April 3, 2006
Encapsulated Polymer Nanoparticles Shine Brightly
In just a few short years, semiconductor quantum dots have become a mainstay of cancer researchers. Their small size and extremely bright fluorescence, or optical brightness, makes them ideal molecular tracers for nearly any study aimed at tracking the behavior of biomolecules in living cells and even organisms. Researchers are considering how these optically active nanoparticles could be used to improve cancer imaging in humans, but the heavy metals used to make quantum dots — cadmium, in particular — have raised questions about the ultimate biocompatibility of these particles.
To overcome this potential limitation, investigators at Clemson University, led by Jason McNeill, Ph.D., have created a new type of fluorescent nanoparticle made of what is known as a "conjugated polymer," that is, one whose electrons move freely across the surface of the polymer molecule. Then, to improve the stability and versatility of these nanoparticles, the researchers deposited a thin shell of silica on top of the polymer core.
Reporting its work in the journal Langmuir, the Clemson team describes the methods it uses to create brightly fluorescent polymer nanoparticles with diameters of about 10 nanometers. As a result of the methods the researchers used, each nanoparticle contains one molecule of polymer, a first, and as a result are far smaller than other polymer nanoparticles created until now. The resulting nanoparticles are stable for weeks, and though they are brighter initially than quantum dots, their optical brightness fades much more quickly than that of semiconductor quantum dots.
The investigators remedied this shortcoming by adding a 2 to 3 nanometer-thick shell around the polymer molecule. The shell had no deleterious effect on the nanoparticles' brightness, but it did stop the optical brightness from fading. The silica shell also provides a surface to easily attach biomolecules, including those that could be used to target the new particles to cancer cells.
This work is detailed in a paper titled, "Preparation and encapsulation of highly fluorescent conjugated polymer nanoparticles." An abstract of this paper is available through PubMed.