Avaliação sobre dupla prova forense a partir da análise genética de impressões digitais reveladas pela eletrodeposição de polipirrol

Abstract

Latent fingerprints are frequent and relevant evidences in crime scene investigations. However, success rates are still low regarding the quality of development, especially for metallic surfaces using conventional methods. In this context, the spatially selective electrodeposition of conducting polymers is a promising strategy for developing fingerprints on metallic surfaces. As fingerprints are also potential sources of DNA, this study aims to visualize them by the electrodeposition of polypyrrole (PPy), verifying the impact of the process on subsequent DNA analysis. For this, 120 fingerprints of 24 hours and 7 days, applied on stainless steel surfaces and cartridge cases (brass), were developed by PPy electrodeposition through potentiostatic (PST) and galvanostatic (GST) methods in an electrochemical system with 0.01 mol L-1 pyrrole and 0.1 mol L-1 LiClO4 solution, with platinum counter electrode, reference electrode Ag/AgCl (KCl sat.). The developed fingerprints were photographed and analysed by forensic softwares and protocols, classifying them according to the quality of the developments. Best results in stainless steel, given the contrast between fingerprint and metal, obtained by GST method (j = 1.25 mA cm-2), with 90% utilization. Regarding cartridge cases, the best results were observed by PST method (E = 0,75 V vs. Ag/AgCl (KCl sat.)), showing 90% use. The PPy films showed chemical stability, adhesion and uniformity on the metallic surface. The experimental conditions aimed to avoid the oxidation of DNA and trace constituents. Effects related to the PPy electropolymerization mechanism and the interaction of metallic alloys with the process were investigated. The evaluation of the quality of the results classified the developments as strong and very strong, with full details of the papillary crests and fully identifiable fingerprints for forensic purposes. The methods proved to be reproducible, cheap, fast, non-destructive and advantageous compared to conventional methods, especially for aged fingerprints. DNA analysis were performed on the developed fingerprints, including the steps of extraction (manual and automated), quantification by RT-PCR, amplification by PCR and genotyping by capillary electrophoresis. Evaluating the different times and surfaces, the results did not show significant differences, being possible to recover amounts greater than 3 pg/μL of DNA in more than 50% of the samples from the different extraction methods. The rate of obtaining partial genetic profiles was 12.5%, from recent and aged fingerprints, developed by electrodeposition of PPy under different electrochemical methods and surfaces. Therefore, the general evaluation of the results demonstrated that it is possible to successfully apply PPy electrodeposition as a development method without harm to subsequent DNA tests, with considerable potential for two forensic tests from the same fingerprint.