Optical microscope photo (a) shows arrays of cantilevers of varying lengths. (b) Zoomed-in scanning electron microscope (SEM) image of several cantilevers, and (c) Oblique angle SEM image of a single 90nm thick silicon nitride cantilever with a 40 nm circular gold aperture centered 300 nm away from the free end.  Copyright © Cornell University

The answer: 995,000 Daltons (1 Dalton is about the weight of a single proton or neutron), or a bit over an attogram (10^-18 gram).

Cornell researchers -- who built a scale last year sensitive enough to weigh a virus -- have refined the system enough to be able to weigh a single DNA molecule. The scale actually measures the frequency of the vibration of a solid object, which will vary with its mass. The "nanoelectromechanical system" (NEMS) has not yet reached the limits of its potential sensitivity.

So what do you do with a scale that can measure DNA?

The technology, they suggest, can be combined with microfluidics to perform genetic analysis of very small samples of DNA, even the amount present in a single cell. Current techniques for genetic analysis require small samples of DNA to be replicated many times through a process called PCR amplification. DNA analysis can be used, among other things, to detect genetic markers for cancer susceptibility. [...]

While DNA molecules are fairly large, as molecules go, they are still a step smaller than most viruses, which consist of a DNA core surrounded by a protein coat. The Cornell researchers believe their technology could be used to identify even smaller organic molecules, including proteins, and could have widespread applications in medical and forensic diagnosis.

"The limitation in detecting specific molecules is in the chemistry. The mass resolution of the devices is orders of magnitude better than we're using here," said Harold Craighead, Cornell professor of applied and engineering physics. The ability to identify proteins and other organic molecules could lead to detectors for a variety of diseases, including HIV, he noted.

The ability to do DNA analysis without having to do PCR amplification would be a significant step. PCR amplification takes time and laboratory facilities. It's conceivable that this technique could be faster and require fewer lab resources. Vibration measurements may not be well-suited to field sensors, but this could still move us closer to a point where we can readily sense and analyze even the tiniest part of the world around us.