Near-field Scanner for Moving Molecules

A near-field scanner for moving molecules has been built and demonstrated by a multinational research team (Robert Austin, Princeton, 609-258-4353, rha@suiling.princeton.edu), offering a potentially fast way to make high-resolution images of molecules such as DNA. Traditional scanning-probe microscopes offer molecular-level images, but at the cost of slow scanning speeds for large molecules.

In the new device, molecules travel in a microscopic fluid channel (5 microns wide by 1 micron deep) and pass directly under a trio of 100-nm-wide slits that are just a few hundred nanometers above the molecules. The fluid channel contains an array of posts to stretch out the DNA molecules. A laser causes the molecules to fluoresce, providing light that yields an image. The slits' narrow width, along with their proximity to the molecules, enables high-resolution images, 200-nm resolution in this initial experiment. To ensure high-quality images, the microscope accepts data only from those molecules that pass through the three slits at roughly equal time intervals.

For a DNA molecule with 200,000 base pairs (corresponding to about 74 microns in stretched form), the researchers obtained imaging data in just 100 milliseconds, considerably faster than AFM or traditional near-field optical microscopes. Resolution improvements are possible by narrowing the slits or making them thinner; future versions of the device will employ shallower fluid channels for confining DNA molecules to a greater degree.

Ultimately, the researchers envision massively parallel data acquisition by creating multiple slits that simultaneously scan many molecules. This microscope design could potentially obtain high-resolution maps of the binding sites of repressor/promoter proteins critical for the expression of genes, part of an emerging field called epigenetics. (Tegenfeldt et al., Physical Review Letters, 12 February 2001.)