Capturing The Glow Of Quantum Dots With A Smartphone

Quantum dots—bits of semiconductor materials a few nanometers in diameter—are useful as optical tags for biological assays. They luminesce in different colors depending on their size and composition, have long shelf lives, and don’t fade from repeated exposure to light like other tags. But because labs have needed expensive equipment to detect individual wavelengths of light, they often stick with older tagging methods. Now, researchers have shown that today’s sharper smartphone cameras do just as well as specialized fluorescence instruments for analyzing quantum dots (Anal. Chem. 2014, DOI: 10.1021/ac500131r). The results could lead to cheaper, more sensitive tests that could be performed in doctors’ offices or other places with limited access to laboratory equipment.

Eleonora Petryayeva and W. Russ Algar, chemists at the University of British Columbia, put together an assay to detect three different proteases using a technique called fluorescence resonance energy transfer (FRET). They used dots with a cadmium-selenide-sulfide core surrounded by a zinc-sulfide shell that emitted red, green, or blue light. They then attached the dots to peptides that interact with the three proteases, using a different color for each. Finally, they attached dye molecules to the peptides that quenched the fluorescence of the quantum dots, reducing their light emission.

To test the assay, they shone a handheld ultraviolet light on a solution containing the quantum dots mixed with the proteases. As proteases cut the peptides, the dye molecules moved away from their associated quantum dots, allowing the dots to brighten. An iPhone captured images of the solution at 20-second intervals, with filters to block the UV light, which allowed the chemists to watch the proteases at work.

The study is just a proof of concept, Algar says, and the next step is to optimize the system for an existing diagnostic test. Researchers have yet to develop many of the tests that could benefit from this approach, he says. But because smartphones have sufficiently sensitive cameras—plus the ability to store, process, and transmit data—they offer a way to make tests based on luminescent labeling much more portable and affordable.