Lecture 1-3 Pre-Master Electrochemical Sensors PDF

Electrochemical Sensors Electrochemical sensors are the most versatile and highly developed chemical sensors. They are divided into several types: Potentiometric (measure voltage) Amperometric (measure current) Voltammet...

Electrochemical Sensors Electrochemical sensors are the most versatile and highly developed chemical sensors. They are divided into several types: Potentiometric (measure voltage) Amperometric (measure current) Voltammetry Conductometric (measure conductivity) Sometimes the distinction between these types can be blurred. In all these sensors, special electrodes are used.  Electrochemical Cell The reaction takes place at the cathode where electrons are “pulled” out of the electrode. Nernst Equation E  E  RT log ( C0 ) 0 e nF CR The Nernst equation gives Co is the oxidant concentration the potential of each half CR is the Reduced Product cell. Concentration In a potentiometric sensor, two n is the number of electrons half-cell reactions take place at transferred per redox reaction each electrode. Only one of the F is the Faraday constant reactions should involve sensing the species of interest. The other T is the temperature should be a well understood R is the gas Constant reversible and non-interfering reaction E0 is the electrode potential at a standard state.  Electrodes and Potentiometry Introduction 1.) Potentiometry  Use of Electrodes to Measure Voltages that Provide Chemical Information - Various electrodes have been designed to respond selectively to specific analytes  Use a Galvanic Cell - Unknown solution becomes a ½-cell - Add Electrode that transfers/accepts electrons from unknown analyte - Connect unknown solution by salt bridge to second ½-cell at fixed composition and potential  Indicator Electrode: electrode that responds to analyte and donates/accepts electrons  Reference Electrode: second ½ cell at a constant potential  Cell voltage is difference between the indicator and reference electrode  Electrodes and Potentiometry Reference Electrodes a.) Standard hydrogen electrode  Electrodes and Potentiometry Reference Electrodes  Electrodes and Potentiometry Reference Electrodes C.) Saturated Calomel Reference Electrode (S.C.E) Saturated Calomel Reference Electrode (S.C.E)  Indicator Electrodes (Sensor) I. Metallic IE A. Electrodes of the First Kind B. Electrodes of the Second Kind C. Inert Metallic Electrodes (for Redox Systems) II. Membrane IE A. Glass pH IE B. Crystalline-Membrane IE C. Liquid Membrane IE D. Polymer membrane 1- Polymeric salt membrane 2- polymer-immobilized ionophore membranes 3- Imprinted polymer electrode sensor III. Enzyme selective electrode IV. Gas Sensing Probes  MEMBRANE ELECTRODES p-ion electrodes Consist of a thin membrane separating 2 solutions of different ion concentrations Most common: pH Glass electrode The discovery, that a thin glass membrane develops a potential, called a membrane potential, when opposite sides of this membrane are in contact with solutions of different pH led to the eventual development of a whole new class of indicator electrodes called ion-selective electrodes (ISEs). The electrochemical cell consists of Two Reference Electrodes, one immersed in the sample and the other immersed in the internal standard solution. These solutions are separated by a membrane electrode (ISEs).  Membrane Potentials ISE, such as the glass pH electrode, function by using a membrane that reacts selectively with a single ion (H +). Ref (sample) || [A] sample | Membrane | [A] internal || Ref internal Ecell = E Ref (internal) – E Ref (sample) + E membrane + Elj …….(a) Liquid junction potential and reference electrode potentials are constant, thus any change in the cell's potential is attributed to the membrane potential. Current is carried through the membrane by the movement of either the analyte or an ion already present in the membrane's matrix.  The membrane potential is given by: Emem = Easym -(RT/zF) ln ([A] internal /[A] samp) ….(b) Easym = Asymmetric potential : The membrane potential that develops when the concentrations on both sides are equal. Substituting eq. (b)into eq. (a) gives Ecell = K + (0.059/z) log [A) sample This equation applies to all types of ISE’s. Advantages and limitations of I.S.E. Advantages: 1. Linear response: over 4 to 6 orders of magnitude of signal. 2. Non-destructive: no consumption of analyte. 3. Non-contaminating. 4. Short response time: in sec. or min. useful in indust. applications. 5. Unaffected by color or turbidity. Limitations: 1. Precision is rarely better than 1%. 2. Electrodes can be fouled by proteins or other organic solutes. 3. Interference by other ions. 4. Electrodes are fragile and have limited shelf life. 5. Electrodes respond to the activity of uncomplexed ion. So ligands must be absent or masked. Membrane Electrode (Glass pH)  Glass pH IE The history of ion selective electrodes begins with the creation of the pH electrode in 1906. Sensors  I– What is a sensor? Sensors A sensor is a physical device or biological organ that detects, or senses, a signal or physical condition and chemical compounds.  WHAT IS THE CHEMICAL SENSOR A sensor is a self-contained integrated device, which is capable of providing specific quantitative analytical information using a recognition element which is retained in direct spatial contact with a transducer  1-POTENTIOMETRIC TRANSDUCERS Measuring Ecell requires the ability to measure the difference in electron density between the two electrodes. Potentials at each electrode develop as a result of the summation of all the potential differences at the interfaces of all the phases in the electrochemical cell.  Liquid-Based Ion-Selective Electrodes  Similar to solid-state electrode  Hydrophobic membrane impregnated with hydrophobic ion exchanger  Hydrophobic ion exchanger selective for analyte ion Binds Ca+2 Hydrophobic counter-ion Hydrophobic solvent  Liquid/Polymer Membrane ISEs liquid polymeric electrodes are: Most widely used of any electrochemical sensing devices Can be prepared for both cations and anions!--- Effectively a generic method to make selective ion sensors----only need a new chemistry in the membrane to sense a given target analyte ion! All the original systems were wet liquid membranes where porous organic membrane was wetted with organic liquid containing the membrane active ion- exchanger or ionophore! All the original systems were wet liquid membranes where porous organic membrane was wetted with organic liquid containing the membrane active ion-exchanger or ionophore!. The ion-selective membranes are usually composed of three or four components: polymeric matrix, plasticizer, lipophilic salt and ionophore, all matched in adequate proportions. Complex formation constants for the different ions and the ionophore determine selectivity of the membrane. The plasticizer polarity influences extraction properties of the membrane, while a concentration of anionic sites in the membrane depends on the lipophilic salt content. The polymers such as: poly(vinylchloride), polyurethane, polysiloxane, polyacryloamide, cellulose used for the membrane fabrication play a role of a scaffold maintaining the liquid membrane. The incorporated ion carrier (ionophore) is the key compound of the polymeric membrane that defines the membrane selectivity by interaction with primary ion (cavity size geometry of the molecules and type of functional group which leads to a specific metal-ligand interaction) Hence, the electrode with satisfying potentiometric and electrochemical properties can be achieved by synthesis of highly lipophilic and selective complexing agents that can be doped within the membrane The potentiometric characteristics, , depend on the membrane composition, nature of plasticizer and additive used, combined with optimum lipophilicity of the ionophore and plasticizer that ensure stable potential and long life time.  Polymeric salt membranes Polymeric membranes are made by use of a polymeric binder for the powdered salt About 50% salt and 50% binding material. The common binding materials are PVC, polyethylene and silicon rubber. In terms of performance these membranes are quite similar to sintered disks.  Polymer-immobilized ionophore membranes A development of the inorganic salt membrane Ion-selective, organic reagents are used in the production of the polymer by including them in the plasticizers, particularly for PVC. A reagent, called ionophore (or ion-exchanger) is dissolved in the plasticizer (about 1% of the plasticizer). This produces a polymer film which can then be used as the membrane replacing the crystal or disk in sensors. Plasticizer Polymer membrane preparation  2. Classification of Ion-exchange Membranes Classification based on function is clear; ion-exchange membranes have an electrical charge, which is positive or negative. 1. Cation exchange membranes, in which cation exchange groups (negatively charged) exist and cations selectively permeate through the membranes. 2. Anion exchange membranes, in which anion exchange groups (positively charged) exist and anions selectively permeate through the membranes. 3. Amphoteric ion exchange membranes, in which both cation and anion exchange groups exist at random throughout the membranes. 4. Bipolar ion-exchange membranes which have a cation exchange membrane layer and anion exchange membrane layer (bilayer membranes).  Ion exchange process Ion Exchange Process In the following ion exchange process, a lithium cation displaces a potassium cation from the organic anion, R -: KR + Li+ ⇋ LiR + K+ We can imbed the lipophilic R- in a membrane, as shown in Figure 2, and place it in a solution of Li+ KR(mem) + Li+(aq) ⇋ LiR(mem) + K+(aq) The cation-exchanger is an aliphatic diester of phosphoric acid, (RO)2PO2-, where each R group is an aliphatic hydrocarbon chain containing between 8 and 16 carbons. The phosphate group can be protonated, but has a strong affinity for Ca 2+. The cation exchanger is dissolved in an organic solvent and held in a porous compartment between the analyte solution and internal reference calcium chloride solution. The ion-exchanger uptakes Ca2+ into the membrane by the following mechanism, forming a complex with the structure shown in Examples for ion exchanger sensors Cationic and anionic exchange resins  Lead Sensor The N,N'-bis-thiophene-2-ylmethylene-ethane-1,2-diamine that was tested as a lead(II) ionophore is shown in Figure. The ionophore was synthesized by condensing ethylenediamine with thiophenecarboxaldehyde. High molecular weight PVC, 2- nitrophenyl octyl ether (o-NPOE), dioctyl phthalate (DOP), dioctyl adipate (DOA), dioctyl sebacate (DOS), potassium tetrakis (p- chlorophenyl)borate (KTpClPB) and tetrahydrofuran (THF) were used to prepare the PVC membranes.  Copper(II) ion selective PVC membrane electrode Structure of S,S'-bis(2- aminophenyl)ethanebis(thioate The natural affinity between mercury (II) and sulfur (IV) is directed to design a new membrane electrode for detecting sulfite and/or hydrogen sulfite ions. In preliminary studies, several mercury (II) compounds were examined [ diphenylmercury, mercury (II ) -diphenylthiocarbazone, and Hg (DDC )2.Of these, Hg(DDC),proved most suitable for incorporation into polymeric matrices, yielding membranes with significant sulfite response and selectivity.  Chloride Selective Electrode Cobalt complex of amino methylated polystyrene-salicylaldehyde schiff base has been developed as the ionophore in the fabrication of the electrode. a sensitive electrode, selective to chloride ion has been developed. The fabricated electrode has a stable shelf life of about three months and is inert towards most of the transition metal ions and a variety of anions.  Neutral Carrier Most successful of all organic membrane Neutral carriers are lipophilic ion complexing ligands that are electrically neutral to start but the final complex is charged, the same charge as the ion that interacts selectively (hopefully) with the ligand.. The ionophore is a neutral “carrier” molecule represented by the blue oval. Figure shows the chemical structure of two ionophores. The ionophore cannot diffuse out of the membrane and but can “trap” the analyte ion (A+) at the interface between the solution and membrane. Without the ionophore, the analyte would be unable to partition into the organic membrane. Macro-cyclic Ionphores Metal phthalocyanines  Potentiometric Biosensor Makes use of the selectivity offered by many of the high selective ISEs. Makes use of the very high selectivity of offered by the enzymatic reactions. Either the product or the reactant of the enzymatic reaction is monitored using ISEs. Can be applied for the determination of many neutral and organic species (e.g., Urea). The main limitation is the high cost of enzymes. Examples A simple-potentiometric method for determination of acid and alkaline phosphatase enzymes in biological fluids and dairy products using a nitrophenylphosphate plastic membrane sensor  Enzyme electrodes In the case of the enzymatic membranes, stability of the biosensor depends on proper anchor of enzyme molecules to their surface or in membrane bulk. Various methods of the enzyme immobilisation in and/or within the supports with different functional groups were analysed. To obtain functional groups on the support surface,various methods of chemical modifications were applied. The enzyme converts an analyte into a product that is detected at the sensor surface.To obtain the biosensor response, the analyte must penetrate through the membrane to the catalytically active sites of the enzyme and then the products partially diffuse to the sensor surface and away into the bulk solution. The mass transport trough the membrane is mainly driven by the concentration gradients of species then, diffusive  POTENTIOMETRIC TRANSDUCERS BASED MOLECULARLY IMPRINTED POLYMERS (MIP) A molecular imprinted polymer (MIP) is a polymer that was formed in the presence of a molecule that is extracted afterwards, thus leaving complementary cavities behind. These polymers show a certain chemical affinity for the original molecule and can be used to fabricate sensors, catalysis or for separation methods. The functional mechanism is similar to antibodies or enzymes. Potentiometric Sensors of Molecular Imprinted Polymers for Specific Binding of Chlormequat  Imprinted polymer electrode sensor In the recent years, molecular imprinting technology (MIT) has been considered as an attractive method to produce artificial receptors obtained with the memory of size, shape, and functional groups of the template molecules. MIPs have been widely used as mimetic molecular recognition receptors with recognition sites for a given molecule structure. The different strategies used in the preparation of MIP, including precipitation polymerization, emulsion polymerization, core–shell approaches, and bulk processes.  The specific recognition property of MIPs was based on the formation of complexes between the appropriate functional monomers with the templates which mainly could bind covalently or non- covalently. Non-covalent interactions between the functional monomers and the template are probably the most flexible regarding the selection of the possible template molecules and the functional monomers (ex: acrylic acid, methacrylic acid, acrylamide…etc). These complexes were then immobilized by copolymerization with a high concentration of cross- linkers (Ethylene glycol dimethacrylate (EGDMA)) in the presence of iniator (ex: AIBN, potassium per sulphate.  After the polymerization was complete, the templates were removed, providing binding sites in the MIPs that had complementary shapes, sizes, and functionalities toward the target template. With these tailor-made binding sites, MIPs not only recognize the shape and size of a given template but also respond to the functional groups of the molecule. Molecular imprinting technology Self-assembling approach Pre-organized apporach Non-covalent Metal coordination Covalent interactions interactions interactions semi-covalent imprinting polymers Gas Sensors  pCO2 Electrode The measurement of pCO2 is based on its linear relationship with pH over the range of 10 to 90 mm Hg. H2 O  CO2  H2 CO3  H   HCO3  The dissociation constant is given by  H  HCO    k 3 a  pCO2 Taking logarithms pH = log[HCO3-] – log k – log a – log pCO2 Biosensor - an analytical device that exploits a biocatalytic reaction Consists of: biocatalyst (enzyme, cells, tissue) transducer (converts the biological or biochemical signal into a quantifiable electrical or optical signal)  First biosensor - Clark (1962): glucose sensor with glucose oxidase and oxygen electrode Glucose + O2 Gluconic acid + H2O2 Oxygen electrode (1956) working electrode: Pt cathode (-0.6 V) reference electrode: Ag/AgCl electrodes separated from measured solution with a gas permeable mebrane Leland C. Clark, Jr. with the first enzyme electrode Personal glucose meter for diabetics (Medisense Britain, Ltd.)