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Fluorescent fingerprinting technique boosts sensitivity

10 Jul 2013

To improve IDs derived from "hidden" fingerprints on weapons.

A new way of detecting and visualizing fingerprints from crime scenes using colour-changing fluorescent films could lead to more effective identifications from latent (hidden) fingerprints on knives, guns, bullet casings and other metal surfaces. The technique is the result of a collaboration between the University of Leicester, UK, the Institut Laue-Langevin, Grenoble, France, and the ISIS pulsed neutron and muon source, Oxfordshire, UK.

The technique was first presented publicly at the UK’s Royal Society of Chemistry’s Faraday Discussion in Durham, UK, on 3 July.
When a finger touches a surface, it leaves behind deposits of sweat and natural oils in a pattern that mirrors the ridges and troughs on the fingertips. The odds of two individuals having identical fingerprints are 64 billion to 1, making them an ideal tool for identification in criminal investigations.

The greatest source of fingerprint forensic evidence comes from “latent” fingerprints - those not immediately visible to the eye, because they are less likely to be wiped. However, visualizing these prints with sufficient clarity for positive identification often proves difficult. Despite the availability of several enhancement techniques, only 10% of fingerprints taken from crime scenes are of sufficient quality to be used in court.

The classical approach to enhance latent print visibility is to apply a coloured powder that adheres to the sticky residue and provides a visual contrast to the underlying surface. However, these techniques require significant preservation of fingerprint material and are therefore vulnerable to ageing, environmental exposure or attempted washing of the fingerprint residue.

The Leicester researchers have developed the technique that visualizes fingerprints by exploiting their electrically insulating properties. The fingerprint material acts like a mask, blocking an electric current that deposits a colored electro-active film, which directs the film to the regions of bare surface between fingerprint deposits. The polymers are electrochromic: they change from one colour to another when subjected to an electrical voltage.

The technique is highly sensitive as even tiny amounts of insulating residue, just a few nanometers thick, can prevent polymer deposition on the metal underneath. 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 deposits, it can be used in combination with existing (e.g. powder-based) approaches.

Fluorophore molecules

In their latest paper the team, led by Professor Robert Hillman, have developed this technique further by incorporating within the film fluorophore molecules that re-emit light of a third colour when exposed to light or any other form of electromagnetic radiation such as ultra-violet rays.

Their success in combining the electrochromic and fluorescence approaches provides a significantly wider palette to ‘color’ their films and two sets of ‘levers’ in the form of electricity and light to control and tune this coloration in order to achieve the best possible contrast with the underlying metal surface.

The addition of these large fluorescent tagging molecules required a conducting film that could undergo post-deposition chemical changes. Neutron reflectivity measurements were used to follow and quantify the deposition and functionalisation of the film with the fluorophores.

The measurements were performed on FIGARO and D17 at the Institut Laue-Langevin and on INTER at ISIS (Harwell Oxford, UK).

 Professor Robert Hillman commented, “By using the insulating properties of the fingerprints to define their unique patterns and improving the visual resolution through these colour-controllable films, we can dramatically improve the accuracy of crime scene fingerprint forensics. From the images we have produced so far, we are achieving identification with high confidence using commonly accepted standards.

“This combination of optical absorption analysis with observation based on fluorescence is also opening up fingerprint analysis to a far wider set of samples, particularly those eroded by ageing or aggressive environments. The use of neutrons alongside spectroscopic techniques has been fundamental to understanding how this technique might work in practice and is evidence for what has been a truly collaborative partnership between these three institutions.”

Dr Rob Barker, Instrument Scientist at the Institut Laue-Langevin, added, “Neutrons are an ideal tool for understanding what is going on inside these complex systems. Whilst the mix of polymer and fluorescent molecules might look similar to x-rays and other surface-sensitive techniques, neutrons can easily distinguish between them. This allowed us to non-invasively probe on a nanometer 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.”

About the Author

Matthew Peach is a contributing editor to optics.org.

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