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Dots Within Dots May Hit the Spot

Photonics Spectra
Jun 2006
Hank Hogan

Like nested dolls, quantum dots can reside inside microspheres. Quantum dots are a few nanometers in diameter and have bright, size-dependent emission. A single wavelength can be used to excite dots of different sizes, making multiplexing possible. By incorporating many dots into polystyrene or bioconjugated micron-size spheres, researchers can create structures with the potential to confine electrons and photons in three dimensions.

This arrangement could be useful in a number of research areas as a chromophore. “The main application would be in optical encoding for biological applications,” said Moungi G. Bawendi, a professor of chemistry at MIT in Cambridge, Mass.

Bawendi led a team that packed quantum dots into polystyrene microspheres, achieving a uniform distribution inside each polymer bead through the use of functionalized oligomeric phosphine ligands. In contrast to other methods, this approach resulted in the dots being incorporated throughout the volume of a bead. Also, the dots could not escape this confinement and cause trouble to their surroundings, easing biological applications.

According to Bawendi, the technique arose from earlier work that sought to create biologically compatible quantum dots. A byproduct of that research was the synthesis of oligomeric phosphine ligands that terminated in functional groups. These ligands sealed the quantum dots from the environment and could be polymerized, enabling incorporation into polymer films and microspheres.

Polymerization process

The investigators concocted a successful recipe for this polymerization through trial and error. They first synthesized CdSe, ZnCdS and ZnS core/shell quantum dots and then embedded them in polystyrene microspheres using a growth process lasting longer than a day.

Bawendi noted that simply incorporating the quantum dots into the beads was not enough. “The basic goal is to find methods to create quantum dot/microspheres with a high loading of quantum dots, with microspheres that are well-formed and have a narrow size distribution,” he said.

Measurements indicated that the technique did not attain the ideal. The presence of the quantum dots, for example, resulted in a smaller average size and a wider size distribution of the beads. The diameter of beads without the quantum dots was tightly centered around 1.2 μm, while those with the dots ranged from 0.4 to 1.2 μm in diameter.

Moreover, the quantum efficiency of the dots fell from about 20 percent before incorporation into the beads to 12 percent after. Finally, virtually all quantum dots ended up in the microspheres at low concentrations of dots and ligands, but 90 to 95 percent were not incorporated at higher concentrations.

Bawendi said the method probably will not be able to deliver high-quality beads with a high loading of dots, but he sees the work as a step forward. “It is a beginning and may be useful for some applications.”

Langmuir, online March 17, 2006, doi:10.1021/la051973l.



GLOSSARY
quantum dots
Also known as QDs. Nanocrystals of semiconductor materials that fluoresce when excited by external light sources, primarily in narrow visible and near-infrared regions; they are commonly used as alternatives to organic dyes.  
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