Conducting polymers display a range of interesting properties, from electrical conduction to tunable optical absorption and mechanical flexibility, to name but a few. Their properties arise from positive charges (carbocations) on their conjugated backbone that are stabilised by counterions doped in the polymer matrix. In this research we report hydrolysis of these carbocations when poly(3,4-ethylenedioxy thiophene) is exposed to 1 mM aqueous salt solutions. Remarkably, two classes of anion interactions are revealed; anions that oxidise PEDOT via a doping process, and those that facilitate the SN1 hydrolysis of the carbocation to create hydroxylated PEDOT. A pKa of 6.4 for the conjugate acid of the anion approximately marks the transition between chemical oxidation and hydrolysis. PEDOT can be cycled between hydrolysis and oxidation by alternating exposure to different salt solutions. This has ramifications for using doped conducting polymers in aqueous environments (such as sensing, energy storage and biomedical devices).

Conducting polymers are promising candidates for wearable devices due to mechanical flexibility combined with electroactivity. While electrochemical measurements have been adopted as a central transduction method in many on‐skin sensors, less studied is the stability of the active materials (in particular poly3,4‐ethylenedioxythiophene, PEDOT) in such systems, particularly for “on‐skin” applications. In this study, several different variants of doped PEDOT are fabricated and characterized in terms of their (electrical, physical, and chemical) stability in biological fluid. PEDOT doped with tosylate (TOS) or polystyrenesulfonate (PSS) are selected as prototypical forms of conducting polymers. These are compared with a new variant of PEDOT co‐doped with both TOS and PSS. Artificial interstitial fluid (aISF) loaded with 1% wt/vol bovine serum albumin is adopted as the testing medium to demonstrate the stability in dermal applications (i.e., conducting polymer microneedles or coatings on microneedles). A range of techniques such as cyclic voltammetry and electrochemical impedance spectroscopy are used to qualify and quantify the stability of the doped conducting polymers. Furthermore, this study is extended by using human skin lysate in the aISF to demonstrate proof‐of‐concept for stable use of PEDOT in wearable “on‐skin” electronics.

Ions are present throughout our environment-from biological systems to agriculture and beyond. Many important processes and mechanisms are driven by their presence and their relative concentration. In order to study, understand and/or control these, it is important to know what ions are present and in what concentration-highlighting the importance of ion sensing. Materials that show specific ion interaction with a commensurate change in measurable properties are the key components of ion sensing. One such type are conducting polymers. Conducting polymers are referred to as 'active' because they show observable changes in their electrical and optical (and other) properties in response to changing levels of doping with ions. For example, p-type conducting polymers such as poly(3,4ethylenedioxythiophene) and polypyrrole, can transition from semi-conducting to metallic in response to increasing levels of anions inserted into their structure. Under certain circumstances, conducting polymers also interact with cations-showing their utility in sensing. Herein, recent advances in conducting polymers will be reviewed in the context of sensing ions. The main scope of this review is to critically evaluate our current understanding of ion interactions with conducting polymers and explore how these novel materials can contribute to improving our ion-sensing capabilities.

Conducting polymers are an interesting class of materials that can be tuned to have a range of properties through counterion doping. For most conducting polymers, the insertion of anions (the doping process) leads to the formation of carbocations (positive charge carriers) along the conjugated polymer backbone. In this research, we report on a scenario that arises where certain (commonly used) anions in water induce oxygenation of the conducting polymers heteroatom. This is in contrast to the widely reported doping process, and the recently reported hydrolysis of conducting polymers. We observe that the transition between these different conducting polymer-interactions/reactions is well described by the concept of structure-making and structure-breaking anions. Poly(3,4-propylenedioxy thiophene dimethyl) (PProDOT-Me 2 ), polypyrrole (PPy), and poly(3,4-ethylenedioxy thiophene) (PEDOT) thin films are exposed to a range of anions in water. Both PProDOT-Me 2 and PPy are susceptible to oxygenation, while in contrast PEDOT is doped, when exposed to structure-breaking anions. All the polymers show hydrolysis for structuremaking anions. The knowledge of the interaction and/or reaction of conducting polymers with anions in water is not only critical to their application in devices for aqueous environments (i.e., sensing), but also for their processing and fabrication using water.

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