Biomedical Engineering Reference
In-Depth Information
the bonding interactions among the bases in the DNA backbone. ssDNA can
uncoil freely to expose the component bases while dsDNA has a stable double-
helix geometry where the negatively charged phosphate backbone is exposed
and the bases are shielded.
58,59
Gold citrate NMs in solution are stabilized by
the adsorbed negative citrate ions with repulsive interactions preventing van der
Waals attraction between AuNPs from causing them to aggregate. The phos-
phate backbone of dsDNA repels the citrate ions adsorbed on the AuNPs creat-
ing a distance that prevents the possibility of electrostatic interaction causing
the dsDNA not to adsorb on the gold. On the other hand, the bases in ssDNA
are exposed and are not repelled by the citrate ions allowing interaction with
the AuNPs.
51
Li and Rothberg
60
documented the selective adsorption of ssDNA on AuNPs.
They showed that adsorption of ssDNA stabilizes the AuNPs against aggrega-
tion at salt concentrations that would ordinarily screen the repulsive interactions
of the citrate ions. The color of AuNPs as determined by SPR that is dramati-
cally affected by aggregation of the NPs was used to design a simple ssDNA and
dsDNA sensor using the difference in their electrostatic properties in a simple
colorimetric hybridization assay. The assay can be designed for sequence-spe-
cific detection of untagged ONTs for visual detection at the level of 100 fmol
that can be used to detect single-base mismatches between probe and target.
Wang et al.
51
studied the adsorption of dye-labeled DNA on AuNP. They
compared adsorption with thiol-mediated assembly and determined that DNA
adsorption on AuNP is simple, fast, and convenient. However, nonspecific
adsorption of DNA on AuNPs was highly inhomogeneous and the detection
limit was not very sensitive. Nevertheless, it is obvious that the nonspecific
adsorption of DNA on AuNPs is highly inhomogeneous, leading to broad distri-
bution of both the equilibrium constant and rate constant in desorption.
The group of Ehrenberg et al.
54
demonstrated the capacity of NM surfaces to
adsorb protein on endothelial cells. Quantification of adsorbed protein showed
that high binding NMs were maximally coated in seconds to minutes which
indicated that proteins on cell surfaces can mediate cell association. They
removed the most abundant proteins from culture media that alters the profile
of adsorbed proteins on NMs without affecting the level of cell association.
They concluded that cellular association was not dependent on the identity of
adsorbed proteins which indicted that there is no need for specific binding to
any particular cellular receptors.
Their results indicated that NMs surface chemistry mediates their protein-
adsorbing capacity during cellular binding. They showed that nonspecific inter-
actions account for a large portion of NMs binding to endothelial cell surfaces.
Carboxyl-based surface chemistry that is covalently modified reduced protein-
binding capacity that resulted in decreased cell association. In terms of pro-
tein binding and cell association, Ehrenberg reported that the least effect was
observed from positively charged lysine, followed by neutral groups CH
3
and
cysteine, and the greatest from PEG.
