Advances in Fingerprint Technology.pdf

Saunders. 22 Both workers recognized the importance of having negatively
charged silver colloid particles and a low pH in the Ag-PD solution. Both of
these are present in the UK-PD. The colloidal particles are selectively
adsorbed on the print residue (which is positively charged), become neutralized,
and function as catalytic nucleation sites for silver physical development.
Charge of Latent Print Residue
The reason the print residue is positively charged is that in an acidic environment,
residue components containing amine groups (R–NH 2) and carbon-carbon
double bonds (–C�C–) get protonated. 23 Among the
compounds with carbon-carbon double bonds in the print residue are the
unsaturated lipid components and among the amines are insoluble proteins,
lipoproteins, and even water-soluble proteins and amino acids that get
trapped in the lipid matrix as the print dries and hardens by oxidation. The
protonation reactions are
R–NH 2 + H + → R–NH 3 + (7.11)
(7.12)
Note that another way of providing a positive charge to the residue is by the
complexation of silver ions with the double bonds in alkenes: 24
(7.13)
Formation of Nucleating Sites on Latent Print Residue
As stated, the role of the cationic surfactant in the stabilized UK-PD is to
suppress the growth of the spontaneously formed and negatively charged
silver particles. Without the surfactant, these particles grow uncontrollably;
however, when the surfactant is used, they begin as negatively charged particles
and gradually grow and reverse their charge until they are fully encased
with surfactant molecules (micelle formation). At this point they cease to
grow and are positively charged. These negatively charged particles have an
electrostatic attraction toward the cationic surfactant molecules; however, in
the vicinity of the positively charged print residue, they are also attracted to
the residue. In the proximity of the residue, the attractive forces are competitive.
For particles formed in the bulk solution, micelle formation is dominant