Multi-Functionalized Carbon Nitride with Enhanced Room-Temperature Phosphorescence for Afterglow Visual Biosensing

Metal-free room temperature phosphorescent (RTP) materials in aqueous phase have drawn extensive concern, benefiting from their long-lived luminescence without interference of autofluorescence in biological application. Nevertheless, its construction was restricted by intrinsically weak spin-orbit coupling and intense nonradiative relaxation by aqueous medium, Herein, we report the facile synthesis of CN-Im with stable afterglow in aqueous solution. Using flexible polymeric carbon nitride (CN) as a versatile platform, the RTP emission was realized through the regulation of polymerization degrees. Spectroscopic experiments and theoretic calculations jointly disclose the incompletely condensed residues serve as phosphorescent emission center with efficient intersystem crossing (ISC) process while the highly condensed poly(tri-s-triazine) rigid skeleton effectively stabilize the excited triplet state. Furthermore, the covalent functionalization of engineerable CN conjugated units with imidazolium chloride establish hydrophobic microenvironment to effectively and homogeneously sheltered the quenching from liquid medium, leading to outstanding phosphorescence stability. As a result, a time-dependent color-tunable phosphorescence hydrogel was constructed based on CN-Im for visual detection of Fe3+ with selectivity. This work enriches luminescence behaviors of CN material in solution, contributing to their potential applications in time-resolved biosensing and point-of-cared testing as a soft afterglow material.