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Fingerprints can be visualized using a technique called multi-metal deposition, in which colloidal gold nanoparticles that adhere to greasy skin secretions (sebum) left behind on a surface catalyse the precipitation of metallic silver from a developer solution. Ionic interactions between the gold colloids and the molecules in sebum are thought to drive this process.

Now, researchers at the Hebrew University of Jerusalem in Israel1 have found that when gold nanoparticles are capped with long hydrophobic molecules (alkanethiols), they stick to finger mark ridges through hydrophobic interactions. Daniel Mandler, Joseph Almog and colleagues collected sebum-rich finger marks on pieces of paper and immersed them in solutions containing the nanoparticles. Following treatment with the silver developer, it was found that longer alkanethiol molecules produced clearer fingerprints with greater contrast.

A second experiment was performed in which finger marks on a silicon surface were immersed in a solution containing CdSe/ZnS nanoparticles and a stabilizing organic molecule (octadecaneamine). After drying, fingerprint ridges could be observed under ultraviolet light. The method that forensic scientists use to visualize fingerprints is dependent on the surface that the finger marks were left on. The new one-step CdSe/ZnS method is appropriate for finger marks left on non-porous surfaces, whereas the gold nanoparticle approach can be used on porous surfaces such as paper.