World's Smallest Pen

BBC News Online / Sci-Tech

January 28, 1999

US scientists have produced the world's smallest pen,
capable of drawing lines just a few dozen molecules wide and
just one molecule thick.

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

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

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

"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.

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