Electrodes response time

Acetylcholineesterase and choline oxidase Immobilization of horseradish peroxidase in the redox polymer poly (4-vinylpyridine-chlorobis-(2,2 -bipyridyl) osmium cross-linked by means of polyoxyethylene 400 diglycidyl ether on polished vitreous carbon electrodes. Response time of 30 s and maintained its sensitivity for 24 h at low substrate concentration. Responses were rectilinear up to 10 mM H202 for 100 pM choline with detection limit of 10 nM and 1 pM. [78]... 

Acetylcholineesterase and choline oxidase A rotating graphite-disc electrode was polished, defatted, cleaned and oxidized by immersion in 10% HN03/2.5%, K2Cr207 at 2.2 V versus SCE for 10 s. AChE was covalently immobilized on to the electrode using a standard method. Measurements were made in 0.1 M-universal buffer of pH 7 at 0.8 V versus SCE. Calibration graphs were linear from 0.6-10 pM substrate. RSD were 5% (n = 10). The detection limit was 10 pM acetylthiocholine, electrode response time was 15 s. [89]... 

The electrode response time is increased relative to stationary solution. 

Although the one-layer model is an oversimplification of actual conditions, its application to the case where the oxygen partial pressure is allowed to change with time illustrates how electrode properties affect transient dissolved oxygen measurements. Pick s second law is needed to describe the unsteady-state diffusion in the membrane, and shows that the diffusion coefficient of the membrane directly determines how fast an electrode will respond to a step change in the oxygen partial pressure (Aiba et al., 1968 Lee and Tsao, 1979 Sobotka et al., 1982). Lee and Tsao (1979) showed mathematically that the electrode response time, for the one-layer model, depends on the electrode time constant defined as... 

Factors that influence the electrode response time... 

Equation (6.1.1.1) suggests that the electrode response time should decrease with decreasing electrode radius. Figure 6.1.2.1 shows how the RC cell time constant measured in 0.1 M LiC104 depends on the radius of platinum microdisk electrodes. Consistent with equation (6.1.1.1), as the electrode radius decreases from 25 to 1 pm, the cell time constant decreases linearly from approximately 2 psec to 80 nsec. The slope of the best-fit line is consistent with equation (6.1.1.1) where the double-layer capacitance is about 40 pF cm . Moreover, the intercept is approximately 4.3 nsec indicating that the stray capacitance (Section 6.1.3.3) of these microelectrodes is very small. Thus, cell time constants of tens of nanoseconds can be achieved with UMEs of micron radii. 

Electrode response time is relatively slow as one must wait for oxygen to diffuse through the membrane. The basic reaction at the cathode is probably... 

Various membrane materials can be used including rubber, silastic. Teflon, and polyethylene. Membrane thickness determines electrode response time, with thin membranes (0.001 in.) being the most responsive. Response time is similar to that of pOj electrodes (1-2 min) which is to be expected as the same sorts of processes are involved. 

It was reported that the electrode responded to urea concentrations from 5 X 10 to 1.6 X 10" in tris(hydroxymethyl)aminomethane buffer. Electrode response was not independent of sodium and potassium ions when Na was greater than one-half the urea concentration and when was greater than one-fifth of the urea concentration. Optimal electrode response time was 25 sec. 

Oyama and Hirokawa [804] described a potentiometric pH sensor that used a film of poly(l-amino pyrene) (P(lAPyre)) in which no internal standard solution, as is required in conventional glass pH electrodes, was used. The pH detection was based on the sensor functioning as an ion-sensitive field effect transistor (ISFET). The sensor had high ion selectivity with respect to Na, and Ca, and insensitivity to O2, much like a conventional pH glass-membrane electrode. Response time was in the 30 to 60 sec region, with excellent linearity of response in the pH 4.0 to 10.0 region. 

Although symmetric ISEs have found a wide range of applications,they still have certain inherent limitations they are mechanically complex, and thus difficult to be miniaturized. The internal reference solution increases the system s impedance and the electrode response time, in addition to a shorter lifetime due to leaching of the electroactive material throughout both solutions in contact with the membrane, and finally due to the internal compartment, which cannot withstand high pressure. ... 

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