogen-bonding interactions. Full assignments of the SERS spectrum of pyridine-d₅ are reported. SERS spectra of 4-vinyl pyridine and poly(4-vinyl)pyridine at the silver electrode are reported and assigned. At negative potentials free-radical polymerization of the monomer and short-chain polymers can be initiated by H*. When dissolved oxygen is present, surface bound and organic peroxides may also be formed and bands due to these are assigned. SERRS spectra of [PVP-M(bpy) ₂Cl]Cl (M = Ru or Os) adsorbed layers show typical 2,2'-bipyridine dominated spectra at Ag. Small changes in band positions due the oxidation of Os(II) to Os(III) were observed at a gold electrode for the 1400 -1700 cm⁻ ¹ region.

The metallopolymers are used as model systems for the quantification of the SERRS signal in polymer-modified electrodes, plots for intensity vs surface coverage of metal centres being obtained as a function of loading (py:M ratio) at a constant deposition mass and as a function of deposition mass at a constant loading. At very low surface coverages the attenuation adjusted intensity can be modelled as a Freundlich isotherm with I = 1.41c ⁰.³⁷ (c = the concentration, in moldm⁻³, of Ru centres in the film). At higher coverages surface plasmon damping and the increasing importance of resonance Raman scattering lead to substantial deviation from this equation. The potential of SERS for obtaining partition coefficients and diffusion coefficients of ions into polymer films is considered.

A series of Crᴵᴵᴵ(bpy)₂(L)(L') {L = OH, L' = OH₂ monomer and dimer; L = L' = CI; and L = poly(acrylic acid), no L') complexes are studied at glassy carbon by cyclic voltammetry and at silver by SERRS. Slow electron transfer in the reduction of Cr(III) to Cr(II) means that reduction waves are only observed for adsorbed species. The reduced species undergo ligand substitution reactions, and at a PVP modified electrode the dichloride can be reductively electrolyzed to form [(PVP)₂-Cr(bpy)₂]³⁺ (E0j/2 = -0.32 V vs SCE). Surface Raman spectra of the reduced complexes are thought to be in resonance with a broad ligand 𝜋7  𝜋10* transition of the bipyridine radical anion, resulting in spectra which indicate that the electron is localized on one bipyridine. The aquahydroxo complexes, which have an initial 2+ charge per Cr centre, adsorb strongly at the Ag surface, as evidenced by a broad Cr-0 vibration at around 550 cm-1 and by the fact that they stabilize the surface against chloride desorption and therefore show intense SERRS spectra at -1.1 V vs Ag / AgCl. The dichloro and poly(acrylic acid) complexes, which have an initial 1+ charge, do not show similar stabilization of SERRS sites.