Sensitive naked-eye detection of gaseous ammonia based on dye-impregnated nanoporous polyacrylonitrile mats

Herein, highly sensitive colorimetric detection of gaseous ammonia is reported based on bromocresol green–impregnated polyacrylonitrile (BCG–PAN) nanofibers. The sensing layer with a porous nanofibrous structure was prepared simply by drop-casting a BCG solution containing polyethylene glycol (PEG) and oxalic acid on an electrospun PAN mat. The nanofiber mat exhibited durable structural stability under various conditions of acid, base, acetone and hot water. A smooth nanoporous structure of the sensor provided better color uniformity with a brighter reflectance than the porous paper, thus providing an ideal medium for high-quality colorimetric detection. Upon exposure to 25 ppm ammonia, the yellow BCG–PAN mat quickly changed and became saturated to blue within 1 min. The sensor was confirmed to have a detection limit of less than 1 ppm in the detection by the naked eyes, good selectivity to common volatile organic solvents and reversible color change behavior at ambient conditions. These results straightforwardly demonstrate that sensitive and reversible NH3 detection is possible simply by observing the sensor color without the help of any detecting instruments. The addition of water-soluble PEG into a BCG solution was found to enhance sensor response probably due to its strong hydrophilicity, while the oxalic acid prevented BCG dye from deprotonating by the ambient humidity. Based on the change in reflection spectrum and a great response to amine compounds, the colorimetric detection mechanism was proposed to be the transform of BCG acid form to the base counterpart through the acid-base reaction with basic analytes dissolved into the water-containing nanofiber matrix.