Water Sorption in MXene/Polyelectrolyte Multilayers\nfor Ultrafast Humidity Sensing

Abstract

The\nrapidly growing fields of noncontact medical diagnosis, noninvasive\nepidermal sensing, and environmental monitoring bring forward the\nneed for fast humidity sensors. However, achieving a rapid response\nto dynamic changes in humidity, such as for human respiration, is\nchallenging. This is because the response can be limited by the diffusion\nof water, the sorption of water in the material, and the sensing method\nitself. Here, the water sorption and response mechanism for multilayer\nassemblies made from MXene nanosheets and polyelectrolytes for ultrafast\nhumidity sensing are described. MXenes are a class of two-dimensional\ntransition metal carbides (e.g., Ti₃C₂) possessing\nhydrophilicity and metal-like conductivity. Herein we show that MXene/polyelectrolyte\nmultilayer films prepared using layer-by-layer (LbL) assembly exhibit\nresponse and recovery times exceeding those of most humidity sensors.\nQuartz crystal microbalance and ellipsometry support the mechanism\nthat, upon changing humidity, water molecules intercalate into (or\ndeintercalate from) the MXene/polyelectrolyte multilayer, resulting\nin an increase (or a decrease) in the thickness and sheet-to-sheet\ndistance, which then changes the tunneling resistance between MXene\nsheets. The ultrafast response was further demonstrated by monitoring\nreal-time human respiration using a portable microcontroller for wireless\nsensing.