Dual Functions of Fluorescence and Peroxidase Mimics for Hemin@NH2-UiO-66 and Ratiometric Fluorescence Sensing to l-Cysteine

The bifunctions of fluorescence activity and peroxidase mimics were well-integrated by a hemin chloride-modified Zr-based metal–organic framework (Hemin@NH2-UiO-66), which can catalyze the oxidation of o-phenylenediamine (OPD) to generate 2,3-diaminophenazine (DAP), presenting typical peroxidase mimic properties. The influence of the introduction of hemin chloride upon the enhanced peroxidase mimic activity was clarified, and the nanozyme catalytic parameters were optimized. Interestingly, the oxidized product of DAP presents strong fluorescence emission at 564 nm; combining it with the intrinsic fluorescence emission of NH2-UiO-66 at 452 nm, a dual-emission system could be built up by the resulting Hemin@NH2-UiO-66 in the presence of definite OPD and H2O2. Moreover, the fluorescence quenching effect (564 nm) was observed by adding l-cysteine (l-Cys); based on that, a straight-line dependence of the fluorescence intensity ratio (I452/I564) upon the concentrations of l-Cys was established. The detection limit was 0.21 μmol, and the analytical selectivity for l-Cys was also demonstrated. The work highlights the idea of combining the intrinsic fluorescence property and nanozyme catalytic activity in a functional MOF, and its special usability is found in the ratiometric fluorescence sensing analyses.