Functional DNA-Integrated Nanomaterials for Biosensing - DNA Nanotechnology: From Structure to Function - page 273

Fig. 13.3 Schematic illustration of colorimetric sensor for adenosine. The DNA sequences are
shown in the right side of the figure. The introduction of adenosine stimulates disassembly of
aptamer-linked AuNPs, changing the color of AuNPs from blue to red (Reproduced from Ref. [ 49 ]
by permission of John Wiley & Sons Ltd)

containing the aptamer sequence was used to cross-link the two ssDNAs that
were attached to two different batches of AuNPs, resulting in aggregation of the
AuNPs. In the presence of target, the aptamer switches its structure and binds target
molecules. As a result, much fewer base pairs were left to link AuNPs, which was
unstable and thus disassembled the AuNPs aggregates, with an accompanying blue
to red color change. This method can be extended to detect various targets such as
adenosine, cocaine, as well as multiple targets with controllable cooperativity [ 50 ].
Li group also reported a new type of sensing method based on structure-switching
aptamers and non-cross-linking AuNP aggregation [ 51 ]. A structure-switching
DNA aptamer was first hybridized with a short cDNA attached to AuNPs, which
provided additional negative charge to AuNPs and enhanced their stability at a
chosen salt concentration. Upon binding of the target, the aptamer strand underwent
structure switching and dissociated from the AuNPs, which decreased the salt
stability of the AuNPs and resulted in a red to purple color change. This turn-on
colorimetric sensor had high selectivity and a detection limit of 10

M. Later, it
was found that AuNPs to which folded aptamer structures are attached are more
stable than those tethered to unfolded aptamers, which could be used for designing
colorimetric sensors for adenosine and potassium ions [ 52 , 53 ]. By introducing pro-
tein aptamers into such a system, various colorimetric sensors were developed for
detection of proteins (e.g., liplatelet-derived growth factor and thrombin) [ 54 , 55 ].
On the other hand, it was found that the absorption capabilities of aptamers
on AuNPs is decreased by the target-binding-induced aptamer folding, which
could be used for designing various label-free colorimetric sensors. Dong and
coworkers demonstrated the detection of thrombin based on this principle [ 56 ].
Fan and coworkers reported colorimetric sensors for potassium [ 57 ] and adenosine