Naphthalimide-based fluorescent polymeric probe: a dual-phase sensor for formaldehyde detection

Formaldehyde (FA) is a common pollutant found indoors and outdoors, posing a significant threat to human health. Therefore, developing sensitive and efficient detection methods for FA is essential for environmental monitoring and protecting public health. Herein, we report a naphthalimide-conjugated water-soluble polymeric fluorescent probe for the detection of FA in both aqueous and vapor phases using fluorimetric methods. The aromatic amines present in the side chain of the polymer react with FA, forming a Schiff base (imine bond). This imine formation inhibits the photoinduced electron transfer (PET) process within the polymer, leading to a 'turn-on' fluorescence under 365 nm UV light. The probe is capable of selectively sensing FA with a detection limit as low as 1.36 nM in aqueous medium. The formation of imine is confirmed for the model reaction between 6-(4-aminophenyl)-2-(4-((4-vinylbenzyl)oxy)phenyl)-1 h-benzo[de]isoquinoline-1,3(2 h)-dione and FA by electrospray ionization mass spectrometry (ESI-MS) and nuclear magnetic resonance (NMR) titration methods. The mechanism behind 'turn-on' FA sensing is investigated using density functional theory (DFT) analysis. Additionally, the study demonstrates a facile approach for covalently attaching the polymer on the surface of a filter paper surface via ultraviolet (UV) light-induced cross-linking. Such polymer attached paper exhibits FA vapor sensing through changes in fluorescence intensity.