The Dynamics of Latent Fingerprint Development Using the Multimetal Deposition (MMD) Technique

Fingerprints are the most robust form of identification and are frequently used to establish identities in criminal investigations. Latent (invisible) fingerprints possess high evidential value but require the application of suitable development techniques to render them visible. This thesis evaluates the dynamics of one such technique, referred to as multimetal deposition (MMD); it proceeds via the prior deposition of gold nanoparticles, followed by subsequent silver growth, and displays exceptional performance on polymer films (“clingfilm”) that are used as wraps for illicit materials such as drugs and explosives. In this thesis, MMD was evaluated on clingfilm, and the experimentally more convenient model surface of polypropylene. Since the MMD method was developed primarily based on empirical observations, there is currently a limited understanding of the chemistry of this process. This thesis aims to gain novel fundamental insights into the process and quantify the various behaviours displayed by the MMD technique. The nucleation and growth processes exhibited by the metal particulates were investigated using Dynamic Light Scattering (DLS) and Scanning Electron Microscopy (SEM), and it was revealed that gold particles grow to their maximum size in the solution and selectively adhere to fingerprint ridges as small nucleation sites in both MMDI and MMDII. SEM showed that these small ‘seeds’ are the exclusive sites on which silver particles grow. The exceptional spatial selectivity of the gold particles has been quantified, with the gold deposition rate on the fingerprint residue (representing the ridges) being 15 times faster than that on the background. In addition to this, substituting the Tween 20 in the formulations with a structurally constant surfactant (decaethylene glycol monododecyl ether or DGME) was able to develop fingermarks with high consistency, while also offering the potential to lower the development times. The nucleation and growth dynamics of DGME stabilised gold particles mirror that displayed with Tween 20 stabilised particles, indicating the importance of these characteristics for successful visualisation. Neutron reflectivity (NR) studies of surfactant adsorptions on planar gold and silver was used to determine the composition of the adsorbed surfactants on the metal surfaces; this is what leads to the observed stabilities of the colloidal dispersions. Finally, a technique that possesses the capability to objectively quantify the quality of MMD developed marks has been proposed; this can standardise the comparisons during operational casework and research.