From Rigid to Flexible: Solution-Processed Carbon Nanotube Deposition on Polymeric Substrates for the Fabrication of Transistor-Based Ion Sensors

Electrolyte-gated field-effect transistors (EG-FETs) are widely used in the growing field of biochemical sensing applications, due to their manifold advantages, such as large specific capacitance, low operating voltage, and intrinsic signal amplification. In this work, carbon nanotube (CNT) EG-FET (EG-CNTFET)-based sensors for ammonium ( $\mathrm {NH_{4}^{+}}$ ) detection are reported. The active semiconducting single-walled CNTs layer was manufactured through a cost-effective and scalable spray deposition technique. To achieve high-quality semiconducting networks, the CNT ink was optimized. Atomic force microscopy (AFM) analysis was used to optimize the CNT concentration and significantly reduce the posttreatment time from the previously reported 12 to 1 h. The optimized ink was then used to fabricate EG-CNTFETs first on standard rigid $\mathrm {Si/SiO_{2}}$ substrate and then on flexible polyimide (PI) foils. Both devices showed typical p-type behavior with an on–off ratio in the order of magnitude of $1\,\, {}\times {}10^{3}$ A/A. Furthermore, as proof of concept, we demonstrated the detection of the $\mathrm {NH_{4}^{+}}$ ions with EG-CNTFETs fabricated on a flexible substrate and functionalized with nonactin ion-selective membrane. The calibration curve of the fabricated sensors showed a linear detection range for ammonium from 0.01 to 10 mM, covering the entire range of physiological concentrations of interest, with an average sensitivity of 0.346 $\mu \text{A}$ /decade and a 94.35% coefficient of determination.