Sensors Amperometric

With amperometric sensors, the electrode is polarized by a predetermined potential, and the resulting electrolysis current is measured. Under certain conditions, saturation is achieved, and the limiting current is proportional to the actual concentration. Instead of the transducer principle of energy conversion (with potentiometry)y we now have the current-limiting transduction. 

Under these conditions, an electric current is an expression of the reaction rate. Now the electrode is not necessarily in thermodynamic equilibrium with the analyte. It is not necessary to wait until the equihbrium has been estab-Ushed. Consequently, commonly the response time of amperometric sensors is magnitudes shorter than that of potentiometric sensors (Morf and de Rooij 1995). The strict concentration proportionality of the measured quantity (the Umiting current) is another appreciated property of this sensor group. 

Amperometric sensors can be miniaturized more easily than potentiometric ones. Mass production is usual and yields low prices. Hence there is a tendency in favour of amperometry, in particular with biosensors. 

The principal setup of most amperometric electrochemical cells is based on three electrodes a measuring electrode, the counter electrode and the reference electrode. After applying a voltage the dissolved gas will be electrochemically trans- 

By applying a specific voltage on the reference electrode it is possible to compensate non-linearities during exposure to higher gas concentrations and/or to increase the sensitivity and selectivity to different gas species. The fifetime of this kind of electrochemical cell is limited by the consumption of electrode material. 

Voltammetric measurements are not simply restricted to analytical laboratories. The applications of these methods are more numerous than is at first obvious. A large number of analytical instruments, whether portable or not, intended to make precise measurements of substrates present in gas mixtures, vapours or solutions are equipped with electrochemical sensors. These devices operate on the principle of the 2- or 3- electrode cell enclosed in the sensor housing. 

When the AGS is exposed to a gaseous mixture, the electroactive species for which it was designed reaches the working electrode where a redox reaction occurs. For example, an oxidation reaction results in flow of electrons from the working electrode to the auxiliary electrode through the external circuit. This flow of electrons constitutes an electric current, which is proportional to the gas concentration. The potentiostat can modify the electrode potentials, allowing different analytes to be measured on condition that the detector is changed. 

Two other examples on the figure relate the reactions which occur in AGS for carbon monoxide (CO) and for oxygen (O2). The electrolytes are, for CO, a strong mineral acid such as sulfuric acid, while for O2 it consists in a weakly alkaline solution with potassium acetate. These liquid electrolytes are immobilized by absorbent materials. 

A biosensor contains a compound of biological origin (enzyme, antibody) included in a device holding the electrodes in order to convert a biological signal (e.g. fixation of the antigen to the antibody) to an electric signal. 

To make the biosensor specific for an analyte, a membrane is generally used that contains a recognition layer trapping the specific enzyme that catalyses the 

---

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. 

By changing the enzyme and mediator, the amperometric sensor in Figure 11.39 is easily extended to the analysis of other substrates. Other bioselective materials may be incorporated into amperometric sensors. For example, a CO2 sensor has been developed using an amperometric O2 sensor with a two-layer membrane, one of which contains an immobilized preparation of autotrophic bacteria. As CO2 diffuses through the membranes, it is converted to O2 by the bacteria, increasing the concentration of O2 at the Pt cathode. 

Selecting the Voltammetric Technique The choice of which voltammetric technique to use depends on the sample s characteristics, including the analyte s expected concentration and the location of the sample. Amperometry is best suited for use as a detector in flow systems or as a selective sensor for the rapid analysis of a single analyte. The portability of amperometric sensors, which are similar to po-tentiometric sensors, make them ideal for field studies. 

Sittampalam and Wilson described the preparation and use of an amperometric sensor for glucose. " The sensor is calibrated by measuring the steady-state current when it is immersed in standard solutions of glucose. A typical set of calibration data is shown in the following table. 

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.)... 

Alcohol dehydrogenase, 178 Alkaline error, 149 Alkaline phosphatase, 185 Alkanethiols, 46, 123 Alkoxide precursor, 120 Amino acids, 92, 187 Ammonium sensor, 181, 182 Amperometric sensors, 172 Aniline, 35, 39... 

F. Mizutani, S. Yabuki, T. Sawaguchi, Y. Hirata,Y. Sato, and S. Iijima, Use of a siloxane polymer for the preparation of amperometric sensors 0-2 and NO sensors and enzyme sensors. Sens. Actuator B-Chem. 76, 489—193 (2001). 

C. J. McNeil, Determination of the herbicide chlorsulfuron by amperometric sensor based on separation-free bienzyme immunoassay. Sens. Actual. B 98, 254—261 (2004). 

C. Malitesta, F. Palmisano, L. Torsi, and P. Zambonin, Glucose fast-response amperometric sensor based on glucose oxidase immobilized in an electropolymerized poly(o-phenylenediamine) film. Anal. Chem. 

V. Lvovich and A. Scheeline, Amperometric sensors for simultaneous superoxide and hydrogen peroxide detection. Anal. Chem. 69, 454-462 (1997). 

W.E. Morf and N.F. de Rooij, Performance of amperometric sensors based on multiple microelectrode arrays. Sens. Actuators B. 44, 538-541 (1997). 

Amperometric sensors for redox-inactive cations and electroactive compounds... 

AMPEROMETRIC SENSORS FOR REDOX-INACTIVE CATIONS AND ELECTROACTIVE COMPOUNDS... 

In conclusion, the unique properties of Prussian blue and other transition metal hexa-cyanoferrates, which are advantageous over existing materials concerning their analytical applications, should be mentioned. First, metal hexacyanoferrates provide the possibility to develop amperometric sensors for non-electroactive cations. In contrast to common smart materials , the sensitivity and selectivity of metal hexacyanoferrates to such ions is provided by thermodynamic background non-electroactive cations are entrapped in the films for charge compensation upon redox reactions. 

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). 

Besides these potentiometric sensors there are also amperometric sensors using the principle of ion conductive solid electrolytes. In addition to the heating voltage those sensors are also equipped with a second voltage supply, inducing a current, which varies depending on the concentration of the test gas. Fig. 3.19 shows a schematic view of these so-called saturating current probe. 

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]. 

This was found still to be the case when the other anions were in a tenfold excess over H2PO4, showing conclusively that [92] is acting as a selective H2PO4 amperometric sensor. 

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 ... 

Enormous progress is being made in the development of amperometric sensors, based on enzyme catalysed reactions, in widely varying clinical and immunological diagnostic methods. 

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. 

---