Lead-Selective Electrodes Based on Calixarene ... - ACS Publications

Anal. Chem. 1999, 71, 5544-5550

Lead-Selective Electrodes Based on Calixarene Phosphine Oxide Derivatives Francis Cadogan,† Paddy Kane,† M. Anthony McKervey,‡ and Dermot Diamond*,†

The Biomedical and Environmental Sensor Technology (BEST) Centre, School of Chemical Sciences, Dublin City University, Glasnevin, Dublin 9, Ireland, and School of Chemistry, Queen’s University, Belfast BT9 5AG, Northern Ireland

We have discovered that a series of calixarene phosphine oxide derivatives can produce ion-selective electrodes with excellent response characteristics to the Pb2+ ion. These ligands were previously known to complex lanthanides, but their complexation behavior with transition metal ions and group I/group II ions is relatively unexplored. The extreme stability of these compounds, coupled with interesting selectivity behavior, make them attractive contenders for use in potentiometric poly(vinyl chloride) membrane electrodes. Results show near-Nernstian responses to Pb2+ for electrodes based on three ligands (p-tert-butylcalix[n]arene ethyleneoxydiphenylphosphine oxide, n ) 4, 5, 6) and excellent selectivity against a wide range of common interferences. Ca2+ was found to be the most important interference, but its impact dramatically decreased as the cavity size increased from the tetramer (n ) 4) through the pentamer (n ) 5) to the hexamer (n ) 6). Electrodes based on the hexamer were found to have characteristics superior to similar devices based on several commercially available Pb2+-selective ligands. The importance of the calixarene macrocyclic backbone on the selectivity is highlighted by the poor selectivity of the electrode based on triphenylphosphine oxide. Molecular modeling has been used to provide some insight into the observed selectivity. Over recent years, the importance of controlling the levels of environmental pollutants in natural waterways and potable water has generated increasing interest in the development of novel sensors for the detection of heavy metals. The use of ion-selective electrodes (ISEs) for the detection of lead has received much interest, and many ligands have been investigated as sensing agents in electrodes based on ionophore-doped poly(vinyl chloride) (PVC) membranes. Alexander et al.1 recently reported the use of diazacrown ethers bearing double-armed thenoyl and thiopheneacetyl groups as potential selective agents for Pb2+ ion-selective membranes. A total of five different ionophores were evaluated, and the optimum membrane composition was found to reach steady state in less than 30 s at low concentrations (∼10 µM). A near-Nernstian * Corresponding author: (e-mail) [email protected]. † Dublin City University. ‡ Queen’s University. (1) Yang, X.; Kumar, N.; Chi, H.; Hibbert, D. B.; Alexander, P. W. Electroanalysis 1997, 9, 549-553.

5544 Analytical Chemistry, Vol. 71, No. 24, December 15, 1999

response and a limit of detection of ∼1 × 10-6 M Pb2+ was obtained. It was found that the membrane worked most efficiently between pH 4.0 and 5.5. Above these pH values, hydroxide and nitrate formation within the membrane gave a false increase in the electrode response due to the formation of the PbX+ species. On the other hand, below pH 4, the electrode response gradually fell away. Hg2+ and Ag+ ions were both found to interfere strongly with the electrode response, however (log KpotPbj > 2.0 for these ions). Acyclic amides and oxamides have also been investigated2,3 as potential ionophores for lead. The most selective ligands have log KpotPbj values better than -3 for the alkali and alkaline earth metals, but Ag+ and H+ ions appear to be major interferences. Lead ionophore I,2 available from Fluka,4 is different in that lead is detected as the monovalent species PbX+ (X: OH-, Cl-, NO3-, CH3COO-), which in theory should result in a doubling of electrode slope. A similar compound, uranyl ionophore I,3 from Fluka, is also used as a lead-selective agent in polymeric membrane electrodes. Likewise, macrocyclic oxamides have been studied and although they show good selectivity (log KpotPbj > -2) against a large range of ions, Cu2+, Ag+, Cs+, and Rb+ are major interferences.5 Kamata and Onoyama6,7 used acyclic dithiocarbamates as ionophores. The electrode based on a methylene bis(diisobutyldithiocarbamate) ligand responded well to Pb2+, rejecting alkali and alkaline earth metals by a factor of at least 100. The Cu2+ ion was a major interference, however, and this would be problematic in many applications as Cu2+ tends to be found in association with Pb2+ at relatively high concentrations in water (the maximum legal limit in drinking water is 3 ppm). This ionophore is also available commercially from Fluka Chemicals as lead ionophore II (FII). Thomas et al.,8 have described liquid ion-exchange membranes incorporating the tetraphenylborate salts of nonionic surfactant polyoxylates, the most selective of which was found to have poor selectivity for Pb2+ in the presence of Ba2+. Anuar and Hamdan9 (2) Lindner, E.; To´th, K.; Pungor, E.; Behm, F.; Oggenfuss, P.; Welti, D. H.; Ammann, D.; Morf, W. E.; Pretsch, E.; Simon, W. Anal. Chem. 1984, 56, 1127-1131. (3) Malinowska, E. Analyst 1990, 115, 1085-1087. (4) Fluka Chemika, Selectophore Catalogue, 1996. (5) Malinowska, E.; Jurczak, J.; Stankiewicz, T. Electroanalysis 1993, 5, 489492. (6) Kamata, S.; Onoyama, K. Anal. Chem. 1991, 63, 1295-1298. (7) Kamata, S.; Onoyama, K. Chem. Lett. 1991, 653-656. (8) Jabar, A. M. Y.; Moody, G. J.; Thomas, J. D. R. Analyst 1988, 133, 14091413. 10.1021/ac990303f CCC: $18.00

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reported the use of poly(hydroxamic) acid as the active material in lead ISEs, but Ni2+ and Hg2+ exhibited a very large interference. The electrode was also found to be poisoned by Fe3+ and was reported to take between 50 and 120 s to reach a steady-state response. Attempts have also been made to develop lead-selective agents based on crown ether structures,10 in particular, Jain et al.11 developed a PVC membrane based on the 15-crown-5 ligand (lead ionophore V, Fluka Chemicals; see Figure 1, FV). The membrane was targeted specifically for lead determination in nonaqueous environments, but interference from alkali metal ions and Ag+ ion is significant. Arnaud-Neu et al.,12 recently described how the chemical modification of the p-tert-butylcalix[n]arene (where n ) 4, 5, and 6) lower rim with various thioamide podands can effect their ability to bind cations. The findings suggest that, unlike the corresponding ester,13 ether,14 ketone,15 carboxylic acid,16 amide,17 crown ether,18 and hyperspherand19 derivatives, these compounds have no affinity for the alkali or alkaline earth metals. Picrate extraction experiments12 revealed that Cu2+, Pb2+, and Ag+ ions are extracted efficiently by all thioamides tested, while Cd2+ was extracted to a significant level by the pentamer derivative only. X-ray molecular structures of the calix[4]arene thioamide-PbII(ClO4)2 revealed that the complex exists in the cone conformation with the lead ion being bound by heteroatoms on the lower rim of the calixarene. Beer et al. reported20 transition metal complexes of a tetraamide calix[4]arene. Crystals were obtained of the Fe2+, Zn2+, Ni2+, Cu2+, and Pb2+ complexes. The results were in agreement with those of Arnaud-Neu et al.,12 discussed above. In work carried out by Reinhoudt et al.,21,22 lead-selective PVC electrodes based on di- and tetrathioamide-functionalized calix[4]arenes were examined. The electrode containing the tetrathioamide-functionalized calix[4]arene (Fluka lead ionophore IV; see Figure 1, FIV) 4 was claimed to have good selectivity (-log KpotPbj > 3) against the alkali metals and Cu2+, Zn2+, and Cd2+. The selectivity against Cu2+ is surprising (but verified in our experiments), given the well-known affinity of thio groups for Cu2+ ions.12 (9) Anuar, K.; Hamdan, S. Talanta 1992, 39, 1653-1656. (10) Sheen, S. R.; Shih, J. S. Analyst 1992, 117, 1691-1695. (11) Srivastava, S. K.; Gupta, V. K.; Jain, S. Analyst 1995, 120, 495-498. (12) Arnaud-Neu, F.; Barrett, G.; Corry, D.; Cremin, S.; Ferguson, G.; Gallagher, J. F.; Harris, S. J.; McKervey, M. A.; Schwing-Weill, M. J. J. Chem. Soc., Perkin Trans. 2 1997, 575-579. (13) Arnaud-Neu, F.; Barrett, G.; Cremin, S.; Deasy, M.; Ferguson, G.; Harris, S. J.; Lough, A. J.; Guerra, L.; McKervey, M. A.; Schwing-Weill, M. J.; Schwinte, P. J. Chem. Soc., Perkin Trans. 2 1992, 1119. (14) Bocchi, V.; Foina, D.; Pochini, A.; Ungaro, R.; Andreetti, C. D. Tetrahedron 1982, 38, 373. (15) Ferguson, G.; Kaiter, B.; McKervey, M. A.; Seward, E. M. J. Chem. Soc., Chem. Commun. 1987, 584. (16) Arnaud-Neu, F.; Barrett, G.; Harris, S. J.; McKervey, M. A.; Owens, M.; Schwing-Weill, M. J.; Schwinte, P. Inorg. Chem. 1993, 32, 2644-2650. (17) Muzet, N.; Wipff, G.; Casnati, A.; Domiano, L.; Ungaro, R.; Ugozzoli, F. J. Chem. Soc., Perkin Trans. 2 1996, 1065-1075. (18) Ghidini, E.; Ugozzoli, F.; Ungaro, R.; Harkema, S.; Abu El-Fadl, A.; Reinhoudt, D. N. J. Am. Chem. Soc. 1990, 112, 6979-6985. (19) Reinhoudt, D. N.; Dijkstra, P. J.; in’t Veld, P. J. A.; Bugge, K. E.; Harkema, S.; Ungaro, R.; Ghidini, E. J. Am. Chem. Soc. 1987, 109, 4761-4762. (20) Beer, P. D.; Drew, M. G. B.; Lesson, P. B.; Ogden, M. I. J. Chem. Soc., Dalton Trans. 1995, 1273-1283. (21) Mailinowska, E.; Brzo´zka, Z.; Kasiura, K.; Egberink, R. J. M.; Reinhoudt, D. N. Anal. Chim. Acta 1994, 298, 253-258. (22) Wroblewski, W.; Brzozka, Z.; Janssen, R. G.; Verboom, W.; Reinhoudt, D. N. New J. Chem. 1996, 4, 419-426.

Figure 1. General structures of butylcalix[n]arene ethyleneoxydiphenylphosphine (n ) 4, 5, and 6 for ligands L1-L3, respectively), Fluka lead ionophores FII, FIV, and FV, and triphenylphosphine oxide (L4).

Calix[n]arene phosphine oxides were first reported by McKervey et al.23 in 1995 as a new series of cation receptors for the extraction of certain lanthanides and actinides from simulated nuclear waste, which employed HNO3 at concentrations of up to 4 M as background matrix. Subsequent work by our group revealed their ability to produce PVC membrane sensors with interesting selectivity selectively within and between these series.24 More recently, we have shown that p-tert-butylcalix[4]arene ethylenediphenyloxyphosphine oxide (L1; see Figure 1), can produce PVC membrane Ca2+-selective electrodes with excellent discrimination against magnesium and alkali metal ions.25 In this paper, we report the hitherto unknown selective complexation of the larger calixarene phosphine oxides (particularly the hexamer p-tert-butylcalix[6]arene ethyleneoxydiphenylphosphine oxide) for Pb2+ ions at pH >3. Below this pH, a very strong interaction with Hg2+ ions is apparent, which also has not been reported previously in the literature. The response characteristics of the electrodes produced and the effect of changing cavity size on selectivity (by variation of the number of repeat units in the calixarene macrocycle through 4 (tetramer), 5 (pentamer), and 6 (hexamer)) is discussed. This, coupled to a comparative study between the hexamer (L3, see Figure 1) and a selection of commercially available Pb2+ ligands (purchased from Fluka Chemicals4), suggests that this ligand has certain advantages over those currently available. EXPERIMENTAL SECTION Chemicals and Reagents. All membrane components with the exception of the calixarenes were Selectophore grade obtained (23) Malone, J. F.; Mars, D. J.; McKervey, M. A.; O’Hagan, P.; Thompson, N.; Walker, A.; Arnaud-Neu, F.; Mauprivez, O.; Schwing-Weill, M. J.; Dozol, J. F.; Rouquette H.; Simon, N. J. Chem. Soc., Chem. Commun. 1995, 21512153. (24) Grady, T.; Maskula, S.; Diamond, D.; Marrs, D. J.; McKervey, M. A.; O’Hagan, P. Anal. Commun. 1995, 32, 471-473. (25) McKittrick, T.; Diamond, D.; Marrs, D. J.; O’Hagan, P.; McKervey, M. A. Talanta 1996, 43, 1145-1148.

Analytical Chemistry, Vol. 71, No. 24, December 15, 1999

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from Fluka Chemicals. Metal nitrates were also obtained from Fluka and were of the puriss p.a. standard or higher. Ultrapure water from a Barnstead EASYpure water purification system was used throughout. Ligands L1, L2, and L3 (Figure 1) were synthesized as described previously.23,26 Ligands FII, FIV, FV, and L4 (Figure 1) were obtained from Fluka Chemicals. Procedure. All PVC membranes used were fabricated using the method of Diamond et al.27 using the following composition: ionophore (10 mg), potassium tetrakis(4-chlorophenyl)borate (KTpClPB; 2 mg), plasticizer (o-nitrophenyl octyl ether (o-NPOE; 1 g), and high-molecular-weight PVC (500 mg); 10-2 M PbCl2 was used as the internal filling solution. A double-junction calomel electrode, model E8091 (EDT, Lourne Rd., Dover, Kent, U.K.), was used as the reference electrode, with the outer junction containing saturated KNO3 and the inner reference containing saturated KCl. All measurements were carried out at 25 ( 0.5 °C under constant stirring. Selectivity values were estimated using both the separate solution method (equal activities, 10-2M) and the fixed interference method (by nonlinear curve fitting of the Nikolskii-Eisenmann equation to the experimental data using the optimization add-on Solver available in Microsoft EXCEL28). For the purpose of the comparative study between L3 and the ligands obtained from Fluka, four electrodes based on PVC membranes were fabricated using the same composition with each ligand. The selectivity coefficients of these electrodes for Pb2+ against a series of ions were evaluated under identical conditions to allow a close comparison of the observed characteristics. The modeling calculations were carried out using Spartan29 SGI Version 5.1.1. The simulations were run on a Silicon Graphics O2 workstation with a MIPS R10000 Rev. 2.7, 195-MHz CPU running an IRIX operating system, Release 6.3, with 128 MB RAM. Monte Carlo conformational searches were carried out on each of the complexes formed by L1, L2, and L3 with Ca2+ and a substitute atom type for Pb2+ (see below). Each conformer found was geometry optimized in vacuo with molecular mechanics using the Merck molecular force field (MMFF).30 This force field does not have parameters for Pb2+ and so a substitute atom type (type X) was created to simulate the effect of placing a large doubly charged cation similar to lead in the calixarene cavity. The van der Waals parameters used for X are shown in Table 1. 1H NMR spectra of the complexes of L2 modeled indicated that they were in the “cone” conformation and because of this, we felt justified in our decision to model the complexes as cone conformers. All torsional angles in each of the pendant groups in the lower rim of each ligand (i.e., in the region from the phenoxy oxygen atoms to the phenyl groups) were specified as torsional angles to be varied in the conformational searches. The conditions used for the modeling process are summarized in Table 1. (26) Arnaud-Neu, F.; Browne, J.; Byrne, D.; Marrs, D. J.; McKervey, M. A.; O’Hagan, P.; Schwing-Weill, M. J., Walker, A. Chem. Eur. J. 1999, 5 (1), 175-186. (27) Lu, J.; Chen, Q.; Wang, J.; Diamond, D. Analyst 1993, 118, 1131-1135. (28) Kane, P.; Diamond, D. Talanta 1997, 44, 1847-1858. (29) Spartan, Wavefunction Inc., 18401 Von Karman Ave., Suite 370, Irvine, CA 92612; http://www.wavefun.com/. (30) Halgren, T. J. Am. Chem. Soc. 1992, 114, 7827.


Table 1. Conditions Used for Monte Carlo Conformational Searches and van der Waals Parameters Used in the Creation of the Atom Type for X conformational search conditions conditions d-maxa simulation temp nb c-maxc no. of bonds rotated per Monte Carlo cycle terminating gradientd

value 0.25 Å 298 K 35 100 1

van der Waals parameters parameter

value

alpha-ie

1.000 2.500 5.600 5.083

n-if a-ig g-ih

1 × 10-5 kcal mol-1 Å-1

a The maximum difference in correlated distances between two conformations that are considered to be duplicates. b The number of conformers involved in the search. c The maximum number of cycles. d When the gradient becomes less than this value, the minimization ceases. e Related to atomic polarizabilities. f Slater Kirkwood effective number of valence electrons. g Scale factor. h Scale factor.

RESULTS AND DISCUSSION Electrode Response Characteristics (Ligands L1-L4). Studies on the response characteristics of PVC membrane electrodes based on ligands L1-L4 revealed excellent response characteristics to Pb2+. Results from three replicate studies gave near-Nernstian slopes of 29.5 ( 0.2 mV/decade change in activity to Pb2+ with the limit of detection as low as 1 × 10-6 M (5σnoise) and a Nernstian response down to 1 × 10-5 M under steady-state conditions. It is anticipated that this limit of detection could be considerably improved upon by varying the composition of the sensor inner filling solution, as described in the literature recently.31 The response time (t95%) was found to be less than 10 s over the range of activities used (8 × 10-7-6 × 10-2 M Pb(NO3)2) and was probably limited by the rate of stirring during spiking experiments. Upon further examination of the response of ligands L1-L3 to Pb2+, a trend in selectivity was found to exist; i.e., as the number of repeat units in the macrocycle was increased, the selectivity in favor of Pb2+ was improved. This was particularily evident in the case of the Ca2+ ion, which L1 was known to selectively complex in preference to magnesium and the alkali metal ions.25 Figure 2 illustrates the response obtained to a series of additions of Pb(NO3)2 to 10-2 M Ca(NO3)2 obtained with electrodes based on ligands L1-L3. Clearly, the Ca2+ ions have the largest effect on the electrode based on the tetramer (L1) as the response to Pb2+ is supressed until relatively high concentrations are reached. Evidence of a “best-fit” recognition mechanism is convincingly demonstrated by the dramatic improvement in the response to Pb2+ spikes obtained with the larger pentamer (L2), and hexamer (L3), with the best response being obtained from the latter. On the basis of these results, ligand L3 was chosen for further studies. Figure 3 shows typical Pb2+ response curves obtained in the presence of fixed backgrounds (0.01 M) of Na+, K+, Ca2+, and Ni2+ (for an electrode based on L3). Log KpotPbj values for a range of possible interferences obtained by fitting the NikolskiiEisenman equation to curves of this type28,32 are illustrated (31) Sokalski, T.; Ceresa, A.; Zwickl, T.; Pretsch, E. J. Am. Chem. Soc. 1997, 119, 11347-11348.

Table 2. Selectivity Coefficients for Electrode Based on L3a fixed inteference method log KpotPbj

Figure 2. Response of electrodes based on ligands L1-L3 (Figure 1) to successive spikes of Pb2+ ions (8 × 10-7-6 × 10-2 M) in a constant background of 1 × 10-2 M Ca(NO3)2. The inset shows an expanded view of one of the steps obtained with L3 and demonstrates the very rapid dynamic response (t95% < 10 s).

zj ) +2 Ca Cu Ni Co Cd Zn Mg Sr Hgb zJ ) +1 K Na Agc

separate solution method log KpotPbj ∆E (Ej - Ei)d

-2.47 -2.48 -3.33 -3.22 -2.81 -2.78 -2.97 -2.72

-2.26 -2.48 -3.81 -3.43 -2.87 -3.40 -3.43 -3.40 7.68

-68.3 -75.0 -101.6 -91.6 -76.6 -90.6 -91.6 -90.6 194.3

-1.13 -0.97

-1.38 -1.91 1.19

-98.3 -112.4 -28.2

a Selectivity coefficients obtained for the electrode containing ligand L3 (See Figure 1) using the fixed interference method (0.01 M interference) and the separate solution method (0.01 M). b pH 2.0.c pH 4.0. d ∆E values for the separate solution method are also included to allow direct comparison with other electrodes.

Figure 3. Fixed interference Pb2+ response curves in the presence of K+, Na+, Ca2+, and Ni2+ (0.01 M) obtained with an electrode based on L3.

Figure 4. Graphical representation of selectivity coefficients (as log KpotPbj) for Pb2+ over other cations tested. Method 1 represents the fixed interference method (0.01 M interference) while method 2 represents the separate solution method (all cations at 0.01 M).

graphically in Figure 4. Values obtained with the separate solution method (method 2) using eq 1 are also included for comparison (all ions at 0.01 M).

log Kpoti,j )

( )

Ej - Ei zi + 1log ai S zj

(1)

where Kpoti,j is the selectivity coefficient, i is the primary ion (Pb2+), j is the interfering ion, E is the measured potential (mV), S is the (32) Walsh, S.; Diamond, D. Talanta 1995, 42, 561-572.

Nernstian slope factor (mV/decade), z is the charge of the ion, and a is the activity calculated from activity coefficients estimated by the Davies equation. It should be appreciated that the numerical values for selectivity coefficients obtained for interfering ions with differing charges (i.e., for the differing classes of ions, zi ) zj or zi * zj in eq 1) are not directly comparable. For example, upon examination of the response curves obtained during the fixed interference measurements with L3 (Figure 3) it is evident that calcium shows the greatest interference despite the fact that its selectivity coefficient is numerically smaller than those of the sodium and potassium (log KpotPbCa ) -2.47 compared to log KpotPbK ) -1.13 and log KpotPbNa ) -0.97 (see Table 2). This point is further highlighted by comparison of the response to Pb2+ ions with electrodes based on L3 obtained in the presence of 0.01 M K+ (Figure 3), which is very similar to the Pb2+ response obtained in the presence of 0.01 M Ni2+, despite the fact that the selectivity coefficient for the former is 2 orders of magnitude larger (log KpotPb,K ) -1.13 compared to log KpotPb,Ni ) -3.33; see Table 2). Figure 4 compares the selectivity coefficients obtained for Pb2+ against a number of interfering ions using the fixed interference and separate solution methods. Although the values of the coefficients differ quite significantly, close examination reveals that the order is very similar. In both cases, Ca2+ yields the largest interference of all the divalent ions tested while among the monovalent ions Na+ and K+ are the main interferences. This is to be expected due to the known interaction between phosphine oxide ligating groups and the calcium ion on one hand25 and the well-established binding of Na+ ions by calix[4]arene tetraesters on the other.33 Nevertheless, Figure 3 clearly shows that the combination of the hexamer cavity size with the phosphine oxide ligating groups results in a very selective Pb2+ ligand capable of producing excellent lead-selective electrodes. While the separate solution method is useful for rapid screening of interferences, it has the disadvantage of assuming that the (33) Cadogan, A.; Diamond, D.; Smyth, M. R.; Deasy, M.; McKervey, M. A.; Harris, S. J. Analyst 1989, 114, 1551.


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Table 3. Comparative Studya

zj ) +2 log KpotPbCu log KpotPbZn log KpotPbMg log KpotPbCo log KpotPbCa log KpotPbSr log KpotPbCd log KpotPbNi log KpotPbHgb zj ) +1 log KpotPbNa log KpotPbK log KpotPbAgc

FII

L3

FV

FIV

L4

0.74 -2.27 -3.47 -2.97 -1.11 -2.52 -2.02 -3.17 3.26

-2.48 -3.40 -3.43 -3.43 -2.26 -3.40 -2.87 -3.81 7.68

-1.98 -3.39 -4.32 -4.23 -0.88 -3.17 -2.91 -4.32 9.99

-4.28 -4.51 -5.68 -5.38 -2.33 -4.20 -5.29 -4.85 1.24

-0.32 -1.58 -1.81 -2.38 -1.57 -1.61 -0.91 -2.01

-1.04 0.36 16.31

-1.91 -1.38 1.19

-1.46 -0.12 3.92

-0.76 -2.46 -0.24

0.58 2.93

a Selectivity coefficients, obtained using the separate solution method (0.01 M), where L3, FII, FV, and FV represent the different ionophores used;, see Figure 1. b pH 2.0 c pH 4.0.

interactions between the primary and interfering ions are of no consequence. Consequently, the fixed interference method gives a more realistic prediction of electrode response in real situations, where mixtures of ions are invariably encountered.28 Furthermore, it is very likely that the inherent selectivity of these ligands will be considerably better than that estimated by the conventional methods employed in this study, as the fundamental selectivity is often masked by artifacts arising from ion transport from the internal filling solution.34,35 Comparative Study of Ligands FII, FIV, FV, L3, and L4. Selectivity against Cu2+, Zn2+, Mg2+, Ca2+, Sr2+, Cd2+, Ni2+, Hg2+, Na+, K+, and Ag+. Due to the difficulty in comparing selectivity coefficients quoted in the literature7,11,21 arising from different experimental conditions used, three commercially available ionophores were obtained from Fluka Chemicals for direct comparative studies. The structures of these ligands can be seen in Figure 1. Five membranes were prepared containing ligands L3, L4, FII, FIV, and FV. The selectivity of these electrodes for the Pb2+ ion against a series of ions was estimated using the separate solution method at concentrations of 0.01 M for all ions tested. The coefficients obtained are tabulated in Table 3 and shown graphically in Figure 5. The following conclusions may be drawn from these results: (1) Acyclic thioamide ligand (FII) is more responsive to Cu2+ and K+ ions than Pb2+ ions. The electrode therefore can only be used as a lead sensor if the concentrations of these ions are several orders of magnitude less than that of Pb2+ in the sample. (2) The 15-crown-5 ligand (FV) suffers from relatively poor selectivity against Ca2+ and K+, which will severely limit its effectiveness in many natural water assays, which commonly contain high levels of Ca2+ in particular. (3) Of the three ligands available from Fluka, we found that calix[4]arene thioamide (FIV) gave the best overall selectivity. Sodium was the most significant interference, and for reliable results, high levels of sodium could not be tolerated with this electrode. (4) For L3, good selectivity was obtained for Pb2+ against all these ions. Although the selectivity against the divalent ions is, (34) Bakker, E. Anal. Chem. 1997, 69, 1061. (35) Sokalski, T.; Ceresa, A.; Fibbioli, M.; Zwickl, T.; Bakker, E.; Pretsch, E. Anal. Chem. 1999, 71, 1210.


Figure 5. Selectivity coefficients (as log KpotPbj) obtained with electrodes based on L3, L4, FII, FIV, and FV for a range of possible interferences (see Figure 1 for ligand structures) using the separate solution method (all cations at 0.01 M).

in general, not as good as for FIV, we feel that the better selectivity against Na+ ions, and several other factors outlined below, make L3 a better choice for most applications. (5) Triphenylphosphine oxide (L4) is much less selective for lead than any of the calixarene derivatives, with K+, Na+, and Cu2+ in particular having much greater effect than for the macrocyclic calixarenes. This confirms that the observed selectivity arises from the 3-D spatial arrangement of the phosphine oxide ligating groups generated by the calixarene backbone. Selectivity against H+, Hg2+, and Ag+. As H+, Hg2+, and Ag+ ions can prove to be particularly troublesome with lead-selective electrodes, a separate study was set up to investigate the effect of these ions on electrodes based on the four ligands. Figure 6 shows data obtained with electrodes based on the four ligands (L3, FII, FIV, FV) to Ag+ and Hg2+ ions, in comparison to the response to Pb2+ ions. Mercury(II) compounds are generally covalent in nature and therefore insoluble in water. An exception is Hg(NO3)2, which is soluble, but only at low pH values (around pH 2 or lower). In contrast, AgNO3 is soluble at pH 4. Pb(NO3)2 is soluble up to alkaline conditions, at which lead is precipitated as Pb(OH)2. In Figure 6, the numbered regions show the response of each electrode in turn to the following solutions: (1) 0.01 M Pb(NO3)2 at pH 4.0 (offset to 0mV); (2) 0.01 M Ag(NO3) at pH 4.0; (3) 0.01 M Pb(NO3)2 at pH 2.0; (4) 0.01 M Pb(NO3)2 at pH 2.0. The main conclusions from this study are as follows: (1) The electrode based on ligand FII shows a huge response (∼+300 mV) to Ag+ ions at pH 4. The electrode based on ligand FV also responds to Ag+ ions, although the degree of interference is less at ∼+40 mV (compare regions 1 to 2 in each case). In contrast, the electrode based on L3 shows a negative response to Ag+ ions (∼-25 mV) indicating a preference for Pb2+ ions. The electrode based on ligand FIV shows the most negative response to Ag+ ions, but its high dependence on pH (see below) and lack of selectivity against sodium ions are severe limitations. (2) Comparing regions 1 and 3 in each case, the electrodes based on ligands FII and FIV have a very large response to pH, meaning that it will be impossible to use these devices unless the pH is carefully buffered to prevent any change. The sensor

Figure 6. Real-time traces illustrating the response of a PVC membrane-based electrode containing ligands L3, FII, FIV, and FV to the following: (1) Pb2+ at pH 4.0, (2) Ag+ at pH 4.0, (3) Pb2+ at pH 2.0, and (4) Hg2+ at pH 2.0.

based on FV also shows some dependence on pH, although it is not as severe as for FII and FIV. In contrast, the electrode based on L3 shows negligible response to changes in the range pH 2-4. This is an important characteristic, as it means that small shifts in pH will not affect the electrode signal significantly. (3) Comparing regions 3 and 4, the effect of changing from Pb2+ to Hg2+ at pH 2 is demonstrated. A large response is generated in every case, which is proof of a very high affinity for Hg2+ ions for all four ligands at pH 2. This means in situations where Hg2+ is a problem, measurements must be made at pH >2 in order to ensure that Hg2+ ions are precipitated out of the sample, and under these conditions, it can be assumed that the signal is dominated by Pb2+ ions. Stability and Lifetime. Three electrodes were tested over a period of 60 days to investigate stability. During this period, the electrodes were in daily use over extended periods of time (several hours per day) and stored overnight in 0.01 M Pb(NO3)2. A slight gradual decrease in slope was observed, as is usual in these plasticized PVC membrane electrodes, and probably arises from leaching of the membrane components. At the end of the 60-day period, the electrode slopes were found to have fallen to 22.5 ( 0.2 mV/decade from the initial value of 29.5 ( 0.2 mV/decade, but were still functioning well as lead-selective electrodes. Phosphine oxides are among the most stable organic compounds knownsfor example, unlike amides in general, the calixarene phosphine oxide derivatives used in this study are not susceptible to hydrolytic or oxidative cleavage, which was an important factor in their selection for investigation in the nuclear waste program. In contrast, thioamides such as ligand FIV are prone to decomposition via these mechanisms, and hence storage under argon is recommended.36 We therefore feel that these derivatives and similar compounds could form the basis of a new (36) Storage under argon is recommended by Fluka for ligand IV.

series of ion sensors with exceptional long-term stability under extreme acid or alkaline conditions. For example, ligand L3 can clearly be used to sequester Hg2+ ions with high efficiency in acidic solutions that would destroy most other ligands. Molecular Modeling. Monte Carlo conformational searches were used in this study to model the complexes formed between L1-L3 with Ca2+ and the substitute atom type (X) to simulate the effect of Pb2+ (see Experimental Section). While the cavity size is very different for the three calixarenes, the models obtained for all six complexes (models representative of the L1-X2+ and L3-X2+ complexes are shown in Figure 7) show similar modes of complexation, which gives some insight into the trends in selectivity obtained. The models clearly show that Ca2+ and X2+ are complexed in the region intermediate between the two types of oxygen atom (the phosphine oxide and the phenoxy) in all cases. These results suggest that all three calixarenes bind the Ca2+ and X2+ ions in an 8-fold coordination whereas other ions such as Na+ and K+ ions show 3-fold coordination. This means that with the tetramer (L1) all four pendant groups are involved in binding Ca2+ and X2+. In the case of the pentamer (L2), binding also involves four of the pendant groups, with the fifth group being forced out of the binding cavity region by the rearrangement on binding. This behavior has been reported previously in molecular modeling studies and X-ray structures of calix[5]arene-ion complexes37 and therefore is a reasonable explanation for the observed improvement in lead selectivity. A very similar mechanism is suggested for the hexamer (L3) in which two of the pendant groups are this time forced from the cavity to facilitate the formation of the 8-fold cavity involving the remaining four pendant groups. It appears that, for Pb2+ ions, the (37) Bell, S. E. J.; McKervey, M. A.; Fayne, D.; Kane, P.; Diamond, D. J. Mol. Model. [Electronic Publication] 1998, 4, 44-52.


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Figure 7. Tube rendering of the side-on view (A, C) and the view through the cavity (B, D) of the L1-X2+ and L3-X2+ energy-optimized models, respectively. Hydrogen atoms are not shown for the sake of clarity. An arrow is used in each case to identify the position of X2+. Color scheme: carbon, 90% black; oxygen, 50% black; phosphorus, gray; X, black.

8-coordination cavity involving four pendant groups is maintained in all three cases (L1-L3), whereas for other ions, such as Na+ and K+, a much weaker 3-fold coordination occurs. In the case of the smaller Ca2+ ion, rearrangement of the binding groups in the pentamer and hexamer is progressively more expensive in energy terms, and this leads to the corresponding improvement in selectivity for Pb2+ ions against Ca2+ ions obtained with electrodes based on L2 and L3 (see Figure 2).


ACKNOWLEDGMENT The authors gratefully acknowledge grant aid from Enterprise Ireland (Grants SC96/412 and ST96/607a) and Dublin City Corp. (F.C.). Received for review March 22, 1999. Accepted August 11, 1999. AC990303F

Lead-Selective Electrodes Based on Calixarene ... - ACS Publications

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