Dye-soaked cotton pads for latent blood fingerprint development

Abstract Visualization of latent blood fingerprints (LBFPs) at violent crime scenes is very important for identifying criminals, while current reagents/methods for developing LBFPs still have more or less limitations. Here, we advance a new strategy for LBFP development based on the dye-soaked cotton pads as “ensemble” materials. The cotton pad was first soaked in the dye solution and then placed on the substrate with deposited blood fingerprint for development. After peeling off the pad, uniform and legible fingerprint patterns were obtained in all cases where different dyes and substrates were employed. The dye-soaked pads displayed superior developing effect compared with those using conventional methods (immersing, smearing and spraying). The pad-based method exhibited many other merits, such as little damage to the fingerprint patterns and less dye stains left on fingerprint and substrate, as well as the reusability. This pad-based method was also applicable for developing sebaceous latent fingerprints by replacing the small molecular dyes with fluorescent conjugated polymer nanoparticles. In all, the dye-soaked cotton pad developing protocol is feasible, cost-effective and universally applicable for developing different types of latent fingerprints on various substrates. Graphic abstract