A convenient fluorescent assay for quinolones based on their inhibition towards the oxidase-like activity of Cu2+

In this work, we proposed a convenient fluorescent assay for the detection of quinolones based on their inhibition towards the oxidation of o-phenylenediamine (OPD) by copper ions (Cu2+). Free Cu2+ can oxidize OPD into fluorescent 2,3-diaminophenzine followed by the formation of copper nanoparticles (CuNPs), which would further catalyze the oxidation of OPD. When quinolones were added, their strong binding affinity to Cu2+ inhibited the oxidase-like activity of Cu2+. So, the fluorescence intensity of the Cu2+–OPD system decreased obviously. The determination of quinolones can be achieved by using this fluorimetric assay without the use of complicated instruments and protocols. As demonstrated, this assay offered a highly sensitive readout for quinolones including nalidixic acid, cinoxacin, ciprofloxacin, and moxifloxacin in the linear ranges of 1.0–100 nM, 0.5–100 nM, 1.0–100 nM, and 0.1–100 nM, respectively. In addition, the developed assay was successfully applied to the direct analysis of quinolones in real amples.