Colorimetric Bioassay Using Noble Metal Nanoparticles - From Bioimaging to Biosensors: Noble Metal Nanoparticles in Biodetection - page 47

3.4.2.1 DNA assays
Poly( N -isopropylacrylamide)-DNA-conjugated Au NPs are useful
for the detection of single-stranded DNA above the physiological
temperature. The conjugated Au NPs are stable at 1.5 M NaCl.
Because of the great negative charge density from the conjugated
DNA on the Au NP surface, the repulsion force is dominant. In the
presence of the target DNA that is perfectly complementary to the
probe, in sequence as well as in chain length, the conjugated Au NPs
aggregate, leading to a change in color. When DNA hybridization
occurs, the charge density on the Au NP surface decreases. Similar
phenomena also occurred using bare Au NPs (without capping with
the probe DNA) for detection of DNA. 86,87 Bare Au NPs immediately
aggregate at 0.1 M NaCl while they do not exhibit any visible change
in solutions with concentrations up to 2.5 M NaCl in the presence of
DNA because DNA molecules assemble around the Au NP surfaces
through electrostatic interactions (see Fig. 3.9). In the presence of
a target DNA molecule with complementary sequences to the probe
DNA, a clear colorimetric change from red to purple immediately
occurs (<3 min) at NaCl concentrations greater than 0.5 M. Similar
strategies have been applied to the studies of eukaryotic gene
expression, Mycobacterium tuberculosis DNA, and single nucleotide
polymorphism in clinical samples. 88-90
Figure 3.9 Pictorial representation of the colorimetric method for dif-
ferentiating between single- and double-stranded oligonucle-
otides. The circles represent colloidal gold nanoparticles.