Self-Assembled Monolayers

Objectives for this experiment

  • Synthesize both metal and silicone films that contain features ranging from the nanoscale to the milliscale.
  • Functionalize the surfaces of these films with a series of chemicals that behave differently from each other when in contact with water.
  • Build and understand relationships between the structure of a surface, its chemical functionality, and the resulting interactions with the macroscopic world.

Introduction

Self-assembled monolayers on surfaces modify the physical, chemical, and even biological behavior of those surfaces.  Just a few of the applications include chemical sensing, biosensors, drug delivery, light absorption for solar cells, and making waterproof coatings!

In this lab, students will learn the relationship between chemical groups on a surface, and how they interact when that surface comes in to contact with other materials. We use self-assembled monolayers that interact strongly or weakly with water to build these relationships.

There are a lot of self-assembled monolayer chemistries out there, but we use thiols on noble metals. Thiols are sulfur-containing organic molecules, and noble metals are metals that do not oxidize in air such as the jewelry metals gold, platinum, and silver. We silver in this lab because it’s comparatively inexpensive, and its very easy to make thin films of silver on glass using the Tollen’s test for sugars.  In this case, we’re not “testing” for the presence of a sugar, rather we intentionally feed the mixture a sugar that results in silver plating out on to the glass slide of interest.  From there we can make hydrophilic and hydrophobic thiol solutions, put them on the silvered glass slides, and play!

Chemicals needed for Tollen’s test (must be prepared the day of the lab):

We make up a large enough batch to distribute to 12 groups in amber dropper bottles.  In the lab we use, three groups share a large table, so we will generally put out 2 or 3 bottles of both the silver solution and the sugar solution for students to use.

  • 0.8 M KOH (1.1 g KOH in 25 mL water)
  • 0.1 M silver nitrate (0.85 g AgNO3 in 50 mL water)
  • 15–30 M ammonia (concentrated aqueous ammonium hydroxide)

Add concentrated ammonium hydroxide dropwise to 50 mL of 0.1 M silver nitrate solution until the initial precipitate just dissolves. Mix with a glass stir rod. Add 25 mL of 0.8 M KOH solution; a dark precipitate will form. Add more ammonium hydroxide dropwise until the precipitate just redissolves.

Note: To avoid the formation of explosive silver nitride, discard any remaining active solution by washing/diluting with plenty of water and disposing as necessary.

  • 0.5 M glucose or dextrose (4.50 g in 50 mL water) into many dropper bottles. Table sugar or sucrose does not work.

Chemicals needed for thiols (can be prepared in advance and refrigerated until use):

  • A hydrophilic thiol such as 10 mM mercaptohexadecanoic acid (290 mg in 100 mL ethanol)
  • A hydrophobic thiol such as 1 mM octadecanethiol (28 mg in 100 mL ethanol)

Doing the experiments:

Reagents and equipment for producing a silver mirror

  • 1 M Tollens’ reagent (diammine silver ion — prepared by the lab manager)
  • 5 M glucose solution (prepared by the lab manager)
  • Stirring rod or Pasteur pipette
  • Microscope slides
  • Glass scribe for slides
  • Glass or petri dish
  • Water rinse bottle
  • Beakers to collect rinse waste
  • Warm (~50 °C) hotplate
  • Wide Sharpie/VWR marker

Thiols to react with the silver mirror

  • 10 mM solution of mercaptohexadecanoic acid in ethanol (prepared)
  • 1 mM solution of octadecanethiol in ethanol (prepared)

Optional: reagents for producing textured PDMS surfaces

  • PDMS elastomer and initiator (Sylgard 184)
  • Disposable weighing boat
  • Disposable BD-style syringes
  • Various grits of sandpaper
  • Vacuum desiccator
  • Vacuum pump
  • Hot (~120 °C) hotplate

Preparing a silver mirror with the Tollens’ test for aldehydes and making self-assembled monolayer surfaces

  1. With the scribe, break the microscope slides in to three 1 inch pieces, and place 2-3 pieces in to a petri dish.Writing your name backwards on the underside of the petri dish with a Sharpie may help you to identify it later.
  2. Place two drops of the glucose solution on each glass piece, then add 5 drops of the Tollens’ reagent.
  3. Gently agitate the petri dish to mix the sugar and silver ion solutions.You may carefully stir the solutions with a small glass stirring rod or glass pipette tip.  (Dragging the tip across the glass substrate will remove any deposited silver and likely ruin the film.). Within minutes the solution should darken with a gray-brown precipitate – some of that will produce the silver mirror (the mirror may be hard to see under the gray-brown).
  4. When you think you have deposited enough silver to form a mirror, use water from a wash bottle to remove excess precipitate and reveal the silver mirror.Remove from dish.  Note: silver ion solutions will stain and may burn skin.  Remove the Ag+ solution by rinsing into a beaker for later disposal.  Label that beaker “Tollens’ reagent waste.”
  5. Let the surface dry. You may accellerate this process by placing the petri dish on a warm ~50 °C hotplate.  Multiple dishes can share a hotplate because you named yours, right?
  6. Functionalize one of the silver mirrors with one of the thiol solutions with 1-2 drops of the thiol solution. Tinse any excess off after 5 minutes with ethanol.  Collect all waste in a beaker labeled “Thiol waste.”  Do not heat the mirror to speed the process.
  7. From a water wash bottle, carefully place individual droplets on to the SAM-coated mirror.Characterize the relationship between the SAM composition and the droplet contact angle.  Can you photograph the droplet well enough to quantify that angle?
  8. Repeat steps 6 and 7 for any thiol solution not yet investigated.
  9. Based on intuition and interpretation, how would you expect a droplet to behave on a mixed thiol monolayer? Try to form a mixed monolayer surface to which a water droplet has an intermediate contact angle relative to the hydrophobic and hydrophilic cases.  Does the contact angle change if you functionalize the silver surface with different thiols sequentially or simultaneously?  Does the order of thiol functionalization matter?  Do these results agree with your intuition?  Why or why not?
  10. We are going to be using the silver mirrors for several experiments with pattering.  When you feel like you are very good at the silver deposition, prepare 3-6 extra ~1 in2 mirrors.

Patterning chemical functionality with textured PDMS films and stamps

  1. Place ~1 cm2 pieces of sandpaper from several different grits together in a petri dish, making sure the pieces do not overlap. This will become the template for the PDMS.
  2. Into a disposable weigh boat, measure out a few grams of the PDMS elastomer, and suffient quantities of the curing agent to make a 10 to 1 mixture by weight of the elastomer to the curing agent.The elastomer is very sticky and very messy, so be patient!
  3. With a disposable toothpick, liberally mix the elastomer and curing agent together.Note that several bubbles and air pockets will form in the mixture.
  4. With the toothpick, spread a film of the elastomer mixture over each of the pieces of sandpaper.The next step (vacuum pumping to remove air bubbles) will be more challenging if the elastomer gets trapped between the sandpaper pieces and the petri dish.
  5. (Note: your instructor will help you with this!) Place the petri dish with the elastomer-coated sandpaper samples in to a vacuum desiccator that is connected to a vacuum roughing pump. Gently open the valve between the desiccator and the vacuum pump.  As the pressure in the desiccator drops, air pockets and dissolved gases in the elastomer will bubble to the surface and be similarly pumped away.  If the pressure drops too quickly, the action of the bubbles may be unpredictable, so be patient!
  6. With the elastomer sufficiently degased (no more bubbles appearing under vacuum), close the valve to the pump and gently open the desiccator to air. Place the petri dishes on to a hot plate for curing.  The Sylgard 184 that we use will cure in about 2 days at room temperature, 45 minutes at 100 °C, 20 minutes at 125 °C, and 10 minutes at 150 °C.
  7. When sufficiently cured and cooled, lift the PDMS off of the sandpaper samples and place face up in to a cleaned petri dish.As with the thiol-functionalized silver mirrors, explore the contact angle of water droplets on your PDMS films.  Characterize the behavior and relationships between sandpaper grit, and the resulting droplet contact angle.  Use a camera to capture any images that may assist your analysis.