Wednesday, November 21, 2012

DVD trick makes for cheap boost to imaging and sensing

Researchers in the US have hugely boosted the sensitivity of a widespread imaging and sensing technique, using an off-the-shelf DVD.
Fluorescence spectroscopy is used to diagnose disease, image inside cells or detect toxins - but signals are weak.
A report in Nanotechnology has outlined how to boost those signals 200-fold, making use of a DVD's ready-made pits and grooves.
That could help put the technique in far more labs - or hands - worldwide.
Fluorescence describes the process in which the energy of light put into a molecule is redistributed, leading to a relatively long-lasting "glow" - it is in fact the process behind so-called glow sticks.
This has turned out to be a tremendously successful way to examine the world down at the molecular level. Scientists can "tag" molecules with a part that fluoresces and then use microscopes to see exactly what it is doing.
Some of these tags only switch on when a particular chemical is present - for instance a toxin or an explosive.
Lightning rods At that molecular level, the light signals are profoundly weak. One way to boost them developed in recent years is to make use of what are called surface plasmons - the propensity of light waves from a sample to excite electrons in a surface beneath it, thereby boosting the signal.
This surface plasmon resonance is itself improved when the surface has nanometre-scale structure - tiny bumps or features of a size not too different from the wavelength of the light.
Making these nanometre-scale features for the task has been an inherently complex and expensive business, but there is one industry in which the costs of such precision-made structures have plummeted: DVDs.
Shubhra Gangopadhyay of the University of Missouri-Columbia and her colleagues started with commercial DVDs - sourced from a nearby store for $10 - and removed the protective outer layer to expose the nano-structure beneath.
They coated the DVDs with a stretchy polymer that dries to form something like a rubber stamp with an almost-perfect imprint of the DVD's surface, which can be ink-stamped onto any surface and then coated with a thin layer of metal.
"Almost-perfect" because the process of peeling the stamp from the DVD creates tiny imperfections, what the team call nano-gaps, just billionths of a metre wide.
Prof Gangopadhyay said that these gaps act like "lightning rods", drawing together and intensifying the fluorescence signal, making it easier to detect.
In recent experiments, she said, DVD-stamped surfaces have shown a 30-fold jump in light from the molecules of interest - analytes - whereas the nano-gaps boost that signal by 200 times.
"Usually when people try to look for low concentrations of analytes, they have to use very expensive microscopes, costing over a million dollars," she told BBC News.
"The interesting thing is that this can be integrated with simple microscopes - that's what we're using."
The team is pursuing a two-fold goal with the new technique.
"One is to give it to researchers, so everyone who has a simple microscope can utilise it to do great science - including high school kids," Prof Gangopadhyay said.
Secondly the team aims to put the idea to use in simple, cheap diagnostics for use in the developing world - looking for the molecular markers for the HIV virus, toxins, even explosives.
"This field can go far in the future," she said. "I think it has big implications."

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