Efficiency Electrochemical detection

On a glassy carbon electrode modified with nanowires of polytetrakis (o-aminophenyl)porphyrin, SWCNT and Nation as binders resulted in an efficient electrochemical detection of hydrogen peroxide [228], while detection of H2O2 in beverages was achieved with the picket-fence porphyrin, FeTpivPP, on MWCNTs [229]. 

Depending on their conversion efficiency, electrochemical detectors can be divided into two categories those that electrolyze only a negligible fraction (0.1-5%) of the electroactive species passing through the detector (amperometric detectors), and those for which the conversion efficiency approaches 100% (coulo-metric detectors). Unfortunately, the increased conversion efficiency of the analyte is accompanied by a similar increase for the electrolyte (background) reactions, and no lowering of detection limits is reahzed. 

Liquid chromatographic methods based on ultraviolet and/or electrochemical detection have also been developed (622-625). In the earliest of these methods (623), tissues were extracted with mcthanol/water (1 1), and the evaporated residue was taken up in dichloromethanc. The extract was then injected on a Kieselgel Merckosorb SI-60 liquid chromatographic column, and eluted with dichloromethane/ethanol/water. Monitoring at 280 nm allowed 5 ppb thiouracils to be readily detected in the tissue samples. In the latest method (625), cattle plasma samples were extracted with etliyl acetate in presence of ethylenediamine-tetraacetate (EDTA). The addition of ED I A could significantly improve the efficiency of the extraction process. Remarkable improvement of the ethyl acetate... 

Capillary electrophoresis (CE) has also been employed for the analysis of flavonols.17 In fact, CE methods coupled with electrochemical detection (ECD) are increasingly being employed for the analysis of flavonoids because of the minimal sample volume required, short analysis time, and high separation efficiency.17 The limit of detection for quercetin and kaempferol by CE/ECD methods are comparable to those obtained by LC/MS. The primary drawback of using CE is low repeatability of retention times as compared with HPLC. 

Selective Detectors for Analytical HPLC. Sweetser and Swartzfager (61) demonstrated that either fluorescence or electrochemical detectors are efficient for selective identification of IAA. Fluorescence detection is much more selective than UV detection since fewer compounds fluoresce than absorb UV radiation. Electrochemical detection is also specific because only compounds that may be oxidized or reduced are detected. IAA is oxidized at a low voltage potential (0.7 to 0.9 V) relative to other compounds. In our hands, these two methods of detection appear to be quite accurate, since the same plant sample yields the same quantitative data by both methods. Since they are quite different, using both detection method adds credibility to the assays. 

Electrochemical Detectors. The only electrochemical detector in current use is amperometric. However, some workers have used the term cou-lometric for detectors that operate at a high current efficiency and others have used the term polarographic when the electrode is mercury. The acronym LCEC is in common use to represent LC with electrochemical detection. 

Finckh, B. Kontush, A. Commentz, J. Flubner, C. Burdelski, M. Kohlschiitter, A. 1995. Monitoring of ubiquinol-10, ubiquinone-10, carotenoids, and tocopherols in neonatal plasma microsamples using high-performance liquid chromatography with coulometric electrochemical detection. Anal. Biochem. 232 210-216. Fleshman, M.K. Cope, K.A. Novotny, J.A. Riedl, K. Schwartz, S.J. Jones, P.J. Baer, D.J. Harrison, E.H. 2010. Efficiency of intestinal absorption of P-carotene (BC) is not correlated with cholesterol (CHL) absorption in humans. FASEB J. 24S 539.4. 

Mechanistic aspects of the action of tyrosinase and the usual transduction schemes have been summarized on several occasions [166,170-173]. In short, this copper enzyme possesses two activities, mono- and di-phenolase. Due to the predominant presence of the mono phenolase inactive form (met-form), the enzyme is inherently inefficient for the catalysis of these monophenol derivatives. However, in the presence of a diphenol, the catalytic cycle is activated to produce quinones and the scheme results in an efficient biorecognition cascade. This activation is achieved more efficiently when combined with electrochemical detection through the reduction of the produced quinones [166], as illustrated in Fig. 10.5. Consequently, a change in the rate-hmiting step can be observed through kinetic to diffusion controlled sensors with a concomitant increase in stability and sensitivity, as depicted in Fig. 10.6. 

Microbiological or immunochemical detection systems offer the advantage to screen, rapidly and at low cost, a large number of food samples for potential residues, but cannot provide definitive information on the identity of violative residues found in suspected samples. For samples found positive by the screening assays, residues can be tentatively identified and quantified by means of the combined force of an efficient liquid chromatographic (LC) separation and a selective physicochemical detection system such as UV, fluorescence, or electrochemical detection. The potential of pre- or postcolumn derivatiza-tion can further enhance the selectivity and sensitivity of the analysis. Nevertheless, unequivocal identification by these methods is not possible unless a more efficient detection system is applied. 

Electrochemical detection is very sensitive for the compounds that can be oxidized or reduced at low-voltage potentials. Therefore, it could also be applied in the HPLC analysis of phenolic acids that are present in natural samples at very low concentrations. With the recent advances in electrochemical detection, multi-electrode array detection is becoming a powerful tool for detecting phenolic acids and flavonoids in a wide range of samples. The multi-channel coulometric detection system may serve as a highly sensitive way for the overall characterization of antioxidants the coulometric efficiency of each element of the array allows a complete voltametiic resolution of analytes as a function of their reaction (redox) potential. Some peaks may be resolved by the detector, even if they are unresolved when they leave the HPLC column. ... 

As mentioned in the previous section, the response, the stability and the enzyme activity found greatly enhanced at the MWCNT platform. Other than CNTs, AuNPs also possess some unique properties and recent years it has been widely employed in the biosensors to immobilize biomolecules. Thus in this section we discuss about the application of AuNP matrix for the immobilization of AChE for pesticide sensor development. With the use of AuNPs, the efficiency and the stability of the pesticide sensor gets greatly amplified. Moreover, the nanoparticles matrix offers much friendly environment for the immobilized enzyme and thus the catalytic activity of the enzyme got greatly amplified. Interestingly, Shulga et al. applied AChE immobilized colloidal AuNPs sensor for the nM determination of carbofuran, a CA pesticide [16], The enzyme-modified electrode sensor was also utilized for the sensitive electrochemical detection of thiocholine from the enzyme catalyzed hydrolysis of acetylthiocholine chloride (ATCl). The fabrication and the enzyme catalyzed reaction at the AuNPs coated electrode surface is shown in Fig. 6. 

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