Dye-impregnated Nanofiber-based Colorimetric Sensors for Naked-eye Detection of Gaseous Ammonia

Abstract

Firstly, highly sensitive colorimetric detection of gaseous ammonia was 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. In addition, compare to the porous paper, a smooth nanoporous structure of the sensor provided better color uniformity with a brighter reflectance, thus providing an ideal medium for high-quality colorimetric detection. The sensor was confirmed to have 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 by the naked eyes with a detection limit of less than 1 ppm. The superb sensing characteristics have been explained and discussed in terms of the unique nanofibrous structure and chemical properties of moieties involved in the BCG solution. Next, a naked-eye based colorimetric strip sensor array was reported as an instrument-free simple device possible to visually display a concentration of gaseous ammonia. The array consisted of five sensing elements arranged along the axial direction with increasing response sensitivity to basic NH3. The colorimetric sensors were fabricated by drop-casting a dye solution containing pH indicator, acid compound and hydrophilic PEG on PAN nanofiber mat. The response behavior with respect to different NH3 concentrations was examined for various sensors fabricated with different kinds and amounts of pH indicator and additive acid. When ammonia gas was exposure to the sensors containing a pH indicator of bromophenol blue (BPB), BCG, or chlorophenol red (CPR), an initial yellow color was changed to blue or violet at the lower concentration for the more acidic pH dye: the response sensitivity order of BPB > BCG > CPR. Furthermore, the colorimetric response was retarded and suppressed with increasing an amount of acid compounds through a competitive acid-base reaction to the basic analyte. Two BPB and BCG-based NH3 strip arrays exhibited to be operated in the concentration regions of 1 – 50 and 5 - 100 ppm, respectively. The ammonia concentration was well correlated with the number of color-changing elements in the strip, i.e., the longer length of a blue bar at the higher NH3 concentration. Finally, as a preliminary work, a dye-impregnated PAN sensor array with a format of 3 by 5 has been prepared by using the two patterning methods based on the wax-printing and laser ablation. This colorimetric sensor array will conduct not only to discriminate ammonia and basic amine compounds but also to discriminate other toxic industrial gases such as formaldehyde and acid vapor.