Molecular Self-Assembly on Gold and Metal-Oxide Surfaces

Material Matters Volume 3 Article 2

Molecular Self-Assembly1 is the assembly of molecules without guidance or management from an outside source. Self-assembly can occur spontaneously in nature, for example the self-assembly of the lipid bi-layer membrane, surrounding cells.

An approach to Molecular Self-Assembly that is efficient and that has received ample attention during the last decade is the creation of Self-Assembled Monolayers (or SAMs) using relatively weak intermolecular interactions between certain types of organic molecules, such as thiols or phosphonic acids, and a gold or oxidic surface.


Molecular Self-Assembly on Gold

Several forces are driving assembly of alkyl thiols on a gold surface. First, the sulfur-gold interactions are quite strong, ~45 kcal/mol, which allow for a relatively strong bonding of the film-forming molecules to the surface.

Figure 1 Schematic overview of a thiol molecule on a gold surface

Furthermore, hydrophobic interactions between carbon and hydrogen atoms in the alkyl thiol molecules can significantly lower the overall surface energy thus promoting the formation of a self-assembled monolayer, especially if the alkyl chains contain at least 10 carbon atoms.2,3


Molecular Self-Assembly on Metal-Oxide Surfaces

Recent advances in the area of micro and nano electronic materials have extended SAMs beyond conventional gold/ thiol systems. To expand the choice of the substrates used for the preparation of SAMs, the chemical functionalities in film-forming molecules can be altered by introducing phosphate or phosphonate-groups. Such polar acidic molecules are capable of interacting with diverse metaloxide surfaces (e.g. Al2O3, Ta2O5, Nb2O5, ZrO2 and Ti2) and form films with a similar degree of ordering as for alkyl thiol SAMs on gold.4

Applications of SAMs comprise materials for semiconductor electronics industry such as nano-wires, nano-transistors, and nanosensors in large numbers. A few more examples of SAMs applications include surface wetting, non-fouling property, electrochemistry, surface passivation, protein binding, DNA assembly, corrosion resistance, biological arrays and cell interactions.5-7

Figure 2 Some applications of self-assembled monolayers.


  1. Non-fouling surfaces
  2. SAMs with specific binding receptors
  3. Cell supports for native cell growth and studies
  4. Molecular electronics
  5. Microarrays
  6. Separations

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References

  1. Material Matters™, 2006, Vol.1, No. 2.
  2. M.D. Porter et al. J. Am. Chem. Soc. 1987, 109, 3559.
  3. C.D. Bain et al. J. Am. Chem. Soc. 1989, 7155.
  4. G. Hahner et al. 17(22):7047–7052, 2001.
  5. J.C. Love et al. Chem. Rev. 2005, 105, 1103.
  6. N.K. Chaki et al. Biosensors & Bioelectronics 2002, 17, 1
  7. A. Ulman, Chem. Rev. 1996, 96, 1533.

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