10 pH electrode, membrane electrode, biochemical electrode, ISFET, MOSFET

Dr. Heena Rekhi Rekhi

Electrodes are the primary reference standard for pH measurement. They respond reversibly to the concentration of hydrogen ions. A typical cell may be represented as follows:

 

Electrode | unknown solution || KCl| Hg2Cl2 (s) | Hg

 

The emf of this cell containing standard or unknown solution is given by,

 

EH = Eref – RT/F lnaH

 

Standard hydrogen electrode consists of platinum electrode surrounded by an outer tube along which H2 passes and escaping at the bottom through the test solution. The hydrogen electrode was the first electrode to be used for pH measurement but it required a supply of hydrogen gas and thus its application was limited. Many types of pH electrodes are available but glass electrode is ideal for majority of the tests. The electrode due to the nature of its construction needs to be kept moist at all times. In order to operate properly the glass needs to be hydrated. Hydration is required for the ion exchange process to occur. For the maintenance of an electrode its calibration is required so that it assures that the system is operating properly. The electrodes have junctions which allow the internal fill solution of the measuring electrode to leak out into the solution being measured. This junction can become clogged by particulates in the solution and can also facilitate poisoning by metal ions present in the solution. If the electrodes are clogged then it is best to place them in warm water.

 

Ion selective electrodes (ISEs) have become important and reliable devices for chemical, pharmaceutical and biomedical analysis as they are inexpensive and easy to use and have a wide range of application. A glass electrode is perhaps the most successful and ubiquitous electrochemical sensor providing the information about the activity of hydronium ions, in water. The number of electrodes used in current applications which are described below:

 

pH electrode:

 

pH is measured as the hydrogen activity of a solution. Various methods such as visual, photometric, and potentiometric can be used for its measurement. Visual and photometric methods are based on the change of color of specific organic pigments. In visual methods, visual indicators like pH strips are used while in photometric methods shining of light through the sample is involved. But these methods have limited applications and their results are not so reliable. Therefore potentiometric methods are preferred over these two.

 

In potentiometric metods, pH is measured by electrical potential of pH-sensitive electrodes. Potentiometric sensors are very sensitive and selective. Based upon potentiometric determination there are different pH electrodes such as hydrogen, metal and glass electrodes.

 

Glass electrode is most commonly used electrode. Glass electrode belongs to ion-selective electrode family. Most of the pH electrodes which are commercially available are combination electrodes that have both glass H+ ion sensitive electrode and additional reference electrode. In some particular applications separate pH electrodes and reference electrodes are also used – they allow higher precision. In most of the cases combination electrodes are precise and much convenient to use.

 

Membrane electrode:

 

In this electrode system the membrane potential is selective towards a given ion in the same way as the potential of glass electrode is selective towards the hydrogen ions. Exclusively membrane electrodes are used for the determination of ionic activities in solution. There have been many different experimental approaches to study the ionic interactions with macro biological molecules. Important types of membrane electrodes are glass, solid and liquid junction membrane whose construction is similar to pH electrode.

 

 

Glass membrane electrode: Glasses are generally described as a super cooled liquids or solids. Many oxides, halides form the combination of glass. The typical model of glass electrode used consist of a glass bulb membrane with composition of 72% SiO2, 22% Na2O, 6% CaO (or 80% SiO2, 10% Li2O, 10% CaO for highly alkaline media) and an electrically insulating tubular body,separates the internal solution. The most familiar membrane electrode is pH type used for a wide range of hydrogen ion activities. The active sites on the surface or hydrated layer of the glass are fixed during the pH measurements. The cation exchange and cation mobility leads to an accumulation of charge on the surface of a glass solution. According to the selective properties the glass membrane electrode are subdivided into three types. The selective properties can be produced by the appropriate adjustment of various parameters by altering the composition of glass material. These types are (i) pH type electrode made up of glass in which ions response according to their nature H+ >>Na+>K+, Rb+, Cs+…>>Ca2+; (ii) cation type H+>K+>Na+>NH4+, Li+……>>Ca2+ (iii) sensitive sodium type includes Ag+>H+>Na+>>K+, Li+…>>Ca2+. Cation selectivity can be achieved by adding elements with coordination numbers that are higher than oxidation numbers.

 

 

Solid membrane electrode: The glass membrane can be replaced by an ionically conducting membrane or by a single crystal. The electrode is made up of epoxy formulation composed of pure, non porous material with homogeneous surface of low microporosity that keeps sample retention to a minimum. Anion concentrations are measured by the use of silicon rubber and plastic membrane impregnated with silver salts. Fluoride and iodide ion electrodes are formed by incorporating finely dispersed salts into silicon rubber monomer and then carrying out the process of polymerization. It may be homogeneous or heterogeneous membrane electrode. The single crystal and pellet membranes are homogeneous whereas the membrane consisting of sparingly soluble salt in the binding material is heterogeneous membrane electrode.

 

Liquid membrane electrode: The membrane is formed by a layer of a suitable solvent which does not dissolve in the test solution. The solvent or the liquid saturating the membrane must be quite insoluble in water and must have a low vapor pressure. In addition if the liquid has sufficiently high viscosity, the membrane remains stable for long periods. Accomplishment of ion selectivity occurs through the selective extraction of ions into the membrane phase as well as mobility difference within the membrane. The potential across the two solution membrane are established. The developed potential is:

 

Ecell = Constant + 0.0591/n log [Mn+]If Mn+ is the ion being measured with valency n. The cell potential is given by:Ev = Eref +Ej – E°A + 0.0591/n log1/a

 

 

The membrane electrode responds selectively to single ion on contact with the solution of the ion to be determined on one side and to the solution of fixed activity of ion in contact with reference electrode on the other side. For example PVC matrix membrane electrodes and electrodes sensitive to nitrate ions. The PVC membrane electrode is best exemplified by calcium ion electrode, in which the cation exchange material is based upon the dialkyl phosphate.

 

Gas sensing electrodes: Gas sensing electrodes have a gas permeable membrane. The inner electrode system is formed by a pH glass electrode and Ag/AgCl reference electrode in the form of a combined pH electrode with a flat glass membrane. The glass membrane is covered by a microporous gas permeable filter from the outer side. Polytetrafluroethylene or polypropylene is used to isolate the analyte from possible interferences in the sample. A thin film commonly made up of silicon is used to trap the analyte gas and cover it to some ionic species that can be determined with some analyzing technique like potentiometry. The intermediate solution interacts with the gaseous species in such a way as to produce a change in a measured constituent of the intermediate solution. This change is then sensed by the ion-selective electrode. They are used to assay the gasses dissolved in aqueous solutions. They also have a small reference electrode within the gas permeable membrane. For example ammonia solution diffuses through a fluorocarbon membrane until a reversible equilibrium is established between the ammonia level of the sample and internal solution.

 

NH 3 + H O NH + + OH -4 2

 

The amount of hydroxide ions is measured by the help of internal sensing element and is directly proportional to ammonia level of the sample. Volatile nature of amines may interfere in certain cases.

 

 

Biochemical electrodes: Biochemical electrodes are multilayered composites containing biocatalysts, often enzymes immobilized in gel layer that coats ion selective electrode. At the interface between an electrode and an oxidation-reduction reactions need to occur for a charge to be transferred between the electrode and the solution. The redox reactions occurring between the electrode solution is represented as follows:

 

 

 

Where, n is the valence of cation material C, and m is the valence of anion material A. For most electrode systems, the cations in solution and the metal of the electrodes are the same, so the atoms C are oxidized when they give up electrons and go into solution as positively charged ions. A typical example is urea electrode in which an enzyme urease is employed to hydrolyse urea.

 

 

The final concentration of ammonium ion can be related to the urea present in the reaction. The urease enzyme is incorporated on a gel which is allowed to set on glass electrode then inserted into the urea solution. Thus ammonium ions diffused through the gel and cause a response by probe. These electrodes couple a purified enzyme with an ion-sensing electrode to make a very sensitive and selective probe for a particular biological molecule. In some cases whole bacteria can be used in place of the purified enzyme. Biochemical electrodes can be used tissue slices, living cells for making the electrode sensitive to arginine which converts arginine to urea. Using glucose oxidase and glass membrane electrode, a biochemical electrode for measuring the glucose will be constructed. If an enzyme works then why not the whole organism works. This idea leads to the development of a biochemical electrode probe that uses living cells rather than the isolated enzyme to make it biologically selective. Enzyme electrodes have been used for the determination of glucose, urea and enzymes. For example an amygdoline selective electrode can be made by retaining glucosidase in a gel layer coupled to cyanide sensitive membrane electrode. Penicillin can also be determined by using enzyme penicillinase to destroy the penicillin with production of hydrogen ions which can be determined using normal glass pH electrodes.

 

Ion selective field effect transistors (ISEFT):

 

The ion selective field effect transistor is a miniature ion sensor manufactured using standard micro technology. Its principle is based on the field effect induced by ion across an insulated film acting as a capacitor. A novel development of the use of ion selective electrode is the incorporation of a very thin membrane into a modified metal oxide semiconductor field effect transistor which is encase in a non conducting shield. When the membrane is placed in contact with the solution containing an appropriate ion, the potential is developed. This potential affects the current flowing through the transistor between terminals T1 and T2. By the calibration the activities of ions are measured from the measurement of current flowing through these points. The first report of an ion-selective field-effect transistor (ISEFT) was given by Bergveld (1970) stimulated much research, over the past 15 years. These would seem to be particularly well-suited for biomedical applications with advantages over the use of ion-selective electrodes. It is very similar to ion selective gate field effect transistor with little differences. The ISEFT differs from IGFET in the following respects:

 

The electrolyte solution is brought in direct contact with the gate insulator layer and a reference electrode in the solution replaces the metal gate.

 

A layer of silicon nitride overlying the silica provides a charge-blocking interface, conferring improved pH sensitivity.

 

Mechanism behind the potential generation:

 

Furthermore, the requirements and methodologies used to accomplish the analysis are modified day to day by the researchers working in this field to improve the routine analytical methods. The equipment required for direct measurements includes an ion-selective electrode, a reference electrode, and a

 

potential-measuring device. The reference electrode should provide a highly stable potential for an extended period of time. The ion-selective electrode is an indicator electrode capable of selectively measuring the activity of a particular ionic species. Ion-selective electrodes are mainly membrane-based devices, consisting of selective ion-conducting materials, which separate the sample from the inside of the electrode. The mechanism behind the potential generation is described as:

 

 

During recent decades increasing interest has been shown in the development of bioelectronic sensors based on ion sensitive field effect transistors (ISFETs). Many ISFET– based pH sensors have been commercialized and attempts have also been made to commercialize ISFET based bioelectronic sensors for applications in the fields of medical, environmental, food safety, military and biotechnology areas. The growing interest for development of these sensors is due to the fact that they are manufactured by means of semiconductor technology. The various advantages of this electrode are illustrated as follows:

 

 

Applications of ISFET

 

An ion-sensitive field-effect transistor (ISFET), is one of the most popular electrical biosensors, and has been introduced as the first miniaturized silicon-based chemical sensor. The ISFET, conventionally has referred to as pH sensor, used to measure the ions concentration in a solution. It is a type of potentiometric device that operates in a way similar to that of MOSFET works. Therefore, in order to evaluate the performance of ISFET, it makes sense to first understand the basic principle behind this potentiometric device. In recent years there has been a great progress in applying FET type biosensors for highly sensitive biological detection. The various applications are described as follows:

 

Ion-Sensitive Field-Effect Transistor for Biological Sensing: ISFET biosensors have provided various opportunities for developing a new generation of biosensor technologies. Due to its simple and clear working principle, ISFET biosensors have worked as a powerful sensing tool for detecting DNA, proteins, enzymes and cells. Although various types of ISFET-based biosensors have been developed, still they suffer from number of fundamental and technological problems. To conquer with these difficulties interdisciplinary cooperation from various research fields such as chemistry, biology, electrics, and physics must be required.

 

Selective electrodes for the measurement of pH in seawater: These electrodes were compared directly to the conventional hydrogen electrode and Ag/AgCl electrode in order to report the degree to which they obey ideal Nernstian laws. The Nernst responses of the ISFET to hydrogen ions and chloride ions have been examined over a wide range of pH and halide ion molality. As the commercial production of number of ion selective electrodes are increasing, it will essential to establish the standard calibration protocols ideally implemented by the manufacturers to ensure the data compatibility.

 

ISFET bases sensors for soil analysis: Extraction and pre-concentration techniques involved for the analysis of soil samples makes the work more tedious and complex. Therefore, the chemical sensor warrants the investigation since they can be placed directly in the soil and real time or quasi time analysis produces the results. This result also help in reducing the environmental impact caused by runoff of nutrients into ground and surface waters. The results confirm the feasibility of ISFET based sensors for monitoring of soil.

 

ISFET for the detection of physical parameters in liquids: ISFET a well known chemical sensor utilized as transducer in a hybrid sensor module for the detection of four physical parameters flow velocity, flow direction, diffusion coefficient of ions, level of liquid etc. Liquid type detection is observed with the help of ISFET. The sensor uses the effect of the drain current’s dependence on the level of the liquid contacting with the gate of the ISFET.

 

Biosensor and industrial application of pH ISFET: Encapsulation of electronically sensitive parts can prove to be so problematical that lengthy timelines and costly overruns compromise commercial promise. Intermediate applications, which will be referred to here as industrial appear to be commercially viable, however, and can be used to provide profits that will drive further development in the biosensor area. Thus pH ISFET has so much remarkable applications in various fields. However, the great commercial breakthrough has not occured so far. The problem of miniaturisation of the needed reference electrodes, as well as the, still not understood problem of current drift, which is inhibiting exact measurements on longer time are the main reasons for the missing success.

 

Metal oxide field effect transistor (MOSFET): The MOSFET is a semiconductor device which is widely used for switching and amplifying electronic signals in the electronic devices. It is a core of integrated circuit and because of a very small size can be designed and fabricated in a single chip. It is a four terminal device with source, gate, drain and body terminals. The body of the MOSFET is frequently connected to the source terminal so making it a three terminal device like field effect transistor. Figure below shows a cross sectional diagram and circuit symbol. n-type semiconductors are formed on the surface of the p type substrate, and the surface is then covered by an insulating SiO2. The area on the surface of p type semiconductors between the drain and the source is called the channel. The electrical conductivity of the channel is enhanced by the factor when the electrical potential is applied between the gate and the source. It is the most common transistor and can be used in both analog and digital circuits. The ISFET is very similar in construction and function to n-channel enhancement mode MOSFET. The ISFET differs only in that variation in the concentration of ions of interest provides the variable gate voltage to control the conductivity of the channel. It is naturally sensitive to pH changes but can be modified so that it becomes sensitive to other species by coating silicon nitride gate insulator with a polymer containing molecules that tend to form the complexes with the species other than the hydronium ions.

The operation of a MOSFET device is based on the variation of charge carriers in a channel that exists at the oxide-semiconductor interface between the source and drain diffusion regions. It exists in two forms depletion mode MOSFET and enhancement mode MOSFET correspondingly n-type and p-type semiconductors. Increase gate voltage pushes the p-type holes further away and enlarges the thickness of the channel. As a result current increased on moving from source to drain that is why this kind called enhanced mode MOSFET and vice versa for the depletion mode MOSFET.

 

Working Principle

 

The MOSFET is an important element in the embedded system design which is used to control the loads as per the requirement. Many of the electronic projects developed using it such as light intensity control, motor control and max geenerator applications. The working of MOSFET depends upon the metal oxide capacitor (MOS) that is the main part of the MOSFET. The oxide layer presents among the source and drain terminal. It can be set from p-type to n-type by applying positive or negative gate voltages respectively. When apply the positive gate voltage the holes present under the oxide layer with a repulsive force and holes are pushed downward through the substrate. The deflection region populated by the bound negative charges which are allied with the acceptor atoms.

 

Real life applications

 

This represents a powerful technique with a myriad of different applications in biology, physics as well as chemistry. It was also used for the respiratory gas analysis in the hospitals consisting of respired gas samples from the patients undergoing anesthesia. Due to its speed and sensitivity, has played a pivotal role in space related applications, drug discovery.

 

pH sensor applications in lake, sea and river water monitoring

 

There has been a notable increase in research over recent years into the declining pH level of the world’s oceans. This ongoing ocean acidification is having an impact already being felt by industries such as oyster farming in the

Pacific Northwest, and has prompted actions for new technologies to study the long-term trends.Our pH sensors are perfectly geared for monitoring seawater as well as fresh water sources.

 

Mud/Sediment Analysis, Colored or Turbid Liquids, Fermentation

 

While competitor optical pH sensors rely on the fluorescence of an indicator molecule, our approach uses a more simple yet robust colorimetric reading. This extremely fine gold mesh is biologically and chemically inert, and allows the sensors to be used in applications that would normally be impossible for optical sensors.

 

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