Quantum Dot Functionalization
Functional Water-soluble Quantum Dots
Carefully consider the following- a research group develops methods for synthesizing very bright, water soluble emissive nanocrystals. What next? While pretty to look at, they don’t “do” anything! A good use of such a sample is for biological imaging, and most likely we need to tag the NC to a biological agent. If we want to use the NC as a chemical sensor, we have to attach an organic dye to the NC (see our articles in JACS, 2006, 128, 13320). Our group is increasing the “tool-kit” available to a wide range of researchers by making all of this easy.
Water solubilization. To impart water solubility to NCs made in a hydrophobic solvent, the two most wide-spread approaches are to replace the hydrophobic ligands with hydrophilic ones or to wrap NCs in amphiphilic polymers. Cap exchange makes very small NCs (good for biological studies!) but it comes at a price- the samples are not very stable in water and will precipitate in hours to days at best. Polymer encapsulation makes near-permanently stable NCs, but they can be quite large (>25 nm); this renders them somewhat biologically incompatible. Our group recently demonstrated a technique to make cap exchanged NCs much more stable in water by using an organometallic species as the cap in the exchange process (ACS Nano, 2011, 5, 546). As shown here, our method starts by synthesizing a zinc coordination complex using water-soluble mercaptans. Next, the zinc metallated cap is mixed with phosphonic acid coated NCs. Zinc has very strong interactions with phosphonic acids such that the phosphonic acid is “gently” removed upon coordination to the zinc complex. This process “sheds” the thiol-ligand that now coordinates to the surface of the nanocrystal. This increases the brightness and stability of the water-soluble cap-exchanged NCs by a significant extent- we can coat NCs with small molecules like cysteine to make aqueous nanocrystals that are stable for weeks as opposed to mere hours.
QD functionalization. Whether we cap-exchange or encapsulate NCs with polymers, we now must functionalize them if they are ever to do anything useful. Given that NCs are generally coated with carboxylic acids (-COOH), many research groups activate carboxylic acid coated NCs with 1-ethyl-3-(3-dimethylaminopropyl) carbodiimide (called EDC), which “primes” carboxylic acids to react with amines. However, our group and others have shown that EDC causes NCs to precipitate out of solution permanently, thus destroying the sample. Unfortunately, there are no other commercially available water soluble carbodiimide reagents.
We recently found a solution to the functionalization problem. (ACS Nano, 2009, 3, 915-923). We realized that the carbodiimide functionality (R1-N=C=N-R2) is not responsible for the observed precipitation of NCs; rather, it’s the fact that EDC is cationic and the NCs are highly negatively charged. Thus, the solution is to make a carbodiimide that has no net charge at all! Shown here is the method we used to make a carbodiimide functional methyl polyethylene glycol (PEG) polymer. It does not cause NCs to precipitate and may functionalize NCs with 95% reaction efficiency!
Although PEG-carbodiimides have the highest functionalization yields, they can be difficult to make. We recently followed up this work with a demonstration of neutral carboxylic acid activating reagents, one of which is commercially available (Chemical Communications, 2011, 47, 3532-3534). These reagents were used to create functional nanomaterials useful for biological imaging. Shown here are cells that have been coated with “normal” polymer encapsulated CdSe/ZnS NCs; the fluorescent image shows that the NCs tend to stick to cells although the nanocrystals have not been targeted for this purpose. This can lead to highly erroneous conclusions in cell labeling studies. We used some of our new reagents to coat the same CdSe/ZnS with polyethylene glycol. These PEG-coated NCs do not adhere to cells as shown in the fluorescence image where no NC emission is seen; we are now targeting PEG coated NCs such that they stick to cells only if the NCs are designed to do so.
We have also developed a second high yielding functionalization scheme (JACS, 2008, 130, 3744). We used a method of size controlled free radical polymerization that makes highly uniform polymers that contain a single thiol functional group. We then demonstrated that the SH function may be used to functionalize the NCs without fear of loss of all the sample due to precipitation. Shown here is one example of a BODIPY dye bound to a polymer coated CdSe/ZnS NC where we demonstrated energy transfer from the NC to the surface bound dye. This paves inroads for the development of chemical and biological sensors.
We are also developing new polymer-encapsulation techniques that significantly enhance the solubility of NCs such that they can be functionalized, in high yield, with highly cationic cell-penetrating peptides and other biological materials that are known to cause NC precipitation and quenching.