Impedance and Conductivity

The method has been used for detecting both bacteria and fungi, and is finding increasing use in industry due to its speed, but the cost of the apparatus is high, and can only be justified if there are a large number of samples. The use of this method in the developing world is unlikely due to the cost. 

---

Growth-based technologies ATP hioluminescence impedance and conductivity Hydrophobic grid membrane filter methods... 

The methods reviewed in this chapter, electrochemical impedance and conductivity, have common features the measured object is a specialized electrochemical cell subjected to a periodic electrical perturbation signal (in most cases the sine-wave signal) and the resulting periodic current flowing through the cell is used for the evaluation of the overall cell impedance. This cell impedance consists of the individual impedance contributions of both electrodes and a solution placed in the cell. The techniques mentioned above are aimed at the determination of only one predominant contribution of the overall impedance by using experimental conditions that enable other cell impedances to be ignored. 

The electrical parameters that are measured are current and voltage (amperometric, potentiometric or voltammetric), impedance, and conductivity. In this section, we discuss electrochemical methods applicable to LOC devices. 

Electrochemical sensors can be based on potentiometric, amperometric, or impedimetric transduction principles. Inherent benefits of electrochemical sensors include selectivity, ease of use, low Hmits of detection, and scope for miniaturization. Similar to biosensors, electrochemical immunosensors are also classified into potentiometric, amperometric, impedimetric, and conductometric based on the measured signal such as potential, current, impedance, and conductance, respectively. 

Once these substitutions are made, the electrical formulas and intuition can be applied to thermal problems. Ohm s law, voltage dividers, parallel and series impedances and conductances, and capacitors charging through a resistor all have thermal analogs. 

A.C. impedance (thickness conduction mechanisms and profiles compactness crystallinity)... 

The most important non-faradaic methods are conductometric analysis and (normal) potentiometric analysis in the former we have to deal essentially with the ionics and in the latter mainly with the electrodics. Strictly, one should assign a separate position to high-frequency analysis, where not so much the ionic conductance but rather the dielectric and/or diamagnetic properties of the solution are playing a role. Nevertheless, we shall still consider this techniques as a special form of conductometry, because the capacitive and inductive properties of the solution show up versus high-frequency as a kind of AC resistance (impedance) and, therefore, as far as its reciprocal is concerned, as a kind of AC conductance. 

FIGURE 10.6 Schematic drawings of a lab-on-a-chip system. (Top) Multisensor chip consists of pH, p02, and conductivity (impedance) electrodes incorporated in a microfluidic cell. (Bottom) The layout of electrodes on the chip. (Reproduced from [91], with permission from Elsevier.)... 

Based on GebeTs calculations for Nafion (where lEC = 0.91 meq/g),i isolated spheres of ionic clusters in the dry state have diameters of 15 A and an intercluster spacing of 27 A. Because the spheres are isolated, proton transport through the membrane is severely impeded and thus low levels of conductivity are observed for a dry membrane. As water content increases, the isolated ionic clusters begin to swell until, at X, > 0.2, the percolation threshold is reached. This significant point represents the point at which connections or channels are now formed between the previously isolated ionic clusters and leads to a concomitant sharp increase in the observed level of proton conductivity. 

Thus, at temperatures lower than the liquid us temperature (usually above —20 °C for most electrolyte compositions).EC precipitates and drastically reduces the conductivity of lithium ions both in the bulk electrolyte and through the interfacial films in the system. During discharge, this increase of cell impedance at low temperature leads to lower capacity utilization, which is normally recoverable when the temperature rises. However, permanent damage occurs if the cell is being charged at low temperatures because lithium deposition occurs, caused by the high interfacial impedance, and results in irreversible loss of lithium ions. An even worse possibility is the safety hazard if the lithium deposition continues to accumulate on the carbonaceous surface. 

It usually takes place close to the melting temperature of the polymer when the pores collapse turning the porous ionically conductive polymer film into a nonporous insulating layer between the electrodes. At this temperature there is a significant increase in cell impedance and passage of current through the cell is restricted. This prevents further electrochemical activity in the cell, thereby shutting the cell down before an explosion can occur. 

One limit of behavior considered in the models cited above is an entirely bulk path consisting of steps a—c—e in Figure 4. This asymptote corresponds to a situation where bulk oxygen absorption and solid-state diffusion is so facile that the bulk path dominates the overall electrode performance even when the surface path (b—d—f) is available due to existence of a TPB. Most of these models focus on steady-state behavior at moderate to high driving forces however, one exception is a model by Adler et al. which examines the consequences of the bulk-path assumption for the impedance and chemical capacitance of mixed-conducting electrodes. Because capacitance is such a strong measure of bulk involvement (see above), the results of this model are of particular interest to the present discussion. 

The results obtained in the photostimulated Sj l reaction between carbanions from 2,4,4-trimethyl-2-oxazoline or 2,4-dimethylthiazole and 2-bromopyridine are also consistent with the incomplete formation of the carbanions in KNH2-NH3(ii ) system. In these cases, 2-aminopyri-dine is formed alongside the corresponding pyridyl-2-methylene oxazolinyl or thiazolyl substitution products (Wong et al. 1997). When the Sr I pathway is impeded by conducting the reaction in the dark or in the presence of di(tert-butyl) nitroxide, the ionic amination reaction dominates. 

Microbial metabolism results in an increase in both conductance and capacitance causing a decrease in impedance and a consequent increase in admittance. In the Rapid Automated Bacterial Impedance Technique (RABIT) system, the admittance was plotted against time to provide results (Bolton, 1990). The final electrical signal is frequency- and temperature dependent and it has a conductive and a capacitive component. At present, impedance instruments are able to detect 10 —10 bacteria/ml (Ivnitski et ah, 2000). Several commercially available systems are operated... 

The antiarrhythmic action is due to cardiac adrenergic blockade. It decreases the slope of phase 4 depolarization and automaticity in SA node, Purkinje fibres and other ectopic foci. It also prolongs the effective refractory period of AV node and impedes AV conduction. ECG shows prolonged PR interval. It is useful in sinus tachycardia, atrial and nodal extrasystoles. It is also useful in sympathetically mediated arrhythmias in pheochromocytoma and halothane anaesthesia. 

Frequency-dependent measurements of the materials dielectric impedance as characterized by its equivalent capacitance, C, and conductance, G, are used to calculate the complex permitivity, e = d — id, where co = 2nf, f is the measurement frequency, and C0 is the equivalent air replacement capacitance of the sensor. 

INSIGHT uses the fundamental correlation between the electrical and permeability properties of skin. Skin permeability shows a strong correlation with skin impedance, as shown in Figure 4B. Figure 4B shows 150 independent and simultaneous measurements of mannitol skin permeability and skin impedance for six different enhancer formulations. The relationship between skin impedance and permeability to hydrophilic solutes confirms that the former can be used as a surrogate measure for the later. Skin conductance is quick and easy to obtain and does not require additional sample handling and analysis. 

---