A near-field scanner

For moving molecules has been built and demonstrated by a multinational research team. The scanner offers a potentially fast way to make high-resolution images of molecules such as DNA. Traditional scanning-probe microscopes can produce molecular-resolution images, but at the cost of slow scanning speeds. The new device, shown here, is stationary; molecules travel past an array of posts intended to stretch them, then proceed through a microscopic fluid channel (5 microns wide by 1 micron deep) across a trio of 100-nm-wide slits illuminated with near-field laser light. The laser causes the molecules to fluoresce, and that fluorescence yields a far-field image. To ensure high-quality images, the microscope accepts data only from those molecules that cross the three slits at roughly equal time intervals. The researchers obtained 200-nm-resolution imaging data in just 100 milliseconds for a DNA molecule with 200‥000 base pairs (corresponding to about 74 microns in stretched form). Resolution improvements are possible by narrowing the slits or making the covering plate thinner. Future versions of the device will have narrower and shallower fluid channels for better stretching of the molecules. Such a device 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. (J. O. Tegenfeldt et al., Phys. Rev. Lett. 86 , 1378, 2001 http://dx.doi.org/10.1103/PhysRevLett.86.1378 .)