HE GREAT promise of nanotechnology has come a considerable step closer to being realized, thanks to chemists who have solved the problem of connecting electronic components that are not much larger than molecules.

Nanotechnology, which is molecule sized engineering, promises wonders - from ultra-dense computer memories to cell-sized robots.

The main drawback to be beaten - if size reduction is to continue beyond the next few years - is the challenge of molecular assembly. This seems to be the only way to satisfy the demand for ever smaller electronic devices.

Indeed, "smaller, faster and more efficient" is already the maxim of the international electronics industry as it constantly strives towards the further miniaturization of electronic components.

Conventional micro-electronics fabrication methods based on lithographic techniques are being used to produce electronic components of around 0.1 micrometers or 100 nanometers in size (one nanometer equals a millionth of a millimeter).

The emerging field of nanotechnology aims to produce such functional structures that are an order for smaller magnitude. Nanostructures of interest include metal and semiconductor nanoparticles, quantum dots, fullerenes, nanotubes and biological macromolecules.

One of the challenges of nanotechnology is to organize, using chemical methods, such structures into functional devices. A further challenge is to interface these minuscule objects with electrical contacts so that they can be addressed from the outside world.

In a major breakthrough towards this end a team working at Liverpool University, north-west England, has linked a piece of gold just six nanometers (six millionths of a millimeter) across to a gold electrode to form a device called a redox gate.

Researchers used wires consisting of single molecules that can also act as switches, so that the flow of electrical current through them can be turned on and off.

If computers are to become even smaller and thus faster, the size of the circuit must be reduced towards the nanoscale domain. One prospect is the "bottom up" approach in which smaller electronic components are constructed from even smaller building blocks, preferably by a process of chemical "self-assembly."

An important issue concerning the continual miniaturization of electronics is whether or not the ease of electron transfer across structures of nanometer dimensions can be controlled by injection of a small number of electrons in to the spacer molecules.

This is where the importance lies of the breakthrough of the team under Professor David Schiffrin from the Centre for Nanoscale Science at Liverpool University. In theory, nanoparticles such as these could act as switching elements in a "nanocomputer", allowing components to be packed at incredibly high density. The strategy used at Liverpool was based on using a class of organic molecules called thiols that stick to gold.

At the end of a chain-like molecule, a ghiol group reacts with a gold atom to form a strong link. Molecule, a thiol group reacts with a gold atom to form a strong link. Molecules with thiols at both ends can link themselves between two gold surfaces.

The researchers attached gold nano-particles to a flat gold surface, tethered by two-headed thiol molecules. Each gold particle, they say, is probably linked to the surface by dozens of these molecules.

This process of chemical self-assembly will be crucial to nanotechnology. The alternative - manipulating and securing molecules by hand - is extremely difficult.

Professor Schiffrin's team also reports in the prestigious journal Nature that their linkers can conduct a current. They used a device called a scanning tunneling microscope (STM) to pull a current from the gold surface, up through the linking molecules and into the gold nanoparticles.

The STM has a very fine metal tip. When voltage is applied to this tip and it is brought close to the nanoparticles, a current flows from the particles into the tip. But by injecting electrons into the tip. But by injecting electrons into the linker using well-known elecctro-chemical methods, the researchers were able to switch the current on and off, because their linker molecules were voltage - sensitive.

By injecting one electron per molecule, the linkers existed in a conducting state. But after the voltage became more positive, electrons were stripped from the molecules, and their conductivity plummeted. Then the current could no longer find its way to the gold nanoparticles. If the nanoparticles are connected by, say, 30 or so molecules, this switch is flipped by shifting just this many electrons, a tiny "control" current for an electronic device.

This work represents a step towards decreasing the size of electronic components using chemical means. The challenges for further development are two fold. The design and creation of chemical structures that are able to organize themselves will permit the construction of more elaborate nanoscopic devices by self-assembly.

There is also a need for the development of methods by which these elements may be connected together and interfaced to the macroscopic world using the engineering nano-fabrication techniques that are becoming available. Professor Schiffren says the future will probably involve the conjunction of the two approaches to nanotechnology that are:

- bottom up - which has its roots in the synthetic and physical chemistry of self-assembled Nanostructures.

- and top down - representing the engineering approach to the construction of nanoscopic objects.