Bioelectrodes

There are two basic types of electrodes used as detectors of electrochemical potentials and/ or currents in the biological sciences, i.e., selective and nonselec-tive electrodes. Selective electrodes are those electrodes that exhibit a high degree of specificity to the chemical activity of one type of chemically reactive species in an electrochemical system. Conversely, nonselective electrodes are nonspecific and indicate the electrochemical activity of many different types of chemical constituents of an electrochemical system. The most common examples of these two types are the pH electrode (high selectivity) and the platinum metal (general purpose) electrode. 

Electrodes are used as sensors in either a potentiometric mode or an amperometric mode. As the names imply, potentiometric electrodes measure electrochemical activity by relating it to a potential (voltage). Amperometric electrodes measure electrochemical activity by relating it to a quantity of current (amperes). Both modes have found wide application. Ion-selective electrodes generally operate in the potentiometric mode. Amperometric sensors, conversely, generally use nonselective electrodes which can be made selective by electrochemical and nonelectrochemical modification. Potentiometric electrodes operate via a number of presently ill-defined mechanisms. However, regardless of the mechanism, the measured potential is due to an interfacial chemical equilibrium that does not involve a bulk transfer of material. Amperometric electrodes, on the other hand, do involve the bulk transfer of material. 

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. 

While these electrodes are merely miniaturizations of the familiar Ag/AgCl reference electrode, the very small tips impart a few unfamiliar problems. One of these is very high electrode resistance, with resistance 

Ion-selective electrodes are really a subgroup of glass microelectrodes because the electrodes are identical in many respects. Conventional glass micropipets and filling solutions are used, but in addition, an ion-selective barrier is interposed between filling solution and external solution. 

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Some of these stability issues can be addressed by the use of protective barrier membranes, at the risk of aggravating another fundamental challenge reactant mass transfer. Typical reactants present in vivo are available only at low concentrations (glucose, 5 mM oxygen, 0.1 mM lactate, 1 mM). Maximum current density is therefore limited by the ability of such reactants to diffuse to and within bioelectrodes. In the case of glucose, flux to cylindrical electrodes embedded in the walls of blood vessels, where mass transfer is enhanced by blood flow of 1—10 cm/s, is expected to be 1—2 mA/cm. ° Mass-transfer rates are even lower in tissues, where such convection is absent. However, microscale electrodes with fiber or microdot geometries benefit from cylindrical or spherical diffusion fields and can achieve current densities up to 1 mA/cm at the expense of decreased electrode area. ... 

Dubini-Paglia, R. Galli and T. lAmsini,Experientia Suppl. 18, 259 (1971). Eisenman, Bioelectrodes, in Modem Techniques of Physiological Sciences (ed. J. F. Gross), Academic Press, New York (1974), p. 245. 

Most of the pro-oxidative enzymes of bacteria are stabilized inside the cell, but are very fragile outside the cell. Therefore, the view that analysis may be carried out via isolated enzymes for aromatic processing, perhaps coupled to an electrode of some kind, appears quite impractical. MCA takes advantage of what bacterial cells can actually do, namely to stabilize and protect enzymes, besides the initial synthesis. Hence, MCA is likely to be far more practical than any bioelectrode method for analysis. 

Kaldb T, Sklddal P (1994) Evaluation of mediators for development of amperometric microbial bioelectrodes. Electro analysis 6 1004-1008... 

Electrical Stimulation Devices. Bioelectrodes that transmit electrical signals into the body are generally known as electrical stimulation devices, examples of which include cardiac pacemakers, transcutaneous electronic nerve stimulators (TENs) for pain suppression, and neural prostheses such as auditory stimulation systems for the deaf and phrenic nerve stimulators for artificial respiratory control. In these, and other similar devices, electrodes transmit current to appropriate areas of the body for direct control of, or indirect influence over, target cells. 

Compagnone et al. [5] Glycerol Alcoholic fermentation Glycerokinase (GK) and glycerol-3-phosphate oxidase (GPO)/ covalently immobilised (Bioreactor of GK and bioelectrode of GPO) Pt-based hydrogen peroxide probe/+650mV vs. Ag/AgCl -... 

A bioelectrode functioning optimally has a short response time, which is often controlled by the thickness of the immmobilized enzyme layer rather than by the sensor as well as many other factors (see Table 7). The biosensor response time depends on (1) how rapidly the substrate diffuses through the solution to the membrane surface, (2) how rapidly the substrate diffuses through the membrane cmd reacts with the biocatalyst at the active site, and (3) how rapidly the products formed diffuse to the electrode surface where they are measured. Mathematical models describing this effea are thoroughly presented in the biosensor literature (5, 68). 

Hanazato, Y.-, Shiono, S. Bioelectrode using two hydrogen ion-sensitive field-effect transistors and a platinum wire pseudo reference electrode. Anal. Chem. Symp. Ser. 1983, 513-518. 

Figure3. (A) SEM images of CHIT/ITO electrode (a) NanoZnO-CHIT/ITO electrode (b) and ChOx/NanoZnO-CHIT/ITO bioelectrode (c). (B) Biochemical reaction of the biosensor to cholesterol. Reprinted from Analytica Chimica Acta, 616, R. Khan, A. Kaushik, P. R. Solanki, A. A. Ansari, M.K. Pandy, B.D. Malhotra, Zinc oxide nanoparticles -chitosan composite film for cholesterol biosensor, 209,211,Copyright ( 2008) with permission fom Elsevier.

Figure 13. Electrochemical response of Urs-OLDH/ZnO-CH/fTO bioelectrode with respect to urea concentration (5-100 mg dl"1) at scan rate of 10 mVs"1. Inset, the plot of cuurent vs. urea concentration, (b) The electrochemical reaction at bioelectrode. ( Reused with permission from P.R. Solanki, A.Kaushik, A.A. Ansari, G. Gumana, B.D. Malhotra, Applied Physics Letters, 93, 2008, 163903. Copyrights 2008 American Institute of Physics ...

Sol-gel derived hybrid Ti02 film deposited on glassy carbon electrode has been used to construct the phenol biosensor. The resulting biosensor is selective towards phenol with a linear range from 7.5 x 10 8 - 6 x 10"6 M with detection limit 1 x 10"8 and has response time as 10 s. The biosensor exhibits maximum response at 45 °C. The initial response current of the bioelectrode decreases to 95 % after 2 months [85],... 

Ochratoxin-A (OTA), a mycotoxin produced in unstored food and beverages has recently been detected using CH-Fe304 nanobiocomposite modified indium-tin oxide (ITO) electrode [73], This immunosensor shows a specific response to OTA detection in the range of 0.5 - 6 ng/dL with a detection limit of 0.5 ng dL 1. The sensitivity of the bioelectrode has been found to be 36 pA/ng dL"1 cm 2 with response time of 18s [101],... 

Figure 5. Electrochemical response of Ch0x/NS-Ce02/IT0 bioelectrode at different concentrations of cholesterol 10, 50, 100, 200, 300, and 400 mg/dL at scan rate of 50 mV/s.(Electrochem. Commun.

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