Amperometric Studies

In this section the use of amperometric techniques for the in-situ study of catalysts using solid state electrochemical cells is discussed. This requires that the potential of the cell is disturbed from its equilibrium value and a current passed. However, there is evidence that for a number of solid electrolyte cell systems the change in electrode potential results in a change in the electrode-catalyst work function.5 This effect is known as the non-faradaic electrochemical modification of catalytic activity (NEMCA). In a similar way it appears that the electrode potential can be used as a monitor of the catalyst work function. Much of the work on the closed-circuit behaviour of solid electrolyte electrochemical cells has been concerned with modifying the behaviour of the catalyst (reference 5 is an excellent review of this area). However, it is not the intention of this review to cover catalyst modification, rather the intention is to address information derived from closed-circuit work relevant to an unmodified catalyst surface. 

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J.P. Hart and I.C. Hartley, Voltammetric and amperometric studies of thiocholine at a screen-printed carbon electrode chemically modified with cobalt phthalocyanine studies towards a pesticide sensor, Analyst, 119 (1994) 259-263. 

Fig. 20.2. Amperometric study of the percentage of inhibition as function of the concentration of the enzyme in presence of 50 ppb of methyl mercury using phosphate buffer/toluene mixture and glucose oxidase biosensor. Phosphate buffer pH 6.0 Eapp — 0.60 V vs. Ag/AgCl reaction time = 5 min and incubation time = 10 min.

A PPy/PQQ modified GC electrode was used for amperometric detection of V-type nerve agent decomposition products. The electropolymerization of pyrrole was efficiently used for immobilization of the biocatalyst, PQQ. The introduction of CaCl2 as a supporting electrolyte during electrodeposition significantly improves the response of the sensor to DMAET and DEAET. Amperometric studies targeted to detection of DMAET and DEAET by PPy/PQQ electrode were performed at a constant potential set at 0.25 V, and the electrode characteristics such as sensitivity and the analyte detection limit were determined. 

E is for an electron transfer at the electrode, indicates a homogeneous electron transfer and C is a homogeneous chemical reaction. During a chrono-amperometric study of a process believed to go by a mechanism such as that above, the response did not fit theoretical data for the ECE mechanism and it was necessary to include the homogeneous electron transfer (70) in the mechanism in order to account for the data (Hawley and Feldberg, 1966). A detailed theoretical study of this problem has shown that the E step predominates over the second E step in situations where it would be possible to distinguish between the two (Amatore and Saveant, 1977, 1978, 1979, 1980). 

Overall, both voltammetric and amperometric studies convincingly demonstrated remarkably prolonged lifetime of extracellular DA in DAT-KO mice (36,38). However, both investigations also revealed a significantly decreased (by 75% in voltammetric experiments and by 93% in amperometric studies) amount of... 

The reactivity towards St is higher for the inner terms of the series (DPA+ and p+ ) too stable or too reactive ions are demonstrated to be unsuitable for acting as St polymerization initiators. Mengoli and Vidotto62 have recently published an amperometric study on the reactivity of DPA and p towards some organic monomers. In this study the authors clarified the important role played by the monomer in addressing the mechanism of the evolution of the DPA+ ... 

Amperometric studies, with a working potential of -500 mV, gave a detection limit of 0.75 ppm for carbon tetrachloride. The amperometric calibration curves for carbon tetrachloride and perchloroethylene are shown in Figure 137. Greatest sensitivities were seen for haloalkanes. 

Napier, A. and J.P. Hart (1996). Voltammetric and amperometric studies of selected thiols and dimethyldisulfide using a screen-printed carbon electrode modified with cobalt phthalocyanine Studies towards a gas sensor. Electroanalysis 8(11), 1006-1013. 

Stanca, SE Popescu, IC. Amperometric Study of the Inhibitory Effect of Carboxilic Acids on Tyrosinase. Journal of Molecular Catalysis, 2004 27, 221-225. 

SECM is a scaiming-probe teclmiqiie introduced by Bard et aJ in 1989 [49, and M ] based on previous studies by the same group on in situ STM [ ] and simultaneous work by Engstrom et aJ [53 and M], who were the first to show that an amperometric microelectrode could be used as a local probe to map the concentration profile of a larger active electrode. SECM may be envisaged as a chemical microscope based on faradic current changes as a microelectrode is moved across a surface of a sample. It has proved iisefiil for... 

Selecting the Voltammetric Technique The choice of which voltammetric technique to use depends on the sample s characteristics, including the analyte s expected concentration and the location of the sample. Amperometry is best suited for use as a detector in flow systems or as a selective sensor for the rapid analysis of a single analyte. The portability of amperometric sensors, which are similar to po-tentiometric sensors, make them ideal for field studies. 

Studies of the polarized IBTILE provide a fundamental knowledge that makes it possible to explain phenomena occurring at the membranes of ion-selective electrodes. In addition, the rates of ion transfer and assisted ion transfer reactions are proportional to concentrations, which is a basis of an amperometric ion-selective (sensitive) electrode. 

Enzyme linked electrochemical techniques can be carried out in two basic manners. In the first approach the enzyme is immobilized at the electrode. A second approach is to use a hydrodynamic technique, such as flow injection analysis (FIAEC) or liquid chromatography (LCEC), with the enzyme reaction being either off-line or on-line in a reactor prior to the amperometric detector. Hydrodynamic techniques provide a convenient and efficient method for transporting and mixing the substrate and enzyme, subsequent transport of product to the electrode, and rapid sample turnaround. The kinetics of the enzyme system can also be readily studied using hydrodynamic techniques. Immobilizing the enzyme at the electrode provides a simple system which is amenable to in vivo analysis. 

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. 

SECM employs a mobile UME tip (Fig. 3) to probe the properties of a target interface. Although both amperometric and potentiometric electrodes have found application in SECM, amperometry - in which a target species is consumed or generated at the probe UME - has found the most widespread use in kinetic studies at liquid interfaces, as... 

The electrolyte dropping electrode has found particular application in the study of ion transfer at the polarized ITIES, with an emphasis on analysis. A range of species have been detected amperometrically by measuring the transport-limited current ... 

The nature of mass transport in MEMED has been confirmed with both ampero-metric and potentiometric studies of bromine transfer from an aqueous phase to DCE [79]. Figure 17 shows typical amperometric data for this case, in which the DCE phase acts as a sink for Br2, and a depleted region of Br2 is measured adjacent to the droplet in the aqueous phase. Video images are also provided, which correspond to particular times during the amperometric transient at position (3) the edge of the developing concentration boundary layer, around the drop, reaches the electrode the concentration profile is then mapped out between points (3) and (4). The measured current, i, can be related to the local concentration, c, via... 

In scanning electrochemical microscopy (SECM) a microelectrode probe (tip) is used to examine solid-liquid and liquid-liquid interfaces. SECM can provide information about the chemical nature, reactivity, and topography of phase boundaries. The earlier SECM experiments employed microdisk metal electrodes as amperometric probes [29]. This limited the applicability of the SECM to studies of processes involving electroactive (i.e., either oxidizable or reducible) species. One can apply SECM to studies of processes involving electroinactive species by using potentiometric tips [36]. However, potentio-metric tips are suitable only for collection mode measurements, whereas the amperometric feedback mode has been used for most quantitative SECM applications. 

A microhole-based ITIES has been used by Osborne et al. for amperometric determination of ionic species in aqueous solutions [12]. They studied the assisted ammonium transfer with DB1816 at the water-DCE interface. Because the concentration of iono-phore in the organic phase was high, the measured steady-state current was proportional to the concentration of ammonium in the aqueous phase. The time required to reach a steady state was relatively short (e.g., 5 s for an 11/xm hole). A linear relationship was found between the steady-state plateau current and the ammonium concentration over the range 1 to 500/aM. 

Y. Yamamoto. Studies on Amperometric Ion-Selective Electrode Sensors. PhD Thesis, Kyoto University, Kyoto 1991. 

Amperometric parameters have been studied and optimized for sugar separations.57236 The orcinol reaction has also been used as a post-column detection system for sugars.237... 

Wangfuengkanagul and Chailapakul [9] described the electroanalysis of ( -penicillamine at a boron-doped diamond thin film (BDD) electrode using cyclic voltammetry. The BDD electrode exhibited a well-resolved and irreversible oxidation voltammogram, and provided a linear dynamic range from 0.5 to 10 mM with a detection limit of 25 pM in voltammetric measurement. In addition, penicillamine has been studied by hydrodynamic voltammetry and flow injection analysis with amperometric detection using the BDD electrode. 

M.D. Gouda, M.S. Thakur, and N.G. Karanth, Stability studies on immobilized glucose oxidase using an amperometric biosensor. Biotech. Techniq. 11, 653-655 (1997). 

Due to the broad substrate specificity of AP, and the drive for higher efficiency, several studies have recently investigated the suitability of alternative substrates to the common 4-APP [47, 48], For example, Pemberton et al. compared 4-APP and 1-naphthyl phosphate (1-NP) as AP substrates in an amperometric immunosensor for progesterone [47], The signal generation scheme when 1-NP is used as a substrate is illustrated in Scheme 2. 

A similar study has also been conducted to determine the suitability of ascorbic acid 2-phosphate (AAP) as an alternative substrate to 4-AP for AP under identical conditions [48], Although 4-APP and AAP were suitable substrates for amperometric immunosensors, 4-APP was superior owing to its sixfold faster enzymatic reaction and lower detection potential (approximately 200-400mV). Notably, the lower detection potential for the hydrolysis product of 4-APP minimizes interferences from other species and hence improves the sensitivity of the immunosensor. 

E.J. Moore, M. Pravda, M.P Kreuzer, and G.G. Guilbault, Comparative study of 4-aminophenyl phosphate and ascorbic acid 2-phosphate, as substrates for alkaline phosphatase based amperometric immunosensor. Anal. Lett. 36, 303-315 (2003). 

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