9 Jicun Ren (Angew. Chem. Int. Ed., 2006, 45, 7588-7591) has an interesting paper here on blinking quantum dots. Now, blinking actually isn’t something you want and, according to Jicun Ren, by switching from an organic solvent to make these quantum dots to an aqueous solvent stabilized with thiopropiononic acid the blink-blink goes away. There are clearly no greater annoyances than trying to watch something that disappears every few seconds. While I would think a quantum Christmas tree would be awesome as hell, complete with blinking lights, I suppose people that actually use these colorful quantum dots wouldn’t have much need for it.
I’ve had a secret crush on QDs for a while now, because they’re so filled with buzzwordy goodness. Quantum is all the rage these days as are dots that fluoresce. Not only that but you can attach them to fullerenes in ionic liquids by triazoles that you clicked together to make nanodevices. If someone could find a way to squeeze a cyclodextrin and use it to do a quantitative macrolactamization… wellllll… that would be a ticket straight to Science and Nature and JACS and your mom. But one kitschy scientific advance at a time, please. What Ren did was hardly kitschy at all; however. Ren’s accomplishment illustrated two things for me. One: Solvent effects are big in even the most bizarre reactions and two: no one really has a flippin’ clue about these things. They explain:
To date, the blinking mechanism of QDs is not clear but thought to arise from electron (or hole) ejection from an Auger process, pure electron tunneling into the surrounding matrix, or electron capture in trap states of the surface of the particle It was shown that blinking may exhibit unique kinetics by different degrees of surface passivation or by altering synthesis conditions and that the associated mechanism is related to defect sites or unsaturated dangling bonds located at the surface of the QDs.CdTe QDs used in our study were synthesized in aqueous phases in the presence of TPA as a stabilizer and their surfaces were well coated with TPA.We speculate that TPAhas a critical effect on the optical properties of CdTe QDs synthesized in aqueous solution. The thio moiety in TPA is an excellent electron donor and donateselectrons to the surface traps, thus rendering them incapable of accepting electrons from the carrier or serving as channelsfor efficient charge tunneling from the interior of the QDs to the surrounding matrix.
Whatever. I’m not sure what most of that means… but it sounds incredibly scientific, which means it possibly contains a preponderance of nonsense, but that’s hardly Ren’s fault. It’s just a black box at the moment. Ren’s point is, after you get through it all, that stabilization by thiol groups as ligands seems vital to prevent the blinking.
The only issue I have with the article is the choice of quantum dots. CdTe compared to CdSe with a ZnS coating are comparable? With or without the thiol coating, it seems like the differences may be a little huge to contrast the two – but that’s a minor point and I have no idea if it is even relevant. Point is, he made a QD that doesn’t blink and that’s awesome.
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If they were real chemists they’d isotopically substitute their surface ligands and look for spin as well as mass effects. Hell, append some TEMPO, verdazyl, or nitronyl nitroxide to the surface coat and get a real dose of plasmon-triplet spin-coupling joy. What fun is a 1200 hp engine unless you can inject nitrous?
Then, add a magnetic field. NMR, EPR, cha cha cha! Makes a fella get all Gouy just thinking about it.
The supression of blinking of QDs in aqueous solutions has been known for a while. (Hohng, S.; Ha, T. J. Am. Chem. Soc. 2004, 126, 1324.; done with mercaptoethanol) The suppresion of blinking in aqueous solution is great for biological tags, but one of the dream uses of QDs is photovoltaics, so supression of blinking in an aqueous solution doesn’t get you too far for this use. QDs are a real bitch to work with in the solid state, as they tend to aggregate and self-quench (fluorescence). Many groups have been blending QDs with conjugated polymers for 15 years with mixed results, but one group showed suppression of blinking in the solid state by directly attaching PPV to the QD surface. (J. Phys. Chem. B 2006, 110, 14167-14171)
Then what’s Angew worthy about this research?i
What about heating effects during flourescence? wont that kind of melt the organics away when used over long periods of time?
“If someone could find a way to squeeze a cyclodextrin and use it to do a quantitative macrolactamization…”
Dean Toste can do it…
I’d second that sentiment, Kyle. Even as the layman that I am, I recognize quantum dots as being cooler than well-digger’s ass.
CdSe passivated with a thin layer of ZnS (2-5 nm) is one of the best known ways to remove surface traps.
They are comparing this to their CdTe with the new ligand to demonstrate that organiky ligands can decrease (remove?) blinking.
I should also mention that blinking is not always a bad thing.
Some people are using this blinking effect to get time resolved images of things (proteins in cells typically) that are way below the diffraction limit of light.
For example:
Lidke, K.A., Rieger, B., Jovin, T.M. & Heintzmann, R. Opt. Express 13, 7052–7062 (2005).
Is it easy to make a thin (2-5nm) ZnS layer that is defect free and stoichiometric?