Chemistry Reference
In-Depth Information
particle aggregation. When the networks are formed in the presence of duplex
DNA-binding molecules, such as DAPI (4
0
,6-diamidino-2-phenylindole) or EB
(ethidium bromide), the resulting duplexes connecting the AuNPs become more
stable due to the DNA-binding interactions (p-p stacking, electrostatic, or hydrogen
bonding). This increases the melting temperature so that the presence of different
DNA-binding molecules can be read out from the temperature-dependent color
change.
Based on this fundamental properties of DNA-functionalized AuNPs, Mirkin
and others have developed more sophisticated techniques, including chip-based
scanometric assays, which have been commercialized partially for highly sensitive
molecular diagnostic technologies including clinical disease states, such as
Alzheimer's disease or prostate cancer [
49
]. Furthermore, the unique set of
properties is useful for intracellular applications. This includes the high affinity
for complementary DNA and RNA [
54
], the pronounced stability against nuclease
degradation [
55
], and no observed toxicity [
56
], which allows application as gene
regulation agents [
57
].
Interestingly, despite their large negative charge due to the polyanionic DNA,
the AuNP-DNA were found to easily enter cells. It was demonstrated recently that
uptake of these particles in tumor cells (HeLa) is mediated by scavenger receptors.
Blocking of these receptors with well-known ligands, polyinosinic acid (PolyI) and
Fucoidan, has led to inhibition of particle uptake to a high degree, demonstrating
competitive binding of AuNP-DNA and the inhibitors to the receptors. The extent
of uptake increases with increasing number of DNA ligands per AuNP and is
supported by positively charged scavenger proteins. The presence of serum proteins
reduced particle uptake as the adsorption of these proteins on the AuNP-DNA is
suspected to compete with receptor binding [
58
]. This suggests that the interactions
between AuNP-DNA and scavenger receptors via the scavenger proteins are a
charge-mediated process. These unique features hold great promise for applications
in gene regulation therapies as well as in diagnostics, since it was shown that also
other inorganic particles, in particular iron oxide particles [
59
] as well as the
so-called “coreless spherical nucleic acids,” i.e., cross-linked nucleic acids, which
are oriented in the same way as in the case of AuNP-DNA but do not carry an
inorganic core [
57
], exhibit the same uptake properties.
Recently, DNA-functionalized AuNPs have been used as a platform to study the
interaction of even weaker-binding molecules in a similar colorimetric approach.
Witten el al. introduced glyconanoparticles with thermally addressable DNA
ligands, i.e., they bound glycomodified oligonucleotides to complementary
functionalized DNA-AuNP to form glyco-DNA-gold nanoparticles (AuNP-DNA-
glyco) [
60
]. These particles provide a multivalent presentation of DNA-glyco
ligands that assemble as a result of the binding of carbohydrate-binding proteins,
the so-called lectins, which carry a carbohydrate recognition domain (CRD). These
carbohydrate-protein interactions play an important role in processes such as
cell-cell or cell-matrix interactions and are exploited for the design of glyconano-
particles, e.g., for vaccine development or in vivo imaging [
61
]. In the example
given, the coupling of AuNP-DNA with
N
-acetyl-d-glucosamine (GlcNAc)-modified
