Oxalate sensors

An oxalate sensor by immobilizing spinach tissue as the source of oxalate oxidase was developed by Li [50], The sensor responds linearly to oxalate concentration in the range of 1.0-100 pM with a detection limit of 0.6 pM. The sensor was stable for 30 days when stored at 4°C and a complete analysis for the determination of oxalate could be performed in 1 min including sampling and washing. Considering the low cost of the plant tissue and simple procedure for plant tissue immobilization, this report is most valuable. 

Why is the operational stability of an enzyme-based thermal sensor for oxalic acid expected to be poorer than an amperometric oxalate sensor prepared using the same enzyme, oxalate oxidase ... 

Another type of sensor was based on the utilization of glucose dehydrogenase enzyme coupling with /ra(2,2 -bipyridylruthenium(II) complex [31]. This sensor can be used in the 10-2500-pmol/L concentration range. Several interferences occur, like NADH, oxalate, proline, and tripropylamine. However, gluconic acid and NAD+ do not interfere. 

Most of the sensors using a consumable reagent are based on a chemiluminescence phenomenon that is revealed by a reagent such as trichlorophenyl oxalate (TCPO) and a fluorophore (usually a fluoranthene), or luminol. 

In contrast with the sensors described elsewhere in this Chapter, the device proposed by the authors group uses no reagent, but photons, to induce a photochemical reaction, and involves electrochemical detection of the photochemical product, which allows one to continuously monitor the formation of the electroactive product. Kinetic monitoring increases the selectivity of determinations by eliminating matrix effects and the contribution of side reactions, whether slower or faster than the main reaction. The electrochemical system chosen for implementation of this special sensor was the Fe(II)/C204 couple, which was used for the kinetic determination of oxalate ion based on the following reaction ... 

Ammonium, citrate and oxalate 8 PVC sensors — Fermentation broth [97]... 

Oxalate is determined by using either oxalate decarboxylase (246-248) or oxalate oxidase (242,249) immobilized electrodes. In the former, the CO2 liberated and detected is proportional to the logarithm of the oxalate concentration. Linearity is reported from 0.2 to 10 mM, and the electrodes are stable for more than one month. Human control samples, spiked with oxalate, have been analyzed (246). Oxalate oxidase can be immobilized onto an O2 electrode (242), an amperometric H2O2 sensor (246, 249, 250), and a potentiometric CO2 probe (246) ... 

The use of peroxide sensors has been extended to the sensing of glucose, lactate, alcohol, oxalate, glutamate and other amino acids by coimmobilization of the respective oxidase on top of the peroxidase [7,8,228,229], and furthermore, to afl nity-based assays [130,131,217,273]. An octane sensor was created by layering a porphyrin type P450-mimics on a screen-printed HRP-modified carbon electrode (Fig. 2.13) [106]. The biomimetic catalyst (iron(III)-meso-tetrakis-(pentafluorophenyl)-p-tetrasulfonatopor-phyrin chloride) was linked to the electrode with polyallylamine on the basis... 

It is evident from the equation that potentiometric CO2 electrodes as well as amperometric O2 or H2O2 electrodes can be used as transducers. Both potentiometric and amperometric sensors have been covered by a layer of oxalate oxidase protected by a dialysis membrane (Bradley and Rechnitz, 1986 Rahni et al.f 1986a). The sensors had a pH optimum at pH 3.5-4. Diffusion control was reached at 1 U oxalate oxidase per electrode. Oxalate determination was not affected by ascorbic acid or amino acids. The hydrogen peroxide-detecting sensor (Rahni et al., 1986a) has been used to measure oxalate in urine diluted 1 40. 

Bouropoulos N., Kouzoudis D., and Grimes C. 2005. The real-time, in situ monitoring of calcium oxalate and brushite precipitation using magnetoelastic sensors. Sensors and Actuators B 109 227-232. 

For the determination of AA, two non-enz3miatic sensors based on PANI-CNT composites were developed that differ from each other in the electrode composition. The catal ftic effect of copper nanoparticles (CuNPs] was utilized in one of the approaches [47] and the other was based on an electropolymerized poly(2,5-dimethoxyaniline]-NH2-MWCNT film [48]. Both sensors showed selective oxidation of AA in the presence of DA and UA at 0.4 and 0.28 V, respectively. The interference from 10 pM glucose, oxalic acid, fructose, lactose, NaCl, sucrose, and tartaric acid was negligible in 0.1MAA[47]. 

B ard and co-workers in 1977 observed ECL emission from oxalate in acetonitrile [8]. Later, they observed it in aqueous solutions following mechanism as shown in Eqs. (2.13)-(2.19) for the detection of oxalate and Ru(bpy)3 [10, 14]. In the following years, a fiber-optic-based ECL sensor for oxalate detection was constructed [18], and the reaction mechanism of the oxalate system was further investigated and applied to study the electron transfer at liquid/liquid interface [19]. Simultaneous electrochemical and ECL detection of oxalate was reported in 2000 by Forster et al. [20]. 

Egashira N, Kumasako H, Ohga K (1990) Fabrication of a fiber optic based electrochemiluminescence sensor and its application to the determination of oxalate. Anal Sci 6 903-904... 

Feed water Tracer dye (Malachite Green oxalate) Optical (transmittance sensor) PTFE tube Gunther et al. [13]... 

The ill-defined nature of the temperature at the reaction interface is exacerbated by the enthalpy of the specific reaction. This energy, which is evolved or absorbed by the reaction, is generally large compared with the heat capacity of the reactants and products. Hence, the actual temperature at the reaction interface may differ substantially from that of the nearby temperature sensor. Consider the oxidation of carbon monoxide that occurs during the decomposition of oxalates in air. The thermocouple near the sample may indicate 300 °C while the actual specimen is glowing a bright red (indicating at least 600 °C). Conversely, for the more typical endothermic decomposition it is necessary to supply heat to maintain the reaction. 

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