Redox markers

M. Hromadova, M. Salmain, N. Fischer-Durand, L. Pospisil, and G. Jaouen, Electrochemical microbead-based immunoassay using an (h5-cyclopentadienyl)tricarbonylmanganese redox marker bound to bovine serum albumin. Langmuir 22, 506-511 (2006). 

In principle, electrochemical transducers can be used to detect the formation of a surface-bound affinity complex when the affinity-binding reaction is associated with a change in electrical properties (e.g., ion permeability or capacitance) of the layer immobilized onto the electrode surface. For example, the so-called ion-chemnel sensors detect permeabilily changes of a film immobilized on an electrode surface to an electroactive molecule, which is used as a redox marker. The formation of a surface-bound affinity complex results in a permeability change, which can be monitored by the change of cyclic voltammetric response of the redox marker. 

A Two-step In Situ STM Electronic Transition via a Single DNA-based Redox Marker... 

A farther electrochemical sensor platform development uses methylene bine connected to one end of the aptamer, which is fixed on the electrode surface by the other end as a redox marker for the detection of aptamer-binding reaction (Baker et al., 2006). The measuring principle is comparable to that of aptamer beacons with optical detection In the absence of target, the aptamer is thought to remain partially nnfolded with a greater distance between the redox marker and the electrode snrface in the presence of target, the aptamer presumably folds... 

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]. 

Table 4.1 summarizes data reported between 2007 and 2009 for DNA sensors where the detection strategy is related to the direct detection of redox markers. 

Table 4.1. Gold nanoparticle-based electrochemical DNA biosensors using direct detection of redox markers...

Many standard electrochemical techniques can be used, depending on the biological system to be studied. In the presence of redox markers in solution, modification of the electrode resulting from biomolecular interaction affects the impedance of the system, which can be measured by using electrochemical impedance spectroscopy (ElS). EIS is a very promising technique, in particular for the detection of DNA hybridization. 

In EIS, the impedance of the system is measured by applying a small ac signal and by the frequency scanned (typically between 10-100 kHz and 1 Hz or less). Stable impedance spectra can be obtained with electrically charged redox markers in solution. The data can be fitted with an equivalent electrical circuit, where the most important components are the charge transfer resistance Ret and the double layer/biolayer capacitance Cdi. 

The redox behavior of nucleic acids, especially at mercury and carbon electrodes, has been studied for decades, and a variety of quantitative methods exist [117-120, and references therein]. Recently, under-ivatized nucleic acids in solution have been selectively quantitated by ac voltammetry at copper electrodes [121], while DNA bound to gold electrode surfaces via gold-thiol chemisorption has been quantitated by chronocoulometry using charge-compensating redox markers [122]. Derivative square wave voltammetry has been used to examine chemically induced DNA damage following reaction with styrene oxide [123]. 

The redox marker associates with DNA strictly through electrostatic interactions and the intercalation of the redox marker into the hydrophobic region of DNA should be avoided. 

The amount of electrostatically trapped redox marker can be determined precisely, i.e, all redox ions should be electrochemically contacted. 

The charge compensation for the DNA phosphate groups is complete and provided by the redox marker only. 

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