Anal. Chem. 1002, 64, 1805-1812
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Selectivity of Ion-Sensitive Bulk Optodes Eric Bakker and Wilhelm Simon' Swiss Federal Institute of Technology (ET"), Department of Organic Chemistry, Universitdtstrasse 16, CH-8092 Ziirich, Switzerland
The ulectivlty behavior of solvent polymerk optode membranoswith two dlffwmt ionophoresincarporatedhd k c d . One of thew Is competing reverrlbly for the anaiyie ion; the other (chromoionophore), for a hydrogen ion. The two ions are extracted by mass transfer into the bulk of the organic phase. A novelselectlvltyformalkm Is devdop8d on the bask of dmerenlqu#lbda inthe absence and presenceof intorfwhg Ions. The chromoionophore was ammod to be ideally hydrogen ion seiectlve. For a constant pH, the sekctivlty coefficlenl k given by tho ratio of the two actlvities of the primary and tho Interfering ion, which lead, lf separately calibrated, to the same degree of protonation of tho chromdonophore. The novel theory is eyHciaily helpful for ion$ of dlfferent vaiencies, where no appropriate dercrlption has beonavdlabk sofar. I n these cams, tho 0d.ctlvltycoMkknt k dlfferently defined as compared to the Nicokky-Elsenman formaikm and Is dependent on the sample pH value. The caiclumselectivity of a sodlum wnsor based on the ionophore ETH 4120 is measured and compared to the new theoretical approach. It h shown that the uiectivlty coefficknt of log G&= 0.27 for a sample at pH 4.54 fulfills the requirement of 10.4 for measurements In diluted serum.
INTRODUCTION The development of optical ion sensors has been initiated by the availability of suitable optical hardware,l which promises a wide field of novel applications.2 Although many efforts in the development of ion-sensitive optodes based on the immobilization of the sensing components on the surface of an optical waveguide have been set forth,394 it recently has been recognized that the nonthermodynamic assumptions for the determination of single ion activities are in this case more severe as compared to the corresponding ion-selective electrodes.5 Other workers focused on field-sensitive dyes in multilayers6 to obtain ion-selectivesensors, which were difficult to describe theoretically and have not met the analytical requirements in respect to sensitivity, reproducibility, and measuring range so far. In the past few years, a novel optode principle has been introduced successfully with the idea of making use of ionophores which had been designed for ion-selectiveelectrodes. The optical sensors are bulk optodes, which are extracting the sensed ionic components into a plasticized polymeric membrane by mass transfer where the change of the optical signal of an incorporated component is detected by means of absorption or fluorescence.
* Corresponding author.
(1)Seitz, W. R. Anal. Chem. 1984,56, 16A-34A. (2)Kunz, R. E.h o c . SPZE,in press. (3)Peterson, J. I.; Goldstein, S. R.; Fitzgerald, R. V. Anal. Chem. 1980, 52, 864-9. (4)Wolfbeis, 0.s. Fresenius Z. Anal. Chem. 1986, 325, 387-92. (5)Janata, J. Anal. Chem. 1987,59, 1351-6. (6)Schaffar, B. P. H.; Wolfbeis, 0. S. Analyst 1988, 113, 693-7. 0003-2700/92/0384-1805$03.00/0
First attempts led to an irreversibly working commercial test strip for single use only.7 When Morf and co-workers presented the first theoretical description of such systems, the pathway for reversibly working optodes with a high analytical potential was set.8~9 A large variety of such optical sensors has been developed in our group. Optodes selective for ions such as Ca2+,10K+,ll Na+,12 NH4+,13chiral ammonium ions,14J5 Pb2+?l6C032-,17 N03-,18 Cl-,19 and for anions following the Hofmeister selectivity series20 have been reported recently. Until now, the mathematical description of the selectivity of these systems waa closelyrelated to the selectivityformalism of ion-selective electrodes, based on the Nicolsky-Eisenman formalism.21 Morf et al. could give a straightforward and exact description of the selectivity of ions of the same charge and the same stoichiometry between the analyte and the ionophore? Since no better approach was available at that time, other authors then used the extended Nicolsky-Eisenman terminology to give a description of the selectivity over ions of different charge, though the lack of accuracy was recognized.12922 In this work, a thermodynamically more exact description of the selectivity behavior of cation- and anion-selective bulk optodes is presented. This novel approach is developed by using precisely defined thermodynamic constants and masstransfer equilibria. With the help of this novel theory, it will be possible to optimize the selectivity coefficient by calculation. A method for the graphical representation of the selectivity coefficient is presented. (7) Charlton, S. C.; Fleming, R. L.; Zipp, A. Clin. Chem. 1982,28,185761. (8)Morf, W. E.; Seiler, K.; Lehmann, B.; Behringer, Ch.; Tan, S. S. S.; Hartmann, K.; Ssrensen, P. R.; Simon, W. In Ion-selective electrodes; Pungor, E., Ed.; Akadbmiai Kiadb: Budapest, 1989;Vol. 5, 115-31. (9)Morf, W. E.; Seiler, K.; Ssrensen, P. R.; Simon, W. Inlon-selectiue electrodes; Pungor, E., Ed.; Akadbmiai Kiadb: Budapest, 1989; Vol. 5, p. 141-52. (10)Morf, W. E.; Seiler, K.; Rusterholz, B.; Simon, W. Anal. Chem. 1990,62, 741-4. (11) Wang, K.; Seiler, K.; Morf, W. E.; Spichiger, U.E.; Simon, W.; Lindner, E.;Pungor, E. Anal. Sci. 1990,6, 715-20. (12)Seiler, K.; Wang, K.; Bakker, E.; Mod, W. E.; Rusterholz, B.; Spichiger, U.E.; Simon, W. Clin. Chem. 1991,37, 1350-5. (13)Seiler, K.; Morf, W. E.; Rusterholz, B.; Simon, W. Anal. Sci. 1989, 5, 557-61. (14)Holq, P.; Morf, W. E.; Seiler, K.; Simon, W. Helu. Chim. Acta 1990, 73, 1171-81. (15)He, H.;Uray, G.; Wolfbeis, 0. S. Anal. Chim. Acta 1991, 246, 251-7. (16)Lerchi, M.; Bakker, E.; Rusterholz, B.; Simon, W. Anal. Chem., in press. (17)Behringer, C.; Lehmann, B.; Haug, J.-P.; Seiler, K.; Morf, W. E.; Hartmann, K.; Simon, W. Anal. Chim.Acta 1990, 233, 41-7. (18)Hauser, P. C.; PBrisset, P. M. J.; Tan, S. S. S.; Simon, W. Anal. Chem. 1990,62,1919-23. (19)Tan,S. S.S.;Hauser, P. C.; Wang, K.; Fluri, K.;Seiler, K.; Rusterholz, B.; Suter, G.; Kruttli, M.; Spichiger, U. E.; Simon, W. Anal. Chim. Acta 1991,255, 35-44. (20)Tan, S.S.S.; Hauser, P. C.; Chaniotakis, N. A.;Suter, C.;Simon, W. Chimio 1989, 43, 257-61. (21)Nicolsky, B. P.; Schulz, M. M.; Belijuatin, A. A.; Lev, A. A. In Glass Electrodes for Hydrogen and other Cations;Eisenman, G., Ed.; M. Dekker: New York, 1967. (22)Seiler, K. Zonenselektiue Optodenmembranen;Fluka Chemie AG: Buchs, Switzerland, 1991. 0 1992 American Chemical Society
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THEORY Possible Optode Mechanisms. In the described optode systems, two different ionophores L and C, each selective for a particular ion, are dissolved in a plasticized polymeric membrane, which is assumed to behave like a homogeneous organic solvent phase. The sensor is in complete equilibrium with the contacting probe, usually being an aqueous solution. The analyte ions are extracted by mass transfer into the bulk of the organic membrane phase. One ionophore changes ita optical signal drastically upon complexation with a specific ion and is therefore called chromoionophoreC. As its choice is free in principle, one normally uses a hydrogen ion selective chromoionophore. The pH of a sample can be measured or adjusted by adding a suitable buffer. The structure of the chromoionophorecan be designed with the aid of available pH indicators, which must be lipophilized. The pK, of the chromoionophoreis usually closely related to ita unlipophilized structure and can be shifted to some extent as well. Hydrogen ion selective chromoionophores are probably the most selective ionophores achievable, and log qi = -10.5 has been realized for these compounds.23 This means that the chromoionophores can be assumed to be ideally hydrogen ion selective forming a 1:lstoichiometry in all cases described in this paper. Primary and interfering cations I"+and Jz+ (or Xv-and Yzin the case of anions) are assumed to form exclusivelyl : pand 1:q complexes, respectively, with the neutral ionophore L. This leads to the charged complexes IL;' and JL:' for cations and to XL; and YL:- for anions. Some ionophores can however form complexes with changing stoichiometry, or in some cases, a complete complexation of the primary or interfering ion cannot be assumed. In such cases, the theoretical approach has to be extended, which will lead to more complicated equations. Two classes of hydrogen ion selective chromoionophores will be considered here. The first class is neutral in the basic form and positively charged when protonated (basic chromoionophores, neutral hydrogen carriers); the second class is neutral in the protonated form and negativelycharged when deprotonated (acidic chromoionophores, charged hydrogen carriers). In combination with neutral cation- or anion-selective ionophores this leads to four different phase-transfer equilibria. Cation-Sensitive Membranes. (la)Ion-Exchange System with Basic Chromoionophore:
L, leading to the corresponding ion complexes CH, CH+, IL;', or XLp" within the membrane. The activity symbols ai, a H , and ax refer to species in the aqueous phase. Concentration symbols in the organic phase are given in square brackets. As the concentration unita in the organic phase are usually molalities (moles per kilogram), the corresponding activity coefficients are here expressed by y. In cases l a and 2a, lipophilic ionic sites R- and R+, respectively, are required in order to maintain the electroneutrality condition within the membrane phase. Tetraphenylborate derivatives in case l a and tetraalkylammonium salts in case 2a are usually incorporated. The hydrophilic counterions of the respective additives are replaced by the primary and hydrogen ion upon first contact with aqueous solution and are therefore not further considered in this discussion. The upper mechanisms lead to the correspondingexchange and coextraction constants flex& and KL,, respectively:
(lb)
Anion-Sensitiue Membranes. (2a) Coextraction System with Acidic Chromoionophore:
The value of the equilibrium constant is dependent on the acidity constant of the chromoionophore K , (the charges are given here for the class of basic chromoionophores) The hydrogen ion selective chromoionophore C is competing with the neutral cation- or anion-selective ionophore
(5)
(23) Cosofret, V. V.; Nahir, T. M.; Lindner, E.; Buck, R. P. J.Electroanal. Chern. Interfacial Electrochem. 1992, 327, 137-46.
on the stability constant of the ionophore-cation complex BIL
ANALYTICAL CHEMISTRY, VOL. 64, NO. 17, SEPTEMBER 1, 1992
and of the ionophore-anion complex BXL, respectively, IxLtj-1 BxLg =
YXL;
[X"I [LIP Yx&L)p
(7)
the corresponding activity coefficients y in the membrane phase and the distribution coefficients k H , ki, or kx of the free ions between the aqueous and the membrane phase.
(8)
All activity coefficients are included within the respective concentration constants P:xch and KLx, respectively. The activities of the involved species control the equilibrium, but only concentration values of the chromoionophore are accessible through the optical response. It is therefore essential that these activity coefficients remain constant within the measuring range. Cases l b and 2b do not need further ionic sites and represent a simpler system. Since the ionic strength within the membrane is changing with each change in the activity of the analytes and since it is likely that severe interactions between anions and cations are occurring, these systems are difficult to describe in a thermodynamically exact manner. As a consequence, the respective equilibrium constant undergoes changes while the degree of protonation changes. For a given membrane composition optodes l a and 2a virtually maintain a constant ionic strength, which is defined by the constant amount of added lipophilic ionic sites R- and R+, respectively. According to the Debye-Huckel theory,24 this should lead to a constant mean activity coefficient within the membrane phase. In a first approximation the activity coefficients of the charged species are therefore neglectable in the expressions of the exchange and coextraction constant of eqs 1 and 3. The change of the activity coefficients of the neutral species is assumed to be relatively small within the calibration range of the optode system, if their overall concentration in the organic phase is kept low. When the amount of ionophore is increased, e.g. for selectivity reasons (see ref 121, a change of the activity coefficient and therefore of the equilibrium constant has to be encountered. Since the charge of the added ionic site is shielded and sterically hindered to a great extent, an association to counterions is neglected in this discussion. Although recent research showed that there is a significant ion pair formation in nonpolar membrane solvent^,^^^^^ it turned out that the association constants, e.g. for the so-called free and valinomycin-complexed potassium ion with tetrakis@-chlorophenyl)borate, do not differ significantly from each other. In such a case, the values of the association constants disappear in the respective equations in cases l a and 2a so that they act as if no ion pair formation at all would appear within the (24) Morf, W. E. The Principles of Ion-Selectiue Electrodes and of Membrane Transport; Akad6miai Kiad6: Budapest, 1981. (25) Armstrong, R. D.; Horvai, G. Electrochim. Acta 1990, 35, 1-7. (26) Armstrong, R. D.; Nikitas, P. Electrochim. Acta 1985,30,1627-9.
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membrane phase. However, if strong ion pair formation is assumed, the exchange and coextraction functions remain formally unchanged but the respective association constants take part of the overall equilibrium constant. More details about association constants are presented in ref 27, where potentiometric sensors are discussed. These arguments show that the optode systems l a and 2a are by far better to describe, and an optimization of the selectivity should be relatively precise by simple calculation (see below). The response and selectivity behavior of these two cases are therefore described in detail, whereas cases l b and 2b are not further explained. In principle they lead to the same results, if the respective exchange and coextraction constants are assumed to remain unchanged. Cation-ExchangeMechanism la. The response behavior of this cation-exchange optode can be described by the transformation of eq 1. If the chromoionophore is the detectable species by means of absorption, the ratio of the concentration of deprotonated form C to the total concentration of C([Cbtl) of the chromoionophore is introduced as a,which is connected to the respective absorbance values as follows: (11)
The subscript "tot" denotes the total initial concentration of the respective species. A is the absorbance of the chromoionophore for a given equilibrium, and Aprot and Adeprot are the absorbance values for the totally protonated and deprotonated form of the chromoionophore, respectively. With the condition of electroneutrality [RJ
= [CH'I
+ u[IL~+l
(12)
and the mass balance for the ionophore L within the membrane phase [L,l = [Ll +p[IL;+l (13) The equilibrium activity ai of the primary ion may be expressed according to eq 1 as a function of a,hydrogen ion activity, and the total concentration of chromoionophore, ionophore, and anionic sites: a,,+ = f(a,,+)=
This equation describes the response curve of a cationselective optode for the given parameters, if only the cations Iv+and H+ are involved. If an interfering ion J2+ is introduced, the ion-exchange reaction scheme may be formulated in analogy to eq 1:
leading to the following response curve, if only J z + and H+are involved: aj2+= f(aj,+) =
The symbols f(ai) and f(aj)in eqs 14 and 16 are called singleion response functions and represent the activity values of (27) Rosatzin, Th.; Bakker, E.; Suzuki, K.; Simon, W. Manuscript in preparation.
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the respective ion for a given degree of protonation of the chromoionophore, if no interfering ion is present. These symbols are introduced here to distinguish them from the later equilibrium activity values in the presence of interfering ions. Regarding the system where I"+and J2+are both competing with the same ionophore L giving the complexes IL;' and JL;', respectively, the condition of electroneutrality
[R;,]
= [CH'I
+ u[IL;+l + z[JLi+l
(17) O*l
and the mass balance for the ionophore
&,,I = [Ll + p[IL;+I + a[JLi+l (18) are extended by the terms of the additional cation Jz+,while eqs 1, 11, and 15 remain valid. Equations 15,17, and 18 may be used to find the correct expression for IL;' and L in eq 1, which leads to the response curve for the primary cation Iu+in the presence of interfering cations Jz+. In the case of a one to one stoichiometry of the two ionophore complexes ILu+ and JLz+, one derives the two expressions for the distribution of the free and complexed ionophore within the membrane with
for combining eqs 15 and 17
TJ
