Environmental Engineering Reference
In-Depth Information
particles by isolating them from their environment, where particle growth and
agglomeration by different mechanisms can be prevented.
With the proper terminal functional groups, SAMs can be used to both
physically and chemically attached protein molecules. Physical attachment is
achieved by the hydrophobic or electrostatic interactions between protein
molecules and surfaces. The SAMs can also be tailored to form hydrophobic
or charged layers, and therefore adsorb protein molecules via the hydrophobic
interaction or electrostatic forces [18]. Silin et al. [19] investigated the non-
specific binding of human immunoglobulin G (hIgG) and bovine serum albu-
min (BSA) on gold surfaces modified by alkyl thiol molecules with different
terminal groups: CH
3
,C
6
H
4
OH, COOH, NH
2
, OH, and oligoethylene oxide
(OEO) [19]. They concluded that the capacity to bind protein depends on both
surface chemical group and protein molecules.
Two families of SAMs have received the most attention, SAMs of alka-
nethiols on gold and alkylsilanes on silicon.
3.2.2.1 Alkanethiols on Gold
Alkanethiols are the most popular reagents used to form SAMs on gold
surfaces. The chemisorption of these alkanethiol molecules on the gold sur-
face is based onto the interaction of thiolate molecules with the gold lattice.
Due to the advantages of easy preparation, high stability, and reproducible
closely packed structures, the coupling of organo-sulfur compounds to gold
surfaces has been widely explored and well established. Since thiols on gold
have been particularly well studied it has been used as a model system for a
variety of applications including biomaterial and biosensor surfaces [20]. The
preparation of SAMs on gold is simple. The clean substrate is immersed in a
1-10 mM solution of the desired alkanethiol at room temperature and after
approximately 1 h, the surface is covered with a near perfect monolayer. It is
generally believed that the thiol group binds to the gold as a thiolate [16],
resulting in an extremely densely packed, crystalline monolayer. Detailed
information regarding the formation of thiol monolayers on gold are dis-
cussed elsewhere [21]. The choice of the head group thus determines the sur-
face topography, as the underlying substrate becomes completely inaccessible
to the molecules in solution. For example, clean gold is naturally hydrophilic,
but the formation of SAMs makes it possible to control the contact angle of
water on the surface to any value between 08 (-OH and -CO
2
H groups) and
1188 (-CF
3
groups) depending on the functional group on the surface [22]. The
surface energy of a given SAM can also be altered by making a 'mixed' SAM
with two (or more) components. The problem of bulky head groups is avoided
by mixing with less bulky thiols in the feed solution. When the alkanethiols are
of equal chain length, the ratio of thiols in the SAM will resemble the ratio in
solution [23].
