Fluorescent fingerprint tag aims to increase IDs from 'hidden' prints on bullets and knives

When your finger touches a surface, it leaves behind deposits of sweat and natural oils in a pattern that mirrors the ridges and troughs found on your 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, i.e. 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.

To address this, researchers from the University of Leicester have been working on a new technique that visualizes fingerprints by exploiting their electrically insulating characteristics. Here, the fingerprint material acts like a mask or 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. Unlike conventional fingerprint visualization reagents, the polymers used by the University of Leicester researchers are electrochromic, that is to say 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 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. Also, because it focuses on the gaps between the fingerprint deposits, it can be used in combination with existing (e.g. powder-based) approaches.

In their latest paper in Faraday Discussions 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 'colour' their films and 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 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. These measurements were performed on FIGARO and D17 at the Institut Laue-Langevin (Grenoble, France) and on INTER at ISIS (STFC Rutherford Appleton Laboratory in the UK).

The exact position and distribution of the fluorophores within the film is key. Professor Hillman and colleagues needed the molecules to penetrate the deposited polymer layer without reaching the underlying metal surface, where their fluorescence is diminished. Using isotopic methods, the team were able to use neutrons at ILL and 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.

Using the new technique on laboratory-sourced fingerprints, Professor Hillman and colleagues have already demonstrated an improved ability to make positive identifications due to better sample resolution. However the team are keen to stress these prints were taken under laboratory conditions. The next step is to apply it to fingerprints that have been exposed to more realistic scenarios, such as water, heat from a fire or cleaning agents.