Electronic tongue can taste pollution

Researchers at Cardiff University’s School of Engineering have developed an electronic ‘tongue’ that can be dipped into water or effluent to check for pollution.

The research, funded by the Engineering and Physical Sciences Research Council and headed by micro-component specialist Professor David Barrow, has proved that the part of the system that ‘tastes’ the sample can be made from very small components, so it should be easy and inexpensive to mass-produce.

The scientists hope to link the tongue to a computer so that the signals that it generates can be analysed. The tongue’s ability to taste is based on the principles of chromatography, where a chemical sample, contained within a liquid or gas, is passed over a solid matrix with a large surface area – a glass cylinder filled with silica beads, for example. Various chemicals that react to components within the mixture can be coated onto the beads so that as the sample passes down the column a reaction will take place, enabling the components of the mix to be identified and analysed.

The Cardiff tongue works on a similar principle but on a considerably smaller scale. It is possible to coat a silicon chip with hydrofluoric acid in a controlled way so that it becomes etched with millions of pores, or spots, and channels just nanometres across.

“So if you etch a spot a millimetre square, you actually have several square metres of internal surface area,” says Professor Barrow. “It is, in effect, a tiny chromatographic ‘extraction cartridge’.” For demonstration purposes, the Cardiff team created a chip containing an array of 16 such spots.

Different chemical ‘hook’ themselves into the cavities of each chromatographic spot enabling each one to grab a variety of chemical species with a different strength.

When the sample to be analysed is presented to the chip, its different chemical components bind preferentially in different spots. However, it is too difficult to analyse each chemical component separately. A far quicker, more efficient and cheaper approach is to take some measure of the pattern of binding across the array – that is, to obtain a ‘fingerprint’ of the mixture, which can be related to its composition.

To do this, the researchers sandwiched the chromatographic chip between two gold electrodes. By passing a voltage between the electrodes – across the chip – it has been possible to measure the chip’s impedance. If the voltage is alternating, its frequency can be changed so that a range of impedance values can be measured.

This pattern of impedance changes according to the type of molecules that have been captured by the spots.

The researchers have successfully assembled the microfluidic system necessary to direct the liquid into the chromatographic cell and have shown that an impedance signal can be obtained. Having demonstrated that the ‘tasting’ part of the electronic tongue works, the next stage will be to link it to a computerised system that can be trained to recognise different patterns of impedance signal.

“You could put the system in a river or factory process stream to monitor the mixtures flowing through it,” said Barrow. “If the pattern of the impedance signal deviated suddenly it would indicate that something unexpected had entered the stream and could trigger an alarm.”

Speaking of his work, Professor Barrow recently said: “Microsystems technology is a rapidly growing discipline, that already, in its very infancy, is worth some $28 billion (£19.5 billion), and the market, in terms of application, is as much as ten-fold that figure. The IT revolution we are witnessing now will expand further with the common availability of massive amounts of (bio)chemical information.”

He believes there is an immense requirement for the precision measurement of chemicals across a range of industries and for the management of the natural habitat. He predicted that micro-instrumentation would allow comprehensive fingerprinting of water quality through distributed remote-sensing systems. This would make water supply management more responsive and cleaner, he added, enabling improved pollution control and drinking water supplies.

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