Optical aptasensors

Electronic aptasensors reveal certain advantages when compared to optical aptasensors (1) The possibility of coupling amplifying catalytic or biocatalytic labels to an aptamer-target complex enables amplified detection of the target substrate and thus enhancing the sensitivity of sensing processes and (2) recent advances demonstrated the electronic label-free detection of the substrate by the aptamers, and thus the elimination of fluorescent labels may be excluded. 

Figure 7.6 Experimental setup and construction of LSPR and interferometry-based label-free optical aptasensor with a porous anodic alumina layer chip.

Optical aptasensors include label-based aptamers (using fluorophore, luminophore, enzyme, nanoparticles) or label-free detection systems (e.g. surface plasmon resonance). Aptamers have also been widely used as biorecognition elements in... 

The first aptasensor reported was particularly based on optical detection [66]. The 58-mer RNA aptamer selective to L-adenosine was immobilized onto the core of multimode fiber using avidin-biotin method. The detection was based on competitive binding of FITC-labeled L-adenosine with unlabeled analyte. This sensor also allowed to study the kinetics of binding and determine equilibrium constants. 

The combination of LSPR with interferometry on a PAA layer chip enabled two important aspects of optical sensing systems a shift in and an increment in the absorbance intensity in a new and highly sensitive format. The excitation of optical characteristics and the detection were performed using an optical fiber, which made our aptasensor user friendly and suitable for the simple construction of a handheld diagnostic device. 

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). 

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. 

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