Functional DNA-Integrated Nanomaterials for Biosensing - DNA Nanotechnology: From Structure to Function - page 269

13.2 Functional DNA-Integrated Gold Nanoparticles

(AuNPs) for Biosensing

13.2.1 AuNP-Based Colorimetric Biosensor

Noble metal nanoparticles, especially gold nanoparticles (AuNPs), have found
multiple applications in the sensing and biomedical fields because of their unique
optical and electronic properties [ 23 , 24 ]. Size- and shape-controlled AuNPs with
flexible surface chemistry can be easily prepared based on reduction of metal salt
precursors with reducing agents under proper conditions, either in the absence or
in the presence of a AuNP seed as a mediator [ 25 ]. One remarkable feature of the
AuNPs is that they show interesting distance-dependent surface plasmon properties,
resulting in obviously different colors between the dispersed state and aggregated
state due to interparticle plasmon coupling and their extinction coefficient [ 26 ].
Therefore, AuNPs can be an ideal choice as reporters for colorimetric sensing. Since
the signal is detectable by the naked eye without the need for analytical instruments,
the AuNP-based colorimetric sensors have played an important role toward real-
time sensing with high sensitivity and low cost.
The attachment of thiolated DNA to AuNPs was first reported by both the Mirkin
group and the Alivisatos group [ 27 , 28 ], which open a new area in the field of
bionanotechnology. The aggregation of DNA-modified AuNP can be prevented due
to highly negatively charged phosphate backbone of DNA, while the addition of a
complementary target DNA would lead to the aggregation of AuNPs, resulting in
a color change from red to blue (Fig. 13.1 a,b). Based on this DNA-induced color
change of DNA-functionalized AuNPs, DNA with a concentration down to sub-
picomolar levels could be detected with single-base mismatch resolution [ 29 - 31 ].
Furthermore, it was found that DNA-modified AuNPs aggregates exhibited an
unanticipated sharp melting profile due to the dense attachment of DNA on the
surface of AuNPs and the DNA hybridization in a highly cooperative manner [ 31 ].
While the Mirkin method can be used to detect nucleic acids, it is desirable to
expand it to detect a much broader range of targets, such as metal ions, organic
molecules, and proteins. In addition, the method requires careful control of tem-
peratures in order to take advantage of the difference of sharp melting temperature
between the target DNA and other DNA, such as those with a single mismatch.
To make the method more tolerable to temperature variations, such as those in
outdoor environmental detection, other methods to influence the aggregation states
of the AuNPs are required. To meet these challenges, the Lu group first reported a
colorimetric biosensor for Pb 2C based on DNAzyme-functionalized AuNPs [ 32 ].
The Pb 2C -dependent DNAzyme consisted of an enzyme strand and a substrate