Neutrons help create colour-changing fingerprint technique

Researchers developing a way to identify fingerprints that would be otherwise unusable have made a breakthrough using neutron scattering.

The new technique involves depositing a colour-changing polymer (dark areas) in the spaces between a fingerprint’s ridges (light areas).

The technique, developed at Leicester University, is designed to make poor quality fingerprints good enough for use in court, by combining a polymer film that changes colour when electricity is applied and fluorescent tagging molecules.

The scientists were able to use neutrons to label the different parts of the film-covered fingerprint and observe the system to find the ideal conditions, such as temperature and polymer concentration, for introducing the “fluorophone” molecules.

Fingerprints are made from deposits of sweat and natural oils that are usually revealed by applying a coloured residue that sticks to these deposits. But only 10 per cent of prints taken from crime scenes are good enough to use as evidence.

The new technique is based on the fact that the fingerprint deposits are electrical insulators. When the polymer coating is applied and an electrical current is passed through the surface below (which must be conductive), the film changes colour in the spaces between the fingerprint contours.

This method is highly sensitive because deposits just a few nanometres thick can prevent the polymer from being deposited on the metal below, which means much less residue is required to create a usable fingerprint image.


The researchers have now improved the technique by adding the fluorophone molecules that re-emit light of a third colour when exposed to electromagnetic radiation, in order to achieve the best possible contrast with the surface below.

Prof Robert Hillman, who led the research, said in a statement: ‘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.

‘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.’

He added that the neutron analysis, performed at the ISIS facility in Oxfordshire and at the Institut Laue-Langevin  (ILL) in Grenoble, France, was fundamental to understanding how the technique might work in practice.

Dr Rob Barker, instrument scientist at ILL, said: ‘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 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.’

The research so far, presented this week at the Royal Society of Chemistry’s Faraday Discussion in Durham, was carried out under laboratory conditions. The team now plan to apply the technique to more realistic scenarios involving water, heat from a fire or cleaning agents.

Assistant chief constable Roger Bannister of Leicestershire Police said: ‘This technique potentially offers opportunities for quick results for the more serious crimes in a way that may still permit other forensic analysis to be performed to maximise the opportunities to recover forensic evidence.’