Bimetallic Eu/Fe-MOFs ratiometric fluorescent nanoenzyme for selective cholesterol detection in biological serum: Synthesis, characterization, mechanism and DFT calculations

Cholesterol is one of the most significant indicators to assess health status. Lanthanide metal-organic frameworks (Ln-MOFs) have a unique luminescence mechanism, which makes it possess tremendous potential in biomonitoring. Thus, on the basic of intramolecular charge transfer theory and quenching effect, the metal-organic frameworks (MOFs) doped by europium was constructed to respond to the cholesterol in serum. The charge transfer direction and the sensing mechanism of fluorescent probe were explained in detail by cross-corroboration of experiment and calculation. The possible sensing mechanism was explained at the molecular level with density functional theory (DFT). With the excitation of 350 nm, the emission peaks at 458 nm and 615 nm which corrsponded to MIL-53(Fe) and Eu centers, appeared enhancement and quenching respectively. To investigate the emission law, the density of states and partial density of states were used to explain the internal electron migration of the probe and the possible sensing mechanism. The results showed the electron transfer efficiency of MIL-53(Fe) was enhanced after Eu3+ doping, which brought a remarkable improvement in response sensitivity. Meanwhile, the higher energy possessed by the excited state of Eu will lead to the capture of its own electrons by hydrogen peroxide, thus reducing the number of electrons returned to the ground states of Eu and eventually leading to the fluorescence quenching of 615 nm. We anticipate the proposed research is to be an emerging method in cholesterol determination and have further development in all walks of life.