A strip array of colorimetric sensors for visualizing a concentration level of gaseous analytes with basicity

A colorimetric strip sensor array based on the naked-eye detection has been reported as a simple instrument-free device able to visually display a concentration level of gaseous analytes with basicity. The array has a vertically aligned configuration of five sensing elements and their detection sensitivities to basic analytes are controlled to decrease downwardly. The colorimetric sensors were fabricated by drop-casting a dye solution containing a pH indicator, an acid compound, and hydrophilic polyethylene glycol on a polyacrylonitrile nanofiber mat. The detection sensitivity was regulated by using different amounts of the acid compound in addition to variation in pH Indicator. When sensors containing a pH indicator of bromophenol blue (BPB), bromocresol green (BCG), or chlorophenol red (CPR) were exposed to ammonia gas, coloration happens considerably in the regions of 0–1, 0–5 and 10–100 ppm for BPB, BCG and CPR sensors, respectively. Furthermore, the detection sensitivity could be controlled to decrease by increasing an amount of citric acid, which reveals that the color-changing acid-base reactions between pH indicator and basic analytes are reduced by competitive interactions between the analytes and acid additive. As illustrations, two strip sensor arrays based on BPB and BCG dyes were fabricated and confirmed the successful naked-eye discrimination of a NH3 concentration level in the ranges of 1–50 and 5–100 ppm, respectively. The concentration level can be determined by a number of coloration elements in the strip device while its resolution relies on a total number of sensing elements.