Device Detects the Mass of a Single DNA Molecule

Some rather interesting research is coming out of Cornell University:

Some people are never satisfied. First, nanotechnology researchers at Cornell University built a device so sensitive it could detect the mass of a single bacterium–about 665 femtograms. Then they built one that could sense the presence of a single virus–about 1.5 femtograms. Now, with a refined technique, they have detected a single DNA molecule, weighing in at 995,000 Daltons–a shade more than 1 attogram–and can even count the number of DNA molecules attached to a single receptor by noting the difference in mass.
The devices, which fall in the class of nanoelectromechanical systems (NEMS), could be made even more sensitive through increased miniaturization, the researchers say…
The principle underlying the mass-detection devices is that the frequency at which a solid object vibrates varies with its mass. A big bell rings at a lower tone than a small one. To apply this at the nanoscale, the researchers used the Cornell Nanoscale Facility to create arrays of tiny cantilever oscillators 3 to 5 microns long and 90 nanometers thick on silicon chips — imagine a diving board that would bounce if you dropped a large bucket of atoms on it. At the end of each cantilever they deposited a tiny dot of gold 40 nanometers in diameter. (A nanometer is one-billionth of a meter, or about the length of three silicon atoms in a row. A micron is 1,000 nanometers.).
A solution containing a strand of DNA consisting of 1,578 base pairs was washed over an array of cantilevers. For experimental purposes, the DNA was modified by the addition of a molecule called a thiol, which contains sulfur atoms that tend to bind to gold. As a result, some of the DNA attached to the gold dots.
When excited by energy from a laser, these cantilevers oscillate at frequencies of around 11 to 12 Megahertz (MHz). The frequency is measured by shining another laser on the oscillator and noting interference patterns in the beam caused by the reflected light. In the reported experiments, the change in mass of 1 attogram was enough to shift the frequency of vibration by 50 Hz or more, depending on the size of the oscillator. With the smallest and most sensitive device, the shift was 194 Hz. This allowed the researchers not only to detect the binding of DNA molecules, but also to count the number of molecules attached to a single receptor by the total frequency shift. By diluting the sample solution, they were able to identify cantilevers to which single DNA molecules had attached.

Image caption: 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. The scale bar corresponds to 100 nm.
The press release…