Miniaturized electrodes

Parylene s use in the medical field is linked to electronics. Certain pacemaker manufacturers use it as a protective conformal coating on pacemaker circuitry (69). The coated circuitry is sealed in a metal can, so that the parylene coating serves only as a backup should the primary barrier leak. There is also interest in its use as an electrode insulation in the fabrication of miniature electrodes for long-term implantation to record or to stimulate neurons in the central or peripheral nervous system, as the "front end" of experimental neural prostheses (70). One report describes the 3-yr survival of functioning parylene-coated electrodes in the brain of a monkey (71). 

A major challenge and important application is nano-wiring of electronic circuits mediated by self-assembled DNA or protein structures providing condncting connection between miniaturized electrodes [51,52]. The use of self-assembled DNA for wiring two... 

Myocardial tissue pH measurement has been used in the studies of different approaches to myocardial protection in various cardiac operations. A needle type glass membrane miniature electrode has been used in studies for pH-guided myocardial management [127], As described in the previous section, this electrode was also adapted to measure brain pH [132], The electrode has a right-angled glass electrode... 

A classical approach for a flow-through device with sensors is the clinical-chemical analyzer already introduced in several hospitals. Such devices use standard miniaturized electrodes and a sophisticated fluid handling system which is bulky [90]. 

For the probing of these microenvironments, appropriately-sized electrochemical sensors and electrodes are necessary, often of nanometre linear dimension, unless they can be incorporated in the walls of the electrochemical cell. By etching, disc microelectrodes with radii of as small as 2 nm have been fabricated [15] nevertheless, problems of interelectrode reproducibility can be large at such miniature electrodes. 

There has been a trend toward electrochemical reactions in lab-on-a-chip devices in the last few years.51,52 This is mainly because miniaturized electrodes can be fabricated using microfabrication methods and solutions can be transferred by microfluidics approaches.5354 Flow injection analysis and sequential injection analysis techniques were also employed for electrochemical enantioselective high-throughput screening of drugs.55... 

On the basis of these initial considerations, a plate-based electrochemical robotic system was conceived (Fig. 14.14). The system was described in detail by Erichsen et al. in 2005.66 However, the first prototype was introduced already in 2001.67 Standard microtiter plates are used as reaction wells in which the miniaturized electrodes are immersed in an automated fashion to perform electrochemical experiments. Accurate positioning of the miniaturized electrodes is achieved by... 

Fig. 14.15 Miniaturized electrodes for usage in the electrochemical robotic system. From left to right 3M Ag/AgCl reference electrode, platinum microelectrode, dual platinum microelectrode, stainless steel cannula that serves as pump outlet and counter electrode 14...

Many reactions that take place in living cells are redox reactions. These reactions can be studied with miniature electrodes. 

In a biosensor application, Wolfson and his research group [10] employed microfabrication techniques to produce miniature electrodes for potential implantable, non-enzyme-based glucose sensors. Thin-film techniques, such as DC magnetron-sputtered processing, were used to deposit platinum film or platinum film enhanced with titanium film on quartz substrates. The geometric shapes of the quartz substrate include cylindrical rods and rectangular plates in a... 

Clark GJ, Langley D, BushneU ME (1995) Oxygen limitation can induce microbial secondary metabolite formation investigations with miniature electrodes in shaker and bioreactor culture. Microbiology 141 663 - 669... 

An interesting approach to miniature electrodes is the coating of a fine platinum wire with a mixture of a liquid ion-exchanger and poly(vinyl chloride) or poly(methyl methacrylate) [14]. Although the mechanism by which these electrodes function is not known (there is no internal reference electrode), they do appear to function almost as well as the larger commercial varieties. 

We define microelectrodes as miniature electrodes in which the critical electrode dimension is of the order of 10 pm yet it remains much greater than the thickness of the electrical double layer, which is typically 10 to 100 A. 

Sometimes more specialized results can be achieved by using more than four electrodes Rabbani et al. (1999) combined two orthogonal four-electrode systems to achieve a more localized zone of measurement sensitivity in what he called focused impedance measurements. Kwon et al. (2012) showed that a system using 16 miniature electrodes was able to outperform the four-electrode system in sensitivity and impedanee estimation. With this method, they were also able to recover anisotropic properties from the measured object. 

Careful design of the front end (Table 2) can considerably increase the performance. The advantage is most prOTiounced for miniaturized electrodes with low sensitivity. Since the electrode impedance depends considerably on current density, it is advisable to hold the current density constant or the same in the MUT and reference branch, as it is accomplished in Table 2 A (current excitation). A symmetric Howland-type current source supplies both the MUT and the reference with a current of the same amplitude but opposite polarity. Although, a single current source with current mirror can be used as well. Availability of integrated circuits like ADS 129/8130 (Analog Devices) makes the approaches, as shown in Table 2, favorable. 

Figure 5 Configuration of capillary isotachophoresis. The arrangement shown is close to a real Instrument. The capillary is often made of plastic. The semipermeable membrane prevents electro-osmotic and hydrodynamic flows of the electrolyte but allows electromigration of ions. A conductivity detector is often used. It has miniature electrodes placed inside the capillary.

Detection limits reach ca 1 nM when background or capacitive currents are filtered or compensated, in transient methods such as differential pulse polarography (DPP) or voltammetry. Numerous redox molecules, either metabolites or drugs, may be determined in biological samples either directly or after an extraction procedure[3,4]. Particularly miniaturized electrodes using carbon fibers or paste may be implanted and allow the continuous monitoring of some metabolites and drugs in neuronal tissues or arteries[5-8]. 

Disks are the most common miniaturized electrode geometry largely because of the relative ease with which a microwire or fiber can be... 

For slow scan rates, i.e., a few volts per second or slower, the voltammetric response of a miniaturized electrode is a sigmoid-shaped steady-state voltammogram [8]. Peak-shaped responses are observed (Mily when the scan rate is sufficiently high so that the depletion layer thickness is smaller than the critical dimension of the electrode. For example, a 50-nm-radius nanoelectrode requires the sweep rate to be of the order of 10,000 Vs to show the classical peak-shaped responses typically observed for electrochemically reversible systems at macroelectrodes at mVs scan rates. The theory describing mass transport to micro- and... 

As it can be seen, the number of publications is enormous. The aim of this chapter, rather than attempting a systematic review of the field, is to show in a didactic manner the possibilities of electrochemical immunosensing for food analysis. There are some parts that could be found in other chapters of this book, such as, for example, the use of miniaturized electrodes, nanomaterials, or electrochemical techniques. However, a focus on the knowledge required for understanding their application to food analysis is always made, leaving a deeper treatment for those chapters. On the other hand, the reader can find many important references to the field in recent reviews, from the most general to the particular ones. Thus, recent trends in antibody-based sensors [2] (immunosensors) or smart electrochemical biosensors (from advanced materials to ultrasensitive devices)... 

The important trend of assay parallelization (similarly to the ELISAs (enzyme linked immunosorbent assays) performed in microtiter plates) has increased the use of miniaturized electrodes and, film electrodes, especially those fabricated by screen-printed technology are widespreading. By their importance, these will be discussed in a separate section. 

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