p-Orbital Electron Spin Modulation in Carbon Nitrides for Enhanced Near-Infrared Photoelectrochemical Biosensing

Living systems incredibly convert light of different wavelengths into bioelectric signals via photosensitive molecules for survival. However, matters for efficient photoelectrochemical (PEC) biosensing, particularly under near-infrared (NIR) lights are limited. Here, we report a C3N2-based D-π-A doublet system for high-performance PEC biosensing under NIR irradiation. The unpaired electrons in the dangling bonds of C3N2 facilitated p-orbital electron spin modulation, thus establishing a new spin-unrestricted α-HOMO→α-LUMO charge transfer pathway. As a result, this strategy dramatically boosts electron exchange efficiency between interfaces of the photosensitive material and aqueous solution by 269-fold. As a demonstration, the system enabled sensitive tetracycline detection directly within a single drop of non-transparent whole blood. This work pioneers spin-state engineering in metal-free p-orbital semiconductors for NIR responsiveness, unlocking a powerful and groundbreaking platform for advanced in vivo PEC biosensing.