Introduction 1 2 3 4 5 6 9 1 Fig. 1 MIP  Aptamers as molecular recognition elements 10 11 12 12 aptus 13 2 14 15 15 Fig. 2 14  1 16 4 17 18 19 20 Table 1 16  14 15 21 22 23 24 25 26 27 35 36 37 Electrical aptamer sensors Electrochemical aptamer sensors 27 35 28 29 31 30 38 3 39 40 41 Fig. 3 MB 31  4 28 29 25 2 Fig. 4 31  Table 2 The detection limits of an electrochemical aptamer sensor for thrombin   Detection limit Characteristics Reference 1 3 nM Uses an MB-tagged "holding" DNA sequence 31 2 80 nM/3.5 nM Peroxidase-labeled thrombin gives an 80 nM detection limit. Biotin-labeled aptamer with horse radish peroxidase labeled streptavidin gives a detection limit of 3.5 nM 27 3 1 μM Sandwich assay using two aptamers with different binding sites. Secondary aptamer labeled with glucose dehydrogenase 28 4 10 nM Same as for 3, but pyrroquinoline quinine glucose dehydrogenase is used for labeling 29 5 6.4 nM MB-tagged thrombin aptamer (signal-off sensor) 30 6 0.1–0.15 nM Thrombin aptamer with ferrocene moiety (square-wave voltametry or chronopotetiometry used for the measurement) 32 7 11 nM MB-intercalated thrombin aptamer 25 MB 34 42 5 34 Fig. 5 a b PDGF a 34 b 35  35 14 43 5 44 Aptamer sensors with carbon nanotube field-effect transistor transducers 45 46 6 47 7 9 6 6 Fig. 6 The working principle of field-effect transistor (FET) sensors based on carbon nanotubes  48 49 7 7 1 \documentclass[12pt]{minimal}
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\begin{document}$$ \lambda _{{\text{D}}}  = {\left( {\frac{1} {{4\pi l_{{\text{B}}} {\sum\limits_i {\rho _{i} z_{i} ^{2} } }}}} \right)}^{{1 \mathord{\left/  {\vphantom {1 2}} \right.  \kern-\nulldelimiterspace} 2}} . $$\end{document} Fig. 7 Electrical double layer at the sensor surface  l B 2 \documentclass[12pt]{minimal}
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\begin{document}$$ l_{{\text{B}}}  = \frac{{ke^{2} }} {{\varepsilon k_{{\text{B}}} T}} $$\end{document} k B k 1/4πɛ 0 ρ i z i The Debye screening length, therefore, is proportional to the inverse square root of the ion number density, i.e., to ionic strength. For pure water, the Debye screening length is about 1 μm, while it is only 0.3 nm in 1 M NaCl solutions. In 1 M NaCl solution, therefore, two ions or charged species separated by 1 nm do not interact with each other. If we consider the sensor surface, any interactions occurring outside the Debye screening length cannot be detected using FET sensors. The biggest disadvantage of FET sensors is that they may not work with blood samples or body fluids, because the ionic strength of such physiological solutions is about 150 mM, which yields a Debye screening length of about 1 nm. 50 51 8 52 Fig. 8 IgG 52  36 2 9 9 9 Fig. 9 a b c CDI 36  37 10 10 53 Fig. 10 a b IgE PBS 37  As discussed, aptamers may allow a wider range of analytes in FET sensors owing to their small size. SWNT-FET sensors with aptamers as recognition elements showed high sensitivity and selectivity, and could be readily regenerated. Highly sensitive carbon nanotube sensors combined with small, economic, highly selective and stable aptamers could provide cost-effective point-of-care testing devices. Concluding remarks This review has focused on the advantages of electrochemical and FET sensor types with aptamers as recognition elements. Through use of aptamers for recognition, no labeling is required for electrochemical sensors, and signal-on architecture that is only possible with aptamers has made possible the sensitivity improvements. Also, as pointed out already, sensors can be recycled because of the reversibility of aptamer configurations. Alternatively, bound proteins may be simply washed off, without damaging the aptamers. In immunosensing, by contrast, it is practically impossible to remove bound antigens from antibodies without damaging the antibodies, because both antibodies and antigens are proteins in nature. Aptamers have proved their superiority over antibodies in nano-FET sensors. When no signal can be measured with an antibody–antigen pair because of the large size of the antibody, the binding of small aptamers to various targets can occur inside the electrical double layer where the nanotube can "feel" the change. Owing to the rapid aging of society, there is great demand for paradigm-breaking biosensors to detect the onset of disease with speed, convenience and accuracy. In a few more years, electrical nanosensors, with engineered aptamers, will become prominent in the market. They will be small in size, high in sensitivity and competitive in price.