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Microcontact Printing experimental details
For a more general overview of microcontact printing, click here
Creating the stamps
The most common silicone elastomer used for microcontact printing is Dow Corning's Sylgard 184. We
have had good success mixing 10:1 by weight elastomer:curing agent, as per the package directions. We cure
in an oven at ~65-70 °C for at least 4 hours, but typically overnight.
The cured stamps should be at least rinsed in deionized water and dried before use. There seems to be a slight
surface residue that is removed by water. This was first noticed when a freshly cured stamp was dropped into
a clean (hyrdophilic) glass vial with a few milliliters of water in the bottom. As soon as the stamp touched
the bottom, the water instantly moved away from the stamp and formed a ball at the edge of the vial. The entire
bottom of the vial had been rendered hydrophobic. This same stamp did not have the same effect on subsequent
vials, but other fresh stamps showed the same effect in new vials. Rinsing a fresh stamp for a few seconds under
flowing DI water seems to remove this residue.
This
simple cleaning procedure should be sufficient for most purposes. Still, to be sure, I generally soak new stamps
in DI (~18Mohm-cm) for several hours, followed by shaking in methanol for several minutes to remove any traces
of water. The stamps are then dried in an oven for ~1 hour at 65 °C. The stamps are then soaked in hexane for
~1 hour, then dried for > 1 hour at 65 °C. This procedure is only done if I have many stamps to clean at
once. Otherwise, it would take too long.
Inking the stamps
We are primarily interested in non-covalent stamping of molecules like lipids and proteins. Due to their relatively
weak interaction with the substrate, non-covalently bonding molecules are often sensitive to environmental conditions
like temperature, relative humidity and sometimes solvent vapors remaining in the stamp.
We typically ink the stamps from ~200 micromolar solutions of an appropriate solvent. For tristearin, we have had
good success with toluene. Other lipids like DiPalmitoyl PhosphatidylCholine (DPPC)
were found to be sufficiently soluble in ethyl alcohol.
Clean stamps are immersed in the ink solutions for 30 seconds with tweezers, then blown dry with compressed nitrogen
for 30 seconds. We began to notice occasional reproducibility problems which were consistent with the idea of the
ink solvent coming out of the stamp during stamping. This "problem" seems to have been solved by letting the
freshly inked stamps sit at room temperature for an hour before use, partially covered to protect from dust.
The inked stamps are then placed on the sample surface using tweezers, and allowed to sit only under their own weight
for 30 seconds. It is important to use a very clean, smooth surface, because a small piece of dust can "prop up" the
stamp in the region surrounding the dust for many tens of microns, preventing proper ink transfer in that region.
We primarily use freshly cleaved muscovite mica as our substrates.
Contact info:
Srin Manne
Physics Dept
PAS 575
520-626-5305
smanne@physics.arizona.edu
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Figure 1. Basic steps for microcontact printing
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Figure 2. A small stamp created from a 3 micron pitch linear grating master
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Figure 3. A pattern of lines composed of molecules of tristearin created
from a 3 micron pitch linear grating stamp which was immersed in a solution containing tristearin
molecules. The lines are only ~3 nm tall, about the length of a tristearin molecule.
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