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Last Updated: Wednesday, 7 April, 2004, 12:57 GMT 13:57 UK
Scientists weigh tiny bacterium
By Dr David Whitehouse
BBC News Online science editor

Cantilever, Cornell
A test weight tips the scales
Scientists have developed a device able to measure the weight of a single cell, and they intend to weigh a virus next.

Made at Cornell University, it is a small cantilever whose vibration depends upon tiny masses placed on it.

The mass of a single cell of the E coli bacterium is 665 femtograms, the researchers say in the Journal of Applied Physics.

A femtogram is one-thousandth of a picogram, which is one-thousandth of a nanogram, which is a billionth of a gram.

Zepto science

The scale of the researcher's work is straining the number of prefixes needed to describe the world of the very small.

They have moved beyond the prefixes "nano", "pico" and "femto" to "atto," and now they have "zepto" in their sights. Officially zepto means one sextillionth of something, or one prefixed by 20 zeros.

The Cornell University group, headed by Professor Harold Craighead, report that they have used tiny oscillating cantilevers to detect masses as small as 6 attograms by noting the change an added mass produces in the frequency of vibration.

An attogram is one-thousandth of a femtogram, or a thousandth, millionth, billionth of a gram.

Their submicroscopic devices, whose size is measured in nanometres (the width of three silicon atoms), are called nanoelectromechanical systems, or Nems.

The attogram precision is important to weigh objects smaller than cells. The mass of a small virus, for example, is about 10 attograms.

The work is an extension of earlier experiments that detected masses in the femtogram range, including a single E. coli bacterium.

Eventually, the researchers say, the technology could be used to detect and identify micro-organisms and biological molecules.

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