A Rapid Response Humidity Sensor for Monitoring Human Respiration with TiO 2 -Based Nanotubes as a Sensing Layer

The use of TiO 2 -based samples with nanotube (NT) shape for humidity sensing has been investigated. Sample characterization was carried out using x-ray diffraction analysis, scanning electron microscopy (SEM), and Fourier-transform infrared spectroscopy. x-Ray diffraction analysis showed that the nanotubes had TiO 2 , NaO 3 , and H 2 Ti 2 O 5 phases. SEM revealed that the NTs had diameters ranging from 20 nm to 200 nm and very different lengths. TiO 2 -based nanotubes were coated using the drop-casting method onto a quartz crystal microbalance (QCM). The synthesis of TiO 2 -based NTs was performed using a hydrothermal process. Humidity sensing measurements showed that the resonant frequency of TiO 2 -based NTs deposited onto the QCM was very sensitive to humidity changes. It was also shown that the sensor could be used for respiratory monitoring purposes.