Potentiometric probes applications

Most scanning electrochemical microscopy (SECM) experiments are conducted in the amperometric mode, yet microelectrodes have for many years been used as potentiometric devices. Not surprisingly, several SECM articles have described how the tip operated in the potentiometric mode. In this chapter we aim to present the background necessary to understand the differences between amperometric and potentiometric SECM applications. Since many aspects of SECM are covered elsewhere in this monograph, we have focused on the progress made in the held of potentiometric microelectrodes and presented it in the context of SECM experiments. Starting with an historical perspective, the key discoveries that facilitated the development and applications of micro potentiometric probes are highlighted. Fabrication techniques and recipes are reviewed. Basic theoretical principles are covered as well as properties and technical operational details. In the second half of the chapter, SECM potentiometric applications are discussed. There the differences between the conventional amperometric mode are developed and emphasized. 

When performing potentiometric measurements with a traveling probe, one needs to take into account the effect of ohmic drop. Whereas this effect is the basis of SRET measurements, it becomes a nuisance in potentiometric SECM applications. If the substrate is an electrode involved in a Faradaic process, the current flowing between the substrate and the counterelectrode leads to potential gradients in solution. The tip will be sensitive to the potential distribution, and this may overcome the signal due to the concentration change for the ion of interest. This is particularly pronounced if the reference electrode associated to the tip is located far away in the bulk and of course if the solution conductivity is low. To remedy this situation some researchers have used double barrel electrodes where one channel acts as the ion-sensitive element and the other acts as a reference electrode (81,82). In the life sciences intracellular measurements are usually carried out in this way. Alternatively, it is possible to subtract the ohmic drop from the tip... 

In the following sections we consider several potentiometric applications. Many articles do not refer directly to scanning electrochemical microscopy, but all are closely related to the SECM principles and were therefore included in the present chapter. We have not included a section on applications related to potentiometric probing of biological substrates since they are covered in Chapter 11 of this volume. 

To determine if a dye will be a useful potentiometric probe in biological applications, information on its chemical, physical, spectral, and toxicological properties must be gathered in addition to data on the sensitivity to membrane potential. A number of model membrane systems are employed to characterize dye properties as well as some simple biological preparations. 

In addition to the amperometric feedback mode described above, other amperometric operation modes are also possible. For example, in the substrate generation/tip collection (SG/TC) mode, ip is used to monitor the flux of electroactive species from the substrate and vice versa for the tip generation/substrate collection (TG/SC) mode. These operation modes will be described in Section 12.3.1.3 and are useful in studies of homogeneous reactions that occur in the tip-substrate gap (see Section 12.4.2) and also in the evaluation of catalytic activities of different materials for useful reactions, e.g., oxygen reduction and hydrogen oxidation (see Section 12.4.3). In addition to the amperometric methods, other techniques, e.g., potentiometric method is also applicable for SECM and will be discussed in Section 12.3.2. We will also update the techniques suitable for the preparation of SECM amperometric tips in Section 12.3.1.1 and potentiometric probes in Section 12.3.2.2. 

This section deals with the fabrication of potentiometric probes and their use in SECM studies. Potentiometric probes (see Chapter 7) can detect many non-electroactive species not accessible to amperometric techniques. They are highly selective and have found widespread application in clinical chemistry, in environmental studies and the food industry. A general review of potentiometric probe fabrication has been presented previously, and several publications have demonstrated the utility of potentiometric probes in SECM studies (55). This section will provide the reader with a highlight of potentiometric probe fabrication techniques taken from the literature. The section will also include a discussion of the basic concepts, fabrication steps, necessary equipment, and characterization of ion-selective micropipettes applied in SECM studies. 

Other applications snch as the measuranent of local concentrations in environmental analy-sis44,45 using potentiometric probes as detectors in various types of scanning electrochemical microscopies inclnding probes with dnal functionality are emerging, but are stiU largely limited to micrometer-size electrodes due to inherent difficulties of using nanoelectrodes. 

The Ohmic drop should be taken into account when performing potentiometric measurements with a traveling probe. While this effect is the basis of SRET measurements, it becomes a nuisance in potentiometric SECM applications. If the substrate is an electrode involved in a faradaic process. 

SECM employs a mobile UME tip (Fig. 3) to probe the properties of a target interface. Although both amperometric and potentiometric electrodes have found application in SECM, amperometry - in which a target species is consumed or generated at the probe UME - has found the most widespread use in kinetic studies at liquid interfaces, as... 

In scanning electrochemical microscopy (SECM) a microelectrode probe (tip) is used to examine solid-liquid and liquid-liquid interfaces. SECM can provide information about the chemical nature, reactivity, and topography of phase boundaries. The earlier SECM experiments employed microdisk metal electrodes as amperometric probes [29]. This limited the applicability of the SECM to studies of processes involving electroactive (i.e., either oxidizable or reducible) species. One can apply SECM to studies of processes involving electroinactive species by using potentiometric tips [36]. However, potentio-metric tips are suitable only for collection mode measurements, whereas the amperometric feedback mode has been used for most quantitative SECM applications. 

In addition to their use as reference electrodes in routine potentiometric measurements, electrodes of the second kind with a saturated KC1 (or, in some cases, with sodium chloride or, preferentially, formate) solution as electrolyte have important applications as potential probes. If an electric current passes through the electrolyte solution or the two electrolyte solutions are separated by an electrochemical membrane (see Section 6.1), then it becomes important to determine the electrical potential difference between two points in the solution (e.g. between the solution on both sides of the membrane). Two silver chloride or saturated calomel electrodes are placed in the test system so that the tips of the liquid bridges lie at the required points in the system. The value of the electrical potential difference between the two points is equal to that between the two probes. Similar potential probes on a microscale are used in electrophysiology (the tips of the salt bridges are usually several micrometres in size). They are termed micropipettes (Fig. 3.8D.)... 

Where R is the gas constant, T is the temperature, and F is the Faraday constant. Caused by the logarithmic correlation between the gas concentration and the voltage signal, the potentiometric measurement is best suited for measurements of small amounts of oxygen. A well-known application of this principle has been realized in the so called lambda-probe for automotive applications where they are used to control the lambda value within a small interval around 1 = 1. The lambda-value is defined by the relation between the existing air/fuel ratio and the theoretical air/fuel ratio for a stoichiometric mixture composition ... 

While most gas sensors rely on potentiometric detection, the important oxygen probe is based on amperometric measurements. In particular, membrane-covered oxygen probes based on the design of Clark et al. (105) have found acceptance for many applications. The sensor is based on a pair of electrodes immersed in an electrolyte solution and separated from the test solution by a gas-permeable hydrophobic membrane (Fig. 6.22). The membrane is usually... 

The above authors coimmobilized choline oxidase and AChE on a nylon net which was fixed to a hydrogen peroxide probe so that the esterase was adjacent to the solution. The apparent activities were 200-400 mU/cm2 for choline oxidase and 50-100 mU/cm2 for AChE. The sensitivity of the sequence electrode for ACh was about 90% of that for choline, resulting in a detection limit of 1 pmol/l ACh. The response time was 1-2 min. The parameters of this amperometric sensor surpass those of potentiometric enzyme electrodes for ACh (see Section 3.1.25). Application to brain extract analysis has been announced. 

The first analytical instruments adapted for use in in-process measurements were electrochemical pH meters used as immersion probes. On-line potentiometric analysers can give continuous, real-time results for various analytes in a process. They are rugged ISEs that are not affected by the colour or turbidity of the process stream. Arrays of potentiometric sensors can even be used in fermentation broths. The sample can be taken into a loop, passed through a filter to protect the ISE surface and measnred. A feedback mechanism allows control of other parameters in order to keep the process in check. Applications... 

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]. 

Potentiometry—the measurement of electric potentials in electrochemical cells—is probably one of the oldest methods of chemical analysis still in wide use. The early, essentially qualitative, work of Luigi Galvani (1737-1798) and Count Alessandro Volta (1745-1827) had its first fruit in the work of J. Willard Gibbs (1839-1903) and Walther Nernst (1864-1941), who laid the foundations for the treatment of electrochemical equilibria and electrode potentials. The early analytical applications of potentiometry were essentially to detect the endpoints of titrations. More extensive use of direct potentiometric methods came after Haber developed the glass electrode for pH measurements in 1909. In recent years, several new classes of ion-selective sensors have been introduced, beginning with glass electrodes more or less selectively responsive to other univalent cations (Na, NH ", etc.). Now, solid-state crystalline electrodes for ions such as F , Ag", and sulfide, and liquid ion-exchange membrane electrodes responsive to many simple and complex ions—Ca , BF4", CIO "—provide the chemist with electrochemical probes responsive to a wide variety of ionic species. 

Reports of new materials and formulations and resin properties are prolific. Articles of a topical or applied interest include probes for in-situ hardness measurements on adhesives, photobase generators for image recording devices, oxygen inhibition in packaging applications, resins for sign boards, potentiometric sensors, new photodefinable polyimides, visible curable resists, " clay composites, putties, silica fillers, curable paints, soluble photocurable systems, fluorinated coatings and in-... 

In the potentiometric sensing of H2, (ii) is useful for H2 in inert gases while (i) is especially suited for H2 in air. On the other hand, only (iii) is applicable to the amperometric sensor and the four-probe type sensor. 

Guilbault and Montalvo were the first, in 1969, to detail a potentiometric enzyme electrode. They described a urea biosensor based on urease immobilized at an ammonium-selective liquid membrane electrode. Since then, over hundreds of different applications have appeared in the literature, due to the significant development of ion-selective electrodes (ISEs) observed during the last 30 years. The electrodes used to assemble a potentiometric biosensor include glass electrodes for the measurement of pH or monovalent ions, ISEs sensitive to anions or cations, gas electrodes such as the CO2 or the NH3 probes, and metal electrodes able to detect redox species some of these electrodes useful in the construction of potentiometric enzyme electrodes are listed in Table 1. 

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