Advances in Fingerprint Technology.pdf

Figure 7.6 Comparison of an Ag-PD print on a counterfeit $20 note before (left)
and after hypochlorite bleach treatment (right). Ag-PD print darkens and background
lightens.
involves treating the test species (proteins in the biochemical case and latent
prints in our case) with a colloidal gold solution (having a pH ~ 2.7) and
then with a weak solution of silver physical developer. The colloidal gold
particles are highly negatively charged and thus bind strongly with the target
species to form catalytic nucleating sites for silver physical development. In
this technique, one speaks of silver amplifying the gold image.
Porous vs. Nonporous Surfaces
As indicated above, compared to the silver physical development method,
which visualizes latent prints on porous rather than nonporous surfaces, the
MMD method is able to visualize prints on both surfaces. Silver physical
developers form and supply the colloidal particles that bind to the latent
print residue and these become the critical catalytic nucleating sites. Once
these particles bind, they then grow in the development process. The MMD
method, on the other hand, supplies already-formed colloidal particles that
do not grow and can thus build up if given sufficient time. They grow only
during the silver physical development stage as silver builds up on them.
Recall that for binding to occur, the silver or gold particles must reside on
the latent print residue for a sufficient amount of time without growing
significantly. Thus, for nonporous surfaces (where the surface area of the
latent print residue is less expansive than on a porous surface), the MMD
method must be given enough time to form a sufficient number of gold