Nanoparticles appear in several areas of forensic science including security documents, paints, inks, and reagents that
develop latent prints. One reagent (known as the silver physical developer) that visualizes the water insoluble
components of latent print residue is based on the formation of highly charged silver nanoparticles. These attach to and
grow on the residue and generate a silver image. Another such reagent involves highly charged gold nanoparticles.
These attach to the residue forming a weak gold image which can be amplified with a silver physical developer.
Nanoparaticles are also used in items such as paints, printing inks, and writing inks. Paints and most printing inks
consist of nano-sized pigments in a vehicle. However, certain modern ink jet printing inks now contain nano-sized
pigments to improve their light fastness and most gel inks are also based on nano scale pigments. These nanoparticlecontaining
materials often appear as evidence and are thus subject to forensic characterization. Both luminescent
(quantum dots), up-converting nano scale phosphors, and non luminescent nanoparticles are used as security tags to label
product, add security to documents, and as anti counterfeiting measures. These assist in determining if an item is
fraudulently made.
Latent prints on substrates of varying porosity are visualized by a host of physical, optical, and chemical techniques that are often used sequentially. Optical techniques include viewing the scattering of UV radiation by latent print residue on smooth surfaces and inducing natural (inherent) fluorescence of such residue; physical techniques include visualizing the latent print using diverse powders and vapors that selectively polymerize on the residue; and chemical techniques include reacting components of latent print residue with reagents that render color and/or fluorescence to the residue. Recent developments in visualizing techniques include the use of nanoparticles, hyperspectral imaging, and the development of more sensitive reagents for visualizing the water-soluble components of latent print residue (e.g., amino acids) and the water-insoluble components (e.g., lipids, dried proteins, etc.). The chemistry of latent print residue and current research in the methods to visualize its components will be discussed.
Silver physical development, a now-abandoned technique used for developing photographic film or paper, is one of the most powerful methods for visualizing latent prints on paper. The method develops the water-insoluble components in the print residue. These components include the “fats and oils” or lipids found on the skin of fingers. The resulting developed print, referred to as a silver physically developed (Ag-PD) print, is made up of (gray to black) silver particles adhered to the fingerprint residue. Such prints are usually intensified (made darker) with a hypochlorite treatment. This process converts silver to silver oxide making the Ag-PD print become a Ag2O-PD prints. Often such (Ag-PD or Ag2O-PD) prints are found on areas with heavy or patterned printing making them difficult to see. This work resolves this problem by chemically lightening the print and darkening (suppressing) the interfering background.
The U.S. Secret Service has been doing and supporting research in several areas of fingerprint visualization. The following is discussed: (1) developing ninhydrin analogues for visualizing latent prints on porous surfaces such as paper (with Dr. Madeleine Joullie, University of Pennsylvania); (2) exploring reflective UV imaging techniques as a no-treatment-required method for visualizing latent prints; (3) optimizing 'gun bluing' methods for developing latent prints on metal surfaces (such as spent cartridges); (4) investigating aqueous metal deposition methods for visualizing latent prints on multiple types of surfaces; and (5) studying methods of transferring latent print residues onto membranes.
Conference Committee Involvement (6)
Optics and Photonics for Counterterrorism and Crime Fighting
19 September 2011 | Prague, Czech Republic
Optics and Photonics for Counterterrorism and Crime Fighting
20 September 2010 | Toulouse, France
Sensors, and Command, Control, Communications, and Intelligence (C3I) Technologies for Homeland Security and Homeland Defense VI
9 April 2007 | Orlando, Florida, United States
Sensors, and Command, Control, Communications, and Intelligence (C3I) Technologies for Homeland Security and Homeland Defense IV
28 March 2005 | Orlando, Florida, United States
Sensors, and Command, Control, Communications, and Intelligence (C3I) Technologies for Homeland Security and Homeland Defense III
12 April 2004 | Orlando, Florida, United States
Sensors, and Command, Control, Communications, and Intelligence (C3I) Technologies for Homeland Defense and Law Enforcement II
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