Lanthanide-based metal-organic framework nanosheets with unique fluorescence quenching properties for two-color intracellular adenosine imaging in living cells

Two-dimensional (2D) materials have attracted tremendous interest as fluorescence quenchers of dye-labeled biomolecules for application in biosensing. Metal-organic framework (MOF) nanosheets, as a new type of 2D material, have rarely been studied as bioanalytical platforms. Herein, we synthesize a series of ultrathin lanthanide-based MOF (MOF-Ln) nanosheets as a dye-labeled aptamer platform. The fluorescence quenching or recovery on the MOF-Ln nanosheets is determined by the charge properties (positive or negative) of the labeled fluorophores. The negatively charged fluorophores experience a fluorescence ‘turn-down followed by turn-down’ process, whereas the positively charged fluorophores experience a fluorescence ‘turn-down followed by turn-up’ process. The interesting fluorescence quenching properties of the MOF-Ln nanosheets make them an excellent two-color sensing platform for the intracellular detection of biomolecules. Nanosheets that use rare earths to modify dye-labelled DNA fluorescence can selectively detect biomolecules in living cells. Xing-Hua Xia from Nanjing University in China and co-workers designed a biodegradable sensor based on nanoscale metal-organic frameworks containing lanthanide elements. After using exfoliation to produce two-dimensional nanosheets, the researchers modified them with single-stranded DNA aptamers bearing fluorescent dyes. Charge transfer with lanthanide ions either quenches or enhances the dye's fluorescence, depending on its polarity. The team exploited this effect to produce a two-colour sensor. As a demonstration, they synthesized nanosheets functionalized with aptamers targeting adenosine triphosphate (ATP) and then incubated them with cancer cells. Confocal microscopy imaging revealed distinct red-to-green colour changes at regions in the single cancer cells where DNA probes form duplexes with ATP and cause the dye to leave the nanosheet. A series of ultrathin lanthanide-based MOF (MOF-Ln) nanosheets were synthesized and characterized. These MOF-Ln nanosheets exhibit interesting fluorescence quenching ability toward dye-labelled DNA. The fluorescence quenching or recovery on the MOF-Ln nanosheets is determined by the charge properties (positive or negative) of the labeled fluorophores. In aptameric sensing, the negatively charged fluorophore experiences a fluorescence ‘turn-down followed by turn-down’ process; while the positively charged fluorophore follows a fluorescence ‘turn-down followed by turn-up’ process. This work reveals the novel properties of MOF nanosheets.