Rapid One-Pot Syntheses Developed For Quantum Dots

Efficient and highly scalable new chemical synthesis methods developed at the University at Buffalo's Institute for Lasers, Photonics and Biophotonics have the potential to revolutionize the production of quantum dots for bioimaging and photovoltaic applications.

A confocal microscope image shows quantum dots, developed at UB, uptaken by cancer cells.

Quantum dots are tiny semiconductor particles generally no larger than 10 nanometers that can be made to fluoresce in different colors depending on their size. Scientists are interested in quantum dots because they last much longer than conventional dyes used to tag molecules, which usually stop emitting light in seconds. Quantum dots also are of great interest for energy applications because they can produce electrons when they absorb light, making possible extremely efficient solar-energy devices.

Both fabrication methods developed by the UB researchers involve using a single container, or "pot," and take just a few hours to prepare.

The UB scientists report that one of their rapid-solution synthesis methods enabled them to prepare robust, water-dispersible quantum dots for bioimaging, while the other one allowed them to prepare organically soluble quantum dots ready to be sequestered into a polymer host. According to the researchers, the new synthesis methods are truly scalable and can be used to produce large quantities of quantum dots.

This is believed to be the first successful demonstration of so-called III-V semiconductor quantum dots as luminescence probes for bioimaging that appear to be non-toxic. "Three-five," and other such classifications refer to the position on the periodic table of the elements that make up semiconductors. Until now, only II-VI quantum dots have been produced for these applications. However, they are highly toxic to humans.

Composed of indium phosphide, the nanocrystals developed at UB demonstrate luminescence efficiencies comparable to other quantum dots, but they also emit light in longer wavelengths in the red region of the spectrum.

Like those cadmium selenide quantum dots, the nanocrystals also exhibit two-photon excitation, absorbing two photons of light simultaneously, which is necessary for high-contrast imaging.

The UB group's quantum dots are composed of an indium phosphide core surrounded by a zinc selenide shell to protect the surface. An organic group then is attached to this shell, as well as a targeting group, in this case, folic acid. Folate receptors are targeted commonly by drugs in diseases such as cancers of the breast, ovary, prostate and colon.

In their experiments, UB researchers showed that the quantum dot system recognized the folate receptor and then penetrated the cell membrane. The entire system is water dispersible, which is critical if quantum dots are to be widely used for bioimaging.

The other scalable chemical fabrication procedure developed by the UB researchers allowed them to prepare quantum dot-polymer nanocomposites that absorb photons in the infrared region.

In addition to broadening the applications for solar energy in general, the UB research is likely to have applications to nighttime imaging systems used by the military that must absorb and emit light in the infrared.

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