CHEMISTRY: SCRATCHING THE SURFACE Using what could be the sharpest needle in the world, a team of researchers in Japan have pricked the first few layers of water molecules close to the surface of a solid material, and found them unusually resilient. The findings imply that proteins and other biomolecules might be influenced by the water molecules around them, that are arranged in a more orderly manner than in the rest of the liquid. Suzanne Jarvis of the Joint Research Center for Atom Technology in Tsukuba, Japan, and colleagues have used a device called the atomic force microscope (AFM) to measure the force needed to press a needle through water lying just above a surface coated with a film of organic molecules. They find that the force oscillates, alternately rising and falling as the needle's tip advances towards the surface. This behaviour, surprising though it may seem for a needle penetrating a liquid, is precisely what previous experiments had predicted. Very close to an obstacle like a wall or surface, liquids stop looking like a disorderly, featureless medium in which molecules move at random. Instead, molecules are arranged into layers lying parallel to the surface. An impenetrable barrier forces them to adopt a more orderly structure. Orderliness typically decreases rapidly with increasing distance from the surface. The first layer is well-defined, the second, a little more smeared out, and by perhaps the fourth or fifth all traces of order have vanished and the molecules' positions are random. Where it is organized into a layered structure, a liquid has properties similar to those of a solid material. If two flat surfaces separated by a liquid are pressed together, these surface layers are squeezed out one by one just before the surfaces come into contact. This means the force required to push the surfaces together oscillates - rising and falling abruptly as each layer of intervening molecules is removed. These force oscillations have been seen before where the region of contact between two surfaces is about the surface area of a bacterial cell (many thousandths of a millimetre across). Jarvis and colleagues wanted to see if the same behaviour applied when the contact area was much smaller - nearer, say, to the diameter of a single protein molecule. This is important because the shell of water that surrounds a biomolecule such as a protein may influence the way it interacts with other such molecules in the cell. Various experimental methods have suggested that proteins are indeed surrounded by a shell of `structured' water. But before now, no one had measured the forces between surfaces of molecular-scale dimensions. Jarvis and colleagues' AFM consists of a sharp, microscopic pyramid-shaped tip affixed to a flexible `cantilever' arm, like the stylus of a record player. The forces acting on the tip as it nears a surface bend the arm. This flexure can be measured with great accuracy, and used to deduce the strength of the force. An ordinary AFM tip, fine though it is, ends in an point much larger than a molecule. So the researchers fixed a stiff, rod-like single molecule called a carbon nanotube to their tip. This hollow tube of pure carbon is sealed by caps at each end, just a few nanometres (millionths of a millimetre) diameter - about the same as a medium-sized molecule. The improvement in sharpness is like erecting a flagpole on top of one of the Great Pyramids. Jarvis' group measured the force needed to bring the nanotube tip towards a gold surface covered with a `carpet' of organic molecules and immersed in water. As they explain in the Journal of Physical Chemistry [6 July 2000], the force increased as the nanotube needle had to push through the layers of `structured' water. Penetration through each successive layer was signalled by a rise and then a sharp fall in the force. The researchers plan to use this technique to investigate the effects of dissolved molecules on the degree of structuring of water near the surface: some additives are thought to be `structure-breakers', which disrupt layer-like ordering. Philip Ball Copyright Nature News Service 2000