CRIME INVESTIGATION Tutorial IV – Silent Witnesses talk.

A fingerprint developed with luminescent nanoparticles (green) has ridges with better definition than one dusted with commercial black powder (gray), 20 days after deposition on aluminum foil.

Latent fingerprints create Silent Witnesses

A new fingerprinting technique that uses long-lived luminescent nanoparticles provides sharp images of otherwise invisible prints. The method offers better resolution than standard fingerprinting for forensic investigation, the researchers say.

When collecting fingerprints at crime scenes, investigators choose from a handful of reagents to reveal the patterns deposited on surfaces by skin oils and proteins. They dust or spray surfaces with black carbon-based powder, organic dyes, or fluorescent molecules

The choice depends on what will maximize the prints’ visibility on a given surface, whether porous, nonporous, dark, or multicolored.

To enhance the contrast of fingerprints against surfaces, researchers like to stain them with molecules or metallic nanoparticles that glow when light is shined on them. But some surfaces—including metal and plastic—themselves fluoresce under ultraviolet light, which can obscure the details of a glowing print.

Researchers at China’s Wuhan University wanted to eliminate this background fluorescence. Surface fluorescence typically disappears once the UV light is turned off, so they sought a material with long-lasting luminescence that would continue to shine even after UV exposure was over.

Nanoparticles can reveal totally hidden fingerprints.

To examine the particles as a fingerprint stain,  the researchers tested a playing card and a soda can that someone had touched, leaving fingerprints. They pipetted an aqueous solution of the nanoparticles onto each surface, removed the liquid after 30 minutes, and rinsed the surface with phosphate buffer.

When they shined UV light on each sample, the blue fluorescence of the playing card and the red fluorescence of the can outshone the glow from the nanoparticles. But once the light was turned off, only the green glow from the particles remained, revealing the whorls, branches, and other patterns in the fingerprints on each surface.

Fingerprints developed with glowing nanoparticles showed more defined ridges than ones dusted with conventional black powder. The particles also revealed clear fingerprints left 60 days earlier.

Evaporated gold and zinc – new fingerprint tactic.