A Step-by-Step Guide for Solution Based Self-Assembly

By: Dr. Dan Graham, Dr. Sean Dingman, Material Matters 2006, 1.2, 18.

Material Matters 2006, 1.2, 18.

Dr. Dan Graham, Assemblon; Dr. Sean Dingman, Sigma-Aldrich

Self-assembled monolayers of thiols are prepared by immersion of a clean gold substrate into a dilute solution of the desired thiol. Although self-assembly takes place very quickly, good experimental procedures are needed to produce highly ordered films. A recommended protocol for preparing SAMs is outlined in this article.

Equipment and Materials

Equipment Checklist and Considerations

  1. Gold coated substrates (See pg 10 for our substrates)
  2. Thiol compound(s)
  3. Fresh 200 proof ethanol (or appropriate solvent)
  4. Calibrated micropipettes
  5. Container for mixing thiol solution (solution container)
  6. Tweezers for sample handling
  7. A dedicated ethanol solvent bottle
  8. Parafilm for sealing containers
  9. Containers for sample preparation (sample containers)
  10. Petri dishes for transporting and storing SAMs
  11. Dry nitrogen
  12. Analytical Balance
  13. Sonicator
  14. pH paper

Environment: A clean environment is key to preparing quality SAMs. Low levels of contaminants can affect monolayer quality. Avoid rooms or hoods in which silanes or poly(dimethyl siloxane) (PDMS) have been used. These compounds easily cross contaminate a variety of surfaces. Iodine adsorbs readily onto gold and should be avoided.

Handle all thiols in a fume hood. Most thiols have an obnoxious odor and are toxic (check MSDS before using).

Containers: Appropriate containers include glass or polypropylene (e.g., scintillation vials, polypropylene test tubes and centrifuge tubes). Glass containers must be cleaned thoroughly to avoid solution contamination1.

Containers that can be easily sealed are recommended. For the highest quality films, oxygen exposure should be minimized during the assembly process. This is achieved by reducing the headspace above the thiol solution and backfilling with an inert gas. Each substrate is placed in its own container to avoid interactions that would be detrimental to film quality.

1One option for glass cleaning is the use of piranha solution (30:70 v/v solution of 30% hydrogen peroxide (H2O2) and concentrated sulfuric acid (H2SO4)). Extreme caution has to be taken when using piranha solution. It is a very strong oxidant and reacts violently with organic matter.

Containers can be reused, as long as they are rinsed well with solvent after each use and dedicated to the same thiol to avoid cross-contamination.

Solvent: For most thiols, pure ethanol (200 proof) is required for successful assembly. Alternately, denatured alcohol, containing up to 5% isopropanol and/or methanol, is a suitable substitute. Solvent purity should be verified due to potential contamination by copper. Copper disrupts the assembly of the thiols and may affect the performance of the resulting SAM.

Sample Slides: Gold slides must have an adhesion layer of chromium (Cr) or titanium (Ti) under the gold layer. If this layer is missing the gold will delaminate and ruin the monolayer during sonication.

Dedicated wash bottle: It is best to have a dedicated solvent wash bottle used for rinsing containers, substrates and SAMs. Store the bottle empty and only fill it with fresh ethanol when needed.

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Step-by-Step Procedure

This general protocol is appropriate for most thiols. Thiols containing amine, carboxy groups require modifications to the protocol as noted in green below. The properties of PEG thiol monolayers depend on the method of self-assembly. Researchers should review references 5–7 before using these materials.

  1. Determine necessary amounts and concentration of thiol solution.
    a. Calculate the total volume of thiol solution needed to make the number of samples desired.
    [Total volume of solution (mL)] = [total number of samples] x [Sample solution volume (mL)]
    b. Calculate the total amount of thiol needed to prepare desired amount of solution. (where C = 1-5 mM solutions)
    [Mass of thiol (g)] = [Total Volume (mL)] x [C x 10-6 mol/ml] x [MW(g/mol)]
    If the thiol is a liquid, you can convert the mass to a volume using the density of the thiol. Use a calibrated micropipette for measuring and dispensing liquid thiols.
  2. Preparing the thiol solution. Prepare enough solution for all samples to ensure the solution concentration is constant across the sample set. When preparing mixed thiol solutions, prepare a stock solution of each thiol separately, then mix them at the proper proportions for the final stock solution.
    a. Rinse all assembly containers with solvent by squirting ~3 to 5 mL around the inside of the containers. Repeat 2–3 times and re-cap each container. Rinse all beakers, tweezers, etc., to be used in the experiment with solvent. Label all containers.
    b. Measure the appropriate volume of solvent into the clean solution container.
    c. Dispense the mass (or volume) of thiol, to the solvent.
    d. Sonicate the container 5–10 min to dissolve.
    e. Once dissolved, dispense the planned volume of solution into each sample container.

    • Carboxy terminated thiols: Adjust the solution pH to ~2 with a few drops of concentrated HCl. Then sonicate stock solution.
    • Amine terminated thiols: Adjust the solution pH to ~12 with concentrated NH4Cl or triethylamine. Then sonicate stock solution.
  3. Sample Self-Assembly.
    a. Immerse gold substrate in container containing the thiol solution. Handle gold substrates with tweezers and minimize expose to air, to reduce surface contaminants.
    b. Backfill each container with dry nitrogen and seal the cap and wrap the cap with Parafilm.
    c. Store the sample for at least 24 to 48 hours. In general, longer assembly times tend to result in better packing of the monolayers.
  4. Terminating self-assembly: Functional groups on the thiols affect self-assembly termination.
    For simple alkanethiols:
    a. Hold the sample with clean tweezers and rinse with solvent for 10 to 15 seconds from a clean solvent bottle.
    b. Dry sample with a stream of dry nitrogen. For thiols with hydrogen-bonding, polar or bulky head groups:
    c. Hold the sample with clean tweezers and rinse with solvent for 10 to 15 seconds from a clean solvent bottle. (Solvent should be pH adjusted for carboxy and amine terminated SAMs)
    d. Place each sample in a container with fresh solvent and close the cap. (Again solvent should be pH adjusted if needed)
    e. Sonicate the samples for 1–3 minutes.
    f. Remove the samples individually and rinse again for 10–15 seconds under a steady stream of ethanol. (At this stage no pH adjustment is necessary. Use pure solvent)
    g. Dry sample with a stream of dry nitrogen.
  5. Sample Storage
    a. Place in clean Petri dish.
    b. Backfill Petri dish with dry nitrogen.
    c. For long-term storage: Place the Petri dishes in a jar backfilled with dry N2 and sealed with Parafilm.

If you are going to use the monolayers for further experimentation, plan your experiments so you can rinse the samples right before use. Minimize time between preparation and use since SAMs can oxidize over time.

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  1. Love, C.; Estroff, L.; Kriebel, J.; Nuzzo, R.; Whitesides, G.; Chem. Rev., 2005, 105, 1103–1170.
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