Thiol-labeled aptamers

In this part we will describe recent achievements in the development of biosensors based on DNA/RNA aptamers. These biosensors are usually prepared by immobilization of aptamer onto a solid support by various methods using chemisorption (aptamer is modified by thiol group) or by avidin-biotin technology (aptamer is modified by biotin) or by covalent attachment of amino group-labeled aptamer to a surface of self-assembly monolayer of 11-mercaptoundecanoic acid (11-MUA). Apart from the method of aptamer immobilization, the biosensors differ in the signal generation. To date, most extensively studied were the biosensors based on optical methods (fluorescence, SPR) and acoustic sensors based mostly on thickness shear mode (TSM) method. However, recently several investigators reported electrochemical sensors based on enzyme-labeled aptamers, electrochemical indicators and impedance spectroscopy methods of detection. 

A similar approach was conducted by using quantum dots (QDs) for signal amplification [51]. In this case the hybrid formed by a thiol-labeled oligonucleotide and the thrombin aptamer was immobilized onto a gold electrode. When binding to thrombin the aptamer adopts its G-quartet structure and only the single-stranded probe remained onto the electrode, which is now available for hybridization with a QD-labeled complementary oligonucleotide (Fig. 2.11). 

Another aptasensor based on the displacement of a complementary strand from an aptamer was developed for the detection of ATP by coupling this approach to signal amplification by Au-NPs [50]. In this work the hybrid was formed by a reporter DNA labeled with Au-NPs, a thiol-modified DNA anchored to an electrode and a target-responsive DNA (the aptamer) (Fig. 2.10). Moreover, [Ru(NH3)e] ... 

---