Microcontact Printing with Thiols

This experiment was adapted by John Kania from one developed by Michael Davis, Microcontact Printing on Gold CD-Rs.

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 or the printed side of a transparency sheet, 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.

Safety:

• Wear eye protection
• Chemical gloves required
• PDMS monomer can be messy—cover work surfaces with foil.

Procedure:

Dispensing the viscous liquid can be messy. Cover the work surface and the balance with aluminum foil. Wear gloves.

1. 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.
2. 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.
3. Place a quarter or a printed transparency sheet on a metal plate and place the square metal tubing around it in order to contain PDMS. Slowly pour about 8 g of 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.
4. 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 the oven so the PDMS components can cure before disposal.)Preparation of a silver coated surface
   Pretreatment will make the silver stick better. Fill a clean plastic Petri dish with dilute tin(II) chloride solution, SnCl2•3H2O. After 30 seconds return the solution to the stock. Rinse the Petri dish with water.
5. Add 18 drops of 0.5 M glucose solution and 60 drops of active-silver-ion solution into the empty pretreated Petri dish, mixing as you go and swirling to cover the entire surface.
6. Gently agitate the Petri dish for several minutes to thoroughly mix the solutions. A dark precipitate will begin to form and a silver coating will deposit on the dish.
7. Rinse with pure water to reveal the silver coating. Avoid contact with the solution which will stain your hands.
   Preparation of PDMS stamp
8. 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.
9. Remove the metal plate and the PDMS from the tubing. Remove the master from the PDMS stamp.
10. Cut out around the face of the stamp. The highest relief feature should be part of the image
    Inking the PDMS stamp
11. Apply several drops of an ethanol solution of alkanethiol to entirely cover the surface of the stamp.
12. 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
13. Place the dry stamp face down on the silver-coated slide, applying gentle pressure across entire stamp surface 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.
14. 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.

Conclusions

1. 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?
2. How could you make a square water drop on the silver surface?
3. If you make a fresh stamp and use HSCH2CH2CH2CH2CH2CH2CH2CH2CH2CH2COOH
   instead of HSCH2CH2CH2CH2CH2CH2CH2CH2CH2CH2CH2CH2CH2CH2CH2CH3 the inverse image is produced upon exposure to water vapor. Why?
4. Materials for 25 students
5. 0.8 M KOH (Dissolve 0.45 g KOH in 10 mL of water.)
6. 0.1 M silver nitrate (Dissolve 0.17 g AgNO3 in 10 mL of water.)
7. 15 M ammonia (concentrated aqueous ammonium hydroxide.)
8. 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.
9. 0.5 M glucose (Dissolve 0.90 g glucose in 10 mL of water.)
10. Alkanethiol solution. Add a very small amount (just barely visible) of a long-chain alkanethiol, such as octadecanethiol, to 20 mL of absolute ethanol.

Equipment

• Aluminum foil
• Petri dish (Disposable plastic dishes can be reused.)
• Microscope slides or petri dish (Clean plastic objects can be silvered if first soaked in dilute tin(II) chloride, SnCl2•3H2O, sensitizer solution for about 30 seconds and then rinsed well with water.)
• PDMS base and curing agent (Dow Corning Sylgard Elastomer 184 Kit, available from Ellsworth Adhesive)
• Weighing boats (100 mL)
• stir sticks
• Molds (Cut from 1.5×1.5 inch square aluminum tubing, 1/8 inch wall)
• Optional: Aluminum squares, approximately 4 or 5 inches square.
• Oven set at 250 °F (120 °C) or 200 °F (95 °C)