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, 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.
You will need a coin to do this experiment.
Procedure | Wear eye protection |
Chemical gloves REQUIRED |
PDMS monomer can be messy. Cover work surfaces with foil. |
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.Conclusions
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
instead
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?