Disc microelectrodes

Galceran, J., Salvador, J., Puy, J., Cecilia, J. and Gavaghan, D. J. (1997). Analytical solution for the steady-state diffusion towards an inlaid disc microelectrode in a multi-layered medium, J. Electroanal. Chem., 440, 1-25. 

The solutions of the stationary diffusion equations for spherical and disc microelectrodes are deduced in Appendix C. 

In this section, microdisc electrodes will be discussed since the disc is the most important geometry for microelectrodes (see Sect. 2.7). Note that discs are not uniformly accessible electrodes so the mass flux is not the same at different points of the electrode surface. For non-reversible processes, the applied potential controls the rate constant but not the surface concentrations, since these are defined by the local balance of electron transfer rates and mass transport rates at each point of the surface. This local balance is characteristic of a particular electrode geometry and will evolve along the voltammetric response. For this reason, it is difficult (if not impossible) to find analytical rigorous expressions for the current analogous to that presented above for spherical electrodes. To deal with this complex situation, different numerical or semi-analytical approaches have been followed [19-25]. The expression most employed for analyzing stationary responses at disc microelectrodes was derived by Oldham [20], and takes the following form when equal diffusion coefficients are assumed ... 

Table 5.2 Kinetic parameters obtained from the fitting of experimental voltammetry at mercury hemispherical microelectrode and platinum disc microelectrode with the asymmetric MH model...

As is well known, the steady-state behavior of (spherical and disc) microelectrodes enables the generation of a unique current-potential relationship since the response is independent of the time or frequency variables [43]. This feature allows us to obtain identical I-E responses, independently of the electrochemical technique, when a voltammogram is generated by applying a linear sweep or a sequence of discrete potential steps, or a periodic potential. From the above, it can also be expected that the same behavior will be obtained under chronopotentiometric conditions when any current time function I(t) is applied, i.e., the steady-state I(t) —E curve (with E being the measured potential) will be identical to the voltammogram obtained under controlled potential-time conditions [44, 45]. 

A similar discussion can be made, although only in an approximate way, for disc microelectrodes by assuming as valid the analogy between the disc and sphere radius, i.e., by making the change rs = nr /A in the expressions of the diffusion and reaction layers (see Sect. 3.4.7). 

Fig. 7.27 Influence of the difference between the formal potentials (AEf1 values indicated in the graphs) on the DSCV voltammograms of an EE mechanism at disc microelectrodes of different...

The evolution of the peak current (/ dlsc,peak) with frequency (/) is plotted in Fig. 7.37 for the first-order catalytic mechanism with different homogeneous rate constants at microdisc electrodes. For a simple reversible charge transfer process, it is well known that the peak current in SWV scales linearly with the square root of the frequency at a planar electrode [6, 17]. For disc microelectrodes, analogous linear relationships between the peak current and the square root of frequency are found for a reversible electrode reaction (see Fig. 7.37 for the smallest kx value). 

Acetylcholineesterase and choline oxidase Co-immobilizing AChE and ChO on to a Pt disc microelectrode (200 tM diameter) with glutaraldehyde vapor. Sensor was used in flow-injection and LC system at a potential of 0.6 V versus Ag/AgCl/ KC1 (saturated) with mobile phase containing 0.1 M-phosphate buffer (pH 8) (for FIA) and 0.05 M phosphate buffer at pH 7.5 containing 0.03 mM SDS and 3mM tetramethyl ammonium chloride (for LC). The LC separations were carried out on an ODS-5 column (25 cm x 0.5 mm i.d.). The microsensor exhibited a linear response for acetylcholine and choline for 0.05-103 pmol. [87]... 

Finally, we consider the case of a plane disc microelectrode. In this... 

Nevertheless there is an important difference, as shown in Fig. 5.8. In these cases the current density is not uniform. However, it is easier to make disc microelectrodes of solid materials than hemispheres. In fact, the similarity of (5.36) and (5.38) indicates that, in terms of d, the theory for the more easily tractable hemisphere can be applied without significant error, at least in the steady state, to the disc equivalent—this is found to be the case. 

For the probing of these microenvironments, appropriately-sized electrochemical sensors and electrodes are necessary, often of nanometre linear dimension, unless they can be incorporated in the walls of the electrochemical cell. By etching, disc microelectrodes with radii of as small as 2 nm have been fabricated [15] nevertheless, problems of interelectrode reproducibility can be large at such miniature electrodes. 

The response of SECM is distance-dependent as shown in Fig. 16.8. At large distances from the substrate the current measured is that of the microelectrode tip. The tip is a disc microelectrode, so that far from a substrate in the steady state the diffusion-limited tip current, /T., will be measured. For a simple n-electron electrode reaction this is... 

The situation at a miniature disc microelectrode embedded in a flat insulator surface (such as an RDE of very small size) can be approximated by spherical symmetry, obtained for a small sphere situated at the center of a much larger (infinitely large, in the present context) spherical counter electrode. How will the change of geometry influence the diffusion-limited current density This is shown qualitatively in Fig. 18L. As time progresses, the diffusion layer thickness increases, causing, in the planar case, a proportional decrease in the diffusion current density. In the spherical configuration the electroactive... 

In an attempt to improve the selectivity of local dopamine measurements in the complex extracellular matrix of brain fluid, an implantable enzyme-based dopamine microbiosensor has been constructed based on the immobilization of tyrosinase in a thin-film chitosan coating of carbon-fiber disc microelectrodes [357]. o-Dopaquinone, which is the product of the tyrosinase reaction with dopamine, was monitored via its reduction at the modified microelectrode surface. The application of these cathodic tyrosinase dopamine microbiosensors was reported for the continuous real-time in vivo visualization of electrically stimulated dopamine release in the brain of anesthetized laboratory rats. Remarkably, due to the cathodic potential the sensor response was not significantly disturbed by the presence of typical interferences such as ascorbic and uric acid, serotonin, norepinephrine, and epinephrine. 

V. Rathod, B. Doloi, B. Bhattacharyya, Parametric investigation into the fabrication of disc microelectrodes by electrochemical micromachining, J. Micro Nano-Manuf., ASME 1 (2013) 041005-1-11, http //dx.doi.org/ 10.1115/1.4025977. 

Fig.6. Cyclic voltammograms of encapsulated carbon fiber disc microelectrode. Scan rate is 150 mV/s a) 8 fim diameter of the disk, b) approximately 1 //m diameter of the disk.

Patching schemes are helpful in situations where there are singularities in the concentration profiles in certain regions, such as the edge of disc microelectrodes (see Chapter 9) or the boundary of the diffusion domain in thin-film voltammetry, amalgamation processes and electrode arrays (see Chapter 10). In the latter case, the domain is confined to a distance d... 

A. Molina, C. Serna, Q. Li, E. Laborda, C. Batchelor-McAuley, and R. G. Compton. Analytical solutions for the study of multi-electron transfer processes by staircase, cyclic and differential voltammetries at disc microelectrodes, J. Phys. Chem. C 116, 11470-11479 (2012). 

Fig. 2. Influence of the radial diffusion on the current-time response under diffusion-limited conditions. (A) at the static spherical electrode of the radius ro = 0.5mm (B) at the disc microelectrode of the radius a = 5 m. Simulated currents in dimensionless scales ipi = 1/ (nFTrrocXD ) and I/(nF7racXD ), respectively 1, currents according to the Cottrell equation 2, currents corrected for the radial diffusion 3, steady state currents. (A) = 2000, (B)...

In the paper [74] oxidation of ferrocene and anthracene in acetonitrile and dichloromethane was successfully studied using the NPV technique at 5 pm Pt disc microelectrodes. The pulse widths were very short (5 to 20 ps) combined with the waiting times of duration 25 ps. Besides NPV also RPV has been applied. The resulting NP and RP waves for the oxidation of 9,10-anthraquinone are demonstrated in Fig. 28. The model of quasi-reversible charge transfer was fitted and parameters of both processes (k , and E1/2) were estimated. The results show that NP and RP voltammetric experiments retain the advantages over fast CV method even at pulse times as short as 5 ps. They provide effective discrimination against the double-layer charging current as well. 

Very promising is the use of microelectrodes in pulse voltammetry. The theory describing the reversible electrode reaction in RPV at a disc microelectrode was derived and verified in [77]. Due to the steady-state... 

A comparison of the chronoamperometric response at inlaid and recessed disc microelectrodes. J Electroanal Chem 249 1-14... 

Molina A, Olmos J, Laborda E (2015) Reverse pulse voltammetry at spherical and disc microelectrodes characterization of homogeneous chemical eqrulibria and their impact on the species diffusivities. Electrochim Acta 169 300-309... 

Bond AM, Oldham KB, Zoski CG (1988) Theory of electrochemical processes at an inlaid disc microelectrode under steady-state conditions. J Electroanal Chem 245 71-104... 

Matysik FM (1997) Voltammetric characterization of a dual-disc microelectrode in stationary solution. Electrochim Acta 42 3113-3116... 

Harvey SLR, Parker KH, O Htire D (2007) Theoretical eveduation of the collection efficiency at ring-disc microelectrodes. J Electroaned Chem 610 122-130... 

Harvey SLR, Coxon P, Bates D, Ptirker KH, O Htire D (2008) Metallic ring-disc microelectrode fabrication using inverted hollow cyfindrictd sputter coater. Sens Actuators B... 

A more common approach that also allows one to control a critical parameter, the separation between neighbouring microelectrodes, is through the use of photolithography which is based upon photoresists (light sensitive chemicals) and exposure tools such as UV sources [1, 16]. The fabrication process of gold disc microelectrode arrays is highlighted within Figure 6.2 [17]. 

The gold disc microelectrode arrays are fabricated using 4-in. silicon wafers and standard microfabrication techniques in a class 100 clean room. The first step in the fabrication is the growth of a thermal oxide layer across the wafer (Figure 6.2.2). This thermal oxide is grown in a furnace at 1100 °C within a water atmosphere, and the resulting oxide layer... 

Figure 6.2 Typical fabrication process of gold disc microelectrode arrays. Reproduced from Ref [17] with permission from Elsevier...

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