Traces of DNA exposed by twisted light

PUBLIC RELEASE DATE:

28-Oct-2013

Contact: Kate McAlpine kmca@umich.edu 734-763-4386 University of Michigan

ANN ARBORStructures that put a spin on light reveal tiny amounts of DNA with 50 times better sensitivity than the best current methods, a collaboration between the University of Michigan and Jiangnan University in China has shown.

Highly sensitive detection of DNA can help with diagnosing patients, solving crimes and identifying the origins of biological contaminants such as a pathogen in a water supply.

"It really does not matter where the target DNA is from," said Nicholas Kotov, the Joseph B. and Florence V. Cejka Professor of Chemical Engineering at U-M. "In order to detect a specific DNA, we just need to know a small portion of its sequence."

Current DNA analysis methods rely on copying segments of a strand of DNA. The process unzips the double helix and then short, lab-made 'primer' DNA strands attach to each half of the original DNA. These primers kick-start the copying process, using the unzipped DNA as a template. Targeted segments of DNA can be replicated in this way, doubling every cycle. If enough DNA is produced before copying errors become a major problem, then further analysis can show whether the sample matches a suspect, for example.

But if the primers were very selective for the suspected DNA sequence, then a match could be determined by simply detecting whether the DNA had copied or not. Studies revealed that small amounts of DNA could be observed when spherical gold nanoparticles were attached to the primers. If the DNA matched suspicions, strings of particles bound together with DNA would form in the replication process. The nanoparticle solution would change color from red to blue, due to the way the strings of particles interact with light.

"Impressive detection limits were attained for short DNAs with nanoparticles; however, not for long DNA," Kotov said.

The problem, he explained, is that if the particles are further apart than a few nanometers, or millionths of a millimeter, "they do not interact strongly and the blue color does not happen." Longer strands are needed to differentiate between species and individuals with greater accuracy.

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Traces of DNA exposed by twisted light