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 from HU. Photo: 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
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
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
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
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
“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.