4-chloro-1-naphthol to an insoluble precipitate, the impedance is further increased.
Hereby, Peng et al. reached a detection limit of 7 pM for NF-κB.
Compared to the standard polymerase chain reaction (PCR), the hybridization
of the DNA fragments to a nanoladder produces multiplex nanowires which
increase the negative charge on the surface significantly and thus the impedance.
As oligonucleotides can be synthesized cost-effective in big amounts, this
amplification method is a valuable alternative to other labels.
The artificial synthesis of oligonucleotides not only enabled the use of aptamers
but also revolutionized the field of nanomedicine by engineering of three-dimensional
DNA structures, also called DNA origami, for its use as synthetic vaccine, as vehicle
in drug delivery or marker in cancer therapy [23]. Therefore, short oligonucleotides
with specific sequences which overlap each other are used to form different structures
like squares or triangles. Sheng et al. developed an impedimetric aptasensor for
the detection of interferon-γ by using a triangular pyramidal oligonucleotide nanostructure [24]. The basement of the pyramidal structure is formed by four different
oligonucleotides that are immobilized via thiol groups on the gold electrode surface,
while the interferon-γ aptamer is the fifth oligonucleotide fragment which closes
the pyramidal structure on top. The closed pyramid hinders the electron transfer
to the redox mediator in solution. In the presence of the analyte, the aptamer is
released from the surface and electron transfer is enabled, thus impedance decreased
(see Fig. 7). By the simple addition of free aptamer, the sensor is regenerated. With
this technique Sheng et al. reached an impressive wide linear range from 1 nM to 2 μM
and a detection limit of 520 pM. Additionally, they were able to detect interferon-γ in
diluted serum (50%).
The same principle was used by Sheng et al. for the detection of cocaine with
a detection limit of 210 pM and a linear range from 1 nM to 2 μM [25], which
shows that this technique can be applied for different aptamers and especially is
suitable for the sensitive detection of small molecules. DNA origami offers great
prospects for surface engineering in the future as there is no limit to the design of
DNA nanostructures.
DNA origami can be also used to design target-induced hydrogel formation
as described by Yang et al. [26]. Therefore a DNA strand was modified with acrydite
to obtain a polymeric DNA structure, of which the arms are complementary to
the aptamer for heparanase, a potential target in cancer treatment. In presence of
the target, the aptamer is released, and the arms can hybridize with each other to
form a hydrogel leading to a significant increase in impedance due to the hindrance
of charge transfer between electrode and redox mediator in solution. With this
method, they were able to detect heparanase down to 3 fg/mL and to detect the
target in diluted serum samples with recovery rates of 90–110%. Another advantage
of the use of DNA origami is that the biocompatibility of the developed biosensors
is increased.
To reach lower detection limits, one strategy is to increase the effective surface,
for example, by the deposition of gold nanoparticles. Gold nanoparticles exhibit
special optical and physical properties, such as surface plasmons, enhanced Raman
signals, fast electron transfer, magnetism, and catalytic activity [27]. If the
Impedimetric Aptamer-Based Biosensors: Principles and Techniques
25
Hereby, Peng et al. reached a detection limit of 7 pM for NF-κB.
Compared to the standard polymerase chain reaction (PCR), the hybridization
of the DNA fragments to a nanoladder produces multiplex nanowires which
increase the negative charge on the surface significantly and thus the impedance.
As oligonucleotides can be synthesized cost-effective in big amounts, this
amplification method is a valuable alternative to other labels.
The artificial synthesis of oligonucleotides not only enabled the use of aptamers
but also revolutionized the field of nanomedicine by engineering of three-dimensional
DNA structures, also called DNA origami, for its use as synthetic vaccine, as vehicle
in drug delivery or marker in cancer therapy [23]. Therefore, short oligonucleotides
with specific sequences which overlap each other are used to form different structures
like squares or triangles. Sheng et al. developed an impedimetric aptasensor for
the detection of interferon-γ by using a triangular pyramidal oligonucleotide nanostructure [24]. The basement of the pyramidal structure is formed by four different
oligonucleotides that are immobilized via thiol groups on the gold electrode surface,
while the interferon-γ aptamer is the fifth oligonucleotide fragment which closes
the pyramidal structure on top. The closed pyramid hinders the electron transfer
to the redox mediator in solution. In the presence of the analyte, the aptamer is
released from the surface and electron transfer is enabled, thus impedance decreased
(see Fig. 7). By the simple addition of free aptamer, the sensor is regenerated. With
this technique Sheng et al. reached an impressive wide linear range from 1 nM to 2 μM
and a detection limit of 520 pM. Additionally, they were able to detect interferon-γ in
diluted serum (50%).
The same principle was used by Sheng et al. for the detection of cocaine with
a detection limit of 210 pM and a linear range from 1 nM to 2 μM [25], which
shows that this technique can be applied for different aptamers and especially is
suitable for the sensitive detection of small molecules. DNA origami offers great
prospects for surface engineering in the future as there is no limit to the design of
DNA nanostructures.
DNA origami can be also used to design target-induced hydrogel formation
as described by Yang et al. [26]. Therefore a DNA strand was modified with acrydite
to obtain a polymeric DNA structure, of which the arms are complementary to
the aptamer for heparanase, a potential target in cancer treatment. In presence of
the target, the aptamer is released, and the arms can hybridize with each other to
form a hydrogel leading to a significant increase in impedance due to the hindrance
of charge transfer between electrode and redox mediator in solution. With this
method, they were able to detect heparanase down to 3 fg/mL and to detect the
target in diluted serum samples with recovery rates of 90–110%. Another advantage
of the use of DNA origami is that the biocompatibility of the developed biosensors
is increased.
To reach lower detection limits, one strategy is to increase the effective surface,
for example, by the deposition of gold nanoparticles. Gold nanoparticles exhibit
special optical and physical properties, such as surface plasmons, enhanced Raman
signals, fast electron transfer, magnetism, and catalytic activity [27]. If the
Impedimetric Aptamer-Based Biosensors: Principles and Techniques
25