Biology Reference
In-Depth Information
target the aptamer is in an unfolded conformation, allowing rapid
interaction between the MB redox label and the electrode. Upon
target binding the aptamer forms a stable structure, reducing the
distancebetweenthelabelandtheelectrode,therefore,reducingthe
electron transfer between MB and the electrode.
An alternative strategy has been developed, based on a “signal-
on” configuration [46]. A thiolated thrombin aptamer modified at
the non-thiolated end with a ferrocene group was immobilized
onto a polycrystalline gold electrode. The long and flexible aptamer
chain prevented contact between the redox label and the electrode,
inhibiting the generation of the electrochemical signal. The binding
of thrombin to the aptamer caused the formation of the characteris-
tic G-quadruplex aptamer structure, orientating the ferrocene units
toward the electrode and leading to a positive amperometric signal.
The differential pulse voltammetry measurements demonstrated a
thrombin detection limit of 0.5 nM and a detection range between
5 and 35 nM. A similar “signal on” approach [47] utilizing a cocaine
aptamerandMBaselectrochemicallabelwasusedforthedetection
of cocaine.
More recently, a detailed study was conducted on an electro-
chemical biosensor based on a “signal- on” approach for the detec-
tion of theophylline [36, 37]. The RNA aptamer for theophylline
was first labeled with ferrocene and anchored to a gold electrode:
its conformation switching upon binding of theophylline caused
the formation of a folded structure with an increased electron
transfer between ferrocene and the electrode. In this approach
theophylline could be detected at the micromolar range, but the
biosensorresponse wasinhibitedinserum. Thebiosensorwasthen
optimized by substituting ferrocene with MB, shifting the redox
potential from positive to negative potential (
−
0.25 V vs. Ag/AgCl).
The modified biosensor could detect theophylline in the relevant
range 2 to 100
μ
M and the biosensor response in serum was sim-
ilar to the response in buffer.
A very interesting biosensor was developed for the detection
of cocaine by using a microfluidic electrochemical device [42]
(Fig. 2.8).
Cocaine at the micromolar range was detectable in continuous,
real-time, and in undiluted and untreated bloodsamples.
