Electronic engineers may soon have a new way to design miniature circuits, actually drawing them directly using a new kind of "pen" that writes a line just a few dozen molecules wide.

Lines this small are down to the nanoscale, measured in billionths of a metre, where an atom or a molecule is a countable object. This is also the realm sought out by the electronics industry which believes there is only one way to go - absolutely as small as technically possible.

"This should open up many ways to explore the nanoworld of electronics based on molecules," said Prof Chad A Mirkin, Professor of Chemistry at North-western University, based near Chicago, who lead the research on the nanopen. "It is engineering, but when you get down to the nanoscale, it is really chemistry."

The size of the line is almost beyond comprehension, 30 nanometres or about three millionths of a centimetre wide. The line is a few dozen molecules across and can be laid down at just one molecule thickness. At this scale a human hair would represent a massive structure.

Prof Mirkin's team included graduate students, Mr Jin Zhu and Mr Feng Xu and Dr Seunghun Hong of North-western, and Dr Richard D. Piner of Washington University. He published his findings recently in the journal, Science. The researchers described finding a new use for a long established scientific device, the atomic force microscope (AFM). The AFM is a commonplace lab tool normally used to trace the contour of a surface. Like a phonograph needle tracing the bumps and grooves on a record, the AFM uses an extremely fine stylus of silicon nitride which follows the molecular topography of a sample and delivers a three dimensional image of the surface in atomic detail.

The AFM has a problem however, it just will not stay dry. The very fine tip tends to attract moisture from the air and this forms a tiny blob of water that connects the tip and the surface directly underneath it. The inspiration for the research project arose from a study of this problem. Prof Mirkin and his group realised that this water was in constant motion, either flowing down towards the surface or back up towards the tip. They decided to examine whether this flow could be used to transport other molecules onto the surface like an old fashioned quill or dip pen.

"The dip pen is a 4,000-year-old technology," Prof Mirkin said. "This is a little different, because our `ink' is not just flowing from the tip right onto the surface, it is going through the water. The water forms a nanocapillary, which lets us write a very narrow line."

The width of line depends on the type of chemical ink being used and its affinity for the surface material. Prof Mirkin chose combinations that suited later chemical identification and measurement of the ink lines, so lines finer than 30 nanometres were achievable.

He used an oily, sulphur-containing chemical called octadecanethiol (ODT) as the ink and a "paper" of granular gold particles fused to silica. The sulphur allowed the ODT to absorb the gold to produce the line. The minute movements of the pen are controlled by computer.

The researchers called their new process dip-pen nanolithography (DPN) and have already filed provisional patents covering their techniques. The research team readily pointed out that DPN was not the only lithographic method for producing such lines. Microcontact printing uses a stamp to deposit molecules directly onto surfaces and has the advantage of being able to deliver an entire pattern or series of patterns very quickly.

DPN has advantages of its own, however, including being able to place different types of molecules at specific sites within a particular type of nanostructure. It is also a much simpler technology and does not require complicated processing methods or sophisticated non-commercial instrumentation.

Prof Mirkin sees a variety of uses for this new device. It could immediately complement existing lithographic techniques used for nanotechnology and molecular electronics. "This technique will be even more technologically useful once we convert our dip pen to a fountain pen, and once we can draw multiple lines in parallel rather than serial fashion," he said, in other words drawing several lines at once with multiple tips.

He also sees potential for a new type of intelligent chemical sensor using this technology. "Suppose I have a computer chip that will form the basis for a chemical sensor, and I need to put onto its nanocomponents some chemical that will tell me whether or not some chemical agents are around. I could use this type of technique to do that. I can go in and just paint those components with different types of molecules."