The method is designed to overcome the problem that more than half of the fingerprints on paper cannot be identified
Paper has been tested more than once in the criminal identification process in order to detect fingerprints on it, but fingerprints on paper are considered difficult to detect. Research done at the Hebrew University presents for the first time a new method developed specifically for this purpose. The method produces a "negative" of a fingerprint and does not depend on the sweat composition of the fingerprint, on which the method used today is based.
A team of researchers led by Prof. Yosef Almog and Prof. Daniel Mandler from the Institute of Chemistry at the Hebrew University developed the new method, which was published this week in the important journal Angewandte Chemie International Edition.
"In many criminal cases, paper evidence plays an important role," notes Prof. Almog. "A prominent case was the deciphering of the murder of the late minister Rehabam Zeevi based on a fingerprint found on a newspaper." He further adds, "Studies have shown that less than half of the fingerprints imprinted on paper can be detected in a way that will allow identification. The reason for this is the great variation in the amount of sweat remaining on the paper and its composition."
The advanced method used today to detect prints on paper consists of two steps: first, the paper is dipped in a solution of gold nanoparticles, which bind to the proteins in the sweat, and then in a solution of silver ions, which settle as a black powder on top of the print ridges.
In the new method developed by the team of researchers, the gold particles stick exclusively to the paper and not to the fingerprint. The researchers developed a special chemical reagent whose one end binds to the cellulose from which the paper is made and the other end to the gold particles. The reagent acts as an intermediary between the paper and the gold. The fat secreted by the skin in sweat serves as a kind of mask for the paper underneath and protects it from the gold particles. The deposition of the silver on the gold in this case colors the paper black and highlights the fingerprint, to which no material is attached. The result is a negative image of the fingerprint.
"Since the method only refers to the fatty components of the fingerprint, the composition of the sweat does not play a central role in this process. This method makes it possible to overcome another problem - the detection of fingerprints on paper wetted with water. The proteins in the sweat, the main component for the chemical reaction in the existing method, melt and wash away in contact with water, but the fatty components are almost unaffected", clarifies Prof. Almog.
