by: Dr. Maxi Boeckl and Dr. Daniel Graham
In the 1980’s, scientists discovered that alkanethiols
spontaneously assembled on noble metals. This new area of
science opened the doors to a simple way of creating surfaces
of virtually any desired chemistry by placing a gold substrate
into a millimolar solution of an alkanethiol in ethanol. This
results in crystalline-like monolayers formed on the metal
surface, called self-assembled monolayers (SAMs).1
Over the years, the mechanism of the self-assembly process
has been well studied and elucidated. Researchers have found
that a typical alkanethiol monolayer forms a (√3 × √3)R30°
structure2 on gold with the thiol chains tilted approximately 30
degrees from the surface normal.3–6 The exact structure of the
monolayer depends on the chemistry of the chain.
Self-assembly forms the basis for many natural processes
including protein folding, DNA transcribing and hybridization,
and the formation of cell membranes. The process of selfassembly
in nature is governed by inter- and intra-molecular
forces that drive the molecules into a stable, low energy
state. These forces include hydrogen bonding, electrostatic
interactions, hydrophobic interactions, and van der Waals forces.
As with self-assembly in nature, there are several driving forces
for the assembly of alkanethiols onto noble metal surfaces. The
first is the affinity of sulfur for the gold surface. Researchers
have found that the sulfur-gold interaction is on the order of 45
kcal/mol,3 forming a stable, semi-covalent bond; in comparison,
the C—C bond strength is ~83 kcal/mol.
The next driving force for assembly is the hydrophobic, van
der Waals interactions between the methylene carbons on
the alkane chains. For alkanethiol monolayers, this interaction
causes the thiol chains to tilt in order to maximize the
interaction between the chains and lower the overall surface
energy. A well-ordered monolayer forms from an alkane chain
of at least 10 carbons. With carbon chains of this length,
hydrophobic interactions between the chains can overcome the
molecules’ rotational degrees of freedom.6,7
A simple alkanethiol molecule is shown in Figure 1. (next
page) An alkanethiol can be thought of as containing 3 parts:
a sulfur binding group for attachment to a noble metal surface,
a spacer chain (typically made up of methylene groups, (CH2)n),
and a functional head group. As mentioned above, the sulfur
atom and the carbons in the methylene groups act as the main
driving forces for assembly of the alkanethiols. The head group
then provides a platform where any desired group can be used
to produce surfaces of effectively any type of chemistry.
By simply changing the head group, a surface can be created
that is hydrophobic (methyl head group), hydrophilic (hydroxyl
or carboxyl head group), protein resistant (ethlylene glycol head
group), or allows chemical binding (NTA, azide, carboxyl, amine
head groups). This enables a researcher to custom design a
surface to serve any desired function.
