A low-cost titanium suboxide pH sensor with competitive operational lifetime assessed with electrochemical impedance...

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 (TiO_x) pH sensor with silver (Ag) electrodes is presented. A sensitivity of -27.8 Ω pH^-1 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 TiO_x 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 TiO_x 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 TiO_x's chemical stability relative to PANI in both acidic and alkaline media. Further, the sensitivity of the TiO_x sensors is recovered to 95.7% after 200 °C 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 TiO_x 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.