Ratiometric fluorescence sensor based on cholesterol oxidase-functionalized mesoporous silica nanoparticle@ZIF-8 core-shell nanocomposites for detection of cholesterol.

A novel ratiometric fluorescence sensing system based on cholesterol oxidase-functionalized dual-color mesoporous silica nanoparticles (MSNs)@metal-organic framework core-shell nanocomposite is demonstrated for cholesterol detection. MSNs were first loaded with 5-aminofluorescein (AF) inside pores and then wrapped with red-emission CdTe quantum dots (QDs) on the surface to seal in the dye molecules, forming the signal displaying unit (AF-MSN-QDs). Next, AF-MSN-QDs were encapsulated with zeolitic imidazolate framework (ZIF-8) to form a transition layer with distinct size-selectivity, which not only protected the cores from corrosion but also greatly decreased background interference from large molecules. More significantly, the ZIF-8 shells showed high affinity for most enzymes, which made it possible for cholesterol oxidase (ChOx) to self-organize on the surface of ZIF-8-encapsulated AF-MSN-QDs via chemo-physical adsorption, forming novel core-shell nanocomposites (AF-MSN-QD@ZIF-8-ChOx) as a sensing platform for cholesterol detection. The detectable signal was monitored by enzymatic product-quenching fluorescence of the QDs. The fluorescence changes of I520/I618 showed excellent linearity with H2O2 concentrations in the range of 5-100 nM, with a limit of detection (LOD) as low as 0.89 nM. As a proof-of-concept, cholesterol was selectively detected with beneficial LOD as low as 0.923 μg/mL, demonstrating the great potential of this biosensor platform for other biologically important molecules with H2O2-producing oxidases.