Microcantilevers bend to accommodate GC

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  • Published: Jan 8, 2007
  • Author: Jon Evans
  • Channels: Detectors
thumbnail image: Microcantilevers bend to accommodate GC

For the first time, a team of US chemists has successfully coupled a microcantilever array to a gas chromatograph (GC) column and shown that this set-up allows the array to overcome its normal inability to analyse complex mixtures.

Microcantilevers are microscopic strips of silicon (usually around 400 µm long and 100 µm wide) coated with a specific chemical compound, which acts as a molecular receptor. Any interaction between a passing molecule and this chemical coating causes the microcantilever to bend. This tiny movement can then be recorded by noting changes in the deflection of a laser beam shone onto the microcantilever, in much the same manner as in atomic force microscopy.

Understandably, researchers have been busy exploring the potential for using arrays of microcantilevers, each with a different coating, as chemical sensors. The idea is that the unique pattern of responses produced by the array on exposure to a specific compound can act as a 'fingerprint' for that compound.

One of the pioneers in this area is Michael Sepaniak, a professor of chemistry at the University of Tennessee, Knoxville. Over the past six years, he has developed microcantilver arrays for detecting a variety of chemical and biological compounds, including explosives and amino acids. But he has also discovered that, although these arrays are highly effective at detecting individual compounds, they tend to struggle when analysing even quite simple mixtures. This is because the set of responses from the microcantilever array quickly becomes too complex to discern any pattern.

For instance, at the beginning of 2006, Sepaniak and his team reported that an array of 10 microcantilevers could measure the ratio of two compounds in a mixture. But Sepaniak has since found that a similar array of six microcantilevers is unable to analyse a mixture of four volatile organic compounds (VOCs), only being able to identify three of the compounds with any confidence.

Now while this level of performance is clearly insufficient for analysing complex mixtures, it is more than sufficient for distinguishing co-eluting compounds in chromatography (where usually only two compounds co-elute at any one time). So Sepaniak and his team decided to investigate the possibility of using a microcantilever array as a GC detector.

To do this, the researchers simply coupled a single microcantilever, coated with tetrabutylammonium p-toluenesulfonate, to a GC column, without making any special modifications. They then tested its ability detect a variety of VOCs, including acetone, ethanol and trichloroethylene, after separation in the GC column.

Sepaniak and his team discovered that the microcantilever produced clear responses for each of the VOCs. Furthermore, this response did not vary too much with changes in the column operating temperature or flow rate. The microcantilever was not particularly sensitive (achieving limits of detection of 7-14 nmol), but Sepaniak argues that this proof-of-concept study was designed simply to see whether a microcantilever could be coupled to a GC column. He claims that an optimised system should be able to detect VOCs at concentrations below parts per billion.

He and his team therefore conclude that 'with the combination of pre-sensor separation, microcantilever array detection, and pattern recognition of array response signatures, the components of complicated mixture could readily be identified.' They also claim that these tiny arrays could assist with ongoing efforts to reduce the size of GC systems. Nevertheless, at the moment they are not conducting further research in this area.

The views represented in this article are solely those of the author and do not necessarily represent those of John Wiley and Sons, Ltd.


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