Ultrafast detection of dynamic variations in the carbon dioxide (CO 2 ) gas concentration with good sensitivity is crucial in many applications that range from respiration monitoring to fire detection. However, the chemical inertness of the CO 2 sensor makes its detection with high sensitivity quite difficult, and only a few materials were reported that can sense CO 2 effectively. Nevertheless, the sensors based on these materials exhibited very low sensitivity with large response times, and thus they are not suitable for many practical applications. Here, we report a highly sensitive, reversible, and ultrafast detection of CO 2 gas in air using a resistive gas sensor based on ZnO nanoflakes. Excellent sensitivity (0.1125 ppm −1 for 600 ppm) with ultrafast response (<20 s) is observed upon exposure of the sensor to 200−1025 ppm of CO 2 at 250 °C. The sensing mechanism of the device is explained by the oxygen vacancy model. Further, the effect of the temperature and cross-sensitivity of the sensor to other gases were experimentally investigated. Being highly sensitive and faster, this CO 2 sensor can be utilized in numerous applications where high response and recovery times along with good sensitivity are extremely important.

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This paper describes a fully‐drawn pencil‐on‐paper based low‐cost capacitive sensor for non‐invasive respiration monitoring. The sensor utilizes the hygroscopic character of the paper to measure the breathing rate and pattern. The adsorption and desorption of water molecules on paper during inhalation and exhalation results in variation in its dielectric constant. This change in dielectric constant during respiration reflects the change in capacitance of the sensor. By interfacing the sensor with the microcontroller, the capacitance data was acquired and transferred to a smartphone through Bluetooth communication. Being a low cost, wearable, non‐invasive and disposable sensor, it holds tremendous potential in healthcare technology and can be commercialized into a viable product for easy‐to‐use diagnostic purpose.

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(a) Schematic diagram of a fabrication method of a pencil on paper FET device; (b) image of flexible and wearable FET based strain sensors attached to the fore-finger for human motion detection; (c) strain sensing considering the resistance in stretched condition as the reference.