UK chemists have stumbled across a fascinating application for a polymer precursor, which could provide a new technique for the ancient forensic science of fingerprinting.

Invisible fingerprints on surfaces exposed to the disulfur dinitride vapour turn black

Paul Kelly and colleagues at Loughborough University were investigating the polymerisation of disulfur dinitride (S₂N₂) to form the conducting polymer (SN)_x when they discovered that invisible (or latent) fingerprints on surfaces exposed to the S₂N₂ vapour turned black. They found that the darkening effect was due to the spontaneous formation of a layer of (SN)_x polymer on the prints.

Latent prints are normally detected by forensic scientists using a soft brush and fine aluminum powder. Usually they only show up on hard surfaces such as glass, metal or polished wood. Kelly says his technique is unique because it detects prints on almost any surface. 'It soon became apparent that S₂N₂ can "develop" prints from a vast range of media, from metal surfaces through to cotton and plastics. A one-covers-all versatile system like this has obvious potential,' says Kelly.

Fingerprints appear to initiate the polymerisation process, claims Kelly, although traces of inkjet printer ink can also act as the initiator. The detection limit is so low that details of a printed letter previously contained inside an envelope could be read off the inside of the envelope after exposure to S₂N₂.

'This work has demonstrated that it is possible to obtain fingerprints from surfaces that hitherto have been considered extremely difficult, if not impossible, to obtain,' says Colin Lewis, scientific advisor at the UK Ministry of Defence. 'The method proposed has shown that this system could well provide capabilities which could significantly enhance the tools available to forensic scientists in the future.'

James Hodge

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