Enzyme electrodes listed

Enzyme electrodes for other substrates of analytical significance have been developed. Representative examples are listed in Table 6-1. Further advances in enzyme technology, and particularly the isolation of new and more stable enzymes, should enhance the development of new biocatalytic sensors. New opportunities (particularly assays of new environments or monitoring of hydrophobic analytes) derive from the finding that enzymes can maintain then biocatalytic activity in organic solvents (31,32). 

Although the primary focus of oxidase based enzyme electrodes has been the determination of glucose, the list of extensions to other analytes is considerable. Systems have been described for cholesterol 123,132-136) galactose ii -i35-i37) dd i38.i39) lactatepyruvatecreatinine serum lipaseethanoland amino acids... 

As is evident from the numerous publications on enzyme electrodes in the literature, a tremendous effort has been expended in this field resulting in a long list of applica-... 

Because each enzyme sensor has its own unique response, it is necessary to construct the calibration curve for each sensor separately. In other words, there is no general theoretical response relationship, in the same sense as the Nernst equation is. As always, the best way to reduce interferences is to use two sensors and measure them differentially. Thus, it is possible to prepare two identical enzyme sensors and either omit or deactivate the enzyme in one of them. This sensor then acts as a reference. If the calibration curve is constructed by plotting the difference of the two outputs as the function of concentration of the substrate, the effects of variations in the composition of the sample as well as temperature and light variations can be substantially reduced. Examples of potentiometric enzyme electrodes are listed in Table 6.5. 

Enzyme electrodes for other substrates of analytical significance have been developed. Representative examples are listed in Table 6.1. Further advances in enzyme technology, particularly the isolation of new and more stable... 

Various research groups have developed enzyme electrodes for the determination of sucrose. The operational parameters of these sensors are listed in Table 10. 

Table 1 lists a few of the enzyme electrodes reported/ Both poten-tiometric and amperometric devices have been employed with these electrodes. Test results with enzyme electrodes have been reported as ranging from excellent to poor. Problems reported deal principally with lack of stability of the enzymes used, extended response times, and temperature sensitivity. There does not appear to be any pattern to the problems reported. It is apparent however, that (a) care must be taken in the choice of enzyme immobilization method, (b) the immobilization technique should provide a minimal barrier to reactant and product diffusivity, and (c) temperature stabilization is desirable. 

Guilbault and Montalvo were the first, in 1969, to detail a potentiometric enzyme electrode. They described a urea biosensor based on urease immobilized at an ammonium-selective liquid membrane electrode. Since then, over hundreds of different applications have appeared in the literature, due to the significant development of ion-selective electrodes (ISEs) observed during the last 30 years. The electrodes used to assemble a potentiometric biosensor include glass electrodes for the measurement of pH or monovalent ions, ISEs sensitive to anions or cations, gas electrodes such as the CO2 or the NH3 probes, and metal electrodes able to detect redox species some of these electrodes useful in the construction of potentiometric enzyme electrodes are listed in Table 1. 

Table 1 lists enzyme electrodes that have been prepared for analysis of common substrates together with the enzyme used, the sensor, and the range of concentrations determinable. The most important features of some of these are described here. 

While potentiometric enzymatic electrodes for detection of phosphate have been developed [121,122], no application has been made of these to water analysis because of the relatively poor sensitivity. However, amperometric enzyme electrodes have been reported that they have high sensitivity, selectivity, and long operational life and it is expected that the use of these for water analysis will become more widespread (see Table 8.2). For example, a sensitive enzyme electrode based on the amperometric detection of hydrogen peroxide produced by membrane coimmobilized nucleoside phosphorylase and xanthine oxidase has been reported for the detection of phosphate by D Urso and Coulet [123]. Other similar enzyme electrode systems suitable for water analysis are listed in Table 8.2. 

The relay-modified enzyme electrodes vary in their chemical and electrochemical stability. In the group listed in Table 2, the enzyme with 14 of its histidines bound to ruthenium-pentaammine was both the most and the least stable. When the enzyme-bound ruthenium was predominantly in its trivalent state, the modified enzyme showed stable electrochemistry for over a week. When the ruthenium was reduced, for example by adding glucose to the solution, some of the enzyme-bound ruthenium-pentaammine complex dissociated and the glucose-concentration dependent current dropped rapidly The modified enzyme solution lost 10 % of its current in less than 5 min. Furthermore, assay of the number of bound ruthenium atoms after incubation of the modified enzyme (at 25 C and at 30 mM glucose concentration) for 20 min showed a drop from 14 ruthenium atoms/enzyme molecule to 7 ruthenium atoms/enzyme molecule. 

A number of potentiometric enzyme electrodes are listed in Table 4.2 to provide an overall view. The components required, the enzyme, and the transducer are listed for each analyte that is detectable by biosensors. The performance of the electrodes are presented using two essential parameters, the response time and the range of concentrations in which the signal obtained is linear as a function of the logarithm of the concentration. More detailed descriptions of each biosensor can be found in the references indicated [23-137]. Immobilized enzyme measuring systems that use microcolumns associated with electrochemical detection are excluded from this table [138] because they are not biosensors but analytical techniques. 

Figure 18.6 Energetics of the ORR at the heme/Cu site of CcO the enzyme couples oxidation of ferroc3ftochrome c (standard potential about —250 mV all potentials are listed with respect to a normal hydrogen electrode) to reduction of O2 (standard potential at pH 7 800 mV). Of the 550 mV difference, only 100 mV is dissipated to drive the reaction 220 mV is expanded to translocate four protons from the basic matrix compartment to the acidic IMS (inter-membrane space). In addition 200 mV is converted into transmembrane electrostatic potential as ferroc3ftochrome is oxidized in the IMS, but the charge-compensating protons are taken from the matrix. The potentials are approximate.

The diffusion-reaction problem in the more general case occurs in a system containing n — 1 inactivated enzyme layers adjacent to the electrode surface on top of which N — n active layers have been deposited. Table 6.9 lists the equations that govern the fluxes of the two forms of the cosubstrate in such systems. 

However, these reports of multitudinous enzyme-based biosensors should be viewed with some caution, as it is much easier to demonstrate the possibility of using an enzyme in a laboratory prototype than to convert these observations into a reliable, and reproducible, device that can meet commercial product requirements. This is illustrated by table 7.1, which lists the few enzymes that have been reported to have been used in commercial biosensors only about two dozen enzymes have been used commercially. Most of the enzymes are oxidases partly because of the stability of this class of enzyme, and partly because of ease of linking this type of enzyme with a Clark-type oxygen electrode. 

Table 1 lists the main sensors used for diagnosis or cure. Moreover, classifications, measurement principles, and applications for diagnosis or cure also will be described. If commercial availability can be confirmed, it is so indicated. For enzyme sensors, the detection method (electrode) for enzyme reaction products is stated in the supplemental column. 

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