Gradient-Based Colorimetric Array Sensor for Continuous Monitoring of Multiple Gas Analytes

Colorimetry is widely used in chemical sensing due to its high sensitivity and high selectivity. However, most colorimetric sensors are one-time use because the color-producing reactions or bindings are usually irreversible. In addition, traditional colorimetric sensors like the detection tubes are bulky and packed individually, making parallel sensing of multiple analytes difficult. Here, we demonstrate a gradient-based colorimetric array sensor (GCAS) to overcome these limitations. Different colorimetric sensing elements are inkjet-printed as parallel straight lines on a porous substrate. Lateral transport of analytes across the substrate creates color gradients on the sensing elements. The color gradients shift along the transport direction over time, and GCAS tracks the gradient shifts and converts them into analyte concentrations in real time. Using a low-cost complementary metal-oxide semiconductor imager, we show detection of three air pollutants using a single GCAS chip and 24 h continuous monitoring of ambient ozone.