PDMS is cured by an organometallic crosslinking
reaction to give an optically transparent polymer with the ability to reproduce
surface features. In the experiment the polymer is cured in contact with a coin, inked with a dilute thiol solution, and then used
as a stamp to deposit molecules on a silver surface in the image of the coin.
Procedure
Wear eye protection
Chemical gloves recommended
PDMS monomer can be messy. Cover work surfaces with foil.
Preparation of silver coated glass slide
Add concentrated ammonium hydroxide dropwise to 10 mL of 0.1 M silver
nitrate solution until the initial precipitate just dissolves. Add 5 mL
of 0.8 M KOH solution; a dark precipitate will form. Add more ammonium
hydroxide dropwise until the precipitate just redissolves. This "active
silver" solution should be used within an hour of preparation. To
avoid the formation of explosive silver nitride, discard any remaining
active solution by washing down the drain with plenty of water.
Place a clean microscope slide in a Petri dish. Add 12
drops of 0.5 M glucose solution and 40 drops of active-silver-ion solution
onto the slide.
Gently agitate the Petri dish for several minutes to mix the solutions.
A dark precipitate will begin to form and a silver coating will deposit
on the glass.
Rinse the slide with pure water to reveal the silver coating. Avoid
contact with the solution which will stain your hands.
Preparation of PDMS stamp
Dispensing the viscous liquid can be messy. Cover the work surface
and the balance with aluminum foil. Wear gloves.
Add 8.00 g of Sylgard polymer base to a large weighing
boat using a disposable plastic spoon. Add 0.80 g of curing agent using
a disposable dropper.
Thorough mixing of the PDMS components is essential for good
curing. Improper mixing can result in a polymer that is a sticky mess. On
the order of 100 strokes with a stir stick are needed to mix the polymer
components so that they will yield an adequately cured sample.
Most of the trapped bubbles from
mixing of the components will eventually rise to the top of the liquid
where they may be broken by blowing across the surface. Sonicating
the mixed liquid can help remove the bubbles more quickly. If an
ultrasonic cleaning bath is available, float a weighing boat containing
the mixed PDMS components on the surface of the water
in the sonicated bath.
Place a quarter on the metal plate and place the metal tubing around it in order
to contain PDMS. Slowly pour about 8 g of the uncured PDMS mix over the quarter,
starting in the middle and allowing the PDMS to spread out evenly.
(Leave any
remaining PDMS sticking to the walls of the weighing boat; too many bubbles
are created during attemps to remove it. Let the assembly sit at room temperature
for a few minutes so that bubbles incorporated during pouring can rise
out of the PDMS. Gentle blowing over the surface may also eliminate bubbles.)
Place the whole assembly in a toaster oven. Set the oven to 250°F
for 20 minutes for the PDMS to cure.
Turn off the toaster oven and allow the assembly to cool for a few
minutes until it is safe to handle, then remove the assembly from the
toaster oven.
Remove the metal tubing then use a single-edge razor blade to assist
in gently peeling the PDMS from the metal plate. Remove the quarter from
the PDMS stamp.
Use a razor blade to cut out around the face of the stamp.
Inking the PDMS stamp
Apply several drops of an ethanol solution of alkanethiol to entirely
cover the surface of the stamp.
Allow the alkanethiol to sit in contact with the stamp for
about a minute. Then rinse the stamp
with ethanol to remove excess alkanethiol. Allow the stamp to dry by evaporation.
Microcontact printing and testing
Place the dry stamp face down on the silver-coated slide, applying
gentle pressure across entire stamp surface for a few seconds. The soft
PDMS stamp makes good surface contact and transfers molecules from the
stamp to the surface. Carefully lift off the stamp while trying
not to “smudge” the imprint.
Hold your breath and then exhale deeply onto the silver to lightly
mist the surface with water vapor and reveal the transferred hydrophobic
self-assembled monolayer. Repeat several times. On humid days the surface
could also be cooled to collect water vapor.
Can you use some of the scraps of PDMS and the alkanethiol solution
to make a square water drop on the silver surface?
If you make a fresh stamp and use HSCH2CH2CH2CH2CH2CH2CH2CH2CH2CH2COOH
instead
of HSCH2CH2CH2CH2CH2CH2CH2CH2CH2CH2CH2CH2CH2CH2CH2CH3 the
inverse image is produced upon exposure to water vapor. Why?
