Visual detection of tetracycline antibiotics with the turned on fluorescence induced by a metal–organic coordination polymer

Here, a rapid, simple and sensitive method for visual detection of tetracycline antibiotics (TCs) was developed based on the turned on fluorescence induced by a metal–organic coordination polymer of Zn(bix) [bix=1,4-bis(imidazol-1-ylmethyl)benzene]. Tetracycline antibiotics, such as tetracycline hydrochloride (TC), doxycycline hyclate (DC), chlortetracycline hydrochloride (CTC) and oxytetracyline hydrochloride (OTC), showed very weak fluorescence in Tris–HCl buffer of pH 8.68–9.40. However, the fluorescence intensities of these TCs were greatly enhanced in the presence of Zn(bix) and were found to be proportional to the concentrations of TCs. Accordingly, TCs could be quantitatively determined with the detection limits of 12.0, 6.0, 19.0 and 19.0 nM for TC, DC, CTC and OTC, respectively. Furthermore, this fluorimetric method was successfully applied to the determination of DC contents in doxycycline hyclate tablets from three factories with satisfactory results compared with the HPLC method. Also, the interaction mechanism between TCs and Zn(bix) was investigated by the absorption spectra, fluorescence spectra and circular dichroism spectra. The results showed that TCs could interact with Zn(bix) to form TCs–Zn(bix) ternary complexes and the turned on fluorescence was mainly ascribed to the restricted conformational rotation of TCs molecules through coordination with Zn(II) from the polymeric networks of Zn(bix).