Nanotechnology-Enabled Sensors

7.2 Surface Interactions 375
Carboxylic acids can be covalently coupled with other functional groups
to form several types of bonds such as amides, thioesters, acyl esters and
aryl esters (see Fig. 7.4). In this figure, R′ denotes the rest of the compound
(such as a protein) which is supposed to be targeted. R is the compound
which is bond to the carboxylic acid (whose other end is attached to
the sensor surface), and hence the covalent coupling bond joins both R and
R′ which makes the sensing links.
R
O
OH
Functional group Bond formed
H2N Amine
R'
Fig. 7.4 Interactions between carboxylic acids and other functional groups.
Carboxylic acids can be placed on a sensor surface by several different
means. Many water soluble molecules contain amine functional groups.
When a carboxylic acid covalently bonds with an amine group, it forms an
amide bond. The resulting molecule is called an amide. In fact, the amide
bond is one of the most important bonds in nature, and is the basis of the
formation of polypeptides (which will be described later in this chapter and
its importance in proteins). Therefore, these bonds are called peptide
bonds. The direct reaction between a carboxylic acid and amine functional
groups are very slow at room temperature, and hence activation is needed
R
O
N R'
H
Amide
HS R'
Thiol R
O
S R'
Thio ester
HO R'
Alcohol R
O
HO R'
O R'
Acyl ester
O
R
O R'
Phenol
Aryl ester