Readers, who are more interested in nanocomposites, are encouraged to read the
review of Muñoz et al. [10] about the usage of nanocomposites for impedimetric
biosensors.
Another interesting method for the detection of PSA was developed by Jolly et al.
[34] who used the so-called hybrid molecular imprinting system. Molecularly
imprinted polymers (MIPs) are synthetic receptors made by polymerization around
a template. After removing the template, the created cavities recognize the
target molecule. The weaknesses of MIPs are low specificity and reproducibility.
By the utilization of immobilized aptamer-target complexes as template for MIPs,
it is expected that these weaknesses are overcome due to the higher flexibility
of aptamers compared to the usually used functional monomers. Indeed, Jolly
et al. were able to increase sensitivity to 1 pg/mL (~29 fM) for PSA compared
to 91 pg/mL (2.7 pM) by a MIP without aptamers [35].
In conclusion, only a small fraction of new developments in biosensing strategies
was presented here. The recent advances in oligonucleotide synthesis procedures
opened up a new field of surface modification. Working with oligonucleotides is,
like playing with toy bricks, only limited by your imagination. Thus, using DNA
origami for application-specific surface modifications is promising. Also
nanocomposites are advantageous, as the combination of different materials can be
used to form the characteristics of the surface according to the desired application.
But it should be kept in mind that for a successful transfer of scientific developments
into products, a thorough understanding of the whole mechanism is needed. Therefore, surface modifications are favored that are completely characterized and
understood.
3.2 Non-Faradaic Impedimetric Aptasensors
Non-faradaic systems do not need any redox mediator and thus are favored in realtime measurements. They detect the accumulation of charge on the electrode surface.
The measurement principle is based on the change of the thickness and/or dielectric
properties of the material between the electrodes upon binding of the analyte,
measured as a decrease in capacitance. To avoid any leaking current, proper
insulation of the electrode surface is needed. And as the capacitance measured in
non-faradaic systems is the sum of all capacitances, the passivation layer should
be as thin and its dielectric constant as high as possible to reduce its impact on
overall capacitance [36]. For details on the principles and strategies of non-faradaic
biosensors, the reader is addressed to Daniels et al. [37] who published a substantial
review on label-free non-faradaic biosensors including all aspects of this technology.
Herein, we will review non-faradaic impedimetric aptamer-based biosensors
since 2005.
In non-faradaic biosensors, mainly interdigitated electrodes (IDEs, Fig. 8) are
used due to their advantages for capacitive sensing such as high contact area
and collection efficiency. Interdigitated electrodes are often formed like two
28
P. Reich et al.
review of Muñoz et al. [10] about the usage of nanocomposites for impedimetric
biosensors.
Another interesting method for the detection of PSA was developed by Jolly et al.
[34] who used the so-called hybrid molecular imprinting system. Molecularly
imprinted polymers (MIPs) are synthetic receptors made by polymerization around
a template. After removing the template, the created cavities recognize the
target molecule. The weaknesses of MIPs are low specificity and reproducibility.
By the utilization of immobilized aptamer-target complexes as template for MIPs,
it is expected that these weaknesses are overcome due to the higher flexibility
of aptamers compared to the usually used functional monomers. Indeed, Jolly
et al. were able to increase sensitivity to 1 pg/mL (~29 fM) for PSA compared
to 91 pg/mL (2.7 pM) by a MIP without aptamers [35].
In conclusion, only a small fraction of new developments in biosensing strategies
was presented here. The recent advances in oligonucleotide synthesis procedures
opened up a new field of surface modification. Working with oligonucleotides is,
like playing with toy bricks, only limited by your imagination. Thus, using DNA
origami for application-specific surface modifications is promising. Also
nanocomposites are advantageous, as the combination of different materials can be
used to form the characteristics of the surface according to the desired application.
But it should be kept in mind that for a successful transfer of scientific developments
into products, a thorough understanding of the whole mechanism is needed. Therefore, surface modifications are favored that are completely characterized and
understood.
3.2 Non-Faradaic Impedimetric Aptasensors
Non-faradaic systems do not need any redox mediator and thus are favored in realtime measurements. They detect the accumulation of charge on the electrode surface.
The measurement principle is based on the change of the thickness and/or dielectric
properties of the material between the electrodes upon binding of the analyte,
measured as a decrease in capacitance. To avoid any leaking current, proper
insulation of the electrode surface is needed. And as the capacitance measured in
non-faradaic systems is the sum of all capacitances, the passivation layer should
be as thin and its dielectric constant as high as possible to reduce its impact on
overall capacitance [36]. For details on the principles and strategies of non-faradaic
biosensors, the reader is addressed to Daniels et al. [37] who published a substantial
review on label-free non-faradaic biosensors including all aspects of this technology.
Herein, we will review non-faradaic impedimetric aptamer-based biosensors
since 2005.
In non-faradaic biosensors, mainly interdigitated electrodes (IDEs, Fig. 8) are
used due to their advantages for capacitive sensing such as high contact area
and collection efficiency. Interdigitated electrodes are often formed like two
28
P. Reich et al.