Miniaturization of electrochemical

Microfabrication technology has made a considerable impact on the miniaturization of electrochemical sensors and systems. Such technology allows replacement of traditional bulky electrodes and beaker-type cells with mass-producible, easy-to-use sensor strips. These strips can be considered as disposable electrochemical cells onto which the sample droplet is placed. The development of microfabricated electrochemical systems has the potential to revolutionize the field of electroanaly-tical chemistry. 

Miniaturization of electrochemical power sources, in particular batteries and fuel cells, has been described as a critical—but missing—component in transitioning from in-lab capability to the freedom of autonomous devices and systems. - In top-down approaches, macroscopic power sources are scaled to the microlevel usually by the use of fabrication methods, often in combination with new materials. Power generation schemes that can themselves be microfabricated are particularly appealing, as they can lead to a one-stop fabrication of device/machine function with an integrated power source. 

Developments in electroanalytical chemistry are driven by technical advances in electronics, computers, and materials. Present scientific capabilities available in a research laboratory will be applicable for field measurements with the advent of smaller, less expensive, more powerful computers. Miniaturization of electrochemical cells, which can improve perfonnance, especially response time, can be implemented most effectively in the context of miniaturization of control circuitry. Concomitant low cost could make disposable systems a practical reality. Sophisticated data analysis and data handling techniques can, with better facilities for computation, be handled in real time. 

Electrochemistry is the basis of many important and modem applications and scientific developments such as nanoscale machining (fabrication of miniature devices with three dimensional control in the nanometer scale), electrochemistry at the atomic scale, scanning tunneling microscopy, transformation of energy in biological cells, selective electrodes for the determination of ions, and new kinds of electrochemical cells, batteries and fuel cells. 

Currently, there is a need for high-throughput determination of nucleic acid sequences. At present, detection systems most commonly employ fluorescence-based methods. However, wide spread applications of such methods are limited by low speed, high cost, size, and number of incubations steps, among other factors. Application of electrochemical methods in affinity DNA sensors presents likely a promising alternative, allowing miniaturization and cost reduction, and potentially allowing application in point-of-care assays. 

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. 

Pulse polarographic studies have been described using a microcell of 0.5 mL capacity, which analyzed two 1,4-benzodiazepines, with the lowest detection limit reported to date being 10-20 ng/mL of blood [199]. Detailed construction of the cell and electrode assembly was also described (shown in Fig. 26.16). Further miniaturization of this type of three-electrode cell is not practical hence further increases in sensitivity will have to rely on electrochemical detector flow cells of microliter capacity such as those used in conjunction with liquid chromatography (see Chap. 27). 

Electrochemical detection offers also great promise for CZE microchips, and for other chip-based analytical microsystems (e.g., Lab-on-a-Chip) discussed in Section 6.3 (77-83). Particularly attractive for such microfluidic devices are the high sensitivity of electrochemical detection, its inherent miniaturization of both the detector and control instrumentation, low cost, low power demands, and compatibility with micromachining technologies. Various detector configurations, based on different capillary/working-electrode... 

More recent trends aim in the direction of fabricating electrochemical protein array systems (for detecting multiple protein targets) and miniaturization of such immunoassays. These include an electrochemical protein chip with an array of 36 platinum electrodes on a glass substrate (64) and electrical immunoassays using microcavity formats down to the zmol antigen level (65). 

Electrochemical detection has attracted considerable interest for miniaturized analytical systems (134,135). The extremely small dimensions of electrochemical detectors, coupled with their remarkable sensitivity, compatibility... 

Advances in technology have permitted the miniaturization of many electrochemical systems [2]. In particular, application of modern fabrication techniques permits the manufacture of electrochemical cells of extremely small volume, Fig. 16.2. The first step in this direction was probably the development of thin-layer electrochemical cells. These contain large (2-3 mm) electrodes in a rectangular cell of dimensions typically 2 cm x l cm but of small height produced by a spacer of thickness 2-100 xm, which corresponds to a cell volume of 4-200 p.L [3]. Cell volumes have been progressively reduced, so that now measurements often have to be made with microelectrodes. 

By exploiting the microdialysis technique, the miniaturization of the whole instrumentation was achieved, also through the use of a peristaltic pump which turned out to be the only type of pump suitable for coupling to an electrochemical cell. The high sensitiviiy of the latter, in fact, revealed that all the syringe pumps tested were not really pulse-free . 

An example of miniaturized on-chip EC-CE is given in Ref. 7. The theoretical aspects of electrochemical detection have been discussed in detail in Ref. 8. 

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