A self-calibrating logic system and oxidase-based biosensor using Tb³⁺-doped carbon dots/DNA conjugates

Tb³⁺-doped carbon dots (Tb³⁺@CDs) were prepared in a facile hydrothermal method by using ammonium citrate as carbon source and Tb³⁺ as dopant. A 15-bp GT-rich single-strand DNA (ssDNA) was introduced to sensitize Tb³⁺ via the antenna effect for generating two fluorescence signals (CDs and Tb³⁺), forming a conjugate of Tb³⁺@CDs/ssDNA. The ratiometric fluorescence of Tb³⁺@CDs/ssDNA could be reversibly regulated by Ag⁺ and Cys, in which the fluorescence peak at 546 nm of Tb³⁺ could be switched to “On” or “Off” as the signal indicator while the fluorescence peak at 444 nm of CDs remained constant as the build-in reference. The proposed Ag⁺/Cys-mediated reversible fluorescence changes in Tb³⁺@CDs/ssDNA was also proven for the design of a self-calibrating ratiometric fluorescence logic system. By integrated with the specific reaction between H₂O₂ and Cys, Tb³⁺@CDs/ssDNA was applied for ratiometric fluorescence detection of H₂O₂. More importantly, the sensing strategy could be further successfully extended to the monitoring of H₂O₂-produced oxidase-related reactions, such as GOx-biocatalyzed oxidation of glucose (the limit of detection: 0.06 μM) and was well applied in rat serum compared to commercial kits. This work unveiled a novel ratiometric fluorescent design, which is cost-effective, simple to prepare and easy-to-use without chemical modification or fluorescence labeling.