A Photoelectrochemical Biosensor with On-Off-On Type Signal Conversion for Pb²⁺ Detection

Herein, a photoelectrochemical (PEC) biosensor with on-off-on type signal conversion was designed by the signal quenching of Cobalt tetraphenyl porphyrin (CoPP) to gold nanoparticles@graphitic carbon nitride (Au@g-C₃N₄) and the signal recovering triggered from a DNAzyme-mediated amplification strategy for sensitive detection of lead ion (Pb²⁺) related to environmental monitoring. Impressively, compared to pure g-C₃N₄, the Au@g-C₃N₄ prepared by an in situ reduction method presented a significantly enhanced photocurrent response, which was named as the initial "on" PEC signal. Furthermore, with the incubation of S1, S2, and CoPP, the PEC signal generated from Au@g-C₃N₄ could be tremendously quenched (signal-off state) owing to the electron withdrawing characteristic of CoPP and the steric hindrance effect of the CoPP-S1-S2 complex. Additionally, the tiny target Pb²⁺ could initiate the Pb²⁺-dependent DNAzyme-mediated cyclic cutting reaction, leading to S1-S2 duplex spinning and CoPP dropping, thus producing a marked restored PEC response for sensitive detection of Pb²⁺. The limit of detection of the proposed PEC biosensor with on-off-on type signal conversion was calculated to be 16.67 pM. Notably, this work proposed a sensitive strategy for Pb²⁺ analysis, which was expected to detect trace heavy metal ion for medical diagnostics, environmental monitoring, and food safety.