Good vibrations

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  • Published: Jun 16, 2008
  • Author: Jon Evans
  • Channels: Detectors
thumbnail image: Good vibrations

A team of US engineers has shown that its newly-developed suspended microchannel resonator (SMR) can act as a universal detector for liquid chromatography and electrophoresis. Not only is this SMR highly sensitive but it is also microscopic, meaning that it should make an ideal detector for microchip-based separation systems.

The SMR consists of a thin strip of silicon, shaped like a diving board. It looks much like a microcantilever (see Microcantilevers bend to accommodate GC), but works in a slightly different way.

A microcantilever detects molecules based on the fact that they cause it to bend when they land on its surface. The SMR, on the other hand, takes advantage of the fact that a thin strip of silicon will naturally vibrate at a characteristic frequency, known as its resonant frequency.

Any particle landing on the silicon strip will alter the resonant frequency, with larger particles producing greater changes. This means that not only can the frequency change be used to detect the particle, it can also provide a measure of its mass.

A number of research groups have developed resonators that utilise this effect and shown that they can accurately weigh individual molecules. But all these resonators need to be surrounded by a vacuum, in order to prevent interference from any other molecules. This means that they can't be used to weigh living cells or any biological material suspended within a fluid.

To overcome this limitation, a team of engineers led by Scott Manalis from the Massachusetts Institute of Technology (MIT), Cambridge, etched a tiny channel into the silicon strip. The channel was 3µm deep and 8µm wide, and extended from the foot of the silicon strip up to the far end, where it turned round and came back (see image). Manalis and his team filled this channel with fluid, allowing cells and other biological material to travel along it, but then surrounded the whole strip with a vacuum.

This set-up allows the silicon strip to measure the masses of cells and other biological materials based on the changes they induce in its resonant frequency as they travel through the channel. Manalis and his team found that the SMR could weigh cells, proteins and nanoparticles with masses as low as one femtogram (10-15 grams).

But Manalis soon realised that the SMR can do much more than just weigh cells. 'The SMR was originally developed for detecting the mass of biomolecules that bind to the channel walls,' Manalis told separationsNOW. 'We then realized that it could be used to weigh particles such as cells. From these measurements, we found that the sensitivity as a densitometer was good and this is what led to the paper on universal detection.'

Manalis and his team first showed that the SMR could detect compounds such as polyethylene glycol (PEG), glucose and glycine, which don't absorb UV light and so can't be detected with UV detectors, at concentrations as low as 2.5µg/mL. They then coupled the SMR to a gel filtration chromatography column and showed that it could produce an accurate chromatogram for the separation of five different-sized PEG particles at concentrations of 5mg/mL.

Manalis claims that the fluid volumes and flow rates utilised by the SMR are compatible with those used in current microchip-based analytical systems, meaning that the SME would make an ideal on-chip detection technology. Nevertheless, Manalis doesn't have any immediate plans to develop the SMR as a universal detector, although he admits that this may be something he revisits in the future.

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

Suspended microchannel resonator
Silicon strip with channel shown in dark blue.
Photo courtesy of Thomas Burg, MIT.

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