Carbon nitride of five-membered rings with low optical bandgap for photoelectrochemical biosensing

As an analog of graphene, carbon nitrides have drawn increasing attention, ranging from artificial photosynthesis to synthetic organic chemistry. Notably, the most studied polymeric, C3N4, and other stoichiometric ones, such as C3N5, C4N3, and C3N, are commonly employed repetitive units of six-membered rings. Building blocks of five-membered rings (FMRs) are also fascinating and prevalent from a fundamental point of view, but FMR-based carbon nitrides are scarcely reported. Here, we propose a stable FMR-based C3N2 and the kinetic-oriented synthesis method via a pre-stabilization strategy using abundant-available zeolitic imidazolate framework as a precursor. The FMR topological structure and the associated dangling bonds were disclosed in C3N2, leading to an interesting p-π conjugation and splitting molecular orbitals, which remarkably narrowed the optical bandgap unusually down to 0.81 eV. The as-obtained C3N2 was further successfully applied to real-time, dynamic, and quantitative photoelectrochemical biosensing for practical non-transparent biosamples under near-infrared light irradiation.