Electrodes amperometric sensors

It is commonly assumed that application of these methods in sensors has started from invention of oxygen Clark electrode,2 and in biosensors from first glucose biosensor.3 At present, main sensor application of amperometric and voltammetric detections include, with wide use of oxygen Clark electrode, amperometric sensors based on modification of working electrodes with various materials, and biosensors employing practically all biorecognition species. With the very wide use of the term sensors, applications of voltammetric detections include also miniaturized screen-printed devices for stripping determinations of, e.g., heavy metal ions. 

Previous efforts relevant to oxygen transport have used two-electrode amperometric sensors such as the Clark and Mackereth cells (Clark, 1959). These sensors are inexpensive, accurate, and small, but they require frequent calibration, consume oxygen, and suffer from long-term drift. Other options include cyclic voltammetric sensors with remote three-electrode cells, and variations on galvanic techniques to monitor limiting currents at steady state (Fan et al., 1991 Haug and White, 2000 Utaka et al., 2009). 

One important application of amperometry is in the construction of chemical sensors. One of the first amperometric sensors to be developed was for dissolved O2 in blood, which was developed in 1956 by L. C. Clark. The design of the amperometric sensor is shown in Figure 11.38 and is similar to potentiometric membrane electrodes. A gas-permeable membrane is stretched across the end of the sensor and is separated from the working and counter electrodes by a thin solution of KCl. The working electrode is a Pt disk cathode, and an Ag ring anode is the... 

Figure 11.39 summarizes the reactions taking place in this amperometric sensor. FAD is the oxidized form of flavin adenine nucleotide (the active site of the enzyme glucose oxidase), and FAD1T2 is the active site s reduced form. Note that O2 serves as a mediator, carrying electrons to the electrode. Other mediators, such as Fe(CN)6 , can be used in place of O2. 

FIGURE 6-24 Response pattern of an amperometric sensor array for various carbohydrates. The array comprised carbon-paste electrodes doped with CoO (1), Cu20 (2), NiO (3) and Ru02 (4). (Reproduced with permission from reference 84.)... 

K.N. Thomsen and R.P. Baldwin, Evaluation of electrodes coated with metal hexacyanoferrate as amperometric sensors for non-electroactive cations in flow systems. Electroanalysis 2, 263—271 (1990). 

D.R. Shankaran and S.S. Narayanan, Amperometric sensor for thiols based on mechanically immobilised nickel hexacyanoferrate modified electrode. Bull. Electrochem. 17, 277-280 (2001). 

R.M. Ianniello and A.M. Yacynych, Immobilized enzyme chemically modified electrode as an amperometric sensor. Anal. Chem. 53, 2090-2095 (1981). 

Although not strictly relevant to amperometric sensor technology, various metalloporphyrins [Co(III), Mn(III), Fe(III) Fig. 45] have been shown to sense anions potentiometrically with selectivity sequences dependent on the centrally bound metal (Amman et al., 1986 De et al., 1994). For example the anti-Hofmeister selectivity sequence SCN" > I" > CIO4 > N02 > Br > Cl- > NOJ was exhibited by PVC membrane electrodes containing [87]. 

As far as the use of ferrocene molecules as amperometric sensors is concerned, they have found wide use as redox mediators in the so-called enzymatic electrodes, or biosensors. These are systems able to determine, in a simple and rapid way, the concentration of substances of clinical and physiological interest. The methodology exploits the fact that, in the presence of enzyme-catalysed reactions, the electrode currents are considerably amplified.61 Essentially it is an application of the mechanism of catalytic regeneration of the reagent following a reversible charge transfer , examined in detail in Chapter 2, Section 1.4.2.5 ... 

Sensor A device having a response (ideally) for one particular analyte. Poten-tiometric sensors are typically ion-selective electrodes, while amperometric sensors rely on Faraday s laws. 

Figure 1.11 — Average number of papers on (bio)chemical sensors published annually, based on data from Janata s biannual review. E electrochemical sensors ISEs ion-selective electrodes P potentiometric sensors A amperometric sensors C conductimetric sensors O optical sensors M mass sensors T thermal sensors. (Adapted from [23] with permission of the American Chemical Society).

Voltammetric/Amperometric Sensors Thick-film carbon-containing screen-printed electrodes modified with formazan [473] were used for zinc determination. 

Methods to electrically wire redox proteins with electrodes by the reconstitution of apo-proteins on relay-cofactor units were discussed. Similarly, the application of conductive nanoelements, such as metallic nanoparticles or carbon nanotubes, provided an effective means to communicate the redox centers of proteins with electrodes, and to electrically activate their biocatalytic functions. These fundamental paradigms for the electrical contact of redox enzymes with electrodes were used to develop amperometric sensors and biofuel cells as bioelectronic devices. 

As we have seen already, there are two kinds of selectivity thermodynamic selectivity and kinetic selectivity (Chapter 2). Let us first consider how we could use thermodynamic selectivity for amperometric sensors. The electrode and the solution form a double-layer capacitor. The minimum energy of this capacitor occurs at... 

Manipulation of the mass transfer resistances is another possibility. Let us assume that the analyte is an electrically neutral species, but the major interferant is charged. By placing an ion-exchange membrane with immobile charge of opposite polarity to that of the interferant in front of the electrode, the access of the charged interferant becomes blocked by the electrostatic repulsion. These selectivity design strategies can be summarized in a statement that applies also to other life situations. In amperometric sensors, the information is obtained from the current path of least resistance. 

In general, traditional electrode materials are substituted by electrode superstructures designed to facilitate a specific task. Thus, various modifiers have been attached to the electrode that lower the overall activation energy of the electron transfer for specific species, increase or decrease the mass transport, or selectively accumulate the analyte. These approaches are the key issues in the design of chemical selectivity of amperometric sensors. The long-term chemical and functional stability of the electrode, although important for chemical sensors as well, is typically focused on the use of modified electrodes in energy conversion devices. Examples of electroactive modifiers are shown in Table 7.2. 

We must remember that with amperometric sensors, the analytical information is obtained from the mass transport limiting current. One important consequence of the current-voltage equation is that one can always apply a potential high enough in order to transfer electrons to or from the electrode to a given species of interest. 

The enzyme can be incorporated into an amperometric sensor in a thick gel layer, in which case the depletion region due to the electrochemical reaction is usually confined within this layer. Alternatively, enzyme can be immobilized at the surface of the electrode or even within the electrode material itself, in which case the depletion region extends into the solution in the same way as it would for an unmodified electrode. In the latter case, the enzyme can then be seen as a true electrocatalyst that facilitates the interfacial electron transfer, which would otherwise be too slow. The general principles of the design and operation of these biosensors is illustrated on the example of the most studied enzymatic sensor, the glucose electrode (Fig. 2.14, Section 2.3.1). 

There are several species in this reaction that can be used for electrochemical sensing. Detection of proton released from the gluconic acid was used in the poten-tiometric glucose electrode (Section 6.2.1). The amperometric sensor can be based on oxidation of hydrogen peroxide, on reduction of oxygen, or on the oxidation of the reduced form of glucose oxidase itself. 

The three types of glucose electrode discussed here illustrate the major facets of design and operation of enzymatic amperometric sensors. Examples of amperometric enzyme electrodes for other substrates are shown in Table 7.3. The actual design details of these sensors depend on the enzyme kinetics involved and on the operating conditions under which they are used. 

Sensing performance for H-,. Sensing performance of the amperometric sensor was examined for the detection of H2 in air. Figure 3 shows the response curve for 2000 ppm H2 in air at room temperature. The response was studied by changing the atmosphere of the sensing electrode from an air flow to the sample gas flow. With air the short circuit current between two electrodes was zero. On contact with the sample gas flow, the current increased rapidly. The 90% response time was about 10 seconds and the stationary current value was 10yUA. When the air flow was resumed, the current returned to zero within about 20 seconds. 

Modification of the sensor structure. The above amperometric sensor has a rather complicated construction, because the sample gas (H2 + air) is separated from the reference air. So, we tried to simplify the sensor structure as shown in Figure 9. As proton conductor we used a thin antimonic acid membrane (mixed with Teflon powder) of 0.2 mm thickness. This membrane is thin and porous enough to allow a part of the sample gas to permeate. On the other hand, the counter Pt electrode was covered with Teflon and Epoxy resin in order to avoid a direct contact with the sample gas. 

One of the major deterrents to the successful application of electroanalytical sensors has been the lack of long-term stability of the polymer films. At least three factors effect the stability of these amperometric sensors. These factors are the mode of polymer film attachment to the electrode surface (adsorption vs. covalent bonding), solubility of the film in the contacting solution, and finally, the mode of attachment of the catalyst in the polymer film (electrostatic vs. covalent). 

If one wishes to verify whether the prewave can form the basis for an amperometric sensor, one would preferably dispose of as much information as possible concerning the nature and the properties of this wave. An obvious technique for diagnosis is cyclic voltammetry. Hydrogen peroxide can be oxidised as well as reduced at glassy-carbon electrodes however, the potential ranges within which the reactions occur are situated relatively distant from each other, as can be seen in Fig. 4.3 and Fig. 4.5. 

E. Turkusic, V. Milicevic, H. Tahmiscija, M. Vehabovic, S. Basic and V. Amidzic, Amperometric sensor for L-ascorbic acid determination based on Mn02 bulk modified screen printed electrode, Fresenius J. Anal. Chem., 368 (2000) 466-470. 

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