Quantum dots to detect DNA

Engineers from Johns Hopkins University have developed a new ultrasensitive, quick and easy technique to detect DNA. This system is the first one to use quantum dots to detect DNA. This method could help to identify people at risk of developing cancer in very early stages.

Detection of DNA is critical in diagnosing genetic diseases, but current approaches are somewhat cumbersome and time-consuming. So engineers from Johns Hopkins University have developed a new ultrasensitive, quick and easy technique to detect DNA. This system is the first one to use quantum dots to detect DNA. The device, based on fluorescence resonance energy transfer (FRET), was already tested to detect a critical mutation of ovarian cancers. This method could have important uses in medical research, such as identifying people at risk of developing cancer in a very early stage.

Here are some quotes from the lead researcher, Jeff Tza-Huei Wang, about methods used to identify samples of DNA.

"Conventional methods of finding and identifying samples of DNA are cumbersome and time-consuming," said Jeff Tza-Huei Wang, senior author of the paper and supervisor of the research team. "This new technique is ultrasensitive, quick and relatively simple. It can be used to look for a particular part of a DNA sequence, as well as for genetic defects and mutations."
The technique involves an unusual blend of organic and inorganic components. "We are the first to demonstrate the use of quantum dots as a DNA sensor," Wang said

But what are quantum dots and what can they be useful for?

Quantum dots are crystals of semiconductor material, whose sizes are only in the range of a few nanometers across. They are traditionally used in electronic circuitry. In recent years, however, scientists have begun to explore their use in biological projects.
Wang, an assistant professor in the Department of Mechanical Engineering and the Whitaker Biomedical Engineering Institute at Johns Hopkins, led his team in exploiting an important property of quantum dots: They can easily transfer energy. When a laser shines on a quantum dot, it can pass the energy on to a nearby molecule, which in turn emits a fluorescent glow that is visible under a microscope.

Below is a diagram describing how work these single-QD-based DNA nanosensors (Credits for image and caption: Johns Hopkins University).

Single-Quantum-Dot-Based DNA Nanosensor

. a, Conceptual scheme showing the formation of a nanosensor assembly in the presence of targets. b, Fluorescence emission from Cy5 on illumination on QD caused by FRET between Cy5 acceptors and a QD donor in a nanosensor assembly. c, Experimental setup.

Here are some more details about these DNA probes.

Each DNA probe also has an important partner. Attached to one is a Cy5 molecule that glows when it receives energy. Attached to the second probe is a molecule called biotin. Biotin sticks to yet another molecule called streptavidin, which coats the surface of the quantum dot.
To create their nanosensor, the researchers mixed the two DNA probes, plus a quantum dot, in a lab dish containing the DNA they were trying to detect. Then nature took its course. First, the two DNA probes linked up to the target DNA strand, holding it in a sandwich-like embrace. Then the biotin on one of the probes caused the DNA "sandwich" to stick to the surface of the quantum dot.

The research work has been published by 'Nature Materials' under the title "Single-Quantum-Dot-Based DNA Nanosensor" (Volume 4, Issue 11, Pages 826-831, November 2005). Here are two links to the first paragraph and to the full paper (PDF format, 6 pages, 356 KB) from which the above diagram has been extracted.

And for more information, you can visit Jeff Wang's BioMEMS and Single Molecule Dynamics Lab.

Sources: Johns Hopkins University news release, December 5, 2005; and various web sites

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