Electrodes in Flow Systems

In Chap. 2, the concept of the diffusion layer was established. It is a thickness, within which a large fraction of diffusional changes take place, and at a distance of several times this thickness, practically no more diffusional changes are observed. This layer will here be given the symbol 8b (D for diffusion). In fluid dynamics, there is a similar layer, within which most of the velocity changes occur. This is the hydrodynamic layer 8. It turns out that for diffusive mass transfer, 8b is usually much smaller than 5/,. This is fortunate, because it justifies to some extent the linearised velocity profiles often assumed near walls, making analysis easier. These relations are very lucidly discussed in a classic paper by Vielstich [560]. 

Single electrodes in a flow where a steady state is attained act much like the DME or RDE, in that a sigmoid current/voltage curve is measured, from which information about the electrochemical reaction can possibly be gleaned. Heterogeneous rate constants can for example be measured if the flow is sufficiently fast. This was the aim of Bernstein et al. [86] and their 

Another application of double, generator/collector, electrodes is what is called diffusion layer titration. This can be used for a quantitative analysis of some species in solution. The technique was first suggested by Bruckenstein and Johnson [157], and has been followed up since then, with theory [33,458] and simulations [81,458,541,554] (naming just a selection of works). 

It may be added that generator/collector cells can be implemented without convection. Double microbands [65] and interdigitated bands [294,447, 500] have been considered for this purpose, for titrations [458, 541] and for studies in electrochemically generated chemiluminescence (ECL) [539], See also the review by Amatore [46] with more references therein. 

In flow systems that necessitate consideration of two-dimensional geometry, Flanagan and Marcoux s work is an early example [38]. They examined a variety of conditions, among them the importance of axial diffusion in a tube. They found that neglecting axial diffusion is justified for most flows except the slowest. This is because transport due to the flow dominates in the axial direction, and this holds for electrode lengths that are small compared with the tube radius. This is often called the Levich approximation. Levich [1] related the diffusion layer thickness, which is a function of distance along the electrode and flow velocity, to the tube radius. The condition can then be reduced to the condition 

Most of the flow systems used in these channel experiments attain steady state, and, as will be seen below, the x direction can take the place of time. There is a multitude of works on the simulation of the channel electrode system, dominated to a large extent by the Compton school but many others have simulated these systems, and this work continues to the present [65, 85, 97-141], of which [100-103, 115, 121, 122] were published in the last 10 years or so. 

Tube electrodes have inspired simulations since 1974 [38, 46, 87, 88, 107, 142-144], and the wall-jet or impinging jet electrode, following the theory works mentioned above, has been simulated by many [46, 91, 145-158]. 

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Male K, Luong J, Gibbs B and Konishi Y 1993 An improved FIA biosensor for the determination of aspartame Appl. Biochem. Biotechnol. 38 189-201 Martin G and Meyerhoff M 1986 Membrane-dialyzer injection loop for enhancing the selectivity of anion-responsive liquid-membrane electrode in flow systems Anal. Chim. Acta 186 71-80... 

G. B. Martin and M. E. Meyerhoff, Membrane-Dialyzer Injection Loop for Enhancing the Selectivity of Anion-Responsive Liquid-Membrane Electrodes in Flow Systems. Part I. A Sensing System for NOx and Nitrite. Anal. Chim. Acta, 186 (1986) 71. 

There are two general problems with the use of electrocatalytic modified electrodes in flow systems. In order for the method to have a wide linear dynamic range, the rate of electrolysis must be controlled by mass transport of the analyte from the bulk solution to the electrode surface. Second, the modified electrode must be stable during both experimental operations and storage. 

Enzyme electrode in flow system Closed-loop Packed bed... 

K.N. Thomsen and R.P. Baldwin, Amperometric detection of non-electroactive cations in flow systems at a cupric hexacyanoferrate electrode. Anal. Chem. 61, 2594—2598 (1989). 

Fig. 3. Diagrams of electrochemical cells used in flow systems for thin film deposition by EC-ALE. A) First small thin layer flow cell (modeled after electrochemical liquid chromatography detectors). A gasket defined the area where the deposition was performed, and solutions were pumped in and out though the top plate. Reproduced by permission from ref. [ 110]. B) H-cell design where the samples were suspended in the solutions, and solutions were filled and drained from below. Reproduced by permission from ref. [111]. C) Larger thin layer flow cell. This is very similar to that shown in 3A, except that the deposition area is larger and laminar flow is easier to develop because of the solution inlet and outlet designs. In addition, the opposite wall of the cell is a piece of ITO, used as the auxiliary electrode. It is transparent so the deposit can be monitored visually, and it provides an excellent current distribution. The reference electrode is incorporated right in the cell, as well. Adapted from ref. [113],...

Trojanowicz, M., Electrochemical Detectors in Automated Analytical Systems , in Modem Techniques in Electroanalysis, Vanysek P. (Ed.), Wiley-Interscience, New York, 1996, pp. 187-239. This chapter contains a fairly thorough discussion of the possible arrangements of electrodes within flow systems, and for a variety of applications. 

Analysers for clinical purposes have been designed [129-131] and even a bedside analyser for monitoring Na, K, Cs and jJ-D-glucose in patients blood [127] or a blood potassium analyser for use during open-heart surgery [109]. A computer-controlled interference correction has been proposed [44], in which the standards are mixed to match the electrode potential obtained in the test solution. A simple caUbration in flow systems [61] involves dilution of the standard solution and monitoring of the ISE potential as a function of the diluent volume and dilution time. 

A thin film of manganese oxides deposited over a glassy earbon electrode dramatically lowers the overpotential for oxidation of various hydrazines ad hydrogen peroxide, thereby facilitating their amperometric detection in flow systems. Sensors based on this principle are highly sensitive and provide... 

It is used in IC systems when the amperometric process confers selectivity to the determination of the analytes. The operative modes employed in the amperometric techniques for detection in flow systems include those at (1) constant potential, where the current is measured in continuous mode, (2) at pulsed potential with sampling of the current at dehned periods of time (pulsed amperometry, PAD), or (3) at pulsed potential with integration of the current at defined periods of time (integrated pulsed amperometry, IPAD). Amperometric techniques are successfully employed for the determination of carbohydrates, catecholamines, phenols, cyanide, iodide, amines, etc., even if, for optimal detection, it is often required to change the mobile-phase conditions. This is the case of the detection of biogenic amines separated by cation-exchange in acidic eluent and detected by IPAD at the Au electrode after the post-column addition of a pH modiher (NaOH) [262]. 

Levodopa (2-amino-3-(3,4-dihydroxyphenyl)-propanoic acid) (IV) is administered to Parkinson s sufferers to alleviate the reduced levels of dopamine commonly seen in such patients. Dopamine itself is not administered, as this compound cannot cross the blood-brain barrier unlike levodopa which is then converted in the body to dopamine [137]. Bergamini et al. [138] have utilised a gold screen-printed electrode as a sensor for monitoring levodopa both in stationary solution and in flow systems. This was achieved by using the oxidation of levodopa at +0.63 V in acetate buffer pH 3.0. A linear response was reported from 9.9 x 10-5 to 1.2x10 3M and an associated detection limit of 6.8 x 10-5M. The sensor was successfully used to detect the levodopa... 

Arrays can be divided into three groups depending on their design (Fig. 32.2). In the first design, each electrode is independently addressed (Fig. 32.2a). This type of array allows several practical applications. On one hand, these devices can be fabricated for obtaining the same information at different sites of a medium. For example, studies about the responses of neuronal cell cultures under different stimulus [85-87]. On the other hand, these devices can be designed to obtain different information at reduced spaces, i.e. multianalyte determination. Those arrays can be employed directly like detectors in flow systems [88] (where each electrode is maintained at a different potential) or they can be individually modified for specific analytes [89],... 

From the fact that the dehydrogenases depend on a soluble cofactor follows that the enzyme need not be immobilized in close proximity to the detection device. There are several examples in flow systems (FLA and LC) where the enzymatic reaction and NADH production occur in an immobilized enzyme reactor (18,66, 104-106,118,119). The NADH is then transported downstream to the electrode situated in an amperometric flow through cell. As we have reported, the enzyme... 

Not all redox titrations have a well-defined equivalence point, and amperometric titrations1, in which a potential corresponding normally to that necessary to attain the mass-transport-limited current is applied to the working (indicator) electrode, permit the calculation of the titration endpoint through measurements done far from the equivalence point. Titrations can be done in flow systems, and in this sense it is possible to alter the quantity of added titrant so as to obtain greater accuracy in the determination of the equivalence point2. 

The use of hydrodynamic electrodes in these experiments has been very important in that they increase sensitivity because of higher mass transport, ensure good reproducibility, and sometimes gives better resolution in solutions of mixtures. The use of cells such as the wall-jet in flow systems is particularly useful, as response is fast and it is easy to introduce them at any point in the flow system. 

The utilization of a DNA matrix to aid enzyme immobilization is a promising alternative for the development of new biosensors and its application in flow systems. A DNA-tyrosinase carbon paste electrode [95] showed excellent performance for detection of cathecol in a FIA system and suggests that other enzymatic biosensors can benefit from the presence of DNA. 

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