Electrode polarization microelectrodes

Part—III exclusively treats Electrochemical Methods invariably and extensively used in the analysis of pharmaceutical substances in the Official Compendia. Two important methods, namely potentiometric methods (Chapter 16) deal with various types of reference electrodes and indicator electrodes, automatic titrator besides typical examples of nitrazepam, allopurinol and clonidine hydrochloride. Amperometric methods (Chapter 17) comprise of titrations involving dropping-mercury electrode, rotating—platinum electrode and twin-polarized microelectrodes (i.e., dead-stop-end-point method). 

It can be a further advantage of microelectrodes that they often increase the electrode resistance to bulk resistance ratio Rei/Rbuik- This is so because Re 1 frequently scales with the inverse area of the electrode, whereas the bulk resistance between a circular microelectrode and a counter-electrode is proportional to the inverse microelectrode diameter dme (see Sec. 4.1). Hence Rei/Rb iik ocbulk resistance decreases with decreasing microelectrode diameter. This is particularly helpful in order to investigate electrode polarization phenomena below the detection limit in experiments using macroscopic electrodes. (The reduced importance of the electrolyte resistance is also one of the reasons for ultramicroelectrodes to be applied in liquid electrochemistry [33, 34].)... 

Fig. 43. Double-logarithmic plot of the electrode polarization resistance versus the microelectrode diameter measured with impedance spectroscopy (ca. 800 °C) at (a) a cathodic dc bias of -300 mV, and (b) at an anodic dc bias of +300 mV. In (b) the first data point of the 20-pm microelectrode is not included in the fit. (c) Sketch illustrating the path of the oxygen reduction reaction for cathodic bias, (d) Path of the electrochemical reaction under anodic bias the rate-determining step occurs close to the three-phase boundary.

Bias-dependent measurements were performed in order to check to what extent the mechanism depends on the electrical operation conditions. Fig. 43 shows double-logarithmic plots of the electrode polarization resistance (determined from the arc in the impedance spectrum) versus the microelectrode diameter observed at a cathodic bias of —300 mV and at an anodic bias of +300 mV respectively. In the cathodic case the electrode polarization resistance again scales with the inverse of the electrode area, whereas in the anodic case it scales with the inverse of the microelectrode diameter. These findings are supported by I-V measurements on LSM microelectrodes with diameters ranging from 30-80 pm the differential resistance is proportional to the inverse microelectrode area in the cathodic regime and comes close to an inverse linear relationship with the three-phase boundary (3PB) length in the anodic regime [161]. 

Many types of mammalian cells are dependent on attachment to a surface in order to grow and multiply. Exceptions are the different cells of the blood and cancer cells which may spread aggressively (metastases). To study cell attachment, a microelectrode is convenient as shown in Chapter 5.2, the half-cell impedance is more dominated by electrode polarization impedance the smaller the electrode surface is. Figure 10.25 shows the setup used by Giaevers group (Giaever and Keese, 1993) ... 

Alternating-Current Electrode Polarization in Microelectrode Systems... 

Figure 4.11. Two lumped-parameter circuit models for studying the effects of ac electrode polarization in microelectrode systems. simulated biological signal source Cj,Q, contact or source capacitances Cp, electrode polarization capacitance = /(co) shunt capacitance of electrode to electrolyte and reference electrode C, cable leakage capacitance and input capacitance to amplifier system ...

In connection with associated electronic circuitry, fluid-filled microelectrodes behave as low-pass filters, while metal microelectrodes act as high-pass filters (Gesteland et a/., 1959). The reasons for this are simply that glass electrodes exhibit high shunt capacitance and series resistance (Figure 4.10) while electrode polarization impedance associated with metal microelectrodes produces a frequency-dependent RC combination whose series impedance decreases as frequency increases. 

At a recording microelectrode, ac electrode polarization, in addition to producing a frequency-sensitive voltage drop, may also introduce signal distortion. 

A rudimentary representation for an input circuit with glass microelectrodes (using a dc amplifier) is shown in Figure 7.5. represents the series resistance of the glass micropipette. A detailed description of electrode polarization problems was presented in Chapter 4. For this discussion it is sufficient to consider only the elements shown in Figure 7.5 since any signal distortion produced by electrode polarization can be incorporated into the description of. ... 

When metal microelectrodes are used, Figure 7.7 indicates an approximate input circuit configuration. is nearly zero and is omitted. C, the electrode polarization capacitance, now becomes important. When direct coupling to the amplifier is used, only Cp is present. For ac coupling, the series capacitance is given by... 

A new approach is to assemble a large number of microelectrodes together. Studies and applications of such micro-arrays are a growth area at present. In these assemblies, if each electrode is polarized to a different potential, then (in principle, at least) each one could then monitor the amounts of different analytes. 

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