Criminals are smuggling billions in US bank notes into Mexico every year, but help could be on the way for border guards.

A machine that can "smell" dollars - like a sniffer dog senses narcotics - is being developed.

For the first time, chemists have captured the unique fragrance of US paper money.

They announced their discovery at the American Chemical Society meeting.

"Money sniffing is an unknown art. No-one had ever tried to find these aromas," said Dr Joseph Stetter, of KWJ Engineering.

"We found that US currency emits a wide range of volatile organic compounds that make a 'fingerprint' we can identify in less than a minute."

His firm is developing a handheld cash detector for border police, called the Bulk Currency Detection System (BCDS).

In the past fiscal year, US officials seized more than $106m in smuggled cash heading to Mexico - the bulk of it laundered drug money.

But that's only a whisker of the estimated $39bn that crosses the border undetected every year - hidden in clothing, baggage and vehicles.

Current checks are done by guards with dogs - but training is expensive and time-consuming.

To read the complete article, see:'Smell of dollars' could catch smugglers(www.bbc.com/news/science-environment-28802706)