A carbon dots-doped europium-MOF for dual-signal ratiometric fluorescence and colorimetric detection of flumequine

A signal probe, Eu-2MIM@CDs, was prepared by embedding carbon dots (CDs) into a porous europium-based metal–organic framework (Eu-2MIM) composed of europium and 2-methylimidazole, and was deployed for the dual-signal detection of flumequine (FLU) antibiotic. This single probe provided two distinct signal outputs for FLU ratiometric sensing: colorimetric and fluorescence signals. In the colorimetric channel, the Eu-2MIM@CDs complex reacted with Eriochrome Black T (EBT) to form a magenta Eu 3 ⁺–EBT complex. Upon the introduction of FLU, FLU competitively displaced EBT from the complex, releasing free EBT and restoring the blue color. This displacement process was accompanied by ratiometric absorbance changes at 538 nm and 610 nm, enabling multi-color visual detection. In the fluorescence channel, the Eu-2MIM@CDs probe initially exhibited emission from CDs at 436 nm. Upon adding FLU, the characteristic Eu 3 ⁺ fluorescence was significantly enhanced through a coordination-environment change, which simultaneously quenched the emission at 436 nm and generated a new emission peak at 612 nm, yielding a concentration-dependent ratiometric fluorescence response for FLU detection. In the colorimetric channel, as the FLU concentration increased, the absorbance at 538 nm progressively decreased while that at 610 nm increased, producing a well-defined ratiometric colorimetric signal. This channel provided a wide linear detection range from 0.01 to 2.5 µM with a detection limit of 3.3 nM. The visible color of the solution shifted from magenta to blue with rising FLU concentration. In the fluorescence channel, the intensity at 436 nm gradually decreased along with an increase at 612 nm, showing a linear range from 0.005 to 2.0 µM and a detection limit of 1.7 nM. The fluorescence response was rapid, completing within 1 min. As the FLU concentration increased, a visible color shifted from blue to red. The fluorescence channel exhibited high selectivity toward FLU over other antibiotics, organic compounds and metal ions, while the colorimetric channel showed fluoroquinolone-class selectivity. Furthermore, the probe showed good practicability in actual samples. Collectively, this dual-signal probe provides a simple, fast and efficient method for both visual and quantitative FLU detection in complex matrices. Graphical abstract