ion selective sensors and electrodes technologies for industrially

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.comTel: 714-978-2837

Thomas A. D. PatkoDecember 2003

Introduction

Ion selective sensors have been used for analytical determination of a wide variety of ions since the 1900 s. Ionselective sensor s utility and simplicity has replaced other wet analytical methods that were often far slower and morecumbersome to perform. Although it would be impossible to give a comprehensive review of previous work here, (oreven to give a complete overview) a brief overview is desirable to properly assess the areas of interest in the currentliterature and establish its place amongst previous research. In particular, very limited consideration is given to ionselective field effect transistor technologies, due primarily to their very poor commercial acceptance and extensivetechnological difficulties of their fabrication for the relatively low volume demand in the ion selective measurementindustry (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 ionelectrode of the first kind [M|M+], speed of response and reversibility is of critical importance for accuracy andreproducibility of measurements. In fact, the issue of reversibility and consideration of all electrochemical systems asequilibrium processes was one of the major contributions of Nernst. The Nernst equation describes that a change inpotential of an electrochemical system is linear to the change of the ion activity (in logarithmic units) of the selectedanalyte 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, andhaving 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 andselectivity over interfering ions. In addition, the operational pH, temperature and pressure limits of the sensor greatlydetermine its usefulness in real world industrial and laboratory applications. Another very important criterion for theutility of any given sensor is the expected lifetime under constant use. This consideration will be discussed at some depththroughout this paper. In the more recent literature, biocompatibility of ion selective sensors for both in-vivo and in-vitrouse has been a very important consideration as the use of biosensors is expected to be one of the fastest growing fields inelectrochemistry [3].

The Reference Electrode

The reference electrode is the holy grail of electrochemical measurements from the inception of the first pH glassand silver halide sensors. Both the measuring electrode and the reference electrode are redox half-cell reactions that existin equilibria, and cannot be measured separately from each other. In much the same way, the Debye-Huckel ion activitymodel is most commonly used in electrochemistry as it is accurate to about 10-2 or 10-1 Molarity, which is the limit ofmost ion selective sensors. The Debye-Huckel model expresses the fact the only the ion activity of the cation-anionsystem can be determined, but not the individual activity for only the cation or anion. The ability to measure the relative

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Thomas Patko, ASTI, Dec 2003, Ion Selective Sensors Overview; Website: www.astisensor.com

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potential difference between measuring ion selective sensor and standard reference electrode whose potential isinvariant with any changes in ion concentration of the measured solution is the foundation of modern electrochemistry. Itshould be noted that reference junction potential, which is generated by the difference in ion mobility between cations andanion species in solution. The single largest source of errors, for most pH and ion selective measurements, is this junctionpotential. To complicate real measurement further, often the reference systems exhibiting the smallest junction potentialhave some of the shortest lifetimes. Often the more rugged systems, with lower ion mobility that is required for moreaggressive measured solutions, exhibit higher junction potentials that must be compensated by calibration methods. Themost mobile ions in solution are the hydrogen ion and hydroxyl ion. This explains why some of the highest junctionpotentials are observed in very high and very low pH solutions.

The invariance of the referenceelectrode potential is the basis of allelectrochemical measurements. Deviationsfrom this (stable) invariant potentialconstitutes a large percentage ofmeasurement uncertainty, and are thesubject of many papers regardingcalibration methods and the realisticaccuracy of electrochemical systems. Theclassical reference electrode employs aporous ceramic or plastic interface which isimpregnated with an electrolyte solutionsuch as potassium chloride or a gel thatcontains such an electrolyte solution.

The reference potential is (mostcommonly) generated by a silver wire thathas been chloridized or been dipped intomolten silver chloride. The potential of thereference electrode, while ideally invariantto the ion activity of the measured solution,is also a function of the inner fillingsolution of the reference electrode. Similarsystems using mercurous chloride, mercuryliquid and a platinum or silver electrode cansubstituted for the typical, Ag/AgCl/Cl- forsome laboratory applications requiring highaccuracy.

Both the membrane formulations and EMF are from reference [4]

Many novel papers discussfabrication of solid state contact referenceelectrodes, avoiding the known problemsassociated with traditional electrolytereference systems [4, 5]. These papersdiscuss the use of plasticized PVC,polyurethane and silicone rubbermembranes, with a variety of additives tocreate a stable potential over wide range ofpH and ionic strengths. The typicalproblems of drift, stability and longevity areaddressed for solid contact electrodes.

The potential of these solid statereference electrodes is a function of thematrix employed, and both the amount andtype of plasticizers added. Selectedlipophilic salts serve to reduce the bulkresistance of the membrane, whereas theplasticizer increases the solubility of the



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lipophilic salts added. EMF potentials inmany common weak electrolytes solutionsfor various silicone and polyurethane basedmatrix formulations are shown on theprevious page and to the top right. Inaddition, the pH dependence is shown to theright. The pH dependence is of tantamountimportance, particularly in many in-vivoand in-vitro medical diagnosticmeasurement. All of these membraneswere intended primarily as referenceelements for blood and serum media.

Both the EMF graphs and pH dependencecurves are from reference [4]

Traditional Laboratory Ion Selective Measurement Setup

To the left appears a traditional ion selectivelaboratory setup. Note that the Calomelreference electrode is used for stability andaccuracy in many laboratory measurements,although it is all but absent for meaningfulindustrial and medical measurement, due tomercury toxicity and its inability to operateover a wide range of temperature (usually notused above 40 oC). The ion selectivemembrane is traditionally applied over aporous ceramic that has been bound to thePVC plastic body, with a weak electrolyteinner filling solution and an Ag/AgClelectrode. It should be noted that the ionselective electrode potential is a function ofthe inner electrode filling solution, just as isthe 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 ionselective sensors are compared. Only the pH glass electrodes have gained widespread acceptance although many othercation selective (Na+, K+, Ca++) glass electrodes have been formulated [6]. This is primarily due to problems of lowselectivity and high drift associated with these cation selective glass electrodes. Many of these measurements are nowalmost 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 pressurehave made it the undisputed standard for hydrogen ion [H+] activity determination. Recent research in pH sensortechnology has focused primarily on developing the stability and lifetime of the reference electrode.



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Although the introduction of the ion selective field effect transistor (ISFET) as a measuring element for pH isnovel development in the sensing element of the pH sensor, this has had little commercial impact in laboratory andindustrial 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) equipmentcost of manufacturing, sophistication level of the required software to interface with ISFET s, problems associated withaggressive and fouled industrial media and the fact that most pH measurement difficulties arise from problems related tothe reference system [1,2]. In addition, ISFET based sensors will only function with the software provided by the OEMinstruments to mate with these sensors. The widespread employment of conventional pH and ion selective sensors is duegreatly to the standardization of the laboratory and industrial meters such that most any manufacturer s pH or ion selectivesensor can be interfaced to most any meter. There are some notable exceptions to this general premise, although they arepurely 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 formeasurement of halide ions, and other related measurements that are possible with this class of electrodes. These ionmeasurements include Silver (Ag+), Chloride (Cl-), Bromide (Br-), Iodide (I-), Sulfide (S-2), Cyanide (CN-), andThioCyanate (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|MSco-precipitates based electrodes where M is any divalent heavy metal cation that forms a stable sulfide precipitate such aslead, copper or cadmium have been investigated for quite some time. Although these ion selective electrodes have beencommercially available, their problems with drift, stability and redox sensitivity have prevented them from beingemployed to any significant extent. The environmentally significant copper and lead measurements, in particular, will bediscussed in the subsequent sections on ionophore based PVC, polyurethane and silicone based ion sensors. Simpleillustrations of some older and newer electrode assemblies are shown below and will be discussed in greater depththroughout this paper.[6] [8]

To the far left, thetraditional ion selectivecombination electrode isillustrated, in this casewith a calomel styleinternal electrode half-cellfor the ISE. The otherillustrations are of ISFETbased (top) and a solidcontact type (bottom) ionselective electrodes. Thebinding of the ISEmembrane to the siliconegate and Ag/AgClsubstrate is one of theprimary difficulties forboth solid state electrodes.



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Ionophore Based Passive Membrane Electrodes Overview

Many measurements that cannot beperformed by use of ionically conductivesparingly soluble inorganic salts (such as silverhalides) nor by ion selective glass electrodes (suchas pH). For the vast majority of measurements,the use of ionophore based sensor is required. Anionophore is a neutral or charged carrier (usually alarge organic molecule) that enables the selectivereversible binding of an ion [7, 9].

Ion-Exchanger based ion selectiveelectrodes are quite useful for conversion of non-measurable species such as ammonia and carbondioxide and converting them into measurablequantities such ammonium and carbonate. Theionophore must have a number of desirableproperties to be considered a good ionophore. Itmust exhibit a high binding constant to the ion ofinterest over a wide range of concentration,usually from 10-1 to 10-6 Molar for most ions (seeattached detection limit graph below) [7].

The illustration to the right is from reference [7]

When an ionophore is embedded into a matrix(PVC, polyurethane or Silicon) with suitableadditives (plasticizer to improve the solubility ofthe ionophore), as may be required, it is able toselectively transport ions across a lipophilicmembrane. The addition of plasticizer not onlyincreases the solubility of the ionophore, but alsoincreases the leaching rate of the ionophore andlowers the overall bulk resistance of themembrane. Additives are often also required toimprove the lipophilicity of the membrane, andwill be discussed in subsequent sections. The improved membrane with a lowerimpedance and higher selective ion mobility isrequired to produce a stable electrochemicalpotential for the ion meter. This fabricated ionselective membrane, in conjunction with asuitable half cell (such as the traditional Ag/AgCl)constitutes the ion sensing portion of an ionselective sensor (a reference electrode is alwaysrequired for all electrochemical ionmeasurements). As shown on the right, the linearmeasurement range as well as the high and lowdetections limits are defined by the ISE membranesensitivity.

The graph above is from reference [7].



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Selectivity Coefficient Determination and Significance

One of the most technically complex and dynamicareas of ionophore assessment is the determination ofmeaningful selectivity coefficients. Often, thedetermination of these coefficients can make the differencebetween a paradigm change from previous ionophores to anew generation of ionophores. The most traditional ofthese methods is the Nickolsii-Eisenman formalism. Thissemiempricial method determines coefficients thatcharacterize the relative selectivity of any given ionophoreto a (interfering) given ion. These coefficients are usuallyobtained through the separate solutions method. Thegeneral equations describing the Nickolsii-Eisenmannequation are given to the right.

Although the Nickolsii-Eisenmann methodologydoes have some inherent errors and limitations due to thelack of allowance for multiple ion interactions with theionophore it is still a very useful model and the basis for allsubsequent semiempirical selectivity determinationmethods such as the mixed solutions method, matchedpotential method and fixed interference method. Due to thecomplexity of these subsequent methods, they will not bedescribed in great depth (mathematically) in this paper.

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 givenionophore are only valid for the matrix and plasticizer composition in which they are tested. In addition, each method willgenerate different selectivity coefficients. The only reasonable way to quantitatively compare selectivity coefficients fromdifferent ionophorea (or research groups) is when both similar membranes types are employed and selectivity coefficientsare evaluated using the same method. Due to the many limitations and dependencies on the empirical determination ofselectivity 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 employingdifferent membrane matrix and plasticizer technologies.

The fixed interference method is amongst the most commonly employed for its good mix of realism andsimplicity. One can consider the interference coefficient (as determined by the fixed interference method) to mean thepoint at which the uncertainty of the measurement of the analyte matches the deviation due to the interfering ion. In thisway, the interference point is not only dependent upon the ionophore selectivity to other ions, but also the uncertainty ofthe measurement for a given concentration. The fixed interference concentration is commonly set to 0.1 or 0.01 Molar forsystems with good selectivity and lower concentrations (0.001 or 0.0001 Molar) for systems that demonstrate poorerselectivity [7]. Shown below is a demonstration of the highly dependent nature of the selectivity coefficient. Thevariance between the selectivity coefficient values for the different ions under the Nickolskii Eisenmann (separatesolutions) formalism to the new (mixed solutions) formalism. Some variances are so large, that not only the values, butthe 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):

Separate Solutions Method· Advantages:

§ Speed and ease of determination§ Can determine a large array of interfering

ion selectivity coefficients very quickly§ Used for simple flow injection

potentiometry applications (simple and welldefined systems)

· Disadvantages:§ Does not account for any error due to

multiple ion interaction§ Overly simplistic method for real solutions,

often giving very different coefficients thanother methods

The graph to the right is an example of separate solutions sensitivityto an array of different (interfering) ions from reference [10]

Fixed (Constant) Interference Method· Advantages:

§ Accurate for a larger variety of systems thanseparate solutions

§ Relatively simple to perform for areasonable set of potential interfering ions ofinterest

§ More accurate than separate solutions§ Method gives good (reasonable) data for

most real world systems§ Coefficients translate fairly well to many

observed application selectivity performance· Disadvantages:

§ Does not account for all multiple ion-ioninteractions, only interfering ion-analyteinterference

§ Poor match of performance for mostphysiological fluids (serum, whole blood,urine ) The graph above is taken from reference [7]

Mixed Solutions Method· Advantages:

§ Accurate for almost all stable systems, evenif complex

§ More accurate than fixed interferencesolutions

§ Method gives very good data for complexsystems

· Disadvantages:§ Very cumbersome to perform if the system

has any variance the ionic background§ Laboratory technique and uncertainties of

measurement are of great importance

The complex graph to the right demonstrates the behavior of anionophore to multiple ion environment as given in reference [7].



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Ionophore (Ligand) Binding Mechanisms and Visualization

Visualization of the role and function of anionophore is central to the ability to engineer newionophores and improve the performance ofexisting ionophore. Although crown ether basedionophores are an area of intense research incurrent literature and are the easiest ionophore tovisualize, they will not be shown here ordiscussed in this paper due to their intenselypoisonous characteristics and resultant difficultyin commercial implementation. Rather theionophore valinomycin will be shown. Thisnaturally occurring ionophore has been the mostwidely used ionophore for the detection ofpotassium. Although many other potassium ionophorenow exist which demonstrate superior selectivityand performance to valinomycin, its stability,excellent lifetime and reproducible results in awide range of matrix and plasticizers continue tomake it a widely used and researched ionophore[11].It should be recalled that although a high (andselective) binding coefficient to the analyte ion ishighly desirable, this binding must be of areversible nature (and not of a pseudo-permanentnature such as CO-Heme binding).

The illustration of valinomycin above is taken from reference [11].The X-Ray crystal structure below of complexed and uncomplexedvalinomycin 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 itis able to retain it properties over a long period of time. Novel fabrication technologies seek to move away from the useof solvent polymeric PVC based ISE membranes and use polyurethane and silicon based matrix substrates that exhibitsuperior properties in terms of lower ionophore (ligand) leakage, less plasticizer requirements and better overallselectivities and lifetime [3, 13]. Very often fabrication technologies can determine whether a given ionophore hasdesirable enough quality to be widely employed for their intended applications. Ionophores have been reported for a verywide 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+). Ionophoreshave 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].


ion selective sensors and electrodes technologies for industrially

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