Synergistic PEDOT:PSS and N-GQD Composite for Fast and Sensitive Electrochemical pH Sensing

This work reports a high-performance flexible electrochemical pH sensor based on a poly(3,4-ethylenedioxythiophene):polystyrene sulfonate (PEDOT:PSS) and nitrogen-doped graphene quantum dot (N-GQD) composite fabricated on a PET substrate using scalable printing techniques. The synergistic interaction between PEDOT:PSS and N-GQDs markedly enhances proton-coupled electron transfer (PCET), yielding a super-Nernstian sensitivity of 68.25 ± 1.2 mV/pH across a wide linear range (pH 2–12). Comprehensive material and electrochemical characterization—using X-ray photoelectron spectroscopy (XPS), Raman spectroscopy, and impedance spectroscopy—confirmed that optimal nitrogen doping (9 at%) minimizes charge-transfer resistance and promotes rapid interfacial proton–electron equilibration. The composite sensor exhibits a fast response time (t95= 5.8 ± 0.4 s), negligible short-term drift (0.42 mV·h−1), and excellent long-term stability with >92% signal retention after 28 days. It also demonstrates high selectivity against common sweat constituents (Na+, K+, lactate, and glucose) and stable operation under mechanical deformation. These results establish a reproducible materials–electrochemistry framework that extends PEDOT:PSS-based sensing beyond the classical Nernst limit. The proposed PEDOT:PSS/N-GQD composite platform represents a promising route toward next-generation wearable pH sensors for real-time, non-invasive healthcare monitoring.