Water Sorption in MXene/Polyelectrolyte Multilayers for Ultrafast Humidity Sensing

The rapidly growing fields of noncontact medical diagnosis, noninvasive epidermal sensing, and environmental monitoring bring forward the need for fast humidity sensors. However, achieving a rapid response to dynamic changes in humidity, such as for human respiration, is challenging. This is because the response can be limited by the diffusion of water, the sorption of water in the material, and the sensing method itself. Here, the water sorption and response mechanism for multilayer assemblies made from MXene nanosheets and polyelectrolytes for ultrafast humidity sensing are described. MXenes are a class of two-dimensional transition metal carbides (e.g., Ti3C2) possessing hydrophilicity and metal-like conductivity. Herein we show that MXene/polyelectrolyte multilayer films prepared using layer-by-layer (LbL) assembly exhibit response and recovery times exceeding those of most humidity sensors. Quartz crystal microbalance and ellipsometry support the mechanism that, upon changing humidity, water molecules intercalate into (or deintercalate from) the MXene/polyelectrolyte multilayer, resulting in an increase (or a decrease) in the thickness and sheet-to-sheet distance, which then changes the tunneling resistance between MXene sheets. The ultrafast response was further demonstrated by monitoring real-time human respiration using a portable microcontroller for wireless sensing.