Plasmodium LDH covering the necessary range from 13 to 206 pM. Depending on
the pH, the required linear range can be adjusted. Moreover, the regeneration of the
sensor was demonstrated [32].
For influenza virus H1N1, Kiilerich-Pedersen et al. presented an impedimetric
aptasensor that is potentially applicable as a point-of-care test [33]. Standard
methods include RT-PCR, antigen staining, and virus cultivation. Especially virus
cultivation requires several days and is therefore not suitable for rapid testing.
However, by way of example, an influenza virus that is responsible for a potentially
pandemic flu, as arisen in 2009, needs to be determined in a rapid and reliable
manner. In a buffer system, a linear range of 10
1
–10
6 Plaque-forming units (PFU)
per ml was achieved. Briefly, a suitable DNA-aptamer was immobilized on a
conductive polymer in a microchannel. A saliva sample spiked with 10
3 PFU/ml
(100-fold lower than in real samples) caused a signal within 15 min, which was not
induced by a H1N1 virus-free saliva sample.
Cyclic adenosine monophosphate (cAMP) is a secondary messenger with various
functions in different physiological contexts [34]. cAMP levels are usually low and
stable, while an increase in urine or plasma points to different diseases. The detection
of cAMP is an example of detecting relatively small molecules by means of
impedance increase. Zhao and coworkers applied a RNA aptamer in a simple
approach that uses gold electrodes [35]. These gold electrodes were enhanced by
gold nanoparticles that allowed for the detection of cAMP in buffer and spiked
diluted serum. In spiked serum, a linear range of 50 nM to 1 μM was achieved. Three
orders of magnitude lower, in a linear range of 50–250 pM, were detectable in a
buffer system. Selectivity was proven, as the sensor was not triggered by ATP,
AMP, or c-diGMP.
For the detection of adenosine, Wang et al. used a methodically similar design
[36]. The study demonstrates the influence of backfiller such as DTT and MCH
which are added to functionalized aptamer solutions (see Fig. 3). Depending on the
backfiller composition, linear ranges of 0.1–61 nM for pure MCH backfilling (LoD
0.03 nM), 0.5–27 pM for pure DTT backfilling (LoD 0.2 pM), and 0.05–17 pM for a
mixture of DTT and MCH (LoD 0.02 pM) were achieved. For testing clinical
suitability, diluted clinical samples were analyzed, resulting in recoveries of
95–101% (lowest sample concentration 0.066 pM).
2.2 Cancer Detection
The uncontrolled proliferation of the body’s owns cells and the inhibition of natural
cell death (apoptosis) as present in cancer require early diagnosis in order to prevent
the formation of metastases. Since aptamer sequences with a high and specific
affinity to different tumor-related markers and cancer cells have been selected lately,
the development of impedimetric aptasensors for the detection of low level tumor
markers at an early point is pursued [37, 38]. Beside the benefits of specific
Impedimetric Aptamer-Based Biosensors: Applications
51
the pH, the required linear range can be adjusted. Moreover, the regeneration of the
sensor was demonstrated [32].
For influenza virus H1N1, Kiilerich-Pedersen et al. presented an impedimetric
aptasensor that is potentially applicable as a point-of-care test [33]. Standard
methods include RT-PCR, antigen staining, and virus cultivation. Especially virus
cultivation requires several days and is therefore not suitable for rapid testing.
However, by way of example, an influenza virus that is responsible for a potentially
pandemic flu, as arisen in 2009, needs to be determined in a rapid and reliable
manner. In a buffer system, a linear range of 10
1
–10
6 Plaque-forming units (PFU)
per ml was achieved. Briefly, a suitable DNA-aptamer was immobilized on a
conductive polymer in a microchannel. A saliva sample spiked with 10
3 PFU/ml
(100-fold lower than in real samples) caused a signal within 15 min, which was not
induced by a H1N1 virus-free saliva sample.
Cyclic adenosine monophosphate (cAMP) is a secondary messenger with various
functions in different physiological contexts [34]. cAMP levels are usually low and
stable, while an increase in urine or plasma points to different diseases. The detection
of cAMP is an example of detecting relatively small molecules by means of
impedance increase. Zhao and coworkers applied a RNA aptamer in a simple
approach that uses gold electrodes [35]. These gold electrodes were enhanced by
gold nanoparticles that allowed for the detection of cAMP in buffer and spiked
diluted serum. In spiked serum, a linear range of 50 nM to 1 μM was achieved. Three
orders of magnitude lower, in a linear range of 50–250 pM, were detectable in a
buffer system. Selectivity was proven, as the sensor was not triggered by ATP,
AMP, or c-diGMP.
For the detection of adenosine, Wang et al. used a methodically similar design
[36]. The study demonstrates the influence of backfiller such as DTT and MCH
which are added to functionalized aptamer solutions (see Fig. 3). Depending on the
backfiller composition, linear ranges of 0.1–61 nM for pure MCH backfilling (LoD
0.03 nM), 0.5–27 pM for pure DTT backfilling (LoD 0.2 pM), and 0.05–17 pM for a
mixture of DTT and MCH (LoD 0.02 pM) were achieved. For testing clinical
suitability, diluted clinical samples were analyzed, resulting in recoveries of
95–101% (lowest sample concentration 0.066 pM).
2.2 Cancer Detection
The uncontrolled proliferation of the body’s owns cells and the inhibition of natural
cell death (apoptosis) as present in cancer require early diagnosis in order to prevent
the formation of metastases. Since aptamer sequences with a high and specific
affinity to different tumor-related markers and cancer cells have been selected lately,
the development of impedimetric aptasensors for the detection of low level tumor
markers at an early point is pursued [37, 38]. Beside the benefits of specific
Impedimetric Aptamer-Based Biosensors: Applications
51