See above. This is a considerable misunderstanding of the Smartflare technology: the gold is not supposed to stay where the mRNA was. In fact, to address this 'problem', Mirkin (the inventor of the SmartFlare) even developed another version where the mRNA stay bound to the mRNA (the famous stickyflare).

The SmartFlares are supposed to enable the measurement of mRNA levels in cells. Nobody has ever claimed that they can target mRNA, even less so target specific cells on the basis of their mRNA levels. There is no reason to believe that the amount of gold in different cells should correlate with the amount of mRNA. In fact the working principle of the sensor assumes that all cells uptake the same amount of SmartFlare but the fluorescence levels are different depending on the amount of mRNA because of the release of a reporter strand from the nanoparticle. [the technology does not work, but that is another story]

Reference 3 does not include any study of passive diffusion through membranes. To the contrary, it includes a liposomal transfection agent to help oligonucleotide-capped nanoparticles go inside cells. Reference 14 does not mention passive diffusion whatsoever. It is an article about active transport (endocytosis and exocytosis). Reference 15 includes the sentence "GNPs smaller than 30 nm are leaving the cell again by passive diffusion (15)" (that's right, identical word for word)... where (15) is reference 14 above.