(Invited) Exploitation of Electron Transfer Reactions for Latent Fingerprint Visualization: Fundamental Concepts and Technology Transfer

Abstract

A fingerprint is a unique time-invariant attribute of an individual and is the most frequent physical evidence used to identify a suspect in a criminal investigation. An image of the fingerprint left on a surface by the transfer of material between the fingertip and the surface can establish contact between the individual and an object (e.g. weapon) or a fixed location (crime scene). In practice, most marks are so-called latent (non-visible) fingermarks that require chemical treatment to generate a visible image. This presentation describes the contribution of electron transfer reactions (redox processes) to this endeavor and reveals the variety of reaction type, reagent materials, and substrate surfaces.The key to a successful fingermark reagent is the facility to deliver it with high spatial selectivity to either the deposited residue (representing ridges on the finger) or the bare substrate (representing furrows on the finger), such that there is visual contrast between the ridge and furrow areas. The lateral (horizontal) patterning of so-called second level features (such as ridge endings, crossovers and bifurcations) is the basis of the identification. Superficially, the distance scale is macroscopic (ridge width ca. 100 mm), but the aspiration of a suitably sharp image involves so-called third level detail at the mesoscopic level (ridge edge shape resolution ca. 1 mm). However, this lateral image quality requires vertical control of reagent deposition at the nanoscale. Here we explore how latent fingermark visualization can be accomplished by controlled molecular mechanisms involving metallic and polymeric materials.Two established (but previously poorly mechanistically understood) metal deposition processes are physical developer (PD) and multi-metal deposition (MMD). PD involves redox generation of silver nanoparticles that deposit selectively on fingermark residue on paper substrates (e.g., documents, correspondence, paper currency). We show how mechanistic understanding [1] of a recent PD re-formulation [2] has been used to overcome the environmental outlawing of a critical surfactant in the classical process, revealing latent marks up to a century after deposition. MMD is a more sophisticated variant, in which gold nanoparticles are pre-deposited on the residue and act as nucleation sites for silver deposition to reveal marks on thin plastic films (e.g., drugs and explosives wrappings). We show how the kinetics of redox-driven silver deposition on ridge vs furrow regions define image contrast.In a complementary strategy, we use the fingerprint residue as a template (“mask”) to direct electrochemically generated reagent to the bare surface between the deposited ridges, creating a negative image of the fingerprint. When the surface is a reactive metal (Cu or Fe), a more noble metal (Ag) can oxidize exposed substrate atoms. The electron transfer process results in solution oxidant (here, Ag) being reduced to elemental metal that deposits on the surface and dissolution of an equivalent amount of substrate. Complexing agents influence both reaction thermodynamics and kinetics. Optical and SEM imaging and EDAX surface analysis reveal high quality images with clear third level detail.When the surface is an inert metal, exemplified by stainless steel (forensically relevant to knife crime), the above galvanic exchange cannot take place. In this instance, electrochemically driven oxidation of aromatic monomers creates negative images of latent marks via polyaniline [3], PEDOT [4] or poly(pyrrole-co-EDOT) [5] films. We show how these electrochromic systems offer the prospect of optimization of visual contrast via light absorption. More recently, we have explored the prospect of incorporating fluorescent dyes (Methyl Red, Methyl Orange, Indigo Carmine, Basic Yellow 40) in the polymer films, using functionalization, ion-exchange and physical encapsulation strategies [6].We discuss the practicalities and prospects for technology transfer of these approaches to the practitioner environment; even for the nominally established PD and MMD processes, this is not trivial. Positive outcomes lie at the core of collaborative expertise provided by practitioners, researchers, regulatory authorities and investigators. We discuss how this can be realized through engagement of these parties and researcher use of the image capture and enhancement technologies used by practitioners.