Ultrafast Colorimetric Humidity Sensors with Mesoporous Titania Microspheres

Fast humidity monitoring is becoming increasingly important for a wide range of applications including emerging healthcare solutions and fuel cell engineering. However, most commercially available sensors suffer from slow response times ranging from seconds to minutes, limiting their use. Herein, we propose mesoporous solid and yolk-shell titania microspheres as ultrafast colorimetric humidity sensors. Different microsphere morphologies were synthesized through a solvothermal process with varying reaction times, and their performances were compared to determine the optimal structure for humidity sensing. We found that the pore volume, pore diameter and crystallinity, respectively, determine the responsive RH range, response time and stability of the materials. The optimal mesoporous solid microsphere had a fast response time of 20 ms and was structurally-stable for at least 180 days, which was the duration of this study. These microspheres were integrated into a prototype sensor system, consisting of a pair of LEDs and a photodiode. The system was sensitive and fast enough to detect unique humidity profiles generated from human speech, demonstrating an alternative approach to speech recognition. These results suggest that mesoporous titania microspheres are promising for ultrafast humidity detection.