Self-assembled carbon nanotube multilayer resistors and nanotube/nanoparticle thin-film transistors as pH sensors

In this paper, we report the fabrication and characterization of two pH sensors using layer-by-layer self-assembled thin films. The first sensor uses a thin film as the sensing component which consists of alternating layers of single-walled carbon nanotubes (SWNTs) and polyelectrolytes. The second sensor is based on a thin-film transistor which has a nanotube/polyelectrolyte semiconducting film and a SiO₂ nanoparticle/polyelectrolyte dielectric film. The device fabrication process is inexpensive, solution-based, and can be done at room temperature. The sensors are highly sensitive to hydrogen ions in aqueous solutions with measured sensitivities of 9.7 and 179.8 μA/pH, respectively. The sensors respond linearly to pH solutions in the range of 5-9. This work introduces a new approach to fabricate inexpensive and high-performance pH sensors.