Conductometric biosensors

A. Steinschaden, D. Adamovic, G. Jobst, R. Glatz, and G. Urban, Miniaturised thin film conductometric biosensors with high dynamic range and high sensitivity. Sens. Actuators B. B44, 365-369 (1997). 

Electrode modification by the attachment of various types of biocomponents holds considerable promise as a novel approach for electrochemical (potentiometric, conductometric, and amperometric) biosensors. Potentiometric sensors based on coupled biochemical processes have already demonstrated considerable analytical success [26,27]. More recently, amperometric biosensors have received increasing attention [27,28] partially as a result of advances made in the chemical modification of electrode surfaces. Systems based on... 

The electrochemical based biosensors are the most widely used format in biosensing. Typically the reaction under investigation would either generate a measurable current (amperometric), a measurable potential or charge accumulation (potentiometric) or measurably alter the conductive properties of a medium (conductometric) between electrodes [55]. 

Enzyme sensors can measure analytes that are the substrates of enzymatic reactions. Thermometric sensors can measure the heat produced by the enzyme reaction [31], while optical or electrochemical transducers measure a product produced or cofactor consumed in the reaction. For example, several urea sensors are based on the hydrolysis of urea by urease producing ammonia, which can be detected by an ammonium ion-selective ISE or ISFET [48] or a conductometric device [49]. Amperometric enzyme sensors are based on the measurement of an electroactive product or cofactor [50] an example is the glucose oxidase-based sensor for glucose, the most commercially successful biosensor. Enzymes are incorporated in amperometric sensors in functionalised monolayers [51], entrapped in polymers [52], carbon pastes [53] or zeolites [54]. Other catalytic biological systems such as micro-organisms, abzymes, organelles and tissue slices have also been combined with electrochemical transducers. 

The electrochemical biosensors, based on the parameter measured, can be further classified into potentiometric, amperometric, conductometric, impedimetric and ion charge or field effect biosensor [18],... 

Conductometry is an electrochemical technique used to determine the quantity of an analyte present in a mixture by measurement of its effect on the electrical conductivity of the mixture. It is the measure of the ability of ions in solution to carry current under the influence of a potential difference. In a conductometric cell, potential is applied between two inert metal electrodes. An alternating potential with a frequency between 100 and 3000 Hz is used to prevent polarization of the electrodes. A decrease in solution resistance results in an increase in conductance and more current is passed between the electrodes. The resulting current flow is also alternating. The current is directly proportional to solution conductance. Conductance is considered the inverse of resistance and may be expressed in units of ohm (siemens). In clinical analysis, conductometry is frequently used for the measurement of the volume fraction of erythrocytes in whole blood (hematocrit) and as the transduction mechanism for some biosensors. 

Enzyme-Based Biosensors with Potentiometric AND Conductometric Detection... 

Clearly, electrochemical indication prevails over all other methods of transduction. Potentiometric and amperometric enzyme electrodes are at the leading edge of biosensor technology with respect to the body of scientific literature as well as to commercially available devices (Schindler and Schindler, 1983). Only a few conductometric biosensors have been described, but the relevance of this sensor type may increase because of the relative ease of their preparation and use. Furthermore, the development of biochemically sensitized field effect transistors, being at present only at an initial stage, offers new prospects (Pinkerton and Lawson, 1982). 

The same research group described a small-scale electronic urea biosensor based on a urease-covered conductivity sensor (Watson et al., 1987/88). The conductometric sensor was prepared on a silicon wafer by applying the following sequence of steps thermal oxidation, deposition... 

Vianello, R, Boscolo-Chio, R., Signorini, S. and Rigo, A. (2007) On-line detection of atmospheric formaldehyde by a conductometric biosensor. Biosens Bioelectron, 22 (6), 920-925. 

Dzyadevych, S.V., Soldatkin, A.P. and Chovelon, J.-M. (2002) Assessment of the toxicity of methyl parathion and its photodegradation products in water samples using conductometric enzyme biosensors. Analytica Chimica Acta 459, 33-41. 

Mainly electrochemical (amperometric, potentiometric, impedimetric, or conductometric) and optical (IR, Raman, fluorescence, absorption, reflection, evanescence field, or surface plasmon resonance) transducers are used as the basis for biosensors. However, beside these there are other, less often employed transducers that make use of the piezoelectric effect, surface acoustic waves, or detection of heat generated in enzyme reactions [40, 41]. In the context of this work, the focus is on the specific features of electrochemical transducers. An overview showing the different fields of apphcation can be found in Sect. 2.11.1.5 (Table 2). 

Biosensors based on conductometric or impedimetric measurements are comparably rarely described in literature. They have two main fields of application sensors utilizing enzyme reactions in which ionic substances are formed and sensors employing receptors (or bilayer lipid membranes containing receptors) or intact cells of higher organisms. 

In enzyme-based conductometric biosensors the modulation of the conductivity of a conducting layer connecting two electrodes upon enzymatically catalyzed formation of ionic species is measured. In these conductometric enzyme sensors which are often miniaturized. 

Conductometric MEMS biosensors (conductometry) are based on measuring conductance between two electrodes in a solution. Conductance is measured by applying a small quantity of AC potential to block a polarization. The existence of ionic elements is measured as an increase in conductance. 

Conductometric MEMS Biosensors Electrolytic conductance is a non-faradaic process that can give useful chemical information. Electrolytic conductance originates from the transport of anions to the anode and cations to the cathode. In order to complete the current path, electrons are transferred at the electrode surface to and from the ions. The conductance of an electrol3Te is measured in a conductance cell consisting of two identical nonpolarizable electrodes. To prevent polarization, an AC potential is applied to these electrodes and the AC current is measured [8]. 

MEMS-Based Biosensor, Fig. 5 A cross-sectional view of a conductometric sensor [9]... 

Figure 10.9 Response curve of a conductometric urea biosensor...

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