Microelectrodes frequency response

The first frequency response of the function I/Q (disk response) follows an asymptotic power law dependence I/Q [Pg.231]

Fleischmann What actually limits the frequency response of your microelectrode ... 

An ac impedance technique based on changes in solution resistance is also available and has been appUed to enzyme-coated microelectrodes (64). In this case, a sinusoidal potential is applied between the electrode and the auxiliary electrode. The measured sinusoidal current is sent to a frequency response analyzer that monitors the change of the real impedance with distance to the substrate. Using equivalent circuits, a theoretical approach curve can be obtained and fitted to the experimental solution resistance profiles with distance. 

If the electrode reaction (1.1) is kinetically controlled, the response depends on both the parameter p and the kinetic parameter k [26,27]. If the electrode size is constant and the frequency is varied, both parameters p and k ate changed. Also, if a certain reaction is measured at constant frequency, with a range of microelectrodes having various diameters, the apparent reversibility of the reaction decreases with the decreasing diameter because of radial diffusion. So, the relationship between... 

Figure 2.21 shows the dependence of dimensionless net peak currents of ferrocene and ferricyanide on the sphericity parameter (note that A0p = AT], andy = p)-The SWV experiments were performed at three different gold inlaid disk electrodes (ro = 30, 12.5 and 5 pm) and the freqnencies were changed over the range from 20 to 2000 Hz [26]. For ferrocene the relationship between AT], and p is linear A Fp = 0.88 + 0.74p. This indicates that the electrode reaction of ferrocene is elec-trochemically reversible regardless of the frequency and the electrode radius over the range examined. For ferricyanide the dependence of AT], on p appears in sequences. Each seqnence corresponds to a particular value of the parameter The results obtained with the same freqnency, but at different microelectrodes, are cormected with thin, broken lines. The difference in the responses of these... 

However, at the highest frequencies the diffusion layers are muchthiimer, the microdroplets tend to behave as independent hemispherical microelectrodes and the response differs significantly from the theory of the thin-film electrode [49]. 

The phase shifts are wholly reducible versus p and are displayed in Fig. 5-5. The response of the controlled microelectrode increases monotonically with the frequency and equals the 7t/4 limiting value of the isolated microelectrode [57] increased by 2 7t, a case theoretically predicted for a small gap. 

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]. 

The forward and reverse currents i/rf and i//( of the square wave voltammograms corresponding to Fig. 7.5c are shown in Fig. 7.6a for microelectrodes of the four electrode geometries considered. From these curves, it can be seen that both currents present a sigmoidal shape and they are separated by 2Esw in the case of spheres and discs. This behavior clearly shows that the steady state has been attained. On the other hand, in the case of cylinders and bands, y/f and i/// show a transient behavior under these conditions. From Fig. 7.6b, c, it can be verified that a decrease in the radius, ((w/2) = rc = 0.1 pm) and that of both radius and frequency (Fig. 7.6c, (w/2) = rc = 0.1 pm and/= 10 Hz) do not lead to a stationary SWV response at cylinder and band microelectrodes. 

The effect of frequency on the SWV voltammograms of a two-electron electrochemical reaction is shown in Fig. 7.31 for different AEf1 values at T = 298 K. The responses for disc (solid line) and (hemi)spherical (dashed line) microelectrodes calculated from Eq. (7.65) are considered. 

Oscillations of [Ca2+][ have been reported following initiation of insulin release by nutrients and sulphonylureas (Heilman etal., 1992). The frequency of these large-amplitude oscillations corresponds to 0.2-0.5 min-1 in mouse B-cells, which is similar to the slow cyclic variations in burst activity recorded with intracellular microelectrodes in intact islets and also the periodicity of insulin release. However, this oscillatory pattern of the electrical and [Ca2+]j responses induced by glucose is not accompanied by, and thus probably not due to, similar oscillations in metabolism (Gilon and Henquin, 1992). However, Longo et al. (1991) reported oscillations with similar periods in insulin secretion, oxygen consumption and [Ca2+]j. Since oscillations appear in vivo as well as in vitro there must be a pacemaker in the islet tissue itself (Goodner et al., 1991). 

To test this hypothesis, we administered an uptake inhibitor, nomifensine, to rats that had been pretreated with a drug, sulpiride, which blocks the D2 dopamine receptor. In animals pretreated with this drug, the uptake inhibitors are less effective at decreasing the action potential frequency in dopamine neurons, strongly implying that the D2 receptors mediate the homeostatic response by providing negative feedback information to dopamine neurons [85]. In the sulpiride-pretreated rats, an increase in dopamine was observed with carbon fiber microelectrodes immediately after the adrninistration of... 

Sohn and co-workers reported a novel device which can quantify the DNA content within the nucleus of single eukaryotic cells [3]. Since DNA molecules are highly charged in intracellular environment, they will he polarized in an applied low-frequency AC electric field. This polarization response can he measured as a change in total capacitance ACj, across a pair of microelectrodes as individual cells suspended in buffer solution flow one by one through a microchannel (as shown in Fig. 1 and Fig. 2). They found that there is a linear relationship between the capacitance and the DNA content of a cell. And they further showed that this relationship is not species-dependent. This innovative technique is termed as capacitance cytometry, which helps to reveal changes in cellular internal properties and determine the phase of individual cell in cell-cycle. 

Figure 2 ACh-induced responses in the cat petrosal ganglion and in isolated neurons in culture, (a) Increases in the carotid sinus nerve frequency discharge ( csn) evoked by the application of increasing AQi doses (2-500 pg) to the ganglion (arrowhead), (b) Dose-response relationship for the significant increases in csn (Afew) observed in (a), (c) Depolarization and firing of multiple action potentials, recorded with intracellular microelectrode, elicited by application of an ACh (200 pM) bolus (arrowhead), (d) Inwardly directed inactivating current, recorded in whole-cell voltage-clamp configuration at Vm = —60 my induced by a 4-sec ACb (500 pM) pulse (continuous line).

Fig. 14.2 illustrates the determination of an equimolar low concentration (0.1 pM) of three neurotransmitters with the diamond-based CE detector. The very low and stable noise levels in the background current enabled us to attain the lowest detection limits from the diamond-based electrochemical detector. Under the same CE separation conditions, the diamond microhne electrode showed lower noise levels (0.5 - 1 pA), and a more stable background current, than that of the carbon fiber electrode (whose minimum noise level was 2 pA), even though its surface area was 25 times larger. Variations in the background current (low-frequency noise) were also much smaller and less irregular than those for the carbon fiber microelectrodes. In addition, the diamond microelectrodes exhibited greater stability in the amperometric response compared to that for the carbon fiber microelectrodes. 

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