Construction of a Dual-Mode Biosensor with Ferrocene as Both a Signal Enhancer and a Signal Tracer for Electrochemiluminescent and Electrochemical Enantioselective Recognition

We demonstrate for the first time the construction of a dual-mode biosensor for electrochemiluminescent (ECL) and electrochemical chiral recognition of l- and d-isomers of amino acids, with ferrocene (Fc) as both a signal enhancer and a signal tracer. With the dissolved oxygen as a coreactant, ZnIn₂S₄ acts as the ECL emitter to generate a weak cathodic ECL signal. Fc can enter into the β-cyclodextrin (β-CD) cavity on ZnIn₂S₄-modified electrode as a result of host-guest interaction. Since Fc can promote H₂O and O₂ to produce abundant reactive oxygen species (ROS) (e.g., O₂^·- and ^·OH), the ECL signal of ZnIn₂S₄ can be further amplified with Fc as a coreaction accelerator. Meanwhile, Fc molecules on the β-CD/ZnIn₂S₄-modified electrode can be electrochemically oxidized to Fc⁺ to produce a remarkable oxidation peak current. When l-histidine (l-His) is present, the matching of the l-His configuration with the β-CD cavity leads to the entrance of more l-His into the cavity of β-CD than d-histidine (d-His), and the subsequent competence of l-His with Fc on the Fc/β-CD/ZnIn₂S₄-modified electrode induces the decrease in both Fc peak current and ZnIn₂S₄-induced ECL intensity. This dual-mode biosensor can efficiently discriminate l-His from d-His, and it can sensitively monitor l-His with a detection limit of 7.60 pM for ECL mode and 3.70 pM for electrochemical mode. Moreover, this dual-mode biosensor can selectively discriminate l-His from other l- and d-isomers (e.g., threonine, phenylalanine, and glutamic acid), with potential applications in the chiral recognition of nonelectroactive chiral compounds, bioanalysis, and disease diagnosis.