Immobilization and Interaction Strategies in DFS of Biomolecular Partners - Dynamic Force Spectroscopy and Biomolecular Recognition - page 141

4.4.2.1 Functionalization of Gold Surfaces
Gold is a convenient substrate material because it does not oxidize in conditions
of DFS experiments, gold-coated substrates, or probes are commercially available
or can be prepared using standard deposition techniques. In addition, chemical
immobilization of molecules on gold is relatively well researched in comparison
to many other substrates (Ulman, 1991; Love et al. 2005). As an alternative, gal-
vanic displacement can be used to deposit copper on AFM probes (Fritz, Carraro, &
Maboudian, 2001). However, it remains unclear whether this technique offers signif-
icant advantages for DFS experiments.
As noted above, molecules containing thiols can be immobilized on gold surfaces
directly. Molecules that do not contain thiol groups can be immobilized using cross
linkers equipped with thiols and another functional group that can be used in the
subsequent chemical reactions. Table 4.1 shows several chemicals that have been
used in DFS experiments to functionalize gold surfaces.
Mercaptohexadecanoic acid and mercaptoundecanoic acid are the most common
compound for functionalization of gold surfaces in DFS (Schmitt et al. 2000; Ratto
et al. 2004; Friedsam et al. 2004; Odorico et al. 2007; Averett et al. 2008; Friddle
et al. 2008; Averett et al. 2009). These molecules form stable self-assembled mono-
layers on gold (Ulman, 1991; Friedsam et al. 2004) that can be made amino-reactive
using coupling reactions shown in Figure 4.2c. Other commercially available thi-
ols that can be used for functionalization of gold surfaces include 1,8-octanedithiol
and cysteamine (Hukkanen et al. 2005; Sulchek et al. 2005; Odorico et al. 2007).

TABLE 4.1
Compounds for Functionalization of Gold Surfaces
Name of Compound

Chemical Formula Functional Group

for Further
Modification

Mercaptoundecanoic acid and mer-
captohexadecanoic acid

Carboxyl

HS CH 2 COOH

10

HS CH 2 COOH

15

1,8-Octanedithiol Thiol

HS CH 2 SH

8

Cysteamine Amine

HS NH 2

11,11'-Dithio-bis-(undecanoic acid

O

Amino-reactive NHS
ester

O

N -hydroxysuccinimide ester)

N

N

O

O

O O

O O

CH 2
S

CH 2

10 10

S