Aptasensors electrochemical sensors

There is a limited number of studies on aptasensors (aptamer sensors), particularly for the detection of small organic molecules. An electrochemical aptasensor was developed by Kim et al. " for the detection of tetracycline using an ssDNA aptamer that selectively binds to tetracycline as the recognition element. The aptamer was highly selective for tetracycline and distinguishes minor structural... 

An electrochemical sensor with a nonlabeled redox probe shakes off probes labeled on the apatmers but still requires that the probes indicate changes on the sensing interface. One commonly nsed probe, methylene bine (MB), belongs to the phenothiazine family and is an aromatic cationic dye showing optically and electrochemically active properties (Tuite and Norden, 1994). Usually, MB can bind with dsDNA or tRNA via intercalation, electrostatic absorption, or G-base binding, which has been nsed widely in DNA sensors and recently, in aptasensors (Tuite and Norden, 1994 Bang et ah, 2005). 

Key words Aptamer, Electrochemical sensor. Gold nanoparticles, Aptasensor, Signal-amplification,... 

It is not surprising that wide range of aptamer-based sensors (aptasensors) have been reported in the literature.118-120 Recently, electrochemical aptamer sensors gained considerable attention.121,122 Since, nucleic acids aptamers fold their structure upon binding to the target molecule, formation of the aptamer-target complex can be... 

Sandwich-type sensing platforms are also nsed widely in electrochemical aptasensors (Willner and Zayats, 2007), especially for common model molecnles such as a-thrombin (Mir et al., 2006 Polsky et al., 2006 Centi et al., 2007) and platelet-derived growth factor (PDGF) (Zhou et al., 2007), which possess two active aptamer-binding sites. This type of sensor usually... 

Obviously, the sensors above are fabricated relatively complicatedly, because most sensors need multiple steps of modification, label, or separation. These steps might display their advantages, such as high selectivity and sensitivity, but still not avoid the defaults that label usually faces. As a matter of fact, most electrochemical aptasensors have been designed for speed and simplicity, which exemplify the inherent advantages of electrochemistry. Hence, most electrochemical aptasensors reported are generally simpler. 

In a similar methodology, methylene blue (MB) has recently been used as electrochemical indicator for the preparation of an adenosine triphosphate (ATP) aptasensor [45]. Au-NPs claimed to make more MB interact with DNA on the sensing interface. The DPV peak current corresponding to the MB oxidation decreased with ATP concentration in the 1 x 10 M to 1 x 10 M range, with a detection limit of 0.1 nM. The sensor showed advantages regarding low-cost, rapidity, simple detection, and reusability. 

DNA probes, the first one linked with paramagnetic beads and the second one modified with Au-NPs via biotin-streptavidin affinity reactions. Zheng et al. [67] employed this detection methodology for the development of a specific electrochemical aptasensor for the detection of thrombin. The sensor was based on a sandwich format of magnetic nanoparticle/thrombin/Au-NP and signal amplification by forming network-like thiocyanuric acid/Au-NPs. A detection limit of 7.82 aM was achieved. 

Here we describe two types of label-free electrochemical aptasensors for small molecule detection based on gold nanoparticle (AuNP) amplification (13,14). These sensors are designed with either signal-off or signal-on modes, respectively (see Fig. 1). 

Aptamer-based biosensors, also called aptasensor have gain a wide interest in the last years due to the advantages of aptamers compared to antibodies. Similar to antibodies, a variety of immobilization methods is available to bind aptamers to the sensor element. Aptasensors can be coupled to an electrochemical, optical or mass-sensitive transducer [13]. One of the successful examples for aptasensor was the detection of thrombin which was widely investigated [14]. Xiao et al. [15] have made an interesting development a redox compound (methylene blue) was inserted into the thrombin aptamer. When the target bound to the aptamer, the induced conformation change inhibited the electron transfer from the methylene blue to the electrode. This change could be detected amperometrically. 

Multisensor chemical arrays and aptasensors are generally electrochemical-based sensor devices (eg, potentiometry, amperometry, voltammetry, and impedance spectroscopy). 

This chapter discussed the versatility and broad range of applicability of electrochemical based-sensors, namely sensor arrays/E-tongues and aptasensors, for biomedical and pharmaceutical applications. The potential use of these low-cost, fast, and green bioinspired analytical techniques for medical purposes has been demonstrated in standard solutions, complex biological matrices, and in real samples. 

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