Scientists from Australia and Israel have discovered a new, safe way of detecting invisible fingerprints.

Amino acids react with lawsone to give strongly luminscent fingerprints.

Fingerprints on porous surfaces such as paper are usually detected by a chemical called ninhydrin. Ninhydrin reacts with amino acids secreted by the fingers and turns invisible fingerprints dark purple. But ninhydrin is an irritant and so scientists are looking for safer alternatives.

Now, Simon Lewis at Curtin University of Technology, Perth, Australia and colleagues have found that a component of henna can work in a similar way to ninhydrin but without causing irritation. Henna is a traditional skin and hair dye, made from the leaves of the plant Lawsonia inermis. It has been used for more than a thousand years without ill effect.

Lawsone is the compound thought to be responsible for the staining properties of henna. It is a naphthoquinone, a group of compounds already known to react with amino acids. Lewis' team found that amino acids in invisible fingerprints turn a brown-purple colour when exposed to lawsone and are strongly luminescent under a forensic light source.

'The discovery will no doubt generate lots of activity in the global forensic identification community,' says Della Wilkinson, a forensic expert from the Royal Canadian Mounted Police. 'Lawsone has interesting spectroscopic characteristics that could prove to be very useful when examining surfaces that fluoresce under excitation wavelengths used for existing detection reagents.'

'This research opens the possibility of a whole suite of new analogues that may lead to further improvements in fingerprint detection,' says Lewis. His team are currently testing other closely related compounds.

Freya Mearns