Electrochemical sensor types amperometric

There are three types of electrochemical sensors potentiometric, amperometric, and potentiodynamic sensors. 

Electrochemical sensors include amperometric cells, especially galvanic fuel cells, and polarographic cells. Both types are available from several manufacturers world-wide in large numbers for clinical use. Higher stability with fewer technical problems makes the galvanic cell currently the most popular oxygen sensor in the operating room environment [12]. 

With this group of electrochemical sensors, information is obtained from the current-concentration relationship. The two most important issues to discuss are (1) the origin of the signal for various types of amperometric sensors and (2) the origins of selectivity. To begin our examination of these issues, we briefly reiterate some of the information presented in the Introduction to Electrochemical Sensors (Chapter 5). 

Introduction of room-temperature ionic liquids (RTIL) as electrochemical media promises to enhance the utility of fuel-cell-type sensors (Buzzeo et al., 2004). These highly versatile solvents have nearly ideal properties for the realization of fuelcell-type amperometric sensors. Their electrochemical window extends up to 5 V and they have near-zero vapor pressure. There are typically two cations used in RTIL V-dialkyl immidazolium and A-alkyl pyridinium cations. Their properties are controlled mostly by the anion (Table 7.4). The lower diffusion coefficient and lower solubility for some species is offset by the possibility of operation at higher temperatures. 

An electrochemical sensor is generally an electrochemical cell containing two electrodes, an anode and a cathode, and an electrolyte. Electrochemical sensors in general are classified, based on the mode of its operation, and they are conductivity sensors potentiometric sensors, and voltammetric sensors. Amperometric sensors can be considered as a special type of voltammetric sensors. The fundamentals of these sensors operational principles are described exceptionally well in several excellent electro-analytical books. In this entry, only the essential features are included. 

Clearly, electrochemical indication prevails over all other methods of transduction. Potentiometric and amperometric enzyme electrodes are at the leading edge of biosensor technology with respect to the body of scientific literature as well as to commercially available devices (Schindler and Schindler, 1983). Only a few conductometric biosensors have been described, but the relevance of this sensor type may increase because of the relative ease of their preparation and use. Furthermore, the development of biochemically sensitized field effect transistors, being at present only at an initial stage, offers new prospects (Pinkerton and Lawson, 1982). 

It has been proved that the selectivity of an analytical method is directly connected to the complexity of the matrix from which the analyte must be determined. As a result, the same method can be more selective or less selective, depending on the qualitative and quantitative composition of the matrix from which the analyte must be determined. For example, two types of electrochemical sensors are described for the assay of thyroid hormones L-T3 and L-T4. The first is an amperometric biosensor based on L-amino acid oxidase (l-AAOD),270 whereas the other is an amperometric immunosensor based on anti-L-T3 and anti-L-T4.271 If T3 and T4 have to be determined from phramaceutical products, both types of sensors have the necessary sensitivity and selectivity. When it is required to determine both hormones in biological fluids or in thyroid tissue, the proposed biosensors are not selective enough because l-AAOD catalyzes the reactions of both thyroid hormones. The amperometric biosensors can only make the discrimination between the two thyroid hormones, namely, L-T3 and L-T4, since the specific antibody reacts only with the specific antigen. 

Electrochemical sensors can be classified according to their mode of operation, e.g. conductivity/capacitance sensors, potentiometric sensors, and voltammetric sensors. Amperometric sensors can be considered a specific type of voltammetric sensor. The general principles of electrochemical sensors have been extensively described in other chapters of this volume or elsewhere. This chapter will focus on the fabrication of electrochemical sensors of micro or miniature size. 

Amperometric electrodes are a type of electrochemical sensor, as are potentiometric electrodes discussed in Chapter 13. In recent years there has been a great deal of interest in the developmenfoTv ious types of electrochemical sensors that exhibit increased selectivity or sensitivity. These enhanced measurement capabilities of amperometric sensors are achieved by chemical modification of the electrode surface to produce chemically modified electrodes (CMEs). 

Different electrochemical sensors have been developed for cell concentration measurement. The most promising of these sensors are based on impedimetric measurements. A commercial version of a sensor that measures the frequency-dependent i)ermittivity is available from Aber Instruments Ltd [137-139]. Another type of electrochemical probe measures the potential changes in the cell suspension caused by the production of electroactive substances during cell growth [140-143]. To date, no on-line applications of these potentiometric sensors under real cultivation conditions have been reported. Other types of probes, such as amperometric and fuel-cell sensors, measure the current produced during the oxidation of certain compounds in the cell membrane. Mediators are often used to increase the sensitivity of the technique [143-145]. 

Electrochemistry made its first contribution to biolo and medicine with the works of L. Clark in the early 1950 s. He came up with the first electrochemical sensor based on an amperometric method in order to measure out dissolved oxygen in water, especially in biological fluids. It was only after the early 1970 s that on the back of this type of device, other electrochemical biosensors were developed (specifically for detecting glucose, or chemical neurotransmitters). In the case of neurobiology, these developments reflected the need to understand how mechanisms worked for the chemical transmission of neuronal information, a field of study which has been in evidence since the late 1960 s. 

Nitrite Nitrite is an important indicator of fecal pollution in natural waters as well as a potential precursor of carcinogenic species. A rush of flow and sequential injection spectrophotometric method based on Griess-type reactions has been proposed, also coupled to online sorbent enrichment schemes. The catalytic effect of nitrite on the oxidation of various organic species constitutes the basis of fairly sensitive spectrophotometric methods. Fluorometric methods based on the formation of aromatic azoic acid salts, quenching of Rhodamine 6G fluorescence, and direct reaction with substituted tetramine or naphthalene species have been also reported. Indirect CL methods usually involve conversion into nitric oxide and gas-phase detection as mentioned in the foregoing section. The redox reaction between nitrite and iodide in acidic media is the fundamental of a plethora of flow injection methodologies with spectrophotometric, CL, or biamperometric detection. New electrochemical sensors with chemically modified carbon paste electrodes containing ruthenium sites, or platinum electrodes with cellulose or naphthalene films, have recently attracted special attention for amperometric detection. 

The basic configuration of an amperometric electrochemical sensor [14], sometimes called a limiting current-type sensor, is illustrated in Fig. 5. It is composed of a working electrode, a counter electrode and, optionally, a reference electrode in electrolytic contact (through an ionic conductive electrolyte) embedded within a cell enclosure containing a small aperture (or diffusion hole) or a porous, gas-permeable membrane. The main role of the aperture or membrane is to control the rate of gas flow into the sensor and thus control the amount of gas molecules reaching the electrode surface. Additionally, gas... 

Several decades of industrialization have changed the enviromnent drastically, leading to all sorts of pollution. Water pollution, being one of most important issues related to daily life, has always been addressed and mtmitored by various means of analytical tools. Different electrochemical sensors for the detection of pollutants in water have been well established, which can be categorized into the following (i) poten-tiometric sensors, (ii) amperometric sensors, (iii) voltammetric sensors, and (iv) conductometric sensors. In this chapter, we will introduce the fundamentals, applications, advantages, limitations, and recent trends for the development of each type of sensors. 

Naturally, in other areas with different technical requirements for the measuring system, preference will be given to other gas sensors with other sets of advantages, which depend not only on the sensor type but also on its version. This facet can be well illustrated using the example of electrochemical sensors, which can be realized either as conductometric, potentiometric, or amperometric (voltammetric) sensors (Brett and Brett 1998 Brett 2001). 

The idea of separating the gas sample by a gas-permeable membrane from the actual internal sensing element is common to several types of electrochemical and some optical sensors. The potentiometric Severinghaus electrode and the amperometric oxygen Clark electrode have already been discussed. Actually, most types of sensors can be used in this configuration and the conductometric sensor is not an exception (Bruckenstein and Symanski, 1986). 

Electrode modification by the attachment of various types of biocomponents holds considerable promise as a novel approach for electrochemical (potentiometric, conductometric, and amperometric) biosensors. Potentiometric sensors based on coupled biochemical processes have already demonstrated considerable analytical success [26,27]. More recently, amperometric biosensors have received increasing attention [27,28] partially as a result of advances made in the chemical modification of electrode surfaces. Systems based on... 

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