Intracellular fluorometric determination of microRNA-21 by using a switch-on nanoprobe composed of carbon nanotubes and gold nanoclusters.

A sensitive and rapid fluorometric "switch on" assay is described for the detection of microRNA-21. It is based on the use of a fluorescence resonance energy transfer pair consisting of lysozyme-modified gold nanoclusters (Lys-Au NCs) and carbon nanotubes (CNTs). The Lys-Au NCs can be synthesized by a microwave-assisted technique within 2.5 min. They were modified with the ss-DNA probe (a 22-mer) for microRNA-21. Once the ss-DNA associates with the CNTs due to π stacking, the orange-red fluorescence (with excitation/emission peaks at 500/610 nm) is quenched. Nevertheless, the quenched fluorescence can be recovered after addition of microRNA-21 because of the stronger affnity between ss-DNA and microRNA-21. On the basis of the fluorescence recovery at 610 nm caused by microRNA-21, the latter can be quantified in the 0.01 to 100 nM concentration range, with a 36 pM detection limit. The method was applied to the determination of microRNA-21 in spiked serum with recoveries ranging from 98.6% to 110.0%. It also enables normal and cancer cells to be differentiated by direct imaging of intracellular microRNA-21. Graphical abstract A sensitive "switch on" FRET-based fluorometric assay for microRNA-21 is described. It is based on the use of  lysozyme-modified gold nanoclusters (Lys-Au NCs) and carbon nanotubes (CNTs) as energy donor and energy acceptor, respectively.