Using specially designed molecules that have the capacity to assemble themselves, Professor Fraser Stoddart and colleagues have created miniature molecular switches that could be used to store information. This raises the prospect of the molecular microchip - an electronic circuit board on which the components are individual molecules.
The driving force is the desire to miniaturise electronics. So far, miniaturisation has been conducted from the top down: electronic engineers start with a lump of material and carve it into an appropriate shape. But when the size of the components is perhaps no more than a few hundred atoms across, this approach runs up against the limits of precision of the available patterning methods, or against other problems such as 'cross talk' between closely spaced components.
To overcome these problems, researchers are adopting the bottom-up approach of building from individual molecular parts. It is now possible to manipulate atoms one by one to form structures that can only be seen by the world's most powerful microscopes. But this is a slow and laborious process. It might take minutes to build a single structure of a dozen atoms and a circuit might require millions of them. Hence chemists are hoping to get round this by designing molecules that assemble themselves into complex structures without human intervention.
The catchwords of this new science are 'self-assembly' and 'self-organisation'. It is possible to design molecules with specific shapes, or interactions, that allow them to come together spontaneously in a unique self-organised arrangement like the pieces of a jigsaw puzzle.
But what exactly does a molecular device consist of? There are, in principle, many possibilities. It might be a molecule that lets an electric current pass through in one direction but not another. This behaviour is analogous to that of a diode, a ubiquitous component in microelectronics. Natural molecular diodes can be found in the walls of cells where they help regulate the flow of electrically charged salts in and out.
Or maybe it is a molecule that captures light and converts the energy into electricity - a molecular solar cell. Plant cells use such a device for photosynthesis. Alternatively, it could be a molecule that changes shape or moves its parts around when a stimulus is applied, such as light or an electrical charge.
Such molecules might act as switching mechanisms. Again, there are analogous molecules in biology. Chemists are generally interested in devising much simpler molecular devices than their biological counterparts.
A striking example is the electrically powered molecular switch made in collaboration between Professor Stoddart and a group at the University of Miami led by Dr Angel Kaifer. The device contains a dumb- bell-shaped molecule on which is threaded a hoop-like molecule. The hoop can reside at one of two stations on the dumb-bell's axle, and is prevented from coming unthreaded by bulky groups at each end. Professor Stoddart has spent many years perfecting the construction of these threaded molecules, called rotaxanes. He has devised a set of standardised molecular components that will self-assemble into a threaded form when mixed together in water.
The process is akin to threading a needle at about one millionth of the scale. The molecular switch to achieve it is flipped by giving one of the stations a positive electric charge. The hoop then shuttles along the axle to the other station. Remove the positive charge and the hoop shuttles back.
A device like this could be used to store information. In computer memories, information is stored as a sequence of binary digits (bits), a string of ones and zeros that records the data in a coded form. An array of molecular switches could encode binary data with the two positions of the hoop corresponding to the ones and zeros.
Another example is the molecular brake devised by Dr T Ross Kelly and colleagues of Boston College in Massachusetts. They constructed a molecule containing a pinwheel scarcely one millionth of a millimetre across, which spins on an axle as the molecule floats in solution. When a metal salt is added to the solution, another part of the molecule swings round to stick between the spokes of the wheel, stopping it from spinning.
A molecular circuit will have to be robust - its components cannot simply float around in water. Chemistsare now developing methods for organising molecular components into ordered arrays on a solid surface. Professor George Whitesides, ofHarvard University, has shown that film just one molecule thick can be patterned on a scale finer than that obtainable for semiconductor microchips.
The biggest hurdles to the organic computer are likely to lie with the problem of ensuring that the molecular components do not break apart or lose their information through thermal vibrations, and with finding ways to write information into the system and extract it again.
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