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Glowing tags to reveal hidden prints

By Paul Rincon
Science editor, BBC News website

image captionThe technique, demonstrated here on stainless steel, produces a negative image of the print

Scientists have described a new system for visualising "hidden" crime scene fingerprints.

Despite several enhancement techniques already in use, only about 10% of fingerprints from crime scenes are of sufficient quality to be used in court.

The technique is based around fluorescent chemical "tags" and works on metal surfaces, meaning it could be used on knives, guns or bullet casings.

Details were outlined at the Faraday Discussions lecture series in Durham.

"Notwithstanding DNA, fingerprints are still the major source of identification in criminal investigations," co-author Prof Robert Hillman, from the University of Leicester, told BBC News.

"When someone asks: 'Haven't we been doing this for a century, why do we need another method?' Our answer is: 'To image the 90% we don't currently get'."

Although the technique currently works only with metallic objects, Prof Hillman said these were "quite signficant" forensically - especially in violent crimes.

The classical approach to enhancing the visibility of hidden, or - to use the correct forensic terminology - latent, prints is to apply a coloured powder that adheres to the oily residue left by the finger surface.

This then provides a visual contrast to the underlying surface.

The new technique visualises fingerprints by exploiting the fact that their ridges do not conduct electricity.

Here, the fingerprint material acts like a stencil, blocking an electric current that is used to deposit a coloured film.

The film is directed to the regions of bare surface between the ridges of a fingerprint, thereby creating a negative image of the print. The substances used to do this are "electrochromic", which means they change from one colour to another when subjected to an electric voltage.

The researchers say the technique is highly sensitive, as even tiny amounts of insulating residue, just a few billionths of a metre, can prevent the coloured film from sticking to the metal below.

As a result, much less fingerprint residue is required than is typical for other techniques. Also, because it focuses on the gaps between the fingerprint ridges, it can be used in combination with existing, powder-based approaches.

"In principle, it allows us to have two bites of the cherry," said Prof Hillman.

"If one mechanism doesn't work, you haven't jeopardised another."

image captionExisting techniques to retrieve latent prints involve "dusting" surfaces

The team has developed this technique further by incorporating within the film what are known as fluorophore molecules.

These re-emit light of a third colour when exposed to a source of visible or ultra-violet (UV) light.

The researchers claim that combining the electrochromic and fluorescence approaches provides a significantly wider palette with which to "colour" their films.

They verified that the technique worked using neutrons - subatomic particles with no net electric charge. The researchers used neutron-producing reactors at the Institut Laue-Langevin (ILL) and ISIS facilities in Grenoble, France, and Didcot, UK, respectively.

Dr Rob Barker, instrument scientist at the ILL, said: "Neutrons are an ideal tool for understanding what is going on inside these complex systems."

He added: "This allowed us to non-invasively probe on a nanometre scale deep into the sample from the top surface of the polymer to the metal below and follow the marker molecules as they entered the polymer film."

"All the technology to do this fluorescence imaging is there [in forensic labs]," Prof Hillman told BBC News.

"If we get this to work a little further down the road, we don't have to persuade people to buy large amounts of imaging equipment, they already have it.

"That has capital expenditure benefits and scientists doing this work are already familiar with it, which means we don't have to re-train a workforce." and follow me on Twitter

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