Response time, electrochemical electrode

It would obviously be informative if the release of insulin could be monitored spatially and on a useful time scale. In many microelectrode studies in biology, the substances concerned are easy to reduce or oxidize electrochemically. This is not so with sulfur-containing insulin, and it was not until a suitable (if complex) electrocatalyst was discovered that fast scan cychc voltammetry with microelectrodes could be used to monitor it. The substance that promotes reaction with the sulfide atoms in insulin is a mixed-valent RuOz-cyanoruthenate mixture. The catalyst is prepared in situ and deposited onto the carbon fiber of the microelectrode. The response time of electrodes thus prepared (Kennedy and Huang, 1995) is < 100 ms, and detection limits are as low as 5 pM. 

Other important alternate electrochemical methods under study for pCO rely on measuring current associated with the direct reduction of CO. The electrochemistry of COj in both aqueous and non-aqueous media has been documented for some time 27-29) interferences from more easily reduced species such as O2 as well as many commonly used inhalation anesthetics have made the direct amperometric approach difficult to implement. One recently described attempt to circumvent some of these interference problems employs a two cathode configuration in which one electrode is used to scrub the sample of O by exhaustive reduction prior to COj amperometry at the second electrode. The response time and sensitivity of the approach may prove to be adequate for blood ps applications, but the issue of interfering anesthetics must be addressed more thorou ly in order to make the technique a truly viable alternative to the presently used indirect potentiometric electrode. 

The high specificity required for the analysis of physiological fluids often necessitates the incorporation of permselective membranes between the sample and the sensor. A typical configuration is presented in Fig. 7, where the membrane system comprises three distinct layers. The outer membrane. A, which encounters the sample solution is indicated by the dashed lines. It most commonly serves to eliminate high molecular weight interferences, such as other enzymes and proteins. The substrate, S, and other small molecules are allowed to enter the enzyme layer, B, which typically consist of a gelatinous material or a porous solid support. The immobilized enzyme catalyzes the conversion of substrate, S, to product, P. The substrate, product or a cofactor may be the species detected electrochemically. In many cases the electrochemical sensor may be prone to interferences and a permselective membrane, C, is required. The response time and sensitivity of the enzyme electrode will depend on the rate of permeation through layers A, B and C the kinetics of enzymatic conversion as well as the charac-... 

CNTs offer an exciting possibility for developing ultrasensitive electrochemical biosensors because of their unique electrical properties and biocompatible nanostructures. Luong et al. have fabricated a glucose biosensor based on the immobilization of GOx on CNTs solubilized in 3-aminopropyltriethoxysilane (APTES). The as-prepared CNT-based biosensor using a carbon fiber has achieved a picoamperometric response current with the response time of less than 5 s and a detection limit of 5-10 pM [109], When Nation is used to solubilize CNTs and combine with platinum nanoparticles, it displays strong interactions with Pt nanoparticles to form a network that connects Pt nanoparticles to the electrode surface. The Pt-CNT nanohybrid-based glucose biosensor... 

There are no electrolyzers developed specifically for operation with wind turbines. However, the rapid response of electrochemical systems to power variations makes them suitable "loads" for wind turbines. Industrial electrolyzers are designed for continuous operation, mainly because their elevated investment cost requires high-capacity factors for reasonable payback times, but they are subject to a considerable number of current interruptions through their lifetime due to occasional power interruptions, accidental trips of safety systems, and planned stops for maintenance. Current interruptions are more frequent in specialty applications, where electrolyzers supply hydrogen "on demand." Therefore, the discontinuous use of the equipment is not new, and most commercial electrolyzers may be used in intermittent operation although a significant performance decrease is expected with time. In fact, it is not power variation, but current interruptions that may cause severe corrosion problems to the electrodes, if the latter are not protected by the application of a polarization current when idle. 

Electrochemical electrodes are subject to interference from a number of substances and sampling conditions Can have a response time of 60-90 s, depending on temperature, sample viscosity and stirring speed Typical lifetime of just 3 months membranes must be changed frequently as they become fouled, damaged or clogged... 

It is probably the complexity of these theories that prohibited this particular aspect of electrode kinetics from being attractive for application in the study of homogeneous reaction kinetics per se. Yet it must be clear that the electrochemical techniques, together providing an extremely wide range of time scales, should be preeminently suited for investigations of both slow and (very) fast homogeneous reactions. This is the more true since, nowadays, the problem of the non-availability of a closed-form expression for the response—perturbation or response—time relation has been overcome by numerical analysis procedures conducted with the aid of computers. 

Ion-sensitive electrodes are finding increasing use and are superseding atomic absorption techniques in certain cases, partly because of the limited requirements of these electrochemical methods. Electrodes for Na+, K+ and Ca2+ are particularly important, but suffer from the drawback of slow response time. They may be of less value, therefore, in.continuously monitoring changes in concentration levels with time. Thus Ca2+-sensitive electrodes have a detection limit of about 10-8 mol dm-3, but a response time of about 2 s. 

The amperometric biosensor based on carbon paste electrode ensures proximity at the molecular level between the catalytic and electrochemical sites because the carbon electrode is both the biocatalytic phase and the electrode sensor (Table 17.2). The tissue containing carbon paste can be incorporated in various electrode configurations and these have very rapid response times, extended lifetimes, high rigidity, mechanical stability and very low cost. 

The group of Ruhr pursued an approach where enzymes were only immobilized on specific areas of the electrode. The electrochemical detection was performed on the unmodified regions of the same electrode leading to faster response times of the sensor [58]. The authors used SECM to show the different kinetics at the modified and unmodified regions of the sensor surface [57]. SECM was (among other techniques [58,70]) used for microderivatization of the surface [63]. 

Acetylcholineesterase and choline oxidase Prepared by mounting a carbon fiber (200 pm diam) in a glass capillary with silver paste and epoxy resin, electrochemically pretreating the electrode from 0 to 1.2 V for 15 min, and dipping the electrode in 11% PVA-Styryl pyridinium solution containing AChE and ChO. The calibration graph was rectilinear from 0.2 to 1 mM of ACh. The response time was 0.8 min. [76]... 

Radiometer pOz electrode, type E 5046 consists of a platinum cathode (20 /am diameter) and silver-silver chloride reference electrode placed in an electrochemical solution behind a 20 /am thick polypropylene membrane. A polarizing voltage of about 650 mV is applied. The polarographic current is about 10 " A per mm Hg of oxygen tension at 38°C. Zero current is lower than 10 A, response time less than 60 sec at 38°C 99% of full deflection. The PO2 electrode is used with the pH-Meter 27 GM or the Astrup Micro-Equipment, in conjunction with the Oxygen Monitor. The scale can be calibrated to the range 0-100 mm Hg p02. Thermostated cells provide measurements at constant temperature of volumes down to 70 /al. The small volume makes this cell useful to measure the PO2 of capillary blood. The cell is supplied with accessories for blood sampling. 

Nanoparticles have also been used successfully as amperometric gas sensors. Chiou and co-workers developed a dispersed catalyst gas-diffusion electrode for SO2 sensing [195]. Chloroauric acid is adsorbed on carbon black and subsequently reduced in a stream of hydrogen to obtain nanometer-sized particles. These are then shaped in the form of an electrode and used as a sensor. The electrochemical oxidation of SO2 gas is catalyzed by the nanoparticles with a fast response time. A... 

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