Flavylium-Containing Polymeric Probe as Encryption Ink for Reversible Sensing of Bisulfite Ions and Formaldehyde

Prolonged absorption of sulfur dioxide (SO 2 ) and formaldehyde (FA) inflicts severe long-term damage to human health. Here we report a fluorescent polymeric probe with flavylium pendants ( CP5-FLA ) for detecting bisulfite (HSO 3 – ) and FA in an aqueous medium. The probe displays remarkable photophysical characteristics with rapid response times (30/60 s), high sensitivity (detection limit of 1.1/2.6 nM), and the capability for colorimetric and fluorimetric detection for HSO 3 – /FA. The sensing mechanism is based on the intramolecular charge transfer (ICT) “off-on” process in CP5-FLA tuned by the Michael/reversible Michael addition reactions between the flavyluim groups and HSO 3 – /FA, as confirmed by electrospray ionization mass spectrometry (ESI-MS) and time-dependent density functional theory (TD-DFT) analysis for the reaction of the model compound toward HSO 3 – and FA. This work presents a facile approach for developing versatile flavylium-based fluorescent probes through flexible molecular engineering, with potential applications in data encryption.