Selectivity of amperometric sensors

An amperometric sensitivity coefficient has been proposed (Wang 1994). 

Permselective membranes Dialysis membranes Cellulose acetate Small molecules 

Gas-permeable membranes Macroporous PIPE foil Dissolved gases (O2 CI2) 

Ion exchanger membranes NAFION (cation exch.) diverse cations 

Membranes with steric recognition Layers with intermolecular channels Calixarenes crown ethers Alkali metal ions 

---

The equivalent circuit corresponding to this resistive network is shown in Fig. 7.8. The current that carries the information about the analyte flows through the path of the lowest resistance. This seemingly trivial circuit can help us to design the best strategy for selectivity of amperometric sensors. 

Fig. 7.8 Origins of selectivity of amperometric sensors based on charge-transfer resistance and mass transport 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. 

Numerous catalytic layers made by different immobiUzing techniques have been used to enhance the selectivity of amperometric sensors. One example chosen from a large variety was a platinum microelectrode coated with polyvinylpyridine carrying a layer of anodicaUy deposited paUadium/iridium oxides (Shi et aL 2001). Polyvinylpyridine is an electricaUy conducting polymer. The sensor has been used to determine sulphur dioxide in gaseous phase as weU as sulphite in solution. 

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. 

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). 

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... 

Amperometric sensors monitor current flow, at a selected, fixed potential, between the working electrode and the reference electrode. In amperometric biosensors, the two-electrode configuration is often employed. However, when operating in media of poor conductivity (hydroalcoholic solutions, organic solvents), a three-electrode system is best (29). The amperometric sensor exhibits a linear response versus the concentration of the substrate. In these enzyme electrodes, either the reactant or the product of the enzymatic reaction must be electroactive (oxidizable or reducible) at the electrode surface. Optimization of amperometric sensors, with regard to stability, low background currents, and fast electron-transfer kinetics, constitutes a complete task. 

Different other attractive way to modify the active electrode of amperometric sensors, besides deposition of polymers or electropolymerization, is formation of self-assembled structures, e.g., self-assembled monolayers (SAM) on solid supports 88 89 or bilayer lipid membranes (BLM) on various types of support.90 Both types of theses structures can either induce selectivity of sensor to particular analytes and... 

Since in most cases only one enantiomer possesses a desired pharmacological activity, it is necessary to construct enantioselective sensors to improve the quality of analysis due to the high uncertainty obtained in chiral separation by chromatographic techniques.315 For this purpose, enantioselective amperometric biosensors and potentiometric, enantioselective membrane electrodes have been proposed.264 The selection of one sensor from among the electrochemical sensor categories for clinical analysis depends on the complexity of the matrix because the complexity of different biological fluids is not the same. For example, for the determination of T3 and T4 thyroid hormones an amperometric biosensor and two immunosensors have been proposed. The immu-nosensors are more suitable (uncertainty has the minimum value) for direct determination of T3 and T4 thyroid hormones in thyroid than are amperometric biosensors. For the analysis of the same hormones in pharmaceutical products, the uncertainty values are comparable. 

Amperometric selectivity coefficients are reported for the validation of amperometric sensors. The ratio between the analyte and interferent must be 1 10, and the mixed solution must be used in the determination of these selectivity coefficients. Only under these conditions will the results of the selectivity test be validated, and only then will the method attain the status of validated or not validated. 

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). 

All chemical sensors consist of a transducer, which transforms the response into a signal that can be detected (a current in the case of amperometric sensors) and a chemically selective layer. The transducer may be optical (e.g., a fiberoptic cable sensor), electrical (potentiometric, amperometric), thermal, and so on. We are concerned here with amperometric transducers. 

Some selected applications of ICP for the formation of amperometric sensors based on MIP strategy are reported in Table 2.2. 

As mentioned above, many amperometric sensors rely on one or more of the nanosized materials described above [1-32]. In the following sections, we highlight how the performance of amperometric sensors, in terms of sensitivity, detection limits, and selectivity, can be significantly improved by the use of materials under the form of nano-objects. Table 6.6 lists the most significant classes of analytes determined through amperometric sensors, together with a few representative references. 

Another strategy to enhance selectivity of amperometric solid electrolyte sensors consists in the development of new electrode materials with tailored electrocatalytical activities for the desired components to be measured. It has been proven e.g., [45], that the introduction of Pt/Au/Rh alloys enable the selective NO measurement even at elevated oxygen concentrations. 

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. 

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).

This form of selectivity applies to sensors that operate in the steady-state regime. The prime examples are thermal and amperometric sensors. It is somewhat limited for potentiometric sensors and it is least suitable for mass sensors. The minimum necessary kinetic background information can be found in Appendix B. 

Selectivity can be viewed as the fraction of the overall response of a sensor to the species of interest in the presence of interfering species. For multiple electroactive species detectable by amperometric sensors we can write... 

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. 

---