Guanine oxidation, label-free electrochemical

Fig. 6 Label-free electrochemical detection of DNA hybridization. The inosine (I) substituted probe shows no electrochemical signals, since inosine is not electroactive (a) After hybridization with the target DNA, the appearance of the guanine (G) oxidation signal (around + 1 V vs. Ag/AgCl) provides specific detection (b)...

Kerman et al, 2004 also described the label-free electrochemical detection of DNA based on the direct attachment of adenine probes to the sidewall and end of functionalized MWCNTs. The MWCNTs were attached onto the carbon paste electrode surface modified with thymine probes by hybridization between adenine and thymine. The combination of sidewall and end functionalization of MWNT showed enhancement of the guanine oxidation signal in the direct measurement compared to the ones from only end-modified MWCNT. 

The ability of CNTs to facilitate the adsorptive accumulation of the guanine nucleobase can lead to a dramatic amplification of label-free electrochemical detection procedures based on the intrinsic electroactivity of DNA [52]. The coupling of a CNT nanoelectrode array with the Ru(bpy)3+ -mediated guanine oxidation has facilitated the detection of subattomoles of DNA targets [53, 54], Such CNT array was also applied for the label-free detection of DNA PCR amplicons, and offered the detection of less than 1000 target amplicons. 

Figure 7.1 Label-free voltammetric detection of aptamer-protein interactions. (A) The electrochemical oxidation response from the guanine bases in the aptamer is observed at about 1 V (peak G) on the surface of the screen-printed electrodes. (The inset shows gold and carbon-based screen-printed electrodes with a three-electrode system.) (B) After the binding event with the target protein, an oxidation signal appears at about 0.6 V (peak P), and the oxidation signal of the aptamer decreases.

Label-free detection technique Procedure that utilizes electrochemical and/or surface activity of DNA (reduction and tensammet-ric responses of DNA at mercury and some amalgam electrodes, guanine oxidation at carbon electrodes, detection by using nonco-valent DNA redox indicators, etc.). The label-free technique uses no chemical modification of a DNA probe or target or another anal3fre interacting with NA. 

On the other hand, changes to the electrical properties of an interface, change in flexibility from ssDNA to the rigid dsDNA, or the electrochemical oxidation of guanine bases are used approaches for label-free methods for detection of DNA hybridization. There... 

Figure 10.2 Scheme of label-free hybridization commonly used in electrochemical DNA hybridization sensors, (a) Target DNA guanine bases on carbon electrode after the hybridization. (b) Representation of the electrocatalytic oxidation of guanine on ITO electrode by means of redox mediator, (c) Increase the electron transfer resistance by use an electroactive redox couple and a gold electrode surface. Reproduced from [25], with permission from American Chemical Society... 

DNA oxidation at carbon electrodes is associated with the irreversible oxidation of guanine and adenine [33], For example, the G oxidation signal observed at -i-1.0 V, without external labels, has been used to monitor telomerase activity by using a carbon graphite electrode (CGE) as an electrochemical transducer [5], Telomerase activity has been detected in cell extracts containing as low as 100 ng pl of protein. This label-free assay is practical in the quantitative determination of telomerase activity providing a cheap and simple detection protocol for the diagnosis of cancer that can also be extended to the analysis of food related to DNA. 

In this chapter, we have reported some examples related to DNA sensors, especially used in food applications, by employing electrochemical detection techniques. A variety of sensing systems based on label-free techniques utilizing electrochemical and/or surface activity as well as direct methods that rely on the intrinsic electrochemical properties of DNA (the oxidation of purine bases, particularly guanine) have been presented in the different sections of this chapter. 

Eskiocak, U., Ozkan-ariksoysal, D., Ozsoz, M. and Oktem, H.A. (2007) Label-free detection of telomerase activity using guanine electrochemical oxidation signal. Anal. Chem.,19 (22), 8807-8811. 

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