Environmental Engineering Reference
In-Depth Information
interfacial surface chemical reactions (two-dimensional reactions). A variety
of terminal functional groups and their chemical transformations on SAMs
after their assembly have been examined [59, 60, 62, 63, 64, 65, 66, 67, 73,
77, 78, 79, 80, 81, 82, 83, 84, 85]. These studies have shown that many
organic reactions that work well in solution are difficult to apply at surfaces
because of steric hindrance. In such a hindered environment, backside
reactions (e.g., S
N
2 reaction) and reactions with large transition state (e.g.,
esterification, saponification, Diels-Alder reaction and others) often proceed
slowly [17]. All these methodologies also use organic solvents and involve
tedious multi-step protection deprotection chemistries [63, 86, 87, 88, 89].
Cooks and co-workers [90] have carried out studies on silylation of
OH-terminated self-assembled monolayer surface through low-energy colli-
sions of ions. Using a multi-sector ion-surface scattering mass spectrometer,
reagent ions of the general form, SiR
3
+ were made to collide with a hydroxy-
terminated self-assembled monolayer (HO-SAM) surface at energies of nearly
15 eV. These ion-surface interactions resulted in covalent transformation of the
terminal hydroxy groups at the surface into the corresponding silyl ethers due to
Si-O bond formation. This result demonstrates that multi-step reactions can be
performed at a surface through low-energy ionic collisions. Other gas phase
methods have been reported for surface modification of SAMs, however, they
do not provide the versatility and control over surface chemistry that chemical
reaction provide, possibly due to vapor pressure requirements and the absence
of catalysts [91, 92].
These studies show that the rules that govern chemical reactions in solution
would be different from that at the interfaces. The intimate study of reactions
and interactions within such films and with external reagents is sure to widen
our understanding of the molecular behavior of such surfaces - an area that has
not received sufficient attention for organic chemists.
3.3 Proposed Technology for Surface Reactions on SAMs
3.3.1 Biocatalysis on Surfaces
Relatively, very few reports exist on use of biocatalytic methodologies for
carrying out surface modification of SAMs. Use of enzymes in organic synthesis
[93] and polymer science [1] is well established and has been discussed elsewhere
within comprehensive reviews. The rapidly increasing interest in in vitro
enzyme-catalyzed organic and polymeric reactions has been due to the fact
that several families of enzyme utilize and transform not only their natural
substrates but also a wide range of unnatural compounds to yield a variety of
useful products. Recent advances in non-aqueous enzymology have signifi-
cantly expanded the potential conditions under which these reactions can be
