Sulfur Quantum Dot-Based Electrochemiluminescence Resonance Energy Transfer System for a Dual-Mode Ultrasensitive Detection of MC-LR Using Ag+ as a Signal Amplification Switch

Improving the sensitivity and accuracy of bioimmunoassays has been the focus of research into the development of electrochemiluminescence (ECL) sensing platforms, as this is a critical factor in their application to practical analysis. In this work, an electrochemiluminescence–electrochemistry (ECL–EC) dual-mode biosensing platform based on an “off–on–super on” signals pattern strategy was developed for the ultrasensitive detection of Microcystin-LR (MC-LR). In this system, sulfur quantum dots (SQDs) are a novel class of ECL cathode emitter with almost no potentially toxic effects. The sensing substrate is made from rGO/Ti3C2Tx composites, whose huge specific surface area greatly reduces the possibility of aggregation-caused quenching of SQDs. The ECL detection system was constructed based on the ECL-resonance energy transfer (ERET) strategy, where methylene blue (MB) with an ECL receptor function was bound to the aptamer of MC-LR by electrostatic adsorption and the center actual distance between the donor and the acceptor was calculated to be 3.84 nm, which was verified to be in accordance with the ERET theory. Meanwhile, the introduction of Ag+ as an ECL signal-amplifying molecule greatly improved the sensitivity of sensing analysis. Based on the specific binding of MC-LR to the aptamer, the concentration of MC-LR was found to have a positive correlation with the ECL signal. Also, EC detection was realized with the benefit of the excellent electrochemical properties of MB. The dual-mode biosensor greatly improves the confidence of the detection, examination areas of 0.001–100 pg/mL with MC-LR for ECL and EC were obtained, and the detection limits are 0.17 and 0.24 pg/mL, respectively.