Carbon fiber electrodes selectivity

Figure 1 Electrochemical detection of catechol, acetaminophen, and 4-methyl catechol, demonstrating the selectivity of differential pulse detection vs. constant potential detection. (A) Catechol, (B) acetaminophen, and (C) 4-methylcatechol were separated by reversed phase liquid chromatography and detected by amperometry on a carbon fiber electrode. In the upper trace, a constant potential of +0.6 V was used. In the lower trace, a base potential of +425 mV and a pulse amplitude of +50 mV were used. An Ag/AgCl reference electrode was employed. Note that acetaminophen responds much more strongly than catechol or 4-methylcatechol under the differential pulse conditions, allowing highly selective detection. (Reproduced with permission from St. Claire, III, R. L. and Jorgenson, J. W., J. Chromatogr. Sci. 23, 186, 1985. Preston Publications, A Division of Preston Industries, Inc.)...

Surface modified NO sensors incorporate an electrode surface that has been modified or treated in some way so as to increase the selectivity of the sensor for NO and promote catalytic oxidation of NO. An early example of such a sensor was presented by Malinski and Taha in 1992 [27], In this publication an —500nm diameter carbon fiber electrode was coated with tetrakis(3-methoxy-4-hydroxyphenyl)porphyrin, via oxidative polymerization, and Nation. This electrode was shown to have a detection limit of — lOnM for NO and great selectivity against common interferences. However, recently it has been shown that this electrode suffers severe interference from H202 [28],... 

Electrochemical detectors for liquid chromatography have reached a level of maturity in that thousands of these devices are used routinely for a variety of mundane purposes. Nevertheless, the technology is advancing rapidly in several respects. Multiple electrode and voltammetric detectors have been developed for more specialized applications. Small-volume transducers based on carbon fiber electrodes are being explored for capillary and micropacked columns. Recently, electrochemical detection has also been coupled to capillary electrophoresis [47]. Finally, new electrode materials with unique properties are likely to afford improved sensitivity and selectivity for important applications. 

Exocytosis that results in the secretion of 5-hydroxytryptophan and insulin has been monitored from single pancreatic -cells (human, porcine, canine, mouse, and cultured tumor cells), using 9-gm-diameter carbon fiber electrodes [110]. Unmodified electrodes allowed 5-HT quantitation, while modification with a ruthenium ox-ide/cyanoruthenate film allowed selective quantitation of insulin at electrodes positioned 1 pm away from cell surfaces. 

Recently, another type of sensor has been developed to directly detect individual exocytotic events. Xin et al. have simultaneously measured Ca + and catecholamine following their secretion from individual cells using a multidimensional microsensor based on both electrochemistry and fluorescence [45], The surface of a carbon fiber microelectrode is modified with the fluorescent dye calcium green-1 dextran, as this dye is a selective chelator for Ca. The fluorescence response linearly increases with bound Ca +, and the large size of this molecule prevents overconcentration of the reagent at the sensor tip. The dye is attached to the tip of a carbon fiber electrode by cross-linking with 5% glutaraldehyde. 

Another miniaturized pH-measuring device was manufactured by modifying a carbon-fiber electrode to develop a reagentless sensor system to measure in vivo pH values. Carbon-based electrodes are ideal candidates to fabricate such devices due to their biocompatibility. In addition, carbon-fiber electrodes are conductive and their surface can easily be modified as desired, in this case by ion-selective materials. A number of methods are available for tethering the ion-selective molecules such as acidic oxidation in combination with plasma treatment, physical adsorption, and covalent bonding of amines via oxidation or bonding of diazonium groups via reduction. ... 

Figure 13 Electropherogram of selected amino acids with end-column addition of 1 mM Ru (bpy)32+. Separation conditions 20 kV with injection of analytes for 8 s at 20 kV. Capillary, 75 im id, 62 cm long with a 4-cm detection capillary. Buffer 15 mM borate, pH 9.5. The electrode used for in situ generation of Ru(bpy)33+ was a 35-jlm-diameter carbon fiber, 3 mm long held at 1.15 V versus a saturated calomel electrode. The PMT was biased at 900 V. Peak identification (1) 100 fmol TEA, (2) 70 fmol proline (3) 1.6 pmol valine, (4) 50 pmol serine. Injection points. (From Ref. 97, with permission.)...

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