Detection of latent fingermarks : different approaches to targeting amino acids in the deposit

Abstract

Amino acids are a common organic component of human perspiration and are often used as a reliable target for latent fingermark enhancement on porous surfaces as they are highly persistent over long periods of time. Ninhydrin, and its analogues 1,8-diazafluoren-9-one (DFO) and 1,2-indanedione, remain the benchmark for chemical fingermark enhancement reagents. When applied to latent fingermarks, ninhydrin produces a non-fluorescent deep purple product on the fingermark ridges, while 1,2-indanedione and DFO produce a faint pink colouration that luminesces strongly in the yellow region of the spectrum. However, prior to the research presented in this thesis, little was known about the reaction of these reagents with amino acids within the fingermark and paper matrices, particularly with respect to the effect of low concentrations of zinc chloride on the indanedione-amino acid reaction. The experimental results obtained indicated that nine α-L-amino acids commonly present in perspiration formed the same product when reacted with ninhydrin on cellulose substrates, although the reaction kinetics were highly dependent on the structure of the amino acid. Similar results were observed with 1, 2-indanedione, indanedione-zinc and DFO. Mechanism studies into the indanedione-amino acid reaction determined that indanedione reacts with amino acids in fingermark deposits following a similar mechanism to ninhydrin. Zinc chloride stabilised a crucial reaction rate limiting step in low humidity conditions, acting as a Lewis acid catalyst and accelerating the reaction to a rate equal to that observed in high humidity conditions. The nature of the porous substrate and the heating conditions used to accelerate the reaction were also determined to significantly affect the luminescence of the reaction product. The preliminary evaluation of a redox cycling dye, nitroblue tetrazolium, as a potentially sensitive fingermark enhancement reagent on paper was also undertaken. The poor colour and luminescence obtained upon development of amino acid standards and the failure of the reagent to develop latent fingermarks indicated that solvent-based initiation of the redox reaction is not compatible with this form of forensic evidence. The synthesis and trial of a novel reagent incorporating anti-L-amino acid antibodies conjugated to gold nanoparticles indicated the potential for the detection of amino acid-rich fingermarks on non-porous surfaces such as glass and metal. When enhanced with a fluorescent dye, the anti-L-amino acid reagent successfully developed latent fingermarks that had been aged up to 12 months on glass, foil and ceramic tile substrates.