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Nanocantilevers studied for quick pathogen detection

By Ahmed ElAmin , 29-Aug-2006

Nanocantilevers could be crucial in designing a new class of ultra-small sensors for the quick detection of viruses, bacteria and other pathogens, say researchers at Purdue University.

Nanotechnology has been touted as the next revolution in many industries, including food manufacturing. It holds the promise of helping manufacturers produce novel products and improve their processes and packaging.

Nanotechnology is the study of the way materials behave at a scale that can be as small as one billionth of a metre. Industry is interested in nanoscale materials because at this size their properties can be very different from those of the same material at a larger scale. Nanocantilevers, which look like tiny diving boards made of silicon, are one type of nanoscale materials that are being studied as part of general research into nanotechnology.

The nanocantilevers could be used in future detectors because they vibrate at different frequencies when contaminants stick to them, revealing the presence of dangerous substances.

Because of the nanocantilever's minute size, it is more sensitive than larger devices, promising the development of advanced sensors that detect minute quantities of a contaminant to provide an early warning that a dangerous pathogen is present, the Purdue University scientists say.

They say they were surprised to learn that the cantilevers, coated with antibodies to detect certain viruses, attract different densities - or quantity of antibodies per area - depending on the size of the cantilever. The devices are immersed into a liquid containing the antibodies to allow the proteins to stick to the cantilever surface.

"But instead of simply attracting more antibodies because they are longer, the longer cantilevers also contained a greater density of antibodies, which was very unexpected," stated Rashid Bashir, a researcher at the Birck Nanotechnology Center and a professor of electrical and computer engineering and biomedical engineering at Purdue University.

The research also shows that the density is greater toward the free end of the cantilevers. The engineers found that the cantilevers vibrate faster after the antibody attachment if the devices have about the same nanometer-range thickness as the protein layer.

The longer the protein-coated nanocantilever, the faster the vibration. This effect could only be explained if the density of antibodies were to increase with increasing lengths, Bashir stated in releasing the results of the study yesterday.

The research group also proved this hypothesis using optical measurements. They then worked with Ashraf Alam, a researcher at the Birck Nanotechnology Centre and professor of electrical and computer engineering, to develop a mathematical model that describes the behaviour.

The information will be essential to properly design future "nanomechanical" sensors that use cantilevers, Bashir said. So-called "lab-on-a-chip" technologies could make it possible to replace bulky lab equipment with miniature sensors, saving time, energy and materials, he stated. Thousands of the cantilevers can be fabricated on a one-square-centimeter chip.

The cantilevers studied in the work range in length from a few microns to tens of microns, or millionths of a meter, and are about 20 nanometers thick, which is also roughly the thickness of the antibody coating. A nanometer is a billionth of a meter, or approximately the length of 10 hydrogen atoms strung together.

A cantilever naturally "resonates," or vibrates at a specific frequency, depending on its mass and mechanical properties. The mass changes when contaminants land on the devices, causing them to vibrate at a different "resonant frequency, " which can be quickly detected.

The work, funded by the National Institutes of Health, is aimed at developing advanced sensors capable of detecting minute quantities of viruses, bacteria and other contaminants in air and fluids by coating the cantilevers with proteins, including antibodies that attract the contaminants.

The study is outlined in a research paper published online yesterday in Proceedings of the National Academy of Sciences.

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