Inverse Fingerprints on Paper
Visualization
of Latent Fingermarks by Nanotechnology: Reversed Development on Paper: A
Possible Remedy to the Variation in Sweat Composition
Paper is one of the surfaces most
commonly tested for fingerprints in forensics. Unfortunately, it is
particularly difficult to make fingerprints on paper visible. In the journal Angewandte
Chemie, Israeli scientists have now introduced a new method developed
specifically for use on paper. It produces a “negative” of the fingerprint and
is, in contrast to conventional methods, independent of the composition of the
sweat residue left behind.
In many criminal cases, paper
evidence plays an important role and it would be useful to know through whose
hands checks, documents, or paper currency have passed. Studies have shown that
only about half of the fingerprints present on paper can be made sufficiently
visible. The main reason that this does not work consistently seems to be the
highly variable composition of the sweat left behind on the paper.
A team led by Daniel Mandler and
Joseph Almog at the Hebrew University of Jerusalem has now developed a
procedure that avoids these problems. It involves a sort of inversion of an
established method in which gold nanoparticles 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 get stuck to components of the sweat in fingerprints. In
contrast, the gold nanoparticles in the new method stick directly to the paper,
not the sweat. This technique uses the sebum from the fingerprints, which
effectively shields the paper beneath it from the gold nanoparticles. Treatment
with a developer containing silver, which turns the areas with gold on them
black, results in a negative image of the fingerprint.
The secret to the success of these
researchers is a special bifunctional reagent. The head of this molecule is an
acylpyridazine group, which can bind to cellulose. The tail is made of
hydrocarbon chains with a sulfur-containing group at the end, which binds to
gold and attaches the molecule to the surface of the gold nanoparticles. When
gold particles coated with these molecules are deposited onto paper with a
fingerprint on it, the heads bind to the cellulose in the paper, avoiding the
fat-containing lines.
Because only the fatty components of
the fingerprints are used, the possibly unfavorable composition of the sweat in
the fingerprint plays no role in this method. This technique also promises to
alleviate another problem: if paper has become wet, it has previously been
nearly impossible 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. The fatty components are barely effected.
About the Author
Joseph Almog is a Professor of
forensic chemistry at the Casali Institute of Applied Chemistry, The Hebrew
University of Jerusalem, and formerly the director of the Division of
Identification and Forensic Science (DIFS) of the Israel Police. His main
research topics are development of crime-scene techniques, explosives detection
and identification and enhancement of latent fingermarks. He is the 2005
laureate of the American Academy of Forensic Sciences' Lucas Medal for
outstanding achievements.
http://onlinelibrary.wiley.com/journal/10.1002/%28ISSN%291521-3773/homepage/press/201244press.html
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