Label-free detection methods electrochemical techniques

So far, there are two types of label-free DNA molecule detection techniques widely studied in laboratories. In the past decades, optical-based label-free detection of DNA molecules gains great development. The sensitivity of the technique has reached the order of pico-mole level. In contrast, although electrochemical method has obtained more and more attentions in the area of detection of DNA molecules because of the effectiveness and costless of the method, the sensitivity of DNA sensors with sihcon electrodes is in the order of p to nano-mole [4], which is low for the propose of detection of ultra-low concentration of DNA molecules, although sensitivity of electrochemistiy based sensors has been greatly improved with the incorporation of microelectronic techniques. 

Besides these potentiometric-based methods, a series of electrochemical techniques can be applied to the detection of biomolecular interactions. Depending on the desired dynamic detection range and the specific properties of the system under study, techniques such as electrochemical impedance spectroscopy, voltage step capacitance measurements, amperometry, differential pulse voltammetry, square wave voltammetry, AC voltammetry, and chronopotentiomet-ric stripping analysis can be used for label-free detection of DNA, proteins, and peptides [1]. Often these techniques require the use of redox mediators. Electrochemical impedance spectroscopy (EIS), in particular, is a very promising technique for DNA biosensing [2,3]. 

An alternative approach to the intrinsic DNA electrochemical activity utilizes electroactive species as redox indicators of the presence of immobilized DNA as well as its interaction events such as hybridization, damage, and association with another substance [14]. This mode was also used in a pioneering work on the DNA biosensor used for sequence detection [7]. In this case, it is still a label-free method in the sense that DNA probes or targets are not chemically modified by a special label however, as the indicator has to be added to a test S5 em as an additional reagent, we cannot speak more about the reagent-less technique. Redox indicators typically possess electrochemical responses at a "safe" electrode potential and often reversibly. The terms redox probe and redox marker are sometimes used in the literature to mean the redox indicator, which is confusable with the DNA capture probe used as a recognition element at hybridization and with markers used in medical diagnostics [8]. 

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. 

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