Environmental Engineering Reference
In-Depth Information
3.2.6.2 Reactions of Carboxylic Acid-Terminated Monolayers
Carboxylic acid-terminated monolayers self-assembled onto Au substrates
have been studied by Leggett and co-workers [60]. The reaction between a
carboxylic acid functionalized SAM and trifluoroethanol in the presence of
di-tert-butylcarbodiimide, an activator added to make the carboxylic acid
monolayer more susceptible towards nucleophilic attack, [65] was found to
proceed slowly with only a 60% rate of conversion after several days. These
sluggish reaction rates are in agreement with comparably slow reactions on
poly(methacrylic acid) where steric interactions are believed to be responsible
for the long reaction times [74]. The authors concluded that the slow reaction on
the monolayer was due to a combination of (a) bulky tert-butyl groups on the
diimide combined with the lack of space within the carboxylic acid SAM
preventing attack, (b) the sterically hindered nature of backside attack from
the approaching alcohol directed towards the carbonyl group, and (c) the
adsorption of alcohol contaminants due to hydrogen bonding between the
carboxylic acid and the ethanol used in preparation of the monolayer.
Terminal carboxyl groups inmonolayers can also be activated by treatment with
carbodiimides such as dicyclohexylcarbodiimide (DCC) or 1-ethyl-3-(3-dimethyla-
minopropyl)carbodiimide (EDC) [15]. Alternatively, conversion to a mixed anhy-
dride can be effected by reaction of a carboxyl-terminated film with ethyl
chloroformate [64]. Exposure of the surface to gaseous SOCl
2
has been reported
to produce carboxyl chloride groups [63, 75]. These activated acid derivatives then
react smoothly with alcohols or amines to form esters or amides (Fig. 3.3).
Coster et al. have investigated immobilization of dendrimers on Si-C linked
carboxylic acid-terminated monolayers on silicon(111) [76]. Poly(amidoamine)
dendrimers were attached to activated undecanoic acid monolayers, covalently
linked to smooth silicon surfaces via Si-C bonds. XPS results suggest amide
bond formation between the dendrimer and the surface carboxylic acid groups.
3.2.6.3 Other Organic Reactions on SAMs
Studies have been carried out to investigate the similarities and differences
between chemical reactions in solution (three-dimensional reactions) and
Fig. 3.3 Reaction of carboxyl-terminated SAM
