Colorimetric Bioassay Using Noble Metal Nanoparticles - From Bioimaging to Biosensors: Noble Metal Nanoparticles in Biodetection - page 45

fM and 500 zM, respectively. The last detection approach provides
sensitivity that is comparable to PCR-based approaches.
3.4.1.3 Small analyte assays
Colorimetric assays using conjugated Apt-Au NPs are sensitive
and selective for the determination of a number of small analytes,
including metal ions such as Hg 2+ and K + . 82,83 Selective and sensitive
(LOD = 100 nM) detection of Hg 2+ ions in aqueous solutions in the
presence of 2,6-pyridinedicarboxylic acid has been realized by using
3-mercaptopropionic acid functionalized Au NPs (MPA-Au NPs),
based on aggregation-induced color changes. Figure 3.8A shows the
UV-visible absorption response to Hg 2+ of a 3.0 nM suspension of
MPA-Au NPs in 50 mM Tris-borate buffer at pH 9.0. The dispersed
MPA-Au NPs display a LSPR band at 520 nm, which undergoes
redshifts with decreased extinction (the extinction at 650 nm
increases) upon aggregation. The extinctions at 520 and 650 nm are
related separately to the quantities of the dispersed and aggregated
MPA-Au NPs. The inset A to Fig. 3.8A displays the color changes from
red to purple following the addition of 1.0 mM Hg(NO) 3 (100 μL) to
the MPA-Au NP solution (900 μL), and the aggregated MPA-Au NPs
precipitate after 1 h. As shown in inset B of Fig 3.8A, the Hg 2+ induced
aggregation of the MPA-Au NPs reaches completion within 60 min
according to the monitoring of the extinction ratios (Ex 650/520 ) in
the time frame of these measurements. Figure 3.8B,C displays the
transmission electron microscopy and optical dark-ield scattering
images of the MPA-Au NP solutions in the absence and presence
of Hg 2+ , respectively. Owing to low scattering intensity and faster
diffusion of the Au NPs (13.3 nm), the scattering light from single
MPA-Au NPs is not easily observed by the dark ield spectroscopy
system. It is important to highlight that strong scattering occurs once
the MPA-Au NPs aggregate; the orange and red spots correspond to
the scattering images of the MPA-Au NP aggregates.
Apt-Au NPs have also been developed for the detection of
small solutes such as adenosine and cocaine based on reversed
sensing mechanisms (from aggregation to suspension). 84,85 The NP
aggregates contain Apt-Au NPs (3 ′ - and 5 ′ -thiol-modiied DNA) and
a linker DNA molecule that is divided into two segments, including
hybridized and recognized sites. In the presence of adenosine, the
aptamer changes its structure to bind adenosine, leading to the
disruption of the aggregates and thus a change in the solution color