Acta Chim. Slov. 2005, 52, 309–316 309 Scientific Paper Novel Calcium Sensor Based on [2-(2-Hydroxyphenyl)imino]-l,2-diphenylethanone Mohammad Reza Ganjali,"* Hassan Ali Zamani,* Parviz Norouzi," Mehdi Adib," and Mohammad Accedj^ " Department of Chemistry, Tehran University, Tehran, Iran, E-mail: garijali@khayam.ut.ac.ir b Department of Chemistry, North Tehran branch, Azad University, Tehran, Iran c Department of chemistry, Tarbiat Modarres University, Tehran, Iran Received 03-12-2004 Abstract In this work, a novel calcium PVC-based membrane sensor based on 2-[(2-hydroxyphenyl)imino]-l,2-diphenvlethanone (HD) as a new ionophore is presented. The sensor displavs a linear dvnamic range betvveen 1.0x10^ andl.0xl0~6 M, with a near Nernstian slope of 28.5 ± 0.5 mV per decade and a detection limit of 8.0x 1CT7 M. The best performance was obtained with a membrane composition of 30% poly(vinyl chloride), 62% nitrobenzene, 5% sodium tetraphenyl borate and 3% HD. The potentiometric response of the proposed electrode is independent of the pH of the solution in the pH range of 4.0 -11.5. The sensor possesses the advantages of short conditioning tirne, fast response tirne (<20 s), and especially, very good selectivity tovvards alkali and alkaline earth, and some mono, di and trivalent cations, such as Li+, Na+, K+, Mg2+, Sr2+, Ba2+, Ag+, Cu2+, Al3+, Ta3+ and Ce3+ ions. The electrode can be used for at least 10 weeks vvithout any considerable divergence in the potentials. It was used as an indicator electrode in potentiometric titration of Ca(II) ions with EDTA. Key words: potentiometry, calcium sensor, PVC membrane, 2-[(2-hydroxyphenyl)imino]-l,2-diphenylethanone Introduction Potentiometric sensors have been shown to be very effective tools for analysis of a wide variety of cations and anions. They are very simple to use, inexpensive, and capable of reliable responses in a wide concentration range. A wide variety of chemically, clinically or environmentally important analytes are now routinely monitored using ion-selective sensors based on impregnated polymeric membranes.13 During the past decade, an extensive effort has been made in the synthesis and characterization of neutral ion carriers with high selectivities for specific cations to develop new potentiometric sensors for the monitoring of the respective cations in real samples.35 Literature survey revealed that a number of Ca(II) membrane sensor based on different ion carrier have been reported.617 Most of these reported calcium sensors have one, two, or in some cases aH following problems; A) relatively, high detection limit, B) narrow working concentration range, and finally, C) serious interfering from Na+, K+, Sr2+, Ba2+,...ions. We were interested in the preparation of a highly selective and sensitive sensor for determination of Ca(II). We have recently reported a number of highly selective and sensitive membrane sensors for various metal ions.18-22 In this work, we report a novel Ca(II) membrane sensor base on a new selective ionophore HD (Figure 1). O HO Figure 1. Structure of HD Experimental Reagents Reagent grade benzyl acetate (BA), nitrobenzene (NB), sodium tetraphenyl borate (NaTPB), tetrahydrofuran (THF), and high relative molecular weight PVC (ali from Merck and Aldrich) were used as received. The nitrate and chloride salts of aH cations used (aH from Merck and Aldrich) were of the highest purity available and used without any further purification except for vacuum drying over P2Os. Doubly distilled de-ionized water was used throughout. Ganjali et al. Novel Calcium Sensor 310 Acta Chim. Slov. 2005, 52, 309–316 Synthesis of HD The procedure for the preparation of 2-[(2-hydroxyphenyl)imino]-l,2-diphenylethanone: A mkture of benzil (0.01 mol, 2.10 g), 2-aminophenol (0.01 mol, 1.09 g) and catalytic amount of toluene-4-sulfonic acid was refluxed in benzene (70 mL) for 6 h. Then the solvent was evaporated and the residue was crystallized from 95% ethanol as pale yellow crystals, mp 118-120 °C, 2.56 g, yield 85%; IR (KBr) (umax/cm-1): 3246 (br, OH), 1610 (C=0), 1474, 1450, 1344, 1259, 1232, 1092, 1042, 989, 933, 750, 694; MS, m/z (%): 301 (M+, 2), 196 (10), 104 (23), 77 (100), 65 (89), 51 (77), 39 (56). Anal. Calcd. for C20H15NO2: C, 79.72; H, 5.02; N, 4.65. Found: C, 79.7; H, 5.1; N, 4.7%. JH NMR (80 MHz, acetone-d6): 87.40 (t,/ = 7.7 Hz, 1H), 7.43 (t,/ = 7.8 Hz, 1H), 7.60-7.90 (m, 8H), 8.00 (t,/ = 7.5 Hz, 2H), 8.05 (t,/ = 7.4 Hz, 1H), 8.30 (dd, / = 7.7, 2.0 Hz, 2H). 13C NMR: S 118.0, 122.4, 123.0, 128.6, 129.0, 129.7, 130.0, 130.9, 134.5, and 135.2 (10 CH), 136.5, 139.5, 140.9, and 154.3 (4 C), 163.2 (C=N), 182.1 (C=0). Electrode preparation The general procedure to prepare the PVC membrane was to mix thoroughly 30 mg of powdered PVC, 62 mg of plasticizer NB, 5 mg of additive NaTPB, and 3 mg of ionophore HD 4 mL of fresh THF. The resulting mkture was transferred into a glass dish of 2 cm diameter. The solvent was evaporated slowly until an oily concentrated mkture was obtained. A Pyrex tube (3-5mm o.d.) was dipped into the mkture for about 10 s, so that a nontransparent membrane of about 0.3 mm thickness was formed. The tube was then pulled out from the mkture and kept at the room temperature for about 12 h. The tube was then filled with internal filling solution (1.0 xl0~3 M of CaCl2). The electrode was finally conditioned by soaking in al.0xl0 2 M CaCl2 solution for 24 h.2325 A silver/silver chloride wire was used as an internal reference electrode. The emf measurements Ali emf measurements were carried out with the following assembly: Ag-AgCl | internal solution (1.0xlO~3 M CaCl2) | PVC membrane | sample solution | Hg-Hg2Cl2, KC1 (satd.) A Corning ion analyzer 250pH/mV meter was used for the potential measurements at 25.0 ± 0.1 °C. Procedure of complexation study of HD and metal ions Conductivity measurements were carried out with a Metrohm 660 conductivity meter. A dip-type conductivity celi made of platinum black, with a celi constant of 0.84 cm 1 was used. In aH measurements, the cell was thermostated at the desired temperature 25.0 ± 0.05 °C using a Phywe immersion thermostat. In typical experiments, 50 mL of a metal ion nitrate solution (1.0x10 4 1.0Xl0-5 M) was placed in water jacketed celi equipped with magnetic stirrer and connected to the thermostat circulaing water at the desired temperature. In order to keep the electrolyte concentration constant during the titration, both the starting solution and titrant had the same metal ion concentration. Then, a known amount of the HD (1.0x10 2 - 1.0xl0-3 M) solution was added in a stepwise manner using a calibrated micropipette. The conductance of the solution was measured after each addition. Addition of the HD was continued until the desired HD-to-metal ion mole ratio was achieved. The 1:1 binding of the metal ions with HD can be expressed by the following equilibrium: Mn++L Kf -)MLn+ (1) The corresponding equilibrium constant, Kt, is given by K [MLn+] f{MLn+) X [Mn+ ][L] f(M n+) f(L) (2) where [MLn+], [M n+], [L] and/represent the equilibrium molar concentration of complexes, free cation, free HD and the activity coefficient of the species indicated, respectively. Under the dilute condition we used, the activity coefficient of the unchanged ligand, /(L) can be reasonably assumed as unity.26 The use of Debye-Hückel limiting law of 1:1 electrolytes,27 lead to the conclusion that f M n+, ~ f/j^n+v so^e activlty coefficient in equation (2) is canceled out. Thus, the complex formation constant in terms of the molar conductances, A, can be expressed as:28 K [MLn+] (AM-Aobs) ------------ -------------------- [Mn+][L] (Aobs-AML)[L] Where Kf=CL CM (?M - ?obs) (?obs - ?ML ) (3) (4) Here, AM is the molar conductance of the metal ion before addition of HD, AML the molar conductance of the complexed, Aobs the molar conductance of the solution during titration, CL the analytical concentration of the HD added, and CM the analytical concentration of the metal ion salt. The complex formation constant, Kt and the molar conductance of the complex, A^, were obtained by computer fitting of equations (3) and (4) to the molar conductance-mole ratio data using a nonlinear least-squares program KINFIT29 Ganjali et al. Novel Calcium Sensor Acta Chim. Slov. 2005, 52, 309–316 311 Results and discussion Theoretical studies Many experimental and theoretical investigations have been carried out to better understand the fundamental interaction between metal ions and neutral molecules and their relationship to molecular recognition. Computational models capable of reliably predicting ligand selectivity in a variety of cations have been shown to be valuable tools for the advancement of practical \vorks.1-4'30"33 In order to have a clear picture about the selectivity of (HD) for various metal ions, in this work, we investigated its binding to Li+, K+, Ba2+, La3+, Sr2+, Rb+, Be2+, Cs+, Pb2+, Bi3+, and Ca2+ ions by using the ab initio theoretical calculations. The influence of the nature, size and charge of metal ions on the complexation reaction with the neutral ligand is explained on the basis of the calculation of gas - phase binding energies. The molecular structures of the uncomplexed ligand and its complexes with a number of metal ions were optimized using the lanl2mb basis set for ali atoms at restricted Harfree Fock (RHF) level. No molecular symmetry constraint was applied. Rather, full optimization of aH bond lengths, angles and torsion angles was carried out using the Gaussian 98 program.34 The binding energy (AE) was calculated with the enlarged basis sets using equation (1): Table 1. Binding energy of HD with various metal ions. complex v ligand cation/ (1) Where, E^ lex, Eligand and E^,;^ are the total energies of the complex, uncomplexed HD and metal ion, respectively. Optimization of the uncomplexed HD was also carried out with the semi-empirical PM3 method using Hyper Chem software (Version 6.01). No adequate parameterization of the metal ions was available in PM3, so that semi-empirical calculations could not be carried out on the complexes. Table 1 summarizes the theoretical data relating the stability of the HD complexes with Li+, K+, Ba2+, La3+, Sr2+, Rb+, Be2+, Cs+, Pb2+, Bi2+, and Ca2+ ions. Inspection of Table 1 reveals that the cation binding energy with HD shows a pronounced dependence on the nature of metal ions used. In fact, the stability of the resulting complexes is expected to decrease in the order Ca2+> La3+> Sr2+> K+> Ba2+ > Rb+> Cs+> Be2+> Li+> Bi3+> Pb2+. The optimized structures of free HD and its Ca2+ complexes are shown in Figure 2 and some of their selected geometric parameters are summarized in Table 2. As can be seen from Figure 2b, in the complexed form, the benzo groups are not coplanar. It is immediately obvious that while the optimized of free Metal ion Total energy of Metal ion /Hartree Total energy of the complex /Hartree Binding energv (AE)7kcalmor1 Ca2+ -35.6102894 -994.0233323 ^14842.29757 La3+ -29.5417988 -988.287661 -37435.18598 Sr2+ -29.5354119 -987.9049245 -37191.01105 K+ -27.6277966 -985.8765068 -34721.15034 Ba2+ -24.3878965 -982.7080712 -30699.91969 Rb+ -23.4435331 -981.6778679 -29460.87908 Cs+ -19.4792122 -977.7001223 -24477.23235 Be2+ -13.439758 -972.4282974 -17379.40472 Li+ -7.1354476 -965.5392815 -9100.592466 Bi3+ -3.5615768 -962.5547625 ^1985.202866 Pb2+ -2.6011289 -961.1170647 -3480.366439 a The total energv of the uncomplexed ligand is -809.625146 Hartree. a) b) Figure 2. Optimal comformation of HD before (a) and after (b) complexation with Ca+2. Ganjali et al. Novel Calcium Sensor 312 Acta Chim. Slov. 2005, 52, 309–316 Table 2. Selective Geometric Parameters for Free, and its Ca+2 Complex of HD.a Torsion angle of