Ratiometric detection of tetracycline based on gold nanocluster enhanced Eu³⁺ fluorescence

Tetracycline (TC), a widely-used antibiotic to treat bacterial infections, combines with Eu³⁺ to form a stable EuTC complex that has a low fluorescence (FL) yield because of Eu³⁺ coordination with water molecules. We report a simple, label-free ratiometric fluorescent platform for sensitively and selectively sensing of TC, using L-histidine caped gold nanoclusters (His-AuNCs) as a FL indicator and an enhancer of Eu³⁺ FL. The His-AuNCs were prepared via chemical reduction of Au(III) by L-histidine, which was a reducer and a protecting agent. The His-AuNCs exhibited good photostability, outstanding stability toward high ionic strengths, storage stability, and favorable optical properties. In the absence of TC, the AuNCs-Eu³⁺ system displays strong FL emission at 475 nm (F₄₇₅) from the His-AuNCs and weak FL at 620 nm from Eu³⁺ (F₆₂₀) with excitation at 375 nm. TC quenches the His-AuNC FL and greatly enhances the Eu³⁺ emission. This is attributed to an enhancement of the EuTC complex fluorescence by the His-AuNCs. Thus, a ratiometric F₆₂₀/F₄₇₅ FL signal can be used for TC detection by simply mixing AuNCs and Eu³⁺. Under the optimized conditions, the linear range from 10 nM to 60 μM and a detection limit of 4 nM (S/N = 3) for TC were obtained. The application of the assay platform for the detection of TC in environmental and biological samples was demonstrated. The sensing platform has advantages of easy preparation, rapid response, high sensitivity, and good selectivity.