Making quantum dots less toxic broadens users’ options

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Aug. 2, 2005 — Today quantum dots shine in the life sciences, where researchers tack them onto molecules in cells and use their fluorescing properties to track their movement. The tags allow scientists to spy on cellular processes and better understand the inner workings of biological systems.

But quantum dot manufacturers have their sights on much larger and more lucrative markets: solar cells, electronics and even diagnostics. Manufacturers have recognized that in order to achieve their goals, though, they’ll need a much different quantum dot than those used in research.

It’s one thing to put potentially toxic nanocrystals in a Petri dish. It’s another to introduce cadmium or other heavy metals found in quantum dots into solar devices, displays and the human body.

In late spring, two companies based in New York independently announced that they had found a solution, one by engineering around the problem and the other by creating a new kind of fluorescing nanoparticle. A third company in California also is working to produce a cadmium-free quantum dot.

“The need has been driven by our customers,” said Clinton Ballinger, chief executive of Evident Technologies in Troy, N.Y. Evident made a quantum dot that contains no heavy metals commercially available in late May. The new line of its EviTags is being targeted at life science researchers who want to use labels within living organisms. But Ballinger said that companies in Japan and Europe also were interested in testing the new line for energy and lighting applications.

“Japan and Europe have a strict limit on cadmium and lead in electronic and optoelectronic devices like displays. They don’t want to have cadmium in those devices,” Ballinger said. Pharmaceutical and other biomedical researchers also worry that cadmium may poison cells in assays.

“In life science applications, the customers question how smart it is to put cadmium in,” he said. “Cadmium kills cells.”

Evident devised a new method for making the nanoparticles that are the core of a quantum dot. Dots, if they are sufficiently small, emit specific colors when exposed to light waves. The size and composition of the dot determines the color. Making dots typically requires two materials such as dimethyl cadmium and selenium.

Evident uses a three-material system instead to produce indium gallium phosphide quantum dots that are then encased in a zinc-metal shell. The shell is designed to bind to the core to produce a bright and long-lasting fluorescence. Evident adds a coating to the shell to make it more stable in water and in harsh environments.

Its near-term goal is to provide dots for life science research. But the company’s mid-term and long-term plans include the larger and more lucrative markets of diagnostics, sensors and eventually light emitting diodes and solar cells. “This is mostly driven by Japan’s optoelectronics,” Ballinger said. “They’ll need more dots than biotech will ever use.”

In the future, customers may have other options as well. Researchers at Cornell University announced in May that they had made a silicon-based nanoparticle that fluoresced like quantum dots. The nanoparticles, which are akin to glass beads with dye in them, were made at room temperature using solvents like water and alcohol rather than heavy metals.

“The advantages clearly are that the synthesis is easy and the conditions are relatively benign,” said Ulrich Wiesner, a professor of materials science and engineering at Cornell. He also is co-founder of Hybrid Silica Technologies, a startup based in Ithaca, N.Y., that intends to commercialize what they’ve named Cornell dots. “It’s a nice approach. … The surface chemistry of silica is well known, so we can functionalize (add onto) them with everything under the sun.”

Wiesner and his team attached dye molecules to a nanoparticle core that is then encased in silica. While that technique is not novel, the Cornell team has been able to use it to make dots in the 20- to 30-nanometer range. Then they mastered a system for making dots of a uniform size and dispersion.

The dots initially produced only a dim light. But when they added the silica shell, “the brightness went through the roof,” Wiesner said. They realized they had a possible substitute for quantum dots when a brightness test showed their dots to be a contender. “Now we hope to close the gap or even exceed it,” he said.

Wiesner, former student Hooisweng Ow and Cornell alumnus Kenneth Wang created Hybrid Silica Technologies in late 2003 and remain in the early stages of building a business. But Wiesner said they already have corporate support and have been approached by companies that see possible uses for their dots. Their next challenge is to develop quality control mechanisms.

In the meantime, Quantum Dot Corp. also is exploring ways to eliminate cadmium from biotech products. The Hayward, Calif.-based company has been involved in medical projects that could lead to diagnostic applications such as a quantum dot-based test for detecting and monitoring diseases of the eye. By removing the potentially toxic cadmium, the company hopes to avoid time-consuming and expensive tests that would be required by the U.S. Food and Drug Administration.

Ballinger and Wiesner agree that the quantum dot industry will benefit from having several suppliers whose products can stand up to customers’ scrutiny. “We can’t have a quantum dot industry without competition,” Ballinger said. “This is new to the end users, and if they only see our company, that doesn’t bode well.”


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