Pollen could be used to identify the perpetrators of gun crimes, thanks to developments in nanotechnology. The microscopic grains can be coated onto bullets during manufacture and are sticky enough to hold on even after the gun has been fired. Each 'nanotag' is made up of pollen and a unique chemical signature that can be used to identify the batch of ammunition.

The pollen grains – from one of two species of lily – are around 30 micrometres in diameter and are invisible to the naked eye. Thousands can be attached to each cartridge.

"The tags primarily consist of naturally occurring pollen, a substance that evolution has provided with extraordinary adhesive properties," said Prof Paul Sermon from the University of Surrey, who led the research. "It has been given a unique chemical signature by coating it with titanium oxide, zirconia [zirconium dioxide], silica or a mixture of other oxides. The precise composition of this coating can be varied subtly from one batch of cartridges to another, enabling a firm connection to be made between a particular fired cartridge and its user."

According to the UK Home Office, there were 18,489 crimes involving firearms in England and Wales in 2006-7. But DNA profiling using skin cells or other biological material stuck to gun cartridges is only successful in about 10% of cases. DNA is damaged by the heat produced when the bullet is fired and also by copper released from the cartridge case by mildly acidic sweat.

If all ammunition were marked with pollen tags, they could help police find out when and where a bullet had been bought. The nanotags are also difficult to remove from clothes and hands, so it would be possible to prove that a suspect had handled a particular batch of ammunition.

The pollen tags could be combined with another method developed by the same team that involves roughening the surface of the bullet at a very fine scale so as to capture dead skin cells from the thumb of someone handling it.

"We're currently focusing on understanding the precise requirements of the police and cartridge manufacturers," said Sermon, "but our work clearly could make a valuable contribution not only to solving gun crime but also to deterring criminals from resorting to the use of firearms in the first place."