Redox activation, electrochemical mode

One should not see the potentiometric mode as an alternative but as a complement to the amperometric mode. The potentiometric mode offers the possibility of performing measurements on non-redox active species, e.g., not detectable amperometrically in aqueous solutions. Another advantage is the increased selectivity of the potentiometric response compared to the Faradaic response. The logarithmic response is also useful when the species of interest is in low concentrations. Since the tip is passive, only substrate generation-tip detection experiments are possible. However, the tip does not alter the concentration profile of the chemically or electrochemically generated species. The potentiometric micropipette tips have very small dimensions, typically 1 /xm or less, and the shielding is minimal. 

Investigated systems include a broad variety of dissolved organometallic species, electrochemically active organic molecules and redox active polymers like polyani-line (for a review, see [142]). Both dissolved species and species attached by adsorption, covalent bonding or film-forming deposition have been studied. Dissolved poly aniline dispersions as prepared by chemical oxidation [145] show various transitions in the NIR, as depicted in a set of NIR spectra in Fig. 5.40. The bands around X = 1490 nm and 1950 nm are overtones of the N-H stretch mode of an aromatic amine, whereas the band around X = 2300 nm is caused by the oligomer itself, which presumably indicates the presence of mobile charge carriers. 

The ion transfer that occurs on electrochemical switching of redox-active polymers has been studied by a variant of the GC mode, tip/substrate cyclic voltammetry (T/S CV). In this experiment, ij is monitored as E is cycled to switch the polymer film. Er is poised at a constant value to detect particular ions near the polymer film-solution interface. Ejection of Os(bpy)3 from NaFion-coated elec-... 

One particular SECM experiment is the approach curve in feedback mode. In this experiment, a redox active salt, the mediator, is introduced into the electrolyte. A single potentiostat polarizes the tip to cause an electrochemical reaction however, the sample itself is not polarized. The resulting current is recorded as the tip is moved closer towards the sample. When the tip is positioned appropriately close to the sample, a local response is seen. If the specific location on the sample is conductive, the resulting nernstian response observed at the surface sample causes the current to increase when compared to the bulk current (i.e., when the tip is far from the substrate). This is called positive feedback . If the specific location on the sample is insulating, then mass transport to the electrode of the tip is hindered, and the current decreases when compared to the bulk current. This is called negative feedback . A range of intermediate types of behavior may also occur with different samples. The c uantitative analysis of such approach curves allows for a very accurate analysis of local surface kinetics to be carried out. 

Nakano, K., Nakamura, K., Iwamoto, K., Soh, N., Imato, T. Positive-feedback-mode scanning electrochemical microscopy imaging of redox-active DNA-poly(l,4-benzoquinone) conjugate film deposited on carbon fiber electrode for micrometer-sized hybridization biosensor applications. J Electroanal Chem 2009, 628, 113-118. 

In the transient RC mode, both the UME tip and the enzyme-modified surface sample compete for the same analyte present in the microenvironment between the tip and the sample. Here, both tip and sample are held at the same potential, enabling both of them to electrochemically convert the analyte. When tip and sample are not along the same vertical axis, and are sufficiently separated laterally, the currents at the tip and sample are determined by the bulk analyte concentration and kinetics at the respective electrodes. However, as the tip moves closer to the sample and/or when it is directly above the sample, both electrodes (tip and sample) compete for the limited quantity of the analyte present between them. This results in a reduced tip current, as the analyte concentration available at the tip is reduced by its reaction at the sample. The decrease in the tip current is imaged as enzymatic redox activity in RC mode of SECM [63]. RC mode does not impose any limitation on the quality of the SECM activity image. Therefore, there are no restrictions to both the sample size and the minimum size of the UME tip. 

Other coordination modes of trans-diammac have been identified where one (154) or both (155) primary amines are free from the metal.721 725 An extension of this concept involves attachment of active functional groups such as crown ethers selectively at one primary amine to generate ditopic ligands capable of electrochemically sensing alkali metal ions through their inductive effect on the Co11111 redox potential. One example is provided by (156) further, the 15-crown-5 and 18-crown-6 analogs were also prepared.726... 

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]. 

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