Diagnosis on the Dot
Quantum dots are really, really small.
But University of Toronto scientist Ulli Krull is showing how they’ll make a really big difference in diagnosis.

They’re unimaginably tiny, oddly behaved—and incredibly versatile. “Quantum dots” are artificial assemblages of atoms that are revolutionizing fields as diverse as biology, medicine, computing and power generation.

Depending on its structure, a quantum dot can be as small as 20 atoms across, or about 2 billionths of a meter. “You’re getting down to molecular dimensions in size,” explains Dr. Ulli Krull of the University of Toronto at Mississauga.

“And when you do that you get some interesting properties that you don’t see at the level of the everyday world.”

One of those properties involves the way quantum dots respond to light. In the world of our everyday experience, the colour of light that objects reflect or emit is based on what they’re made of. But at scales involving billionths of a metre, we enter the unintuitive realm of quantum effects. When you shine a light on a quantum dot, the colour of light that comes back is determined by the dot’s size. And since the size can be precisely set during fabrication, dots can be “tuned” to emit a specific colour when excited by light.

This combination of diminutive size and “tunability” has made these particles very useful as optical “biosensors”—beacons that light up to indicate the presence of even the tiniest amounts of a particular substance—a virus, for example, or a marker for a type of cancer. Scientists accomplish this magic by wrapping a quantum dot with a layer designed to bond chemically with the target substance, and by adding a biological switch that disables the light-emitting capability of the dot unless this bonding has taken place. Once the target substance bonds to the particle, however, the modified dot glows with a distinctive colour when it’s exposed to light.

These biosensors could, in theory, be released in quantity into the body to find and then illuminate their quarry. But it’s difficult to read the light of the beacons accurately through layers of tissue and fluid.

Dr. Krull is also looking at a solution to this problem. At UTM’s Centre for Applied Biosciences and Biotechnology— a facility funded partially through an investment by the Ontario Innovation Trust—he’s developing a way to attach the nanoscale sensor particles to the outer surface of a fibre optic probe. The probe can be inserted into a sample—and perhaps one day, into a human body. Light pumped into the fibre will cause the beacon dots to glow if the target substance is present, and the same fibre carries their tell-tale colour back to the observer.

By covering the probes with a variety of sensor particles designed to detect different substances, they could be very versatile tools for diagnosing a range of illnesses at their earliest stages. Two other big advantages: the results would be available in minutes, and the probes would be reusable. Neither is the case with current methods of lab testing.

The technology has many other applications as well, including testing for pathogens in food and water, and for environmental pollution. While it will be a few years before these kinds of probes are in daily use, Dr. Krull can see them on the horizon—and he’s excited.

“There are all these things going on at the molecular level, and as we learn to harness them, we’re going to see some very exciting technical and chemical advances.

“That’s what turns me on. We’re at the stage now of being about to harness those properties.”