Electrochemical SERS for <i>in Situ</i> Monitoring the Redox States of PEDOT and Its Potential Application in Oxidant Detection

In response to recent developments for applying conducting polymers on various biomedical applications, the development of characterization techniques for evaluating the states of conducting polymers in liquids is beneficial to the applications of these materials. In this study, we propose a platform using electrochemical surface-enhanced Raman scattering (EC-SERS) technology, which allows a direct measurement of the redox states of conducing polymers in liquids. A thiophene-based conducting polymer, hydroxymethyl poly­(3,4-ethylene­dioxythiophene) or poly­(EDOT-OH), was used to demonstrate this concept. Poly­(EDOT-OH) films were coated on Au nanoparticle-coated ITO glass as SERS-active substrates. Taking the advantage of Raman enhancement, we can in situ and clearly monitor the redox behavior of poly­(EDOT-OH) in aqueous solutions. The Raman peak intensity decreases as the poly­(EDOT-OH) film is oxidized. Furthermore, we demonstrated our idea to utilize this phenomenon as the sensing mechanism for oxidant detection. The Raman intensity of conducting polymers reduces faster when oxidants exist, and we obtain a quantitative analysis for the detection of oxidants. Moreover, the oxidized poly­(EDOT-OH) films can be reused for detection of oxidants simply by applying a reduction potential to activate the poly­(EDOT-OH) films. The film stability was also confirmed, and the detection of two other oxidants, namely ammonium persulfate and iron chloride, were also demonstrated. The results show different SERS spectra of poly­(EDOT-OH) films oxidized by using different oxidants. Besides, the oxidized films can be easily recovered simply by applying a cathodic potential, which allows repeating usage and makes it possible for continuous monitoring applications. To the best of our knowledge, this is the first time to apply PEDOT’s Raman feature for detection purposes