In conclusion, several groups have reported aptamer-based impedimetric biosensors for lysozyme detection at picomolar concentrations (see Table 1). By using
electrochemical chemiluminescence and aptamers as recognition element Dong et al.
showed an astonishing detection limit of 0.4 fM in spiked serum [19]. Due to the
high sensitivity of EIS aptasensors presented by Chen et al. and Zhang et al. which is
magnitudes lower than the physiological concentrations, sample dilution in an
appropriate buffer might be a promising strategy. Nonetheless, a proof of concept
in a diluted clinical sample still needs to be provided.
Thrombin, as coagulation agent in human blood, plays a central role in
human physiology. Usually, thrombin levels are low and additionally inhibited by
anti-thrombin agents in order to prevent non-functional blood clogging (thrombosis). Moreover, thrombin is involved in different neurodegenerative diseases such as
Alzheimer’s disease or Parkinson [20].
To increase sensitivity, signal amplification is a possible strategy. For instance, a
strategy presented by Deng et al. utilized the two binding sites of thrombin and
consists of a multiple step detection enhancement [21]. First, thrombin is captured by
immobilized aptamers on a gold surface. Second, gold nanoparticles coupled with
aptamers build a sandwich with captured thrombin. Third, the gold nanoparticles
serve as seed for further nanoparticle growth and thus increased steric hindrance
(reduction of HAuCl 4 ). Fourth, SDS (sodium dodecyl sulfate) builds a selfassembled monolayer on the gold nanoparticles. Their negative charge further
enhances the charge-transfer resistance resulting in a linear range from 50 pM up
to 35 nM (LoD 100 fM) and showing fair recoveries in spiked serum.
Lu et al. developed a design suitable for EIS as well as ECL determination with
the same aptamer sequence as Deng et al. The aptamer is immobilized on a gold
electrode while a complementary strand coupled to a Quantum Dot is supposed to be
released in the presence of thrombin (target induced dissociation). Thus, the impedance increases while the ECL signal decreases. The limit of detection of both
measurement strategies is 2.7 aM. Both measurement methods result in wide linear
ranges from 2.7 aM up to 2.7 μM for EIS and 2.7 aM up to 27 nM for ECL
[22]. Although there is no investigation in serum, it might work since the same
sequence was applied by Deng et al. for biosensor experiments in human serum.
An aptasensor based on another sequence was reported by Heydari-Bafrooei et al.
in 2016 [23]. Based on a nanocomposite consisting of TiO 2 , MWCNT (multiwalled
carbon nanotube), chitosan, and a Schiff base applied on a glassy carbon electrode, a
LoD of 1 fM was achieved due to increased surface (linear range from 50 fM up to
10 nM). The aptamer is immobilized by simple π-π stacking. Compared to an ELISA
kit on healthy serum and serum of patients with different diseases (e.g. Parkinson,
Epilepsy), similar results were obtained showing the applicability of the design even
in complex matrices.
Interleukins, member of the cytokines, are peptide hormones that regulate
immune cells. The analysis of different interleukin levels in human blood allows
inferences of cell state and cell-to-cell communications.
For instance, interleukin 17A (IL17A) is a biomarker for different autoimmune
diseases like arthritis or multiple sclerosis [1]. The synthesis of IL17A strictly
Impedimetric Aptamer-Based Biosensors: Applications
49
electrochemical chemiluminescence and aptamers as recognition element Dong et al.
showed an astonishing detection limit of 0.4 fM in spiked serum [19]. Due to the
high sensitivity of EIS aptasensors presented by Chen et al. and Zhang et al. which is
magnitudes lower than the physiological concentrations, sample dilution in an
appropriate buffer might be a promising strategy. Nonetheless, a proof of concept
in a diluted clinical sample still needs to be provided.
Thrombin, as coagulation agent in human blood, plays a central role in
human physiology. Usually, thrombin levels are low and additionally inhibited by
anti-thrombin agents in order to prevent non-functional blood clogging (thrombosis). Moreover, thrombin is involved in different neurodegenerative diseases such as
Alzheimer’s disease or Parkinson [20].
To increase sensitivity, signal amplification is a possible strategy. For instance, a
strategy presented by Deng et al. utilized the two binding sites of thrombin and
consists of a multiple step detection enhancement [21]. First, thrombin is captured by
immobilized aptamers on a gold surface. Second, gold nanoparticles coupled with
aptamers build a sandwich with captured thrombin. Third, the gold nanoparticles
serve as seed for further nanoparticle growth and thus increased steric hindrance
(reduction of HAuCl 4 ). Fourth, SDS (sodium dodecyl sulfate) builds a selfassembled monolayer on the gold nanoparticles. Their negative charge further
enhances the charge-transfer resistance resulting in a linear range from 50 pM up
to 35 nM (LoD 100 fM) and showing fair recoveries in spiked serum.
Lu et al. developed a design suitable for EIS as well as ECL determination with
the same aptamer sequence as Deng et al. The aptamer is immobilized on a gold
electrode while a complementary strand coupled to a Quantum Dot is supposed to be
released in the presence of thrombin (target induced dissociation). Thus, the impedance increases while the ECL signal decreases. The limit of detection of both
measurement strategies is 2.7 aM. Both measurement methods result in wide linear
ranges from 2.7 aM up to 2.7 μM for EIS and 2.7 aM up to 27 nM for ECL
[22]. Although there is no investigation in serum, it might work since the same
sequence was applied by Deng et al. for biosensor experiments in human serum.
An aptasensor based on another sequence was reported by Heydari-Bafrooei et al.
in 2016 [23]. Based on a nanocomposite consisting of TiO 2 , MWCNT (multiwalled
carbon nanotube), chitosan, and a Schiff base applied on a glassy carbon electrode, a
LoD of 1 fM was achieved due to increased surface (linear range from 50 fM up to
10 nM). The aptamer is immobilized by simple π-π stacking. Compared to an ELISA
kit on healthy serum and serum of patients with different diseases (e.g. Parkinson,
Epilepsy), similar results were obtained showing the applicability of the design even
in complex matrices.
Interleukins, member of the cytokines, are peptide hormones that regulate
immune cells. The analysis of different interleukin levels in human blood allows
inferences of cell state and cell-to-cell communications.
For instance, interleukin 17A (IL17A) is a biomarker for different autoimmune
diseases like arthritis or multiple sclerosis [1]. The synthesis of IL17A strictly
Impedimetric Aptamer-Based Biosensors: Applications
49