Active-ester-based post-functionalized nanofibers with UV-switchable colorimetric response toward HCl and NH₃ vapors

Colorimetric gas sensors capable of rapid and reversible visual detection are attractive for the monitoring of hazardous acidic and basic vapors. In this work, UV-switchable responsive nanofibers were fabricated by electrospinning poly(pentafluorophenyl acrylate-co-4-acryloyloxybenzophenone) followed by photo-crosslinking and post-functionalization with spiropyran alcohol. Owing to the highly reactive pentafluorophenyl ester groups, spiropyran moieties were covalently immobilized onto the nanofibrous matrix, which effectively suppressed dye leaching and improved the reusability of the material. The obtained nanofibers exhibited reversible photochromic switching under alternating UV and visible light irradiation, accompanied by a characteristic absorption band change centered at approximately 550 nm. After UV activation, the spiropyran-functionalized nanofibers showed distinct and reversible colorimetric responses toward HCl and NH₃ vapors, with response time of 3.2 s and 1.8 s, respectively, accompanied by characteristic changes in the UV-Vis absorption spectra. In addition, the fibrous morphology and sensing performance were well maintained after 10 repeated UV/visible and acid/base cycles. These results demonstrate that post-functionalized electrospun nanofibers based on active ester chemistry provide an effective platform for the construction of reusable colorimetric vapor-responsive materials.