Abstract
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.