Precise synthesis of narrow bandgap carbon nitrides for near-infrared photoelectrochemical biosensing

The application of cost-effective and readily available near-infrared (NIR)-responsive materials for photoelectrochemical (PEC) biosensing of opaque samples remains limited. Herein, we report a precise synthetic strategy for narrow optical bandgap polymeric carbon nitrides (pCNs) with dangling bonds. Interestingly, the final product C/N ratio and repeating unit of the formed pCNs precisely match those of the original N-heterocyclic ligand molecule. It was revealed that the inert Zn²⁺ with filled d-orbitals is crucial for guiding radical polymerization towards desired dangling-bond pCN formation, providing insights for future pCN synthesis. By following this strategy, rather than traditional C₃N₄, a new pCN with a C/N ratio of 7 : 2 and a bandgap of 1.17 eV was synthesized using two distinct precursors. The integration of experimental and DFT data confirmed that dangling bonds significantly narrowed the bandgap <i>via</i> β-HOMO to LUMO transitions, imparting a distinct NIR PEC response. Leveraging this unique capability, sensitive tetracycline detection was achieved in opaque human whole blood, demonstrating its significant clinical potential.