Carbon Nanotube-Based Ion-Sensitive Field-Effect Transistors with an On-Chip Reference Electrode Toward Wearable Sodium Sensing

Ion-sensitive field-effect transistors (ISFETs) are a promising platform for real-time, continuous monitoring of sodium ions in human sweat. However, realization of true wearable sensors is restricted owing to bulky reference electrodes. Herein, we demonstrate carbon nanotube (CNT)-based ISFETs with an on-chip reference electrode suitable for wearable sodium detection. An Ag/AgCl ink and a polyvinyl butyral membrane are utilized to fabricate the integrated reference electrode with a compact size. The CNT surface is functionalized with a polyvinyl chloride-based sodium-selective membrane composed of a specific ionophore to capture sodium ions. The sodium sensor shows the sensitive detection of sodium ions in a range from 0.1 to 100 mM, which covers human sweat sodium concentrations and good selectivity for interfering ions existing in sweat such as potassium, calcium, and magnesium ions. The miniaturized reference electrode can provide a stable potential comparable to that of a commercial reference electrode. The sensor has the ability to selectively detect the sodium ions in artificial sweat.