Evident Technologies' quantum dots come in many colours.

As an alternative to organic dyes, quantum dots (QDs) may be worth a closer look. These semiconductor nanometre-sized crystals, typically with a cadmium-based core, avoid some of the shortcomings associated with traditional organic dyes and fluorescent proteins (see Nature 413, 450; 200110.1038/35097256). QDs are brighter, not prone to photobleaching, and come in a wide range of colours. By changing the size of the particle, the emission can be tuned to any wavelength, from ultraviolet to infrared. QDs also have narrow emission spectra, which means more colours can be used at a time with minimal channel overlap. Multiple colours of QDs can be simultaneously excited using a single light source. QDs have, for example, been used to to track the activity and diffusion of individual glycine receptors in the neuronal membrane for up to 20 minutes — considerably longer than is possible with Cy3 dyes.

“We don't expect QDs to completely displace organic dyes,” says Clint Ballinger, chief executive of Evident Technologies in Troy, New York. “They make an attractive alternative for many different applications.” Evident began by making ultrafast optical switches for the telecom industry. But when the bottom fell out of that business, the company turned to alternative applications for its nanocrystals. It sells its QDs (EviDots) as EviTags, which are water-stabilized, conjugation-ready QDs that can be coupled to proteins or antibodies, and as EviFluors, which have biotin groups.

Another company with know-how in this area is Quantum Dot Corporation (QDC) of Hayward, California. Its product line includes basic Qdot nanocrystals, Qdot bioconjugates coupled to proteins, oligonucleotides and small molecules (streptavidin, protein A and biotin), and Qbead microspheres. In June, QDC introduced a kit that enables researchers to conjugate their own antibodies to Qdot nanocrystals — a procedure that the company says typically takes 3–4 hours. The kit can also be used to couple other thiol-containing molecules to QDs.

So what of the future? “Some customers don't like cadmium,” says Ballinger, and Evident plans to launch indium phosphide-based EviDots in the next few months as an alternative to cadmium for the visible range.

Diane Gershon