Ultrafast, Flexible, and Selective Water Sensor Based on Multiwalled Carbon Nanotubes/Poly(vinylidene fluoride) Screen-Printed on Cotton Fabric

An insulating cotton fabric was transformed into an electroconductive fabric through screen printing of a multiwalled carbon nanotubes (MWCNTs)/poly(vinylidene fluoride) (PVDF) composite. Structural, morphological, thermal, wettability, and electrical properties were analyzed using scanning electron microscopy, high-resolution transmission electron microscopy, X-ray diffraction, Raman analysis, thermogravimetric analysis (TGA), contact-angle measurement, and resistance measurement, respectively. The resistance change of cotton fabric coated with MWCNTs/PVDF (Fab-MWCNTs/PVDF), approximately 1120 kΩ (change of resistance is 24% of the base resistance), was 3 times higher in a water medium compared to that for cotton fabric coated with MWCNTs (Fab-MWCNTs). The sensor demonstrated excellent reproducibility, repeatability (rapid measurement of up to 28 cycles with a standard deviation of 6%), ultrafast response times (0.6 ± 0.2 s), and self-recovery (time 0.8 ± 0.2 s). Additionally, the developed sensor demonstrates good flexibility because the sensor shows excellent water-sensing characteristics upon both forward and backward bending conditions. The sensing properties of Fab-MWCNTs/PVDF were also explored against different liquids, such as ethanol, methanol, and hexane. The sensing mechanism of this unique water sensor was discussed in detail, showcasing its potential for wearable water sensors in various fields. The sensor’s reversibility, reproducibility, quick response, and recovery time make it an attractive candidate for wearable water sensors.