Microelectrodes chemical sensors

The reduction in size of conjugated polymer chemical sensors has been shown to lead significant performance improvements. In one example, Seeber and coworkers [23] compared the electrochemical detection of ascorbic acid with both 3 mm and 10 pm diameter platinum electrodes coated with polythiophene. The microelectrode was found to give lower detection limits and to operate in poorly conductive electrolytes, which are commonly encountered in practical applications. 

Tan, F., Metiers, J.P. and Banks, C.E. (2013) Electro analytical applications of screen printed microelectrode arrays. Sensors and Actuators B Chemical, 181, 454-462. 

Wohltjen H, Barger WR, Snow AW, Jarvis NL (1985) A vapor-sensitive chemiresistor fabricated with planar microelectrodes and a Langmuir-Blodgett organic semiconductor film. IEEE Trans Electron Dev ED-32 1170-1174 Wolfbeis OS (1991) Fiber optic chemical sensors and biosensors, vol 1. CRC, Boca Raton, FL Wolfbeis OS (1992) Fiber optic chemical sensors and biosensors, vol 2. CRC, Boca Raton, FL Wolfbeis OS (2005) Materials for fluorescence-based optical chemical sensors. J Mater Chem 15 2657-2669 Wollenstein J, Plaza JA, Can6 C, Min Y, Bottner H, Tuller HE (2003) A novel single chip thin film metal oxide array. Sens Actuators B 93 350-355... 

Fig. 2b represents a simultaneous recording obtained with a Cl -selective microelectrode. As the double-barreled microelectrode impales the peritubular membrane, there is a steep rise in the membrane potential of about - 70 mV associated with a fall in the Cl potential. The fall in the chloride potential is less steep than the rise in the peritubular membrane PD. This is due to the fact that the chemical sensor has a slower response time than the electrical cell. After recording a stable peritubular membrane PD and cell Cl potential, the double-barreled electrode was advanced a few micra into the lumen of a late proximal tubule to record two stable potentials a transepithelial PD of about 10-15 mV and a tubular fluid Cl potential roughly equivalent to the Cl" potential in the amphibian Ringer s solution covering the kidney. 

Microelectrodes can be designed in different shapes (Fig. 2.30). Their response time vs. potential or concentration changes is much shorter than that of classical electrodes. Microelectrodes are not sensitive to convection from external sources, and they need a lower supporting electrolyte content sometimes they even work without any background electrolyte. All these positive properties make them ideal chemical sensors. The problem is that they exhibit high current densities, however low total currents. Special low-amplitude current amplifiers are necessary to measure current values between femtoam-... 

Electrodes modified by electrodeposition of poly(3-methylthiophene) were used as chemical sensors of AA Multichannel electrochemical detection system fabricated using a 16-channel microelectrode array... 

Figure 24. A chemical sensor composed of polyaniline on two Au microelectrodes.

An electrochemical sensor using an array microelectrode was tested for the detection of allergens such as mite and cedar pollen (Okochi et ah, 1999). Blood was used in the assay and the release of serotonin, a chemical mediator of allergic response, which is electrochemically oxidized at the potential around 300 mV, was monitored for electrochemical detection by cyclic voltammetry. 

In Fig. 2.10, the boundary between the enzyme-containing layer and the transducer has been considered as having either a zero or a finite flux of chemical species. In this respect, amperometric enzyme sensors, which have a finite flux boundary, stand apart from other types of chemical enzymatic sensors. Although the enzyme kinetics are described by the same Michaelis-Menten scheme and by the same set of partial differential equations, the boundary and the initial conditions are different if one or more of the participating species can cross the enzyme layer/transducer boundary. Otherwise, the general diffusion-reaction equations apply to every species in the same manner as discussed in Section 2.3.1. Many amperometric enzyme sensors in the past have been built by adding an enzyme layer to a macroelectrode. However, the microelectrode geometry is preferable because such biosensors reach steady-state operation. 

Figures 1IC-E show SEM images of test patterns of silver that were fabricated using pCP with hexadecanethiol, followed by selective chemical etching [102], The SAMs protect the underlying substrates from dissolving by blocking the dilSisional access of etchants. The ability to generate arrays of microstructures of coinage metals with controlled shapes and dimensions is directly useful in fabricating sensors and arrays of microelectrodes.

H.F. Cui, J.S. Ye, Y. Chen, S.C. Chong, X. Liu, T.M. Lim and F.S. Sheu, In situ temporal detection of dopamine exocytosis from 1-dopa-incubated nm9d cells using microelectrode array-integrated biochip, Sensors and Actuators B-Chemical, 115(2), 634-641 (2006). 

Electrochemical sensors have been used as the basis or as an integral part of many chemical and biosensor developments. The introduction of microelectrode assembly added a new dimension to electrochemical sensors, and, consequently, to chemical and biosensor research. In recent years, the advancement of microelectronic fabrication technology has provided new impetus to the development of micro or miniature electrochemical sensors. 

Microfabrication and micromachining techniques have also been used in the manufacture of electrochemical sensors. This includes po and pco sensors. Zhou et al [9] describe an amperometric CO2 sensor using microfabricated microelectrodes. In this development, silicon-based microfabrication techniques are used, including photolithographic reduction, chemical etching, and thin-film metallization. In Zhou s study, the working electrodes are in the shape of a microdisk, 10 pm in diameter, and are connected in parallel. In recent years, silicon-based microfabrication techniques have been applied to the development of microelectrochemical sensors for blood gases, i.e. P02. Pcoj and pH measurements. 

In this chapter, we discuss voltammetric methods and associated electrochemical sensors, including chemically modified electrodes. Voltammetric techniques use a microelectrode for microelectrolysis. Here, the potential is scanned and a dilute solution of the analyte produces, at a given potential, a limiting current (microampere range or less), which is proportional to the analyte concentration. Am-perometry is the application of voltammetry at a fixed potential to follow, via the current, changes in concentration of a given species, for example, during a titration. Amperometric measurements also form the bases of electrochemical sensors. 

One sensor which we have developed, the carbon-fiber microelectrode, has been shown to be especially useful to obtain Information on chemical heterogeneities (4). Carbon fibers are highly conductive and are commercially available in a variety of diameters ranging from 5 to 30 pm. The sensor is prepared by sealing a single fiber with epoxy Into a glass pipette which Is pulled to a tip diameter comparable to that of the fiber. The sensing area of the... 

However, these in vivo studies are based on the principle of detection experiments carried out in the brain s extracellular fluid. They therefore do not allow for the mechanism of the chemical transmission of nervous stimuli to be analysed. In the early 1990 s, several teams were able to study this phenomenon on a living cell by placing the surface of an electrochemical sensor, such as a carbon microelectrode, a few micrometers away from a cell membrane. Currently, when the results of this detection process are analysed in (Quantitative terms, fluxes of about a thousand molecules per millisecond are shown. Therefore, the various kinetic steps involved in the release process, which is called exocytosis, can be then differentiated and analysed in terms of the biological and physicochemical parameters of the cell and its environment. 

The selective and facile incorporation of sensing materials into sensor devices is an important issue. From this point of view, PANI nanoframework was electro chemically grown between the two platinum microelectrodes [399]. The nanoframework comprised munerous intercrossing nanowires with diameters of 40-80 nm, and was utilized as resistive sensors for detecting HCl, NH3, C2H5OH, and NaCl. 

See also DNA Sequencing. Enzymes Enzyme-Based Electrodes. Forensic Sciences Blood Analysis. Immunoassays, Techniques Enzyme Immunoassays. Microelectrodes. Polarography Techniques Organic Applications. Purines, Pyrimidines, and Nucleotides. Sensors Chemically Modified Electrodes. Voltammetry Organic Compounds. 

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