In traditional microcontact printing - also called soft lithography or
microstamping - an elastic stamp�s end is cast from a mold created via
photolithograpy � a technique used to generate microscopic patterns
with light. Those patterns are then transferred to a surface by
employing various biomolecules as inks, rather like a rubber stamp.
Microcontact printing was first reported by Ralph Nuzzo and Dave
Allara at Pennsylvania State University, and developed extensively in
the laboratory of George Whitesides at Harvard.
A shortcoming of traditional microcontact printing is that pattern
transfer relies on the diffusion of ink from the stamp to the surface.
This same diffusion spreads out beyond the limits of the pattern as
the stamp touches the surface, degrading resolution and blurring the
feature edges, Clark and Toone said.
Because of this mini-blurring, the practical limit to defect-free
patterning is �in excess of 100 nanometers,� said the report, whose
first author, Phillip Snyder, is a former Toone graduate student now
working as a postdoctoral researcher in Whitesides� group.
A 100 nanometer limit of resolution is about 1,000 times tinier than a
human hair�s width. While that seems very precise, the Duke team now
reports it can boost accuracy limits to less than 2 nanometers by
entirely eliminating inking.
Clark and graduate student Matthew Johannes crafted a microstamp out
of a gel-like material called polyacrylamide, which compresses more
uniformly than the silicone material known as PDMS which is normally
used in microstamping.
In lieu of ink, Snyder, Toone and graduate student Briana Vogen
suspended a biological catalyst on the stamp with a molecular �tether�
of amino acids. For this proof-of-principle demonstration, Toone�s
team chose as a catalyst the biological enzyme exonuclease I, derived
from the bacterium E. coli.
In one set of experiments, the polyacrylamide stamp pattern bearing
the tethered enzymes was then pressed on a surface of gold that had
been covered with a uniform coating of single-stranded DNA molecules.
The DNA molecules had also been linked to fluorescent dye molecules to
make the coating visible under a microscope.
Wherever the enzyme met the DNA, the end of the DNA chain and its
attached dye were broken off and removed. That created a dye-less
pattern of dots on the DNA coating, each dot measuring about 10
millionths of a meter diameter each.
The microdots are very precise because the catalyst that created them
could not shift its position more than the length of its chemical
tether - less than 1 nanometer, the Duke team reported. "Whether the
stamp was left on for a short period of time, or for days, the pattern
did not change,� Clark said.
The inkless microstamp could also re-use the same suspended catalyst
molecule repeatedly. �Enzymes can deteriorate with extended use,�
Clark acknowledged. �But because of our tether attachment chemistry,
we can easily wash the old enzyme off, put on a new one and keep going,�
Clark said.
In follow-up research, Clark and Toone are now evaluating more durable
microstamping materials attached to longer lasting catalysts that are
non-enzymatic.
By using different catalysts in succession, future versions of the
inkless technique could be used to build complex nanoscale devices
with unprecedented precision, the two predicted.
�Soft lithography has really revolutionized the field of surface
science over the last 30 years,� said Toone. �And I honestly believe
that using catalysts instead of diffusive processes is going to become
the way that soft lithography is done in the future.�
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