Seers who seek ways to save lives by detecting cancerous tumours early on using nanotechnology can smile. Last week, American nanotechnology company Evident Technologies announced it would now sell an expanded range of quantum dots, which seem promising for such research. Their products are based on research work done at U of T, by chemistry professor Dr. Greg Scholes.
To understand what the fuss is about, you will first a quick primer in semi-conductor theory. A piece of semi-conductor has two important properties, called conduction bands and valence bands. Think of them as two rows of musical staffs-the upper staff representing the conduction band, the lower the valence band; and some distance separating them, called the bandgap.
Now imagine electrons residing as various levels in these staffs, much like individual notes. By nature, electrons prefer to stay as close to the bottom of the valence band-their so-called lowest energy configuration. They’re natural tenors, you might say.
But one can entice them to go soprano, as it were, by giving them just the right flick of energy. This allows them to clear the gap between the staffs, and make it into the upper staff-the conduction band. While in this band, electrons have broken free of their parent atoms and float around freely, producing a current in the semi-conductor.
When electrons drop down to tenor again, they tend to fall down from the bottom of the upper staff to the top of the lower staff. So in falling from one staff to the other, they nearly always cover the same distance. At the same time, they release they energy-as photons of light of a specific frequency.
In common semi-conductor crystals, the distance between the two staffs is fixed. When manufacturing quantum dots, however, it is possible to set this distance-or bandgap value-which gets them to glow at a specific frequency when subjected to the proper energy stimulus.
“It’s a very small pure crystal of a semi-conductor,” explained Scholes. “When you make these crystals that small, properties become size dependent.”
“We developed a way to make a very high-quality preparation of the lead sulfide that Evident is now selling,” Scholes said. “We can make those particles any particular size,” he added, with great accuracy.
Until now, Evident had been selling based on Lead Selenide (PbSe), but the new dots, said Scholes, glow brighter and last longer.
Since quantum dots can be gotten to attach themselves to certain cells in a tissue, and then made to glow with the appropriate zap, said Dr. Warren Chan, at U of T’s Institute of Biomaterials and Biomedical Engineering, “you can use different colours to screen or label cells in different stages [of health].”
“Based on the colour coding you can tell which are tumorous,” he said. But while demonstrating this concept in vitro has been easy, said Chan, in vivo will take much longer, since materials such as lead are toxic.
