Herein, a simplified fabrication method for the producing of a pH-sensitive iridium electrode is developed. The in situ electrochemical fabrication of an iridium oxide film is optimized and shown to be achievable under neutral conditions rather than the acidic conditions hitherto employed. The formation of a pH sensitive Ir(III/IV) hydrous film is confirmed via XPS. The amperometric pH-sensing properties of this electrochemically generated material were investigated using square wave voltammetry. In the pH range 2–13, the iridium oxide redox signal has a pH dependency of 86.1 ± 1.1 mV per pH unit for midpoint potentials with uncertainties being ± 0.01–0.05 pH. Finally, the newly developed pH sensor was used to measure the pH of a natural water sample with excellent results as compared to a conventional glass pH probe.

Poly(3, (PEDOT), a wellcharacterized conducting polymer, has been applied for coating metal neural electrodes to improve their stimulating or recording performance. The coated electrodes possess advantages in better neuron attachment, lower impedance, and larger capacitance compared to the bare metal substrate due to the biocompatibility and porous surface of the polymer. However, the PEDOT-coated electrodes have frequently reported issues associated with mechanical instability, such as cracking and delamination. Solving this problem is crucial for stimulating electrodes, whereas a massive film is unnecessary for recording purposes. Moreover, the thickness control for the latter has rarely been investigated. In this work, we systematically studied and characterized poly(3,4ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) with cyclic voltammetry and atomic force microscopy (AFM) to evaluate the electropolymerization of PEDOT:PSS from the basis and analyze the surface morphology for a range of deposition times. The polymerization potential was obtained, and the deposition charge density was optimized for recording neural electrodes. In addition, high-resolution AFM height and phase images reveal the heterogeneity of the polymer surface. The modified electrode was also tested for its electrochemical performance in a small potential window with both a standard electrochemical cell setup and stainless steel microscrews. The results showed that despite a shift of potential (0.42 V) due to the change of setup, the electrode functions well in the capacitive region without triggering redox reactions.

A bespoke calibration-free pH sensor using an in situ modified Ir electrode for applications in seawater is reported. The electrochemical behaviour of an iridium wire in air-saturated synthetic seawater was studied and the formation of pH-sensitive surface layers was observed that featured three pH-sensitive redox couples, Ir(III/IV), IrOxOI−/IrOxOII−H, and Hupd/H+, where Hupd is adsorbed hydrogen deposited at underpotential conditions. The amperometric properties of the electrochemically activated Ir wire were investigated using linear sweep voltammetry first, followed, second, by square wave voltammetry with the formation conditions in seawater for the optimal pH sensitivity of the redox couples identified. The sensor was designed to be calibration-free by measuring the “super-Nernstian” response, in excess of ca 60 mV per pH unit, of Ir(III/IV) relative to the less sensitive upd H oxidation signal with the pH reported on the total pH scale. The pH dependency of the optimised sensor was 70.1 ± 1.4 mV per pH unit at 25 °C, showing a super-Nernstian response of high sensitivity.

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