Aggregation-induced emission of 4-formyl-3-hydroxybenzoic acid for the ratiometric fluorescence detection of tetracycline antibiotics

In this work, 4-formyl-3-hydroxybenzoic acid (FHBA), which does not contain a large conjugated ring structure, exhibited aggregation-induced emission (AIE) in aqueous solvents. On this basis, FHBA and a metal-organic framework material (i.e., ZIF-8) form into a complex (FHBA@ZIF-8), which presents the AIE characteristics of FHBA. Then, lanthanide metal (i.e., Eu3+) coordinates with guanosine 5 '-monophosphate disodium (GMP) salt to synthesize a coordination polymer (LnCP), which combines with FHBA@ZIF-8 do afford a novel double-emitting fluorescence probe (FHBA@ZIF-8@Eu-GMP, FZE) for tetracycline antibiotics (TCAs). TCAs coordinate with Eu3+ and act as antenna ligand. Thus, an "antenna effect" comes into being and effectively enhances the emission intensity of Eu3+ at 616 nm. Moreover, the UV-vis spectra of TCAs largely overlap with the excitation spectrum of FHBA, resulting in an inner filter effect (IFE) and quenching of blue-green fluorescence from FHBA molecules at 473 nm. The ratio of fluorescence signal (F616/F473) showed a good linear relationship with the concentration of tetracycline hydrochloride (TC) in the range of 0.1-200.0 mu M, and the detection limit was 60 nM (S/N = 3). This fluorescence probe can also effectively respond to oxytetracycline hydrochloride (OTC) and chlortetracycline hydrochloride (CTC). The probe was applied to determine TC in milk, and the recoveries ranged from 94.0% to 101.4%, indicating the application potential of this method. This study provides a novel idea for the construction of ratiometric fluorescence sensing platforms and shows the application potential of the AIE effect of FHBA.