As quantum dots have better quantum yield than organic dyes, many are being developed as a substitute for them. Nonetheless, could these substitutes be small enough to enter inside cells as current dyes do, or would they be operate outside a cell?
As far as I can tell, they do not (readily) cross the plasma membrane. This paper, "Cell-Penetrating Peptide-Functionized Quantum Dots for Intracellular Delivery", discusses strategies to deliver quantum dots into cells. These include:
- Passive delivery done by coating the quantum dots in something that allows them to be endocytosed more readily (the authors do not seem to be optimistic about this strategy)
- Mechanically faciliated delivery (e.g. microinjection)
- Peptide-facilitated delivery by complexing the quantum dots with a cell-penetrating peptide
Since people are going through all this effort to get them in, it does not seem like they are cell-permeable without substantial modification.