A Redox‐Switchable Au₈‐Cluster Sensor

Abstract The proof of concept of a simple sensing platform based on the fluorescence of a gold cluster consisting of eight atoms, which is easily manipulated by reduction and oxidation of a specific molecule in the absence of chemical linkers, is demonstrated. Without using any coupling reagents to arrange the distance of the donor–acceptor pair, the fluorescence of the Au₈‐cluster is immediately switched off in the presence of 2‐pyridinethiol (2‐PyT) quencher. Through an upward‐curving Stern–Volmer plot, the system shows complex fluorescence quenching with a combination of static and dynamic quenching processes. To analyze the static quenching constant (V) by a “sphere of action” model, the collisional encounter between the Au₈‐cluster and 2‐PyT presents a quenching radius (r) ≈5.8 nm, which is larger than the sum of the radii of the Au₈‐cluster and 2‐PyT. This implies that fluorescence quenching can occur even though the Au₈‐cluster and 2‐PyT are not very close to each other. The quenching pathway may be derived from a photoinduced electron‐transfer process of the encounter pair between the Au₈‐cluster (as an electron donor) and 2‐PyT (as an electron acceptor) to allow efficient fluorescence quenching in the absence of coupling reagents. Interestingly, the fluorescence is restored by oxidation of 2‐PyT to form the corresponding disulfide compound and then quenched again after the reduction of the disulfide. This redox‐switchable fluorescent Au₈‐cluster platform is a novel discovery, and its utility as a promising sensor for detecting H₂O₂‐generating enzymatic transformations is demonstrated.