Chad Mirkin: This should open up many ways to explore nano-world of electronics based on molecules

The researchers created a picture of a tiny cube to demonstrate their instrument. Each line on the cube has a width of just 30 nanometres (30 millionths of a millimetre).

The pen's designers believe the new tool could be useful in the manufacture of nanoelectronic circuitry - components 1,000 times smaller than microcircuits.

They envisage, for example, molecules being painted on to circuitry that would form the basis of chemical sensors. These could be inserted into the body on the end of a needle to test for the presence of certain chemical substances and perhaps even diagnose medical conditions.

Old tool, new trick

The pen is really a familiar laboratory tool that has been given a new application. The Atomic Force Microscope (ATM) has been used for some years to image materials at the molecular level.

To create the image, it has an extremely fine stylus of silicon nitride that traces the contours on a surface in much the same way as a phonographic needle follows the bumps and grooves in a record.

But the ATM stylus has an irritating habit of attracting moisture from the atmosphere. This forms a meniscus, or bridge, of water that moves between the surface and the tip of the ATM, and vice versa.

A team from Northwestern University (NWU) realised that they could use the meniscus to transport across other molecules from the ATM to the surface material. In so doing, they made the tip of the ATM into the "nib" of a pen, and turned the surface material into "paper".

"To be honest, I'm surprised no one has done it before," Professor Chad Mirkin, who directed the study, told BBC News Online.

Line thickness

The stylus is dipped into a substance that will act as the "ink". A reaction between that and the "paper" draws more of the ink across the meniscus to build a structure that resembles a line or a dot.

"The size of the meniscus depends on the relative humidity," said Chad Mirkin, "and so by turning up the relative humidity, we can draw wider lines, or, by turning down the relative humidity, we draw more narrow lines."

For their research, NWU used a compound called octadecanethiol, ODT, as the ink. The paper was made of granular gold particles fused to silica. Although not ideal for drawing the thinnest possible lines, the combination allowed for easy identification and measurement of the lines.

A provisional patent covering the technique has now been filed and the NWU team are confident they can take the technology much further.

Possible applications

"We'd like to learn how to feed this type of system ink through the tip so that it acts like a fountain pen, and then we can have different types of cartridges with different types of ink," Professor Mirkin said.

Ultimately, he thinks it could be developed into a dot matrix-type of plotter or printer. Professor Mirkin also expects this "dip-pen nanolithography" (DPN) to take its place alongside other lithographic techniques that are used to mass-produce computer chips, and should prove extremely useful in the customisation of the very small chips that will be the hallmark of the fast-developing field of nanotechnology.

"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," he said.

The small scales involved mean such sensors could be put inside the body or even inside a cell. "This is small enough that you can think about doing that.

"Nobody wants to shove a probe into somebody's arm that's an inch and a half in diameter. Instead you'd like something that would fit on the end of a needle."

The NWU team report their research in the journal Science.