the inability to detect mass transfer effects (e.g., Warburg impedance) due to
the shortness of the applied signal (normally ~2 ms) [16]. Prolongation of the signal
time is complicated because the current decreases exponentially and rapidly is
not distinguishable from noise. However, prolongation might be applicable for
systems with a very low rate constant that slows down the decrease of current
and enables approximate determination of the charge-transfer resistance. However,
no impedimetric aptasensor using the potential step technique has been developed
yet, and further studies on this technique are necessary.
The impedimetric aptasensors, reviewed in this article, are all based on
sequentially applied single-sine waves due to the absence of articles about
impedimetric aptasensors using multisine or potential step techniques.
3 New Developments in Detection Strategies
In impedimetric biosensors the flow of current is measured, and depending on how this
flow of current is influenced by the aptamer-target binding, impedimetric biosensors
are subdivided in faradaic (FIS) and non-faradaic impedance spectroscopy (nFIS).
The commonly used faradaic impedance systems are based on a charge transfer from
electrode to bulk or vice versa, whereas in non-faradaic systems, no charge transfer
occurs.
In this paragraph, we will discuss innovative developments in detection strategies
for impedimetric aptasensors in the last decade subdivided into FIS and nFIS. To
date, there are only a few publications on nFIS-based biosensors (Sect. 3.2), whereas
for FIS-based biosensors (Sect. 3.1), there are several reviews on detection strategies
as mentioned in the introduction, to which the reader is appointed for more details.
3.1 Faradaic Impedimetric Aptasensors
Due to the fact that in impedimetric measurements low voltages are used, no charge
transfer occurs in simple buffer solutions. To enable charge transfer in faradaic
measurements, a redox mediator is added to the buffer or immobilized on the
surface (see Fig. 4). As redox mediators are in general easily available in big
amounts and low-priced, faradaic systems with a redox mediator in solution are
seen as label-free. But the addition of a redox mediator in a separated step disables
online measurements. Beyond this, it is difficult to distinguish between labeled
and label-free impedimetric systems. As the main advantage of label-free techniques
is to spare time and costs for the labeling, the use of attached redox mediators or
other electroactive molecules shall be termed labeled. But also in case of other
amplification strategies, the question for time and cost consumption shall be asked.
The more complex the mechanism for signal amplification is, the less useful are
label-free measurements. A good overview to different detection strategies based
22
P. Reich et al.
the shortness of the applied signal (normally ~2 ms) [16]. Prolongation of the signal
time is complicated because the current decreases exponentially and rapidly is
not distinguishable from noise. However, prolongation might be applicable for
systems with a very low rate constant that slows down the decrease of current
and enables approximate determination of the charge-transfer resistance. However,
no impedimetric aptasensor using the potential step technique has been developed
yet, and further studies on this technique are necessary.
The impedimetric aptasensors, reviewed in this article, are all based on
sequentially applied single-sine waves due to the absence of articles about
impedimetric aptasensors using multisine or potential step techniques.
3 New Developments in Detection Strategies
In impedimetric biosensors the flow of current is measured, and depending on how this
flow of current is influenced by the aptamer-target binding, impedimetric biosensors
are subdivided in faradaic (FIS) and non-faradaic impedance spectroscopy (nFIS).
The commonly used faradaic impedance systems are based on a charge transfer from
electrode to bulk or vice versa, whereas in non-faradaic systems, no charge transfer
occurs.
In this paragraph, we will discuss innovative developments in detection strategies
for impedimetric aptasensors in the last decade subdivided into FIS and nFIS. To
date, there are only a few publications on nFIS-based biosensors (Sect. 3.2), whereas
for FIS-based biosensors (Sect. 3.1), there are several reviews on detection strategies
as mentioned in the introduction, to which the reader is appointed for more details.
3.1 Faradaic Impedimetric Aptasensors
Due to the fact that in impedimetric measurements low voltages are used, no charge
transfer occurs in simple buffer solutions. To enable charge transfer in faradaic
measurements, a redox mediator is added to the buffer or immobilized on the
surface (see Fig. 4). As redox mediators are in general easily available in big
amounts and low-priced, faradaic systems with a redox mediator in solution are
seen as label-free. But the addition of a redox mediator in a separated step disables
online measurements. Beyond this, it is difficult to distinguish between labeled
and label-free impedimetric systems. As the main advantage of label-free techniques
is to spare time and costs for the labeling, the use of attached redox mediators or
other electroactive molecules shall be termed labeled. But also in case of other
amplification strategies, the question for time and cost consumption shall be asked.
The more complex the mechanism for signal amplification is, the less useful are
label-free measurements. A good overview to different detection strategies based
22
P. Reich et al.