Micro electrode techniques

An attempt to confirm the partial ion binding to macromolecules is given by Carpenter et a/.254. They found by a micro-electrode technique that the internal conductivity of plysia neurons is only 5% of the conductivity of squid axoplasm or normal sea water254. They concluded that an extensive binding of ions to cellular macromolecules existed. 

Brown, K. T. Elaming, D. G. New micro-electrode techniques for intracellular work in small cells. Neuroscience 1977, 2, 813-827. 

Electrical methods of analysis (apart from electrogravimetry referred to above) involve the measurement of current, voltage or resistance in relation to the concentration of a certain species in solution. Techniques which can be included under this general heading are (i) voltammetry (measurement of current at a micro-electrode at a specified voltage) (ii) coulometry (measurement of current and time needed to complete an electrochemical reaction or to generate sufficient material to react completely with a specified reagent) (iii) potentiometry (measurement of the potential of an electrode in equilibrium with an ion to be determined) (iv) conductimetry (measurement of the electrical conductivity of a solution). 

This technique involves gradually increasing the potential applied to a micro electrode immersed in a soln of inert electrolyte contg a small quan-of an eleetroactive species. While the potential is gradually increased, the associated increase in diffusion current is monitored. An X-axis asymp-... 

Optical read out will compete with micro electrode arrays. New developments in array detectors will open new perspectives. Direct optical detection techniques will add new possibilities to bioanalytical applications in addition to fluorescence measurements presently preferred in biochips read out. 

It is possible to use cyclic voltammetry in the presence of ferrocene carboxylic acid to confirm the presence of micro-electrodes due to the typical sigmoidal-shaped profile produced [2] (Fig. 24.3). Twenty different sensors comprising micro-electrode arrays formed by this technique were analysed for reproducibility. This analysis can be performed by holding the sensors at a potential of +100 mV for 60s and recording the... 

Enzyme micro-electrode arrays, on exposure to differing concentrations of the substrate acetylthiocholine chloride (Fig. 24.4), demonstrate that above concentrations of 1 mM, responses tend towards a plateau. For this reason, all sensory inhibitory responses to pesticides were recorded in the presence of 2 mM acetylcholine. It should be noted that since sensor responses are recorded in the order of hundreds of nA, it is clear that some current amplification must be operating to achieve currents of this order of magnitude. This is particularly obvious when working electrodes of 0.5 cm2 were used, which only present a combined microelectrode array area of approximately 1 x 10 5 cm 2 per screen-printed electrode (if the total number of micro-electrodes that can be produced by this technique is 2 x 105 cm 2 [2-4]). 

Another micro-destructive technique is laser-induced breakdown spectroscopy (LIBS) which is used to analyse the paint layers. Nanosecond laser pulses vaporise a small amount of material from the surface of the painting and the amounts so lost are only 50 billionths of a gram and too small to be seen with the naked eye. The vapour passes between two high-voltage electrodes which excite the atoms and these then emit a pattern of light energy bands which identify the elements. LIBS together with Raman spectroscopy has been used to examine Russian icons which are multilayered. 

Amperometric sensors — A class of electrochemical sensors based on amperometry. A - diffusion-limited current is measured which is proportional to the concentration of an electrochemically active analyte. Preferred technique for - biosensors with or without immobilized enzymes (biocatalytic sensors). The diffusion layer thickness must be kept constant, either by continuous stirring or by means of an external diffusion barrier. Alternatively, micro electrodes can be... 

The use of CA in combination with other techniques has also lead to a general methodology for the determination of and D for electrode processes. Whereas the current in the CA technique for ordinary micro-electrodes is proportional to ns/D... 

The continuous measurement of histamine released from rat basophilic leukaemia cells has been recently reported by Niwa [190], who coupled a microdialysis probe with a histamine oxidase-based carbon electrode obtained by coating the enzyme on the top of an Os-gel-HPR layer. For the exclusion of electrochemically active compounds, they suggested an overcoating of Nafion. In addition, a sensor using micro-machining techniques has been fabricated by the authors and apphed to the monitoring of histamine release from a small mast cell colony. 

FIGURE 9.2. Photographs of a single naicrodroplet (a) injected into a solution and (b) contacted on a micro-electrode by microcapillaiy manipulation technique. 

Further work is required in order to confirm the intuitively reasonable belief that a more composite measure, such as the DS of plaque fluid calculated by Margolis et al. and discussed in section 6.2, is a more applicable (inorganic) parameter for assessing, for example, caries risk. To date, little work has been done and those studies that have been reported have, for practical reasons, invariably used small numbers of subjects and often pooled plaque samples. The availability of semi-automated micro-analytical techniques, such as capillary electrophoresis, and the development of micro-electrodes, for example, those ofVogel et al., should enable researchers to analyse site-specific samples more easily. 

The biggest pyrolytic carbon micro-electrodes obtained from this technique have a diameter of a few microns. Their exact size could be measured using an optical microscope and compared with the apparent size derived from the steady state current obtained by cyclic voltammetry. This apparent... 

Micro-electrode array mapping technique 292 Micro-gene-gun 1318... 

L6. Lloyd ClafE, C., and Swenson, O., Micro glass electrode technique for determination of hydrogen ion activity of blood and other biological fluids.. Biol. Chem. 162, 519-522 (1944). 

The ion-selective electrode is the basis of instruments that are used for the rapid determination of potassium during surgical procedures or the subsequent recovery. This method is relatively cheap and convenient to use at the patienfs bedside and, in comparison with reference methods, is both accurate and reproducible. The direct potentio-metric determination of potassium (and sodium) in small blood samples is also possible this is a useful development of micro-analytical techniques. The naturally occurring radioactive isotope °K is used to measure total body potassium (Schmidt 1992, Birch and Padgham 1993). 

The Tohoky University, Japan (Min et al.) reported a p-PEFC using micro-fabrication techniques [3]. This FC is based on two different designs, the alternatively inverted structure and the coplanar structure . These structures use Si substrates with porous SiOj layers with Pt-based catalytic electrodes and gas feed holes, glass substrates with micro-gas channels and a polymer membrane (Flemion S). In spite of a reported enhancement [4], the FC reached poor results, the power density is only 0.8 mW cm . ... 

---