Fluorescent fingerprint

Images of a fingermark left on a stainless steel substrate following enhancement by electro-deposition of polypyrrole. The light regions are stainless steel protected by the sweat residue.

A NEW technique based on colour-changing fluorescent films could lead to higher-confidence identifications from hidden fingerprints picked up from crime scenes and objects used in crime such as knives, guns and other metal surfaces. This technique is the result of a collaboration between the University of Leicester in the United Kingdom, the Institut Laue-Langevin (ILL) in France and the Science and Technology Facilities Council (STFC) in the U.K.

A fingerprint on a surface is basically the deposits of sweat and natural oils left behind in a pattern that mirrors the ridges and troughs found on the fingertips. The chance of two individuals having identical fingerprints is one in 64 billion, which makes it an ideal forensic tool for identification.

However, visualising the latent prints—those that are not immediately visible to the eye—with sufficient clarity for positive identification often proves difficult. Despite the availability of several enhancement techniques, only 10 per cent of fingerprints are of sufficient quality to be used as evidence in court.

The classical approach to enhance the fingerprint’s 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 and environmental exposure.

The new technique exploits the electrically insulating characteristics of the latent fingerprints. Here, the fingerprint material acts like a mask or a stencil, blocking an electric current that is used to deposit a coloured electro-active film. This directs the coloured film to the regions of bare surface between the fingerprint deposits, thereby creating a negative image of the print. The University of Leicester researchers, led by Robert Hillman, have used polymers that change from one colour to another when subjected to an electrical voltage—electrochromic—to enhance the image of the fingerprint.

The technique is highly sensitive as even tiny amounts of insulating residue, just a few nanometres thick, can prevent polymer deposition on the metal below. As a result, much less fingerprint residue is required than is typical for other techniques.

This technique has been further enhanced 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 ultraviolet rays. Their success in combining the electrochromic and fluorescence approaches provides two sets of “levers” in the form of electricity and light to control and tune this colouration in order to achieve the best possible contrast.

The exact position and distribution of the fluorophores within the film is the key. The molecules need to penetrate the deposited polymer layer without reaching the underlying metal surface, where their fluorescence is diminished. Using isotopic methods, the team was able to use neutrons at ILL and STFC’s pulsed neutron source ISIS to label the different parts of the system and observe the behaviour of each to find the ideal conditions (temperature, polymer concentrations, reaction time) for the introduction of the fluorophores.

R. Ramachandran