Rotating disk electrode detection

Despite the importance of the ORR and long history of study, very little is known about the reaction mechanism.126,130,131 Mechanistic information has been derived almost exclusively from rotating disk electrode (RDE)131,132 and rotating ring disk electrode (RRDE)133-136,62,128 studies. The rotating electrode minimizes mass transfer effects and allows a kinetic current density to be extracted. In the RRDE setup, the ring surrounding the disk electrode detects species weakly adsorbed to the electrode that are ejected due to electrode rotation. The ORR reaction (eqn 4) is... 

Two types of cell have been described. In Fig. 1 a cell with a rotating disk electrode is shown. Connections to a pH-stat and to the interface are indicated simultaneously with recording CMT measurements, the metal and reference electrode and a counter electrode (not shown in Fig. 1) can be connected to a potentiostat, so that electrochemical measurements can be recorded intermittently. The volume of solution in the cell is ca. 400 ml. What matters for safe and reliable conditions of measurement is that the disk electrode rotating at a speed of no less than 1000 rpm ensures efficient stirring, so that the effect of alkali formed at the corroding metal (or sometimes at the counter electrode located ca. 1 cm below and parallel to the metal electrode) is immediately sensed effectively by the glass electrode and also the effect of acid supplied from the autoburet is quickly detected. 

Since products of the electrode process are quickly transported out of the vicinity of the electrode disk, use of the rotating disk electrode complements the more complex rotating ring disk electrode (RRDE) [32]. Here, redox active products can be detected at the ring electrode, which is held at a separately controlled potential. 

Although new spectroscopic methods have reduced the need for some functions of the rotating disk electrode, there are a number of situations in which it remains the most effective technique. The detection of H202 as an intermediate in the reduction of 02, described above, is one example. Here are a few others. 

C.A. Wijayawardhana, H.B. Halsall and W.R. Heineman, Microvolume rotating disk electrode (RDE) amperometric detection for a bead-based immunoassay, Anal. Chim. Acta, 399 (1999) 3-11. 

Adsorbed Mn6g and Mn6i were employed for the detection of oxygen [94], Rotating disk electrode voltammetry revealed two electron reduction in acidic and slightly... 

Electrochemical sensors based on amperometric detection are popular small instruments that can be used to directly probe samples. The electrodes in these sensors do not always have to be made of metal, e.g. platinum or mercury, and do not always have to be bare. A commonly employed working electrode is the rotating disk electrode, which is preferred over the DME for easily reduced species and anodic reactions. It works by convection mixing of the solution so that fresh sample is constantly passed over the surface. 

The advantages of controlled mass transport have long been recognized in electrochemistry, as discussed in Chapter 2.4. In the present context, the elegant combination of the rotating disk electrode (RDE) [48-50] and wall-jet electrode (WJE) [51, 52] solution hydrodynamics with EQCM interfacial detection has proved to be very effective. 

Note HRP Horseradish peroxidase TMB Tetramethylbenzidine ACV Alternating-current voltammetry amole le-18 moles DPV DifTerential pulse voltammetry E Detection limit for unconjugated enzyme E Detection limit for conjugated (Ab/Ag) enzyme FIA-EC Flow-injection analysis with electrochemical detection CC Classy carbon LC-EC Liquid chromatography with electrochemical detection P Detection limit for final enzymatic product P Detection limit for standards of P RDE Rotating disk electrode SPE Screen-printed electrode SQV Square-wave voltammetry zmole le-21 moles. 

While the voltammograms can be quantitatively analyzed to yield permeant transport parameters - for example, via numerical simulations - we have instead utilized rotating disk electrode (RDE) voltammetry for this purpose. Sequential solution and film transport problems are generally well described by a variant of the Koutecky-Levich analysis [11]. This analysis represents the series process Idnetically as a reciprocal sum of pure solution-phase convection/dilFusion (cd) and pure film permeation (p) rates or fluxes. Since detection is electrochemical, the fluxes can be written in terms of currents, where ilim is the observed overall, mass-transport limited, current [12] ... 

Source Adapted from C. A. Wijayawardhana, H. B. Halsall, W. R. Heineman, in Encyclopedia of Electrochemislry (Vol. 8) (Eds. A. J. Bard, P. Stratmann), Wiley-VCH, New Yoric, submitted. ALP alkaline phosphatase C competitive assay E enzime FIA-EC flow-injection analysis with electrochemical detection GC glassy carbc He heterogeneous Ho homc eneous LC-EC liquid chromatography with electrochemical detection N non-enzyme PAPP 4-amin< henyl phosphate PP j enyl j osjdiate RDE rotating disk electrode S sandwich assay SECM scanning electrochemical microscopy. 

Five types of amperometric detection have been applied for enzyme-based ECIA. They are flow-injection analysis with electrochemical detection (FIAEC), liquid chromatography with electrochemical detection (LCEC), amperometric detection with interdigitated array electrodes (IDA), rotating disk electrode (RDE) amper-ometry, and scanning electrochemical microscopy (SECM). Of these, the two conventional types, FIAEC and LCEC, shall be discussed in this section, leaving the discussion of the other types to Section V on miniaturized immunoassays. 

Figure 14 Schematic of the experimental setup for the rotating disk electrode (RDE) detection in a bead-based immunoassay using the ALP-PAPP enzyme-substrate pair.

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