Highly Stable, Flexible ISFET pH Sensor Using Sorted Semiconducting SWNTs in Aqueous Solution

Flexible sensors enable the gathering of information from nonplanar surfaces, which facilitates huge data collection tasks. Monitoring of pH levels in industrial wastewater is one example. For this purpose, stable, reliable flexible pH sensors are required to monitor pH in pipes or containers. Carbon nanotube field‐effect transistors (CNT‐FETs) are attractive transducers for flexible sensors because of their multiple functionalities, such as good electrical properties and mechanical flexibility. However, semiconducting single‐walled carbon nanotubes (s‐SWNTs) must be separated from metallic SWNTs of high purity for channel materials to be used in CNT‐FETs. Herein, a simple method to sort s‐SWNTs in an aqueous system using only dispersion and centrifugation is developed. This sorting method is much less expensive than other techniques, such as density gradient ultracentrifugation, agarose‐gel chromatography, or selective sorting by conjugated polymers. These sorted s‐SWNTs result in high‐performance CNT‐FETs with electrical hysteresis reduced to almost zero and improved bias stability. This method demonstrates the importance of the solubility parameter between polymers and water for CNT‐FET hysteresis. The resulting carbon nanotubes also have high uniformity of characteristics, making them valuable as transducers for flexible sensors. Furthermore, real‐time pH monitoring using ion‐sensitive, field‐effect transistors (ISFETs) based on CNT‐FET is demonstrated.