Dew Point Measurement Using a Carbon-Based Capacitive Sensor with Active Temperature Control Jing Nie,* ,† Yichuan Wu, †,§ Qiyang Huang, ‡,§ Nirav Joshi, †,⊥ Ning Li, ∥ Xiaofeng Meng, ∥ Sunxiang Zheng, ‡ Min Zhang, § Baoxia Mi, ‡ and Liwei Lin † † Department of Mechanical Engineering & Berkeley Sensor and Actuator Center and ‡ Department of Civil and Environmental Engineering, University of California, Berkeley, Berkeley, California 94720, United States § Tsinghua-Berkeley Shenzhen Institute, Tsinghua University, Shenzhen 518055, China ∥ Science and Technology on Inertial Laboratory, Beihang University, Beijing 100191, China ⊥ Sã o Carlos Institute of Physics, University of Sã o Paulo, Sã o Carlos 13560-970, Brazil ABSTRACT: Laser-induced graphene (LIG) has both good electrical conductivity and three-dimensional porous struc- tures. Here, porous graphene interdigital electrodes (IDE) were constructed as a capacitive sensor from commercial polymer films by the laser ablation process and transferred to the polydimethylsiloxane (PDMS) substrate. The graphene oxide (GO) adsorption layer was electrosprayed as a humidity sensing structure, and a Peltier device was used to control the temperature to produce the condensation of water vapors. The dew point was identified by the equilibrium state of the capacitor when the adsorption layer and the surface air reached the saturation equilibrium. The performances of the hydrophilic dew point sensing system under different envi- ronmental conditions were investigated. The results show that the precision of the carbon-based dew point sensor of ≤±0.8 °C DP with good stability and repeatability is better than those of other dew point instrument based on electrical sensing parameters at ±1.0 °C DP. KEYWORDS: laser-induced graphene, graphene oxide, reduced graphene oxide, dew point sensor, humidity ■ INTRODUCTION Humidity is an important environmental parameter which quan- tifies the amount of water vapor in the atmosphere and affects the physical, chemical, and biological processes of nature. For example, humidity affects the properties of heat, electricity, light, and transport of gases and could result in changes in material size, corrosion of metallic materials, and even the survival of organisms. Humidity measurement and control are widely used in aerospace, microelectronics, atomic energy, petrochemical, electric power, meteorology, storage, and other fields. 1−4 In view of the importance of humidity measurement, many countries are constantly updating their humidity measure- ment standards. Among the many physical humidity expressions, the measurement of dew point temperature (DP) has been inter- nationally recognized as the most accurate humidity measure- ment method. Dew point temperature is absolute humidity, independent of ambient temperature and atmospheric pressure which can have an effect on it. The dew point temperature is defined as when air must be cooled to become saturated with water vapor, and when further cooled, the airborne water vapor will condense to form liquid water (dew). As the air cools to its dew point through contact with a surface that is colder than the air, water will condense on the surface. 5,6 Humidity measurement instruments can be divided into two categories: (1) hydrophilic type and (2) nonhydrophilic type. Most relative humidity sensors are hydrophilic as they determine the relative humidity through the physical and chemical changes of the water molecules absorbed by the adsorption materials. However, the typical dew point sensors are nonhydrophilic. For example, in a standard humidity instrument, the main sensor is a cold mirror dew point meter with the general accuracy of ±0.3 °C DP. For the QCM (quartz crystal microbalance)-based dew point sensor, the highest accuracy can be reached is ±0.1 °C DP. For dew point instrument based on electrical parameters, hydrophilic materials have been used as a medium for electrical capacitance or resistance. When the water-containing gas flows through, the dielectric constant or conductivity changes accord- ingly to calculate the moisture content of the gas. This kind of dew point sensor converts relative humidity into dew point measurement, and the highest accuracy at present is ±1.0 °C DP, while there is no hydrophilic dew point sensor to directly measure the dew point temperature with similar precision. 7 Received: October 23, 2018 Accepted: December 18, 2018 Published: December 18, 2018 Research Article www.acsami.org Cite This: ACS Appl. Mater. Interfaces 2019, 11, 1699-1705 © 2018 American Chemical Society 1699 DOI: 10.1021/acsami.8b18538 ACS Appl. Mater. Interfaces 2019, 11, 1699−1705 Downloaded via UNIV OF CALIFORNIA BERKELEY on January 18, 2019 at 04:46:37 (UTC). See https://pubs.acs.org/sharingguidelines for options on how to legitimately share published articles.