Electrode amperometric

Amperometric sensors are based on heterogeneous electron transfer reactions, i.e., the oxidation and reduction of electroactive substances (Fig. 10). Oxygen and H2O2, being the cosubstrate and the product of several enzyme reactions, as well as artificial redox mediators, such as ferricyanide, N-methylphenazinium ion (NMP+), ferrocene, and benzo-quinone may be determined amperometrically. 

By an increase of the overvoltage, i.e. the deviation from the redox potential, the rate of the heterogeneous charge transfer process is enhanced so as to cause the rate of the whole process to become controlled by mass transfer. Under these conditions the diffusion current, Id, is proportional to the concentration of the substance to be determined, So- 

Detection limits as low as 10 nrnol/1 and a linear measuring range of 3-6 concentration decades are the main advantages of amperometric techniques. 

Oxygen electrodes are being used for the direct determination of oxygen concentration in biomedicine and biotechnological processes. For the development of biosensors they can be combined with a large number of biocatalytic processes such as the respiration of microorganisms, plant photosynthesis, and oxidase-catalyzed reactions. 

Amperometric sensors for hydrogen, nitrous oxides, and carbon dioxide have been developed by modification of the Clark-type electrode (Hanus et al., 1980 Albery and Barron, 1982). 

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The final method of coupling enzyme reactions to electrochemistry is to immobilize an enzyme directly at the electrode surface. Enzyme electrodes provide the advantages already discussed for immobilization of enzymes. In addition, the transport of enzyme product from the enzyme active site to the electrode surface is greatly enhanced when the enzyme is very near to the electrode. The concept of combining an enzyme reaction with an amperometric probe should offer all of the advantages discussed earlier for ion-selective (potentiometric) electrodes with a much higher sensitivity. In addition, since the response of amperometric electrodes is linear, background can be selected. 

Hitherto, there are primarily three ways in which an amperometric electrode can be used to simultaneously induce and monitor interfacial processes. These are illustrated schematically in Fig. 2 for the most general case where diffusion may occur in both of the liquid phases, which comprise the interface of interest. 

A. Karaykin, O. Gitelmacher, and E. Karaykina, Prussian blue-based first-generation biosensor. A sensitive amperometric electrode for glucose. Anal. Chem. 67, 2419—2423 (1995). 

L. Campanella, G. Favero, and M. Tomassetti, A modified amperometric electrode for the determination of free radicals. Sens. Actuators B. 44, 559-565 (1997). 

G.J. Moody, G.S. Sanghera, and J.D.R. Thomas, Modified platinum wire glucose oxidase amperometric electrode. Analyst 111, 1235-1238 (1986). 

When using an amperometric electrode as the measuring technique, interference from the solutes and solvent can occur. The solutes and solvent can adsorb to the semipermeable membrane of the electrode, therefore giving an additional resistance to the diffusion of ozone through this membrane to the electrolyte chamber. The use of an amperometric electrode is not recommended for water containing particles. In these cases the ozone consumption can only be calculated from the ozone gas balance. 

The graphical representation of this protocol is shown schematically in Fig. 10.15. Signals from two amperometric electrodes, representing channel 1 (blue) and channel 2 (red) detect to electroactive species, which is delivered to them with frequency modulation of, for example, 1 Hz. The experiment is performed in the benchtop fluid setup shown in Fig. 10.16. The first interesting observation is the presence of higher harmonics in the coherence spectrum. They arise as the effect of nonsinusoidal modulation. A pure sine wave would transform to the frequency domain as a single line. Any other waveform of the same frequency will contain higher harmonics in the spectrum. 

C. Loechel, A. Basran, J. Basran, N. S. Scrutton and E. A. Hall, Using trimethylamine dehydrogenase in an enzyme linked amperometric electrode. Part 1. Wild-type enzyme redox mediation, Analyst, 128(2) (2003) 166-172 Part 2. Rational design engineering of a wired mutant, Analyst, 128(7) (2003) 889-898. 

The tissue-based amperometric electrodes for the determination of catechol [13,14], dopamine [15] and phenol [16] shown in Table 17.2 operate on the same principle as the biosensor for L-ascorbic acid [11],... 

Because the generator electrodes must have a significant voltage applied across them to produce a constant current, the placement of the indicator electrodes (especially if a potentiometric detection system is to be used) is critical to avoid induced responses from the generator electrodes. Their placement should be adjusted such that both the indicator electrode and the reference electrode occupy positions on an equal potential contour. When dual-polarized amperometric electrodes are used, similar care is desirable in their placement to avoid interference from the electrolysis electrodes. These two considerations have prompted the use of visual or spectrophotometric endpoint detection in some applications of coulometric titrations. 

In the field of biosensor technology, immobilized enzyme electrode development occupies a place of prominence due to the attractive performance of this hybrid device. Coupling an immobilized enzyme layer with an electrochemical sensor combines the advantages of using an insolubilized enzyme system (see below) with the sensitivity of readily available potentiometric and amperometric electrodes. The resulting biosensor enables direct, reliable, and reproducible... 

Sol-gels containing electroactive species have been used in the development of both amperometric and potentiometric electrodes. Films coated with anionic poly-(dimethyldiallylammonium chloride) (PDMDAAC) and cationic poly(vinylsulfonic acid) were used to concentrate Ru(bpy)3 + and the hexacyanoferrate anion, respectively, for use as amperometric electrodes [208a]. The detection limit by square-wave voltammetry improved by up to 50-fold compared with uncovered electrodes. In Figure 41, curve 1 corresponds to a bare graphite electrode, curve 2 to a sol-gel-covered electrode and curve 3 to a sol-gel-PDMDAAC-modified electrode after 10 min of exposure to Fe(CN)g. ... 

The current measured by an amperometric electrode is directly proportional to the flux described in Eq. 7.11, with proportionality constants n (electrons in the stoichiometric electrochemical reaction), F (Faraday s constant, 96,487 C/mol), A (electrode area) and B (fractional collection efficiency) ... 

Phenol and the three dihydroxybenzenes (20, 42, 66) in water were determined by LLE with a hydrophilic solvent followed by amperometric titration. LOD was in the ppm range . A dual electrode in a FIA system has been used as detector for total phenols in wastewater. The upstream coulometric electrode has a large surface area and is used to eliminate compounds that cause interference and the second one is an amperometric electrode for oxidative detection of all phenols. Optimal results were found working with a phosphate buffer at pH 6.8, at potentials of +0.35 V and +0.78 V for the coulometric and amperometric electrodes, respectively. A high sample throughput of 60 per hour can be attained with RSD of 0.1-4%. This method is more reliable than the colorimetric method . The concentration of fenobucarb (142) in drinking water was determined after a short alkaline hydrolysis, and oxidation of the resulting 2-s-butylphenol with a GCE at 750 mV, pH 3.5 LOD was 3.6 x 1Q- M, RSD 3.74% for 1 x IQ- M (n = 11, p = 0.05)37 . 

The physicochemical change of the biologically active material resulting from the interaction with the analyte must be converted into an electrical output signal by an appropriate transducer. On the one hand, unspecific, but broadly applicable transducers may be used, which indicate general parameters such as reaction enthalpy (thermistor), mass change (piezoelectric crystal), or layer thickness (reflectometry). On the other hand, a specific indication may be achieved with potentio-metric or amperometric electrodes for species such as H+, OH-, CO2, NH3, or H2O2, or with optical methods such as photometry or fluori-metry. 

Fig. 18. Principles of electrochemical modulation of sensor characteristics by amperometric electrode reactions. E enzyme, S substrate P product M mediator.

Therefore the concept of chemically modified electrodes has been developed, in which the mediator is integrated with the amperometric electrode (Fig. 19). The following methods for preparing mediator-chemically modified electrodes (MCME) have been described (Murray, 1984) ... 

An alternative to the application of mediators is the direct transfer of electrons between the prosthetic group of the enzyme and the amperometric electrode (Fig. 19). In this heterogenous reaction the electrode acts as an electron transferase. 

An exciting method for accelerating the electron transfer between redox proteins and amperometric electrodes has been described by Heller and Degani (1987), who modified oxidoreductases with electrontunneling relays. The same mediators as are used in chemically modified electrodes are directly bound to groups of the protein molecule. The... 

The advantages of amperometric electrodes, such as the greater sensitivity and precision and the lower measuring time, have prompted several research groups to study the adoption of this measuring principle to the assay of urea. Altogether four different approaches have been investigated ... 

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