Colorimetric Bioassay Using Noble Metal Nanoparticles - From Bioimaging to Biosensors: Noble Metal Nanoparticles in Biodetection - page 35

a repulsive electric double layer that stabilizes the NPs against van
der Waals attractive forces. 54 A characteristic feature of electrostatic
repulsion among NPs is the high sensitivity to the bulk ionic strength.
The force of electrostatic repulsion diminishes signiicantly at high salt
concentrations when the electric double layer is highly suppressed,
explaining why citrate-capped Au NPs are stabilized in water but
aggregate at elevated salt concentrations (e.g., 50 mM NaCl). 55 In the
case of steric stabilization (see Fig. 3.1B), macromolecules grafted
on colloidal surfaces in a “good solvent” — that is, a solvent in which
steric stabilization diminishes with decreasing solubility — impart
a polymeric barrier that prevents collision among NPs. Usually, van
der Waals attractive forces are dominant in this case. The penetration
of polymer chains on the NP surfaces when they approach each
other results in a loss of polymer conigurational entropy, which is
unfavorable to the aggregation process. Steric stabilization is much
less sensitive toward ionic strength than electrostatic stabilization.
Rather, the molecular weight of the macromolecule and surface graft
density are more important factors. In general, thicker polymer
layers (ones with longer polymer chains) and higher graft densities
lead to more effective steric stabilization. Electrosteric stabilization
is probably the most effective strategy to stabilize Au and Ag NPs
(see Fig. 3.1C). Bovine serum albumin-stabilized Au NPs are stable
in phosphate solution (pH 7.0) at ionic strengths up to 0.1 M
NaCl, 56 which is much more stable than mercaptoundecanoic acid-
stabilized Au NPs (which aggregate at >50 mM NaCl). DNA-modiied
Au NPs with high DNA graft densities are stable even at very high
salt concentrations (e.g., 300 mM MgCl 2 ). Steric factors are expected
to play a major role in stabilizing Au NPs at salt concentrations in
which the electrostatic repulsion is signiicantly diminished. 57
3.2.3 Functionalization
Most prepared Au and Ag NPs, such as those capped with citrates, are
not selective for most biomolecules of interest. Thus, functionalization
of Au and Ag NPs is extremely important for providing recognition
sites for speciic (selective) interactions with target analytes such
as DNA and proteins. Bifunctional chemicals, in which a moiety is
used for anchorage to the NPs while the other is directed to the outer
surface for a speciic interaction with biomolecules or for further