Why PEDOT:PSS Should Not Be Used for Raman Sensing of Redox States (and How It Could Be)

Abstract

Poly(3,4-ethylenedioxythiophene):poly(styrene sulfonate) (PEDOT:PSS) has been recently proposed for Raman sensing of redox-active species in solution. Here, we investigated the rationale of this approach through systematic experiments, in which the Raman spectrum of PEDOT:PSS was analyzed in the presence of either nonoxidizing or oxidizing electrolytes. The results demonstrated that Raman spectra precisely reflect the conformation of PEDOT units and their interactions with PSS. Two different responses were observed. In the case of oxidizing electrolytes, the effect of charge transfer is accurately transduced in Raman spectrum changes. On the other hand, reduction induces a progressive separation between the PEDOT and PSS chains, which decreases their mutual interaction. This stimulus determines characteristic variations in the intensity, shape, and position of the Raman spectra. However, we demonstrated that the same effects can be obtained either by increasing the concentration of nonoxidizing electrolytes or by deprotonating PSS chains. This poses severe limitations to the use of PEDOT:PSS for this type of Raman sensing. This study allows us to revise most of the Raman results reported in the literature with a clear model, setting a new basis for investigating the dynamics of mixed electronic/ionic charge transfer in conductive polymers.

Mixed Ionic-Electronic Transport for PEDOT:PSS-Based Zero-Gated Organic Electrochemical Transistors Using Impedance Spectroscopy and Micro-Raman Imaging

“…One of the most interesting features is that a blueshift is clearly observed in the presence of NaCl for those vibrational modes associated with thiophene ring stretching in the 1350–1500 cm –1 region, which is typically associated with PEDOT dedoping mechanism. It is also important to note that the most relevant Raman activities corresponding to PEDOT model are remarkably larger than those observed for the PSS model, which is consistent with the typical PEDOT:PSS Raman spectra observed in the literature. – Now, we will discuss the modeling of the effect of water and NaCl on the joint PEDOT:PSS system, for which the optimization structures and their corresponding computed Raman spectra are shown in the right panel of Figure S6. First of all, the anhydrous PEDOT:PSS model [(EDOT) 6 (SSH) 2 ] showed the presence of the most intense thiophene ring symmetric stretching modes at 1497 and 1409 cm –1 , in addition to the presence of other characteristic vibrational modes, as depicted in Table S3.…”

“…The fitted position of the thiophene ring CC and the oxyethylene ring C–C (as a reference) stretching modes for PEDOT:PSS OECT at the swollen (∼ +3 μm), compacted (∼1 μm), and interface with glass (∼ −1 μm) regions under exposure to air and aqueous solutions with [NaCl] = 0, 1, 10, and 100 mM, is shown in Figure . Particularly, the peak associated with the thiophene ring CC stretching mode showed a notorious blueshift with increasing [NaCl] from air to 0, 1, 10, and 100 mM, which is well-known to be indicative of the PEDOT:PSS dedoping process. – It is interesting to note that the exposure to the aqueous solution with [NaCl] = 0 mM (with hypothetically zero Na + species) also showed a partial dedoping behavior. It is also noticeable that the dedoping behavior due to increasing NaCl concentration is more pronounced in the compacted and interface with glass regions and less pronounced in the swollen region, where the PEDOT:PSS is already partially dedoped even in the absence of aqueous media.…”

“…35 Also, a quite recent study on OECT devices comprising PEDOT:PSS in different electrolytic environments revealed that the Raman response can be altered by different factors, not necessarily related to the redox state of a target analyte. 36 Nonetheless, the technique has demonstrated the high sensitivity of Raman scattering toward the structural conformation of PEDOT:PSS and mutual interactions between both ionomers. 36 Although in-plane and out-of-plane micro-Raman imaging studies have provided rich information on PEDOT:PSS electrodes in organic solar cells and humidity sensors, 37,38 up to now, no studies have been reported for OMIECs materials comprising OECT devices directly evaluating the swelling processes.…”

“…36 Nonetheless, the technique has demonstrated the high sensitivity of Raman scattering toward the structural conformation of PEDOT:PSS and mutual interactions between both ionomers. 36 Although in-plane and out-of-plane micro-Raman imaging studies have provided rich information on PEDOT:PSS electrodes in organic solar cells and humidity sensors, 37,38 up to now, no studies have been reported for OMIECs materials comprising OECT devices directly evaluating the swelling processes. Here, we report for the first time Raman imaging characterization of the swelling processes for the compacted and swollen regions in PEDOT:PSS-based OECT channel materials.…”

“…There have been advances in the use of in-plane micro-Raman imaging studies revealing insights on the presence of positive polarons at the channel of an ionic liquid-gated transistor fabricated with regioregular poly­(3-hexylthiophene) and their dependence on source-gate and source-drain voltages . Also, a quite recent study on OECT devices comprising PEDOT:PSS in different electrolytic environments revealed that the Raman response can be altered by different factors, not necessarily related to the redox state of a target analyte . Nonetheless, the technique has demonstrated the high sensitivity of Raman scattering toward the structural conformation of PEDOT:PSS and mutual interactions between both ionomers .…”