Heterogeneous Fluorometric Detection of pH and Metal Cations by

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Ind. Eng. Chem. Res. 2004, 43, 6064-6069

MATERIALS AND INTERFACES Heterogeneous Fluorometric Detection of pH and Metal Cations by Amphiphilic Zeolite Modified with Anthracene-Substituted Azamacrocycle Go Nishimura, Yasuhiro Shiraishi,* and Takayuki Hirai Research Center for Solar Energy Chemistry and Division of Chemical Engineering, Graduate School of Engineering Science, Osaka University, Toyonaka 560-8531, Japan

N-(9-Anthrylmethyl)-1,4,7,10-tetraazacyclododecane (AC), which acts as an efficient fluorescent chemosensor for detection of pH and metal cations in aqueous solution, was covalently grafted on a NaX zeolite modified with octadecyl group (ZO). This grafted zeolite (ZOAC) is highly dispersed in aqueous solution and hardly precipitates even after 10 min standing, because of the modification with the hydrophobic octadecyl group. Consequently, even though ZOAC is employed for fluorescence measurement in heterogeneous systems, the same high fluorescence intensity as that for AC employed in homogeneous systems is obtained. ZOAC demonstrates as good a fluorescent response toward pH and metal cations as AC does. The metal cations coordinated by the AC group on ZOAC are removed easily by washing with aqueous HCl solution, and ZOAC can be reused for further measurement. The stability of the AC group on ZOAC is significantly higher than that of the homogeneous AC sensor under humid atmosphere, suggesting the potential utility of ZOAC for practical sensing of pH and metal cations in aqueous solution. Introduction Fluorometric analysis is a convenient method for prompt detection of pH and metal cations in aqueous solution. Considerable efforts have therefore been made to develop selective and sensitive fluorescent chemosensors.1-3 Most of the sensors consist of a fluorophore (e.g., anthracene) covalently linked to an ionophore (e.g., amino group), which can bind H+ and metal cations, and are thus called fluoroionophores. The fluorescence of the fluorophore “lights” when the ionophore binds H+ or metal cations. In the absence of H+ and metal cations, the fluorescence is, on the contrary, quenched by an electron transfer from a nitrogen lone pair on the ionophore to the fluorophore. Based on the above electron transfer mechanism, several chemosensors have been synthesized.1-3 These sensors are, however, soluble in aqueous solutions and cannot be reused for further measurement. From a practical point of view, recyclable heterogeneous sensors should be used in place of homogeneous sensors. One of the possible approaches for the synthesis of the heterogeneous sensor is to graft the homogeneous fluoroionophore onto insoluble solid materials. Various heterogeneous sensors have been synthesized based on a stable inorganic support, such as silica and zeolite.4-7 These sensors are recovered simply by decantation from aqueous solution and can be reused for further measurement. However, the sensors, when added to aqueous * To whom correspondence should be addressed. Tel.: +816-6850-6271. Fax: +81-6-6850-6273. E-mail: shiraish@ cheng.es.osaka-u.ac.jp.

solution, precipitate quickly to the bottom of the analytical cell, because of the hydrophilicity of the amino group and the support material itself. Therefore, when using the sensor in aqueous solution, ultrasonication or agitation must be employed during the fluorescent measurement in order to disperse the sensor well in the solution. Synthesis of a heterogeneous sensor, which is well-dispersed in solution without the requirement of ultrasonication or agitation, is needed for the development of more practical fluorometric analytical methods. For the synthesis of the well-dispersing heterogeneous sensor, to increase the hydrophobicity of the sensor may be one feasible solution. In the present work, the hydrophobicity of the sensor was increased by a chemical modification of the surface of the support material with a hydrophobic octadecyl group. A NaX zeolite having a large surface area was used as the support material, and N-(9-anthrylmethyl)-1,4,7,10-tetraazacyclododecane (AC; Scheme 1) was used as the fluoroionophore; it is the representative fluorescent chemosensor employed for detection of pH and metal cations in aqueous solution.8 The dispersion behavior of the zeolite modified with octadecyl and AC groups (ZOAC) in aqueous solution was examined in detail, and suitable quantities of the octadecyl and AC groups on the material were determined. The fluorescent response of the ZOAC toward pH and metal cations was studied, comparing it with that of the homogeneous AC sensor. The reusability and stability of the ZOAC were also studied, and the applicability of the ZOAC as a practical heterogeneous chemosensor was examined.

10.1021/ie0499106 CCC: $27.50 © 2004 American Chemical Society Published on Web 08/19/2004

Ind. Eng. Chem. Res., Vol. 43, No. 19, 2004 6065 Scheme 1. Schematic Diagram for the Synthesis of ZOAC

Table 1. Quantities of Octadecyl and AC Groups on ZOAC and Contact Angle of the Particles amt of n-octadecyl- octadecyl contact trichlorosilane group AC group angle of added (mL) (mmol/g) (mmol/g) water (deg) ZAC ZOAC(R) ZOAC(β) ZOAC(χ) ZOAC(δ)

0.5 2.0 3.0 5.0

4.89 9.46 11.4 14.8

0.320 0.090 0.087 0.110 0.030

39 68 82 93 110

ments were carried out using a BELSORP 18PLUS-SP analyzer (BEL Japan, Inc.) at 77 K. The contact angle of water on the surface of zeolite samples was measured on a Phoenix 150 (SEO Corp., Korea), using a sessile drop technique on a pressed disk of the samples, which were prepared by compression at a pressure of 60 MPa.13 1H NMR spectra of the homogeneous AC sensor in CD3OD were obtained by a JEOL JNM-GSX270 Excalibur.

Experimental Section

Results and Discussion

1. Materials and Synthesis. All of the reagents were supplied by Wako and used without further purification. AC was synthesized according to the procedure described in the literature.8 ZOAC was synthesized according to the procedure as shown in Scheme 1 and as follows:9-13 (i) A NaX zeolite (1 g, Tosoh Corp., average particle size 5 µm) was added to toluene (60 mL) and dispersed well by ultrasonication. An n-octadecyltrichlorosilane (0.5-5 mL) was added to the mixture and stirred for 12 h at 298 K. The resulting material (ZO) was recovered by filtration, washed with acetone and water, and dried in vacuo at 333 K for 5 h. (ii) The ZO was then stirred with (3-chloropropyl)triethoxysilane (3 mL) in toluene (50 mL) at 383 K for 12 h under dry N2. The material obtained was washed with acetone and water and then dried in vacuo at 333 K. (iii) This was then stirred with AC (0.5 g) in toluene (50 mL) at 383 K for 24 h under dry N2. The ZOAC obtained may have contained 4- and 7-substituted AC, but was used without further purification. The quantities of octadecyl and AC groups on ZOAC, synthesized in the presence of different amounts of n-octadecyltrichlorosilane, were estimated by elemental analysis, with the results being summarized in Table 1. 2. Procedure and Analysis. Fluorescence measurement was carried out on a Hitachi F-4500 fluorescence spectrophotometer. Excitation wavelength of 368 nm and emission wavelength of 417 nm were employed for all of the experiments, where both excitation and emission slit width were 3 nm. The measurement was done at 298 ( 1 K using a 10 mm path length quartz cell. For homogeneous measurement, an aqueous solution (3 mL), in which AC (0.1 µmol) was dissolved, was employed. For heterogeneous measurement, ZOAC was suspended in an aqueous solution (3 mL), shaken by hand for 3 s, and used for analysis, where the amount of AC group on ZOAC used was the same as that of AC used for the homogeneous measurement. The pH of the aqueous solution was adjusted by the addition of HCl and/or NaOH. The fluorescent measurement in the presence of metal cations was carried out using an aqueous solution containing a metal chloride (0.3 µmol). All measurements were carried out in the presence of NaCl to maintain the ionic strength of the solution (I ) 0.5 mol/L). N2 adsorption-desorption measure-

1. Properties of ZOAC. The dispersion behavior of ZOAC in water was studied first, comparing it with that of the zeolite modified only with an AC group (ZAC), where ZOAC(χ) having 11.4 mmol/g of octadecyl group was used for the investigation (Table 1). Parts a and b of Figure 1 show photographs of ZAC and ZOAC(χ), respectively, when added to water, shaken by hand for 3 s, and then left to stand for 3 min. For ZAC, almost all of the particles precipitate to the bottom of the cell. In contrast, the ZOAC particles are found to be dispersed finely in water. This suggests, as expected, that the modification of the hydrophobic octadecyl group on the zeolite surface increases the hydrophobicity of the particle, thus allowing the fine dispersion of the ZOAC particles in water. Figure 2 shows a comparison of excitation and emission spectra of ZOAC suspended in aqueous solution (pH 3.0) with those of AC dissolved in aqueous solution (pH 3.0). The ZOAC shows almost the same excitation and emission spectra as the AC, indicating that the immobilization of the fluoroionophore on the zeolite surface and the presence of the octadecyl group do not affect the electronic properties of the anthracene moiety in the AC group. As shown in Figure 1c, when ZOAC(δ), having 14.8 mmol/g of octadecyl group, was added to water, almost all of the ZOAC particles floated on the surface of the water. This is because excess quantity of the octadecyl group increases the hydrophobicity of the ZOAC par-

Figure 1. Photographs of (a) ZAC, (b) ZOAC(χ), and (c) ZOAC(δ), when added to water, shaken by hand for 3 s, and left to stand for 3 min.

6066 Ind. Eng. Chem. Res., Vol. 43, No. 19, 2004 Table 2. Properties of ZOACa

NaX zeolite ZO ZOAC(χ)

av pore diam (nm)

sp surf. area (m2/g)

pore vol (m3/g)

0.863 0.442 0.307

409.3 52.1 14.8

94.05 × 10-6 11.97 × 10-6 3.39 × 10-6

a The N adsorption-desorption data are shown in the Sup2 porting Information (Figure S1).

Figure 2. (i) Excitation and (ii) emission spectra for (a) AC dissolved in aqueous solution (pH 3.0) and (b, c) ZOAC(χ) suspended in aqueous solution (pH 3.0). The spectra for the ZOAC(χ) samples were taken by hand shaking of the sample for 3 s followed by standing for (b) 3 and (c) 10 min, respectively.

Figure 4. Variation in fluorescence spectra of ZOAC(χ) (0.9 mg) when dispersed in aqueous solution of different pH values. Each spectrum was obtained following 3 min standing.

Figure 3. Relationship between contact angle and fluorescence intensities of ZOAC (417 nm) obtained following 3 min standing in aqueous solution (pH 3.0). The ZOAC samples, denoted in the figure, correspond to those shown in Table 1.

ticles too much. To study the relationship between the hydrophobicity of the ZOAC particles and their dispersion behavior in aqueous solution, the contact angle of ZOAC particles having several quantities of the octadecyl group was determined. As shown in Table 1, the contact angle of the ZOAC particles was found to increase with an increase in the quantity of octadecyl group on ZOAC. The respective ZOAC particles were added to aqueous solution (pH 3.0) and left to stand for 3 min, and the fluorescence intensity was analyzed. As shown in Figure 3, the fluorescence intensity of ZAC is very weak, but it is increased with an increase in the contact angle of the ZOAC. Maximum intensity is obtained for ZOAC(χ), with 93° contact angle (having 11.4 mmol/g of octadecyl group), where the intensity is 50-fold higher than that obtained using ZAC. For the case of ZOAC(δ), with 110° contact angle (having 14.8 mmol/g of octadecyl group), the fluorescent intensity is, however, decreased significantly. This is because most of the ZOAC(δ) particles float on the surface of the solution owing to high hydrophobicity (Figure 1c). These findings suggest that the quantity of octadecyl group significantly affects the dispersion behavior of the ZOAC particles in solution and the ZOAC, with about 80-95° contact angle, is the most desirable material for the achievement of high fluorescent intensity. As shown in Figure 2, when comparing the fluorescence intensity of the ZOAC(χ) in a heterogeneous system and the AC in a homogeneous system, the intensity of the ZOAC is

found to be 70% as much as that for AC, indicating that the fluorescence intensity of ZOAC is fairly strong even in a heterogeneous system. The 30% lower fluorescence intensity of ZOAC is because some ZOAC particles, having an exceptionally lower or higher quantity of octadecyl group, precipitate to the bottom of the cell or float on the surface of water, as shown in Figure 1b. As shown in Figure 2c, the ZOAC(χ) sample, when left to stand for 10 min, showed almost the same fluorescence intensity as that obtained following 3 min standing (Figure 2b). This suggests that the ZOAC particles are highly dispersed in aqueous solution and the degree of the dispersion does not change even after 10 min standing, thus indicating that quantitative fluorescence analysis may be possible using ZOAC(χ). The following experiments were therefore carried out using ZOAC(χ). The properties for ZOAC(χ), obtained by N2 adsorption-desorption measurement (Supporting Information available, Figure S1), are summarized in Table 2. 2. Detection of pH and Metal Cations. ZOAC(χ) was suspended in aqueous solution of different pH values, and the variation in the fluorescence spectra was measured. As shown in Figure 4, the fluorescence intensity of ZOAC decreases with an increase in the pH of the solution. The variation in the fluorescence intensity of ZOAC at 417 nm, when plotted against the pH of the solution, is shown in Figure 5 (closed circle symbols). The intensity is nearly zero at pH >10, but increases linearly with a decrease in the pH of the solution. The intensity becomes almost saturated at pH 10 occurs via the donation of an electron on the highest occupied molecular orbital (HOMO) of the nitrogen atom of the ionophore to the HOMO of the fluorophore. At lower pH, upon protonation of the nitrogen atom of the ionophore, the HOMO level of the nitrogen atom becomes lower than the HOMO level of the fluorophore. Consequently,

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Figure 5. Variation in fluorescence intensity of AC and ZOAC(χ) (0.9 mg) at 417 nm in aqueous solution with pH.

Figure 6. Variation in ratio of fluorescence intensity of AC and ZOAC(χ) (0.9 mg) at 417 nm in aqueous solution (pH 10.0) in the absence (I0) and presence (I1) of various metal cations.

the electron transfer does not occur, thus allowing the fluorescence emission from the fluorophore. The AC group anchored on ZOAC acts therefore in the same way as does the AC in homogeneous solution, thus suggesting that ZOAC has potential as an effective heterogeneous fluorescent sensor for the detection of pH. The fluorescent response of the ZOAC toward metal cations was then studied. Figure 6 shows the ratio of the fluorescence intensity of ZOAC at 417 nm in the absence (I0) and presence (I1) of metal cations (Ca2+, Al3+, Zn2+, Cd2+, Cu2+, Co2+, Ni2+, and Hg2+) at pH 10.0, where the variations in the fluorescence spectra of ZOAC are summarized in the Supporting Information (Figure S2). In the presence of Ca2+ and Al3+, no change in the fluorescence intensity was observed for both cases using AC and ZOAC. This is because these cations scarcely form a complex with the azamacrocycle ionophore, while the other metal cations form stable complexes.14,15 In the presence of Zn2+ or Cd2+, the fluorescence intensity of ZOAC was enhanced significantly, as was also the case using AC. This is because the complexation of Zn2+ or Cd2+ with the ionophore leads to the decrease in the HOMO level of the nitrogen atom lower than the HOMO level of the fluorophore, as is also the case when binding H+, thus suppressing the electron transfer from the nitrogen atom to the fluorophore.1-3,8 However, in the presence of Cu2+, Co2+, Ni2+, or Hg2+, the fluorescence intensity of ZOAC was found to be decreased, as was also the case using AC. In the case of Cu2+, Co2+, or Ni2+, the complexation of the metal ion

with the ionophore also leads to the decrease in the HOMO level of the nitrogen atom. However, an electron on the lowest unoccupied molecular orbital (LUMO) of the fluorophore is transferred to a low-lying empty d-orbital of the metal ion,16-18 thus quenching the fluorescence of ZOAC. In the case of Hg2+, the complexation of Hg2+, of high atomic number, near the anthracene moiety enhances the rate of intersystem crossing of anthracene from the singlet excitation state to the triplet excitation state (the spin-orbital coupling effect),19,20 thus decreasing the fluorescence of ZOAC. As shown in Figure 6, the fluorescence intensity of ZOAC is lower than that of AC. This is because some ZOAC particles, having an exceptionally lower or higher octadecyl group, precipitate to the bottom of the cell or float on the surface of the water, as shown in Figure 1b. However, ZOAC shows the same selectivity toward metal cations as does the AC. These findings suggest that the ZOAC acts as a fluorescent chemosensor for the detection of metal cations as well as the pH of the solution. 3. Reuse and Stability of ZOAC. It is necessary to remove the metal cations coordinated by the ionophore on ZOAC and reuse the ZOAC for further fluorescence measurement. The ZOAC, used for the above metal response experiment for Zn2+, was recovered by filtration. The ZOAC (10 mg) was then added to aqueous HCl (1.5 mol/L) solution (5 mL) and stirred for 10 min at room temperature. X-ray fluorescence measurement of the resulting ZOAC sample and inductively coupled argon plasma atomic emission spectrometric analysis of the resulting HCl solution showed that Zn2+ (0.103 mmol/g) coordinated originally by AC groups on ZOAC are removed completely and dissolved successfully to the HCl solution. Elemental analysis of the resulting ZOAC showed no composition changes as compared to the fresh ZOAC. To clarify the reusability of ZOAC, detection of Zn2+ in aqueous solution and subsequent washing with aqueous HCl solution were repeated three times. As shown in the Supporting Information (Figure S3), the change in the fluorescence intensity of ZOAC was scarcely observed for each stage. When the recovered ZOAC was also used for pH detection, the same fluorescence response as that obtained using fresh ZOAC was observed. These findings suggest that ZOAC is recyclable without the loss of selectivity. The chemical stability of the sensor was also examined. AC and ZOAC were left to stand in the dark and in the presence of humid air at room temperature for 30 days. The fluorescence spectra of the resulting AC and ZOAC in aqueous solution are shown in Figure 7. As shown in Figure 7b, the fluorescence of the AC in aqueous solution (pH 10.0) containing Zn2+ was scarcely observed, although AC left in the dark and in dry N2 atmosphere shows strong fluorescence (Figure 7a). When the AC, left in the humid atmosphere for 5 days, was dissolved in CD3OD and analyzed by 1H NMR spectroscopy (Supporting Information Figure S4), new signals appeared at 7.9 and 8.3 ppm. For the AC left for 30 days, signals attributable to aromatic hydrogen on anthracene at 7-9 ppm, aliphatic hydrogen on ionophore at 2.8-3.2 ppm, and also methylene hydrogen of anthrylmethyl group at 4.9-5 ppm were found to disappear completely, leaving with the signal at 7.9 ppm. In addition, although AC left in dry N2 atmosphere displays yellow color in CD3OD, AC left in humid atmosphere is colorless. The above findings suggest that

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G.N. acknowledges the Student Fellowship from the Center of Excellence (21 century COE) program “Creation of Integrated EcoChemistry” of the Chemistry Departments, Osaka University. Supporting Information Available: N2 adsorption-desorption data (Figure S1); changes in fluorescence spectra of ZOAC in the presence of different metal cations (Figure S2); fluorescence intensity of ZOAC in the presence of Zn2+ when reused for further sensing (Figure S3); 1H NMR spectra of AC stored in humid atmosphere (Figure S4). This material is available free of charge via the Internet at http://pubs.acs.org. Figure 7. Fluorescence spectra of AC left (a) in dry N2 and (b) in air for 30 days and ZOAC(χ) (0.9 mg) left (c) in dry N2 and (d) in air for 30 days, when measured in aqueous solution (pH 10.0) containing Zn2+. All of the AC and ZOAC(χ) sensors were stored in the dark.

AC is unstable in humid atmosphere and decomposes easily, although the characterization of the decomposition products of AC by NMR and IR analyses has not been successful to date. When ZOAC left in humid atmosphere for 30 days was used for the detection of Zn2+ at pH 10.0, as shown in Figure 7c, the fluorescence intensity of the ZOAC was almost the same as that of the ZOAC left in dry N2 atmosphere (Figure 7d). Elemental analysis of the ZOAC left in humid atmosphere showed no composition changes as compared to the fresh ZOAC. The results indicate that ZOAC does not lose its sensing ability even left to stand for 30 days in the humid atmosphere and the chemical stability of ZOAC is significantly higher than that of AC. ZOAC therefore has potential as a practical heterogeneous fluorescent sensor for the detection of pH and metal cations in aqueous solution. Conclusion NaX zeolite modified with octadecyl and N-(9-anthrylmethyl)-1,4,7,10-tetraazacyclododecane (AC) groups (ZOAC) was newly synthesized for use as a heterogeneous fluorescent sensor for the detection of pH and metal cations in aqueous solution, and the following results were obtained. (1) ZOAC particles were dispersed finely in aqueous solution by a slight hand shaking, owing to the presence of the hydrophobic octadecyl group, and the degree of the dispersion scarcely changed even after 10 min standing. The ZOAC particles with 80-95° contact angle showed the highest fluorescence intensity, where the intensity was about 70% as high as that for AC employed in a homogeneous system. (2) The fluorescence responses of ZOAC toward pH and metal cations were almost the same as those obtained using AC in a homogeneous system. The metal cations, coordinated by the AC group on ZOAC, were removed completely by washing with aqueous HCl (1.5 mol/L) solution, and the recovered ZOAC could be reused for further measurement. The chemical stability of ZOAC is significantly higher than that of AC, suggesting that ZOAC is potential as an effective and recyclable heterogeneous fluorescent sensor. Acknowledgment The authors acknowledge the Division of Chemical Engineering for the Lend-Lease Laboratory System.

Literature Cited (1) Czarnik, A. W. Chemical Communication in Water Using Fluorescent Chemosensors. Acc. Chem. Res. 1994, 27, 302. (2) de Silva, A. P.; Gunaratne, H. Q. N.; Gunnlaugsson, T.; Huxley, A. J. M.; McCoy, C. P.; Rademacher, J. T.; Rice, T. E. Signaling Recognition Events with Fluorescent Sensors and Switches. Chem. Rev. 1997, 97, 1515. (3) Valeur, B.; Leray, I. Design Principles of Fluorescent Molecular Sensors for Cation Recognition. Coord. Chem. Rev. 2000, 205, 3. (4) Ayadim, M.; Jiwan, J. L. H.; de Silva, A. P.; Soumillion, J. P. Photosensing by a Fluorescing Probe Covalently Attached to the Silica. Tetrahedron Lett. 1996, 37, 7039. (5) Descalzo, A. B.; Jimenez, D.; Marcos, M. D.; MartinezManez, R.; Soto, J.; El Haskouri, J.; Guillem, C.; Beltran, D.; Amoros, P.; Borrachero, M. V. A New Approach to Chemosensors for Anions Using MCM-41 Grafted with Amino Group. Adv. Mater. 2002, 14, 966. (6) Leung, M. K.-P.; Chow, C.-F.; Lam, M. H.-W. A Sol-gel Derived Molecular Imprinted Luminescent PET Sensing Material for 2,4-Dichlorophenoxyacetic Acid. J. Mater. Chem. 2001, 11, 2985. (7) Corma, A.; Galletero, M. S.; Garcia, H.; Palomares, E.; Rey, F. Pyrene Covalently Anchored on a Large External Surface Area Zeolite as a Selective Heterogeneous Sensor for Iodide. Chem. Commun. 2002, 1100. (8) Akkaya, E. U.; Huston, M. E.; Czarnik, A. W. ChelationEnhanced Fluorescence of Anthrylazamacrocycle Conjugate Probes in Aqueous Solution. J. Am. Chem. Soc. 1990, 112, 3590. (9) Nur, H.; Ikeda, S.; Ohtani, B. Phase-Boundary Catalysis: a New Approach in Alkene Epoxidation with Hydrogen Peroxide by Zeolite Loaded with Alkylsilane-Covered Titanium Oxide. Chem. Commun. 2000, 2235. (10) Ikeda, S.; Nur, H.; Sawadaishi, T.; Ijiro, K.; Shimomura, M.; Ohtani, B. Direct Observation of Bimodal Amphiphilic Surface Structures of Zeolite Particles for a Novel Liquid-Liquid Phase Boundary Catalysis. Langmuir 2001, 17, 7976. (11) Gros, C.; Rabiet, F.; Denat, F.; Brandes, S.; Chollet, H.; Guilard, R. New Silica-gel-bound Polyazacycloalkanes and Characterization of their Copper(II) Complexes Using Electron Spin Resonance Spectroscopy. J. Chem. Soc., Dalton Trans. 1996, 1209. (12) Shiraishi, Y.; Nishimura, G.; Hirai, T.; Komasawa, I. Separation of Transition Metals Using Inorganic Adsorbents Modified with Chelating Ligands. Ind. Eng. Chem. Res. 2002, 41, 5065. (13) Nishimura, G.; Shiraishi, Y.; Hirai, T. Selective Recovery of Metal Ions Based on Selective Sinking of Zeolite Modified with Octadecyl and Diethylenetriamine Groups. Chem. Lett. 2003, 32, 1096. (14) Poonia, N. S.; Bajaj, A. V. Coordination Chemistry of Alkali and Alkaline Earth Cations. Chem. Rev. 1979, 79, 389. (15) Kodama, M.; Kimura, E.; Yamaguchi, S. Complexation of the macrocyclic hexaamine ligand 1,4,7,10,13,16-hexaazacyclooctadecane (‘18-azacrown-6’). J. Chem. Soc., Dalton Trans. 1980, 2536. (16) Parker, D.; Williams, J. A. G. Luminescence Behavior of Cadmium, Lead, Zinc, Copper, Nickel and Lanthanide Complexes of Octadentate Macrocyclic Ligands Bearing Naphthyl Chromophores. J. Chem. Soc., Perkin Trans. 2 1995, 1305. (17) Balzani, V.; Scandola, F. Supramolecular Phtochemistry; Ellis Horwood: Chichester, England, 1991.

Ind. Eng. Chem. Res., Vol. 43, No. 19, 2004 6069 (18) Bergonzi, R.; Fabbrizi, L.; Licchelli, M.; Mangano, C. Molecular Switches of Fluorescence Operating through Metal Centred Redox Couples. Coord. Chem. Rev. 1998, 170, 31 (19) Vaidya, B.; Zak, J.; Bastiaans, G. J.; Porter, M. D.; Hallman, J. L.; Nabuisi, N. A. R.; Utterback, M. D.; Strzelbicka, B.; Bartsch, R. A. Chromogenic and Fluorogenic Crown Ether Compounds for the Selective Extraction and Determination of Hg(II). Anal. Chem. 1995, 67, 4101. (20) Foley, S.; Berberan-Santos, M. N.; Fedorov, A.; Bensasson, R. V.; Leach, S.; Gigante, B. Effect of Halogenated Compounds on

the Photophysics of C70 and a Monoadduct of C70: Some Implications on Optical Limiting Behaviour. Chem. Phys. 2001, 263, 437.

Received for review January 31, 2004 Revised manuscript received May 6, 2004 Accepted June 4, 2004 IE0499106