Bifunctional Fe-CDs@MOF composite nanozyme based ratiometric fluorescent probe for the detection of xanthine in fish

A novel bifunctional Fe-CDs@MOF composite nanozyme was synthesized successfully by loading Fe doped carbon dots (Fe-CDs) onto metal-organic framework (MOF) MIL-101 (Fe). The introduction of Fe-CDs enhanced the peroxidase (POD)-like activity of MOF and endowed it with excellent fluorescence properties. Considering the dual functions of Fe-CDs@MOF, we combined it with o-phenylenediamine (OPD)/xanthine oxidase (XOD) to construct a ratiomeric fluorescence probe for xanthine (XAN) detection. XAN was first oxidized by XOD to produce H₂O₂, then Fe-CDs@MOF exerted its POD-like activity to catalyze the reaction between OPD and H₂O₂ to form 2,3-diaminophenazine (DAP). DAP in turn quenched the fluorescence of Fe-CDs@MOF as a result of internal filtration effect. The ratio of the fluorescence intensity of DAP and the fluorescence intensity of Fe-CDs@MOF indicated the XAN content. The XAN detection range was 1-100 μM with a limit of detection of 0.36 μM. The ratiometric fluorescence probe was utilized to analyze XAN in fish samples, and the results obtained were in good agreement with those assayed by high performance liquid chromatography. The method also had the ability to test the freshness of frozen fish. This study provides a novel approach for the design and application of Fe-CDs@MOF composite nanozyme.