Electrochromic enhancement of latent fingerprints by poly(3,4-ethylenedioxythiophene)

Abstract

Spatially selective electrodeposition of poly-3,4-ethylenedioxythiophene (PEDOT) thin films on metallic surfaces is shown to be an effective means of visualizing latent fingerprints. The technique exploits the fingerprint deposit as an insulating mask, such that electrochemical processes (here, polymer deposition) may only take place on deposit-free areas of the surface between the ridges of the fingerprint deposit; the end result is a negative image of the fingermark. Use of a surfactant (sodium dodecylsulphate, SDS) to solubilise the EDOT monomer allows the use of an aqueous electrolyte. Electrochemical (coulometric) data provide a total assay of deposited material, yielding spatially averaged film thicknesses, which are commensurate with substantive filling of the trenches between fingerprint deposit ridges, but not overfilling to the extent that the ridge detail is covered. This is confirmed by optical microscopy and AFM images, which show continuous polymer deposition within the trenches and good definition at the ridge edges. Stainless steel substrates treated in this manner and transferred to background electrolyte (aqueous sulphuric acid) showed enhanced fingerprints when the contrast between the polymer background and fingerprint deposit was optimised using the electrochromic properties of the PEDOT films. The facility of the method to reveal fingerprints of various ages and subjected to plausible environmental histories was demonstrated. Comparison of this enhancement methodology with commonly used fingerprint enhancement methods (dusting with powder, application of wet powder suspensions and cyanoacrylate fuming) showed promising performance in selected scenarios of practical interest.

Nanoscale control of interfacial processes for latent fingerprint enhancement

“…The qualitative conclusion of the preliminary experiments shown above is that polypyrrole can be added to the set of electrochromic materials (to date, polyaniline 34 and PEDOT 35 ) that permit visualization of latent ngermarks on metallic substrates by means of their optical absorption properties; note that the work of Bersellini 36 in this respect demonstrated the facility to electrodeposit polypyrrole, but did not go on to exploit its electrochromic properties. We therefore proceed to the more challenging goal of developing a polymeric system with suitable absorption and uorescence characteristics.…”

“…The PEDOT enhanced image acts as a control; this material has previously been demonstrated to provide good visualization of latent ngermarks, with high delity and controllable visual contrast. 35 We have quite deliberately shown examples that might be typical of real evidence, rather than highly controlled ("groomed") model examples. Thus, one can see evidence of damage to the ngermark and of adjacent ngermarks in Fig.…”

“…1-2 nm) of ngerprint residue is required -far less (by an order of magnitude) than required in the conventional strategies listed above. We have recently demonstrated proof-of-concept for this strategy 34,35 in the context of electropolymerization of aromatic solution precursors 36 to generate conducting polymer lms. Other applications of electrochemically-based methods of latent ngerprint visualization include imaging using the SECM [38][39][40][41] and deposition of Au nanoparticles.…”

“…Typical ngerprint deposits may be a few microns thick as-deposited, but evaporative and other environmental losses will typically decrease this to 100s of nm before enhancement is undertaken. This denes the vertical resolution required for the (electro) chemistry: there is an optimum to be found between the lower limit of detection of deposited material and over-lling 35 of the trenches between ngerprint ridge deposits. To summarize, this singular analytical challenge of uniquely identifying an individual based on electroanalytical visualization of their ngerprint requires simultaneous control of (electro)chemical processes on length scales from the nanometre to the centimetre regime and in both lateral and vertical directions.…”

“…This leads naturally to our second generic goal, use of electropolymerized lms permitting excellent spatial control but with sub-optimal optical properties. In particular, we wish to move from thiophene 35 and aniline 34 based materials to pyrrole-based systems, for which there is much greater opportunity for manipulation of properties by substituent chemistry on the pN-H functionality. Here we focus on the underlying fundamental (electro)chemistry of this approach.…”