2.2 Surface Reactions of Organic and Polymeric Films
13
For instance, diacetylenes were successfully polymerized in SAMs [19]. Similarly,
mercaptomethyl styrene was photopolymerized on a gold surface, all monomers
being consumed during the reaction. Another type of polymerization reaction
between adjacent chains combines the chemistry of gold-thiol and silane monolayers.
Hydrolysis of the monolayers of (3-mercaptopropyl)-trimethoxysilane assembled on
a gold surface produces a siloxane polymer [20]. In addition to polymerization, many
other reactions can be performed between adjacent chains in monolayers. A recent
study described the formation of an interchain anhydride from the monolayer of
mercaptohexadecanoic acid. Reaction with trifluoroacetic anhydride in dimethylformamide in the presence of triethylamine probably leads first to the formation of a
mixed anhydride intermediate, which then reacts with the adjacent carboxylate group
to produce the interchain product [21].
2.2.1.2 Functionalization of Monolayers
To control the surface properties of a SAM, a variety of functional groups have been
introduced. Attachment of the targeted functional molecules to a gold surface has
been achieved mostly by self-assembly of the appropriate ω-functionalized thiol or
disulfide, leading to the desired function in a single step. Some functional groups can
only be introduced via a corresponding surface reaction from a suitable precursor.
As mentioned, most useful surface reactions are done on halogen-bearing, amino[22], hydroxyl-, [23] and carboxyl- [24] terminated SAMs because of quantitative
yields of the reaction.
Carboxylic Acid- and Anhydride-Terminated SAMs: An activation step is necessary for these SAMs if chemical modification is performed, for example, by treatment with carbodiimides, such as dicyclohexylcarbodiimide (DCC) or 1-ethyl-3(3-dimethylaminopropyl) carbodiimide (EDC). Alternatively, conversion to a mixed
anhydride can be affected by reaction of a carboxyl-terminated film with ethyl chloroformate. Exposure of the surface to gaseous SOCl 2 has been reported to produce
carboxyl chloride groups. Hydroxyl terminal groups are quantitatively converted
to trifluoroacetates by exposure to the vapors of trifluoroacetic anhydride. These
activated acid derivatives then react smoothly with amino groups functionalized
bio(molecules) to form esters or amides (Fig. 2.4).
Amino-Terminated SAMs: Amidation is one of the mostly used surface reactions
on SAMs. Surface amino groups can be easily converted to amides by coupling
with a carboxylic acid as shown in Fig. 2.5. The active amino-terminated monolayer can also react with N-hydroxysuccinimidyl groups functionalized molecule
through amidation reaction (Fig. 2.5), which can be used for the further attachment
of biomolecules onto gold surfaces [25].
In addition, hydroxyl-terminated SAMs and halogen-bearing SAMs have also
been studied. In general, hydroxyl-terminated SAMs are more difficult to derivatize
as compared to amino- or carboxylic acid-terminated ones because of the higher
activation energies. The terminal hydroxyl groups of the SAMs can be functionalized
13
For instance, diacetylenes were successfully polymerized in SAMs [19]. Similarly,
mercaptomethyl styrene was photopolymerized on a gold surface, all monomers
being consumed during the reaction. Another type of polymerization reaction
between adjacent chains combines the chemistry of gold-thiol and silane monolayers.
Hydrolysis of the monolayers of (3-mercaptopropyl)-trimethoxysilane assembled on
a gold surface produces a siloxane polymer [20]. In addition to polymerization, many
other reactions can be performed between adjacent chains in monolayers. A recent
study described the formation of an interchain anhydride from the monolayer of
mercaptohexadecanoic acid. Reaction with trifluoroacetic anhydride in dimethylformamide in the presence of triethylamine probably leads first to the formation of a
mixed anhydride intermediate, which then reacts with the adjacent carboxylate group
to produce the interchain product [21].
2.2.1.2 Functionalization of Monolayers
To control the surface properties of a SAM, a variety of functional groups have been
introduced. Attachment of the targeted functional molecules to a gold surface has
been achieved mostly by self-assembly of the appropriate ω-functionalized thiol or
disulfide, leading to the desired function in a single step. Some functional groups can
only be introduced via a corresponding surface reaction from a suitable precursor.
As mentioned, most useful surface reactions are done on halogen-bearing, amino[22], hydroxyl-, [23] and carboxyl- [24] terminated SAMs because of quantitative
yields of the reaction.
Carboxylic Acid- and Anhydride-Terminated SAMs: An activation step is necessary for these SAMs if chemical modification is performed, for example, by treatment with carbodiimides, such as dicyclohexylcarbodiimide (DCC) or 1-ethyl-3(3-dimethylaminopropyl) carbodiimide (EDC). Alternatively, conversion to a mixed
anhydride can be affected by reaction of a carboxyl-terminated film with ethyl chloroformate. Exposure of the surface to gaseous SOCl 2 has been reported to produce
carboxyl chloride groups. Hydroxyl terminal groups are quantitatively converted
to trifluoroacetates by exposure to the vapors of trifluoroacetic anhydride. These
activated acid derivatives then react smoothly with amino groups functionalized
bio(molecules) to form esters or amides (Fig. 2.4).
Amino-Terminated SAMs: Amidation is one of the mostly used surface reactions
on SAMs. Surface amino groups can be easily converted to amides by coupling
with a carboxylic acid as shown in Fig. 2.5. The active amino-terminated monolayer can also react with N-hydroxysuccinimidyl groups functionalized molecule
through amidation reaction (Fig. 2.5), which can be used for the further attachment
of biomolecules onto gold surfaces [25].
In addition, hydroxyl-terminated SAMs and halogen-bearing SAMs have also
been studied. In general, hydroxyl-terminated SAMs are more difficult to derivatize
as compared to amino- or carboxylic acid-terminated ones because of the higher
activation energies. The terminal hydroxyl groups of the SAMs can be functionalized