Electronics based on paper could be useful for a variety of applications, e.g., as sensors, due to their low cost and weight. Paper-based sensors often use solid sensing materials. They can break and lose their function as the paper is bent or folded. Liquid sensing materials can solve this problem and are also easier to apply to the paper, for example, by printing or writing with a pen.

Yapei Wang and colleagues, Renmin University of China, Beijing, have developed a paper-based thermometer which uses an ionic liquid as a sensing ink. The team used regular paper as a base and deposited gold electrodes on it by magnetic sputtering evaporation. Then they used either a ball pen or an inkjet printer to apply the ionic liquid 1-ethyl-3-methyl imidazolium bis(trifluoromethylsulfonyl)imide ([EMIm][Tf₂N]) between the electrodes. The conductivity of the ionic liquid changes with the temperature. This allowed the researchers to use the coated paper as a thermometer by applying a current to the electrodes and measuring the relative change in conductivity.

The paper thermometer is sensitive, has fast response times, and is stable during multiple heating-cooling cycles and under mechanical stress. The team also produced a 2D sensing array, which can measure temperature changes in a spatially resolved way. Using Bluetooth, the device was connected to a smartphone to give a real-time wireless readout of the measured data. According to the researchers, the use of inkjet printing to apply the ionic liquid could allow the large-scale production of such sensing devices.

• Ultrafast Paper Thermometers Based on a Green Sensing Ink,
  Xinglei Tao, Hanyu Jia, Yonglin He, Shenglong Liao, Yapei Wang,
  ACS Sensors 2017.
  DOI: 10.1021/acssensors.7b00060