Characteristics of Microelectrode

In traditional electrochemistry, electrodes with typical size on the order of mm have been used. In comparison, electrodes of micrometer size will be called microelectrodes. Their characteristics include [162-167]  

independent detection current for long term electrolysis  

When an electrode functions as a probe for electrochemical analysis, local analysis is possible by reducing the size of the electrode. In other 

Electrode reaction is controlled by mass transport, that is, the diffusion to and from the electrode. The diickness of die diffusion layer is roughly given by where D is die diffusion coefficient and t is die 

acting electrode RE, reference electrode CE, counter electrode Rg, Rg, resistance of solution (gel) Rp charge transfer resistance and electrical double layer capacity 

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One of the main characteristics of microelectrodes that makes them so interesting is the unusual mass transport properties that they exhibit. To explain this behaviour, it is simplest to start by considering a simple reduction reaction such as... 

Pritchard et al. [131] describe the development of a single thiocholine enzyme-based biosensor. This biosensor is a sonochemically fabricated enzyme microelectrode array in order to impart stir-independent (convection) responses that are characteristic of microelectrodes. Microelectrode arrays with up to 2 x 10 microelectrode elements may be fabricated via the sonochemical ablation of noncondnc-tive polymer films [132,133], which coat and thereby insnlate nnderlying condnctive snrfaces [134]. Paraoxon is determined down to concentrations of 1 x 10 M via the nse of sonochemically fabricated acetylcholine/polyaniline microelectrode array-based sensors. These sensors were fabricated via the electropolymerization of thin... 

One of the characteristics of microelectrodes is an enhanced rate of mass transport of reactants and products to and from the electrode surface. Nonlinear diffusion prevails at microelectrodes two-dimensional diffusion at microcylinder or microband electrodes and three-dimensional diffusion at microspherical and microdisk electrodes. The enhanced mass transport at microelectrodes, which is often called the edge effect, causes increased current density and results in steady-state current responses at sufficiently slow potential sweep rates in potential sweep voltammetry, as will be illustrated in section 3. 

The practical characteristics of microelectrodes are discussed in Chapter 6 of this handbook hence, this section is restricted to a brief theoretical background followed by a discussion of key experiments where microelectrodes have made significant improvements. 

The experimental results confirm the characteristics of microelectrodes theoretically predicted In comparison with a conventional macroelectrode the array exhibits a decreased influence of stirring and an increased current density resulting in an excellent signal to noise ratio Furthermore, there is no peak-shaped cyclic voltammogram allowing measurements in quiescent as well as in stirred solutions... 

A significant characteristic of the ISE is the feasibility of fabricating miniature sensors for measurements in samples of very small volume or in vivo Microelectrodes... 

Liu, Y, Zou, X., and Dong, S., Electrochemical characteristics of facile prepared carbon nanotubes-ionic liquid gel modified microelectrode and application in biochemistry, Electrochem. Commun., 8, 1429-1434, 2006. 

This chapter focuses on the approach we followed for developing a novel electrochemical sensor platform based on disposable polymer microchips with integrated microelectrodes for signal transduction. It presents the development of the so-called Immuspeed technology, which is dedicated to quantitative immunoassays with reduced time-to-results as well as sample and reagent volumes. Prior to presenting the specific characteristics of Immuspeed, the basic principles integrated in this platform are first presented and illustrated with reference to... 

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 ... 

The effect of the electrode geometry and size is shown in Fig. 6.16, where the curves are plotted for co = 5 and different values of the characteristic dimensions of microelectrodes of different geometries (rs, rd, and w/2 for spheres, discs, and bands respectively with rs = rd = w/2). For large electrodes (Fig. 6.16a), the curves (i.e., the current density) show small differences because diffusion is almost planar and... 

Such electrodes have been used to examine insulin secretion from single pancreatic P cells. A stimulant is introduced to contact a single P cell adhering to the bottom of a petri dish. The microelectrode is brought into contact with the cell. The result (representing insulin secretion) is shown in Fig 14.45. The peaks shown are Ca2+ dependent, and this is characteristic of an exocytotic process (Section 14.10.1). The area under the peak represents 360,000 insulin molecules. The results show that the spikes correspond to the ejection of packets of insulin secreted in exocytosis. 

The slope of this straight line is 16.91 x n V-1 at 25 °C. However, it is more common to use the inverse function E = Ei/2 + 2.303 x (RT/nF) log [(fi, - I) /I], with the slope 0.059/nV. Both functions are called the logarithmic analysis of DC polarogram. They both cross the potential axis at the half-wave potential, which corresponds to I = Ii/2. The main characteristic of fast and reversible electrode reactions is that the half-wave potential is independent of the drop life-time in DC polarography, or the rotation rate of the rotating disk electrode, or the radius of microelectrode. If this condition is satisfied, the slope of the logarithmic analysis indicates the number of electrons in the electrode reaction. 

The applicability of the method was further ascertained by the analysis of real samples such as blood serum using an array-type micropattemed gold electrodes of rectangular and circular geometries, exhibiting voltammetric characteristics of bulk and microelectrodes... 

Standard microelectrode techniques were used to study the effects of isocorydine on potential characteristics of canine cardiac Purkinje fibers and ventricular myocardium in vitro. In the Purkinje fibers, the action potential durations APDjj and APD were prolonged at 3 pmol/1 but shortened at 30 pmol/1 by isocorydine. The action potential amplitude and maximal upstroke velocity were decreased at 100 pmol/1. In the ventricular myocardium, the action potential characteristics were changed by isocorydine at concentrations above 30 pmol/1. The APDJ0 was shortened, the APD90 was prolonged, and the maximal upstroke velocity was decreased at 30 pmol/1. The effective refractory period was prolonged by the alkaloid in Purkinje fibers and ventricular myocardium. These results indicated that the alkaloid may interfere with K+, Na+, and Ca+2 currents in myocardial cell membranes at different concentrations [287]. 

The polarization characteristic of a partially covered inert macroelectrode is easy to determine, but it is very difficult or even impossible to do the same for microelectrodes placed on it. On the other hand,30 the morphology of metal electrodeposits indicates the conditions of deposition. Hence, the type of process control on the microelectrodes can be derived from their morphology and correlated with the polarization curve for the partially covered macroelectrode. 

Selection of fertilizable oocytes is one of the most important issues in in vitro fertilization (IVF) process. To date, the oocyte selection has been manually conducted by a skillful expert with a labor-intensive and time-consuming process. Recently, a new method for DEP-based separation of normal oocytes has been demonstrated using a microelectrode device [45]. The normal oocytes showed higher DEP velocity compared to the abnormal ones, which were cultured without medium for 3 days. This result shows that the DEP characteristics of oocytes can be a new criterion for selecting healthy oocytes in IVF. However, the conventional separation method based on the microfabricated electrodes has some limitation such as difficulty of manipulating samples before and after the selection processes. To develop a fully-automated system for the discrimination of normal oocytes for IVF, an ECD-based optoelectrofiuidic platform, which allows the programmable cell manipulation based on the optically induced DEP and the image-driven virtual electrodes, has been utilized [26]. 

Application of the presumed transmitter substance to a neurone must produce the characteristic change in membrane permeability. The sine qua non of transmitter action is the production of the membrane changes characteristic of excitation or inhibition. The multibarrelled microelectrode allows active substances to be applied, by iontophoresis or microinjection, directly to the neurone while the membrane potential is simultaneously recorded. It was hoped that the... 

The important feature of this technique is that the working electrode is a microelectrode, which restricts the current to a few microamperes and allows limiting currents to be reached. Conditions are maintained so that the current is diffusion controlled i,e., the only way the electroactive test substance (one that can be reduced or oxidized at the available potentials) can reach the electrode is by diffusion. When the applied potential reaches the decomposition potential of the test substance a current flows, and it increases linearly as the potential is increased, in accordance with Ohm s law (since the resistance of the circuit remains constant). But with a microelectrode and under diffusion-controlled conditions, the number of ions or molecules of test substance that can diffuse to the electrode and maintain the electrolysis current is limited. Thus, with a dilute solution (10 to 10" M and less) a limiting current is ultimately reached and an S-shaped plot of ciurent versus applied potential results. The limiting current is proportional to the concentration of test substance. The potential at which the current is one-half of the limiting current (called the diffusion current, id) is the half-wave potential (E1/2) and is independent of concentration. The half-wave potential is characteristic of the particular test substance being electrolyzed. 

Microelectrodes, as the name implies, are miniaturized versions of conventional electrodes. The term micro usually denotes electrodes whose tip diameters are less than 10 /x while ultramicro refers to dip diameters less than 1 fjL. There are four basic types of microelectrodes commonly used in bioelectrochemistry glass, ion-selective, metal, and carbon. This section deals with their fabrication and characteristics. 

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