Gold Nanocluster Probe-Based Electron-Transfer-Mediated Electrochemiluminescence Sensing Strategy for an Ultrasensitive Copper Ion Detection

Exploration of a novel and efficient sensing mechanism of Au nanocluster (AuNC)-based electrochemiluminescence (ECL) sensors is still a great challenge and opportunity for further applications. Herein, we proposed that the electron transfer (ET) could be used as a novel sensing regulation factor for the construction of an ECL-sensing platform based on the AuNC probe. As a proof-of-concept, the ECL quenching effect and mechanism of Cu²⁺ on pre-oxidation-treated l-methionine-capped AuNCs (OM-AuNCs) was investigated in detail. The results revealed that after the electrochemical excitation of the AuNC probe, the electron is transferred from the highest occupied molecular orbital (HOMO) of Met-Cu²⁺ to that of the OM-AuNCs, along with the ET from lowest-unoccupied molecular orbital (LUMO) of the OM-AuNCs back to the HOMO of Met-Cu²⁺, leading to the ECL quenching of OM-AuNCs. Since the ECL intensity of OM-AuNCs is sensitively affected by the ET process, a preferable linear dependence was obtained in the concentration range from 1.0 × 10^-18 to 1.0 × 10^-14 M with high selectivity. More importantly, a record low detection limit (LOD, 2.3 × 10^-20 M) at the single copper ion level has been realized without any other amplification technique. Furthermore, the actual sample detection for Cu²⁺ exhibited satisfactory results. Therefore, this study enriches an ET-mediated ECL application and promotes a more rational design of ECL sensors.