Rapid One-pot Syntheses Developed For Quantum Dots

A patent has been filed on themethods, which were described last month in papers in the Journal ofthe American Chemical Society and Applied Physics Letters.

Quantumdots are tiny semiconductor particles generally no larger than 10nanometers that can be made to fluoresce in different colors dependingon their size. Scientists are interested in quantum dots because theylast much longer than conventional dyes used to tag molecules, whichusually stop emitting light in seconds. Quantum dots also are of greatinterest for energy applications because they can produce electronswhen they absorb light, making possible extremely efficientsolar-energy devices.

Both fabrication methods developed by theUB researchers involve using a single container, or "pot," and takejust a few hours to prepare.

The UB scientists report that one oftheir rapid-solution synthesis methods enabled them to prepare robust,water-dispersible quantum dots for bioimaging, while the other oneallowed them to prepare organically soluble quantum dots ready to besequestered into a polymer host.

The new synthesis methods aretruly scalable and can be used to produce large quantities of quantumdots, according to Paras N. Prasad, Ph.D., executive director of the UBInstitute for Lasers, Photonics and Biophotonics, SUNY DistinguishedProfessor in the Department of Chemistry, and co-author on both papers.

"Thisfast-reaction chemistry will allow us to exploit the true potential ofquantum dots, whether it be for delivery into human cells for imagingbiological processes in unprecedented detail or for the development offar more efficient devices for solar conversion," he said.

OnAug. 17, the UB researchers reported in a paper in the Journal of theAmerican Chemical Society what is believed to be the first successfuldemonstration of so-called III-V semiconductor quantum dots asluminescence probes for bioimaging that appear to be non-toxic."Three-five," and other such classifications refer to the position onthe 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.

Composedof indium phosphide, the nanocrystals developed at UB demonstrateluminescence efficiencies comparable to other quantum dots, but theyalso emit light in longer wavelengths in the red region of the spectrum.

"Thisis a key advantage because red-light emission means these quantum dotswill be capable of imaging processes deeper in the body thancommercially available quantum dots, comprised of cadmium selenide,which emit mostly in the lower wavelength range," said Prasad.

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

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

Intheir experiments, UB researchers showed that the quantum dot systemrecognized the folate receptor and then penetrated the cell membrane,Prasad explained.

The entire system is water dispersible, which is critical, Prasad said, if quantum dots are to be widely used for bioimaging.

Theother scalable chemical fabrication procedure developed by the UBresearchers allowed them to prepare quantum dot-polymer nanocompositesthat absorb photons in the infrared region.

The work wasdescribed in the paper, "Efficient photoconductive devices at infraredwavelengths using quantum dot-polymer nanocomposites," published onlineAug. 11 in Applied Physics Letters.

"Current solar cells act onlyin the green region, thus capturing only a fraction of the availablelight energy," Prasad said. "By contrast, we have shown that these leadselenide quantum dots can absorb in the infrared, allowing for thedevelopment of photovoltaic cells that can efficiently convert manytimes more light to usable energy than can current solar cells."

Inaddition to broadening the applications for solar energy in general,the UB research is likely to have applications to nighttime imagingsystems used by the military that must absorb and emit light in theinfrared.

"Because of the efficient photon harvesting ability ofquantum dots, in the immediate future we will be able to incorporate afew different types of them simultaneously into a plastic host materialso that an efficient and broad band active solar device is possible,"said Yudhisthira Sahoo, Ph.D., research assistant professor in the UBDepartment of Chemistry and co-author on the APL paper.

Co-authorswith Prasad on the paper in the Journal of the American ChemicalSociety are Dhruba J. Bharali, Ph.D., and Derrick W. Lucey, Ph.D.,postdoctoral associates, and Haridas E. Pudavar, Ph.D., senior researchscientist, all of the Department of Chemistry in the UB College of Artsand Sciences, and Harishankar Jayakumar, a graduate student in theDepartment of Electrical Engineering in the UB School of Engineeringand Applied Sciences.

The research was supported by a DefenseUniversity Research Initiative in Nanotechnology (DURINT) grant fromthe Air Force Office of Scientific Research and by the John R. OisheiFoundation, as well as by UB's New York State Center of Excellence inBioinformatics and Life Sciences.

Co-authors with Prasad andSahoo on the Applied Physics Letters paper are K. Roy Choudhury,graduate student in the Department of Physics in the UB College of Artsand Sciences, and T.Y. Ohulshanskyy, Ph.D., senior research scientistin the UB Department of Chemistry. The research was supported by theDURINT grant and by the National Science Foundation.

Research atUB's Institute for Lasers, Photonics and Biophotonics has beensupported by special New York State funding sponsored by State Sen.Mary Lou Rath.

The University at Buffalo is a premierresearch-intensive public university, the largest and mostcomprehensive campus in the State University of New York.