Why Can A Silver Electrode Be Used As An Indicator Electrode For Ag And Halides?

Electrochemical methods

P. WESTBROEK , in Belittling Electrochemistry in Textiles, 2005

two.2.3 Electrode systems in potentiometry

The indicator electrodes for potentiometric measurements have traditionally been categorised into three dissever groups. A first group of electrodes consist of a metallic immersed in a solution which contains ions of the same metallic as the electrode, for instance, a copper electrode immersed in a Cu(Two) solution. These electrode systems provide a direct response to the ion or species to exist measured:

[2.17] ${\text{M}}^{\mathrm{northward}+}+n{\text{e}}^{-}\leftrightarrows G$

[ii.18] $E=E^0+\frac{\text{R}T}{\mathrm{northward}\text{F}}\text{ln}{a}_{{\text{M}}^{\text{n}+}}$

Therefore, the primary electrode reaction includes the sensed species. Such electrodes give a direct response co-ordinate to the Nernst equation for the logarithm of the activity of the species.

Electrodes classified in the second group of electrode systems are those in which the metal electrode is coated with a layer of a sparingly soluble salt of the electroactive species and the metallic ion of the metal electrode, such that the potentiometric response is indicative of the concentration of the inactive anion species. Thus the silverish/silver-chloride electrode system, which is representative of this class of electrodes, gives a potential response that is straight related to the logarithm of the chloride ion activeness (run across also Chapter 1, section 1.v), fifty-fifty though information technology is not the electroactive species:

[2.nineteen] $\text{AgCl}+{\text{eastward}}^{-}\leftrightarrows \text{Ag}+{\text{Cl}}^{-}$

[ii.20] $\mathrm{East}=E^0-\frac{\text{R}T}{n\text{F}}\text{ln}{a}_{{\text{Cl}}^{-}}$

This is truthful because the chloride ion concentration, through the solubility product, controls the activity of the silvery ion, which is measured straight by the potentiometric silvery-electrode system.

Finally, there is a third group of electrodes, which are a more specialised instance of the electrodes belonging to the second grouping. They consist of the metal existence in direct contact with a sparingly soluble salt of the metal, which is then used to monitor the activity of an electroinactive metal ion in equilibrium with a more soluble table salt that includes the aforementioned anion as the electrode–salt organisation. For example, the concentration of calcium ions in equilibrium with solid calcium oxalate may exist monitored using a silver/silver oxalate electrode organization. The concentration of calcium ion affects the concentration of oxalate ion, which in turn controls the concentration of silvery ion; the latter is monitored by the potentiometric silver-electrode organization. A rather more than important example of this type of electrode is the Hg|Hg(II)–EDTA electrode organisation, which is used equally a sensing arrangement for the potentiometric titration of electroinactive metal ions with EDTA ^9–10 . The stability constant of the Hg(II)–EDTA complex is so high that just a pocket-size fraction is dissolved. Hence, when calcium ion is titrated with EDTA, the concentration of calcium ion controls the equilibrium concentration of the EDTA anion in solution, which in turn directly controls the gratis concentration of Hg(II). The latter is monitored by a Hg electrode organization to give a direct measure of the calcium ion concentration. This type of system tin be practical to most of the divalent ions that form moderately strong complexes with EDTA.

One of the about of import and extensively used indicator electrode systems is the glass-membrane electrode that is used to monitor hydronium ion activity. Although developed in 1909, information technology did not become popular until reliable electrometer amplifiers were developed in the 1930s. When the exterior surface of the glass membrane is exposed to an ionic solution, a response for the hydronium ion activity meets with the Nicholsky equation, which is similar to the Nernst expression. In view of the importance and widespread utilize of the hydronium or pH electrode, this system is discussed in a separate chapter.

Other potentiometric electrode systems are ion-selective electrodes such as fluoride, calcium, magnesium, sodium, potassium and chloride, selective gas electrodes based on membranes such as O₂, CO₂, CO, NO, NO₂ so₂, and enzyme electrodes. These electrodes fall across the scope of this book and are not discussed further.

For well-nigh potentiometric measurements, either the saturated calomel reference electrode or the silvery/silverish chloride reference electrode are used. These electrodes can be made compact, are hands produced, and provide reference potentials that do not vary more than than a few mV. The silver/silver chloride electrode too finds application in non-aqueous solutions, although some solvents cause the silver chloride motion-picture show to become soluble. Some experiments have utilised reference electrodes in non-aqueous solvents that are based on zinc or silver couples. From our own feel, aqueous reference electrodes are as convenient for non-aqueous systems as are any of the prototypes that accept been adult to date. When there is a need to exclude water rigorously, double-salt bridges (aqueous/non-aqueous) are a convenient solution. This is truthful even though they involve a liquid junction between the aqueous electrolyte system and the non-aqueous solvent system of the sample solution. The use of conventional reference electrodes does crusade some difficulties if the electrolyte of the reference electrode is insoluble in the sample solution. Hence, the use of a calomel electrode saturated with potassium chloride in conjunction with a sample solution that contains perchlorate ion can crusade dramatic measurements due to the atmospheric precipitation of potassium perchlorate at the junction. Such difficulties ordinarily can exist eliminated past using a double junction that inserts some other inert electrolyte solution between the reference electrode and the sample solution (eastward.g., a sodium chloride solution).

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Ion-Selective Electrodes

DONALD J. PIETRZYK , CLYDE W. FRANK , in Analytical Chemistry, 1979

INTRODUCTION

In redox methods an indicator electrode is used to sense the presence or change in concentration of the oxidized and reduced forms of a redox couple. Usually, the indicator electrode is an inert noble metallic, such as Pt, and the potential of the cell is measured vs a reference electrode. In this blazon of cell, the Pt does not participate in an actual electrochemical half-cell reaction but acts as a collector of electrons that are role of the one-half-cell reaction. Nevertheless some substances, not simply collect the electrons, only also participate in the half-cell. For example, a zinc rod responds to Zn(Two) concentration, a copper rod responds to Cu(Ii), and mercury to Hg(II). These, and several other metals, can deed every bit "ion-selective" electrodes toward their own ions.

It would be very convenient to exist able to dip an electrode pair (ion-selective electrode and reference electrode) into a solution of the substance to be adamant and obtain the sample's concentration from the observed potential. The problems with using metals are that in many cases electrode response is slow, the Nernst equation is not followed, electron change is non well denned, and the metal electrodes change potential due to amending of the electrode surface. Although there are some useful ion selective metallic electrodes (Zn, Cu, Hg), the vast majority suffer from some combination of the aforementioned problems.

In that location are also many ions that are of belittling involvement that do not participate in a one-half-cell that includes a metal. A typical example is hydronium ion. An accurate measurement of hydronium ion is very important in a wide variety of scientific disciplines. Consequently, it is desirable to be able to brand this measurement accurately, and routinely under a variety of solution conditions and concentration levels. There are also many other ions, such as F⁻, So₄ ⁻, NH_4⁺ , Na⁺, M⁺, and others, which are not part of a redox couple involving a metal or a easily handled metal.

Many different ion selective electrodes have been investigated and shown to be very useful. Most of these are not based on redox one-half-cells, similar the Zn²⁺/Zn half-cell, simply involve membrane or substitution potentials. This chapter considers the modernistic ion selective electrodes and their applications with the master accent directed toward the measurement of pH.

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Oxidation–Reduction Equilibria

DONALD J. PIETRZYK , CLYDE W. FRANK , in Analytical Chemistry, 1979

Indicator Electrodes

There are several unlike kinds of indicator electrodes. Several metals, such every bit silver, copper, lead, cadmium, and mercury, will participate in a reversible electron exchange and can serve equally indicator electrodes for their ions. In this list mercury is mayhap the nearly valuable (see chapter 29). In general, most other common metals, except noble metals, are not satisfactory as indicator electrodes in ordinary applications, usually, considering of oxide coatings on the surface and other surface properties that hinder electron exchange. Noble metals are chemically inert and can conveniently act every bit collector electrodes for one-half-prison cell reactions that involve charged species or gases. Of all the noble metals platinum and gilded are used the most.

Normally, all metallic electrodes are used as a wire, strip, or button set in plastic or drinking glass (see Fig. 10-7).

Fig. 10-7. Typical indicator electrode designs.

Several metals can serve as indicator electrodes for anions that form slightly soluble precipitates with the cation of the metal. A typical instance is the utilise of a silver electrode for indicating chloride ion. Once the solution is saturated with the sparingly soluble AgCl, the silverish wire becomes coated with AgCl and responds to Cl⁻ concentration through the following half-reaction

${\text{AgCl}}_{\left(\text{due south}\right)}+1e={\text{Ag}}_{\left(\text{s}\right)}+{\text{Cl}}^{-}$

There are several other indicator electrodes. Many of these have go then valuable to the analytical pharmacist in modernistic potentiometry that they are treated separately nether the heading "Ion-Selective Electrodes" in chapter thirteen.

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AMPEROMETRY

South.B. Adeloju , in Encyclopedia of Analytical Science (Second Edition), 2005

Titration with a Unmarried Polarizable Electrode

Amperometric titration with a unmarried polarizable electrode utilizes an indicator electrode (polarizable) and a nonpolarizable electrode in the form of a unproblematic working-reference electrode system in which the potential of the former (working/indicator electrode) is held constant relative to the latter (nonpolarizable electrode). As previously illustrated in Figure 2, the current flowing due to the oxidation or reduction of the electroactive species in such a system is recorded as a function of titrant volume. For a precipitation titration in which the current menstruum is acquired by the reduction of the electroactive species, increasing addition of the titrant results in the precipitation of the analyte equally an insoluble substance and, subsequently, in a subtract in the electric current menses. Figure 2A shows that the current for such a titration volition drop to zero and thereafter remain unchanged fifty-fifty when backlog titrant is added. The equivalence signal for such a titration is adamant by drawing the 2 straight lines and extrapolating to the intersection to locate the terminate indicate. In contrast, Figure 2B shows the titration bend for a system in which the titrant is reducible, but the analyte is unreactive. In this case, the potential of the electrode is held at a value where simply the electroactive species tin can exist reduced. Figure 2C illustrates a titration bend for a system in which both the analyte and the titrant are reducible inside the same potential range. In other cases where the electroactive titrant may undergo oxidation at an RPE, the direction of the titration curve will exist opposite to that shown in Figure 2B and, hence, the resulting current beyond the equivalence point will subtract.

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Nanoparticles in electrochemical bioanalytical assay

R.D.A.A. Rajapaksha , ... C.A.N. Fernando , in Nanoparticles in Analytical and Medical Devices, 2021

6.three.1.1 Potentiometry

The potentiometry technique indicates the potential deference of the indicator electrode and the counter electrode in ii electrodes organisation at i=0 (distinct from the reference electrode used in the 3-electrodes system) ( Ambrosi et al., 2008). The sensor is usually, selectively sensitive to the ion of interest, which determines the relationship betwixt the behavior of the concentration of analyte species and the potential response (Yin and Qin, 2013). Co-ordinate to Bratov et al. potentiometric sensors are used to classify and clarify complex samples such as clinical samples, food samples, and biological probes (Bratov et al., 2010). Further, they highlighted rapidity, reproducibility, and simple analysis techniques as common advantages of the potentiometric technique.

Ion-selective thin film electrodes (ISEs) or selective membrane are the most popular potentiometric sensors which are used for recognizing pH, ions (F⁻, I⁻, Na⁺, 1000⁺, etc.), and gases (NH₃, CO_ii) (Ambrosi et al., 2008). Small in size, portability, low cost, piece of cake to handle, and low LOD are some merits of ISEs (Bobacka et al., 2008). Batool et al. reported, biosensors potentiometric technique uses ion-selective electrodes to transduce the biological signal into electrical signal (Batool et al., 2019). Liang et al. developed molecularly imprinted polymers (MIPs) nanobeads-based ISE with a polymeric membrane to detect trace levels of triclosan in toothpaste (Liang et al., 2013). They achieved a ane.9×x⁻⁹  mol/L LOD at pH 10.1.

Israr et al. developed a ZnO nanorods-based potentiometric biosensor on detecting cholesterol every bit shown in the Fig. six.4A and B (Israr et al., 2010). The bore of the rods was 125–250   nm, and the length was well-nigh 1µ. Co-ordinate to their report, high ionicity, fantabulous biocompatibility, and loftier electron communication are the main advantages of ZnO. Here ZnO nanorods facilitate as a faster platform for electron transfer between ChOx and ZnO nanorod surface. Here they used standard Ag/AgCl reference electrode and ChOx/ZnO/Ag equally working electrode. The sensitivity of the biosensor was 35.2   mV/s. Cevik and the squad adult Iron_iiiO_iv nanoparticles-based potentiometric urea biosensor (Çevik et al., 2013). Depression toxicity, excellent biocompatibility, and super magnetic properties are the significant advantages of iron oxide nanoparticles. Farther, they highlighted these nanoparticles support for high enzyme loading and fast electrocatalytical role by enhancing the surface area.

Figure 6.4. Electrochemical analytical techniques. (A) ZnO nanorods comprised working bioelectrode for cholesterol sensing. (B) The output response of ChOx/ZnO/Ag bioelectrode for cholesterol concentration of 1×10⁻⁴  K (Israr et al., 2010). (C) InN QDs coated with ChOx comprised working bioelectrode for cholesterol sensing; Image (D–F) are EMF as a role of time for 500   μM cholesterol concentration detection by InN QDs, InN thin moving-picture show, InGaN layer, respectively. (Thousand) EMF as a office of fourth dimension when adding 50   μM uric acid (UA) and ascorbic acid (AA) to the 500   μM cholesterol solution (Alvi et al., 2013).

Alvi et al. fabricated InN QDs-based potentiometric biosensors for clinical applications equally shown in Fig. 6.4C–One thousand (Alvi et al., 2013). They used InN QDs considering of high electron density effectually the surface, loftier sensitivity to other charges in the surrounding, high chemical stability, planar arrangement, and rapid point response. Due to high positive charge density, InN QDs facilitate the electronic transfer from the molecule and the cholesterol. The concentration is figured by measuring the potential difference between InN QDs-based working electrode and the counter electrode. The sensitivity of the sensor is in the concentration range of one×x⁻⁶  M to ane×10⁻³  One thousand with 97   mV/decade with fast response inside two seconds. The comparison of the InN QDs with InN sparse motion picture revealed that the sensitivity, EMF, and response fourth dimension of the InN QDs-based biosensor is respectively 2 times larger, iii times larger, and five times shorter than the InN thin film-based biosensor. As shown in the Fig. half dozen.4F, EMF of bare InGaN layer does non produce a stable response. The InN QDs-based sensor produced high selectivity and reusability which is confirmed past negligible response to common interferents for uric acid and ascorbic acrid equally shown in Fig. vi.4G.

Kharitonov developed POM-based potentiometric sensor for papaverine hydrochloride detection (Ammam, 2013; Kharitonov, 2006). The researcher used four types of Keggin type POMs as H₃PMo₁₂O₄₀, H_threePW₁₂O₄₀, H_fourSiMo₁₂O₄₀, and H₄SiW₁₂O₄₀. Kharitonov was able to fabricate a high sensitive, rapid, and highly stabilized sensor.

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FOOD AND NUTRITIONAL Analysis | H2o and Minerals

M. González , V. González , in Encyclopedia of Belittling Science (Second Edition), 2005

Electrochemical Methods

In food assay, the about common electrochemical methods are potentiometric and voltametric.

In potentiometric methods, the potential between a reference and an indicator electrode is measured, which corresponds to the analyte activity. Because of their usefulness in nutrient assay, ion-selective electrodes (ISEs) that mensurate anions like bromide, chloride, and fluoride or cations like potassium, sodium, and calcium stand out amid indicator electrodes. The characteristics and advantages of ISE include the ability to measure out unlike anions and cations directly, the fact that they do non consume the analyte, the fact that analyses are independent of sample volume when taking direct measurements, and that moreover turbidity, color, and viscosity practise not affect the measurement. Potentiometric methods are also fast, easy to use, and inexpensive. The disadvantages of ISEs include the following: they have a relatively low sensitivity, proteins or other organic solutes tin can interfere in the determination, and some ions can act like ligands or can poison the electrodes.

Voltametric techniques are based on the relation betwixt current and voltage in an electrochemical process. Among them, anodic stripping voltamperometry permits metallic species decision with detection limits of parts per billion or lower. The equipment used with these techniques is much more inexpensive than that used with spectroscopic techniques that are also used in trace assay.

Table 5 summarizes some of the electroanalytical methods usually employed in mineral analysis of foods.

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Electrochemical detection techniques in biosensor applications

Behzad Rezaei PhD , Neda Irannejad , in Electrochemical Biosensors, 2019

ii.1.2 Potentiometric devices

In potentiometric devices, the accumulation charge potential is measured at the indicator electrode and compared with that at the reference electrode to obtain useful data on ion activity in the electrochemical reaction (nether zero or no significant menstruation currents through the indicator and reference electrodes) (Bakker, 2014). In such conditions, the measured potential is attributed to the number of electroactive species present in the sample. Using the Walther Nernst equation, the reduction potential is related to the concentrations of the analytes (Janata and Josowicz, 1997).

(2.i) $\mathit{Due\; east}={\mathit{Eastward}}^{\circ }-\left(\mathit{RT}\text{/}\mathit{nF}\right)\times \text{ln}\left(\mathrm{\alpha Red}\text{/}\mathrm{\alpha Ox}\right)$

where E° represents the standard-state potential, R is the gas constant, T is the constant temperature of the jail cell in Kelvin degrees, northward is the number of electrons exchanged in the redox reaction, F is Faraday'south constant, and αRed/αOx denotes the activity ratio of the reductant and oxidant species (Janata and Josowicz, 1997). Directly potentiometry is the direct measurement of analyte concentration with the application of the Nernst equation. Potentiometric devices currently have detection limits in the range of 10⁻⁸ to 10⁻¹¹  M. The best and lowest detection limit has been recorded for potentiometric devices based on ion-selective electrodes (ISEs) (Bard and Faulkner, 2001). Because that potentiometric sensors exercise non chemically affect the sample, they are suitable for measurement at very low concentrations and in samples with extremely low volumes. Detailed information on potentiometry and its detection limit may be found in Bakker and Pretsch (2005).

The potentiometric method is an appropriate alternative to electrical determination of the endpoint in a biochemical titration reaction. Potentiometric titration belongs to the chemical methods in which the endpoint of the titration is adamant using a working electrode. In this method, variation in potential is identified as a function of a well-defined quantity (usually volume) of the added titrant of a known concentration (Bard and Faulkner, 2001; Buck, 1974). Titration devices accept nowadays constitute a special place attributable to the availability of suitable indicator electrodes of low price and high reliability for the titrimetric studies of most all chemical reactions. Potentiometric titration curves provide useful information on the endpoint position of the titration, whereas the shape and position of the curves likewise provide advisable data about the processes accompanying the titration reaction (Cadet, 1974).

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Newspaper Based Sensors

Patricia Batista Deroco , ... Lauro Tatsuo Kubota , in Comprehensive Analytical Chemistry, 2020

5.four Potentiometric detection

Potentiometry is based on measurements of the deviation in potential between an indicator electrode and a reference electrode, which are connected to the ii terminals of a voltmeter and immersed in a sample solution. This measurement is performed under conditions of negligible current period betwixt the electrodes [100]. The potential at the indicator electrode, known equally the ion-selective electrode, is selective in its response to species of involvement due to the presence of an ion-selective membrane [141]. The main advantages of this arroyo are its selectivity and its simple instrumentation and functioning. Moreover, it is suitable for characterization-costless measurements. Thus, the utilize of potentiometric techniques in paper-based devices is platonic for fabricating depression-price POC diagnostic devices [142].

Some modern and simple works have been reported for potentiometric determination using newspaper-based analytical devices for awarding in food [143–146], clinical [147–150] and ecology analysis [151–154].

Bell et al. [147] adult, for the first time, an interesting label-gratis and selective newspaper-based potentiometric sensor for free bilirubin in blood serum. The devices constructed on Whatman due north.1 chromatography paper consist of three layers, equally shown in Fig. 7A : (i) one for the sample and reference solutions, which are connected through a table salt bridge; (ii) one containing an ion-selective membrane, which is sandwiched in the sample zone; and (iii) one containing the indicator electrode. The microfluidic zones were patterned into chromatography newspaper using a wax printer, and the electrodes were stencil-printed using a Ag/AgCl ink on the first and third layers. As a proof-of-concept, the authors used this device equally an affordable and dispensable potentiometric sensor to straight find gratuitous bilirubin in human blood serum without the need for whatever separation steps.

Fig. 7. Representation of some potentiometric paper-based devices. (A) Potentiometric paper-based devices for quantification of bilirubin and the obtained analytical curve in a clinically relevant concentration range. (B) A 3D origami potentiometric paper-based device and the folding sequence for the potentiometric determination of methyl parathion.

Panel (A): Reproduced with permission from J.G. Bell, Chiliad.P.S. Mousavi, 1000.K. Abd El-Rahman, East.K.Westward. Tan, S. Homer-Vanniasinkam, Thou.M. Whitesides, Paper-based potentiometric sensing of free bilirubin in blood serum, Biosens. Bioelectron. 126 (2019) 115–121 (Copyright© 2019 Elsevier). Panel (B): Reproduced with permission from J. Ding, B. Li, 50. Chen, West. Qin, A three-dimensional origami paper-based device for potentiometric biosensing, Angew. Chem. Int. Ed. 55(42) (2016) 13033–13037. Copyright © 2016 John Wiley and Sons

A novel newspaper-based 3D origami approach to monitor butyrylcholinesterase (BuchE) activeness was reported by Ding et al. [154]. The authors used this device for quantification of methyl parathion considering the pesticide inhibits BuchE, decreasing the enzymatic activity towards its substrate. The device was composed of four layers (sample, enzyme, substrate and electrode zone), as shown in Fig. 7B, and and then, hydrophobic zones were fabricated past wax printing on chromatography paper (Whatman Grade ane). The indicator and reference electrodes were prepared by driblet-casting carbon and Ag/AgCl ink on the paper substrate (bottom layer), respectively. Next, the ion-selective and reference membranes were formed by dip-coating the cocktail solutions onto the carbon and Ag/AgCl supports, respectively.

The 3D origami device was operated by but folding the three tabs along the fold-line following a specific sequence, as shown in Fig. viiB. As seen in the graph of Fig. sevenB, a unlike potential alter occurred when the sample contained the pesticide, and the potential change increased with increasing methyl parathion concentration. In this proof-of-concept assay, the dynamic range for methyl parathion was from 0.1 to 1.0   nM, and the limit of detection was 0.06   nM.

The authors considered this proposed device as the beginning 3D origami paper device for potentiometric biosensing, which has the advantages of a solid-contact ISE for rapid sensing and for simple functioning of an ePAD with multiple functions.

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TITRIMETRY | Potentiometry☆

A. Hulanicki , ... S. Glab , in Encyclopedia of Analytical Science (Third Edition), 2013

Introduction

Potentiometric titration belongs to chemical methods of analysis in which the endpoint of the titration is monitored with an indicator electrode that records the modify of the potential as a role of the amount (commonly the volume) of the added titrant of exactly known concentration. Potentiometric titrations are especially versatile because indicator electrodes suitable for the study of almost every chemical reaction used in titrimetry are now available. This technique is also frequently used in the written report of operational atmospheric condition of visual titrimetric indicators proposed for full general use in chemical assay, also equally in the report of numerous reactions, such as protonation and complexation, which find their application non particularly in analytical measurements. The course of the potentiometric titration curve provides information non simply nigh the titration end point position, but besides the position and shape of the bend may provide data virtually the processes accompanying the titration reaction. Another advantage is that the necessary apparatus is more often than not not expensive, reliable and readily available in the laboratories.

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Electrochemical Biosensors: Enzyme Kinetics and Role of Nanomaterials

G. Alarcon-Angeles , ... A. Merkoçi , in Encyclopedia of Interfacial Chemistry, 2018

Potentiometric Methods

Potentiometric measurements are based on the conclusion of the electric potential difference between a working electrode (usually called indicator electrode) and a reference electrode. Usually the transducer tin can be an ion-selective membrane, usually known as ion-sensitive field-result transistor (ISFET). ⁴⁴ Among the most mutual ISFETs are the pH electrode, or any other ion-sensing devices such as those for the quantification of NH₃, K⁺, Ca^{2   +}, Na⁺, Cl⁻, etc. ⁴⁵

The electrical potential deviation between the indicator electrode and the reference electrode is proportional to the logarithm of the ion activity, to which the electrode is sensitive, as shown by the Nernst–Donnan equation (Eq. 6):

(6) ${\mathrm{Due\; east}}_{\mathit{cell}}={\mathrm{Eastward}}_{\mathit{jail\; cell}}^0-\frac{RT}{\mathit{nF}}lnQ$

where E _cell (V) represents the equilibrium prison cell potential, East _cell ⁰ (V) is the standard constant potential, R is the universal gas abiding, T (K) is the absolute temperature, n is the number of electrons involved in the REDOX process, F is the Faraday constant, and Q is the mass action law of the REDOX process.

Amidst the advantages of ISFET-based biosensors are: high-sensitivity and real-time label-costless detection of a wide range of chemical and biological species. In addition, they can exist integrated into circuits for mass product. ⁴⁶

Other biosensors were adult nether similar principles, for case, a potentiometric biosensor based on pH-sensitive field-effect transistors and acetylcholinesterase for aflatoxin B1 determination in real samples. ⁴⁷

In general, potentiometric biosensors are based on an ion-selective electrode (ISE) with a membrane containing or in shut contact to an enzyme as a recognition material. ⁴⁸ Here one production of the enzymatic reaction is selectively recognized by the ISE changing the electric potential deviation and assuasive with this an indirect quantification of the target analyte.

For example, sulfite determination for pharmaceuticals and nutrient analysis is based on potentiometric biosensors; really the strategy of enzyme immobilization onto nanotubes or nanoparticles has demonstrated to improve electrocatalysis ⁴⁹ ( Fig. 6A ).

Fig. 6. (A) Typical chronopotentiogram (with the corresponding Nernstian plot in the inset) obtained for sulfite measurement with the PtNPs/PPy-SOx biosensor. Adjusted from Adeloju, S. B.; Hussain, S. Potentiometric Sulfite Biosensor Based on Entrapment of Sulfite Oxidase in a Polypyrrole Motion picture on a Platinum Electrode Modified With Platinum Nanoparticles. Microchim. Acta 2016, 183(4), 1341–1350, with permission from Springer. (B) AChE-based biosensor responses when aflatoxin B1 is present in concentrations of (a) 0   mg/mL, (b)10   mg/mL, and (c) 2   mg/mL, on AChCl concentration. Adapted from Stepurska, K.; Soldatkin, O.; Arkhypova, Five.; Soldatkin, A.; Lagarde, F.; Jaffrezic-Renault, N.; Dzya, S. Evolution of Novel Enzyme Potentiometric Biosensor Based on pH-Sensitive Field-Result Transistors for Aflatoxin B1 Analysis in Existent Samples. Talanta 2015, 144, 1079–1084, with permission from Elsevier. (C) Schematic representation of ERGO/IrxNPs/Tyr for captopril biosensing, based on the inhibition of Tyr. typical amperometric response for the unlike stages of construction of the biosensor. Adapted from Kurbanoglu, S.; Rivas, L.; Ozkan, S. A.; Merkoçi, A. Electrochemically Reduced Graphene and Iridium Oxide Nanoparticles for Inhibition-Based Angiotensin-Converting Enzyme Inhibitor Detection. Biosens. Bioelectron. 2017, 89, 122–129, with permission from Elsevier. (D) Comparison of the experimental Nyquist diagrams (dots) recorded using dissimilar biosensors, immersed in 1   mM Fe(CN)₆ ^{three   −/iv   −}, with the corresponding Randles' circuit (meet the inset). Adapted with permission from Rodríguez-Sevilla, East.; Ramírez-Silva, G. T.; Romero-Romo, M.; Ibarra-Escutia, P.; Palomar-Pardavé, M. Electrochemical Quantification of the Antioxidant Chapters of Medicinal Plants Using Biosensors. Sensors (Basel) 2014, 14(8), 14423–14439. (E) Complex aeroplane plots obtained with an Au-MPA-GOx SAMs electrode for different concentrations of β-d-glucose.

Adapted from Shervedani, R.; Mehrjardi, A.; Zamiri, N. A Novel Method for Glucose Determination Based on Electrochemical Impedance Spectroscopy Using Glucose Oxidase Self-Assembled Biosensor. Bioelectrochemistry 2006, 69(two), 201–208, with permission from Elsevier.

In particular, potentiometric biosensors have been successful when it comes to the measurement of an ion production of an enzymatic reaction, for example, biosensors developed for the urea detection are based on the enzymatic decomposition of urea co-ordinate to the following reaction:

$\mathit{C}\mathit{O}{\left({NH}_{\mathrm{ii}}\right)}_2+H_{2}O\stackrel{\mathit{urease}}{\to }{CO}_{\mathrm{ii}}+\mathrm{ii}{\mathrm{North}H}_3$

Reaction products undergo acid–base interactions in h2o equally follows:

${CO}_2+H_{2}O\to {{HCO}_{\mathrm{three}}}^{-}+H^{+}$

${\mathrm{Northward}H}_3+H_{2}O\to {{NH}_{\mathrm{four}}}^{+}+{OH}^{-}$

Thus, it is possible to determine urea concentration potentiometrically based on sensing pCO₂, pNH_iv, or pH, ⁵⁰ that is, the electroanalytic response is the electric potential, which is a function of the urea concentration. This biosensor has been successfully practical for the detection of urea in saliva, blood, ⁵¹ human being serum, ⁵² and milk. ^{53 , 54}

Similarly, potentiometric biosensors have been developed for the detection of penicillin, where penicillinase is immobilized on the surface of a selective pH electrode. The enzyme catalyzes the hydrolysis of β-lactam compounds, generating an acid product, which causes a decrease in pH, and so the electrode'south potential increases. The analytical signal of the pH sensor is and then proportional to the penicillin's concentration.

Other potentiometric biosensors have been developed for the detection of pesticides, but hither, enzyme inhibition acquired by the analyte is correlated with its concentration ( Fig. 6B ).

For example, in biosensors based on acetylcholinesterase, ⁴⁶ the enzymatic reaction with the substrate produces choline and acerb acid; in this case, acetic acid contributes to a modify in H⁺ concentration, then the transducer measures a change in pH, which can be stoichiometrically associated with the target analyte concentration:

$\mathit{Acetylcholine}+H_{\mathrm{two}}O\stackrel{\mathit{AChE}}{\Rightarrow }\mathrm{Choline}+{CH}_3{\mathrm{C}\mathrm{O}\mathrm{O}}^{-}+H^{+}$

Chemic species, which human activity as enzyme inhibitors, ^{55 , 56} can also be quantified; the subtract of the analytical response tin can be related to the concentration of the analyte, according to Eq. (5).

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Why Can A Silver Electrode Be Used As An Indicator Electrode For Ag And Halides?,

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