Fluorescent Polymer Detects Crime Scene Fingerprints
Images of a fingermark left on a stainless steel substrate, following enhancement with a new 'fluorescent tag.'
CREDIT: University of Leicester
The latest in high-tech crime-scene investigation is a "fluorescent tag" that can help identify fingerprints on bullets, knives and other metal surfaces by creating images that are accurate to the nanoscale.
A significant portion of crime scene investigation involves searching for what are called "latent fingerprints," or deposits of secreted sweat and natural oils transmitted by touch onto a flat surface, and usually invisible to the naked eye.
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 (the chances of two people having the same prints are 64 billion to one), which makes fingerprints an excellent way to identify who was present at a crime scene. [See also: Crime Busters Embrace Social Media]
One of the most common current methods of imaging and analyzing these fingerprints is called dusting. You've probably seen it in movies or on television—investigators brush fine powder onto the fingerprint and then "lift" the prints by applying and then peeling off tape. The imprint of the powder, in the shape of the fingerprint, is preserved on the tape and can be taken to a lab for further analysis.
University of Leicester scientists, however, say that only 10 percent of latent fingerprint images drawn from crime scenes are complete enough to be used in court.
This is partly because latent fingerprints are often incomplete or smudged. Current techniques for imaging latent fingerprints, such as dusting, also run the risk of damaging the fingerprint before it can be accurately imaged.
To that end, 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 today (July 2) 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 polymersthat 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 University of Leicester 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. [See also: 10 Ways You're Using Nanotech Right Now (And Don't Even Know It)]
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.
"[Fluorescent tags] sound like a very interesting approach," said Christopher Hopkins, director of the Forensic Science Program at the University of California, Davis and a former FBI investigator, who is not associated with the University of Leicester research.
However, he added, "Over the course of my career, we see a lot of people developing new techniques [for crime scene investigation], and sometimes we adopt them, and a lot of times we don't, for a number of reasons."
According to Hopkins, whether or not investigators adopt the fluorescent tag depends on how expensive the process is, how long it takes and whether it can be shown to provide more accurate results than current methods.
The technique is "certainly a laboratory approach," he said. "That doesn't mean it can't be used. It just has to be used in a lab setting, which we do quite frequently with a lot of the evidence we collect at crime scenes."
Hopkins was also skeptical of the technique's usefulness on weapons.
"Traditionally, we have very low success at finding fingerprints on weapons," he said of his 25-year career with the FBI. "With firearms specifically. We don't know why that is."