Near-infrared Raman spectra of azo dye produced by a nitrogen-dioxide-gas-selective coloration reaction in a porous glass chip

We directly observed azo dye in a porous glass chip by near-infrared (NIR) Raman spectroscopy. We produced the dye by a modified Saltzman reaction by using sulfanilamide (SFA) and N, N-dimethyl-1-naphthylamine (DMNA) as reagents and used it for the selective sensing of atmospheric nitrogen dioxide gas (NO₂). The use of NIR laser excitation is particularly advantageous for the direct measurement of azo dye because it avoids the problem of interference from fluorescence. We obtained the 1597 cm^-1 band assigned to the phenyl group stretching mode and the 1415 cm^-1 band assigned to the azo group stretching mode. The Raman intensity and visible absorption intensity of the peaks of the azo dye in a porous glass chip increased as we increased the exposure time to NO₂. This allowed us to monitor the NO₂-sensing reaction in a porous glass chip using NIR Raman spectroscopy. Furthermore, we determined the depth profiles of the azo dye concentration in the porous glass chip by confocal NIR Raman spectroscopy and found that they were different in the early and later NO₂-exposure stages. This result indicates that the consumption of the NO₂-sensing reagents that accompany azo dye production occurs preferentially at the surface to the deeper regions of the porous glass chip.