Functional DNA-Integrated Nanomaterials for Biosensing - DNA Nanotechnology: From Structure to Function - page 274

triphosphate (ATP) [ 58 ] using aptamers. Recently, the same group showed that the
aptamer that was cut into two pieces could reassemble into the intact aptamer tertiary
structure in the presence of the target. AuNPs could effectively differentiate between
these two states via the characteristic color change, which was used to selectively
detect cocaine in the low-micromolar range within minutes [ 59 ].
While AuNPs-based colorimetric sensors have taken an important step toward
real-time detection without the need for expensive instrumentation, they still
require laboratory-type operations, such as precise handling and mixing of multiple
microliter-scale solutions. These requirements make the sensors difficult to use,
especially at home. Considering that antibody-based lateral flow devices have been
widely applied in the home pregnancy test [ 60 ], lateral flow devices may provide
an ideal platform to further improve the DNA-AuNP colorimetric sensors. Ioannou
and Christopoulos et al. used the DNA-AuNP system to detect DNA on lateral flow
devices [ 61 ]. To expand on the range of analytes detected, the Lu group developed
a lateral flow device based on labeled AuNP and aptamer system for the detection
of adenosine and cocaine (Fig. 13.4 )[ 62 ]. The lateral flow devices consisted of four
pads: an absorption pad, a membrane, a glass fiber conjugation pad, and a wicking
pad. Biotin-labeled AuNP aggregates linked by adenosine aptamers (Fig. 13.4 a,b)
were spotted on the conjugation pad, and streptavidin was immobilized on the
membrane as a thin line (Fig. 13.4 c). When the wicking pad of the device is dipped
into a solution without adenosine, the rehydrated aggregates will stay at the bottom
of the membrane because aggregates are too large to migrate along the membrane
(Fig. 13.4 d). In the presence of adenosine, the disassembled AuNPs flowed along
the membrane and was captured by streptavidin to form a red line (Fig. 13.4 e).
Compared with solution-phase results, the flow device had at least tenfold higher
sensitivity with the naked eye as a detector, due to the separation of the aggregated
AuNPs from disaggregated ones offered by the solid-state membrane. The Li
group also reported a paper-based bioassay using aptamers and the protein enzyme
DNAase I [ 63 ]. Recently, by immobilizing AuNP-DNAzyme conjugates on lateral
flow devices, the Lu group developed an easy-to-use dipstick test for Pb 2C with a
detection limit of 0.5 M[ 64 ]. Based on similar principle, an easy-to-use dipstick

test for mercury was also demonstrated with such device [ 45 ]. This kind of simple
dipstick test may find wide use in household and other environmental applications.

13.2.2 AuNP-Based Fluorescent Biosensor

In addition to being used as color reporters for colorimetric sensing, AuNPs also
offer an advantage in constructing fluorescence sensors through their quenching
properties. Both theoretical and experimental studies have shown that AuNPs
can serve as a superquencher for a range of chromophores with high efficiency
[ 65 - 67 ]. Chromophores experience strong electronic interactions with the surface
when they are in close proximity to AuNPs, which results in energy-transfer