Colorimetric Bioassay Using Noble Metal Nanoparticles - From Bioimaging to Biosensors: Noble Metal Nanoparticles in Biodetection - page 42

Au NPs, causing the disruption of Au NP aggregates. As a result, the
color of the solution changes from purple to the original red color.
Similarly, a colorimetric bioassay using Ag NPs conjugated
with 2-mercaptoethyl α -d-mannopyranoside has been used for the
analysis of Con A. 34,73 Con A induces aggregation of the Man-Ag NPs,
leading to color changes from yellow to brown. The color changes
correspond to the changes in the LSPR absorption wavelength from
400 to 480 nm. The spectra show an isosbestic point at 440 nm
upon addition of the Con A, demonstrating that the aggregation of
Man-Ag NPs is directly related to the concentration of Con A. Man-Ag
NPs at a concentration of 3 nM provide an assay for Con A with the
largest linear range. One beneicial feature of assays using Ag NPs
are their faster kinetic rates of aggregation than those using Au NPs.
Self-assembled monolayers on Ag and Au surfaces have different
surface orientations. 74 However, this may account for the signiicant
difference in the rate of aggregation between the Man-Ag and
Au NPs.
Protein assays based on DNA-protein interactions are common
in the life sciences. Recently, DNA-conjugated Au NPs have become
interesting nanomaterials for protein assays. Aptamers are
especially interesting because they are easily synthesized by using
an automatic DNA synthesizer and are extremely stable. Aptamers
are nucleic acid macromolecules (DNA and RNA) that bind tightly to
a speciic molecular target (proteins and small molecules). Like all
nucleic acids, a particular aptamer is described by a linear sequence
of nucleotides (A, U, T, C, and G), typically 15-40 nucleotides long. In
solution, the chain of nucleotides forms intramolecular interactions
to fold the molecule into a complex three-dimensional shape. The
shape of the aptamer allows it to bind tightly to the surface of its
target molecule.
Figure 3.6 Schematics of aptamer-directed Au NP-based colorimetric
sensors. Assembly of aptamer-functionalized Au NPs by a target
protein (PDGF). Each PDGF can bind two aptamer molecules.