Anal. Chem. 1999, 71, 3106-3109
A Calixarene-Based Fluorogenic Reagent for Selective Mercury(II) Recognition Galina G. Talanova, Nazar S. A. Elkarim, Vladimir S. Talanov, and Richard A. Bartsch*
Department of Chemistry and Biochemistry, Texas Tech University, Lubbock, Texas 79409-1061
The first calixarene-based fluorogenic Hg(II)-selective extractant, 5,11,17,23-tetrakis(1,1-dimethylethyl)-25,27bis(N-(5-dimethylaminonaphthalene-1-sulfonyl)carbamoylmethoxy)-26,28-dimethoxycalix[4]arene (2), is reported. In solvent extraction from aqueous acidic solutions (HNO3), 2 exhibits excellent selectivity for Hg(II) over a wide range of transition, alkali, and alkaline earth metal cations. Quenching of its fluorescence due to Hg(II) coordination is unaffected by the presence of 100-fold excesses of alkali metal cations, alkaline earth metal cations, Ag(I), Tl(I), Cd(II), Co(II), Cu(II), Ni(II), Pb(II), Pd(II), Zn(II), or Fe(III). Optical chemosensors for the determination of heavy metal ions are receiving ever-increasing attention by researchers.1 Fluorogenic reagents for Hg(II) detection have been reported recently.1a,2-4 These compounds were prepared by attachment of fluorophore moieties to a platform of a macrocyclic or chelating complexant. This approach combines the capability of the parent ligand for selective Hg(II) recognition with the spectral response of the fluorophore to environmental changes upon metal cation coordination. Among the macrocyclic complexing agents for transition metal ions, calixarene-based ionophores currently occupy an important position.5 Therefore, utilization of such ligands as a platform for the development of new optical chemosensors of heavy metal cations was envisioned. Although fluorogenic calixarenes designed for the detection of alkali metal cations,6a Zn(II) and Ni(II),6b or organic species6c have appeared in the literature, calixarene-based fluorosensors for the determination of Hg(II) have not been reported, to the best of our knowledge. Recently, we discovered7 a remarkable extraction selectivity toward Hg(II) for the series of proton-ionizable calixarene[4]arene di(N-X-sulfonyl carboxamides) 1. This encouraged us to incor(1) (a) Czarnik, A. W. Acc. Chem. Res. 1994, 27, 302-308. (b) Fabrizzi, L.; Poggi, A. Chem. Soc. Rev. 1995, 24, 197-202. (c) Chemosensors for Ion and Molecule Recognition; Desvergne, J. P., Czarnik, A. W., Eds.; Kluwer Academic Publishers: Boston, 1997. (d) de Silva, A. P.; Gunaratne, N.; Gunnlaugsson, T.; Huxley, A. J. M.; McCoy, C. P.; Rademacher, J. T.; Rice, T. E. Chem. Rev. 1997, 97, 1515-1566. (2) Yoon, J.; Ohler, N. E.; Vance, D. H.; Aumiller, W. D.; Czarnik, A. W. In Chemosensors for Ion and Molecule Recognition; Desvergne, J. P., Czarnik, A. W., Eds.; Kluwer Academic Publishers: Boston, 1997; pp 189-194. (3) Vaidya, B.; Zak, J.; Bastiaans, G. J.; Porter, M. D.; Hallman, J. L.; Nabulsi, N. A. R.; Utterback, M. D.; Strzelbicka, B.; Bartsch, R. A. Anal. Chem. 1995, 67, 4101-4111. (4) Sasaki, D. Y.; Padilla, B. E. J. Chem. Soc., Chem. Commun. 1998, 15811582.
3106 Analytical Chemistry, Vol. 71, No. 15, August 1, 1999
porate a dansyl fluorophore moiety as part of the N-X-sulfonyl carboxamide group (i.e., in 2) and probe the resultant reagent for Hg(II) sensing.
We now report ionophore 2 as the first calixarene-based fluorogenic extractant for selective Hg(II) recognition. EXPERIMENTAL SECTION Synthesis of 5,11,17,23-Tetrakis(1,1-dimethylethyl)-25,27-bis(N-(5-dimethylaminonaphthalene-1-sulfonyl)carbamoylmethoxy)-26,28-dimethoxycalix[4]arene (2). Calixarene 2 was prepared by adaptation of the published procedure8 for the synthesis of calixarenes 1. Thus di(acid chloride) 3 was reacted (5) Wieser, C.; Dieleman, C. B.; Matt, D. Coord. Chem. Rev. 1997, 165, 93161. Roundhill, D. M. In Progress in Inorganic Chemistry; Karlin, K. D., Ed.; Wiley: New York, 1995; Vol. 43, pp 537-603. (6) (a) Matsumoto, H.; Shinkai, S. Tetrahedron Lett. 1996, 37, 77-80. Jin, T.; Ichikawa, K.; Koyama, T. J. Chem. Soc., Chem. Commun. 1992, 499-501. Aoki, I.; Sakaki, T.; Shinkai, S. J. Chem. Soc., Chem. Commun. 1992, 730732. Aoki, I.; Kawabata, H.; Nakashima, K.; Shinkai, S. J. Chem. Soc., Chem. Commun. 1991, 1771-1773. Ji, H.-F.; Brown, G. M.; Dabestani, R. J. Chem. Soc., Chem. Commun. 1999, 609-610. (b) Unob, F.; Asfari, Z.; Vicens, J. Tetrahedron Lett. 1998, 39, 2951-2954. (c) For a very recent report, see: Bugler, J.; Sommerdijk, N. A. J. M.; Visser, A. J. W. G.; van Hoek, A.; Nolte, R. J. M.; Engbersen, J. F. J.; Reinhoudt, D. N. J. Am. Chem. Soc. 1999, 121, 28-33. (7) Talanova, G. G.; Hwang, H.-S.; Talanov, V. S.; Bartsch, R. A. J. Chem. Soc., Chem. Commun. 1998, 1329-1330. (8) Talanova, G. G.; Hwang, H.-S.; Talanov, V. S.; Bartsch, R. A. J. Chem. Soc., Chem. Commun. 1998, 419-420. 10.1021/ac990205u CCC: $18.00
© 1999 American Chemical Society Published on Web 06/25/1999
Scheme 1
Figure 1. pH profile for Hg(II) extraction from aqueous 0.25 mM mercuric nitrate solutions into CHCl3 by 0.25 mM 2.
with commercially available dansyl amide (5-(dimethylamino)naphthalene-1-sulfonamide) and KH in dry THF under nitrogen at room temperature for 12 h (Scheme 1). Ionophore 2 was separated from the reaction mixture as the dipotassium salt which was purified by column chromatography on alumina with CH2Cl2 and then CH3OH-CH2Cl2 (2.5:97.5) as eluents. After evaporation of the eluent in vacuo, the resultant solid was dissolved in CH2Cl2. The solution was washed with 10% HCl and then with water, dried over MgSO4, evaporated in vacuo, and dried in vacuo to provide 2 as a bright greenish-yellow solid with mp 180-181 °C in 81% yield: 1H NMR (300.13 MHz, CDCl3, 294 K) δ 0.91 (br s, 18H), 1.28 (br s, 18H), 2.86 (s, 12H), 3.10-3.50 (br m, 4H), 3.65-4.55 (br m, 12H), 6.35-6.75 (br m, 4H), 7.10 (s, 4H), 7.16 (d, J ) 7.3, 2H), 7.54-7.68 (m, 4H), 8.48 (br d, 2H), 8.60 (d, J ) 8.4, 4H), 9.85 (br s, 2H). Anal. Calcd for C74H88N4O10S2: C, 70.67; H, 7.05; N, 4.45. Found: C, 70.33; H, 7.21; N, 4.32. General Methods and Instrumentation. Absorption spectra were measured with Shimadzu UV-260 UV-visible spectrophotometer. Concentrations of Hg(II) in the aqueous solutions were determined spectrophotometrically after extraction into CHCl3 containing 14.0 ppm dithizone (λmax ) 495 nm). Concentrations of alkali metal and alkaline earth metal cations in the aqueous phases were determined by ion chromatography with Dionex DX-100 ion chromatograph. Concentrations of Ag(I), Tl(I), Cd(II), Co(II), Cu(II), Ni(II), Pb(II), Pd(II), Zn(II), and Fe(III) were determined with a Perkin-Elmer 5000 atomic absorption spectrophotometer. pH measurements were performed with a Fisher Scientific Accumet 50 pH meter. Fluorescence spectra were measured in a 1-cm quartz cell using a SLM Aminco 8000C photon-counting spectrofluorometer equipped with a 450-W ozonefree xenon lamp as a light source. Single-Species Extraction of Hg(II) by 2 at Different Aqueous-Phase pHs. An aqueous mercuric nitrate solution (0.25 mM) with pH ) 0.0-4.0 (HNO3) was extracted with a 0.25 mM solution of 2 in CHCl3, and the Hg(II) concentration in the aqueous phase was determined (vide supra). Single-Species Extraction of Hg(II) by 2 at pH ) 2.5 with Varying Hg(II) Concentrations. Aqueous mercuric nitrate solutions with concentrations ranging from 0.50 to 1.00 mM (pH ) 2.5, HNO3) were extracted with a 50.0 mM solution of 2 in CHCl3. The Hg(II) concentration in the aqueous phase after extraction
was determined (vide supra), and the fluorescence of 2 in the organic phase was measured. Single-Species Extractions of Alkali Metal Cations, Alkaline Earth Metal Cations, Ag(I), Tl(I), Cd(II), Co(II), Cu(II), Ni(II), Pb(II), Pd(II), Zn(II), and Fe(III) from Aqueous Acidic Solutions by 2. The aqueous metal nitrate solution (5.00 mM) with pH ) 2.0-4.0 (HNO3) was extracted with a 50.0 mM solution of 2 in CHCl3. The metal ion concentration in the aqueous phase after extraction was determined (vide supra), and the fluorescence of 2 in the organic phase was measured. Binary Competitive Metal Ion Extractions from the Aqueous Solutions of Hg(II) and Alkali Metal, Alkaline Earth Metal, Ag(I), Tl(I), Cd(II), Co(II), Cu(II), Ni(II), Pb(II), Pd(II), Zn(II), or Fe(III) Nitrates by 2. An aqueous solution which was 25.0 mM in Hg(II) and 2.50 mM in the other metal nitrate (pH ) 2.5, HNO3) was extracted with a 50.0 mM solution of 2 in CHCl3. The fluorescence of 2 in the organic phase after extraction was measured. RESULTS AND DISCUSSION Solvent Extraction of Hg(II) by Calixarene 2. Extraction of Hg(II) from aqueous nitrate solutions into CHCl3 by 2 was studied as a function of the aqueous phase pH. The pH profile for Hg(II) extraction by 2 is shown in Figure 1. Calixarene 2 efficiently extracts Hg(II) from the aqueous acidic solutions into CHCl3 with pH1/2 ) 1.9. Attempted single-species extractions of alkali metal cations, alkaline earth metal cations, Ag(I), Tl(I), Cd(II), Co(II), Cu(II), Ni(II), Pb(II), Pd(II), Zn(II), and Fe(III) from aqueous nitrate solutions at pH ) 2.0-4.0 into CHCl3 by 2 gave undetectable transfer of the metal ions into the organic phases. Thus, under the specified conditions, calixarene 2 possesses a remarkable extraction selectivity for Hg(II) over a wide variety of other metal cations. Fluorescence Studies of Calixarene 2. The fluorescence spectrum of 2 in CHCl3 solution shows excitation and emission bands at λex ) 340 and λem ) 520 nm, respectively. On extraction of Hg(II) by 2, the dansyl group fluorescence is quenched (Figure 2). Analogous effects of Hg(II) coordination on the emission intensities of other fluorogenic complexants have been observed previously.2-4 Consistent with the earlier-suggested mechanism of transition metal ion-induced fluorescence quenching for ligandimmobilized fluorophores,1b quenching of the emission of 2 upon Analytical Chemistry, Vol. 71, No. 15, August 1, 1999
3107
Figure 4. Dependence of the relative emission intensity at 520 nm (λex ) 340 nm) on the aqueous-phase pH for extraction of 0.10 mM mercuric nitrate with 50.0 mM 2 in CHCl3. Figure 2. Fluorescence emission spectra (λem ) 520 nm, λex ) 340 nm) of 50.0 mM 2 in CHCl3 (a) before and (b-g) after Hg(II) extraction from aqueous 8.00, 10.0, 25.0, and 50.0 mM and 0.10 and 0.25 mM mercuric nitrate solutions, respectively (pH ) 2.5, HNO3).
Figure 3. Dependence on the initial aqueous-phase Hg(II) concentration (pH ) 2.5, HNO3) of (a) Hg(II) loading and (b) relative emission intensity at 520 nm (λex ) 340 nm) of 50.0 mM 2 in CHCl3 after extraction.
Hg(II) coordination is proposed to occur by electron transfer from the exited dansyl moiety to the proximate mercuric ion. The emission intensity of 2 decreases with enhanced Hg(II) extraction into the organic phase. Thus, when the initial aqueousphase Hg(II) concentration is increased at pH ) 2.5, the metal loading of the calixarene is enhanced as shown in Figure 3a and its fluorescence diminishes accordingly (see Figure 2). The dependence of the relative emission intensity I/I0 (where I0 and I are emission intensities observed in the spectrum of 2 before and after the mercuric ion extraction, respectively) on the initial Hg(II) concentration in the aqueous solution is presented in Figure 3b. Under the experimental conditions (i.e., 50.0 mM 2 in CHCl3 and an aqueous-phase pH ) 2.5), the Hg(II) detection limit 3108 Analytical Chemistry, Vol. 71, No. 15, August 1, 1999
in the aqueous solution was ∼5.00 mM with a signal-to-noise ratio of 3 and precision within 1%. A maximum of ∼86% quenching of the initial ligand fluorescence was observed after the extraction of a 0.25 mM aqueous solution of Hg(II) by the solution of 2 in CHCl3 under otherwise identical conditions. Similarly, fluorescence quenching of 2 due to Hg(II) extraction is enhanced as the aqueous-phase pH is increased with the initial Hg(II) concentration held constant. The dependence of I/I0 upon the aqueous-phase pH is presented in Figure 4. In contrast, no change in the emission spectrum of 2 was observed after single-species extraction of aqueous alkali metal, alkaline earth metal, Ag(I), Tl(I), Cd(II), Co(II), Cu(II), Co(II), Ni(II), Pb(II), Pd(II), Zn(II), or Fe(III) nitrate (5.00 mM at pH 2.5) with 50.0 mM solutions of 2 in CHCl3. Also, the fluorescence of 2 was unaffected when extractions of Na(I), Ag(I), Pb(II), and Pd(II) were conducted with the aqueous-phase pH increased to 6.0. To further verify the noninterference of these metal cations with the observed Hg(II)-induced calixarene fluorescence quenching, emission spectra were measured for the organic phases obtained from binary competitive metal ion extractions of 25.0 mM aqueous mercuric nitrate and a 100-fold excess of an alkali metal, alkaline earth metal, Ag(I), Tl(I), Cd(II), Co(II), Cu(II), Ni(II), Pb(II), Pd(II), Zn(II), or Fe(III) nitrate at pH ) 2.5 with 50.0 mM solutions of 2 in CHCl3. Except for Hg(II)-Pd(II) as discussed below, the emission intensity of 2 in the organic extract was the same ((3%) as that observed after single-species Hg(II) extraction under the same conditions. Thus, the Hg(II)-induced fluorescence quenching of the calixarene 2 was unaffected by the presence in the aqueous solution of a large excess of competing metal ions. When the competitive extraction of an aqueous solution of 25.0 µM Hg(II) and 2.50 mM Pd(II) nitrates (pH ) 2.5) with 2 was performed immediately after preparation of the aqueous solution, no change of the ligand emission intensity was observed relative to that measured after the single-species Hg(II) extraction. (This result is consistent with the above-mentioned inability of 2 to extract Pd(II) from an aqueous acidic solution and the corresponding absence of calixarene fluorescence quenching when single-species Pd(II) extraction was attempted.) However, when the competitive Hg(II)-Pd(II) extraction was performed 1 h later using another sample of the same aqueous solution, the emission
intensity of 2 in the organic extract increased nearly to the value observed for the uncomplexed ligand in CHCl3. Thus, Pd(II) appears to show an eventual masking of Hg(II). One conceivable rationalization of this phenomenon is a gradual formation of a multinuclear species in the aqueous phase in which each Hg(II) is surrounded by several Pd(II)-containing particles making the Hg(II) inaccessible for coordination by the calixarene. The detailed mechanistic investigation that would be required to probe the observed masking effect is beyond the scope of the present study. CONCLUSIONS A new optical sensor for Hg(II) recognition has been prepared by incorporating dansyl fluorophore moieties into a calix[4]arene di(N-X-sulfonyl carboxamide). This reagent, which is the first example of a Hg(II)-selective fluorogenic calixarene, exhibits
efficient Hg(II) extraction from aqueous acidic nitrate solutions into chloroform accompanied by fluorescence quenching. The ligand emission quenching due to Hg(II) coordination is unaffected by the presence in the aqueous phase of excess alkali metal, alkaline earth metal, and transition metal cations. ACKNOWLEDGMENT This research was supported by the Division of Chemical Sciences of the Office of Basic Energy Sciences of the U.S. Department of Energy (Grant DE-FG03-9414416).
Received for review February 23, 1999. Accepted May 10, 1999. AC990205U
Analytical Chemistry, Vol. 71, No. 15, August 1, 1999
3109
