Sense of achievement

Sensors designed to detect the various types of environmental problems are vital tools in the clean-up of contaminated land, and in tracking air pollution.

They’re also a focus of much research, with traditional chemistry, the emerging techniques of nanotechnology and engineering all combining to create a new generation of small, sensitive devices that provide more information with less effort and, ultimately, at a lower cost.

The main areas for sensors using nanotechnology are for testing, monitoring and remediating waste dumps and other contaminated sites, said Rajesh Kanan, an analyst with Frost & Sullivan’s Technical Insights team in Singapore.

‘Among some of the promising applications for sensors in this market segment is for the Environment Protection Agency (EPA) environmental analysis, for monitoring and detecting aquatic toxins, like domoic acid, brevetoxins, and textrodotoxin,’ he said.

Such tests, on the surface of the sites and in groundwater, can provide early warning and prevention of heavy metal ion pollution. Of particular interest are remote, in situ, and continuous monitoring devices capable of yielding real-time information, and also those that can detect pollutants in water at very low concentration levels.

One technology which is attracting attention is the chemiresistor, which can detect volatile organic compounds (VOCs) in soil and groundwater. Invented and under development at Sandia National Laboratory in New Mexico, chemiresistors may be used in the monitoring and clean-up of leaks, toxic chemicals, solvent spills and explosives. The technology’s great advantage is that is can carry out this detection in situ.

‘Traditionally the contaminated sites would be monitored by physically collecting large number of samples and transporting them back to an offsite laboratory for analysis — a time-consuming and costly process,’ said Kanan.

The chemiresistor was invented by Sandia researchers Bob Hughes and Cliff Ho, who describes the device as a sort of artificial and very specific nose.

‘It has the capacity of detecting in real time undesirable chemicals being pumped into the water supply either accidentally or intentionally,’ he said.

The construction of the chemiresistor is fairly typical of this type of sensor. The crucial component is the sensitive material, a polymer which swells in the presence of the substance which the sensor is intended to detect. This is dissolved in a solvent and mixed with granules of conductive graphite to form an ink-like material, which is ‘printed’ on to wire-like electrodes. When the polymer swells, the electrical resistance of the ink increases, and the swelling and resistance increase are directly proportional to the concentration of the pollutant.

‘By using four different kinds of polymers — one for each sensor — we think we can detect all solvents of interest,’ said Hughes.

The sensor is housed inside a small, rugged case, generally made of stainless steel, sealed with a waterproof, gas-permeable membrane which keeps water away from the electronics, but allows VOCs to diffuse into the system.

The ‘artificial nose’ concept is attracting a great deal of attention, both in the environmental sector and in medicine, where the instrument’s sensors are used to detect traces of gas in patients’ breath, aiding disease diagnosis.

A more complex type is based on surface acoustic wave (SAW) technology. These systems use crystals which have piezoelectric properties, vibrating at a characteristic frequency when a current is passed through them. The crystals can be coated with a reactive polymer, similar to those used in the chemiresistor; in this case, the swelling of the polymer changes the frequency of vibration.

While VOCs are a major problem in contaminated sites, nonvolatiles are equally troublesome, and can be even harder to detect and handle. One way of dealing with these is offered by ribbon non-aqueous phase liquid (NAPL) samplers which, Kanan believes, are an example of a new technology which may supplant the existing systems.

Currently under development, the systems, as the name suggests, detect dense NAPLs (DNAPLs) — liquids which do not mix with water, and are more dense, including organic solvents such as dichloroethane, carbon tetrachloride, coal tar, fuel oils and aviation fuel. These are a particular problem in contaminated land.

Most contaminants are carried along with water flow, and can be detected by sampling the groundwater. But DNAPLs sink through groundwater under gravity, and tend to be difficult both to detect and to characterise.

The main part of the ribbon NAPL system — under development by Westinghouse — is a simple concept: a long polymer ribbon, impregnated with a dye which changes colour on contact with the DNAPL. This is surrounded by a porous inflatable liner.

Testers drill a narrow borehole at the contaminated site and insert the probe, then inflate the liner against the walls of the borehole. This pushes the ribbon down the hole, and any liquid present in the soil passes through the membrane, where it is absorbed by the ribbon. When the ribbon is removed, telltale staining tells the testers whether DNAPLs are present, and at what depth they occur.

This is a much better option than the current technologies which, as with many others, involve removing samples and testing them at off-site laboratories.

As well as being quicker and cheaper the ribbon NAPL sampler provides ‘a continuous record of the distribution of zones contaminated with separate phase contaminants’.

A typical test costs around $15,000 (£8,500), less than half the cost of sediment sampling and analysis, according to the US Department of Energy, which has tested the technology at several sites, including Cape Canaverel. Another advantage is that it’s safer: operators are not exposed to the contaminants during testing, as they do not have to handle them; and as the test only requires the drilling of a narrow borehole, it produces much less secondary waste than removing and preparing samples for analysis.

Further developments are likely with different drilling techniques, which would allow the system to be used at greater depths, or in other materials, such as fractured rocks. In the laboratory, meanwhile, work is underway on the next generation of nanotechnology-based sensors.

At the University of Leeds, atmospheric chemistry specialist Jim McQuaid is leading a project to develop an ozone sensor based on the phenomenon of chemiluminescence. McQuaid explained that certain chemical reactions produce light as they progress, and as the amount of light — which is easily measurable — is directly proportional to the concentrations of the reactants, this is a promising area for gas detection.

Current ozone detectors are limited because they tend to be slow; McQuaid believes that chemiluminescence-based sensors will be much faster, and could be mounted on aircraft to investigate how ozone pollution travels away from its source.

Other promising research focuses on carbon nanotubes, whose potential — although some distance off — stems from their ability to conduct electricity, and how this is affected by the adsorption of gas molecules on their surfaces. The effect on a single nanotube is very small, but 2D networks of nanotubes seem to have potential as highly sensitive measuring devices, for applications including atmospheric monitoring and medicine.

However, the commercial acceptance of carbon nanotubes depends on the ability to mass produce them, something which, according to Frost & Sullivan, is still beyond the reach of industry. But once that problem has been solved, the new generation of sensors will be poised to change the face of the sector.