Bifunctional protonated g-C3N4 nanosheets as signal switches for smart immobilization-free photoelectrochemical sensing and mechanistic insight

Immobilization-free photoelectrochemical (IF-PEC) sensors have drawn substantial attention due to no need to fix the identification elements on the surface. The signal-regulation molecule plays a crucial role in immobilization-free sensing. In this work, g-C 3 N 4 is employed for the first time as a "switching molecule". The synthesized proton hydrogenation of g-C 3 N 4 exhibits higher affinity compared with traditional graphene functionalized with aptamer, more ideal band structural matching, and stronger electrostatic forces with substrate material, thereby effectively enabling efficient aptamer adsorption as well as precise regulation and amplification of the sensing signal. A well-designed IF-PEC sensor based on this strategy for highly efficient detection of a typical pesticide endocrine disrupting chemical, atrazine (ATZ), has herein been proposed, which achieves charge modulation while improving photoelectric response. This multiple signal amplification strategy synergistically achieves a broad linear range of 0.03–100 nM and a low detection limit of 16.50 pM (S/N = 3). Meanwhile, the interaction mechanism between molecules is further investigated by in situ attenuated total reflectance-surface enhanced infrared absorption spectroscopy (ATR-SEIRAS). A promising platform has therefore been offered to broaden the application of 2D materials for PEC sensors, as well as deepen the insight into the design of subsequent IF-PEC sensors. • Protonated g-C 3 N 4 nanosheets (PCN) are first employed as signal-switching molecules in immobilization-free sensing. • The interaction between PCN and aptamers is investigated in situ using ATR-SEIRAS. • PCN exhibits faster adsorption and a higher reaction rate constant than graphene. • The sensing strategy is simplified by cleverly utilizing electrostatic interactions to switch signals. • The moderate binding affinity of PCN enhances sensor sensitivity and enables a rapid response.