Fast-Response and Flexible Nanocrystal-Based Humidity Sensor for Monitoring Human Respiration and Water Evaporation on Skin Shinya Kano,* Kwangsoo Kim, and Minoru Fujii Department of Electrical and Electronic Engineering, Graduate School of Engineering, Kobe University, Rokkodai, Nada, Kobe 657-8501, Japan * S Supporting Information ABSTRACT: We develop a fast-response and flexible nanocrystal- based humidity sensor for real-time monitoring of human activity: respiration and water evaporation on skin. A silicon-nanocrystal film is formed on a polyimide film by spin-coating the colloidal solution and is used as a flexible and humidity-sensitive material in a humidity sensor. The flexible nanocrystal-based humidity sensor shows a high sensitivity; current through the nanocrystal film changes by 5 orders of magnitude in the relative humidity range of 8-83%. The response/ recovery time of the sensor is 40 ms. Thanks to the fast response and recovery time, the sensor can monitor human respiration and water evaporation on skin in real time. Due to the flexibility and the fast response/recovery time, the sensor is promising for application in personal health monitoring as well as environmental monitoring. KEYWORDS: humidity sensor, silicon nanocrystal, nanocrystal thin film, flexible device, health monitoring M onitoring humidity in air is indispensable for modern industry, agriculture, and medical care. 1,2 Especially, real- time monitoring of humidity from human body becomes more important recently in the field of personal health monitoring. 3,4 Mogera et al. proposed that dehydration of human body was detected from the analysis of humidity in human breath. 5 Miyoshi et al. suggested noninvasive detection of physiological stress to human by monitoring sweat produced from sweat glands on fingers. 6 Fast response and recovery are prerequisite features to apply humidity sensors for personal health monitoring. For respiratory and epidermal medical sensing, a flexible and wearable sensor is desirable because it can be fit directly to human skin. 7 Flexible humidity sensors have been fabricated by depositing a humidity-sensitive film on a polymer substrate. 6-12 As a fast-response humidity-sensitive film, various nanomateri- als, such as porous materials, 13,14 graphene-related materi- als, 10,12,15-17 and organic polymers, 5,17 have been recently investigated. Colloidal quantum dot thin films, which can be prepared on flexible substrates by depositing the colloidal solution at low temperature, are promising candidates for flexible gas-sensitive materials. 18 Because of the large surface-to-volume ratio, electrical characteristics of the film strongly depend on adsorbed molecules on the surface. Gas sensing by using quantum-dot thin films has been reported. 11,19-23 Liu et al. proposed that highly size-regulated quantum dots can be a sensing material for a high-performance gas sensor: fast response, high sensitivity, and good reversibility. 22 Segev-Bar et al. have reviewed state-of-the-art flexible sensors based on quantum dots for smart sensing applications. 18 Inspired by these works, we apply a colloidal quantum-dot film to a flexible humidity sensor. Among many kinds of colloidal quantum dots, colloidal silicon nanocrystals (Si NCs) are suitable for a wearable humidity sensor because of the high compatibility with conventional semiconductor processes and the nontoxicity to human body. 24-26 Furthermore, surface-oxidized silicon is stable against volatile organic compound gas from human body, which degrades the performance of commercial humidity sensors using organic polymers. In this work, we employ all-inorganic colloidal Si NCs developed in our group as a precursor for the formation of a humidity-sensitive thin film. 25 The all-inorganic Si NCs have a heavily boron and phosphorus-codoped surface layer, which induces negative potential on the surface and makes the NCs hydrophilic. 27-30 As a result, the Si NCs are dispersible in polar solvents such as alcohol and water without organic ligands. 31 Because of the perfect dispersion in solution, a transparent and flexible NC thin film can be prepared by spin-coating. In our previous work, we studied the current transport property of the film in different atmosphere and obtained clear evidence that the film was very sensitive to the amount of water molecules in atmosphere. 32 In this study, we produce a flexible humidity sensor on a polymer substrate by using the all-inorganic colloidal Si NCs. We demonstrate that the response/recovery Received: March 29, 2017 Accepted: May 24, 2017 Published: May 24, 2017 Article pubs.acs.org/acssensors © 2017 American Chemical Society 828 DOI: 10.1021/acssensors.7b00199 ACS Sens. 2017, 2, 828-833