Biomedical Engineering Reference
In-Depth Information
important to have monodisperse and stable Au and Ag NPs for
bioassays because poor and irreproducible sensitivity results can
occur when they are unstable. Increasing the hydrophilicity and/or
charge density on the NPs surfaces against van der Waals attraction-
induced aggregation is a common way to stabilize NPs. 53 For instance,
Au NPs prepared by the classic citrate reduction method are stable due
to the existence of charged citrate ions on their surfaces. The stability
of Au NPs can be further controlled to an exceptional degree through
the introduction of other chemicals or polymers by chemical grafting
methods (e.g., Au-thiol and Au-amine), electrostatic adsorption and
physical adsorption, and several other methods. Common capping
agents for Au and Ag NPs include small organic ions, polymers,
and polyelectrolytes, which stabilize the NPs through electrostatic,
steric, and electrosteric (a combination of electrostatic and steric)
interactions, respectively (see Fig. 3.1). 53,54 Mercaptoundecanoic
acid and citrate are common anions for stabilizing Au NPs, mainly
because they can form strong Au-S and Au-O bonds, leaving charged
parts facing the bulk solution, resulting in increased repulsion
forces among the NPs. Common polymers used to stabilize Au and
Ag NPs are hydrophilic polymers such as poly(vinylpyrollidone),
poly(ethylene oxide), and poly(ethylene glycol). Proteins such as
bovine serum albumin and DNA (a negatively charged polymer) are
typical polyelectrolytes used to stabilize Au and Ag NPs.
(A)
(B)
(C)
Figure 3.1 Schematic representation of colloidal stabilization through
(A) small charged molecules on the Au NP surface (electrostatic
stabilization), (B) surface-grafted polymers (steric stabili-
zation), and (C) surface-grafted, charged polymers (electro-
steric stabilization).
With respect to electrostatic stabilization (see Fig. 3.1A), the
surface charges, together with the counter ions in the medium, form
 
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