Direct current amperometry

Direct-current amperometry (the measurement of electrochemical current in response to a fixed electrode potential) continues to be the most widely used finite-current electrochemical technique. Popular applications include endpoint... 

When the purpose of the working electrode is to act as an inert electron sink, as in the detection of catecholamines, carbon is the preferred electrode material. On occasions when the electrode plays a direct role in the reaction, the precious metals are chosen. For example, silver can be oxidized to silver cyanide in the presence of cyanide ions. A major consideration when choosing an electrode material is its ability to maintain an active surface. Electrodes will develop a layer of surface oxide at positive applied potentials. The oxide layer will inhibit the oxidation of the analyte, and the response will decrease with repeated injections. The active surface can be renewed by polishing the electrode. Since glassy carbon electrodes are more resistant to poisoning by oxide formation, they are the electrode of choice for direct current amperometry. 

Voltammetry and amperometry are the common methodologies which are suitable for real time detection of electrochemically active compounds [4]. For wireless transmission of DA level, an electrochemical sensing system based on direct current amperometry unidirectionally transmitted via an infrared (IR) system was developed [5]. However, the IR transmission is easily interrupted between TX and RX if the light transmission pathway has been blocked. An improved one-way telemetric system, called optoelectronic transmission system (OPT), utilized 10 photodiodes as RX to ameliorate the interrupted event [6]. In addition to IR transmission, Bluetooth-based wireless... 

We wish only to remind readers that there are three main methods of electrochemical re-vealment conductivity, direct current (d.c.) amperometry, and integrated amperometry (pulsed amperometry is a form of integrated amperometry). In revealment by conductivity, the analytes, in ionic form, move under the effect of an electric field created inside the cell. The conductivity of the solution is proportional to the mobility of the ions in solution. Since the mobile phase is itself an electrolytical solution, in order to increase the signal/noise ratio and the response of the detector, it is very useful to have access to an ion suppressor before the revealment cell. By means of ionic exchange membranes, the suppressor replaces the counterions respectively with H+ or OH , allowing only an aqueous solution of the analytes under analysis to flow into the detector. 

Direct current (DC) amperometry is used for the analysis of catecholamines, phenols, and anilines, which are easy to oxidize. A single potential is applied, and the current is measured. The current resulting from the oxidation or reduction of analyte molecules is dependent on many factors, including the concentration of the analyte, temperature, the surface area of the working electrode, and the linear velocity of the flowing stream over the surface of the working electrode. 

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. 

An amperometric technique relies on the current passing through a polarizable electrode. The magnitude of the current is in direct proportion to the concentration of the electroanalyte, with the most common amperometric techniques being polarography and voltammetry. The apparatus needed for amperometric measurement tends to be more expensive than those used for potentiometric measurements alone. It should also be noted that amperometric measurements can be overly sensitive to impurities such as gaseous oxygen dissolved in the solution, and to capacitance effects at the electrode. Nevertheless, amperometry is a much more versatile tool than potentiometry. 

Voltammetric methods are based on measurements made using an electrochemical cell in which electrolysis is occurring. Voltammetry, sometimes also called amperometry, involves the use of a potential applied between two electrodes (the working electrode and the reference electrode) to cause oxidation or reduction of an electroactive analyte. The loss or gain of electrons at an electrode surface causes current to flow, and the size of the current (usually measured in mA or pA) is directly proportional to the concentration of the electroactive analyte. The materials used for the working electrode must be good conductors and electrochemically inert, so that they simply transfer electrons to and from species in solution. Suitable materials include Pt, Au, Hg and glassy carbon. 

Amperometry. Amperometric methods measure the current produced at a working electrode in response to an applied potential. Amperometric enzyme assays rely on the production of an oxidizable or reducible species from an enzyme-catalyzed reaction. The applied potential is extreme enough to completely oxidize (at positive potentials) or reduce (at negative potentials) any analyte that contacts the working electrode. In stirred or unstirred solutions, the current produced under such mass-transport-controlled conditions is directly proportional to analyte concentration. 

Coupling between a biologically catalyzed reaction and an electrochemical reaction, referred to as bioelectrocatalysis, is the constructional principle for enzyme-based electrochemical biosensors. This means that the flow of electrons from a donor through the enzyme to an acceptor must reach the electrode in order for the corresponding current to be detected. In case a direct electron transfer between the active site of an enzjane and an electrode is not possible, a small molecular redox active species, e.g. hydrophobic ferrocene, meldola blue and menadione as well as hydrophilic ferricyanide, can be used as an electron transfer mediator. This means that the electrons from the active site of the enzyme reduce the mediator molecule, which, in turn, can diffuse to the electrode, where it donates the electrons upon oxidation. When these mediator molecules are employed for coupling of an enzymatic redox reaction to an electrode at a constant potential, the resulting application can be referred to as mediated amperometry or mediated bioelectrocatalysis. 

Several types of electrochemical techniques have been used in automated systems (see Table 24.1). At first glance, their use in instrument systems appears straightforward, since each transducer converts chemical information directly into an electrical signal. Unfortunately, few applications are found for those methods involving net current flow (e.g., amperometry) because the rate of mass transfer (and hence the current) depends on the sample flow-rate, which may vary, and on how clean the electrode surface is. This discussion will therefore be restricted to potentiometry, a zero-current technique. 

In order to screen mutants with improved direct electron transfer, it is necessary to use an electrochemical screening system. Currently, only a few electrochemical screening methods were described in literature such as the system developed by the Bartlett group used to screen NADH electro-oxidation. This system uses a multichannel potentiostat with sixty electrodes to screen zinc(n) or ruthenium(ii) complexes bearing the redox phenidione as a mediator for NADH oxidation. It allows the complete evaluation of the electrochemical kinetic constants of the mediators and the immobilization procedure. Unfortunately, this system could only be used with a single electrolyte solution for all the electrodes (e.g., when a single reaction condition or enzyme is assayed), and it requires mL-scale reaction volumes. Recently, another system was described which makes it possible to screen bioelectrocatalytic reactions on 96 independent electrodes screen-printed onto a printed-circuit-board. It showed the possibility to screen direct or mediated electron transfer between oxidoreductases and electrode by intermittent pulse amperometry at the pL-scale (Fig. 6). The direct electron transfer assay was validated with laccase and unmodified electrodes.As an example of the mediated electron transfer assay, the 96 carbon electrodes were modified by phenazines to sereen libraries of a formate dehydrogenase obtained by directed evolution. ... 

In amperometry, quantification of species concentration is possible because the magnitude of the current generated in any given experiment is determined by the number of molecules, of that species, oxidized or reduced at the surface of the sensing electrode and therefore is directly proportional to the concentration of the molecule detected as defined by Faraday s law (Eq. (1)). 

With a method in hand for routinely constructing cytochrome c oxidase modified electrodes that exhibited direct electron transfer between the electrode and the oxidase, amperometry was used to detect reduced cytochrome c in solution at the oxidase-modified electrodes in a flow injection analysis format [69]. The dialysis cell was equipped with a wall jet inlet to direct cytochrome c solution past the oxidase-modified electrodes. Figure 12 shows the current response for three sequential reduced cytochrome c injections. Control experiments conducted at bilayer modified electrodes containing no oxidase showed current responses that are about 2% of those shown in Figure 12. This response may be due to changes in electrode capacitance and/or cytochrome c reacting at bilayer defect sites on the electrode. QCM measurements showed that no cytochrome c incorporated into the bilayer. However, cytochrome c was electrostatically held at the surface of the bilayer membrane at lower ionic strength [69]. 

In contrast, constant potential amperometry has allowed the quantitative aspects of single exocytotic release events to be studied in detail. This technique provides specific information on the amplitude, kinetics, and location of individual release events from single cells. Secretion is resolved as a series of current spikes that represent the electrooxidation of released substances. Wightman et al. have shown that each amperometric current spike detected represents the oxidation of neurotransmitter from a single exocytotic event [8]. In addition, the technique holds the potential to provide clues about the fusion pore complex, which manifests itself as a pre-spike foot that is observed directly prior to some release events. A drawback of this technique, however, is that chemical identification must be sacrificed for temporal resolution. This is a concern when one considers the complex biological matrix present in synaptic vesicles. 

There are several measurement modes amperometry, potentiometry, constant current, and impedance. The amperometry mode measures the probe current-position curve. The potentiometry mode measures the probe potential-position curve. The constant current mode will adjust the Z-position of the probe to maintain the current constant. A probe height-position curve is recorded. If the sample or substrate is an insulator, the curve can be translated to the topographic information. The impedance mode measures the probe impedance-position curve. If the probe scan is in the X- or T-direction, amperometry, potentiometry, and impedance are under a constant height mode. 

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