Ion Selective SensorsMembrane Fabrication Technologies for

Industrially, Environmentally &Biologically Significant Measurements

Advanced Sensor Technologies, Inc.603 North Poplar Street, Orange CA 92868 USAWebsite: www.astisensor.com Tel: 714-978-2837

Thomas A. D. PatkoDecember, 2003

Table of Topics for Presentation

ØIntroduction and Significance of IonSelective ElectrochemistryØOverview of Glass, Solid State and

Ionophore Based Ion Selective Sensors andReference ElectrodesØCharacterization of Passive (Ionophore

Based) Membrane Ion Selective SensorPerformance

Introduction to Ion Selective Sensors& Reference Electrodes

vHalf-Cell TypesvReference Electrode

TypesvGlass, Silver Halide

& Ionophore BasedIon SensorsvJunction PotentialsvReversibility

Nernst Equation:E = E0(T) + (RT/zF) ln (c(OX)/c(RED))At T=25, E = E0 (25 oC) + (0.05915/z) log (a(OX)/a(RED))

[7]

Traditional & Solid Contact ElectrodesvTraditional Electrolyte Electrodes with Ag/AgCl or Calomel

half cells offer superior reversibility & performancevSolid Contact Electrodes offer miniaturization & convenience,

primarily for use in in-vivo and in-vitro measurements[6] [8]

Equilibria Ionophore Carriers

vNeutral and ChargedBased Carriers are themost common IonophoretypesvIon Exchanger Based

Carriers are primarily usedfor measurements thatcannot be performeddirectly (carbon dioxide,ammonia)vIonophore Solubility

(leaching) & Ion Mobility

[7]

Ionopore Binding CharacteristicsvIonophores can be

natural or synthetic(natural valinomycinpotassium ionophoreshown to the right)vHigh Binding to analyte

ion and low binding tointerfering ions

vElectrostatic Binding of Selective Ions (illustrated onfollowing slide for valinomycin with and without potassium)vSolubility in lipophilic media is of great importance for

practical use of ionophores (low aqueous solubility desired)

[11]

Valinomycin Binding [11]

Linear Measurement Range &Detection Limits

vTypical linearmeasurement range of 10-5

to 10-1 Molar (activity) formost IonophoresvUsable activity range

often varies greatly fromlinear range for ultra-lowmeasurementsvUncertainty of

measurement increasesgreatly with lowerconcentrations

[11]

Selectivity CoefficientDetermination Methods

ØSeparate Solutions Method (Nickolskii-Eisenman)ØFastest method for simple systems

ØFixed (Constant) Interference MethodØMost common method employed

ØMultiple Solutions MethodØBest method for complex and physiological systems

ØMatched Response Method (Not Discussed)ØComplex mathematical correctional method - an

improved version of the fixed interference andmultiple solutions methods

Separate Solutions Method

vAdvantages:vSpeed and ease ofdeterminationvCan determine a large array ofinterfering ion selectivitycoefficients very quicklyvUsed for simple flow injectionpotentiometry applications (simpleand well defined systems)

vDisadvantages:vDoes not account for any errordue to multiple ion interactionvOverly simplistic method for realsolutions, often giving verydifferent coefficients than othermethods

[10]

Fixed Interference Methodv Advantages:

v Accurate for a larger variety ofsystems than separate solutions

v Relatively simple to perform for areasonable set of potentialinterfering ions of interest

v More accurate than separatesolutions that gives good(reasonable) data for most realworld systems

v Coefficients translate fairly well tomany observed applicationselectivity performance

v Disadvantages:v Does not account for all multiple

ion-ion interactions, onlyinterfering ion-analyteinterference

v Poor match of performance formost physiological fluids (serum,whole blood, urine )

[7]

Mixed solutions Method

v Advantages:v Accurate for almost all stable

systems, even if complexv More accurate than fixed

interference solutionv Method gives very good data

for complex systemsv Disadvantages:v Very cumbersome to perform if

the system has any variancethe ionic background

v Laboratory technique anduncertainties of measurementare of great importance

[7]

Variation of Selectivity CoefficientsDepending Upon Method Employed

[7]

Reference Electrode Uncertainty

[4] [4]

vThe invariance of the reference electrode potential is the basis of all ionselective electrochemical measurements.vDeviations from this (stable) invariant potential (commonly termed referencejunction potential) constitutes the largest source of overall uncertainty for anyelectrochemical measurement.