Directly Using Ti(3)C(2)T(x) MXene for a Solid-Contact Potentiometric pH Sensor toward Wearable Sweat pH Monitoring

The level of hydrogen ions in sweat is one of the most important physiological indexes for the health state of the human body. As a type of two-dimensional (2D) material, MXene has the advantages of superior electrical conductivity, a large surface area, and rich functional groups on the surface. Herein, we report a type of Ti(3)C(2)T(x)-based potentiometric pH sensor for wearable sweat pH analysis. The Ti(3)C(2)T(x) was prepared by two etching methods, including a mild LiF/HCl mixture and HF solution, which was directly used as the pH-sensitive materials. Both etched Ti(3)C(2)T(x) showed a typical lamellar structure and exhibited enhanced potentiometric pH responses compared with a pristine precursor of Ti(3)AlC(2). The HF-Ti(3)C(2)T(x) disclosed the sensitivities of −43.51 ± 0.53 mV pH(–1) (pH 1–11) and −42.73 ± 0.61 mV pH(–1) (pH 11–1). A series of electrochemical tests demonstrated that HF-Ti(3)C(2)T(x) exhibited better analytical performances, including sensitivity, selectivity, and reversibility, owing to deep etching. The HF-Ti(3)C(2)T(x) was thus further fabricated as a flexible potentiometric pH sensor by virtue of its 2D characteristic. Upon integrating with a solid-contact Ag/AgCl reference electrode, the flexible sensor realized real-time monitoring of pH level in human sweat. The result disclosed a relatively stable pH value of ~6.5 after perspiration, which was consistent with the ex situ sweat pH test. This work offers a type of MXene-based potentiometric pH sensor for wearable sweat pH monitoring.