Electrode fouling, resistance electrodes

In recent years, designs for UMEs and UME arrays continue to evolve. For example, works based on microfabricated diamond UMEs and arrays are increasingly common, motivated by this material s attractive properties as an electrode that include mechanical stability, chemical inertness, low background currents, wide potential window, and resistance to electrode fouling. Individual electrodes fabricated with focused ion beam (E1B) ° and arrays fabricated with thin-film technol-ogy2i,22 gj.g demonstrated. 

There are two types of conductometric procedures commonly used. Firstly, a Wheatstone Bridge circuit can be set up, whereby the ratio of the resistance of unknown seawater to standard seawater balances the ratio of a fixed resistor to a variable resistor. The system uses alternating current to minimise electrode fouling. Alternatively, the conductivity can be measured by magnetic induction, in which case the sensor consists of a plastic tube containing sample seawater that links two transformers. An oscillator establishes a current in one transformer that induces current flow within the tube, the magnitude of which depends upon the salinity of the sample. This in turn induces a current in the second transformer, which can then be measured. This design has been exploited for in situ conductivity measurements. 

In an extension of this work biotin-labeled liposomes were also modified with horseradish peroxidase (HRP) via periodate oxidation chemistry [74]. The HRP loaded biotin-labeled liposomes catalyzed oxidation of 4-chloro-l-naphthol in the presence of H2O2 yielding an insoluble product which precipitated onto and fouled the electrode. FIS was used to monitor resistance of electron transfer to the [Fe(CN 6] redox probe resulting a similar detection limit of 650 fM for a DNA sequence relevant to Tay-Sachs disorder. The authors also extended these various approaches to probe and amplify the signal from single-base mismatches in anal3d e DNA [75]. 

There are several challenges associated with the synthesis of BDD suitable for electrochemistry. Since diamond is a semiconductor with exceptional properties, precise control of dopant impurities and extended defects is required to dope the diamond lattice with sufficient boron to make the material conduct. However, as the boron levels increase, it can be harder to maintain crystallinity and control the amount of nondiamond carbon (NDC) both within crystal defects and at grain boundaries. While NDC can increase material conductivity, it is be detrimental to a diamond electrochemist, as the widely recognized electrochemical properties of BDD (wide solvent window, low background currents, reduced susceptibility to electrode fouling, corrosion resistance) are impaired and the electrochemical response becomes more akin to glassy carbon. If the presence of NDC is unaccounted for, electrical resistivity measurements will mislead the user into believing that there is more boron than actually present in the matrix. 

Electrode surfaces are modified in a quest to render an electrochemical function either not possible or difficult to achieve using conventional electrodes. Targeted improvements include increased selectivity, sensitivity, chemical and electrochemical stability, as well as a larger usable potential window and improved resistance to fouling. Furthermore, electrodes with tailored surfaces enhance fundamental studies of interfacial processes. Therefore, the need for improved electrode performance and logically designed interfaces is rapidly growing in many areas of science. 

In general, the electrochemical performance of carbon materials is basically determined by the electronic properties, and given its interfacial character, by the surface structure and surface chemistry (i.e. surface terminal functional groups or adsorption processes) [1,2]. Such features will affect the electrode kinetics, potential limits, background currents and the interaction with molecules in solution [2]. From the point of view of electroanalysis, the remarkable benefits of CNT-modified electrodes have been widely praised, including low detection limits, increased sensitivity, decreased overpotentials and resistance to surface fouling [5, 9, 11, 17]. 

The organic conductor properties of tetrathiaflulvalenetetracyanoquino-dimethane (TTF-TCNQ) as a material for constructing electrodes, viz. its catalytic response and resistance to passivation, are of special interest for the determination of biological compounds, which usually have slow electrode kinetics and a low sensitivity, and tend to foul electrode surfaces. The response of a TTF-TCNQ microarray sensor inserted in a flow system for... 

Why now is diamond so attractive to be used as material for electrodes Firstly, it is a very stable material under both mechanic and chemical aspects, so it can even be employed in highly aggressive media. Secondly, it features favorable electrochemical properties like a very wide potential window (see below) and a low background current. Furthermore, it is resistant to the so-called fouling and does not form oxides passivating its surface. Hence it may be employed as a sensor or in electrosynthesis (Section 6.6.3). The low sp -content causes an inert behavior in many media here. For example, high-quality diamond electrodes are stable even in a melt of KCl/LiCl at 450 °C. 

Amperometric electrodes that are extremely thin (<1 pm diameter) are called ultramicroelectrodes and have a number of advantages over conventional electrodes. Being narrower than the diffusion rate thickness, mass transport is enhanced, the signal-to-noise ratio is improved and the measurements can be made in resistive matrices such as nonaqueous solvents. These have huge applications in medicine as they can lit inside a living cell. Carbon fibre electrodes are coated in insulating polymer and plated with a thin layer of metal at the exposed tip to prevent fouling of the carbon itself. These can then be used to measure analytes of interest in various cells and membranes of the human body. 

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