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.
Surface-enhanced Raman scattering (SERS) has been widely used for bioanalysis because it provides a high sensitivity for detecting analytes of ultralow concentrations. However, the clinical application of a 2D SERS-active substrate remains challenging because of the difficulty of obtaining accurate quantification, especially at low concentration. In this study, we proposed an analytical method that integrates an optimized sample mapping strategy with an electrochemical SERS (EC-SERS) technique to resolve this problem. We adopted this method to detect two metabolites of azathioprine, namely 6-thioguanine nucleotides (6-TGNs) and 6-methylmercaptopurine (6-MMP), as our proof-of-concept experiment. We first prepared a conductive SERS-active substrate by electrochemically depositing Au nanoparticles (AuNPs) on indium tin oxide glass. The two metabolites were then randomly absorbed on the surface of the AuNPs of the SERS-active substrates. When we applied a negative potential on the substrate, we observed a large enhancement of Raman intensity for both metabolites, which was attributed to both the charge transfer effect and reorientation of metabolites on the substrate surface, leading to the formation of Au−S bonds. In addition, by optimizing the mapping range, we were able to efficiently reduce the standard deviation of SERS intensity and achieve a consistent standard deviation lower than 10%. With these two features, we were able to achieve quantitative analysis of 6-TGNs and 6-MMP with a detection limit of 10 and 100 nM, respectively. The integration of EC-SERS and the mapping method provided a reliable and quantitative analytical platform for analytes, which can be electrochemically modulated, like 6-TGNs and 6-MMP.
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