HU: New technique to see fingerprints on paper

Forensic chemists use innovative chemical process to produce a negative of fingerprint image rather than positive.

Fingerprint negative image HU 370 (photo credit: Courtesy Hebrew University)
Fingerprint negative image HU 370
(photo credit: Courtesy Hebrew University)
A new technique for visualizing fingerprints left on paper – usually difficult because of variation in the sweat mingled with the oil on the fingertips – developed by Hebrew University forensic chemists, would have pleased Sherlock Holmes.
Professors Yossi Almog and Daniel Mandler of the HU chemistry institute used an innovative chemical process to produce a negative of the fingerprint image rather than the positive image produced under current methods. This innovation is almost independent of the composition of the sweat residue.
The new method is described in the current issue of the international, English-language edition of the journal Angewandte Chemie, published by the German Chemical Society.
In many criminal investigations, paper evidence plays an important role, and it is useful to know who has handled documents such as checks, paper currency and notes. Studies have shown that fewer than half of the fingerprints on paper items can be made sufficiently visible to enable their identification.
The new procedure avoids these problems. It involves an inversion of an established method in which gold nano-particles are first deposited onto the invisible fingerprints, followed by elemental silver, similar to the development of a black-and-white photograph.
In the conventional technique, the gold particles adhere to the amino acid components of the sweat in the fingerprints, and then silver is deposited onto the gold. The result is quite often low-contrast impressions of the fingerprints.
In the new method, the gold nano-particles stick directly to the paper surface but without the sweat. This technique utilizes the sebum – oil secreted by the sebaceous glands that helps prevent hair and skin from drying out – from the fingerprints as a medium to avoid this interference. Treatment with a developer containing silver then turns the areas with gold on them black, resulting in a clear, negative image of the fingerprint.
“Since our method relies only on the fatty components in the fingerprints, the sweaty aspects play no role in the imaging process,” said Almog. This technique also promises to alleviate another problem, he said. “If paper has become wet, it has previously been difficult to detect fingerprints because the amino acids in the sweat, which are the primary substrate for current chemical enhancement reactions, are dissolved and washed away by water, whereas the fatty components are barely affected.”
Thus, the avoidance of the sweat aspect provides a further enhancement for police laboratory investigation, he said.