Ti4O7-incorporated polypyrrole/activated carbon foam for enhancing long-term stability of polypyrrole in biomedical pH sensing

The poor stability of polypyrrole (PPy) restricts its application in biomedical pH sensing. This study incorporated Ti₄O₇ particles with PPy and deposited them onto activated carbon foam (ACF) via a galvanostatic method, preparing a (Ti₄O₇ + PPy)/ACF composite that significantly enhances the long-term stability of PPy. The prepared Ti₄O₇ was modified with p-toluenesulfonate (pTS^–), forming negatively charged Ti₄O₇-pTS particles via S–O–Ti bonds between pTS^– and Ti³⁺/Ti⁴⁺ on Ti₄O₇. These particles can tightly bind with N^δ+ in PPy chains, effectively inhibiting pTS^– loss and PPy degradation. During a 90-day immersion test in Hank's solution (37℃), the pH sensitivity of (Ti₄O₇ + PPy)/ACF increased from 65 to 74 mV/pH during 0–7 days (activation period), stabilized at 63–64 mV/pH during 30–60 days, then decreased to 58 mV/pH, showing significantly enhanced sensitivity and long-term stability compared to PPy/ACF. In (Ti₄O₇ + PPy)/ACF, PPy provides primary pH response, while Ti₄O₇ enhances its sensitivity and stability. The decline in its pH sensing performance is primarily caused by the degradation of PPy and the loss of Ti₄O₇ particles. The involved mechanisms are discussed in detail.