The oxygen-plasma-functionalized carbon nanotube thin films on the flexible substrates as the pH sensing membranes of extended-gate field-effect transistors are proposed for the first time. The carbon nanotubes are ultrasonically sprayed onto the polyimide substrates followed by an oxygen-plasma functionalization. Such oxygen-plasma-treated carbon nanotube thin films (CNTFs) exhibit superior pH sensing characteristics with the sensitivity of 55.7 mV/pH and voltage linearity of 0.9996 in a wide sensing range of pH 1-13. Moreover, the excellent flexibility of carbon nanotube is also demonstrated and the oxygen-plasma-treated CNTFs still maintain the sensitivity of 53.6 mV/pH and voltage linearity of 0.9943 even after five-cycle bending test. These results reveal that the oxygen-plasma-treated CNTFs have great potentials in the practically disposal and wearable biosensor applications.Index Terms-Biosensors, carbon nanotube (CNT), extendedgate field-effect transistors (EGFETs), flexible substrates.

Abstract-High-performance pH sening membranes of extended-gate field-effect transistors (EGFETs) composed only of oxygen-plasma-functionalized carbon nanotube thin films (CNTFs) are demonstrated. A large number of oxygen-containing functional groups are decorated on the sidewalls of carbon nanotubes (CNTs) after the oxygen plasma treatment. These functional groups act as the sensing sites and accurately respond to the ions of interest in different pH levels. Therefore, these functionalized CNTFs as the pH-EGFET sensing membranes can achieve a high voltage sensitivity of 56.8 mV/pH, a large voltage linearity of 0.9995, and a wide sensing range of pH 1-13. In addition, the oxygen-plasma-functionalized CNTFs also exhibit superior reliability with a small hysteresis voltage of 4.98 mV.Index Terms-Carbon nanotube thin film (CNTF), extendedgate field-effect transistor (EGFET), sensor.