The optical microscope, pioneered by the Dutchman Antony van Leeuwenhoek in the seventeenth century, transformed scientific discovery even more than Galileo's telescope. Through its lens, scientists have seen bacteria and viruses, as well as chromosomes and the many other tiny structures that make up our cells. Optical microscopes are now regarded as pretty blunt probes, compared with instruments such as electron microscopes -- but they are far from obsolete.

As researchers in Germany now report in Nature [18 May 2000]1, single glowing molecules can illuminate samples in an optical microscope. With such a tiny light source, the new device should be capable of revealing individual molecules. Other, non-optical microscopes can already do this, but light-based methods bring certain advantages -- particularly to the study of living cells.

The new instrument, described by Vahid Sandoghdar and colleagues at the University of Konstanz, Germany, is a development of a technique called scanning near-field optical microscopy (SNOM). This method avoids a problem that restricts the resolution of optical microscopes: a light beam cannot be focused to a point finer than its own wavelength -- the 'diffraction limit'.

The diffraction limit of visible light is several hundred nanometres (millionths of a millimetre). Individual cells are typically larger than this by a factor of ten or more, so they can be studied. But small molecules, less than a nanometre across, are much too small to view with conventional optical microscopy.

Near-field optical microscopy beats the diffraction limit by using a light source so small that it does not need focusing. This source is placed very close to the sample, so it only illuminates an area about the size of the source. The source is then scanned across the sample, lighting it up like a roving torch beam to build up an image, little by little.

How are the tiny light sources needed for SNOM made? One way is to pull out a hot glass optical fibre into a conical tip, and then to coat all but the very end with metal. Light inside the fibre emanates from this exposed tip. This approach has produced sources no more than 100 nanometres or so across, which permits optical microscopy with comparable resolution. But the best systems run aground at around 50 nanometres, partly because metal films are leaky 'masks' over this distance and partly because it is hard to make fibre tips any finer.

Sandoghdar's team have now made a tapering optical fibre that ends with a single molecule, which lights up (fluoresces) when illuminated by laser light in the fibre. So the tip becomes essentially a single-molecule torch, and its fluorescent emission illuminates the sample.

The fibre tip itself is much wider than this single molecule. To this tip, the researchers glued a tiny organic crystal, in which a handful of molecules of the fluorescent substance 'terrylene' were dispersed. These terrylene molecules each take up a random position in the organic host material, so they absorb laser light at a slightly different frequency. This means that the laser can be tuned to excite just one of the terrylene molecules.

Using their single-molecule probe, Sandoghdar and colleagues have taken images of a micro-patterned array of metal islands on a glass slide. The islands, about 500 nanometres across, could be seen in the image taken with the light from a single molecule. By tuning the excitation laser to light up molecules right on the edge of the tip (and so nearest the sample), the researchers expect to be able to achieve much higher resolution, even picking out individual molecules.