Portable DNA analyzer to use GaN LEDs
There is little doubt as to the importance of DNA analysis in today s society. A recent example would be the long-awaited conviction last month of the two brothers responsible for killing Damilola Taylor, a 10-year-old boy from London, UK, in 2000. The key piece of evidence – a small blood stain on one of the killer s shoes – was crucial to the prosecution s case.
Now, UK researchers are trying to push the boundaries of DNA analysis through the application of optoelectronics. In a joint project between the University of Hull and the Centre for Integrated Photonics (CIP), together with £721,000 ($1.37 million) of funding from the Engineering and Physical Sciences Research Council, they are set to pioneer the use of high-brightness LEDs in integrated modules to optically detect DNA fragments. If the approach works, these portable DNA analyzers could revolutionize forensic science by giving scene-of-crime officers in situ access to genetic information.
Currently, DNA analysis is a notoriously inconvenient affair. Evidence must be taken to a laboratory and scrutinized, then possible samples must be subjected to a laborious profiling procedure involving fragmentation and polymerase chain reaction amplification. In the final step, the amplified fragments are separated using electrophoresis, whereby an electric field selectively pulls at the molecules based on their size and shape. It is the layout of the dispersed fragments left that characterizes the DNA "fingerprint" of the sample.
Apart from the obvious time expenditure, transporting samples to the laboratory leaves them dangerously susceptible to contamination. The device that CIP hopes to produce will combine all the stages together in one shoebox-sized unit – simultaneously circumventing possibilities of time-delay and contamination. It will also require an automated detection stage to output the fingerprint without human intervention.
According to Steve Oliver, project leader at CIP, units like this could be cheap enough to hand out to police officers for ruling out suspects within minutes. "It won t tell you their name and address, but it will tell you if they re human, Caucasian or not, male or female – that kind of thing."
The project has been split into two – the biological part will be done by the Hull team. The other part, which entails electrophoretic separation and optical detection of the DNA fragments, will fall onto CIP s shoulders.
"As far as CIP goes, the tricky part will be the optical detection," explained Oliver. "The idea is that you tag these pieces of DNA with different dyes. Depending on the mix of dye that comes through, you can tell something about where the DNA came from."
Once the DNA fragments have been tagged, the dyes will be illuminated using the light of a wavelength that they can absorb. This will stimulate fluorescence, where light is re-emitted by the dye at a longer wavelength. The fluorescence signals then have to be filtered and fed to an optical detector and data processing system.
Sensing the color of these dyes will require components that satisfy two important criteria: cost and reliability. Presuming that it works, the only way that such an analyzer can be justified is if investigators can routinely carry them to crime scenes. CIP could produce bespoke light sources in-house, but these run the risk of being delicate and expensive, so instead the team is looking to commercial LEDs for the solution.
"We don t want to design a system and then build a light source to match it," explained Oliver. "In the first instance at least, we want to see how far we can go with what s commercially available."
Given that the tagged DNA fragments will be absorbing visible wavelengths between 400 and 500 nm, the obvious candidates for the light source are undoubtedly GaN LEDs. These could either be made into an array, with each LED corresponding to a different color of dye, or – preferably – the analyzer could rely on a single, superluminescent LED combined with several filters to cover all of the necessary wavelengths.
CIP s strong pedigree in optical telecoms should give it a firm grounding for developing these filters and squeezing the design into a manageable package. But the DNA analysis project is outside of its normal scope.
"There s been a lot of interest in microfluidics, but until now the devices have been quite simple," explained Oliver. "We re trying to do something with the integration technology and then apply it in that field. This is a deviation from the norm for us – we ve never done anything in the biological sciences before."
