Electrodes bioelectrodes

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. ... 

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. 

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. 

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.

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],... 

Immobilized /3-glucosidase served for enzymatically catalyzed hydrolysis of benzene metabolites in urine. End analysis of phenol was by RP-HPLC with ELD at 0.85 V vi. Ag/AgCl electrode. ELD avoids interference from other componnds present in urine. LOD was 10 p.gL" (20 p,L injection, 0.2 ng), with RSD 1.16% and 3.38% for 1.2 ng and 2.0 ng, respectively. A stndy was carried out of two FIA systems for enzymatically catalyzed determination of dopamine (10a). Thus, a combination of a packed bed reactor containing immobilized tyrosinase followed by photometric detection was compared with ELD based on a graphite electrode with its surface covered by immobilized tyrosinase. The former configuration was linear up to 0.75 mM while the latter reached 1 mM. LC separation and post-column detection with the bioelectrode was applied to analysis of spiked serum samples. ... 

F3. Feder, W., Silver-silver chloride electrode as a nonpolarizable bioelectrode. J. Appl. Physiol. 18, 397-401 (1963). 

The future of ISEs in the clinical chemistry instrumentation is quite exciting. As described in subsequent sections of this article, the coupling of enzyme and immunological reagents to ISE detectors to form bioelectrode systems appears to offer manufacturers a new approach toward the detection of metabolites such as creatinine and urea directly in blood and urine samples. Ultimately, such biosensors will be placed into complete electrode-based automated clinical analyzers. In addition, continued research on new membrane formulations, particularly liquid membrane ionophore systems, will result in the development of addition electrodes which can be incorporated into current analyzer systems to expand the electrolyte menu. Indeed, recent efforts have indicated that membranes selective fi)r bicarbonate (F5) and lithium (Z2) are likely additions in the near future. 

Bioselective electrodes are a class of hybrid devices which combine the selective properties of biocatalytic reactions with ISE detection of liberated ions or gases. In contrast to the organic ion liquid membrane electrodes described above, the bioelectrodes can often possess extraordinary selectivity over molecules which are very similar in structure to the analyte. In addition, these devices can respond to nonionic species. Consequently, such electrodes can be used directly in biological samples to determine a wide variety of biochemicals accurately. 

M5. Meyerhoff, M. E., Preparation and response properties of selective bioelectrodes utilizing polymer membrane electrode-based ammonia gas sensors. Anal. Lett. 13,1345-1357 (1980). 

Coupled reactions of two or more enzymes can also be used to minimize interference, as well as to amplify the response and extend the scope of the enzyme electrode towards additional analytes. For example, peroxidases can be coupled with oxidases to allow low-potential detection of the liberated peroxide. Electrocatalytic surfaces, particularly those based on metallized carbon, represent a new and effective approach for minimizing electroactive interference [9]. Such strategy relies on the preferential electrocatalytic detection of the liberated peroxide or NADH species at rhodium or ruthenium dispersed carbon bioelectrodes. 

To first obtain an ECG the patient must be physically connected to the amplifier front end. The patient/amplifier interface is formed by a special bioelectrode that converts the ionic current flow of the body to the electron flow of the metallic wire. These electrodes typically rely on a chemical paste or gel with a high ionic concentration. This acts as the transducer at the tissue-electrode interface. For short-term applications the use of silver-coated suction electrodes or sticky metallic foil electrodes are... 

Good overviews of biopotential electrodes are found in Geddes L. A. 1972. Electrodes and the Measurement of Bioelectric Events, New York, John Wiley 8c Sons and Ferris C.D. 1974. Introduction to Bioelectrodes, New York, Rlenum. Even though these references are more than 20 years old, they clearly cover the field, and httle has changed since these books were written. Overviews of biopotential electrodes are found in chapters of two works edited by John Webster. Chapter 5 of his textbook. Medical Instrumentation Application and Design, covers the material of this chapter in more detail, and there is a section on bioelectrodes in the Encyclopedia of Medical Devices and Instrumentation... 

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