Hydrazone-Based Europium(III) Complexes: Mechanochromic Luminescence and Turn-On Fluorescence Detection of Quinolone Antibiotics in Human Urine

In recent years, alarmingly increased use of quinolone antibiotics has led to a large number of antibiotic residues in the bodies of organisms and threatened their health. It is crucial to develop efficient and sensitive methods for detecting quinolone antibiotics. For the europium (Eu(III)) complex, its characteristic emission is affected by the energy transfer from the excited singlet state and triplet states of the ligands to the emission energy level of the Eu(III) ion. If external stimuli could change its coordination environment or the stacking pattern, the intensity of the characteristic Eu(III) emission would be affected. Considering the abundant potential coordination sites of ciprofloxacin (CIP) and norfloxacin (NOR), we designed and prepared two europium complexes based on hydrazone, [Eu2L12(phen)2(NO3)2] (L1 = (E)-N′-(2-hydroxybenzylidene)nicotinohydrazide; phen = 1,10-phenanthroline) (Eu-1) and [Eu2L22(CH3OH)2(NO3)2] (L2 = (E)-N′-(2-hydroxybenzylidene)picolinohydrazide) (Eu-2). As expected, Eu-1 and Eu-2 showed a sensitive response to quinolone antibiotics (CIP and NOR). Upon the addition of CIP and NOR to the solutions of Eu-1 and Eu-2, the evident fluorescence enhancement corresponding to both the ligand and characteristic Eu(III) emission is clearly observed. The limits of detection (LODs) of Eu-1 for CIP and NOR are calculated to be 6.25 × 10–9 and 1.44 × 10–8 M, respectively, and those of Eu-2 for CIP and NOR are determined to be 2.86 × 10–8 and 4.22 × 10–8 M, respectively. The LODs and high values of Ksv prove their excellent sensing performance. The high sensitivities and anti-interference abilities make Eu-1 and Eu-2 promising turn-on fluorescence probes for detecting CIP and NOR in human urine, based on which Eu-1 and Eu-2 have been successfully employed to developed paper strips for the on-site detection of CIP and NOR. In addition, Eu-1 and Eu-2 both exhibit high-contrast mechanochromic luminescence, which is attributed to the phase transformation from a crystalline to the amorphous state.