Electrochemical-detection

A large number of DTDAFs ( electron-rich olefins ) described above are very efficient donors, e.g., for their application in organic conductors however they are highly sensitive to air. Studies aimed at the preparation of such compounds, especially the aliphatic ones, have so far met with only limited success. For example, a few alkyl-substituted DTDAF derivatives could be detected electrochemically, but an attempt to isolate one of these only led to oxidation products (91JA985). Similarly, an elec-... 

In addition to enzyme activity, the concentration of an nonelectroactive substrate can be determined electrochemically by this technique. By keeping the substrate (analyte) the limiting reagent, the amount of product produced is directly related to the initial concentration of substrate. Either kinetic or equilibrium measurements can be used. Typically an enzyme which produces NADH is used because NADH is readily detected electrochemically. Lactate has been detected using lactate dehydrogenase, and ethanol and methanol detected using alcohol dehydrogenase... 

A homogeneous electrochemical enzyme immunoassay for 2,4-dinitrophenol-aminocaproic acid (DNP-ACA), has been developed based on antibody inhibition of enzyme conversion from the apo- to the holo- form Apoglucose oxidase was used as the enzyme label. This enzyme is inactive until binding of flavin adenine dinucleotide (FAD) to form the holoenzyme which is active. Hydrogen peroxide is the enzymatic product which is detected electrochemically. Because antibody bound apoenzyme cannot bind FAD, the production of HjOj is a measure of the concentration of free DNP-ACA in the sample. 

The high specificity required for the analysis of physiological fluids often necessitates the incorporation of permselective membranes between the sample and the sensor. A typical configuration is presented in Fig. 7, where the membrane system comprises three distinct layers. The outer membrane. A, which encounters the sample solution is indicated by the dashed lines. It most commonly serves to eliminate high molecular weight interferences, such as other enzymes and proteins. The substrate, S, and other small molecules are allowed to enter the enzyme layer, B, which typically consist of a gelatinous material or a porous solid support. The immobilized enzyme catalyzes the conversion of substrate, S, to product, P. The substrate, product or a cofactor may be the species detected electrochemically. In many cases the electrochemical sensor may be prone to interferences and a permselective membrane, C, is required. The response time and sensitivity of the enzyme electrode will depend on the rate of permeation through layers A, B and C the kinetics of enzymatic conversion as well as the charac-... 

Complement induced immune lysis of cells and liposomes to release markers which are then detected electrochemically has been used to detect antibodies and antigens in a homogeneous format at nanomolar levels 252-256) qqjJj amperometric and potentio-metric electrodes have been employed. Unfortunately, major improvements in sensitivity appear unlikely, and instability of liposomes makes development of stable reagents for commercial systems difficult. 

The deposition of metals has also been studied by a large number of electrochemical techniques. For the deposition of Cu2+, for example, it is reasonable to ask whether both electrons are transported essentially simultaneously or whether an intermediate such as Cu+ is formed in solution. Such questions, like those of the ECE problem discussed above, have usually been investigated by forced convection techniques, since the rate of flow of reactant to and away from the electrode surface gives us an important additional kinetic handle. In addition, by using a second separate electrode placed downstream from the main working electrode, reasonably long-lived intermediates can be transported by the convection flow of the electrolyte to this second electrode and detected electrochemically. 

Physicochemical methods The direct spectroscopic detection of intermediates has proved immensely difficult, especially in the infrared, owing to interference by the solvent, but increasingly powerful tools are being developed. These direct techniques undoubtedly offer the most convincing proof of a model mechanism, and they also indicate whether films on electrode surfaces are forming that may not be detectable electrochemically. A detailed description of these techniques is given in chapter 2. 

There are many possible reaction pathways between acrylonitrile and adiponitrile and, in each, there are several possible rate-determining steps. None of the reaction intermediates has yet been detected electrochemically or spectroscopically thus indicating very fast chemical processes with intermediates of half-lives of < 10-5 s. Bard and Feiming Zhou [104a] have recently detected the CH2 = CHCNT radical by Scanning Electrochemical Microscopy (SCEM) using a 2.5 fim radius Au electrode (1.5 mol CH2 = CHCN in MeCN/TBAPF6). The dimerization rate has been determined to 6.107 M-1 S l. 

That the Mn402 is capable of adopting at least three isolable oxidation levels thus lends support, albeit circumstantial, to the possibility that it is to be found as the redox component in the WOC, if only at the lower S states. In support of this, the various oxidation levels of the synthetic materials have been found to be electrochemically interconvertible, and a [Mn402] level has been detected electrochemically (31). 

Immobilized Enzymes. The immobilized enzyme electrode is the most common immobilized biopolymer sensor, consisting of a thin layer of enzyme immobilized on the surface of an electrochemical sensor as shown in Figure 6. The enzyme catalyzes a reaction that converts the target substrate into a product that is detected electrochemically. The advantages of immobilized enzyme electrodes include minimal pretreatment of the sample matrix, small sample volume, and the recovery of the enzyme for repeated use (49). Several reviews and books have been published on immobilized enzyme electrodes (50—52). 

The detection of the individual C vitamers is complicated by their distinctly different properties. Although AA and DHAA are both ultraviolet (UV) absorbers, the absorbance maximum of DHAA is between 210 and 230 nm (15,18,42,43). For practical detection purposes, this makes DHAA particularly susceptible to interferences from a number of naturally occurring food constituents and limits the choice of reagents and solvents. In contrast, AA exhibits a pH-dependent absorbance maximum of 245-265 nm, which makes UV absorbance an ideal choice for detection. On the strength of its reducing capacity, AA can be detected electrochemically, but DHAA is electrochemically inactive. Neither AA nor DHAA fluoresce naturally. However, DHAA readily forms a fluorescent quinoxaline derivative upon reaction with o-phenylenediamine. As a result, chemical derivatization is often used to achieve the sensitivity needed to detect the naturally occurring vitamin C in food. 

As an alternative to simple AChE electrodes, a bienzyme system containing AChE and tyrosinase which utilised phenyl acetate as a substrate has been developed [42], The AChE hydrolyses the phenyl acetate to phenol which the tyrosinase enzyme oxidises to p-quinone which can in turn be detected electrochemically. The bienzyme system... 

The pesticides are detected electrochemically by measuring the degree of inhibition of cholinesterase on the screen-printed electrodes. The degree of inhibition can be thought of as the ratio of the response of electrodes (to substrate) exposed to pesticide (standard or wool extract) to that of electrodes not exposed to pesticide or exposed to a blank (extract from wool which has not been exposed to pesticide). An efficient and convenient way of doing this is to expose the SPCEs to standards and wool extracts prior to measurement of inhibition in the electrochemical cell. 

It was also found that the hybridization kinetics were found to be faster in a moving sample, as compared to a stationary sample [939]. In another report, active acoustic mixing was used to achieve a five-fold faster DNA hybridization rate. Hybridization was detected electrochemically (by AC voltammetry) based on the ferrocene redox chemistry [62],... 

Other than protein, histamine in human blood samples was determined by the immunoassay method. It was detected by its binding with anti-histamine IgG, which was coupled to ferrocene (Fc-IgG). Separation of histamine and its complex with antibody was based on p/ differences, and it was achieved in a PMMA chip consisting of a multichannel matrix column coated with a cation-exchange resin (Nation). Histamine was detected electrochemically a decrease in the current due to Fc-IgG occurred after its binding to histamine [1012],... 

Rgure 2.6 Chromatogram showing the presence of vitamin C in trout feed. Chromatographic conditions column, Nucleosil C flow rate, 1.0 ml/min detection, electrochemical set at 0.75 V, 200 nA full scale. (Reprinted from Ref. 30 with permission.)... 

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