Ion Selective Sensors and Electrodes – Technologies for Industrially, Environmentally and Biologically Significant Ion Measurements
Advanced Sensor Technologies, Inc.
603 North Poplar Street Orange California 92868 U.S.A.
Website: www.astisensor.com
Tel: 714-978-2837
Thomas A. D. Patko
December 2003
Introduction
Ion selective sensors have been used for analytical determination of a wide variety of ions since the 1900’s. Ion selective sensor’s utility and simplicity has replaced other wet analytical methods that were often far slower and more cumbersome to perform. Although it would be impossible to give a comprehensive review of previous work here, (or even to give a complete overview) a brief overview is desirable to properly assess the areas of interest in the current literature and establish its place amongst previous research. In particular, very limited consideration is given to ion selective field effect transistor technologies, due primarily to their very poor commercial acceptance and extensive technological difficulties of their fabrication for the relatively low volume demand in the ion selective measurement industry (including all of the demand for pH sensors) [1, 2].
Types of Electrodes
Most analytical sensors are electrodes of the second kind. As with all electrodes that are not metal-metal ion electrode of the first kind [M|M+], speed of response and reversibility is of critical importance for accuracy and reproducibility of measurements. In fact, the issue of reversibility and consideration of all electrochemical systems as equilibrium processes was one of the major contributions of Nernst. The Nernst equation describes that a change in potential of an electrochemical system is linear to the change of the ion activity (in logarithmic units) of the selected analyte ion. It is clear that the potential is dependent upon temperature.
E = E0(T) + (RT/zF) ln (c(OX)/c(RED))
The equations given below is valid at 25 degrees Celsius, with the activity of all solid and liquids taken to be unity, and having been transformed from natural log to decadic log units:
E = E0 (25 oC) + (0.05915/z) log (a(OX)/a(RED))
Some critical issues that arise with all ion selective sensors are detection limit, linear measurement range and selectivity over interfering ions. In addition, the operational pH, temperature and pressure limits of the sensor greatly determine its usefulness in real world industrial and laboratory applications. Another very important criterion for the utility of any given sensor is the expected lifetime under constant use. This consideration will be discussed at some depth throughout this paper. In the more recent literature, biocompatibility of ion selective sensors for both in-vivo and in-vitro use has been a very important consideration as the use of biosensors is expected to be one of the fastest growing fields in electrochemistry [3].
The Reference Electrode
The reference electrode is the holy grail of electrochemical measurements from the inception of the first pH glass and silver halide sensors. Both the measuring electrode and the reference electrode are redox half-cell reactions that exist in equilibria, and cannot be measured separately from each other. In much the same way, the Debye-Huckel ion activity model is most commonly used in electrochemistry as it is accurate to about 10-2 or 10-1 Molarity, which is the limit of most ion selective sensors. The Debye-Huckel model expresses the fact the only the ion activity of the cation-anion system can be determined, but not the individual activity for only the cation or anion. The ability to measure the relative potential difference between “measuring ion selective sensor” and standard “reference” electrode whose potential is invariant with any changes in ion concentration of the measured solution is the foundation of modern electrochemistry. It should be noted that reference junction potential, which is generated by the difference in ion mobility between cations and anion species in solution. The single largest source of errors, for most pH and ion selective measurements, is this junction potential. To complicate real measurement further, often the reference systems exhibiting the smallest junction potential have some of the shortest lifetimes. Often the more rugged systems, with lower ion mobility that is required for more aggressive measured solutions, exhibit higher junction potentials that must be compensated by calibration methods. The most mobile ions in solution are the hydrogen ion and hydroxyl ion. This explains why some of the highest junction potentials are observed in very high and very low pH solutions.
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The invariance of the reference electrode potential is the basis of all electrochemical measurements. Deviations from this (stable) invariant potential constitutes a large percentage of measurement uncertainty, and are the subject of many papers regarding calibration methods and the realistic accuracy of electrochemical systems. The classical reference electrode employs a porous ceramic or plastic interface which is impregnated with an electrolyte solution such as potassium chloride or a gel that contains such an electrolyte solution. The reference potential is (most commonly) generated by a silver wire that has been chloridized or been dipped into molten silver chloride. The potential of the reference electrode, while ideally invariant to the ion activity of the measured solution, is also a function of the inner filling solution of the reference electrode. Similar systems using mercurous chloride, mercury liquid and a platinum or silver electrode can substituted for the typical, Ag/AgCl/Cl- for some laboratory applications requiring high accuracy. |
Both the membrane formulations and EMF are from reference [4] |
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Many novel papers discuss fabrication of solid state contact reference electrodes, avoiding the known problems associated with traditional electrolyte reference systems [4, 5]. These papers discuss the use of plasticized PVC, polyurethane and silicone rubber membranes, with a variety of additives to create a stable potential over wide range of pH and ionic strengths. The typical problems of drift, stability and longevity are addressed for solid contact electrodes. The potential of these solid state reference electrodes is a function of the matrix employed, and both the amount and type of plasticizers added. Selected lipophilic salts serve to reduce the bulk resistance of the membrane, whereas the plasticizer increases the solubility of the lipophilic salts added. EMF potentials in many common weak electrolytes solutions for various silicone and polyurethane based matrix formulations are shown on the previous page and to the top right. In addition, the pH dependence is shown to the right. The pH dependence is of tantamount importance, particularly in many in-vivo and in-vitro medical diagnostic measurement. All of these membranes were intended primarily as reference elements for blood and serum media.
Both the EMF graphs and pH dependence curves are from reference [4]
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Traditional Laboratory Ion Selective Measurement Setup
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To the left appears a traditional ion selective laboratory setup. Note that the Calomel reference electrode is used for stability and accuracy in many laboratory measurements, although it is all but absent for meaningful industrial and medical measurement, due to mercury toxicity and its inability to operate over a wide range of temperature (usually not used above 40 oC). The ion selective membrane is traditionally applied over a porous ceramic that has been bound to the PVC plastic body, with a weak electrolyte inner filling solution and an Ag/AgCl electrode. It should be noted that the ion selective electrode potential is a function of the inner electrode filling solution, just as is the reference electrode potential
The illustration is from reference [7]. |
Types of Ion Selective Sensors
Glass Ion Selective Sensors, Including the ubiquitous pH Electrodes
Since its discovery in 1906 by Crème, the pH glass electrode has become the standard against which all ion selective sensors are compared. Only the pH glass electrodes have gained widespread acceptance although many other cation selective (Na+, K+, Ca++) glass electrodes have been formulated [6]. This is primarily due to problems of low selectivity and high drift associated with these cation selective glass electrodes. Many of these measurements are now almost exclusively performed through the use of ionophore based ion sensors as described in later sections of this paper. pH glass’s high selectivity, excellent linear measurement range, and wide range of operating temperature and pressure have made it the undisputed standard for hydrogen ion [H+] activity determination. Recent research in pH sensor technology has focused primarily on developing the stability and lifetime of the reference electrode.
Although the introduction of the ion selective field effect transistor (ISFET) as a measuring element for pH is novel development in the sensing element of the pH sensor, this has had little commercial impact in laboratory and industrial applications despite positive test results in the laboratory and some field measurements, most notably the food, diary and cheese industries. The limited acceptance of the ISFET based sensor has been due to high (capital) equipment cost of manufacturing, sophistication level of the required software to interface with ISFET’s, problems associated with aggressive and fouled industrial media and the fact that most pH measurement difficulties arise from problems related to the reference system [1,2]. In addition, ISFET based sensors will only function with the software provided by the OEM instruments to mate with these sensors. The widespread employment of conventional pH and ion selective sensors is due greatly to the standardization of the laboratory and industrial meters such that most any manufacturer’s pH or ion selective sensor can be interfaced to most any meter. There are some notable exceptions to this general premise, although they are purely artifacts of selective engineering and not a fundamental design issue.
Silver Halide Precipitates
Since their discovery, silver halide precipitate based ion selective electrodes have become widely used for measurement of halide ions, and other related measurements that are possible with this class of electrodes. These ion measurements include Silver (Ag+), Chloride (Cl-), Bromide (Br-), Iodide (I-), Sulfide (S-2), Cyanide (CN-), and ThioCyanate (SCN-). While this is not intended as complete list of all currently available silver halide based precipitates, these are the most commonly used and accepted ion selective electrodes of this class [6]. It should be noted that Ag2S|MS co-precipitates based electrodes where M is any divalent heavy metal cation that forms a stable sulfide precipitate such as lead, copper or cadmium have been investigated for quite some time. Although these ion selective electrodes have been commercially available, their problems with drift, stability and redox sensitivity have prevented them from being employed to any significant extent. The environmentally significant copper and lead measurements, in particular, will be discussed in the subsequent sections on ionophore based PVC, polyurethane and silicone based ion sensors. Simple illustrations of some older and newer electrode assemblies are shown below and will be discussed in greater depth throughout this paper.
[6] [8]
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To the far left, the traditional ion selective combination electrode is illustrated, in this case with a calomel style internal electrode half-cell for the ISE. The other illustrations are of ISFET based (top) and a solid contact type (bottom) ion selective electrodes. The binding of the ISE membrane to the silicone gate and Ag/AgCl substrate is one of the primary difficulties for both solid state electrodes. |
Ionophore Based Passive Membrane Electrodes Overview
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Many measurements that cannot be performed by use of ionically conductive sparingly soluble inorganic salts (such as silver halides) nor by ion selective glass electrodes (such as pH). For the vast majority of measurements, the use of ionophore based sensor is required. An ionophore is a neutral or charged carrier (usually a large organic molecule) that enables the selective reversible binding of an ion [7, 9]. Ion-Exchanger based ion selective electrodes are quite useful for conversion of non-measurable species such as ammonia and carbon dioxide and converting them into measurable quantities such ammonium and carbonate. The ionophore must have a number of desirable properties to be considered a good ionophore. It must exhibit a high binding constant to the ion of interest over a wide range of concentration, usually from 10-1 to 10-6 Molar for most ions (see attached detection limit graph below) [7].
The illustration to the right is from reference [7]
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When an ionophore is embedded into a matrix (PVC, polyurethane or Silicon) with suitable additives (plasticizer to improve the solubility of the ionophore), as may be required, it is able to selectively transport ions across a lipophilic membrane. The addition of plasticizer not only increases the solubility of the ionophore, but also increases the leaching rate of the ionophore and lowers the overall bulk resistance of the membrane. Additives are often also required to improve the lipophilicity of the membrane, and will be discussed in subsequent sections. The improved membrane with a lower impedance and higher selective ion mobility is required to produce a stable electrochemical potential for the ion meter. This fabricated ion selective membrane, in conjunction with a suitable half cell (such as the traditional Ag/AgCl) constitutes the ion sensing portion of an ion selective sensor (a reference electrode is always required for all electrochemical ion measurements). As shown on the right, the linear measurement range as well as the high and low detections limits are defined by the ISE membrane sensitivity. |
The graph above is from reference [7]. |
Selectivity Coefficient Determination and Significance
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One of the most technically complex and dynamic areas of ionophore assessment is the determination of meaningful selectivity coefficients. Often, the determination of these coefficients can make the difference between a paradigm change from previous ionophores to a new generation of ionophores. The most traditional of these methods is the Nickolsii-Eisenman formalism. This semiempricial method determines coefficients that characterize the relative selectivity of any given ionophore to a (interfering) given ion. These coefficients are usually obtained through the separate solutions method. The general equations describing the Nickolsii-Eisenmann equation are given to the right. Although the Nickolsii-Eisenmann methodology does have some inherent errors and limitations due to the lack of allowance for multiple ion interactions with the ionophore it is still a very useful model and the basis for all subsequent semiempirical selectivity determination methods such as the mixed solutions method, matched potential method and fixed interference method. Due to the complexity of these subsequent methods, they will not be described in great depth (mathematically) in this paper. |
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The Nicolskii-Eisenman Equations are from reference [7]
Limitation on Interpretation of Selectivity Coefficients
There are several IUPAC approved methods that are used concurrently by researchers. Selectivities for a given ionophore are only valid for the matrix and plasticizer composition in which they are tested. In addition, each method will generate different selectivity coefficients. The only reasonable way to quantitatively compare selectivity coefficients from different ionophorea (or research groups) is when both similar membranes types are employed and selectivity coefficients are evaluated using the same method. Due to the many limitations and dependencies on the empirical determination of selectivity coefficients, often only the ratios of selectivity coefficients from a given paper can be used to evaluate (compare) ionophores from different research groups, using different selectivity determination methods and employing different membrane matrix and plasticizer technologies.
The fixed interference method is amongst the most commonly employed for its good mix of realism and simplicity. One can consider the interference coefficient (as determined by the fixed interference method) to mean the point at which the uncertainty of the measurement of the analyte matches the deviation due to the interfering ion. In this way, the interference point is not only dependent upon the ionophore selectivity to other ions, but also the uncertainty of the measurement for a given concentration. The fixed interference concentration is commonly set to 0.1 or 0.01 Molar for systems with good selectivity and lower concentrations (0.001 or 0.0001 Molar) for systems that demonstrate poorer selectivity [7]. Shown below is a demonstration of the highly dependent nature of the selectivity coefficient. The variance between the selectivity coefficient values for the different ions under the Nickolskii Eisenmann (separate solutions) formalism to the new (mixed solutions) formalism. Some variances are so large, that not only the values, but the hierarchy of selectivity is affected. The new formalism (B) describes the matched potential method.
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Some Currently Employed Selectivity Coefficient Determination Methods Summary (IUPAC Approved):
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Separate Solutions Method
The graph to the right is an example of separate solutions sensitivity to an array of different (interfering) ions from reference [10] |
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Fixed (Constant) Interference Method
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The graph above is taken from reference [7] |
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Mixed Solutions Method
The complex graph to the right demonstrates the behavior of an ionophore to multiple ion environment as given in reference [7]. |
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Ionophore (Ligand) Binding Mechanisms and Visualization
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Visualization of the role and function of an ionophore is central to the ability to engineer new ionophores and improve the performance of existing ionophore. Although crown ether based ionophores are an area of intense research in current literature and are the easiest ionophore to visualize, they will not be shown here or discussed in this paper due to their intensely poisonous characteristics and resultant difficulty in commercial implementation. Rather the ionophore valinomycin will be shown. This naturally occurring ionophore has been the most widely used ionophore for the detection of potassium. Although many other potassium ionophore now exist which demonstrate superior selectivity and performance to valinomycin, its stability, excellent lifetime and reproducible results in a wide range of matrix and plasticizers continue to make it a widely used and researched ionophore [11]. It should be recalled that although a high (and selective) binding coefficient to the analyte ion is highly desirable, this binding must be of a reversible nature (and not of a pseudo-permanent nature such as CO-Heme binding). |
The illustration of valinomycin above is taken from reference [11]. The X-Ray crystal structure below of complexed and uncomplexed valinomycin are also taken from reference [11]. |
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The utility of any ion sensor is based upon the ability to fabricate the ion selective membrane in such a way that it is able to retain it properties over a long period of time. Novel fabrication technologies seek to move away from the use of solvent polymeric PVC based ISE membranes and use polyurethane and silicon based matrix substrates that exhibit superior properties in terms of lower ionophore (ligand) leakage, less plasticizer requirements and better overall selectivities and lifetime [3, 13]. Very often fabrication technologies can determine whether a given ionophore has desirable enough quality to be widely employed for their intended applications. Ionophores have been reported for a very wide variety of cations including but not limited to : Hydrogen Ion (H+), Sodium (Na+), Potassium (K+), Ammonium (NH4+), Calcium (Ca+2), Magnesium (Mg+2), Lithium (Li+), Copper (Cu+2), and Lead (Pb+2), Cesium (Cs+). Ionophores have been reported for a very wide variety of anions including but not limited to: Chloride (Cl-), Perchlorate (ClO4-), Nitrate (NO3-), Carbon Dioxide and Carbonate (CO2 & CO3-2), and Phosphate (PO4-3) [9, 11].