When further checking the chemical structure of BISX, there are two nitrogen atoms contained in the conjugation backbone (Figure ). The lone pair electrons of the sp nitrogen are perpendicular to the plane of the aromatic ring, forming lone-pair π conjugation, thus the sp N is difficult to be protonated. For the sp 2 N, in the contrast, its lone pair does not participate in conjugated systems, as they are parallel to the molecular plane. The sp 2 hybridized nitrogen is more likely to be protonated. The calculated electron density maps based on electrostatic potential further verified our hypothesis, by giving the result that sp 2 N is more electron-rich than sp 3 N (Figure ). We then tested the reversibility and reliability of the system by alternatively exposing the BISX film to TFA and TEA vapors. The bright and dark states could be converted for more than three consecutive cycles with negligible fatigue, revealing the high reversibility and repeatability of this sensor system. Inspired by its "turn-on/ off" response to ammonia vapor after being post-treated with acid, we employed BISX as a food spoilage sensor. We bought fresh clams, crayfish, and fish from the seafood market as detection samples. The fresh seafood was sealed in black-packaged containers with TFA-treated thin films of BISX, and kept at room temperature for 24 h, respectively. After 12 h, faint green emission was observed. After 24 h treatment, the emission became much stronger (Figure ). For crayfish and fish, as shown in Figure and S53, the thin film gave a strong green fluorescence when sealed at 12 h, indicating their faster decomposition rate compared to clams. In this regard, food spoilage sensors could be generated for sensitive detection of ammonia and biogenic vapors generated through the food spoilage process.