Electrochemical Strategies for the Visualization of Latent Fingerprints

A fingerprint is a unique time-invariant biometric attribute of an individual and is the most commonly used means of identification in criminal investigations. Most crime scene marks are so-called latent (non-visible) fingermarks, created by the transfer of colourless sweat residues from the finger to the surface. They require chemical treatment to generate a visible image. The technical challenge is spatially selective delivery of the reagent to either the deposited residue (representing ridges) or the bare substrate (representing furrows).

Complementary to traditional methods, we use the fingerprint residue on metal surfaces as a “mask” to direct electrochemically generated reagent to the bare surface between the deposited ridges; this generates a negative image of the fingerprint. For reactive metal surfaces (Cu or Fe), a more noble metal cation can oxidize exposed substrate atoms, resulting in deposition of the elemental noble metal. Mechanistic aspects of the process are revealed via QCM monitoring. The influence of complexing agents on reaction thermodynamics and kinetics is used to optimize image fidelity, acquired on different length scales using optical and electron microscopies and EDX surface analysis.

When the surface is an inert metal (e.g. stainless steel, in knife crime), images of latent marks are created via electrochemically driven reduction of metal ions or oxidation of aromatic monomers (pyrrole, aniline, EDOT) to form electrochromic (co-)polymer films. Film multi-functionality can be achieved by dye entrapment. Photography, SEM/EDX and AFM imaging are used to evaluate image quality on macroscopic, mesoscopic and nanoscopic length scales, respectively. Images in absorption and emission modes are compared. FTIR hyperspectral analysis provides compositionally selective images based on polymer and/or dye functional group signatures. The potential for use in the practitioner environment will be considered.