Colorimetric Bioassay Using Noble Metal Nanoparticles - From Bioimaging to Biosensors: Noble Metal Nanoparticles in Biodetection - page 48

3.4.2.2 Protein and small analyte assays

Apt-Au NPs are useful for the analysis of small molecules such as
adenosine triphosphate (ATP), cocaine, DNase I, and Pb 2+ . 91-93
Similar to protein assays, the small analytes induced aggregation of
Apt-Au NPs through their speciic interactions with the aptamers.
Figure 3.10 illustrates a representative colorimetric assay for ATP. 94
Aliquots of salt (e.g., 300.0 mM NaCl) are added to Apt-Au NPs in
solutions with and without ATP. The solution without ATP changes
color from wine red to purple as a result of aggregation while the one
containing ATP retains its original color. A decreased zeta potential
due to salt screening is the main cause of the instability of the Apt-
Au NP solutions without ATP. Once ATP binds to the Apt-Au NPs, the
aptamer on the Au NP surfaces folds into a four-stranded tetraplex
structure (G-quartet). 95 In addition, the negatively charged density on
the Apt-Au NP surfaces increases. Polyethylene glycol promotes the
structural rigidity of the G-quartet, and TOTO-3 stabilizes the stem
structure of the aptamer. The linear range and LOD of this sensing
system for ATP are 20.0-100.0 nM and 10.0 nM, respectively.

Figure 3.10 A schematic representation of the sensing mechanism of Apt-
Au NPs for the colorimetric determination of ATP.

Similarly, Apt-Au NPs are useful for the detection of thrombin. 96
When thrombin interacts with its aptamer (29 base single-stranded
DNA), the aptamer is much more inclined to fold into a G-quadruplex/
duplex structure. Unfolded ssDNA can stabilize the Au NPs in the
presence of a given high concentration of salt while the quadruplex/
duplex structure can destroy this protecting function, leading to