The Impossible Water Sensor

The EU Water Framework Directive requires that all surface water be of ‘good status' by the end of 2015. Checking the health of Europe's many water bodies entails understanding the levels of at least 500 chemicals

This year, there is a question hanging over Europe's rivers, lakes and seas. The EU Water Framework Directive, a water management plan adopted in 2000, requires that all surface water be of ‘good status' by the end of 2015. But how will the member states know whether they have met that objective? Checking the health of Europe's many water bodies entails understanding the levels of at least 500 chemicals. And analytical chemist Dermot Diamond says the EU has a problem there.

The only way to measure the levels of those 500 or so chemicals accurately is with a tandem liquid chromatography-mass spectrometry instrument (LCMS). But the use of such instruments is unlikely to be feasible on a Europe-wide scale. It would take multiple batteries of LCMS machines, plus staff and laboratories that meticulously keep track of their samples (to avoid accusations of fraud). That's not feasible, says Dermot, if only because each LCMS machine costs around £500,000.

The answer, he says, is likely to be remote sensors installed in water bodies that sit there quietly measuring chemical levels and broadcasting the data back to Europe's laboratories. The trouble is, sensors that fit this brief do not yet exist.

So chemists are beginning to ask what it would take to build a sensor that could be thrown into a lake or river, where it would sit for years, without harming the environment, remotely reporting chemical concentrations to a central hub. Meanwhile, big research funders are also beginning to recognise the pressing need for these sensors. For example, the X Prize Foundation, a not-for-profit outfit that offers large prizes to incentivise ambitious research projects, is currently offering $2 million in prizes for teams designing sensors that accurately and affordably measure ocean acidity. Another group is offering awards for affordable nitrate and phosphate sensors, though the prize for this has yet to be agreed.

Beginnings in the blood

If anyone is likely to invent these challenging gizmos, it is Dermot. He has a history of developing sensors that can operate in hostile environments - including the soup of chemicals running in our veins.

At the start of the 1990s, he was part of a team that created asensor for sodium in the blood.Blood has some similarities with lake water, in that there are a whole bunch of chemicals swirling around and this makes it difficult to pick out sodium accurately, while avoiding the similarly-sized potassium, for example.

Dermot's research team did it using calixarenes, cup-shaped molecules that sodium ions neatly fit into. They developed a sensor in which the calixarenes were bound onto a polymer membrane, so that when sodium is present in the sample, it is selectively absorbed into the polymer, creating a signal that can be converted into a sodium concentration (see Calixarenes in the blood ).

These days the blood sodium sensor Dermot helped to invent is used to profile electrolyte levels in clinical settings. The doctor takes a blood sample and generates the electrolyte profile using analysers typically based in centralised biochemical labs. The sensor cartridges used in these instruments have to be replaced regularly due to their limited lifespan when exposed to blood. In many cases, biochemical sensors are used only once, then discarded.

Early on in his thinking about water sensors, Dermot wondered if the functional part of this sensor could be used in lakes or rivers. He tried it out, but found that after just a few days a sort of biochemical slime, essentially bacteria and algae, would begin to accumulate on the sensor's surface. This ‘biofouling' effect quickly threw off the sensitive electrochemical measurements. He needed a sensor that could be ‘deployed and forgotten', as he puts it, because the cost of sending a technician to clean a sensor every week or so would quickly mount up with a device in every river in a country. He needed to re-think the design.

At the sewage works

Skip to 2009, and Dermot is at the Oberstown Wastewater Treatment Plant, a sewage works near Dublin, Ireland.

Dermot knew a sensor with a surface that could be rendered insensitive by biofouling would not work. So he had been working on a new type of sensor that used old fashioned chemical reactions to produce colour changes. Since bacteria shouldn't disrupt this kind of test, the sensor would surely work in even highly polluted water, Dermot reasoned. Where better to prove its resilience than a sewage works?

Here's how the sensor works. The method is based on a well-known chemical reaction that produces a colour change. Two reagents, ammonium molybdate and ammonium vanadate in sulfuric acid, are mixed with the water sample. Any phosphate in the water sample will react with the two reagents to produce a compound called vanado-molybdophosphoric acid. This compound is yellow and absorbs light specifically with a wavelength of 370 nm, so it's possible to measure its concentration photometrically: you shine light with a wavelength of 370 nm through the solution and record how much is absorbed. The absorption reading obtained is directly proportional to the amount of phosphate in the water sample. The method was implemented using microfluidics for the sample and reagent handling, and a low cost LED-photodiode to detect the colour.

Thesensor trialran for seven weeks and it gave accurate readings during that whole period, except for on a few days when air bubbles were drawn into the microfluidics, resulting in anomalous spikes. One issue was the ‘limit of detection', the lowest concentration of phosphate that the sensor could detect. This was rather high in the Oberstown trial prototype, but Dermot says he's currently collaborating with a group at the University of Southampton to test a new design that will offer improved sensitivity.

Source: EducationInChemistry

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• Sensor Systems