Criminals might want to think twice before touching anything, now that a new fluorescent tagging technique could yield more usable results from latent fingerprints on metal surfaces. The odds that any two people have identical fingerprints are 64 billion:1, making them an ideal tool for identification in criminal investigations. And the greatest source of fingerprint evidence comes from prints not immediately visible to the eye, because they are less likely to have been wiped away. But visualizing these latent prints with sufficient clarity for positive identification has proved difficult: Despite the availability of several enhancement techniques, only 10 percent of fingerprints taken from crime scenes are usable in court. A 3-D optical microscope image of part of the same fingerprint as seen above. It shows the fingerprint ridges (as light areas) and the deposited polymer (as dark areas). Note that a ridge starts at the bottom right and runs diagonally up and to the left, terminating as a “ridge ending” (an example of a second-level detail that might be used as part of an ID) in the center of the image. The scale is in microns. The traditional approach to enhancing the visibility of latent prints is to apply a colored powder that adheres to the sticky residue, providing a visual contrast to the underlying surface. This type of method requires significant preservation and is vulnerable to aging, environmental exposure or attempted washing of the residue. Researchers from the Science and Technology Facilities Council’s ISIS pulsed neutron and muon source, the University of Leicester, and the Institut Laue-Langevin in France have now developed a technique that can lift latent fingerprints on knives, guns, bullet casings and other metals using a film of electrochromic polymers, which change colors when subjected to an electrical voltage. Here, a finger mark left on a stainless-steel substrate after enhancement by electrodeposition of polypyrrole. The light regions are stainless steel that was protected by the sweat residue, preventing the polymer from being deposited onto it. The dark regions are the polymer in between the fingerprint sweat without the fluorescence “turned on.” Images courtesy of University of Leicester. The investigators exploited the electrically insulating characteristics of the fingerprint material, which acts as a mask or stencil that blocks the electric current used to deposit a colored electro-active film. This directs the colored film to the regions of bare surface between the fingerprint deposits, creating a negative image of the print. The technique is accurate to the nanoscale, which means that much less fingerprint residue is required than is typical for other techniques. And, because it focuses on the gaps between the fingerprint deposits, it can be used in combination with existing powder-based approaches. “From the images we have produced so far, we are achieving identification with high confidence using commonly accepted standards,” said professor Robert Hillman of the University of Leicester. To further the technique, the investigators incorporated fluorophore molecules within the film that re-emit light of a third color when exposed to light or any other form of electromagnetic radiation. The addition of these fluorescent tags required a conducting film that could undergo postdeposition chemical changes. Using isotopic methods, the team used neutrons at ILL and ISIS to label the different parts of the system and to find the ideal conditions (temperature, polymer concentrations, reaction time) for the introduction of the fluorophores. “The newest neutron-reflection instruments, built at both ISIS and the ILL, provide high-intensity beams that enable the real-time study of changes in such complex systems for the first time,” said Dr. Max Skoda, an instrument scientist at ISIS. “Neutrons are an ideal tool for understanding what is going on inside these complex systems,” said Dr. Rob Barker, an instrument scientist at ILL. “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 noninvasively probe on a nanometer 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 researchers have demonstrated an improved ability to make positive identifications in laboratory conditions. Next, they hope to apply the method to fingerprints that have been exposed to more realistic scenarios such as water, heat from a fire, and cleaning agents. “Fingerprints have been around in policing for over 100 years, but this technique opens up new avenues for the detection of crime in the modern era,” said Assistant Chief Constable Roger Bannister of Leicestershire Police. “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 maximize the opportunities to recover forensic evidence.” Results of the study were published in Faraday Discussions (doi: 10.1039/c3fd00053b.