Sensors electrochemical detection

Gazdik, Z., O. Zitka, J. Petrlova et al. 2008. Determination of vitamin C (ascorbic acid) using high performance liquid chromatography coupled with electrochemical detection. Sensors 8 7097-7112. 

H. Baltruschat, N.A. Anastasijevic, M. Beltowska-Brzezinska, G. Hambitzer, and J. Heitbaum, Electrochemical detection of organic gases The development of a formaldehyde sensor, Ber. Buns. Phys. Chem. 94, 996-1000 (1990). 

Multiple electrodes have been used to obtain selectivity in electrochemical detection. An early example involved the separation of catecholamines from human plasma using a Vydac (The Separation Group Hesperia, CA) SCX cation exchange column eluted with phosphate-EDTA.61 A sensor array using metal oxide-modified surfaces was used with flow injection to analyze multicomponent mixtures of amino acids and sugars.62 An example of the selectivity provided by a multi-electrode system is shown in Figure 2.63... 

J.H. Pei and X.Y. Li, Xanthine and hypoxanthine sensors based on xanthine oxidase immobilized on a CuPtCl6 chemically modified electrode and liquid chromatography electrochemical detection. Anal. Chim. Acta 414, 205-213 (2000). 

Anions play key roles in chemical and biological processes. Many anions act as nucleophiles, bases, redox agents or phase transfer catalysts. Most enzymes bind anions as either substrates or cofactors. The chloride ion is of special interest because it is crucial in several phases of human biology and in disease regulation. Moreover, it is of great interest to detect anionic pollutants such as nitrates and phosphates in ground water. Design of selective anion molecular sensors with optical or electrochemical detection is thus of major interest, however it has received much less attention than molecular sensors for cations. 

Research has been done showing that rapid pressnre-driven LC analysis can be done with little solvent consumption, demonstrating this as a viable process analytical tool. Using electrokinetic nanoflow pumps LC can be miniaturized to the point of being a sensor system. Developments in terms of sampling to enable sampling directly from a process stream, to the separation channel on a chip are critical for the application of miniaturized process LC. The components (valves and pumps) required for hydrodynamic flow systems appear to be a current limitation to the fnll miniatnrization of LC separations. Detection systems have also evolved with electrochemical detection and refractive index detection systems providing increased sensitivity in miniaturized systems when compared to standard UV-vis detection or fluorescence, which may require precolumn derivatization. 

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 ... 

An electrochemical sensor using an array microelectrode was tested for the detection of allergens such as mite and cedar pollen (Okochi et ah, 1999). Blood was used in the assay and the release of serotonin, a chemical mediator of allergic response, which is electrochemically oxidized at the potential around 300 mV, was monitored for electrochemical detection by cyclic voltammetry. 

The self-assembly of an imprinted layer on the surface of a transducer was realized through the adsorption of the template on gold, Si02, or ln02 surfaces followed by treatment with an alkylthiol or organosilane (Hirsch et al. 2003). The first example of this type of sensor was reported in 1987 by Tabushi and coworkers (1987). Octadecylchlorosilane was chemisorbed in the presence of n-hexadecane onto tin dioxide or silicon dioxide for electrochemical detection of phylloquinone, menaqui-none, topopherol, cholesterol, and adamantane. Another MlP-based sensor was... 

Berrettoni M, Carpani 1, Corradini N, Conti P, Eumarola G, Legnani G, Lanteri S, Marassi R, Torrelli D (2004) Coupling chemometrics and electrochemical-based sensor for detection of bacterial population. Anal Chim Acta 509 95-101. 

The electrochemical DNA sensor shows impressive sensitivity and selectivity. The detection limit, estimated to be 5 pM, is competitive with previously reported HCMV DNA hybridization assays based on an enzyme label.52 Introduction of a noncomplementary DNA sequence (human ETS2 DNA gene) records an electrochemical signal consistent with background noise. The electrochemical DNA sensor is therefore selective for the HCMV DNA sequence. 

We further addressed the use of the nucleic acids as biopolymers for the formation of supramolecular structures that enable the electronic or electrochemical detection of DNA. Specifically, we discussed the use of aptamer/low-molecular-weight molecules or aptamer/protein supramolecular complexes for the electrical analysis of the guest substrates in these complexes. Also, nucleic acid-NPs hybrid systems hold a great promise as sensing matrices for the electrical detection of DNA in composite three-dimensional assemblies. While sensitive and selective electrochemical sensors for DNA were fabricated, the integration of these sensor configurations in array formats (DNA chips) for the multiplexed analysis of many DNAs can also be envisaged. 

Another recent development is the advent of pulse amperometry in which the potential is repeatedly pulsed between two (or more) values. The current at each potential or the difference between these two currents ( differential pulse amperometry ) can be used to advantage for a number of applications. Similar advantages can result from the simultaneous monitoring of two (or more) electrodes poised at different potentials. In the remainder of this chapter it will be shown how the basic concepts of amperometry can be applied to various liquid chromatography detectors. There is not one universal electrochemical detector for liquid chromatography, but, rather, a family of different devices that have advantages for particular applications. Electrochemical detection has also been employed with flow injection analysis (where there is no chromatographic separation), in capillary electrophoresis, and in continuous-flow sensors. 

An HPLC method using progressive electrochemical detection of SPA was described by McCabe and Acworth (128). Samples were mixed with hexane, and SPA were extracted with acetonitrile. An HPLC analysis of the extracts was performed, without an evaporation step, on a high-pressure Coul Array system in which analytes were detected on two coulometric array-cell modules, each containing four electrochemical sensors attached in series after the column. Analytes were separated on a Supelcosil LC-18, 5-/tm column using gradient elution and detected at potentials of —50, 0, 70, 250, 375, 500, 675, and 825 mV. To remove oxidative impurities to be coeluted with BHT, a guard cell with applied potential of 900 mV was also placed in the system. 

In recent years, gas sensors operating at room temperature are becoming increasingly more important in many fields. These sensors can be used as so called "cordless sensors", because they need no external electric sources to heat the sensor elements. Although electrochemical gas sensors which utilize liquid electrolytes are available to detect inorganic gases, e.g., 02, CO, Cl2, H2S, etc. at room temperature (1-3), they often have time-related problems such as leakage and corrosion. The problems are minimized if solid electrolytes are used in place of liquid electrolytes. 

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