Confocal Raman Microscopy in the Study of Electrode-Supported Polymer Materials

Confocal Raman microscopy is being applied in the study of electrode-supported materials of interest for electrochemistry (i.e., ion conductive polymer membranes (ionomers), redox polymers, catalyst thin films). When measuring within membranes, the confocal probe volume region serves as a fixed volume reservoir for detecting changes in material composition and membrane matrix expansion or contraction in response to variations in conditions such as material hydration and redox state. A high numerical aperture oil-immersion objective is used to achieve tight focusing within samples and efficient collection of Raman scattered light. In the study of redox polymers, in-situ measurements are sensitive to changes in film oxidation state and swelling/deswelling of the polymer framework as solvent and charge compensating electrolyte ions redistribute in response to variations in the applied potential. Recent work extending the approach to processable (3-substituted) polythiophene conducting polymers and an Os-redox polymer that has applications in bioelectrocatalysis will be discussed. In the case of polythiophenes, the ability to focus centrally within films enhances sensitivity toward the development of electronically conductive states as they are accessed during a potential scan. While the neutral state displays resonance Raman scattering, this enhancement is greatly attenuated in oxidized forms of the polymers. The ability to follow the time-evolution of spectra following a potential step and probe at different depths within films relative to the electrode surface is enhancing selectivity toward the electronically conductive forms of polythiophenes and revealing new insights into their Raman spectral features. The strategies are being extended in the study of a redox polymer film containing fixed Os-centers of interest for biosensing and biofuel cell use, where resonance Raman scattering also complicates detection as material redox state is varied.