Water- and oxygen-tolerant phosphorescent carbon nitrides enable visual hydrogel biosensing

Metal-free room temperature phosphorescence (RTP) materials in aqueous environments offer promising applications due to their long lifetimes and large Stokes shifts but face challenges in maintaining stability against water and oxygen. To address this issue, we developed a trinity strategy that integrates efficient emissive units, excited-state stabilization, and protection from quenching agents within a single polymeric carbon nitride (CN) macromolecule functionalized with imidazolium. This approach yielded homogeneous RTP materials exhibiting robust phosphorescence in water and oxygen, with lifetimes up to 80 ms and high quantum yields. Mechanistic studies revealed that incompletely condensed residues enabled efficient intersystem crossing, which was stabilized by the rigid CN skeleton and protected by the hydrophobic microenvironment from covalent imidazolium functionalization. Leveraging these properties, we fabricated a phosphorescent hydrogel for visual detection of Fe³⁺ in human serum. This work demonstrates the potential of engineered carbon nitride as a versatile platform for stable, water/oxygen-tolerant organic RTP materials suitable for biosensing applications.