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Nanocrystals enable diverse applications: the unique ability of nanocrystals to have their properties fine-tuned by altering their size and composition allows for multiple applications, prompting a growth in demand.

Nanocrystals of semiconductor materials, such as quantum dots, can be tailored, similar to bulk light-emitting materials, to absorb and emit different colors. What sets them apart is the ability to fine-tune this optical behavior by modifying their size. An added advantage is the need for only one excitation source, such as a laser, for a variety of diverse-sized dots.

Quantum dots' narrow emission spectra allow a large number of dots to be simultaneously detected, without any overlap in colors. This capacity to readily and simultaneously employ quantum dots of varying colors makes multiplexed analyses possible and enhances their information. Additionally, quantum dots photobleach slowly, leading to improved sensitivity. This sensitivity is further increased by their brightness, stemming from the absorption of the majority of the irradiation light and the subsequent reemission of most of that light. All these characteristics make working with quantum dots attractive, leading to a rise in demand.

Biological beacons

The heightened interest is evident to Quantum Dot Corp. (QDC), Hayward, Calif., who sees a "dramatic growth in the biological uses of nanocrystals," says Andy Watson, VP of business development at QDC. Focusing on biological detection applications in life science and drug discovery, this company offers semiconductor quantum dots, or Qdot nanocrystals, that are covalently linked to various biomolecules. These water-soluble conjugates, such as streptavidin, biotin, protein A, and secondary antibodies, are produced from standard conjugation procedures and employed as probes in biological tests.

Linking different ligands to the Qdot nanocrystals enables multiple assays to be simultaneously performed, reducing cost and time. Such multiplexing was recently implemented in a collaboration between QDC and NASA's Jet Propulsion Laboratory (JPL), Pasadena, Calif., where fluorescent quantum dots were incorporated in lateral flow assays (LFAs). This alliance led to LFAs capable of multiple, quantitative analyses with enhanced sensitivity.

QDC's nanometer crystals consist of color-controlling "cores" formed by healing mixtures of semiconductor materials, generating 2- to 5-nm-dia crystals in just a few minutes. The selected semiconductor material lightly regulates the emission wavelength region. To obtain emission in a specific wavelength area, the size of the nanocrystal is adjusted. For instance, 5.5-nm-dia cadmium selenide (CdSe) yields 630-nm emission, while 3-nm-dia CdSe leads to 520-nm emission, with in-between sizes producing intermediary colors (up to ten separate quantum dot colors can be generated using these two seminconductors).

Once created, these cores are next covered with multiple inorganic "shells" that stabilize the nanocrystals, protecting them from their surroundings and enhancing their optical characteristics--one shell layer can boost the photostability tenfold. The last step involves applying an organic surface coating to make the nanocrystals compatible with biological analyses. The end result are buffer-soluble Qdot nanocrystals, which connect to biomolecules with minimized nonspecific binding, that can be employed as "bar codes" to yield spectrally multiplexed assays.

Currently, QDC is developing numerous multiplexed assays through the use of cells and polymer latex beads.

Versatile applications

A similar increase in interest in semiconductor nanocrystals is experienced at Evident Technologies, Troy, N.Y. Although requested by university research and teaching laboratories, the main demand for these nanomaterials is from the industrial sector. To accommodate its consumers, the company recently opened a nanotechnology production center in Watervliet, N.Y., where kilogram amounts of these nanocrystals are produced weekly to exacting standards.

Similar to QDC, Evident's quantum dots can be attached to secondary molecules like nucleic acids or proteins for biological tests. However, they can also be removed from the solvent to yield self-assembled thin films, or linked with polymers and cast into films for solid-state instrument applications. With such versatility, these nanocrystals "offer an unprecedented degree of freedom to engineer materials for applications," says Steven Talbot, VP of marketing at Evident Technologies. "They provide our customers the ability to tune both optical and electronic properties of their applications."

Evident's semicondutor nanocrystals are 2- to 10-nm-dia, composed of lead selenide, cadmium telluride (CdTe) with or without cadmium sulfide shells, and CdSe with or without zinc sulfide shells. To extend the usefulness of the nanocrystals, extra surface materials are added to make them water stable and compatible with solvents, insulating and conducting polymers, as well as other matrix materials.

Finally, although not yet commercialized, Evident Technologies has "demonstrated" nanocrystals that consist of zinc selenide, zinc sulfide, lead sulfide, and cadmium sulfide.

Shifting to a smaller scale of production, NanoCo, Manchester. UK, generates gram quantities of quantum dots. It specializes in the formation of nanocrystals consisting of the selenides or sulfides of the majority of metals, particularly zinc, cadmium, gallium, and indium. These nanomaterials are yielded through a proprietary scalable method that uses single-source precursors and does not require highly toxic chemicals. The company anticipates the application of its nanoproducts to be mainly centered on the biomedical, security, and electronic market.

Rational fabrication

At Nanosys, Inc., Palo Alto, Calif., cone-, teardrop-, and tetrapod-shaped nanocrystals of industrially significant inorganic semiconductor materials (groups II-VI, III-V, IV), their alloys, as well as transition metal oxides are rationally configured and manufactured. This technology uses proprietary computer modeling and synthetic techniques to custom design inorganic semiconductor nanomaterials with specific properties. These characteristics include crystal structure, size, shape, composition, as well as surface and doping chemistry properties. The rational design and synthesis technology "allows us to build new functioning inorganic semiconductor nanostructures with very short development time," says David Zaziski, business development associate at Nanosys.

Once produced, these nanomaterials find initial applications in different areas. One such application employs a new nanocomposite technology that produces lightweight, and efficient solar cells by joining nanocrystals with a flexible host-matrix. This is expected to affect various photovoltaic sectors, ranging from satellite power to solar-incorporated building supplies.

Radiant future

Nanocrystals are set to play a major role in the future as their use becomes more widespread. QDC, for example, is currently pursuing business alliances that will help place its quantum dot products in the medical diagnostics field. At Nanosys, Zaziski feels that nanomaterials like nanocrystals will cause a revolution in numerous fields similar to the one brought about by the invention of plastics.

The industries expected to be affected by such a revolution are varied since "within five years, data is going to be moved with switches based on nanocrystals, diagnostics of disease will be done with nanocrystals, your office and household lighting will be driven by nanocrystals, and many other applications will have functionality improved and cost reduced through the use of nanocrystals," concludes Talbot.

Resources

Angstrom Medica, Inc., 617-454-6026, www.angstrommedica.com

Evident Technologies, 518-273-6266, www.evidenttech.com

Quantum Dot Corp., 510-887-8775, www.qdots.com

Nanoco, 44(0)161-606-7254, www.nanoco.biz

Nanosys, Inc., 650-331-2100, www.nanosysinc.com

NASA Jet Propulsion Laboratory, 818-354-4321, www.jpl.nasa.gov

Sandia National Laboratories, 505-844-8066, www.sandia.gov

RELATED ARTICLE: Nanocrystals imitate nature.

A team of scientists at Sandia National Laboratories, Albuquerque, N.M., is using synthetic processes to generate intricate nanocrystals that resemble seashells and diatoms by following the same principles used in nature. The goal is to attain structural control similar to that of nature by means of environmentally friendly chemical reactions.

Since natural products are formed without considerable waste and at a low temperature, the researchers limit themselves to aqueous studies, involving low temperature and minimal concentrations of chemicals. Under these conditions, full control of how and where nanocrystals develop has been achieved by selective activation of the exact surface to be grown and spontaneous generation of intricate 3-D structures, which cannot be produced otherwise.

Imitation of nature is also taking place at Angstrom Medica, Inc., Newton, Mass., where calcium phosphate biomimetic nanocrystals are being used for the orthopedic field. It is the only company generating synthetic bone nanocrystals that "look, taste, feel, smell and sound just like native bone nanocrystals," says Edward Ahn, CTO at Angstrom Medica. Besides being similar to native bone's morphology, size, chemistry and composition, these nanocrystals are bioactive. Once implanted, they are recognized as bone, and are consequently bonded, resorbed and replaced with native bone tissue.
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Title Annotation:Emerging Technologies
Author:Sidawi, Danielle
Publication:R & D
Date:Oct 1, 2003
Words:1318
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