Rapid Quantum Dot Synthesis Suggests Biomedical, Optoelectronic Uses

Daniel S. Burgess

Scientists at State University of New York at Buffalo have developed rapid, single-pot methods for the chemical synthesis of InP and PbSe quantum dots. The work promises to have an effect on the production and application of quantum dots for biological and medical imaging and in the development of optoelectronic devices that are based on the particles, including photovoltaic cells.

Researchers have developed rapid, single-pot methods for the chemical synthesis of quantum dots, including luminescent InP dots that may be suitable for biological and medical imaging. The III-V quantum dots, which are capped with ZnS, were synthesized in six to eight hours.

“Both synthesis methods are much faster and far more efficient than existing methods and are truly scalable,” said Paras N. Prasad, executive director of the university’s Institute for Lasers, Photonics and Biophotonics. This scalability, he said, will be important for the mass production and widespread application of the semiconductor particles.

For example, although the lower apparent cytotoxicity of III-V quantum dots, such as those composed of InP, makes them superior to their II-VI counterparts for biological purposes, their synthesis has been tedious and slow, Prasad explained. But by using customized precursor materials, the researchers have reduced the need for an external surfactant, so the time required to produce hydrophobic InP dots capped with ZnS and conjugated to folic acid is only six to eight hours.

An additional benefit, he noted, is an increase in the luminescence quantum efficiency to 10 to 15 percent. Common surfactants can adversely influence the optical characteristics of quantum dots.

To demonstrate the potential of the III-V structures for biological imaging applications, the investigators used confocal and two-photon fluorescence microscopy to monitor the dots’ internalization in KB cells, a human nasopharyngeal epidermal carcinoma that overexpresses folic acid receptors, and in A549 cells, a human lung carcinoma that lacks a folic acid receptor.

They found that the quantum dots accumulated in multivesicular bodies in the KB cells and could be imaged using the two-photon technique, which suggests that cellular processes could be imaged over extended periods with less photodamage to the cells than methods that employ ultraviolet excitation.

In another demonstration of the potential of the synthesis procedure, the researchers produced PbSe dots with absorption spectra of 1.1 to 1.9 μm. They blended the semiconductor structures with poly-N-vinylcarbazole and spin-coated the mixture to form 300- to 350-nm-thick films onto an ITO-coated glass substrate, atop which they deposited aluminum electrodes.

Using 1.34- and 1.55-μm CW laser diodes to excite the completed devices in photoconductivity experiments, they measured a maximum photoconductivity quantum efficiency at these wavelengths of approximately 3 percent at an applied device bias of 34 V, which is an improvement of more than two orders of magnitude over previous efforts with such IR-responsive quantum dot/polymer composites.

Employed in a photovoltaic system, such as one featuring quantum dots of the appropriate sizes to absorb the desired wavelengths in the visible and near-IR, the materials could extend the sensitivity of solar cells. Prasad also noted that he and his colleagues have demonstrated quantum dot/polymer composites that are photorefractive at telecommunications wavelengths, which he suggested could be useful in beam cleanup, optical data storage or amplification.

Studies of the toxicity of III-V quantum dots are under way, he said, to determine whether they may be used in vivo. Other work will investigate means of preserving luminescence in complex biological environments and of controlling the surface characteristics of the dots to target particular tissue sites. The researchers also hope to enhance photogenerated carrier mobilities in the quantum dot/polymer composites by engineering the dot/polymer interface and by incorporating other materials, such as pentacene, into the polymer matrix.

Journal of the American Chemical Society, Aug. 17, 2005, pp. 11,364-11,371. Applied Physics Letters, Aug. 17, 2005, 073110.