Polymer used in fluorescent tag can help identify fingerprints

05-Jul-13

The latest in high-tech crime-scene investigation is a "fluorescent tag" that can help identify fingerprints by creating images that are accurate to the nanoscale. The ridges on human fingertips, also called epidermal ridges or friction ridges, make the lines, whorls and swirls in these latent fingerprints. These patterns are virtually unique to each individual person.One of the most common current methods of imaging and analyzing these fingerprints is called dusting. University of Leicester scientists say that only 10% of latent fingerprint images drawn from crime scenes are complete enough to be used in court, partly because latent fingerprints are often incomplete or smudged. The scientists have developed a new method for imaging latent fingerprints that uses a film made of electroactive polymers — long, complex molecules that change shape in response to surrounding electric currents. The researchers' findings were published in the Royal Society of Chemistry's journal Faraday Discussions. Due to the polymers' electroactive properties, this film can be applied via an electric current, which drastically reduces the risk of damaging the fingerprint before an accurate image can be drawn, the researchers say. When the electric current is directed at a latent fingerprint, the film adheres to the gaps between the swirls of deposits that comprise latent fingerprints, and not to the deposits themselves. That's because these deposits are insular, meaning they don't conduct electricity. The polymers that comprise the film are also electrochromic; that is, when exposed to an electrical charge, they change color. The result is an inverted image, or negative, of the fingerprint that is highly detailed and visible, making it easy to photograph and analyze, according to the team. This technique is accurate to the nanoscopic scale; even a few molecules of sweat and oil are enough to prevent the polymer film from being transmitted. Further, the scientists have improved on this technique by adding fluorophores, molecules that glow when exposed to a certain type of light, to the film. This will allow crime scene investigators to tweak the coloration of the fingerprint image in order to achieve the best possible contrast with the background surface.