A Low-Cost Titanium Suboxide pH Sensor with Competitive Operational Lifetime Assessed with Electrochemical Impedance Spectroscopy.

Abstract

Smart Bandages are an emerging field of wearable electronics. Biosensors provide a medium of communication between the wound site’s chemical status and the clinicians. The pH is a critical biomarker, as it is a suitable indicator for bacterial infection. In this paper, a low-cost Electrochemical Impedance Spectroscopy (EIS)-based Titanium Suboxide (TiOx) pH sensor with Silver (Ag) electrodes is presented. A sensitivity of -27.8 Ω/pH is achieved in the pH 4-8 region of interest related to wound infection. The measured pH sensitivity, after a one-month stability study, is lost only 2.25% for the TiOx sensor - reaching the state-of-the-art - compared to the Polyaniline (PANI) sensor, which lost 10% of its pH sensitivity. To further assess stability, we evaluated sensor behaviour at the edge pH values. The TiOx sensor exhibits impedance increases of 5.5% and 4% at pH 4 and 8, respectively, which are approximately half of those reported for PANI devices (10% and 9% at pH 4 and 8, respectively). The stability experiments highlighted TiOx’s chemical stability relative to PANI in both acidic and alkaline media. Further, the sensitivity of the TiOx sensors is recovered to 95.7% after 200℃ thermal annealing, thereby recalibrating the sensors and prolonging their lifetime whilst reducing waste. To better understand the sensing and degradation mechanisms, a model-free method is employed for both TiOx and PANI pH sensors. The reason is to demonstrate that pH sensitivity is related to the film surface (given the circuit complexity of the equivalent models in 2-electrode systems). This work is an important step for a potential translation to real-world wound monitoring, where long-term stability and low cost are critical for an accurate and efficient biomarker detection and sustainable well-being.