Fishy sensor

Researchers at Purdue University have harnessed the sensitivity of days-old fish embryos to create a system capable of detecting a range of harmful chemicals.

By measuring rates of oxygen use in developing fish, which are sensitive to contaminants and stressful conditions, their system could reveal the presence of minute levels of toxic substances before they cause harm.

Marshall Porterfield, an associate professor of agricultural and biological engineering at the university, said: 'It could be used as an early warning system against environmental contamination or even biological weapons.'

Respiration is often the first of a fish's bodily functions affected by contaminants. The Purdue system uses a fibre optic probe to monitor this activity and produce results within minutes.

In the laboratory, Porterfield and his researchers first manually positioned the tiny optical electrode just outside individual embryos of two-day-old fathead minnows. At 1.5mm in diameter, they were slightly smaller than the head of a pin.

A fluorescent substance coated the electrode tip, its optical properties varying predictably with oxygen concentration. This allowed the researchers to take quick measurements at locations only micrometers apart, moving the electrode via a computer-driven motor. These readings then allowed researchers to calculate respiration rates within the eggs.

Using a self-referencing technique Porterfield developed over the last decade, he and his team measured each egg with and without contaminants present. This allowed each embryo to serve as its own control, he said, providing more reliable results.

The system has already detected the presence of several common pollutants such as the widely used herbicide atrazine - even at levels near or below those that the US Environmental Protection Agency deems acceptable for drinking water.

Testing also registered noticeable changes in the respiratory activity of fish embryos when the heavy metal cadmium was present at levels 60 times lower than the EPA limit.

A prototype could be ready to test in the field in four years if improvements continue. The technology currently tests immobilised eggs in a laboratory setting but there are plans to make the system more versatile.