A portable instrument based on an ultrasensitive nanoscale sensor could detect bacteria in minutes, helping to catch infectious diseases early and prevent their spread. The simple low cost device should be available within three years, says Benjamin Miller, professor of dermatology and biomedical engineering at the University of Rochester Medical Center.Right now, diagnosing common bacterial infections requires growing cultures in a laboratory over a period of days, but diagnosis could be greatly speeded by a number of new sensors based on various nanomaterials that are being developed for ultrasensitive, rapid DNA detection. The new instrument would take from 15 minutes to two hours for a diagnosis and could be used in doctor, and hospitals.
Each sensor is a hairpin-shaped strand of DNA, complementary to the genetic sequence being targeted, that is fixed on a gold film. Gold quenches the glow of a fluorescent molecule attached to one end of the DNA. Its unfolding results in the fluorescent molecule moving away from the gold film and glowing, which can be seen under a fluorescent microscope.
A blood or urine sample to be tested would be placed directly on the cartridge. The catridge will be a lab-on-a-chip, with rapid, miniaturized ways to prepare the sample for testing. "In the cartridge there are steps for cleaning up samples, that is, extracting material you're interested in and amplifying the (bacterial) DNA, will then be placed in small portable instrument that does the fluorescence imaging and analysis, each catrige should cost a few dollars, Miller says.
By attaching different DNA strands on the gold film, the same cartridge could screen for multiple pathogens. So far, the researchers have made a sensor to detect antibiotic-resistant staph bacteria that cause skin infections. They are now working on detecting bacteria responsible for common urinary-tract infections. The sensors could also be used to quickly spot bacteria in food or bioterror agents in water supplies or even to screen for genetic disorders or cancer.
The silver nanoparticles make the fluorescent signal 10 times brighter. Plus, because thin layers of silver nanoparticles are transparent, the sensor could be coated on glass and optical fibers to make new types of detecting instruments.
In the other nanosensors being developed for ultrasensitive, rapid DNA detection, researchers are using carbon nanotubes, nanowires and nanoparticles. All of these approaches promise high accuracy, portability, and low cost. "If you could make a portable device that would sit in your doctor's office, then, using a small amount of fluid, your doctor could screen you for a genetic abnormality.
Nanosphere's sensor is a microarray coated with DNA strands complementary to the target DNA and incorporated into a test cartridge. Gold nanoparticles, also coated with complementary DNA, are introduced, followed by target DNA, which binds to both the microarray and a nanoparticle. Then the nanoparticle is coated with silver to amplify the light that is scattered from the particle; the light is captured using a digital camera sensor. This method of detection is 100,000 times more sensitive than detecting fluorescence, says William Moffitt, CEO of Nanosphere.
Miller calls Nanosphere's technology fantastic. However, he adds, Lighthouse Biosciences's diagnostics test is simpler and requires fewer steps.
(by Prachi Patel)
(www.technologyreview.com)
