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2 Surface Reactions and Fabrication of Bioreactive Platforms …
2.2 Surface Reactions of Organic and Polymeric Films
In this part, the surface reactivity of organic and polymeric films will be introduced.
The effects of confinement on, for example, reaction rates, degrees of conversion, as
well as temperature effects will also be treated.
2.2.1 Self-assembled Monolayers
2.2.1.1 Reactions in Self-assembled Monolayers
General Reactions of Monolayers in Solution: Reactions in liquid media, i.e., in the
presence of a solvent, show several advantages compared to cases where there is no
solvent present, such as simple operation and easily controlled reaction conditions.
Simple organic reactions, such as nucleophilic substitutions, free radical halogenations, oxidation/reduction, etc., can also be performed on surfaces in a solution [15].
In practice, however, most synthetically useful reactions are performed on monolayers terminated by carboxyl, amino, or hydroxyl groups. This is mainly due to
surface purification issues. While any soluble contaminants can be easily removed
from surface by rinsing off the solid support, monolayers cannot be purified from
by-products or unreacted materials that are attached to the surface. This becomes
especially complex when monolayers are subjected to a number of successive reactions, as formation of surface-attached by-products at each step leads to accumulation
of defects. This situation is partly alleviated by the fact that, because of a very small
amount of surface-immobilized material, bulk reagents are always present in large
excess with respect to the monolayer [16].
Reactions of Monolayers in the Absence of Solvents: A unique property of SAMs
is that the tail groups are generally in contact with the ambient environment. Reactions
in the absence of solvents have also been studied between monolayers and gas-phase
reagents. Hydroxy- and amine-terminated monolayers react with volatile silyl chlorides to produce silyl ethers and silylamines, respectively. Hydroxyl terminal groups
are quantitatively converted to trifluoroacetic by exposure to the vapors of trifluoroacetic anhydride. Interestingly, even carbodiimide coupling can be performed in
the gas phase by direct exposure of the acid-terminated monolayer to the mixed
vapor of a carbodiimide, an alcohol, and a base [17]. Reactivity of OH-terminated
monolayers toward acylating reagents in solution and the gas phase was found to
be quite similar, although in some instances gas-phase processes also did not lead
to complete functionalization [18]. The viability of gas-phase reactions makes them
promising for industrial fabrication of mono- and multilayers.
“Intrafilm” Reactions: The proximity of the adjacent chains or functional groups in
SAMs makes it possible to perform chemical reactions between them, a phenomenon
conceptually similar to intramolecular reactions in solution chemistry. An obvious
type of such reactions is polymerization of surface-attached unsaturated compounds.
2 Surface Reactions and Fabrication of Bioreactive Platforms …
2.2 Surface Reactions of Organic and Polymeric Films
In this part, the surface reactivity of organic and polymeric films will be introduced.
The effects of confinement on, for example, reaction rates, degrees of conversion, as
well as temperature effects will also be treated.
2.2.1 Self-assembled Monolayers
2.2.1.1 Reactions in Self-assembled Monolayers
General Reactions of Monolayers in Solution: Reactions in liquid media, i.e., in the
presence of a solvent, show several advantages compared to cases where there is no
solvent present, such as simple operation and easily controlled reaction conditions.
Simple organic reactions, such as nucleophilic substitutions, free radical halogenations, oxidation/reduction, etc., can also be performed on surfaces in a solution [15].
In practice, however, most synthetically useful reactions are performed on monolayers terminated by carboxyl, amino, or hydroxyl groups. This is mainly due to
surface purification issues. While any soluble contaminants can be easily removed
from surface by rinsing off the solid support, monolayers cannot be purified from
by-products or unreacted materials that are attached to the surface. This becomes
especially complex when monolayers are subjected to a number of successive reactions, as formation of surface-attached by-products at each step leads to accumulation
of defects. This situation is partly alleviated by the fact that, because of a very small
amount of surface-immobilized material, bulk reagents are always present in large
excess with respect to the monolayer [16].
Reactions of Monolayers in the Absence of Solvents: A unique property of SAMs
is that the tail groups are generally in contact with the ambient environment. Reactions
in the absence of solvents have also been studied between monolayers and gas-phase
reagents. Hydroxy- and amine-terminated monolayers react with volatile silyl chlorides to produce silyl ethers and silylamines, respectively. Hydroxyl terminal groups
are quantitatively converted to trifluoroacetic by exposure to the vapors of trifluoroacetic anhydride. Interestingly, even carbodiimide coupling can be performed in
the gas phase by direct exposure of the acid-terminated monolayer to the mixed
vapor of a carbodiimide, an alcohol, and a base [17]. Reactivity of OH-terminated
monolayers toward acylating reagents in solution and the gas phase was found to
be quite similar, although in some instances gas-phase processes also did not lead
to complete functionalization [18]. The viability of gas-phase reactions makes them
promising for industrial fabrication of mono- and multilayers.
“Intrafilm” Reactions: The proximity of the adjacent chains or functional groups in
SAMs makes it possible to perform chemical reactions between them, a phenomenon
conceptually similar to intramolecular reactions in solution chemistry. An obvious
type of such reactions is polymerization of surface-attached unsaturated compounds.