Anal. Chem. 1996, 68, 3210-3214
Selective Optical Sensing of Silver Ions in Drinking Water Markus Lerchi, Franc¸ ois Orsini, Zvjezdana Cimerman,† and Erno 1 Pretsch*
Department of Organic Chemistry, Swiss Federal Institute of Technology (ETH), Universita¨ tstrasse 16, CH-8092 Zu¨ rich, Switzerland Didarul A. Chowdhury and Satsuo Kamata
Department of Applied Chemistry and Chemical Engineering, Faculty of Engineering, Kagoshima University, Korimoto, Kagoshima 890, Japan
A new thiocarbamate derivative is used in polymeric sensing films together with a lipophilic chromoionophore for determining Ag+ at submicromolar levels. The membrane composition has been optimized with a view to measuring concentrations of Ag+ added as a bacteriostatic agent to drinking water. The results compare well with those obtained by ICPMS. Chemical sensors based on optical signal detection are of increasing importance.1-3 In the case of bulk optodes, measurements are made on plasticized polymeric films after establishing a chemical equilibrium with the sample solution.4 Optical sensors of this type have the advantage of being robust and showing a highly reversible response explainable by means of the chemical equilibria involved.5-7 In addition, the use of the numerous ionophores already developed for ion-selective electrodes (ISEs) is straightforward. It is, therefore, not astonishing that reports on a great number of new ion-selective bulk optodes have appeared within a few years (see refs 8-19 in ref 6). One field in special need of inexpensive sensors is environmental monitoring. Several bulk optodes for detecting toxicologically relevant ions, such as Pb2+,8 Hg2+/Ag+,9 and UO22+,10 have been developed recently. These sensors exhibit the low detection limits required. The slow response at low ion activities (e10-6 M) due to the time required to establish chemical equilibrium with the bulk of the sensing film is of no concern in this context. † On leave from Zagreb University, Laboratory of Analytical Chemistry, 41000 Zagreb, Croatia. (1) Wolfbeis, O. S. Fiber Optic Chemical Sensors and Biosensors; CRC Press: Boca Raton, FL, 1991. (2) Janata, J. Anal. Chem. 1992, 64, 921A-927A. (3) Arnold, M. A. Anal. Chem. 1992, 64, 1015A-1025A. (4) Morf, W. E.; Seiler, K.; Lehmann, B.; Behringer, C.; Hartman, K.; Simon, W. Pure Appl. Chem. 1989, 61, 1613-1618. (5) Morf, W. E.; Seiler, K.; So ¨rensen, P. R.; Simon, W. In Ion-Selective Electrodes; Pungor, E., Ed.; Akade´miai Kiado´: Budapest, 1989; Vol. 5, pp 141-152. (6) Bakker, E.; Simon, W. Anal. Chem. 1992, 64, 1805-1812. (7) Seiler, K.; Simon, W. Anal. Chim. Acta 1992, 266, 73-87. (8) Lerchi, M.; Bakker, E.; Rusterholz, B.; Simon, W. Anal. Chem. 1992, 64, 1534-1540. (9) Lerchi, M.; Reitter, E.; Simon, W.; Pretsch, E.; Chowdhury, D. A.; Kamata, S. Anal. Chem. 1994, 66, 1713-1717. (10) Lerchi, M.; Reitter, E.; Simon, W. Fresenius J. Anal. Chem. 1994, 348, 272276. (11) McKee, J. E.; Wolf, H. W. Water Quality Criteria; State Water Resources Control Board: Sacramento, CA, 1963. (12) Train, R. E. Quality Criteria for Water; U.S. Environmental Protection Agency: Washington, DC, 1976.
3210 Analytical Chemistry, Vol. 68, No. 18, September 15, 1996
Since water is frequently disinfected with silver salts,11 there is a real need for simple and robust sensors to monitor low Ag+ activities. The Ag+ concentration used (∼10-6 M) is rather close to the allowable level. According to U.S. EPA norms,12 the maximum Ag+ level allowed in domestic water supply is 50 µg/L (4.6 × 10-7 M).12 Although a number of S-containing ionophores for the detection of Ag+ have been used in optodes9,13 and ISEs,14 the sensors described so far are inadequate for monitoring drinking water because of their limited selectivity and/or insufficient sensitivity. In view of the potential of S-containing compounds as ligands in ISEs,15,16 further investigations have been carried out with the aim of designing an improved optical sensor for Ag+. Here, we report optode films based on a new bis(thiobenzothiazole) compound and their application for measuring Ag+ concentrations in drinking water. EXPERIMENTAL SECTION Reagents. Aqueous solutions were prepared with doubly quartz-distilled water and metal chlorides or nitrates of the highest purity available (E. Merck, Darmstadt, Germany and Fluka AG, CH-9471 Buchs, Switzerland). For preparing the optode films, poly(vinyl chloride) (PVC; high molecular weight), potassium tetrakis[3,5-bis(trifluoromethyl)phenyl]borate [KTm(CF3)2PB], bis(2-ethylhexyl) sebacate (DOS), and tetrahydrofuran (THF, freshly distilled prior to use) were purchased from Fluka AG. The syntheses of the chromoionophores 4-(octadecylamino)azobenzene (ETH 5315) and benzoic acid 4-[[9-(dimethylamino)-5Hbenzo[a]phenoxazin-5-ylidene]amino]-11-[(1-butylpentyl)oxy]-11oxoundecyl ester (ETH 5418) are given elsewhere.8 Synthesis of Methylene Bis(2-thiobenzothiazole) (MBTBT). The sodium salt of 2-mercaptobenzothiazole (0.03 mol) was dissolved in ethanol (200 mL; 99 vol %) in a round-bottomed flask fitted with a mechanical glass stirrer, a reflux condenser, and a dropping funnel. The stirred solution was refluxed, and dibromomethane (0.015 mol) in ethanol (15 mL) was added dropwise. After 4 h, the reaction mixture was allowed to cool. Evaporation of the solvent at 35 °C gave the crude product as a (13) Hisamoto, H.; Nakagawa, E.; Nagatsuka, K.; Abe, Y.; Sato, S.; Siswanta, D.; Suzuki, K. Anal. Chem. 1995, 67, 1315-1321. (14) Casabo´, J.; Flor, T.; Romero, M. I.; Teixidor, F.; Pe´rez-Jime´nez, C. Anal. Chim. Acta 1994, 294, 207-213. (15) Kamata, S.; Bhale, A.; Fukunaga, Y.; Murata, H. Anal. Chem. 1988, 60, 2464-2467. (16) Kamata, S.; Onoyama, K. Anal. Chem. 1991, 63, 1295-1298. S0003-2700(96)00147-3 CCC: $12.00
© 1996 American Chemical Society
basicity (pKa(ETH 5418) - pKa(ETH 5315) ) 3.819) had to be taken into account.
Table 1. Composition (mmol/kg) of Optode Filmsa film
MBTBT
chromoionophore
KTm(CF3)2PB
DOS
I II III
38.7 267 260
(ETH 5315) 9.94 (ETH 5315) 18.3 (ETH 5418) 18.8
10.1 18.3 18.8
1500 1400 1400
a The ionophore (MBTBT), chromoionophore, and borate salt [KTm(CF3)2PB] together with 80 mg of PVC and 160 mg of plasticizer (DOS) were dissolved in 1.5 mL of freshly distilled THF (for compound names, see Experimental Section).
white mass together with crystals of NaBr, which were dissolved by adding water (300 mL). The product was extracted twice with 60-mL portions of diethyl ether. Recrystallization from benzene/ ethanol (1:1) yielded the pure product as white crystals (87%), mp 81 °C. Anal. Calcd for C15H10N2S4 (346.50): C, 51.99; H, 2.91; N, 8.08. Found: C, 51.69; H, 3.10; N, 7.99. 1H NMR (CDCl3,, 400 MHz) δ 5.34 (s, 2 H (CH2)), 7.34 (t, J ) 8 Hz), 7.44 (t, J ) 8 Hz), 7.77 (d, J ) 8 Hz), 7.91 (d, J ) 8 Hz, each 2 H (aromatic)); FT-IR (KBr) 3029, 2990 (C-H str); 820 cm-1 (C-S of SCH2S); EI-MS m/z (%) 348.0 (1.2), 346.0 (M•+, 5.4), 300.0 (M•+ - SCH2, 3.8), 180.0 (C8H6NS2+, 100), 167.0 (C7H5NS2•+, 53.2), 136.0 (C7H6NS+, 27.8). Preparation of Optode Films. The membrane components (see Table 1) were dissolved in THF (1.5 mL). With the help of a spin-on device,17 two films of the same thickness (∼1-3 µm, depending on the rotation frequency) were cast on glass plates and mounted in a flow-through cell of ∼600 µL inner volume.17 Apparatus. UV/visible spectra were measured with a Uvikon Model 810 double-beam spectrophotometer (Kontron AG, CH8010 Zu¨rich, Switzerland). The pH values were measured with a combination glass electrode (Orion Ross Model 8103, Orion Research represented by Hu¨gli-Labortec AG, CH-9030 Abtwil). Solutions. Stock solutions (10-3 M) were prepared by dissolving the salts in diluted magnesium acetate buffer of pH 4.5 or 5.4 (ionic strength, I ) 10-3 M) and further diluted by weight to obtain the sample solutions, which were stored at room temperature in polyethylene bottles to avoid contamination by ions. Absorption Experiments. The flow-through cell with two identical optode films on their glass plates was mounted in the sample path of the spectrophotometer and connected to a peristaltic pump, assuring a constant flow of ∼5 mL/min. An empty cell with two glass plates was placed in the reference path. All spectra were recorded at 21 ( 1 °C in the transmittance mode, for films I or II (with ETH 5315) and for III (with ETH 5418) in the range of 700-350 and 800-400 nm, respectively. Calculations. All computed curves were fitted to the experimental data points by varying Kexch in eq 2 (with Mz+ ) Ag+). The maximum and minimum absorbance values necessary for calculating the degree of protonation (1 - R) of the chromoionophore from the spectra were determined after equilibrating the optode films with 0.01 M solutions of HCl and NaOH, respectively. Activities according to Debye-Hu¨ckel18 were only considered for calculating the selectivity coefficients because all measurements were made at highly buffered ion concentrations as compared with osel those of the analyte ion (Ag+). In order to compare log KAg,M obtained with the two chromoionophores, the difference in their (17) Seiler, K.; Morf, W. E.; Rusterholz, B.; Simon, W. Anal. Sci. 1989, 5, 557561. (18) Meier, P. C. Anal. Chim. Acta 1982, 136, 363-368.
RESULTS AND DISCUSSION Principle of Operation. The polymer films presented here for measuring metal ion concentrations are used in chemical equilibrium with the analyte sample and thus belong to the group of bulk optodes.4 The principle of operation is only briefly summarized here since the details are already given elsewhere.4,6,7 The organic phase (org) contains the Ag+-selective ionophore L, the chromoionophore C, and a salt of a highly lipophilic anion R-. The total concentration of cations in the membrane is defined by that of R-. For the sensing film in contact with the aqueous solution (aq) of a silver salt, the following equilibrium holds: Ag Kexch
Ag+(aq) + p L(org) + CH+(org) \ y z + AgL+ p (org) + C(org) + H (aq) (1)
where p is the stoichiometric factor of the AgL+ complex. The sample activities of H+ and Ag+ determine their concentrations in a given membrane. By monitoring the absorbance of the chromoionophore at the wavelength of maximum absorption of its protonated form, the degree of protonation 1 - R ) [CH+]/ Ag Ctot is obtained. With Kexch as the overall equilibrium constant, the response of the optode to Ag+ is given by6
aAg+ )
1 KAg exch
( )
RaH+ Rtot - (1 - R)Ctot (2) 1 - R (L - p{R- - (1 - R)C })p tot tot tot
with activities a referring to the aqueous sample, whereas Ltot, Ctot, and Rtot are the total concentrations, respectively, of ionophore, chromoionophore, and anionic sites in the membrane phase for which changes in activity coefficients are neglected by this model. Calibration curves (see Figures 1-3 and 5) were calcuAg lated with eq 2, adjusting Kexch for an optimum fit of the experimental data. If various metal ions (Mz+) are present in the sample, the lefthand side of eq 2 has to be replaced by a weighted sum of their osel activities, with the optical selectivity coefficients, log KAg,M , as the weighting factors.6 Their values correspond to the ratio of Ag+ and Mz+ activities that yield the same degree of protonation (1 - R) of the chromoionophore. Hence, they are directly accessible from the experimental data so that there is no need for assuming the nature and stoichiometry of the MLz+ complexes and determining equilibrium constants. The selectivity coefficient thus defined is related to that of ISEs obtained by the matched potential method.20-22 However, since it may vary with 1 - R, it is advantageous to determine the values of the individual exchange constants for full characterization of the optode system. The corresponding selectivity coefficients are then calculated for any value of 1 - R using the expression6 (19) Bakker, E.; Lerchi, M.; Rosatzin, T.; Rusterholz, B.; Simon, W. Anal. Chim. Acta 1993, 278, 211-225. (20) Gadzekpo, V. P. Y.; Christian, G. D. Anal. Chim. Acta 1984, 164, 279282. (21) Bakker, E.; Meruva, R. K.; Pretsch, E.; Meyerhoff, M. E. Anal. Chem. 1994, 66, 3021-3030. (22) Umezawa, Y.; Umezawa, K.; Sato, H. Pure Appl. Chem. 1995, 67, 507-518.
Analytical Chemistry, Vol. 68, No. 18, September 15, 1996
3211
( )
KM exch RaH+ osel KAg,M ) z Ag 1-R K exch
q q - {R- (1 - R)Ctot} z tot (3) p (Ltot - p{Rtot - (1 - R)Ctot})
1-z
(L
tot
)
where z is the charge of the interfering ion Mz+ and q, the stoichiometric factor of its complex with the ionophore L. Equations 2 and 3 present several possibilities for optimizing the response range and selectivity behavior of the optode film by altering its composition as well as the measuring conditions. For example, an increase in the sample pH and/or the use of a chromoionophore of lower basicity shifts the response curve of an ion of charge z toward lower sample ion activities by z × ∆pH or z × ∆pK. Thus, the selectivity for a monovalent primary relative to a polyvalent interfering ion decreases when a sample of higher pH or a less basic chromoionophore is used. The response function may also be shifted by varying the concentrations Ltot, Ctot, and Rtot (cf. eq 2). The size of the shift depends both on the charge of the cations involved and on the stoichiometric factors of their complexes with the ligand (cf. eq 3). Highly charged ions and/or those forming a complex of higher stoichiometry are favored by increased ligand concentration. This is analogous to the influence observed on potentiometric selectivity values by varying the molar ratio of borate/ligand.23 Optimization of the Sensor Response. The response of an optode based on the new ionophore, MBTBT (film I in Table 1) to various concentrations of AgNO3 at pH 5.4 is shown in Figure 1. The three curves are calculated with eq 2 for the stoichiometric Ag ratio of ligand/metal ion, p ) 1, 2, or 3, and adjusting Kexch so that they closely fit the data points. Because the stoichiometry of the complex has a strong influence on the response function for monovalent ions, it can be determined by optical measurements. For ions of higher charge, the effect is much smaller and a clear distinction between different stoichiometries is difficult. It can be seen from Figure 1 that only the theoretical function of a 2:1 complex of ligand/ion fits the experimental data. Response curves were determined for various metal salts with film I at pH 5.4 (not shown) as well as with film II (Table 1), which contains a large excess of MBTBT, at pH 4.5 (cf. Figure 2). The response of strongly discriminated ions was measured at higher pH values and normalized for pH 4.5 as indicated above. The activity scale of Figure 2 is, therefore, only relative since measurements of the strongly discriminated ions (Ca2+, Mg2+, Co2+) are actually carried out at lower values of aM. Moreover, for calculating the response curves, it was assumed that all ions form 2:1 complexes (p ) 2). M The equilibrium constants, log Kexch (cf. eq 2), for the exchange of metal ions for H+ as evaluated from these response curves are given in Table 2. Within experimental error, their values should be the same for both films since the measurement conditions (such as pH) and weighting parameters have no effect on thermodynamic constants. Surprisingly, this was only true for Ag+ and Cu2+ (see columns 2 and 3 of Table 2). For the other ions, large differences of up to 2 orders of magnitude were observed. For a second group of ions (Na+, K+, Mg2+, Ca2+), M however, log Kexch was found to be independent of experimental conditions if the plasticizer (DOS) not the ionophore (MBTBT) is assumed to act as ligand (see columns 4 and 5 of Table 2).
This interpretation is further corroborated by the fact that, for a ligand-free sensing film with the same chromoionophor (ETH K 5315),19 a very similar value was determined for log Kexch (-5.2 compared with -5.4 found here). For a third group of ions, i.e., Hg2+, Pb2+, and Co2+ (due to strong discrimination, the values given for Co2+ are only approximative), neither of the two evaluation modes gave comparable exchange constants, hence, both MBTBT and DOS might be competing as ligands. osel The optical selectivity coefficients, log KAg,M , calculated for films I and II at their highest sensitivity, i.e., at half-protonation (1 - R ) 0.5) of ETH 5315, are presented in Table 3 for three different pH values. The maximum selectivities that allow the Ag+ concentration in drinking water to be measured with a precision of 10% at its highest level permitted by Swiss regulation (9.3 × 10-7 M)24 figure in the last column. It can be seen that
(23) Eugster, R.; Gehrig, P. M.; Morf, W. E.; Spichiger, U. E.; Simon, W. Anal. Chem. 1991, 63, 2285-2289.
(24) Fremd- und Inhaltsstoffverordnung, Swiss Federal Council, CH-3000 Bern, 1990.
3212 Analytical Chemistry, Vol. 68, No. 18, September 15, 1996
Figure 1. Degree of protonation, 1 - R, of two optode films I (Table 1) as a function of log cAg+. Experimental data points (O). Samples: Mg(OAc)2 buffer, pH 5.4 (I ) 10-3 M) with different concentrations of AgNO3. Solid curves are calculated using eq 2 with log KAg exch ) 0.1, 1.7, or 3.5 and p ) 1, 2, or 3, respectively.
Figure 2. Selectivity of the ionophore MBTBT (see Experimental Section). Calibration curves for two optode films II (Table 1) normalized for pH 4.5. The horizontal distance between the calibration curves for Ag+ and any interfering ion Mz+ gives the selectivity coefficient, osel log KAg,M . Experimental data points (O, b). Samples: metal salt solutions in Mg(OAc)2 buffer, i.e., AgNO3, Hg(NO3)2, Pb(NO3)2, and KCl at pH 4.5; Cu(NO3)2, Ca(NO3)2, Co(NO3)2, and NaCl at pH 5.0; Mg(NO3)2 at different pH values. Solid curves are calculated with eq 2 (p ) 2).
M Table 2. Equilibrium Constants, log Kexch ,a for Optode Films I and II
MBTBT as ligand Mz+ Ag+ Hg2+ Cu2+ Na+ K+ Mg2+ Ca2+ Pb2+ Co2+
DOS as ligand
film I
film II
film I
film II
1.7 -6.6 -9.1 -2.4 -2.0 -11.2 -10.5 -9.8 -10.2
1.7 -5.9 -8.8 -4.5 -3.9 -12.6 -12.2 -10.4 -14.0
-2.0 -9.9 -12.4 -5.9 -5.4 -14.4 -13.7 -13.0 -13.4
0.1 -7.4 -10.2 -5.9 -5.4 -14.0 -13.6 -11.8 -15.5
a Obtained by fitting eq 2 (with Mz+ ) Ag+ and p ) 2) to the experimental data and assuming either the ionophore (MBTBT) or the plasticizer (DOS) acts as ligand for Mz+.
osel Table 3. Selectivity Coefficients, log KAg,M , at Maximum Sensitivity (1 - r ) 0.5) of the Optode Films I and II (Table 1) for Different Sample pHs Compared with the Maximum Values Allowed
Mz+ Hg2+ Cu2+ K+ Na+ Pb2+ Ca2+ Mg2+ Co2+
pH 4.5 film I film II
pH 5.4 film I film II
pH 7.8 film I film II
-3.3 -5.8 -3.6 -4.1 -6.5 -7.1 -7.8 -6.8
-2.4 -4.9 -3.6 -4.1 -5.6 -6.2 -6.9 -5.9
0.0 -2.5 -3.6 -4.1 -3.2 -3.8 -4.5 -3.5
-2.7 -5.6 -5.6 -6.0 -7.2 -9.0 -9.0 -10.9
-1.8 -4.7 -5.6 -6.0 -6.3 -8.1 -8.1 -10.0
0.6 -2.3 -5.6 -6.0 -3.9 -5.7 -5.7 -7.6
Figure 4. Absorption spectra of two optode films III (Table 1) equilibrated with 0.01 M HCl and natural drinking water of pH 7.8 before and after spiking with different amounts of AgNO3. Absorption maxima of ETH 5418 665 (protonated) and 521 nm (deprotonated); isosbestic point 577 nm.
max alloweda 2.3 -0.2 -3.0 -4.0 0.7 -4.3 -3.8 -0.2
a Calculated value for log Kosel ) 0.1 a /a with a -7 Ag M Ag ) 9.3 × 10 Ag,M M (the quality criterion for drinking water in Switzerland24) and typical concentrations26 of aM found in Swiss rivers.
Figure 5. Degree of protonation, 1 - R, of two optode films III (Table 1) as a function of log cAg+. Samples: drinking water of pH 7.8 with different concentrations of AgNO3. The solid curve, calculated according to eq 2 with log KAg exch ) 1.7 and p ) 2, reveals that there is only very little interference from ions other than Ag+ contained in the drinking water.
Figure 3. Response of optode films (two each) of compositions I (O) and II (b) (see Table 1) to Ag+ and Na+. Solid curves are computed with eq 2 and normalized for Mg(OAc)2 buffer of pH 4.5 (I ) 10-3 M).
Na+ is the main interfering ion. Therefore, the response of the two optode films I and II to Ag+ and Na+ is shown on an expanded scale in Figure 3. The shift of the Ag+ response curve for film II to lower concentrations in comparison with that for film I is mainly caused by the increase in ionophore concentration. On the other hand, the slight shift of the response curve for Na+ in the opposite direction is ascribed to the combined effects of the reduced concentration of DOS (which acts as ligand for Na+) and an
increase in the amount of lipophilic anions. The influence of the latter depends on the ligand concentration, which is not the same for Ag+ and Na+. When equilibrated with pure magnesium acetate buffer of pH 5.4, the chromoionophore ETH 5315 is not fully protonated in either of the sensing films. This is due to Mg2+ interference which is stronger for film I (cf. Table 3). The osel selectivity of film II for Na+ (log KAg,Na ) -6.0) is sufficient for measurements in drinking water and the sensor has the ideal dynamic range of 10-6-10-7 M for this purpose (see Figure 3). Measurements in Drinking Water. Optodes with films of the optimized composition II (Table 1) show an ideal dynamic range and good selectivities over the relevant interfering ions at pH 4.5 (cf. Table 3). To determine Ag+ activities in drinking water, the samples should be buffered to this pH by adding a highly concentrated acetate buffer. If, however, Ag+ concentrations are to be monitored in natural drinking water which, owing to HCO3-/ CO32- buffering, has a well-defined pH of ∼7.8, ETH 5418 must be chosen as chromoionophore (film III in Table 1). Its pKa in the membrane phase is 9.0, as compared with 5.2 for ETH 5315.19 Thus, the difference in basicity between the two chromoionoAnalytical Chemistry, Vol. 68, No. 18, September 15, 1996
3213
coefficient (N ) 5). The slope of the function indicates that the results obtained with the optode are, in general, slightly lower than those from ICPMS. This is not surprising since the latter method measures the total amount of ions, whereas optodes detect the uncomplexed ones only. Thus, it was demonstrated that optodes are useful for monitoring Ag+ activities in the low ranges demanded to control water samples disinfected with silver salts. Figure 6. Short-time repeatability of the absorbance of two optode films III (Table 1). Samples: natural drinking water of pH 7.8 with, alternatively, log cAg+ ) -6.30 or -6.05.
phores is close to the pH difference between the sample solutions used. The UV/visible spectra of the sensing film III equilibrated with 0.01 M HCl as well as with drinking water before and after spiking with small amounts of AgNO3 are given in Figure 4. The corresponding calibration curve in Figure 5 shows that the response range of about 10-6-10-8 M Ag+ is ideal for monitoring drinking water. The sensor has a detection limit25 of 1.6 × 10-9 M Ag+, the slight interference at low concentrations being due to Ca2+. Its reversibility at submicromolar Ag+ concentrations (Figure 6) is excellent, and the slow response time in the order of 10 min is no handicap for the application envisaged. For comparison, the Ag+ concentrations in the water samples were also measured by ICPMS. The linear regression function corresponded to pAg (optode film III) ) (0.545 ( 0.159) + (0.915 ( 0.025) × pAg (ICPMS), with r2 ) 0.998 as the correlation
CONCLUSIONS It is shown that various possibilities exist to optimize the response range and selectivity of an optical sensing film based on a selective ionophore. Highly discriminated ions may be complexed by the plasticizer instead of the ionophore, in which case the concentration of the latter strongly affects the selectivity behavior. With respect to the detection limit of Ag+, the optode film III is superior to all previously reported optical sensors. Validation of the results by an independent method proved their reliability. The development of inexpensive test strips appears feasible. ACKNOWLEDGMENT This work was supported in part by the Swiss National Science Foundation. We thank Prof. B. Magyar for ICPMS analyses of the water samples as well as Dr. D. Wegmann and a reviewer for careful reading of the manuscript. Received for review February 14, 1996. Accepted June 15, 1996.X AC9601472
(25) Bakker, E.; Willer, M.; Pretsch, E. Anal. Chim. Acta 1993, 282, 265-271. (26) Zobrist, J. Gas, Wasser, Abwasser 1983, 63, 123-131.
3214 Analytical Chemistry, Vol. 68, No. 18, September 15, 1996
X
Abstract published in Advance ACS Abstracts, August 1, 1996.
