Quantum Dot Nanosensor Detects DNA

Utilizing miniature semiconductor crystals, biologic probes, and a laser, engineers have devised a new technique of locating specific sequences of DNA by making them light up under a microscope. The investigators, who report that the method will have important uses in medical research, established its potential in their laboratory by detecting a sample of DNA containing a mutation linked to ovarian cancer.

The investigators, from Johns Hopkins University (Baltimore, MD, USA), published the new DNA nanosensor study in an article in the November 2005 issue of the journal Nature Materials.

"Conventional methods of finding and identifying samples of DNA are cumbersome and time-consuming,” said Dr. Jeff Tza-Huei Wang, senior author of the article and lead investigator 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 method incorporates a unique combination of organic and inorganic components. "We are the first to demonstrate the use of quantum dots as a DNA sensor,” Dr. Wang said. Quantum dots are crystals of semiconductor material, whose sizes are only in the range of a few nanometers across. (A nanometer is one-billionth of a meter.) They are conventionally utilized in electronic circuitry; recently, however, scientists have begun to examine their use in biologic studies.

Dr. Wang, an assistant professor in the department of mechanical engineering and the Whitaker Biomedical Engineering Institute at Johns Hopkins, led his group in exploiting a significant characteristic of quantum dots: they can easily transfer energy. When a laser shines on a quantum dot, it can pass the energy on to a neighboring molecule, which then emits a fluorescent glow that can be seen under a microscope.

But quantum dots by themselves cannot find and identify DNA strands. To do this, the researchers used two biologic probes composed of synthetic DNA. Each of these probes is a complement to the DNA sequence the researchers are looking for. Therefore, the probes seek out and bind to the target DNA.

Each DNA probe also has an important partner. Attached to one probe is a Cy5 molecule that glows when it collects energy. Attached to the second probe is a molecule called biotin. Biotin adheres to even another molecule called streptavidin, which envelopes the surface of the quantum dot.

To devise their nanosensor, the researchers combined the two DNA probes, plus a quantum dot, in a lab dish containing the DNA they were trying to find. After that, the two DNA probes linked up to the target DNA strand, holding it in a sandwich-like hold. Then the biotin on one of the probes caused the DNA "sandwich” to bind to the surface of the quantum dot.

Lastly, when the investigators shined a laser on the mixture, the quantum dot passed the energy on to the Cy5 molecule that was attached to the second probe. The Cy5 released this energy as a fluorescent glow. If the target DNA had not been present in the mixture, the four components would not have joined together, and the characteristic glow would not have appeared. Each quantum dot can attach to up to approximately 60 DNA sequences, making the combined glow even brighter and easier to see.

To evaluate the new technique, the researchers obtained DNA samples from patients with ovarian cancer and identified DNA sequences containing a critical mutation. "This method may help us identify people at risk of developing cancer, so that treatment can begin at a very early stage,” Dr. Wang said.

Johns Hopkins University has filed for a provisional patent covering the DNA nanosensor technology.


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