of 20 mV [51]. They showed that for measurements of distilled water at the
macroelectrode, the relative permittivity approached 10
6 at 10 Hz due to polarization
effects and only reached the value of 80 at high frequencies >1 kHz, whereas at the
nano-gap electrode a relative permittivity of ~80 was reached in the frequency
range of 10–1,000 Hz and at higher frequencies decreased down to a value of ~3.
Thus, the influence of electrode polarization was diminished. Upon aptamer
immobilization the relative permittivity was reduced, and upon addition of thrombin
(0.2 μL), the value decreased another ~20%, while in controls with lysozyme or
random oligonucleotide, no changes were observed.
In summary, the limited amount of research conducted on non-faradaic
aptasensors is mainly based on big targets (molecular weight, >20 kDa) for clinical
application, but also smaller targets (Rev peptides, 2.4 kDa) were detectable [50].
Until now, reached sensitivities are not as good as in faradaic impedance
sensors, but the main drawback of low selectivity was resolved by the use of
aptamers, and additionally it was shown that direct detection in diluted serum is
possible [39–41, 45]. Although further development and research is needed,
non-faradaic aptasensors offer a promising tool for point-of-care diagnostics,
screenings, and online monitoring as a truly label-free technique.
4 Criteria for the Comparison of Impedimetric
Aptamer-Based Biosensors
Due to the almost unlimited variety of impedimetric aptasensors based on the
countless possibilities to choose electrode design, surface modification, immobilization
method, detection strategy, amplification strategy, and furthermore assay design (i.e.,
microfluidic or batch, real time or endpoint), it is difficult to compare the performance
of different biosensors. Thus, guidelines for their characterization are needed as
Thévenot et al. [52] already recommended for electrochemical biosensors based on
enzymes. Based on their publication, recommendations for the characterization of
impedimetric aptasensors are given in this paragraph. Further suggestions are included
and open for discussion.
A biosensor is defined by the integration of a biological recognition element
with a transducer that converts the binding of the analyte into an electrical signal.
By immobilizing the biological recognition element on the transducer, a close spatial
coupling is achieved. The direct coupling enables integrated devices which distinguish biosensors from bioassays. Therefore, we recommend that a biosensor should
be classified by the measurement method of the transducer and its biological
recognition element in the form “receptor-based methodic biosensor”
(e.g., aptamer-based impedimetric biosensor) with the addition of how the direct
coupling was achieved. These key points should be stated in the abstract of a
publication as well as the information if the detection strategy is direct or indirect.
32
P. Reich et al.
macroelectrode, the relative permittivity approached 10
6 at 10 Hz due to polarization
effects and only reached the value of 80 at high frequencies >1 kHz, whereas at the
nano-gap electrode a relative permittivity of ~80 was reached in the frequency
range of 10–1,000 Hz and at higher frequencies decreased down to a value of ~3.
Thus, the influence of electrode polarization was diminished. Upon aptamer
immobilization the relative permittivity was reduced, and upon addition of thrombin
(0.2 μL), the value decreased another ~20%, while in controls with lysozyme or
random oligonucleotide, no changes were observed.
In summary, the limited amount of research conducted on non-faradaic
aptasensors is mainly based on big targets (molecular weight, >20 kDa) for clinical
application, but also smaller targets (Rev peptides, 2.4 kDa) were detectable [50].
Until now, reached sensitivities are not as good as in faradaic impedance
sensors, but the main drawback of low selectivity was resolved by the use of
aptamers, and additionally it was shown that direct detection in diluted serum is
possible [39–41, 45]. Although further development and research is needed,
non-faradaic aptasensors offer a promising tool for point-of-care diagnostics,
screenings, and online monitoring as a truly label-free technique.
4 Criteria for the Comparison of Impedimetric
Aptamer-Based Biosensors
Due to the almost unlimited variety of impedimetric aptasensors based on the
countless possibilities to choose electrode design, surface modification, immobilization
method, detection strategy, amplification strategy, and furthermore assay design (i.e.,
microfluidic or batch, real time or endpoint), it is difficult to compare the performance
of different biosensors. Thus, guidelines for their characterization are needed as
Thévenot et al. [52] already recommended for electrochemical biosensors based on
enzymes. Based on their publication, recommendations for the characterization of
impedimetric aptasensors are given in this paragraph. Further suggestions are included
and open for discussion.
A biosensor is defined by the integration of a biological recognition element
with a transducer that converts the binding of the analyte into an electrical signal.
By immobilizing the biological recognition element on the transducer, a close spatial
coupling is achieved. The direct coupling enables integrated devices which distinguish biosensors from bioassays. Therefore, we recommend that a biosensor should
be classified by the measurement method of the transducer and its biological
recognition element in the form “receptor-based methodic biosensor”
(e.g., aptamer-based impedimetric biosensor) with the addition of how the direct
coupling was achieved. These key points should be stated in the abstract of a
publication as well as the information if the detection strategy is direct or indirect.
32
P. Reich et al.