Electrochemical detection, advantages

One of the most common transduction techniques used in microfluidic LOC systems is electrochemical detection. Advantages of electrochemical detection include easy fabrication, integration, and miniaturization of electrodes within microfluidic devices. Although electrochemical techniques may lag behind optical detection in terms of sensitivity, nevertheless its low cost, ability to work with turbid samples, inertness to ambient light, temperature, etc., prompted researchers to incorporate it in LOC devices for POC applications. 

Whilst for the analysis of plant material for cannabinoids both GC and HPLC are commonly used, in analytical procedures the employment of GC-based methods prevails for human forensic samples. Nonetheless, the usage of HPLC becomes more and more of interest in this field especially in combination with MS [115-120]. Besides the usage of deuterated samples as internal standards Fisher et al. [121] describe the use of a dibrominated THC-COOH (see 7.5). The usage of Thermospray-MS and electrochemical detection provide good performance and can replace the still-used conventional UV detector. Another advantage in the employment of HPLC rather than GC could be the integration of SPE cartridges, which are needed for sample preparation in the HPLC-system. 

UV detection, diode-array detector (DAD) and fluorescence have been the detection techniques used, coupled to HPLC for the analysis of OTC. UV detection is set at 355 nm [49-51], 350 nm [40], or at 353 nm [52], Using the diode array detector [49] offers advantages that the target peak can be identified by its retention time and absorption spectrum. Compared to UV detection, fluorescence detection is generally more specific and is less interfered by other compounds in the sample matrix [51]. A HPLC method with electrochemical detection has also been suggested recently. Zhao et al. [53] described HPLC with a coulometric electrode array system for the analysis of OTC, TC, CTC, DC, and methacycline (MC) in ovine milk. An amper-ometric detection coupled with HPLC was developed by Kazemifard and Moore [54] for the determination of tetracyclines in pharmaceutical formulations. 

Most compounds can be detected directly as they are able to produce a direct analytical signal. Photometric detection, especially UV (including diode array and multi-wavelength UV detection) is by far the most frequently applied detection technique. The application of mass spectrometry (MS) detection in CE is attractive as it can provide structural information [44]. Hologram-based refractive index detection [45] and electrochemical detection [46,47] were also reported. Conductivity [41,48-50] and amperometric [51,52] detection has shown to have advantages for the analysis of both organic and inorganic compounds. 

High-performance liquid chromatographic separation with electrochemical detection may provide the best sensitivity for phenol quantification in biological samples. The use of gas chromatography with a flame ionization detector may be a more versatile method, if other non-ionic pollutants must be quantified. The advantages and disadvantages of different methods available for the quantification of phenol and metabolites in biological and environmental samples have been discussed by Tesarova and Packova(1983). 

Despite being relatively new technology, aptamers have a tremendous potential and can be envisioned to rival antibodies and other traditional recognition elements for molecular detection and recognition, due to their inherent affinity, selectivity, and ease of synthesis. In addition, the combination of aptasensors with electrochemical detection methods has the added advantage of further cost reduction and miniaturization of such systems. 

Immobilized Enzymes. The immobilized enzyme electrode is the most common immobilized biopolymer sensor, consisting of a thin layer of enzyme immobilized on the surface of an electrochemical sensor as shown in Figure 6. The enzyme catalyzes a reaction that converts the target substrate into a product that is detected electrochemically. The advantages of immobilized enzyme electrodes include minimal pretreatment of the sample matrix, small sample volume, and the recovery of the enzyme for repeated use (49). Several reviews and books have been published on immobilized enzyme electrodes (50—52). 

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. 

A number of other carbon materials have been used for electrochemical detection [6] however, at this point none of these appear to have a clear advantage over the electrodes described earlier. Nevertheless, these alternative materials certainly do work and there is little doubt that we will continue to see additional entries as the search for the ideal electrode continues. The chapter on carbon electrodes by McCreery and Kneten (Chap. 10) is a good place to review the fundamental issues. 

Spectroscopic detection techniques (UV, fluorescence) are the most common methods of detection employed in CE. UV detection, although the simplest method of detection to adapt to CE, suffers from a loss of sensitivity due to the extremely small pathlengths involved in CE. Laser-induced fluorescence detection is much more sensitive, but is limited by the number of wavelengths available for excitation. In addition, this technique is very expensive to implement and maintain. Electrochemical detection has several advantages for CE [47]. Since electrochemical detection is based on a reaction at the electrode surface, the cell volume can be very small without loss of sensitivity. The concentration-based limits of detection for capillary electrophoresis with electrochemical detection (CEEC) are comparable to those of LCEC. 

A DNA hybridisation assay with enzymatic electrochemical detection is carried out on a 100 nm sputtered gold thin film that allows working with small volumes. Reducing the cell volume has several advantages [49]. The first one is the decrease in the diffusion distances required for analytes to reach their surface-bound receptor partners. Moreover, in the case of enzymatic detection, the product dilution, a critical factor in achieving low detection limits, diminishes. A simple, cheap and easy-to-handle homemade device that permits to perform simultaneous hybridisation procedures and sequential detection of more that 20 assay sites is presented. 

What are the advantages of carbon over metal for fabrication of electrodes in electrochemical detection (2 marks)... 

Based upon the advantages of the other techniques presented prior to LC-MS, large volume injection HPLC-UV, and HPLC-CAD, the decision to use electrochemical detection would be driven primarily by a unique analytical need, equipment availability and previous experience of the analytical chemist. A complex chemical matrix should not be of concern at most there could be some residual cleaning agent and residual excipients in addition to the active pharmaceutical ingredient. Since the matrix in cleaning verification is typically simple, electrochemical detection would not be the primary detection technique. However, the sensitivity afforded by ECD is excellent and can meet the most stringent of the acceptance limits outlined in Table 15.2. 

In conclusion, we have tried to present the principles of LCEC, describe the present applications that have been made, and survey the areas of potential utility by reviewing pertinent chemistry and related methods. It is our opinion that the success of LCEC in neurochemistry can carry over to pesticides owing not only to the advantages of electrochemical detection but also to the tremendous potential of its adjunct, liquid chromatography. 

Another sensor based on a fiber-optic-based spectroelectrochemical probe uses a DNA/ethidium bromide system to take advantage of the biological recognition processes [92]. The concept of immobilizing electrochemical reagents on the end of an optical fibre is a useful addition to the field of bioanalytical sensors. Before this development, optical and electrochemical detection of DNA were performed separately. Optical and electrochemical detection of DNA are suitable for a DNA detection system [93, 94] and these techniques will enable a production of a cheap DNA biosensor with a rapid and quantitative response. 

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