Lone protein molecule could tip this scale.

Physicists developing exquisitely fine-tuned scales for weighing tiny objects have reached an important milestone--a device sensitive enough to detect individual molecules of biologically active proteins.

To make their protein scale, Michael L. Roukes of the California Institute of Technology in Pasadena and his colleagues fashioned bacterium-size bridges of silicon carbide, a durable semi-conducting compound, onto microchips. Then, they chilled those bridges in a vacuum chamber to temperatures near absolute zero and set them vibrating by means of electromagnetic forces.

By exposing the tuning fork-like devices momentarily to a spray of xenon atoms, the researchers found that the instrument responded--with a slight slowing of its vibrational frequencies--to as few as 30 atoms of xenon settling onto it.

The xenon atoms' collective mass is so small that the scientists had to resort to a little-known unit of measure--the zeptogram--to describe it. At 7 zeptograms, or billionths of a trillionth of a gram, this mass is comparable to that of many small proteins important in functions such as cell-to-cell signaling, Roukes notes.

Such on-chip bridges could prove valuable for investigating the vast and little-charted realm of protein behavior, or proteomics, says Roukes (SN: 12/13/03, p. 371).

The zeptogram-magnitude measurement also moves the team closer to its ultimate goal: a chip-based device capable of weighing a single hydrogen atom. That pursuit promises to open new linguistic territory as well: A hydrogen atom weighs about 1 yoctogram--a thousandth of a zeptogram.--P.W.