Environmental Engineering Reference
In-Depth Information
3.2.2.2 Alkylsilanes on Silicon
The formation of SAMs of alkylsilanes on silicon or glass is more complex.
These monolayers are covalently bound to surface hydroxy groups through
Si-O bonds. The molecules used in the formation of such monolayers are either
chlorodimethyl long chain alkylsilanes, alkyltrichlorosilanes, or trialkoxy(alk-
yl)silanes [24]. The alkylchlorosilane derivatives react spontaneously with clean
Si/SiO
2
or glass, whereas the alkoxysilanes need to be heated, in order to
convert the alcohols into leaving groups. The more commonly used alkylchlor-
osilanes are either deposited from the vapor phase or from solution. These
molecules partially hydrolyze in solution, forming oligomers before settling
down on the surface into a polymeric network. The hydrolysis of trichlorosilane
derivatives can, however, also result in polymer networks 'dangling off' the
surface [25]. Experimental evidence, however, do suggests that long alkylsilanes
form very tightly packed monolayers that are only slightly less dense than
alkanethiols on gold [21].
3.2.3 Mixed Self-Assembled Monolayers
Monolayers comprising a well-defined mixture of molecular structures are
called 'mixed' SAMs [26, 27, 28, 29, 30, 31]. There are three easy methods for
synthesizing mixed SAMs: (1) co-adsorption from solutions containing mix-
tures of thiols (RSH
รพ
R'SH), (2) adsorption of asymmetric disulfides (RSSR'),
and (3) adsorption of asymmetric dialkylsulfides (RSR'). Mixed SAMs provide
a useful methodology for incorporating into a SAM a molecular species whose
own physical dimensions would preclude a direct, well-organized assembly.
Mixed SAMs are also useful for defining gradients of interfacial composition
that, in turn, are useful for studying the properties and biology of cells.
The adsorption ofmixture of thiols allows the formation of SAMs withwidely
varying compositions [22, 32]. The formation of mixed SAMS of methoxy-
tri(ethylene glycol)-undecenyldimethylchlorosilane and dodecyldimethylchlor-
osilane (DDMS) were prepared by self-assembly from organic solution in the
presence of an organic base [31]. The composition of the mixed SAMs was found
to depend directly and linearly on the composition of the silanization solution.
This mixed SAM system prohibited the non-specific adsorption of a variety of
proteins. Mixed SAMs allow the surface properties to be fine-tuned within the
range of the properties of the SAMs formed from the pure compounds.
3.2.4 Surface Characterization
The products of organic synthesis in solution can easily be purified and subse-
quently analyzed with rapid, sensitive techniques such as NMR spectroscopy,
