The spherical nucleic acids mRNA detection paradox

From the 1950s onwards, our understanding of the formation and intracellular trafficking of membrane vesicles was informed by experiments in which cells were exposed to gold nanoparticles and their uptake and localisation, studied by electron microscopy. In the last decade, building on progress in the synthesis of gold nanoparticles and their controlled functionalisation with a large variety of biomolecules (DNA, peptides, polysaccharides), new applications have been proposed, including the imaging and sensing of intracellular events. Yet, as already demonstrated in the 1950s, uptake of nanoparticles results in confinement within an intracellular vesicle which in principle should preclude sensing of cytosolic events. To study this apparent paradox, we focus on a commercially available nanoparticle probe that detects mRNA through the release of a fluorescently labelled oligonucleotide (unquenching the fluorescence) in the presence of the target mRNA. Using electron, fluorescence and photothermal microscopy, we show that the probes remain in endocytic compartments and that they do not report on mRNA level. We suggest that the validation of any nanoparticle-based probes for intracellular sensing should include a quantitative and thorough demonstration that the probes can reach the cytosolic compartment.