Disk ultramicroelectrodes

Feldman BJ, Oserioh JD, Hata BH, D Alessandro A. Determination of lead in blood by square wave anodic stripping voltammetry at a carbon disk ultramicroelectrode. Anal Chem 1994 66 1983-7. 

Scanning electrochemical microscopy (SECM) is a scanning probe technique in which the probe is a disk ultramicroelectrode (typical radius in the 5 to 25 pm range) that scans the surface of the investigated sample facing it [157]. SECM measurements are carried out in... 

The carbon fiber disk ultramicroelectrodes were prepared from pointed fibers which were encapsulated in poly(oxyphenylene) by applying the same procedure as described above. 

Figure 3. Cyclic voltammetry of hexamethylbenzene, 10 mM, in acetonitrile, 0.6 M NBU4BF4, at a 5 m radius gold disk ultramicroelectrode, at the scan rates given in kV.s on each set of on-line ohmic drop compensated or uncompensated voltanunograms. 22 C. ...

The difficulty in determining an electron consumption stems from the fact that only nxD (steady state current plateau at a disk ultramicroelectrode of radius Vq) or nxD (transient chronoamperometry) values can be determined from voltanunetric experiments. However, whenever one is able to measure both parameters within the same time scale (viz, for chron. stcad st. oth n and D values ensue. The above results for pyrylium... 

Mahe (2007) Electrochemistry in confined microsystems. Part 1. CV-SECM of reversible system at a confined disk ultramicroelectrode. Electrochim Acta 52 5018—5029... 

Differential-pulse polarography was reported to evaluate the content of THBQ in edible oils (Tonmanee and Archer, 1982 Cortes et al., 1993). Differential-pulse voltammetry using a carbon paste electrode modified with nickel phthalocyanine was applied to determine BHA in potato flakes (Ruiz et al., 1995). Square-wave voltam-metric methods employing carbon-disk ultramicroelectrodes (UME) (Ceballos and Fernandez, 2000), glassy carbon electrodes (Raymundo et al., 2007), platinum electrodes... 

Electrode processes are often studied under steady-state conditions, for example at a rotating disk electrode or at a ultramicroelectrode. Polarog-raphy with dropping electrode where average currents during the droptime are often measured shows similar features as steady-state methods. The distribution of the concentrations of the oxidized and reduced forms at the surface of the electrode under steady-state conditions is shown in Fig. 5.12. For the current density we have (cf. Eq. (2.7.13))... 

The UMEs used in bioarrays can be divided into three types disk, ring, and strip electrodes. The theory of the disk, ring, and strip UMEs has been extensively studied [97-100], Due to the edge effect, the profile of the mass diffusion to the ultramicroelectrode surface is three dimensional, and can significantly enhance the mass transportation in comparison to the conventional large electrode with one-dimensional mass transportation. The steady-state measurement at a planar UME can be expressed as... 

Diffusion of electroactive species to the surface of conventional disk (macro-) electrodes is mainly planar. When the electrode diameter is decreased the edge effects of hemi-spherical diffusion become significant. In 1964 Lingane derived the corrective term bearing in mind the edge effects for the Cotrell equation [129, 130], confirmed later on analytically and by numerical calculation [131,132], In the case of ultramicroelectrodes this term becomes dominant, which makes steady-state current proportional to the electrode radius [133-135], Since capacitive and other diffusion-unrelated currents are proportional to the square of electrode radius, the signal-to-noise ratio is increased as the electrode radius is decreased. 

This can certainly be done, e.g., by using a rotating disk electrode, or by using ultramicroelectrodes (Section 7) one can increase iL several hundred times compared with its value at a flat plate in a still solution. However, every technique comes with a price tag, which may not only be a dollar one. Thus, if one uses ultramicroelectrodes to give a high iL, one has to recall that the currents there are tiny (picoamperes) and... 

In the case of the ultramicroelectrodes such as the disk electrode, it is necessary to integrate over the surface, and sometimes there will be unequally spaced points along the surface, as for example, in direct discretisation on an unequal grid in the example program UME DIRECT. As mentioned in Chap. 12, it is found that due to the errors in the computed concentration values, the local fluxes are so inaccurate that any integration method better than the simple trapezium method is not justified. The routine U TRAP is thus recommended here. It integrates local current densities, precalculated by using the above routine U DERIV. 

Ultramicroelectrode (UME) — an - electrode having a characteristic dimension less than 25 pm. This characteristic dimension refers to a diameter for a disk, a sphere, a hemisphere, and a cylinder, and a width for a band UME, for example. Conceptually, the lower limit of the characteristic dimension of a UME is about 10 nm, which corresponds to the thickness of the - double layer or the size of molecules. Electrodes of such dimensions are sometimes referred to as - nanoelectrodes or nan-odes. Electrodes with a characteristic dimension ranging from 25 pm up to approximately 1 mm are referred to as -> microelectrodes. Electrodes with dimensions of millimeters, centimeters, or meters have been referred to... 

These methods constitute the frame on which any particular method can be elaborated. Yet in practice, the experimental difficulty is that with standard apparatus, 5 /D cannot be varied over an extremely wide range. For example, with the rotating disk electrode (RDE), which is the most convenient steady-state method (with the exception of ultramicroelectrodes [109]), 8 depends on the rotation frequency w of the electrode (see Chapter 2). Yet to maintain correct hydrodynamic conditions w cannot be varied, with... 

The definition of ultramicroelectrodes is somewhat arbitrary and with reference to the common disk geometry is currently meant to describe electrodes with diameters of about 10 pm or less [125,143,178]. The electrochemical response at ultramicroelectrodes can be classified into two categories depending on the value of v. 

---