Colorimetric Bioassay Using Noble Metal Nanoparticles - From Bioimaging to Biosensors: Noble Metal Nanoparticles in Biodetection - page 43

Aptamer-modiied Au NPs (Apt-Au NPs) are useful for the
detection of PDGF. 23 Since each PDGF can bind two aptamers, the
color of Apt-Au NPs changes from red to purple at low concentrations
(<400 nM) as a result of aggregation with crosslinking between the
Au NPs. Interestingly, the color is reversed to red at very high PDGF
concentrations (>400 nM) due to the repulsion and steric effects
because the surface of the Apt-Au NPs quickly becomes saturated
with PDGF molecules through aptamer-PDGF binding (see Fig. 3.6).
By plotting the ratios of the extinction coeficients of the Apt-Au
NPs at 650 and 530 nm against the concentrations of PDGF-AA, the
linear ranges of the increases and decreases in this extinction ratio
are 25-75 and 75-200 nM, respectively. This assay can be applied to
detect the PDGF receptor- β (PDGFR- β ) at concentrations as low as
3.2 nM on the basis of the competition between the Apt-Au NPs and
PDGFR- β for PDGF-BB. In the presence of PDGFR- β , the interaction
between PDGF and Apt-Au NPs decreases, leading to a lesser extent
of aggregation. Similar strategies have been demonstrated for the
determination of other proteins such as thrombin, β -lactamase, and
phosphatase. 75-77
3.4.1.2 DNA and RNA assays
The irst reported DNA-Au NPs for the detection of target DNA
based on the aggregation of Au NPs through hybridization of
two complementary DNA strands was reported in 1996. 11 Two
noncomplementary DNA oligonucleotides each containing thiol
groups in one end were bound to the surfaces of two batches of 13
nm Au NPs. When DNA, which is complementary to the two grafted
sequences, was added to the solution, a polymer network formed
(see Fig. 3.7). This condensed network causes the conjugated Au NPs
to self-assemble into aggregates with a concomitant red-to-purple
color change. Because of signiicant changes in color, this approach
can detect DNA in concentrations as low as 10 fM. The sensitivity can
be further improved by placing the DNA-Au NP aggregates on solid
substrates such as a C18 reversed-phase thin-layer chromatography
plate. The blue color can be observed by the naked eye after the plate
dries.
For a given sequence, the temperature at which the reverse
color change (melting) takes place depends primarily on the salt
concentration but is also inluenced by the distance between the
particles and the density of the oligonucleotides attached to them. 78