A robust microneedle-based electrochemical sensor for continuous in vivo monitoring of pH

Continuous monitoring of physiological pH is essential for disease diagnosis and health management, yet developing minimally invasive implantable sensors with adequate stability remains challenging. We developed a micro-electrochemical pH sensor fabricated on a flexible gold wire substrate modified with a platinum black and polyaniline nanocomposite. Through a two-step electrochemical deposition process, a porous platinum nanostructure was constructed to increase the electrochemically active surface area, providing a stable scaffold for the subsequent polyaniline coating. The resulting sensor displayed a linear response from pH 4 to 10 with a sensitivity of −45.2 mV/pH (R 2  = 0.998) and intra-batch reproducibility with a coefficient of variation below 1.4%. Specificity tests confirmed negligible potential drift in the presence of common interfering ions. The sensor's practical applicability was validated in complex biological environments; in diluted serum, it maintained a stable baseline and responded rapidly to pH fluctuations, effectively resisting biofouling. In vivo experiments involving subcutaneous implantation in an anesthetized mouse successfully tracked continuous pH variations induced by chemical stimuli. After seven days in phosphate-buffered saline, the sensor retained 78.5% of its initial response. This work presents a robust, easily fabricated microsensor strategy promising for continuous in vivo monitoring of pH.