Water stream detects pollution better

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A microfluidic sensor with a novel design uses unconfined streams of water as a simple, cheap way to test for dangerous chemicals and bacteria in liquid.

The pollutant detector, developed in Italy within the framework of the research project ACQUASENSE, forgoes the channel walls used in today’s more advanced water-testing devices, which can produce background noise that interferes with readings. Instead, it favors a narrow stream of water unconfined by tubes or pipes, with the water jet doubling as both the sample and the collection equipment.

“The innovative aspect of this sensor is related to the simple and tricky way used to collect the light: a water stream,” said Gianluca Persichetti of the Institute for Electromagnetic Sensing of the Environment (IREA) at the National Research Council (CNR) in Naples, Italy.

laser beam
A schematic of the technique used to detect contaminants: A laser beam is directed perpendicularly into the flow of water; fluorescent light from contaminants in the water is carried away to be detected and analyzed. Photo courtesy of Gianluca Persichetti, IREA-CNR.

Typical microfluidic detectors rely on narrow channels to hold and control the water samples with their fluorescing organic compounds. But the laser light that illuminates bacteria and chemicals in the water also shines on the channel walls, where it scatters and obscures the distinction between the fluorescing contaminants and their background. Wear and tear on the walls also weakens the reliability of measurements made using these instruments.

Instead, Persichetti and his colleagues in the group, led by Romeo Bernini, decided simply to do away with them. In their new technique, they pump the water sample through a nozzle at 1.4 m/s, producing a narrow stream that is less than 1 mm in diameter. Then they shine a UV laser onto the exposed jet of water. The fluorescent light produced by the pollutants and bacteria bounces around and is trapped inside the jet, which acts as a waveguide, channeling light through the stream.

The laser light is also a source of background noise that can cloud the signal, Persichetti said, so the amount of light that gets trapped in the jet was minimized by firing the laser beam at an angle perpendicular to the water stream.

After traveling 16 mm, the jet enters a small pipe containing an optical fiber, which collects the fluorescent light signals. The water is pumped back to recirculate through the nozzle, allowing the instrument to analyze even a small sample for an extended period.

The device was tested with varying amounts of some of the main pollutants of groundwater: soil contaminants such as BTX (benzene, toluene and xylene), and polycyclic aromatic hydrocarbons, which are hazardous, carcinogenic chemicals found in tar and petroleum. Persichetti and colleagues found that the device was extremely sensitive: It could detect pollutant levels even lower than those allowed by the US Environmental Protection Agency. The instrument also could sense Bacillus subtilis, a harmless bacterium similar to the one that causes anthrax.

The current device is fitted with a spectrophotometer to measure the optical signatures of specific chemicals. To make the device even cheaper and smaller for future commercialization in water-safety testing, the researchers plan to replace the laser with an LED and to replace the spectrophotometer with a simpler and less expensive setup with a filter to remove unwanted background light, and to add a photodiode to detect the signal.

The paper appears in Optics Express (doi: 10.1364/oe.21.024219).

Published: December 2013

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