If you eat a lot of large predatory oceanic fish, such as tuna or swordfish, then you're at risk of mercury poisoning. The condition affects the central nervous system, and can result in symptoms such as muscle weakness and numbness, or a difficulty in speaking, hearing or seeing. Each individual big fish doesn't necessarily contain a lot of accumulated mercury, though, which is why scientists have created a probe that indicates which ones do.

Building on one of their previous projects, scientists at Spain's University of Burgos developed a polymer known as JG25.

That material has been incorporated into a probe, which is inserted into 2-gram flesh samples of various species of fish. After being left in contact for 20 minutes, it's then taken out and exposed to ultraviolet light. It responds by fluorescing blue – the more intense the color, the greater the amount of mercury present in the sample.

The polymer reacts to the presence of both organic methylmercury, and inorganic Hg2+ mercury salt.

It is now hoped that a product based on the technology could be used on-the-spot in settings such as fish markets. Currently, samples have to be sent off to labs for mercury-content analysis.