Unique water sensors based on carbon nanotube–cellulose composites

Electrically conductive films composed of carbon nanotubes (CNTs) and cellulose matrix were used as water sensors. They exhibited rapid response and high sensitivity, with a relative electrical resistance change of 5500–500% for cellulose-CNT composites with CNT loading from 2 to 10 wt%. This unique performance is well reproducible and high stable even on a long-term scale for 45 days. The sensitivity of cellulose-CNT composites was mainly caused by the hygroscopic swelling of the cellulose matrix, which was investigated by scanning electron microscope and wide-angle X-ray diffraction. The cellulose-CNT composites also showed a high selectivity for water–ethanol solutions with different fraction of water.