An intriguing signal-off responsive photoelectrochemical aptasensor for ultrasensitive detection of microcystin-LR and its mechanism study

A direct formatted photoelectrochemical (PEC) aptasensing platform was constructed for ultrasensitive microcystin-LR (MC-LR) detection with AgI-nitrogen-doped graphene (AgI-NG) composites as photocathode and aptamer as recognition element. The photocurrent of the AgI-NG composites was about 10.9 times higher than that of AgI. Furthermore, to investigate the sensing mechanism, the experiments with photoluminescence (PL) and time-correlated single-photon counting (TCSPC) technique were implemented as supporting evidence. The developed aptasensor can be used for MC-LR detection in fish samples. The experimental and theoretical efforts elucidate that the theory of flow directions of electron give reasonable explanation for the sensing process: prevailing electron-hole recombination constitutes a signal-off strategy, while electron transfer playing the leading role presents a signal-on performance. Impressively, we discovered that the sensing mechanism differed significantly when applied to the same analyte despite the commonality among the detecting configuration themselves, which is dependent by the interaction between the analyte MC-LR and transducer. For the detection process, in the presence of MC-LR, the specific recognition of the targets with the sensing interface would trigger the formation of the aptamer/MC-LR complex, resulting in reduced electron transfer rate, increased charge recombination, and hence quenched photocurrent intensity of the AgI-NG with detection limits of 0.017 pM. This work will help to complement the vital theoretical explanation about the underlying sensing mechanism, and future designing of PEC aptasensors.