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 master.
PDMS monomer can be messy. Cover work surfaces with foil.
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 PDMS components will eventually rise to the top of the liquid
where they may be broken by blowing across the surface.
Place a coin on a metal plate and place the square metal tubing around it in order
to contain PDMS. Slowly pour the uncured PDMS mix over the master,
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 an oven. At 200°F (95°C) it takes around 30 minutes for the PDMS to cure and at 250°F (120°C) it takes around 20 minutes for the PDMS to cure.
When the sample has cured, a dropped stir stick will bounce on the surface.
(Also place the weighing boat and stir stick used for mixing in an oven so the PDMS components can cure before disposal.)
Preparation of a silver coated surface
Pretreatment is needed to make silver stick to plastic. Fill a new clean plastic Petri dish with a saturated tin(II) chloride solution, SnCl2•3H2O.
The liquid level must be deep enough to cover the bottom of the dish. After 30 seconds return the solution to the stock. Rinse the Petri dish with water. After the pretreatment nothing will appear to have changed.
Add 30 large drops of 0.5 M glucose solution and 90 large drops of active-silver-ion solution
into the empty pretreated Petri dish, mixing as you go and swirling to cover the entire surface.
Keep moving the solution over the plastic until all the surface is wet.
A dark precipitate will begin to form and a silver coating will deposit on the dish.
Rinse with pure water to reveal the silver coating. Avoid
contact with the active silver solution which will stain your hands. Shake off water drops and let dry.
Preparation of PDMS stamp
Remove the mold from the oven with tongs or turn off the oven. Allow the assembly to cool for a few
minutes until it is safe to handle.
Remove the metal plate and the PDMS from the tubing. Remove the master from
the PDMS stamp.
Cut out around the face of the stamp. The highest relief feature should be part of the image.
It often works better to push down rather than drag with the single edge razor blade.
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 and let the ethanol evaporate. (You could remove possible excess alkanethiol by either placing the stamp face down on a clean glass or plastic surface and applying pressure OR rinsing the stamp
with ethanol and allowing the ethanol to evaporate.)
Microcontact printing and testing
Place the dry stamp face down on the silver-coated surface, applying
very gentle pressure across entire stamp for a few seconds. A beaker sitting on the stamp is about the correct amount of pressure. 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. Does the thiol-coated part or the silver part become cloudy when you blow on the sample? On humid days the surface
could also be cooled to collect water vapor.
1. Does the thiol-coated part or the silver part become cloudy when you blow on the sample?
2. How could you make a square water drop on the silver surface?
3. If you make a fresh stamp and use HSCH2CH2CH2CH2CH2CH2CH2CH2CH2CH2COOH
of HSCH2CH2CH2CH2CH2CH2CH2CH2CH2CH2CH2CH2CH2CH2CH2CH3 the
inverse image is produced upon exposure to water vapor. Why?
4. David Bergandine, Joe Muskin, and Matt Ragusa at The Center for Nanoscale Chemical-Electrical-Mechanical Manufacturing Systems, University of Illinois, have extended this experiment by using the alkanethiol as a lithography mask to protect the silver from etching by an aqueous solution containing 0.025 M Na2S2O3 (3.95g/L), 0.0025 M K3Fe(CN)6 (0.823g/L) and 0.00025 M K4Fe(CN)6•3H2O (0.106g/L). The etching solution degrades over time (turning from yellow to blue) and should be prepared shortly before use.
The sample is etched only until the pattern is clearly seen. Which part of the pattern will remain?
Materials for 25 students
0.8 M KOH (Dissolve 0.45 g KOH in 10 mL of water.)
0.1 M silver nitrate (Dissolve 0.17 g AgNO3 in 10 mL of water.)
15 M ammonia (concentrated aqueous ammonium hydroxide.)
0.5 M glucose (Dissolve 0.90 g glucose in 10 mL of water. Dispense from a dropper bottle.) Sugar or sucrose does not work.
Add a small amount of tin(II) chloride, SnCl2•3H2O, to water. Not all will dissolve and a slightly milky suspension will be obtained.
Alkanethiol solution. Add a very small amount (just barely visible) of a long-chain alkanethiol,
such as octadecanethiol, to 20 mL of absolute ethanol. Dispense from a dropper bottle.
Active silver ion solution, Ag(NH3)2+
Add concentrated ammonium hydroxide dropwise to 10 mL of 0.1 M silver
nitrate solution until the initial precipitate just dissolves. Mix with a glass stir rod.
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.