Moessbauer spectroscopy - Analytical Chemistry (ACS Publications)

Anal. Chem. 1000, 62, 21 R-33 R

Ion-Selective Electrodes Robert L. Solsky E . I. du Pont de Nemours, Inc., Imaging Systems Department, New James Street, Towanda, Pennsylvania 18848

INTRODUCTION This review covers material that is of interest to those who deal with ion-selective electrodes. Potentiometric means of detection do extend to CHEMFETS and ISFETS but these topics will not be included in this work as they are properly covered elsewhere. Material used in this review was obtained from the major analytical and electrochemical journals and a hand search of Chemical Abstracts yielded the remaining important publications. This review is not meant to simply list all of the articles that pertain to ion-selective electrodes but rather to discuss the major topics that have been ublished during the period from November 1987 to Novemger 1989.

BOOKS AND REVIEWS Potentiometry is based on a measurement phenomenon expressed by the Nernst equation. The derivation of this relationship is detailed in a biography of Hermann W. Nernst by Archer (1). The further development of ion-selective electrodes is described by Hulanicki (2)while Thomas (3) traces the history of potentiometry from the first glass electrodes to the very successful crown compounds that impart high selectivity to the plastic membrane electrodes. Bates (4) rounds out the discussion with a description of the role of potentiometric sensors used for the determination of the single-ion activity. All of this historical background is combined with the current thrust into biosensor development with a discussion by Monroe (5)on present applications and future trends. A continuin series of reviews by Koryta (6)complements his earlier worts by covering the varied a plications of ionselective electrodes. Pungor et al. (7)descri the mechanistic aspects of electrode function and response. The fundamentals of membrane preparation and application are given by Oehme (8)while Evans (9)offers us a textbook on potentiometry and ion-selective electrode use. Recent trends in pH sensors (10) have been reviewed as well as measurements of pH in aqueous and nonaqueous systems (11). The technique of bipolar pulse conductance measurements using ion-selective electrodes for fluoride and calcium ion conductance determination is described (12). The use of electrodes for microsampling biochemical systems is reviewed (13)and illustrates the concentrative tranfer of an analyte for enhanced detection. Another technique is described where arrays of ion-selective electrodes are used for multicomponent anal sis (14).We will hear more about this in sections that follow. ere is a market trend review by Oehme (15)that specifies and defines new types of sensors that are available. Ion-selective electrodes and biosensors have found applications in industrial processes such as fermentation control (16,17)and pharmaceutical analysis (18-23). Electrodes have also found great utility in environmental analysis of effluents and wastewater streams (24-26).Specific methods have been reviewed for the analysis of organofluorine compounds by measuring released fluoride ion (27). Generalized reviews include the applications of ion-selective electrodes in the petroleum industry (28)and for nuclear waste containment monitoring (29). Manv tvDes of Dolvmers have been utilized for the fabrication of &mer:mgmbrane ion-selective electrodes. Man of these polymers are discussed in a review by Moody, SaadI and Thomas (30)while another paper describes the lac uer, Urushi (31).Chelates have been used as mediators in Lese olymer-membrane electrodes (32)whereas the bulk of pubications deal with crown ethers (33)and other neutral carriers (34-39). Work involving advanced forms of this class of electrodes includes developing ion-channel sensors (40)and uphill transport sensors (41).

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0003-2700/90/0362-2 1R809.5010

Increased selectivity and res onse characteristics of ionselective electrodes can be rearized by coupling biological agents to their membranes (42-45). Typically, enzymes, microbes, and specific proteins are coupled with ion- or gasselective electrodes to create biosensors that are capable of respondin to a spectrum of analytes (46-56).Microbial or whole-cell [iosensors have been extensively reviewed in terms of their construction, principle of measurement, and application (57,58).Immuno-based sensors are capable of monitoring antigen or antibody titers (59,60).Animal and plant whole tissues can be similarly coupled with ion-selective electrodes to create novel biosensors (61-66). Neuronal (67) and olfactory (68)sensors have been developed by utilizing animal receptor and taste organ constituents coupled to membrane electrodes. Recent trends and progress in biosensor development have been reviewed in detail (64-80).The design of biosensors and their incorporation into instrumentation have been discussed (81)as well as the market opportunities for biosensor commercialization (82). The ap lications of biosensors have been equally well covered in t rle literature (83-91). Ion-selective electrodes and biosensors have found great utility in clinical and diagnostic applications (92-951,microbiology (%), and medicine (97-100).The use of ion-selective electrodes in clinical analyzers has been reviewed (101)and more specific discussions explain urea (102)and ammonium and chloride ion (103)measurements of biological fluids. The difficulties of measuring ionized calcium by direct potentiometry in blood and urine is described (104),while the influence of proteins and blood cells on the measurement of electrolytes is discussed (105-107). Microelectrodes allow the measurement of both inter- and extracellular electrolyte levels. The components and construction of these microelectrodes have been reviewed for the measurement of ions within cells (108).Small glass capillaries can be filled with neutral carriers to create sensors for all of the common ions (log),or liquid ion exchangers can be used to sense sodium, potassium, or calcium (110). Calcium-selective microelectrodes have been further developed and the limitations of high membrane resistance, low exchange currents, and sensor conditioning and calibration are described (111,112). Ion-selective electrodes are especially suited for detectors in flow analyzers. Flow-injection analysis has made good use of the inherent advantages of potentiometric sensors (113, 114). A multiion flow-injection analyzer that determines cations and anions simultaneously in drinking water illustrates the successful coupling of these two techniques (115).Fouling of the sensing membrane can reduce sensitivity and selectivity but can be corrected through mechanical cleaning on a periodic basis (116). Detectors for chromatographic systems have proved viable as was demonstrated by using chiral crown ethers in liquid membrane electrode sensors to optically resolve amino acids (117).

GENERAL TOPICS Many of the early uses of ion-selective electrodes were to determine formation and bindin constants of ions with a variety of complexing agents. Tiere have been a series of publications dealing with the application of ion-selective electrodes for formation constant experiments. May and Murray (118)have discussed the use of glass electrodes and have warned against the indiscriminate use of computer programs that use unknown wei hing factors for the calculations. They also describe (1197ways to quantify the precision of such methods using a Monte Carlo analysis of error assessment. Besides differences in titration parameters, it is 0 1990 American Chemical Society

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ION-SELECTIVE ELECTRODES

also important to understand chemical effects that occur during formation constant measurements. The conformation of troponin influences calcium binding and is further affected by competitive binding agents in the determination of the formation constant (120). Electrodes have also found similar applications in closely related fields. The diffusion coefficients of ions in soils were measured by plotting observed potentials versus distance in an electrodialyzed soil sample (121). The kinetic parameters of the enzymatic hydrolysis of butyrylcholine by cholinesterase was shown by using a potassium electrode (122). Gaseous systems can also be studied. The second dissociation constant of hydrogen sulfide was measured with a sulfide-selectiveelectrode under varying ionic strengths and sodium sulfide concentrations (123). TRIS buffers can often lead to uncertainties in pH measurements. The pK value for protonated TRIS was calculated in concentrated perchlorate solutions and resulted in a means to calibrate pH electrodes in these solutions with TRIS buffers (124). There are several standard methods of potentiometric analysis that can be found in generalized texts and review papers. However, when one is faced with an analytical system that defies straightforward troubleshooting, a different approach is in order. An easy and fast way to screen many variables at one time takes the form of the experimental desi of Plackett-Burman (125). In this study, the nitrate, chlorig and cyanide electrodes were studied in terms of chemical interferences and instrumental parameters. The condition of the membrane and presence of interfering species overwhelm stirring rate and response time effects. In another approach, a computer program iteratively solves a doublestandard addition method by assuming beginnin potential and slope and by knowing the ionic strength (126f. A blank solution of known composition and ionic strength is similar to the standard addition solution. Blank corrections can also be used to advantage as in the case of measuring fluoride levels a t the low nanogram per milliliter range (127). For potentiometric titrations, linear regression methods are utilized for finding the equivalence volumes to calculate concentrations of unknowns (128). The response of ion-selective electrodes plays a major role in the way they can be applied to measurement processes. Buck and Berube discuss several anomalies in the theory of the time response of liquid membrane electrodes to activity steps (129). They show how the mass-transport equations were unable to describe the time-dependent nature of the response curves. Solvent polymeric membrane electrodes typically display very fast response times. The dynamic response of membranes was studied in parallel with spectroscopic analysis of phase boundary constituents to show that the bulk membrane processes only effect the long-term behavior of the electrode (130). Temperature plays a role in the response characteristics of ion-selective electrodes. Silver and mercury salt electrodes were tested by observing operational temperature range, slope, and standard potential versus temperature, isopotential points, response times, and hysteresis curves (131, 132). Corrections must also be taken into account when dealing with solutions that contain more that just water. Correction factors for potential readings of glass electrodes have been determined for dimethyl sulfoxide solutions (133). The selectivity characteristics of ion-selective electrodes make their use attractive in tough analytical situations. Thus, the measurement of electrode selectivity is important for proper application. Sandifer explains to us that selectivity coefficients may depend on kinetic factors as well as the well-recognized thermodynamic parameters (134). Many have used immiscible oil-water interfaces and extrapolate coefficients from voltammetric data (135) while others use complexing or precipitating agents to determine selectivity that agrees better with thermodynamic values (136). Personally, I prefer a mixed solution method and a new twist on this theme fixes the sum of logarithms of the primary and secondary ions constant so that a graphical treatment of the data yields the selectivity coefficient (137). Reference electrodes contribute to more errors of measurement than any other source. As such, the designs of many free diffusion junctions have been described which eliminate the clogging of conventional reference electrodes (138). Others have combined the functions of the saturated potassium chloride solution and an agar salt bridge in a compact reference electrode (139). Additional efforts have focused on solid-state 22R

ANALYTICAL CHEMISTRY, VOL. 62,NO. 12,JUNE 15, 1990

reference elements that are constructed from pressed and sintered potassium chloride bound together with inorganic salts and fluorinated polymers (140). Potentiometric measurements have been shown to be adversely affected by a suspension effect caused either by potassium chloride solution flowing from the reference junction or by dissolution of silver chloride from the internal reference element (141). Under many conditions an analysis cannot be performed by using a conventional indicating and reference electrode pair. In such cases it is advantageous to use pairs or arrays of ion-selective electrodes. Using this approach, carbonate and bicarbonate were measured simultaneously with a H electrode and a carbon dioxide gas-selective electro& (142). Titrations of fluoride in the presence of high concentrations of chloride were carried out by using a fluoride sensing electrode with a chloride reference electrode (143). Where the selectivity of the sensing electrodes is not suffficient for an application, arrays of electrodes can be used, which shifts the emphasis from selectivity to stability and reproducibility (144). In another example of this technique, four potassium and four ammonium electrodes were used for ion analysis in multicomponent mixtures (145). Additional data handling functions have been applied to arrays that include a new form of regression analysis that is referred to as projection pursuit analysis (146). Mixtures of sodium and potassium solutions were analyzed in this fashion. The response of ion-selective electrodes has been enhanced by placing the sensors in series (147). In this way, three potassium electrodes were connected and displayed response slopes of 58,116, and 174 mvlactivity decade, respectively (148). Even though the slope, sensitivity, and precision increased dramatically, the response time, selectivity, and detection limit remained unchanged (149). Ion-selective electrodes have been applied to a wide variety of analytical problems utilizing some of the principles and techniques that have been outlined above. These include the determination of boric acid in plating baths using a tetrafluoroborate-selective electrode (150),measurements of sodium and potassium in Spanish wines (151), assay of chloride content in fresh concrete (152), and measurements of trace level impurities in nuclear materials (153). Soil (154,155) and water analysis (156) also benefit by the use of enhanced methods with ion-selective electrodes since they represent samples with widely varying ionic strengths and suspended solids. Low conductivity water measurements have been performed with specially designed combination pH electrodes (157). Additional aids to the users of electrode systems include programmable controllers for multichannel measurement systems (158),a computer-controlled titration apparatus (159), a potentiometric ion analyzer (160),and an amplified electrode to reduce problems associated with the high impedance of sensing membrane electrodes (161).

SOLID STATE AND GLASS ELECTRODES Several properties of the many types of solid-state ion-selective electrodes have been recently studied. The ion-exchange processes that occur a t the surface of the lanthanum fluoride crystal membrane electrode have been shown to penetrate into the bulk of the sensing membrane (162). Additionally, water interaction at the membrane surface produces a gel la er much in the same fashion as with glass electrodes (163). &ngle-crystal ion-selective electrodes have been shown to respond in unique ways. The cadmium sulfide single-crystal electrode responds in a crystal-face-specific mode (164). Crystals can be obtained that have crystallographic polarity where one face responds to cadmium while the other responds to sulfide. Others have shown that crystal membrane electrodes are affected more significantly by oxidizing agents than are glass electrodes (165). Lead-selective electrodes prepared from crystalline and glass matrixes were exposed to oxidants and spectroscopically examined for differences in the chemical constituents and charge states on the membrane surfaces. The electrical conductivity of heterogeneous membranes is further affected by the solution that contacts the electrode (166). This was the result of moisture uptake and ionic group distribution within the membrane. Applications of current across copper sulfide membranes alters the composition ratio of the membrane by changing the valence state of the copper (167). The response of solid-state electrodes can be altered or enhanced through a variety of techniques. Silver sulfidesilver halide membrane electrodes have been studied in terms of

IOKSELECTIVE ELECTRODES Rob.rt L. sokky is a Seniw Process C b r n 1st wlh the Imaging Systems DBpanment. E. 1. du Pcmt de N e w s a Co. p r i m to mi posnion. he was a research chemist in the New Technology Research group of the Biomedicai Prcducis hpartment. He received his Ph.0. in Analytical Chemisny in 1980 from the Slate Universky 01 New York at BuflalD with FTOfeSSor G. A. Rechnb and his B.A. in Chemistry in 1975 from lthaca College. Ilhaca. NY. under the guidance of Professor H. F. Koch. His cunent interests include Ion-selective elecnoMt techniques and munidisciplinary analyiical applications Whin a manufacturing environment.

their temperature dependence (268). Measurements and calibrations were performed at different temperatures to evaluate temperature ranges, response slopes, standard and isopotential points, response times, and hysteresis curves. Dissolved oxygen has heen shown to adversely affect the response to silver sulfide membranes to sulfide as compared to silver sulfidesilver iodide membranes (169). Copper-selective electrodes respond better in water solutions containing dimethyl sulfoxide, dimethylformamide, or ethanol for direct and titrimetric analysis of metal content in organometallic materials (170). Silver sulfide electrodes can he used over a wide pH range when they are calibrated in conjunction with pH measurement and correction by plotting potential versus a combined sulfide and pH function (171). Trace fluoride measurements can he performed by calibrating a lanthanum fluoride electrode with buffers of aluminum fluoride complexes to increase accuracy over a much greater linear response ( I 72). Ionic interferences can he suppressed for the copper-selective electrode by coating the membrane with a perfluorosulfonate resin (173). This coating reduces chloride permeation to the copper electrode and allows the sensor to function over much wider chloride ion concentrations. Glass electrodes are relatively well understood in terms of their function and response. The theoretical aspects of the pH glass electrode, the potential generation, and the mechanism of the ionic response have been discussed (174). The work that has occurred of late involves the accuracy and transient response of pH glass electrodes in low ionic strength solutions. The precision and accuracy would appear to correlate well with theoretical calculations if it were not for liquid junction potential errors (175). Combination electrodes using either the high flow rate or conventional low flow rate liquid 'unctions performed similarly when used to measure pH in ow ionic . . strength, dilute acid samples (276). The sluggish response of the glass electrode to dilute water samples reflects low concentrations of cations to a greater degree than the response to the low ionic strength or buffer capacity (177). Several examples of new membrane compositions have appeared that offer alternate approaches to ion analysis. These include heterogeneous membranes selective for potassium ion that are based on precipitates of potassium cohalticarbonate dispersed in a epoxy matrix (178). The polycrystalline material, Nasicon, responds to sodium ion and can utilize a solid-contact internal reference element ( I 79). Electrodes for bromide, iodide, and thiocyanate have been prepared hy using nitron salts in a silicone rubber membrane (180). The functional properties and selectivity coefficients of these electrodes make them useful for potentiometric titrations. The level of chloride ion in metallic sodium was determined by using an electrode based on a mercury(I1) sulfide-mercury(1) chloride membrane (181). When used within a narrow pH range, membranes of tin(1V) arsenate in epoxy respond with good selectivity to ferric ion (182). Dithiocarbamates have been determined by using a heterogeneous membrane composed of silver tetramethylenedithiocarhamate-silver sulfide in a graphite matrix (183). A vanadium oxide bronze alloy has been reported that responds to cesium ion and is selective over alkali and alkaline-earth metals (184). Pressed salt membranes of nickel phosphate and silver carbonate respond to phosphate and carbonate ion, respectively (285,186). Bare carbon fibers show a response to pH (187). The response of these fibers was studied in their pristine state, in the presence of complexing anions and also following electrochemical treatments to understand the re-

1

sponse function. A combination glass electrode that responds to sodium ion was constructed and demonstrated for sodium activity measurements in the temperature range from 100 to 600 "C (188). Bromide-selective electrodes based on chalcogenide glassy-crystalline alloys of silver bromide-silver sultidearsenic sdiide displayed superior response characteristics to their conventional counterparts (189). Solid-state membrane electrodes have found use in a great number of analytical applications. The fluoride electrode has been described in works ranging from microsampling (190) to usage under high temperature and pressure conditions (191). I t has been used for the determination of fluoride content in rice plant leaves using an ultrasonic extraction procedure to solubilize the available fluoride (192). Fluoride has been separated and concentrated from aluminum samples by using hexamethyldisilylamine (193). This reagent forms a volatile trimethylfluorsilane which is then absorbed in water releasing fluoride ion. Another reagent, l-fluoro-2,4-dinitrobenzene, has been used for the kinetic determination of amines (194) and phenols (195) by monitoring the release of fluoride ion. Similarly, acetic acid can he determined in ethanol or vinegar by monitoring the potential of a fluoride-selective electrode that responds to changes in a background electrolyte, tetraethylammonium fluoride (196). Fluoride-containing glucocorticosteroids were analyzed by simply mineralizing the sample and measuring the subsequent fluoride levels that were generated (197). The silver sulfide electrode has been equally well utilized to assay many materials. It is especially useful in potentiometric titration applications such as for acetylene derivatives (198), amines and alkaloids (199), rimary and secondary amines following their reaction with ca&n disulfide (ZOO), and halogen-containing osmium carbonyl derivatives (201). The sulfide content of cements was directly determined with a silver sulfide electrode after distilling the sulfides out by adding hydrochloric acid (202). The use of wire-type supports has been reported for deposited-on-wire sulfide and cyanide sensors (203),silver sulfide and silver iodide coatings on graphite (2041, and sulfided copper wire for sulfide analysis (205). Additional applications of solid-state electrodes illustrate their adaptability to widely varying analytical situations. A copper-selective electrode was used for the titrimetric evaluation of semisynthetic penicillins (206). Hexacyanoferrate(I1) was titrated by using a heterogeneous membrane composed of silver sulfidesilver hexacyanoferrate(I1) (207). A direct electrode was prepared for amphoteric doxycycline by combining anion and cation salts of the analyte in a pressed pellet (208). Other examples include the assay of N-chloro derivatives using a chloride electrode (2091,determination of molybdenum with a lead electrode (210), and measurement of thiosulfate with an iodide sensor (211). The acid cunknt in the paper of old and precioui hooks has been nundestructively determined hy using an inverted pH electrode 1212). A miniature antimony pH electrode has been used to measure the pH at the soil-root interlace uf plants and correlates well to conventional glass electrode results (213). ~~

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POLYMER-MATRIX ELECTRODES Ion-selectiveelectrodes that are constructed from a polymer matrix contain specific ingredients that impart the desired response characteristics. These ingredients are basically the same as have been used in the past to construct liquid membrane electrodes. The liquid membrane electrode was formed by using an absorbent material that contained an ion-exchan e material which was dissolved in a lipophilic solvent. ElectrAe lifetime was limited in this configuration. With the advent of polymer-matrix membranes, noteably poly(viny1 chloride) (PVC), electrode lifetimes increased tremendously. A great deal of research also paved the way for wider application of these sensors to a seemingly unlimited number of analytes through new ion-exchanger and ionophore developments. The understanding of the response mechanism of polymer membrane electrodes begins with an understanding of the ion-exchange processes that m u r within the membrane itself. Many studies have focused on cation-selective membranes by determining the anionic site concentrations of membrane support materials (214). The high selectivities that these electrode display can be defined in terms of both the ionophore that is used and the existence of anion-exchange sites that are present in the membrane (215).It has been shown that inorganic salts which are added to the composition of PVC ANALYTICAL CHEMISTRY. VOL. 62. NO. 12, JUNE 15, 1990

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ION-SELECTIVE ELECTRODES

Table I. Cationic Membrane Electrode Applications ion sensed

hydrogen ammonium

ionophore or exchanger N,N-dioctylaniline and tridecylamine nonactin, narasin, monensin, and

Table 11. Anionic Membrane Electrode Applications

refs 257 258

salinomycin

lithium

1,lO-phenanthroline derivatives substituted crown ethers acyclic ligands sodium calix[4]areneesters and amides 12-crown-4 derivatives 24-crown-8derivatives bis(crown ethers) potassium pyridine macrocycle valinomycin bis(crown ethers) cesium cesium tetrachlorophenylborate magnesium acyclic Aspartamides barium barium (tetraphenylborate-Antarox CO-880 complex) copper(I1) dithiocarbamate derivatives thiuram mono- and disulfides macrocyclic polythiaethers complexing agents with sulfur or nitrogen groups silver monothia-crown ether nickel(I1) bis(2-ethylhexyl)phosphate zinc(I1) tetradecylammonium tetrathiocyanatozincate lead(I1) tetraphenylborate surfactant complexes iron(II1) 1,7-dithia-12-crown-4 thallium(II1) butylrhodamin p-tetrachlorothallate gold(II1) butylrhodamine 8-tetrachloroaurate 1,2,4,6-tetraphenylpyridinium

tetrachloroaurate uranium(V1) uranyl bis(DhosDhates)

259 260 26 1 262, 263 264 265 266 267

268 269 270 271 272

273 274, 275

276

277, 278

279 280 281 282 283 284 285 286 287

membranes act as ion-exchange sites (216) and that anions in general influence the cation response of these electrodes (217). The nature of anions present affects the stability of valinomycin-cation complexes (218). Radiotracer studies using valinomycin electrodes have demonstrated that the plasticized PVC membranes are low-capacity ion exchangers and that the ion-exchange selectivity correlates with potentiometric selectivity (219,220). In an analogous study, a crown ether-PVC membrane was doped with a potassium isotope to determine the exchange of the membrane-bound cation with bulk potassium (221). In this study, no exchange was seen during the 18 min of the experiment. Others have used nuclear magnetic resonance to determine relaxation times and model the cation-exchange rate for sodium ion with crown ethers in an ion-selective electrode (222). The electrical roperties of polymer-matrix electrodes can be further defined y using impedance measurement techniques to investigate both bulk and surface properties (223, 224). The transient behavior and other electrochemical parameters of PVC electrodes can be characterized by using voltammetric techniques (225). The response characteristics of plasticized, polymer membrane electrodes have been investigated. This is partially aided by first understanding the potential response of liquid ion-exchange membrane as was done for pH with weak acid primary anions (226). The overall response has been broken down showing the dependencies of individual membrane components (227). The function of calcium electrodes was studied and correlated to glass transition tempertures by using several solvent mediators, many PVC compositions, and methacrylate-based membranes (228). Aminated, carboxylated, and underivatized PVC were cast into membranes containing three different exchangers to determine the effect of composition on response and selectivity (229). It has been shown that the hydroxy-modified PVC resulted in improved lifetimes and better adhesion to electrode bodies with potassium, ammonium, and calcium sensors (230). Cellulose acetate has also shown promise for membrane construction and offers enhanced performance characteristics for molecular weight response control (231)and immobilization of biological agents on asymmetrical membranes (232). The choice of plasticizer influences the response slope, curve linearity, and

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ANALYTICAL CHEMISTRY, VOL. 62, NO. 12, JUNE 15, 1990

ion sensed

chloride iodide nitrite sulfite carbonate perchlorate

ionophore or exchanger silver chloride-silver sulfide in urushi

vitamin B-12 derivative tetraalkvitin compounds bis(diethy1dithiocarbamato)mercury-

(11) ion-exchanger and derivatized benzoic acid ester complex berberine hydrochloride N-ethylbenzothiazole-2,2’-azaviolene

refs

288 289 290 291 292 293 294

perchlorate phosphate bis(pchlorobenzy1)tin dichloride 295 thiocyanate nitron-thiocyanate complex 296, 297 cobalt porphyrin thiocyanate complex 298 chlorochromate quaternary ammonium 299 chlorochromate periodate 300 -berberine periodate Table 111. Organic Ion Membrane Electrode Applications ion sensed

ATP 1,5-naphthalenedisulfonate 2-methyl-1-4-chlorophenoxyacetic acid trinitrobenzene-sulfonate (TNBS) hydrogen phthalate perrhenate cholate diquat

ionophore or exchanger

ref

macrocyclic polyamines polymethylene

301 302

bis(triocty1phosphonium) trinonyloctadecylammonium iodide

303

tetradodecylammonium TNBS

304

tetraheptylammonium biphthalate nitron perrhenate

305 306

benzyldimethylcetylammonium

307

cholate diquat

308

bis(tetra-4-chlorophenylborate)

diquat, paraquat, and several ion-pairing agents 4,4’-dipyridinium crown ether derivatives guanidinium dibenzo-27-crown-9or guanidinium tetraphenylborate hexadecylpyridinium phosphotungstic acid-HDP bromide (HDP)

309 310 311

312

selectivity of PVC membrane electrodes (233). The extraction properties of the plasticizer and the resultant distribution coefficients of the ion pairs result in the observed potential response of PVC membrane electrodes (234). The effect of the ionophore on the response characteristics of electrodes has been described for membrane systems containing acyclic polyethers (235),carboxylic polyethers (236),azo- and thiocryptands (237),sulfonamides (238),derivatized porphyrins (239),and antibiotics such as monensin (240). The determination of the selectivity coefficients of a membrane electrode is essential to the a plication of the sensor as well as for the understanding of &e processes that occur during a measurement. The ion selectivity can be described by using kinetic arguments to correlate potentiometric selectivity with transport selectivity when ionic trans ort is fast across the solution-membrane interface (241). ifn understanding of selectivity coefficients coupled with detection limits allowed an equation to be derived by taking measurements where both the primary and interfering ions were at low concentrations (242). There are several ways of calculating selectivity coefficients and the effect of interfering ions was explained by performing the calculations using four different methods (243). The selectivity of crown ether based electrodes has been extensively studied. These have included bis(crown ethers) (244,245),propeller crown ethers (246),substituted crown ethers (247-SO),crown ethers with heteroatoms in the ring skeleton (%I), and flow injection methods for crown ether selectivity determination (252). The potentiometric selectivity of tetraphenylborate ionophores for alkali and alkaline-earth metal ions (253) and the effect of solvent mediators on these systems have been studied (254). Acyclic propane neutral carriers have also been used in plasticized PVC membrane electrodes and their response characteristics and selectivity

ION-SELECTIVE ELECTRODES

Table IV. Drug-Responsive Membrane Electrodes species sensed

exchanger

C-reactive protein antazoline trazodone amitriptyline metoclopramide amphetamine primary amine drugs berberine

sample or uses

phosphate ester crown ethers antazoline tetraphenylborate trazodone tetrakisb-chloropheny1)borate amitriptyline tetraphenylborate metoclopramide tetraphenylborate amphetamine dibenzo-crown ethers derivatized macrocyclic polyethers berberine bromomercurate berberine tetraphenylborate naproxen tetraheptylammonium naproxenate 5,5-diethylbarbiturate tetraoctylammonium 5,5-diethylbarbiturate sulfa drugs quaternary phoshonium or arsonium sulfonamide derivatives quaternary ammonium-sulfa complex cocaine cocaine dipicrylaminate salicylate derivatized porphyrin tin dichloride nitron salicylate vitamin B1 fluorinated methylphenylborates cyclamate quaternary ammonium salts saccharin brilliant green-saccharin clonidone clonidone tetraphenylborate anesthetics tetraphenylborate complexes hydralazine hydralazinium tetraphenylborate benzyldimethylcetylammonium benzylpenicillin benzylpenicillin nicotine nicotine tetraphenylborate nicotine-organic anion complex scoDolamine scoDolamine tetraohenvlborate . -

coefficients have been described (255, 256). There have been many exam les of PVC membrane electrode applications in several fie&. Some of these applications have been summarized in Tables I-IV where the analytes have been separated into cationic, anionic, organic ion, and drug species. Tables I and I1 list electrode membrane compositions that are relatively straightforward in their construction. Tables I11 and IV list more obscure membrane formulations that give rise to selective potentiometric response. Examples not found in the tables include pseudo-liquid membrane phases that have been prepared for the determination of cloxacillin (341) and cephalthin (342) where the electrode properties of response range, slope, detection limit, response time, lifetime, and selectivity have been described. Electrodes that respond to cationic antibiotics and sulfa drugs were constructed by using some of the more recent advances in membrane construction techniques (343) as were 12 different drug sensors based on graphite inner contact electrodes (344). Drug-selective electrodes have been used for the kinetic determination of complexation constants of cyclodextrins with chlorpromazine, dicyclomine, imipramine, desipramine, and propranolol (345). Another kinetic study utilized a thioridazine membrane electrode to monitor the photoreaction of this drug to daylight and to determine the rate constant and drug half-life a t various temperatures (346). PVC membrane electrodes have also been constructed that respond to anionic and cationic surfactants (347-349). The vast majority of polymer-matrix ion-selectiveelectrodes that are reported are prepared by using PVC for the membrane support. Other polymers that have shown promise include polypyrrole. A polypyrrole film electrode can respond selectively to either nitrate, chloride, bromide, or perchlorate when appro riate ions are doped into the film during preparation (3505: The conditions during electrochemical polymerization of the film affect the way the electrode responds and influence the mechanism of response (351). Polypyrrole membranes have been further modified by grafting 4-vinylpyridine onto the membrane to investigate changes in the electrochemical behavior of the film (352). These studies lead to pathways for coupling ligands to the polypyrrole to induce ion sensitivity and response. In similar ways, a hydrogen ion selective electrode was prepared that responded well and displayed better selectivity over lithium, potassium, and sodium than a comparable glass pH electrode (353). Additional polymer systems that have been studied include polyurethane for chloride analysis (354),a hardened epoxy resin for several anions (355),and photocured polymers for potassium electrodes (356).

research uses pharmaceuticals tablets response studies pharmaceuticals selectivity studies response and selectivity studies tablets response and selectivity studies tablets response studies selectivity studies response studies response studies biological samples tablets response and selectivity studies food samples tablets response studies response and selectivity stuuies pharmaceuticals pharmaceuticals tobacco products cigarette smoke resDonse and selectivitv studies

refs 313 314 315 316

317 318 319 320 321 322

323 324

325 326 327 328 329

330 331 332, 333

334 335 336

337, 338 339 340

GAS-SELECTIVE ELECTRODES AND BIOSENSORS As useful and informative it is to work on basic ion-selective electrodes it is often more challenging (and more fun) to extend the selectivity and function of electrodes to nonionic species. In analogy to the use of ionophores and exchangers in place of crystalline and lassy interfaces, physical and biochemical agents have moified and shifted the selectivity patterns of more fundamental ion-selective electrodes. This section will focus on these developments in gas-selective, enzyme-catalyzed, and immuno-responsive membrane electrodes. Gas-Sensing Electrodes. A potentiometric membrane electrode that responds to gases does so by physically separating an inner electrolyte solution from the external sample solution. The physical separation is constructed so as to allow for gaseous diffusion to occur between the outside and inside fluids. The diffusion of specific gases alters an equilibrium within the inner electrolyte that is sensed by a fundamental-type ion-selective electrode (i.e., pH). This separation can be as simple as an air gap across which gases diffuse as dictated by their partial pressures in the external sample solution and the inner electrolyte solution. This approach was demonstrated in the construction of both ammonia and carbon dioxide sensors using a pH electrode to sense the hydrogen ion shift during the electrode operation (357). Other indicator electrodes can be used as well. A gas-selective electrode for hydrogen sulfide was prepared by placing a silver sulfide electrode within an internal electrolyte buffer containing a silver complex (358). The more common approach to constructing gas-selective electrodes is to separate the inner buffered electrol@ solution from the external sample with a gas-permeable membrane, such as silicone rubber. The majority of electrode systems that are discussed in the literature rely on glass internal pH electrodes. There are occasional descriptions of other internal elements such as PVC membrane electrodes that utilize ionophores for hydrogen ion to construct gas electrodes for ammonia (359). The response of the ammonia electrode has been modeled in both the steady-state and kinetic modes (360). As the response time of these electrodes is greater than that of other sensors, it is often useful to be able to operate the electrode during the kinetic phase of response to ammonia gas concentrations. The dynamic response of ammonia electrodes has been studied for several volatile amine interferants (361, 362). The response time and corresponding selectivity coefficients correlate well when the difference in ANALYTICAL CHEMISTRY, VOL. 62, NO. 12, JUNE 15, 1990

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diffusion of the amines is compared to that of ammonia gas. Improvements in eliminating the amine interference have been realized by replacing the gas-permeable film with a plasmapolymerized multilayer film that increases selectivity dramatically (363). Ammonia gas sensing electrodes have been used for a wide variety of analytical applications. These include the determination of total nitrogen in dru substances by digesting samples in a Kjeldahl apparatus f364,365). The ammonia electrode replaces the time-consuming distillation and titration procedure that follows the digestion. Meprobate and calcium pantothenate were similarly assayed by decomposition to ammonium salts followed by measuring the evolved ammonia by treatment with base (366). The ammonia nitrogen content in microorganism culture baths was continuously determined by electrolytic alkalization at the tip of an ammonia electrode (367). Ammonia content has also been directly determined in cell culture media (368) as well as in liquid piggery wastes (369). Enzyme kinetics can also be followed as in the case of the oxidation of D-alanine by measuring the increasing concentration of ammonia with a gas-selective electrode (370). This last case leads us into the next class of potentiometric electrodes-the enzyme electrodes. Enzyme Electrodes. Enzyme sensors are constructed by holding a biocatalytic substance near the tip of an ion- or gas-selective electrode. The electrode responds when the specific substrate for the enzymatic reaction diffuses into the catalytic region and is converted into byproducts, one of which the underlying electrode can sense. The analysis is complete when the potential of the electrode system reaches a steady-state condition. The steady-state response of enzyme electrodes that are based on pH internal elements depends strongly on the buffer capacity of the sample as well as on the buffer capacity at the indicating electrode surface (371). Enzyme electrodes have been operated under non-steady-state conditions as well and an equation relates the response to the enzyme film thickness and the diffusion coefficients of the substrate and the products (372). In most cases, the enzyme is physically trapped or held at the sensing electrode surface. There have been other methods of attachment which rely on chemically bonding the enzymes to the membrane surface as well. A novel method for attachin enzymes has been described where the enzyme is modiked with perfluoroalkyl groups, which greatly enhances its ability to adhere to fluorocarbon membranes (373,374). The fluorocarbon membranes are the gas-permeable membranes that cover ammonia gasselective electrodes. The preparation of enzyme electrodes is becoming routine enough that they have been cited for classroom instruction purposes (375). There are many examples of enzyme electrodes that have been based on pH internal elements. An interesting urea sensor was prepared by combining urease enzyme onto a pH-sensitive mixed iron oxide film that was prepared by ferrite plating (376). A more conventional design used urease that was trapped in a cellophane membrane that was placed over a glass pH electrode (377). Air-gap urea electrodes using a pH sensor have been prepared by absorbing urease into filter paper membranes and laying these membranes over the air gap (378). The lifetime of these urease electrodes can be extended by encasing the enz e with a latex rubber coating to increase the stability of the E a t a l y s t (379). The selectivity of the pH-based enzyme electrode is shifted by simply replacing the urease with another enzyme, such as creatine kinase, to create a sensor for creatine (380). Coated wire electrodes have also been utilized to create enzyme electrodes. Thin PVC films containing tri-n-dodecylamineare coated onto silver chloride wires to impart pH response. Penicillinase was adsorbed to the PVC surface to make the electrode responsive to penicillin (381-383). These electrodes are very sensitive to high buffer concentrations, which decreases the response of the sensor. Ammonia gas electrodes have been used as the basis for enzyme sensors for many substrates. Urease has been attached to a modified poly(viny1 alcohol) membrane that is stretched over the gas-permeable membrane of an ammonia electrode (384). The response of an L-aspartate-selective electrode was described where L-aspartase was chemically immobilized onto the surface of an ammonia electrode (385). The linearity, response slope, and lifetime were reported as well as the effect of pH, substrate concentration, and selectivity over other 26R

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common amino acids. The commercially available sweetener aspartamine has been determined in food products by combining the enzyme aspartase with an ammonia electrode (386-388).

Additional enzyme sensors have been prepared by using many other types of indicating electrodes. The carbon dioxide gas electrode has been combined with lysine decarboxylase to detect lysine in fermentation broths (389) and has been further defined in terms of its steady-state response kinetics (390). The fluoride electrode has been used to detect hydrogen peroxide and glucose by immobilizing peroxidase enzyme and using 4-fluorophenol as substrate (391). In a similar manner, peroxidase and lactate oxidase or glucose oxidase and glucoamylase have been coimmobilized and act upon 4-fluoroaniline to respond to lactate or maltose, respectively (392). Urea has been determined by trapping urease within a nylon net and applying this over an ammonium-selective electrode instead of the conventional gas-selective electrode (393). Another PVC-based electrode has been used for the measurement of phosphatidylcholine (394). In this example, a choline-selective electrode is prepared and combined with phospholipase D, which hydrolyzes phosphatidylcholine in the presence of sodium deoxycholate. Bacterial, Tissue, Organelle, and Immuno Sensors. The biocatalytic membrane electrodes that have been discussed previously utilize isolated enzymes to impart activity and selectivity to the biosensor. In many instances, the lifetime of the sensor is compromised because the enzyme has been removed from its natural environment. Evolution has provided the most stable set of conditions for these enzymes to exist and function. It is this understanding that led researchers to construct biosensors based on intact microorganisms that contain the desired biocatalytic functions. A comparison of functional properties was conducted of isolated enzyme versus bacterial electrodes (395). Although the response characteristics were similar, the bacterial electrodes display greatly increased lifetimes due to regeneration of the bacteria at the electrode tip. Very often, the isolated enzyme is not even available for consideration or is unstable in purified, reconstituted forms. The bacteria Proteus vulgaris and Proteus mirabilis have been immobilized on ammonia gasselective electrodes to produce sensors that res ond to DLphenylalanine (396). Proteus vulgaris has also gee, immobilized on both ammonia and carbon dioxide gas electrodes to produce sensors for urea (397). Proteus mirabilis exhibits wide applicability for sensor construction by respondin to cytosine (398),L-asparagine (399),and urea (400)when appted to an ammonia gas electrode. This also illustrates the complexity of response and possibly poor selectivity unless the response characteristics can be tailored by the use of enzyme inhibitors. The response of immobilized cells on electrodes has been used for rapid drug screening to detect drug sensitivity on particular microbial strains (401). The application of organelles and tissues is the next 1 ical extension of biosensor development. Since enzymes havexen extracted and purified from animal and plant tissues, the use of these preparations parallels the use of intact bacterial cells. Mitochondria from rat or swine livers contain monoamine oxidase enzyme which imparts selectivity for serotonin, tyramine, and benzylamine when coupled to an ammonia gas electrode (402). Chemoreceptor structures from the blue crab have been coupled with potentiometric electrodes and respond to amino acids (403). A fluoride electrode has been used to sense diisopropyl fluorophos hate when the giant axon of squid nerve tissue was appliefto the electrode surface (404). A sensor for D-amino acids was constructed by immobilizing a sheep kidney tissue slice over an ammonia gas electrode (405).Similarly, Jack bean tissue (many commercial urease preparations come from Jack bean) has been coupled with an ammonia gas electrode that responds to urea (406). This electrode utilized a PVC-based pH internal sensing element instead of the conventional glass pH sensor. A cyanide electrode was coupled with almond meal tissue containing P-D-glucosidase to create a sensor for amygdalin (407). The almond meal was immobilized on a carbon fiber cloth where the substrate was hydrolyzed to form cyanide ion. Biosensors that directly respond to the analyte of interest continue to drive immunosensor and bioreceptor research (408). The early studies that relied on oxidized metal sensors have been extended by chemically modifying these surfaces.

ION-SELECTIVE ELECTRODES

c

A pol Famine-coated platinum electrode was further modified y binding antibody to human corionic gonadotropin (hCG) to its surface for the detection of hCG (409). The electrode responded to more negative potentials with increasing concentration of hCG in the range 10 ng/mL to 10 pg/mL. Tantalum electrodes respond to IgG when anti-IgG was immobilized to the surface with cyanuric bromide (410). The response of these electrodes has been improved by first treating the tantalum with a silane and then an inert protein (411). The application of ionophore-antigen conjugates in PVC membranes for antibody detection has been extended (412,413), while voltammetric studies of these sensors have shown that the response is due to the immunologic reaction affecting the exchange currents of the membrane (414). Self-assembling monolayer membranes containing synthetic receptor sites have been combined with metal electrodes to create a sensor that responds selectively to metal ions (415). This monolayer membrane electrode may form the basis for new molecular and immunological sensor technologies.

CLINICAL AND FLOW APPLICATIONS Ion-selective electrodes have had a long history for use in biochemical and clinical applications. Organisms exist within a sea of electrolytes and an incredibly complex mixture of low to high molecular weight organic compounds. The ability of ion-selectiveelectrodes to respond to the activity of substances makes them ideal for these measurements, since chemical activity is closely related to the biological activity and functions within these complex matrixes. The discussion of clinical applications includes body fluid analysis using indwelling sensors as well as traditional analyzers that accept discrete, withdrawn samples. Tissue and cellular measurements utilize microelectrodes that can operate within or a t the surface of single cells. Electrodes that are used in flowing stream analyzers are treated separately, which describes their application in chromatography and flow-injection analysis. Clinical Applications. The use of ion-selective electrodes for clinical analysis of electrolytes has not been a straightforward exercise. There were many setbacks attributed to materials of construction, membrane selection, interferences, liquid junction errors, and offsets in reported results when compared with conventional means of analysis. Several instrument manufacturers have introduced electrode-based clinical analyzers that address these difficulties. Studies on the Nova 2 analyzer illustrate the effect of changing the liquid junction electrolyte on ionized calcium determinations (416). Comparisons that are made to flame photometric methods of analysis are favorable when the analyzer uses dilute blood or serum samples as was demonstrated for the Du Pont Na/K/Li analyzer ( 4 1 3 , the IONOMETER EF instrument (418),and the Nova State Profile I analyzer (419). Undiluted serum samples are used in the Kodak Ektachem analyzer for sodium, potassium, and chloride ions and carbon dioxide (420). The Technicon Chem-1, which uses three ion-selective electrodes, was evaluated for plasma and urine analysis (421). All of these instrument correlation studies use similar testing performance criteria of batch precision results, accuracy, correlation, and linearity of response. The determination of electrolytes in blood serum depends on many factors of ion-selective electrode construction and use. Sodium and potassium electrodes were optimized for serum samples using the ionophores bis(l2-crown-4) and bis(benzo-15-crown-5) with several plasticizers on PVC membranes (422). Changing the sodium ionophore to methyl p-tert-butylcalix[4]arylacetateyielded sufficient selectivity but resulted in a bias of reported sodium values for blood plasma (423). Some of these operational difficulties can be overcome by utilizing a certified reference serum for the calibration of ion levels for the common electrolytes (424). This approach would be in keeping with the use of multiple electrodes for the simultaneous determination of analytes in blood sam les (425). A problem that has been identified when using P V 8 membranes for samples containing proteins involves a shift in the standard potential of the electrode (426). This shift results from an asymmetry created when proteins in the sample interact with the membrane surface. Changing the composition of the membrane matrix from straight PVC to copolymers of PVC and vinyl alcohol appears to avoid this asymmetrical shift in the electrode standard potential. A novel approach has been taken for the simultaneous

measurement of electrolyte components and carbon dioxide levels in blood samples (427). A conventional carbon dioxide gas-sensing electrode is constructed that uses a silicone rubber gas-permeable outer membrane. An appropriate ionophore is incorporated into this silicone membrane and the potential across this combination ion-selective and gas-permeable membrane is measured at the same time that the potential of the inner pH membrane is taken. This simultaneous measurement of potentials gives both the ionic and carbon dioxide levels in the blood sample under study. This technique was demonstrated for both in vitro and in vivo blood measurements. Body fluids other than blood samples have been assayed by using ion-selective electrodes with equally good success. A combination electrode array has been used for the determination of sodium, otassium, calcium, and chloride levels in cerebrospinal fluif(428). Adenosine deaminase has been assayed with an ammonia electrode (429) and lecithin with a fluoride electrode (430) in human amniotic fluid. Ion-selective electrodes have been used to study biochemical events occurring within tissues and cells. A carbon dioxide electrode was coupled with the enzyme glutamic acid decarboxylase to measure the release of glutamate from perfused retina tissues (431). In another study, frog kidney distal tubules were held onto the surface of a potassium electrode with a dialysis membrane to measure the rate of potassium transport in renal cells (432). Calcium electrodes have also been used for the determination of the free calcium ion level in erythrocytes (433). Microelectrodes. Ion-selectivemicroelectrodes have shown great utility in the measurement of analytes within cells and in close proximity to cellular surfaces. Both transmembrane potentials and ionic activities can be measured with these devices when prepared and used within certain constraints (434). One of the greatest difficulties encountered with their use arises from their inherently high membrane resistance, which is influenced by shape and composition (435). I t has been shown that the sensitivity and selectivity of the liquid ion-exchanger based chloride electrode can be altered by beveling the tip of the microelectrode housing (436). Thermal gradients that exist between the sample and the internal elements of the microelectrode affect the response and accuracy of the sensor (437). The calibration of these electrodes has been simplified by the construction and use of an automated apparatus that uses a dilution calibration technique (438).

Microelectrodes that respond to pH were some of the earliest sensors that were constructured by using miniature glass pipets, The function and response of the glass pHsensitive microelectrodes compares well with their conventionally sized counterparts as was shown for a series of water and soil measurements (439). Single- and double-barreled electrodes have been prepared and used for the study of hydrogen ion exchange in a variety of tissue samples (440). Microelectrode pipets have been prepared that respond to pH that use neutral carriers as the active agent. The fabrication of these electrodes has been detailed and their use for the direct, in situ examination of ant venom pH is described (441). Others have prepared several of these sensors and optimized the response and dynamic range by replacing the neutral carrier and altering the membrane composition (442). Microelectrodes have been successfully prepared for many other ionic species and have been used in a variety of fields. Potassium ion microelectrodes have found applications for measurements in muscle cells (4431,gallbladders (444),and structures of the eye (445). Ammonium ion microelectrodes that use the neutral carrier nonactin (446,447) in a similar structure have been used for measurin enzymatic product gradients that occur within cross-linkef agar gels (448). An ammonia gas-sensing microelectrode was fabricated that relied on an air-gap structure (449). This electrode was then converted into a urea sensor by immobilizing urease a t its tip. Microelectrodes have been described that respond to calcium ions (450) and magnesium ions (451,452)that use liquid ion exchangers and neutral carriers, respectively. Flow Applications. Electrodes are especially well suited for flowing stream analysis since they are very flexible in terms of their materials of construction, geometry, and a plications. A polymeric pH electrode has been demonstratel as a universal detector for suppressed ion chromatography (453). In a similar application, highly sensitive lead phosphate glass ANALYTICAL CHEMISTRY, VOL. 62,

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membranes containing silver oxide were used in nonsu pressed ion chromatography for the determination of iodiae in seawater (454). Fluoride ion levels at the parts per billion concentration range have been measured in process baths used in the electronics industry (455). The lanthanum fluoride electrode has also been used to detect fluoride ions in highpurity water at submicromolar concentrations (456). Potentiometric detection of carboxylic acids has been accomplished with metallic copper electrodes using ion chromatography (457). Electrodes have also found application in air-segmented continuous flow analyzers to determine the dissolution rates of drug formulations (458). One difficulty that occurs with flowing stream potentiometric anal sis exists with the reference junction. This problem was ilLtrated with an on-line pH monitoring system that resorted to methods to clean the liquid junction or to use different materials of construction in order to maintain an uncontaminated junction (459). The greatest emphasis that has been placed on flow applications of ion-selective electrode detectors has involved flow injection analysis. The performance of ion-selective electrodes can be enhanced in this type of flow analyzer by pushing the electrode outside of its customary operating envelope (460). The electrode can be inserted into the flow path as a wall-jet detector to increase its performance, sensitivity, and response time (461). When low levels of the analyte are to be determined, enhanced sensitivity is often achieved by adding a small amount of the target ion to the carrier stream (462). Detection limits, selectivity, and response times often depend on the nature of the ion-complexing agents used to prepare the sensing membrane as was the case for lead analysis (463). The reparation of calcium sensors for flow injection analysis also lis layed this effect in terms of the response and lifetime of the &tector based on ionophore selection (464). A unique detector assembly was demonstrated for the simultaneous assay of free cyanide and weakly complexed cyanide by separating two silver iodide-silver sulfide electrodes with a gas diffusion cell (465).The potential signals from electrodebased flow injection anal zers can be massaged with an antilog converter that proiuces an output voltage that is directly proportional to the concentration of the sensed ion (466). A series of tubular iop-selective electrode sensors have been prepared by forming a heterogeneous coating within metallic tubes that respond to copper ions (467), sulfide ions (468), cadmium ions (4691,and lead ions (470). Poly(viny1chloride) tubular electrodes have also been prepared by forming sensing elements that contain ionophores and plasticizers for the determination of potassium in soil sample extracts (471). A complete flow and data acquisition s tem has been described that allows the computer-controlledetermination of potassium, calcium, nitrate, and chloride ion levels in soil samples using polymer-matrix electrodes (472). Sulfate concentrations in rainwater were measured with a lead electrode by using a flow titration technique where the carrier stream contained a small amount of lead (473). Chloride ion levels in serum samples were determined in the same manner by using a silver electrode and titrating the chloride ion with a 0.5 mM silver titrant in the carrier stream (474). Two copper-selective electrodes have been used as indicator and reference elements in a flow analyzer to measure copper in wastewater samples (475). The carrier stream consisted of a weak electrolyte containing a 1.0 mM copper ion concentration which was carried to both electrodes for the differential measurement. Enzyme reactions have been successfully coupled with ionselective electrodes in flow injection analyzers to measure nonionic anal s. Horseradish peroxidase has been measured with a fluori e electrode by using a carrier stream containing hydrogen peroxide and p-fluoroaniline as the substrate (476). The enzyme content of vegetable extract and milk samples was demonstrated after optimizing the reaction conditions in the flow manifold. An intact enzyme electrode has also been placed within the flow path of a flow injection analyzer (477). An ammonium ion selective electrode was covered with a membrane containing the enzyme urease. Whole blood samples were analyzed and the pohsium interference was greatly reduced by adjusting the potassium ion level in the carrier stream to match the concentration found in whole blood.

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ACKNOWLEDGMENT R.L.S. thanks the Imaging Systems Department for its support in providing library search facilities and word pro28 R

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31 R

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Chemical Sensors Jii;I Janata Department of Materials Science and Engineering, University of Utah, Salt Lake City, Utah 84112

A. INTRODUCTION The format and rules for selecting the references are similar to the ones used in the first review in this series ( A l ) . It is again based on the search conducted by Chemical Abstracts Service according to the same keywords. The period is limited to the CAS Online abstracts obtained between January 1,1988, and December, 31, 1989. This means that some articles published at the end of 1987 are included but those published at the end of 1989 are not. As before only a fraction of total references could be used. I apologize to those colleagues whose work is not mentioned; it is not intended as a reflection on the im ortance of their work only a necessity dictated by the limited’ amount of s ace available for this review. Publishing essential1 identicarwork in several different journals is unfortunateyy becoming a common practice. As a rule I have cited only the more recent and/or more extensive paper on the same sub’ed from the same author. It is hoped that this uniformity wib provide some measure of comparison of trends and developments in this rapidly moving field. The data on which this review is based are summarized in Table I. As before (A2) ion-selective electrodes are covered in a separate review. They are, however, included in the statistics (Table I) because they provide a point of reference. A closely related reivew of “Dynamic Electrochemistry” is again included in this issue. Table I provides an interesting insight into trends within the sensor field. Perha s the most eye-catching numbers are the increase in the numier of publications devoted to optical sensors and the equally dramatic drop in the number of ISE papers. Although they still re resent the largest group of chemical sensors ISEs seem to {e in decline as the object of research and development. The apparent decrease in the number of review papers is slightly misleading. There are now several multiauthored books in which the reviews are accumulated. The individual entries in these books are not listed here and the reader is strongly advised to check the contents of those books. Mass sensors show a healthy growth as well which is mostly due to the fact that both quartz crystals and surface acoustic devices are commercially available. The trend toward miniaturization is clearly visible within the electrochemical sensor group. While the number of apers on macroscopic ISE decreased the number of those fIevoted to the “other potentiometric” sensors is increasing. The intensive basic research in microelectrodes is reflected in the strength of interest in amperometric sensors. In this respect they are similar to optical and potentiometric sensors because they both draw on a very large pool of basic knowledge, Le., classical electrochemistry and classical spectroscopy. There were almost 900 papers (without ISE) in the original database, of which a proximately 500 are included in this review. A.1. &oks and Reviews. In addition to numerous reviews there are now several sensor books,which is the sure sign that the area of chemical sensors is beginning to mature. I have not included various bound proceedings of meetings and conferences because the rarely contain any new information. There are multiauthordbooks on solid-state gas sensors (A3),

Table I. Summary of the Chemical Sensor Database

topic

reviews

thermal mass

potentio. amp. cond. opt.

ISE

percent ISE av av 1985-1987 1988-1989 with without change 88 6 19 83 93 65 37 336

63 6 24 87 101

66

1 5 17 20 13

99

19

130

25

62 6 23 26 17 26

-29 10 +26 +5

+9 +2 +168 -61

ion-selective electrodes (A4),sensors in immunological analysis ( A @ ,chemical sensors (A6, A n , electroanalytical techniques in clinical chemistry and laboratory medicine ( A @ ,and solid state electrochemical sensors (A9). The first volume of the new international series devoted to chemical sensors has been published (AlO). The whole volume of Methods in Enzymology is devoted to chemical sensors that have to do with enzymes ( A l l ) . There is now also one textbook available (A12). A new bimonthly ’ournal entitled Sensors and Materials (Publisher: MYU k.K.)has appeared. My review is based exclusively on articles published in English. There is only one exception and that is the Practical Dictionary of Sensors published in Japanese (A13). A.2. Selectivity. The issue of selectivity remains central to operation of any chemical sensor. The role of membranes in separation and sensin (A14, A15) has been reviewed. Appearance of highly ortfered selective layers in chemical sensors is a new trend. Amon them Lan uir-Blodgett (LB) films play a prominent role fAl6-18). R i n organometallic films can be used as selective layers in various types of chemical sensors. Both physical and chemical modifications of these materials can result in tayloring their selectivity (A19, A20). Electrochemical modification of solid electrodes is an area of research in itself. From the sensing point of view it represents a powerful tool for design and application of selective layers for all types of chemical sensors (A21). The richest but not necessarily the most successful source of selectivity lies in biology (A22-28). For different types of sensors the selective layers have been made from biological components such as immobilized algae (A29), whole cells (A3&33), receptors (A34, A%), DNA (A36),immunoglobulins (A37,A38), and enzymes (A39). In some cases the lipid bilayers are the natural host matrices for immobilization of these biological units (A40,A41). Specific types of biosensors will be reviewed under the headings of the individual types of sensors. A.3. Packaging. Another important topic common to all chemical sensors is fabrication and packaging. The general use of integrated circuit and micromachining microfabrication techniques for chemical sensors has been reviewed (A42, A43).

0003-2700/90/0362-33R~Q9.50/0 0 1990 American Chemical Society

33 R