Abstract
<title>Abstract</title> <p>Flumequine (FLU), a widely used fluoroquinolone antibiotic, poses serious health risks due to its residual presence in food and the environment, creating a pressing need for on-site visual detection methods. Herein, a dual-mode detection of FLU was achieved using an Eu-2MIM@CDs probe, which was prepared by embedding carbon dots (CDs) into a porous metal–organic framework (Eu-2MIM) composed of europium and 2-methylimidazole. In colorimetric mode, the Eu-2MIM@CDs complex reacted with chromium black T (EBT) to form a magenta Eu³⁺–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 fluorescence mode, the Eu-2MIM@CDs probe initially exhibited emission from CDs at 436 nm. Upon adding FLU, the antenna effect sensitized Eu³⁺, 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 colorimetric mode, 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 mode 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 fluorescence mode, 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 minute. As the FLU concentration increased, a visible color shifted from blue to red. Both detection modes exhibited excellent selectivity for FLU against various interfering antibiotics, organic compounds and metal ions. Furthermore, the probe showed good practicability in actual samples. Collectively, the dual-mode probe provides a simple, fast and efficient method for both visual and quantitative FLU detection in complex matrices.</p>