- Email: [email protected]
A fluorescent probe for zinc ions based on N-methyltetraphenylporphine with high selectivity
Spectrochimica Acta Part A 71 (2009) 1683–1687
Contents lists available at ScienceDirect
Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy journal homepage: www.elsevier.com/locate/saa
A fluorescent probe for zinc ions based on N-methyltetraphenylporphine with high selectivity Qiu-Juan Ma, Xiao-Bing Zhang ∗ , Yan Zhao, Chun-Yan Li, Zhi-Xiang Han, Guo-Li Shen, Ru-Qin Yu ∗ State Key Laboratory of Chemo/Biosensing and Chemometrics, College of Chemistry and Chemical Engineering, Hunan University, Changsha 410082, PR China
a r t i c l e
i n f o
Article history: Received 26 November 2007 Received in revised form 23 May 2008 Accepted 23 June 2008 Keywords: Fluorescent probe Zinc ions NMTPPH Fluorescence enhancement
a b s t r a c t N-Methyl-␣,,␥,␦-tetraphenylporphine (NMTPPH) has been used to detect trace amount of zinc ions in ethanol–water solution by fluorescence spectroscopy. The fluorescent probe undergoes a fluorescent emission intensity enhancement upon binding to zinc ions in EtOH/H2 O (1:1, v/v) solution. The fluorescence enhancement of NMTPPH is attributed to the 1:1 complex formation between NMTPPH and Zn(II) which has been utilized as the basis for the selective detection of Zn(II). The linear response range covers a concentration range of Zn(II) from 5.0 × 10−7 to 1.0 × 10−5 mol/L and the detection limit is 1.5 × 10−7 mol/L. The fluorescent probe exhibits high selectivity over other common metal ions except for Cu(II). © 2008 Elsevier B.V. All rights reserved.
1. Introduction Zinc is the second abundant transition metal after iron in human body with the range of normal concentration from sub-nM to ∼0.3 mM [1]. It plays an important role in various biological systems such as gene expression, protein–protein interaction and neurotransmission [2,3]. However, zinc is an environmental pollutant, significant concentrations of which may reduce the soil microbial activity causing phytotoxic effect [4,5]. Thus, the determination of trace zinc ion is of considerable importance for both biological and environmental applications. Though several analytical techniques such as UV–vis spectroscopy [6], potentiometry [7] and flame atomic absorption spectrometry [8] have been reported for zinc ion assay in various samples, the available detection methods of Zn(II) are still limited due to its 3d10 4s0 electron configuration not giving any spectroscopic or magnetic signals. Owing to the advantages of simplicity, high sensitivity and low cost, past decades have seen increasing interest in the development of fluorescent probes for zinc ions. The Zn(II)-selective receptors in general include the derivatives of quinoline [1,9–12], di-2-picolylamine [13–18], acyclic and cyclic polyamine [19–25], terpyridine [26,27], and iminodiacetate [28–30]. Although some of these fluorescent probes can be applied to detect zinc ions in environmental or biological samples, they have the disadvantages such as insufficient selectivity or sensitiv-
∗ Corresponding authors. Tel.: +86 731 8821916; fax: +86 731 8821916. E-mail addresses: [email protected] (X.-B. Zhang), [email protected] (R.-Q. Yu). 1386-1425/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.saa.2008.06.020
ity. These fluorescent probes usually have interference problems caused by other transition metals cations such as Ni(II), Co(II) or Fe(III). Especially, among the reported fluorescent probes for Zn(II), many cannot distinguish Cd(II) from Zn(II) [31–33] because Zn(II) and Cd(II) have similar chemical properties. Searching for new fluorophores which would react with Zn(II) with sufficient high selectivity is still an active field as well as a challenge for the analytical chemistry research effort. Examples of recent advances along this line have been the synthesis of some new fluorescent probes which show selectively binding with Zn(II) not affected by Cd(II) [34–36]. In the past decades, porphyrins have gained increasing interest as analytical regents for various metal ions [37–39]. The porphyrins possess some characteristic features such as aromaticity, planarity and rigidity. In addition, the porphyrins exhibit nice photophysical properties including strong fluorescence, large Strokes shifts and relatively long excitation (>400 nm) and emission (>600 nm) wavelengths that minimize the effects of the background fluorescence. These make porphyrins potential fluorophores for several metal ions. However, the very slow rate of metal insertion into porphyrin limits the use of porphyrins as fluorogenic ligands for metal ions. To accelerate the reaction rate, several methods have been employed [38,40–42]. In addition, some supramolecular complexes of porphyrin have been used to detect zinc making use of the catalyzing effect of the complexes toward the Zn(II) insertion [43,44]. However, the non-covalent supramolecular complexes of meso-tetrakis(4-N-methylpyridyl)porphine on polyglutamate [43] are less robust than those obtained by covalent approach. Besides, as reported in the literature [44], Cd(II) influences on the fluorescence emission spectra of supramolecular complexes of porphyrin
1684
Q.-J. Ma et al. / Spectrochimica Acta Part A 71 (2009) 1683–1687
Fig. 2. Fluorescence emission spectra of NMTPPH (1.0 × 10−5 mol/L) in the presence of increasing concentration of Zn(II). The concentration of Zn(II) (from bottom to top): 0, 0.5, 0.7, 1, 3, 3.5, 4, 4.5, 5, 6, 7, 8 and 10 M. Measured at 25 ◦ C in CH3 CH2 OH–H2 O (1:1, v/v) and Tris–HCl (0.05 mol/L) buffer (pH 7.10), with ex = 432 nm.
2.2. Preparation of solutions Fig. 1. The structure of NMTPPH.
in the same way as Zn(II) and interferes with the determination of Zn(II). Herein, we report a selective fluoroionophore for Zn(II) based on N-methyltetraphenylporphyrin, which is barely affected by various metal ions except Cu(II). The N-methylporphyrins are a class of porphyrins with a nonplanar coordination site of pyrrolic nitrogens and the rate of complexation of the metal with N-methylporphyrin is much more quickly than that with non-methylated porphyrin [45,46]. The metal complexes of Nmethylporphyrin exhibit very similar visible absorption spectra [47,48]. Lavallee et al. [49] investigated the fluorometric properties of N-methyltetraphenylporphyrin and several derivatives and chose their zinc complexes as standards for determination of chlorophyll a. Tanaka et al. [50] used N-methyltetrakis(4sulfonatophenyl)porphyrin for kinetic determination of copper and zinc by UV–vis spectroscopy in serum. Making use of its relatively fast complexation rate, we studied in detail the use of N-methyl␣,,␥,␦-tetraphenylporphine (NMTPPH, Fig. 1) as a fluorescent probe for zinc ions and showed its outstanding characteristics including large Stokes shifts, nice sensitivity and selectivity for zinc over other metal ions.
A stock solution of 2 × 10−5 mol/L NMTPPH was obtained by dissolving NMTPPH in absolute ethanol. A stock solution of 1 × 10−2 mol/L Zn(II) was prepared by dissolving Zn(OAc)2 ·2H2 O in doubly distilled water. The stock solution of Zn(II) was diluted to lower concentrations of 1 × 10−3 to 1 × 10−7 mol/L stepwise. The wide pH range solutions were prepared by adjustment of 0.05 mol/L Tris–HCl solution with HCl or NaOH solution. The complex solution of Zn(II) and NMTPPH was obtained by mixing 12.5 mL of the stock solution of NMTPPH and 2.5 mL of Zn(II) solution of the different concentrations and diluting the mixture to 25 mL in a volumetric flask. In the solution thus obtained, the concentrations were 1 × 10−5 mol/L in NMTPPH and 1 × 10−4 to 1 × 10−8 mol/L in Zn(II). Blank solution of NMTPPH was prepared under the same conditions without Zn(II). 2.3. Apparatus UV–vis absorption spectra were scanned on a Shimadzu MultiSpec-1501 spectrophotometer. All fluorescence measurements were taken on a PerkinElmer LS 55 Luminescence Spectrometer with excitation slit set at 10.0 nm and emission at 15.0 nm. The measurements of pH were carried out on a MettlerToledo Delta 320 pH meter. Data processing was performed on a Pentium IV computer with software of SigmaPlot.
2. Experimental
2.4. Measurement procedures
2.1. Reagents
The fluorescence intensity was measured at the maximum of excitation wavelength of 432 nm with the emission wavelength varied over 600–750 nm. Before the fluorescence measurement, the complex solution of NMTPPH/Zn(II) was allowed to stand for a few minutes to allow complete formation of metal–ligand complex.
CF3 SO2 OCH3 was obtained from Aldrich Chemical Co. Doubly distilled water was used throughout all experiments. CH2 Cl2 was distilled from calcium hydride and stored over molecular sieves. Before being used, benzaldehyde was simply distilled from K2 CO3 . Pyrrole was distilled at atmospheric pressure from CaH2 . Unless otherwise stated, all other chemicals were of analytical reagent grade and used without further purification. ␣,,␥,␦tetraphenylporphine (TPPH2 ) was prepared from pyrrole and benzaldehyde according to previously described method [51]. NMTPPH was synthesized from tetraphenylporphyrin (TPPH2 ) and CF3 SO2 OCH3 according to documented procedures [49].
3. Results and discussion 3.1. Spectral characteristics The fluorescence emission spectra were recorded from 1 × 10−5 mol/L solution of NMTPPH at 25 ◦ C in CH3 CH2 OH–H2 O (1:1, v/v) and Tris–HCl (0.05 mol/L) buffer (pH 7.10). Fig. 2 shows
Q.-J. Ma et al. / Spectrochimica Acta Part A 71 (2009) 1683–1687
Fig. 3. Absorption spectra of NMTPPH (1.0 × 10−5 mol/L) before (—) and after (- - -) the addition of Zn(OAc)2 ·2H2 O (1.0 × 10−5 mol/L). The inset shows the corresponding spectra magnified between 500 and 700 nm. Measured at 25 ◦ C in CH3 CH2 OH–H2 O (1:1, v/v) and Tris–HCl (0.05 mol/L) buffer (pH 7.10).
the effect of Zn(II) concentration on the fluorescence emission spectra of NMTPPH. As can be seen from Fig. 2, the fluorescence emission spectra of NMTPPH are sensitive to Zn(II). In the presence of various concentration of zinc ion ranging from 5.0 × 10−7 to 1.0 × 10−5 mol/L, significant fluorescence enhancement of fluorescent probe was observed. The fluorescence intensity is enhanced with increasing Zn(II) concentration, which constitutes the basis for the recognition of Zn(II) with fluorescent probe proposed in this work. In the presence of excess of Zn(II), a 3.6-fold fluorescence enhancement at the maximum emission wavelength of 665 nm was observed. This corresponds to the complete conversion of NMTPPH into complexed state with Zn(II). One observes a blue shift of about 20 nm for the emission spectrum peak of NMTPPH from its uncomplexed state (zero Zn(II) concentration) to complexed form. The fluorescence enhancement is caused by the complexation of Zn(II) and the blue shift can be rationalized by the deprotonation of NMTPPH upon Zn(II) binding. The fluorescence quantum yield of NMTPPH in EtOH/H2 O (1:1, v/v) solution is 0.008, as determined by using TPPH2 in Benzene (˚F = 0.11) as a reference [52]. NMTPPH shows larger enhancements (3.4-fold in ˚F ) upon completely coordinating Zn(II). While the mechanism by which this chelation-enhanced fluorescence occurs is still unclear, it is possible that incorporation of Zn(II) enhances the rigidity of NMTPPH thus inhibiting some of non-radiative transitions between electronic states. In order to study the reaction of NMTPPH with Zn(II), the visible absorption spectra of NMTPPH in the absence and presence of Zn(II) were investigated (Fig. 3). From Fig. 3, it can be seen that the absorption spectrum of NMTPPH displays a single maximum in the Soret region and two prominent peaks in the 500–700 nm region and upon zinc binding the spectrum of solution shows a split Soret band and three prominent peaks in the 500–700 nm region. These spectral characteristics coincide with the spectra of NMTPPH and its Zn(II) true complex studied previously [47]. Fig. 4 confirms the 1:1 complexation of NMTPPH with Zn(II). The possible structure of 1:1 complex of NMTPPH and Zn(II) is shown in Fig. 5. The puckered form of ring due to methylation seems to allow rapid metal incorporation [46] and thus decreases the response time of NMTPPH towards Zn(II). The response time for Zn(II) concentration ≤1.0 × 10−5 mol/L is less than 5 min. The linear response of the fluorescence emission intensity toward log CZn(II) was obtained in zinc concentration range of 5.0 × 10−7 to 1.0 × 10−5 mol/L. The response can be expressed by the following
1685
Fig. 4. Plot of log[(F − F0 )/F0 ] as a function of the log CZn(II) . F0 and F are the fluorescence intensity of NMTPPH (1.0 × 10−5 mol/L) in the absence and presence of Zn(II), respectively. Measured at 25 ◦ C in CH3 CH2 OH–H2 O (1:1, v/v) and Tris–HCl (0.05 mol/L) buffer (pH 7.10), with ex = 432 nm.
equation: log
F − F 0
F0
= 1.3779 log CZn(II) + 7.3722 (R = 0.9763)
(1)
Here F is the fluorescence intensity of NMTPPH actually measured at a given metal concentration, F0 denotes the fluorescence intensity of NMTPPH in the absence of zinc metal ion and CZn(II) represents the concentration of zinc ion added. The detection limit was estimated as 1.5 × 10−7 mol/L (calculated as three times standard deviation of blank solution). 3.2. Effect of pH Fig. 6 displays the influence of pH values on the fluorescence emission intensity of fluorescent probe in the presence of Zn(II). The fluorescence intensity measurements were carried out by fixing 1.0 × 10−5 mol/L Zn(II) at various pH values. As can be seen from
Fig. 5. The assumed structure of 1:1 complex of NMTPPH and Zn(II).
1686
Q.-J. Ma et al. / Spectrochimica Acta Part A 71 (2009) 1683–1687
Fig. 7. Fluorescence intensity changes profile of NMTPPH (1.0 × 10−5 mol/L) in the presence of various cations. Cations (10 equiv. relative to NMTPPH) were added as Li2 SO4 ·H2 O, Co(CH3 COO)2 ·4H2 O, NiCl2 ·6H2 O, Mn(CH3 COO)2 ·4H2 O, NaCl, Al2 (SO4 )3 ·18H2 O, Pb(NO3 )2 , AgNO3 , KCl, Ca(CH3 COO)2 ·H2 O, and MgSO4 . Other cations were added as Zn(CH3 COO)2 ·2H2 O (1.0 × 10−5 mol/L), CdCl2 ·2.5H2 O (1.0 × 10−5 mol/L), HgNO3 ·H2 O (1.0 × 10−5 mol/L), FeCl3 ·6H2 O (5.0 × 10−5 mol/L), and CuCl2 ·2H2 O (1.0 × 10−5 mol/L). Measured at 25 ◦ C in CH3 CH2 OH–H2 O (1:1, v/v) and Tris–HCl (0.05 mol/L) buffer (pH 7.10), with ex = 432 nm.
Fig. 6. Effect of pH on the fluorescence intensity of NMTPPH (1.0 × 10−5 mol/L) in the presence of Zn(II) (the concentration of Zn(II) was fixed at 1.0 × 10−5 mol/L). Measured at 25 ◦ C in CH3 CH2 OH–H2 O (1:1, v/v).
Fig. 6, the fluorescence intensity almost did not vary with pH in the range of 6.30–10.50 in CH3 CH2 OH–H2 O (1:1, v/v) solution. When pH is lower than 6.30, the fluorescence intensity decreases with decreasing pH value. At pH higher than 10.50, the fluorescence intensity of the Zn(II)–NMTPP complex also decreases. These experiments can be conveniently explained according to the ligation process. Lower pH leads to protonation of NMTPPH and release of Zn(II) and thus at higher acidity the fluorescence intensity of Zn(II) complex decreases with a red shift of maximum emission wavelength. At higher pH values the concentration of Zn(II) complex decreases owing to the formation of the precipitation of Zn(OH)2 . Taking into consideration the sensitivity and response speed, a pH 7.10 Tris/HCl buffer solution was chosen as optimum experimental condition.
probe was significantly enhanced upon the addition of Zn(II) and was slightly quenched upon binding to Cu(II). Moreover, the addition of other cations did not affect the fluorescence intensity of the fluorescent probe. In order to further test the interference for other common cations on the determination of Zn(II), a competition experiment was performed in which the fluorescent probe was added to a solution of Zn(II) in the presence of other metal ions (Table 1). From Table 1, one can see that the relative error of common species except Cu(II) and Hg(II), such as alkali, alkaline earth, transitional and heavy metal ions, is less than ±5% which is considered as tolerated. Thus, NMTPPH exhibits nice selectivity for Zn(II) over other common cations except Cu(II).
3.3. Evaluation of selectivity To evaluate selectivity of NMTPPH, the fluorescence emission intensity of the fluorescent probe binding to other metal ions was investigated. Fig. 7 demonstrates the selectivity of the fluorescent probe for zinc over various common cations in pH 7.10 Tris–HCl solution. The concentration of Zn(II) was fixed at 1.0 × 10−5 mol/L. Cations were added as chlorides, nitrates, acetate and sulfates. As shown in Fig. 7, the fluorescence intensity of the fluorescent
4. Conclusion In conclusion, a selective fluorescent probe for Zn(II) based on NMTPPH with high selectivity has been reported. The fluorescence emission intensity of NMTPPH is remarkably enhanced upon the addition of zinc which is attributed to the formation of 1:1 complex. The method proposed in this work is simple, rapid, sensitive and
Table 1 Interference of several cations to the fluorescence determination of Zn(II) with NMTPPH Interference +
Li Co2+ Ni2+ Mn2+ Na+ Al3+ Pb2+ Cu2+ Fe3+ Hg2+ Cd2+ Ag+ K+ Ca2+ Mg2+
Concentrationa (mol/L) −4
1.0 × 10 1.0 × 10−4 1.0 × 10−4 1.0 × 10−4 1.0 × 10−4 1.0 × 10−4 1.0 × 10−4 1.0 × 10−5 5.0 × 10−5 1.0 × 10−5 1.0 × 10−5 1.0 × 10−4 1.0 × 10−4 1.0 × 10−4 1.0 × 10−4
Fluorescence (F = F − F0 )b
Relative error (%), (F/F0 ) × 100
1.1151 0.3622 1.2850 1.4911 0.6282 0.4365 1.0017 −45.1434 −2.8169 −9.5198 −0.7692 −2.2692 1.1706 0.5714 0.2833
1.33 0.43 1.20 1.78 0.75 0.52 1.20 −54.00 −3.37 −11.39 −0.92 −2.72 1.40 0.68 0.34
The concentration of Zn(II) is fixed at 1.0 × 10−5 mol/L (pH 7.10). F0 and F indicate the average fluorescence intensities of fluorescent probe NMTPPH (1.0 × 10−5 mol/L) contacting with 1.0 × 10−5 mol/L Zn(II) solution before and after adding the interference (F0 = 83.5671). a
b
Q.-J. Ma et al. / Spectrochimica Acta Part A 71 (2009) 1683–1687
selective. To improve water solubility of NMTPPH without compromising the selectivity, modification of NMTPPH could be achieved by tethering hydrophilic side chains which would allow the study in aqueous and buffer solutions. Further work in this direction is in progress. Acknowledgments This work was supported by the National Natural Science Foundation of China (Grant nos. 20505008 and 20435010), “973” National Key Basic Research Program of China (2007CB310500), Hunan Natural Science Foundation (07JJ3025), and the National 863 High Technology Research Foundation of China (2006AA06Z407). References [1] P.J. Jiang, L.Z. Chen, J. Lin, Q. Liu, J. Ding, X. Gao, Z.J. Guo, Chem. Commun. (2002) 1424. [2] J.M. Berg, Y. Shi, Science 271 (1996) 1071. [3] M.P. Cuajungco, G.J. Lees, Neurobiol. Dis. 4 (1997) 137. [4] A. Voegelin, S. Pfister, A.C. Scheinost, M.A. Marcus, R. Kretzschmar, Environ. Sci. Technol. 39 (2005) 6616. [5] J. Mertens, F. Degryse, D. Springael, E. Smolders, Environ. Sci. Technol. 41 (2007) 2992. [6] C.V. Banks, R.E. Bisque, Anal. Chem. 29 (1957) 522. [7] A.R. Fakhari, M. Shamsipur, Kh. Ghanbari, Anal. Chim. Acta 460 (2002) 177. [8] Q. Li, X.H. Zhao, Q.Z. Lv, G.G. Liu, Sep. Purif. Technol. 55 (2007) 76–81. [9] T. Budde, A. Minta, J.A. White, A.R. Kay, Neuroscience 79 (1997) 347. [10] H. Wang, Q. Gan, X. Wang, L. Xue, S. Liu, H. Jiang, Org. Lett. 9 (2007) 4995. [11] Y. Zhang, X. Guo, W. Si, L. Jia, X. Qian, Org. Lett. 10 (2008) 473. [12] E.M. Nolan, J. Jaworski, K. Okamoto, Y. Hayashi, M. Sheng, S.J. Lippard, J. Am. Chem. Soc. 127 (2005) 16812. [13] G.K. Walkup, S.C. Burdette, S.J. Lippard, R.Y. Tsien, J. Am. Chem. Soc. 122 (2000) 5644. [14] S.C. Burdette, G.K. Walkup, B. Spingler, R.Y. Tsien, S.J. Lippard, J. Am. Chem. Soc. 123 (2001) 7831. [15] T. Hirano, K. Kikuchi, Y. Urano, T. Nagano, J. Am. Chem. Soc. 124 (2002) 6555. [16] S.C. Burdette, C.J. Frederickson, W. Bu, S.J. Lippard, J. Am. Chem. Soc. 125 (2003) 1778. [17] N.C. Lim, C. Brückner, Chem. Commun. (2004) 1094. [18] C.R. Goldsmith, S.J. Lippard, Inorg. Chem. 45 (2006) 555.
1687
[19] T. Koike, T. Watanable, S. Aoki, E. Kimura, M. Shiro, J. Am. Chem. Soc. 118 (1996) 12696. [20] T. Hirano, K. Kikuchi, Y. Urano, T. Higuchi, T. Nagano, Angew. Chem. Int. Ed. 39 (2000) 1052. [21] S. Aoki, S. Kaido, H. Fujioka, E. Kimura, Inorg. Chem. 42 (2003) 1023. [22] A. Bencini, E. Berni, A. Bianchi, P. Fornasari, C. Giorgi, J.C. Lima, C. Lodeiro, M.J. Melo, J.S. Melo, A.J. Parola, F. Pina, J. Pina, B. Valtancoli, Dalton Trans. (2004) 2180. [23] N.C. Lim, J.V. Schuster, M.C. Porto, M.A. Tanudra, L. Yao, H.C. Freake, C. Brückner, Inorg. Chem. 44 (2005) 2018. [24] S. Yunus, S. Charles, F. Dubois, E.V. Donckt, J. Fluoresc. 18 (2008) 499. [25] A.E. Majzoub, C. Cadiou, I. Déchamps-Olivier, F. Chuburu, M. Aplincour, Eur. J. Inorg. Chem. 32 (2007) 5087. [26] W. Goodall, J.A.G. Williams, Chem. Commun. (2001) 2514. [27] C. Goze, G. Ulrich, L. Charbonnière, M. Cesario, T. Prangé, R. Ziessel, Chem. Eur. J. 9 (2003) 3748. [28] O. Reany, T. Gunnlaugsson, D. Parker, Chem. Commun. (2000) 473. [29] O. Reany, T. Gunnlaugsson, D. Parker, J. Chem. Soc., Perkin Trans. 2 (2000) 1819. [30] R. Parkesh, T.C. Lee, T. Gunnlaugsson, Org. Biomol. Chem. 5 (2007) 310. [31] S. Maruyama, K. Kikuchi, T. Hirano, Y. Urano, T. Nagano, J. Am. Chem. Soc. 124 (2002) 10650. [32] M.D. Shults, D.A. Pearce, B. Imperiali, J. Am. Chem. Soc. 125 (2003) 10591. [33] M. Taki, J.L. Wolford, T.V. O’Halloran, J. Am. Chem. Soc. 126 (2004) 712. [34] T. Hirano, K. Kikuchi, Y. Urano, T. Higuchi, T. Nagano, J. Am. Chem. Soc. 122 (2000) 12399. [35] J.N. Ngwendson, A. Banerjee, Tetrahedron Lett. 48 (2007) 7316. [36] Y. Chen, K. Han, Y. Liu, Bioorg. Med. Chem. 15 (2007) 4537. [37] X. Xu, H. Zhang, C. Zhang, J. Cheng, Anal. Chem. 63 (1991) 2529. [38] M. Tabata, M. Kumamoto, J. Nishimoto, Anal. Chem. 68 (1996) 758. [39] Q. Hu, G. Yang, J. Yin, Y. Yao, Talanta 57 (2002) 751. [40] C. Grant, P. Hambright, J. Am. Chem. Soc. 91 (1969) 4195. [41] M. Tabata, M. Tanaka, Inorg. Chem. 27 (1988) 203. [42] Y. Zhang, W. Xiang, R. Yang, F. Liu, K. Li, J. Photochem. Photobiol. A: Chem. 173 (2005) 264. [43] E. Bellacchio, S. Gurrieri, R. Lauceri, A. Magri, L.M. Scolaro, R. Purrello, A. Romeo, Chem. Commun. (1998) 1333. [44] R. Yang, K. Li, K. Wang, F. Zhao, N. Li, F. Liu, Anal. Chem. 75 (2003) 612. [45] B. Shah, B. Shears, P. Hambright, Inorg. Chem. 10 (1971) 1828. [46] M.J. Bain-Ackerman, D.K. Lavallee, Inorg. Chem. 18 (1979) 3358. [47] D.K. Lavallee, Bioinorg. Chem. 6 (1976) 219. [48] D.K. Lavallee, M.J. Bain-Ackerman, Bioinorg. Chem. 9 (1978) 311. [49] D.K. Lavallee, T.J. Mcdonough Jr., L. Cioffi, Appl. Spectrosc. 36 (1982) 431. [50] S. Funahashi, Y. Tto, H. Kakito, M. Inamo, Y. Hamada, M. Tanaka, Mikrochim. Acta 88 (1986) 33. [51] A.D. Adler, F.R. Longo, J.D. Finarelli, J. Org. Chem. 32 (1967) 476. [52] P.G. Seybold, M. Gouterman, J. Mol. Spectrosc. 31 (1969) 1.
Contents lists available at ScienceDirect
Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy journal homepage: www.elsevier.com/locate/saa
A fluorescent probe for zinc ions based on N-methyltetraphenylporphine with high selectivity Qiu-Juan Ma, Xiao-Bing Zhang ∗ , Yan Zhao, Chun-Yan Li, Zhi-Xiang Han, Guo-Li Shen, Ru-Qin Yu ∗ State Key Laboratory of Chemo/Biosensing and Chemometrics, College of Chemistry and Chemical Engineering, Hunan University, Changsha 410082, PR China
a r t i c l e
i n f o
Article history: Received 26 November 2007 Received in revised form 23 May 2008 Accepted 23 June 2008 Keywords: Fluorescent probe Zinc ions NMTPPH Fluorescence enhancement
a b s t r a c t N-Methyl-␣,,␥,␦-tetraphenylporphine (NMTPPH) has been used to detect trace amount of zinc ions in ethanol–water solution by fluorescence spectroscopy. The fluorescent probe undergoes a fluorescent emission intensity enhancement upon binding to zinc ions in EtOH/H2 O (1:1, v/v) solution. The fluorescence enhancement of NMTPPH is attributed to the 1:1 complex formation between NMTPPH and Zn(II) which has been utilized as the basis for the selective detection of Zn(II). The linear response range covers a concentration range of Zn(II) from 5.0 × 10−7 to 1.0 × 10−5 mol/L and the detection limit is 1.5 × 10−7 mol/L. The fluorescent probe exhibits high selectivity over other common metal ions except for Cu(II). © 2008 Elsevier B.V. All rights reserved.
1. Introduction Zinc is the second abundant transition metal after iron in human body with the range of normal concentration from sub-nM to ∼0.3 mM [1]. It plays an important role in various biological systems such as gene expression, protein–protein interaction and neurotransmission [2,3]. However, zinc is an environmental pollutant, significant concentrations of which may reduce the soil microbial activity causing phytotoxic effect [4,5]. Thus, the determination of trace zinc ion is of considerable importance for both biological and environmental applications. Though several analytical techniques such as UV–vis spectroscopy [6], potentiometry [7] and flame atomic absorption spectrometry [8] have been reported for zinc ion assay in various samples, the available detection methods of Zn(II) are still limited due to its 3d10 4s0 electron configuration not giving any spectroscopic or magnetic signals. Owing to the advantages of simplicity, high sensitivity and low cost, past decades have seen increasing interest in the development of fluorescent probes for zinc ions. The Zn(II)-selective receptors in general include the derivatives of quinoline [1,9–12], di-2-picolylamine [13–18], acyclic and cyclic polyamine [19–25], terpyridine [26,27], and iminodiacetate [28–30]. Although some of these fluorescent probes can be applied to detect zinc ions in environmental or biological samples, they have the disadvantages such as insufficient selectivity or sensitiv-
∗ Corresponding authors. Tel.: +86 731 8821916; fax: +86 731 8821916. E-mail addresses: [email protected] (X.-B. Zhang), [email protected] (R.-Q. Yu). 1386-1425/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.saa.2008.06.020
ity. These fluorescent probes usually have interference problems caused by other transition metals cations such as Ni(II), Co(II) or Fe(III). Especially, among the reported fluorescent probes for Zn(II), many cannot distinguish Cd(II) from Zn(II) [31–33] because Zn(II) and Cd(II) have similar chemical properties. Searching for new fluorophores which would react with Zn(II) with sufficient high selectivity is still an active field as well as a challenge for the analytical chemistry research effort. Examples of recent advances along this line have been the synthesis of some new fluorescent probes which show selectively binding with Zn(II) not affected by Cd(II) [34–36]. In the past decades, porphyrins have gained increasing interest as analytical regents for various metal ions [37–39]. The porphyrins possess some characteristic features such as aromaticity, planarity and rigidity. In addition, the porphyrins exhibit nice photophysical properties including strong fluorescence, large Strokes shifts and relatively long excitation (>400 nm) and emission (>600 nm) wavelengths that minimize the effects of the background fluorescence. These make porphyrins potential fluorophores for several metal ions. However, the very slow rate of metal insertion into porphyrin limits the use of porphyrins as fluorogenic ligands for metal ions. To accelerate the reaction rate, several methods have been employed [38,40–42]. In addition, some supramolecular complexes of porphyrin have been used to detect zinc making use of the catalyzing effect of the complexes toward the Zn(II) insertion [43,44]. However, the non-covalent supramolecular complexes of meso-tetrakis(4-N-methylpyridyl)porphine on polyglutamate [43] are less robust than those obtained by covalent approach. Besides, as reported in the literature [44], Cd(II) influences on the fluorescence emission spectra of supramolecular complexes of porphyrin
1684
Q.-J. Ma et al. / Spectrochimica Acta Part A 71 (2009) 1683–1687
Fig. 2. Fluorescence emission spectra of NMTPPH (1.0 × 10−5 mol/L) in the presence of increasing concentration of Zn(II). The concentration of Zn(II) (from bottom to top): 0, 0.5, 0.7, 1, 3, 3.5, 4, 4.5, 5, 6, 7, 8 and 10 M. Measured at 25 ◦ C in CH3 CH2 OH–H2 O (1:1, v/v) and Tris–HCl (0.05 mol/L) buffer (pH 7.10), with ex = 432 nm.
2.2. Preparation of solutions Fig. 1. The structure of NMTPPH.
in the same way as Zn(II) and interferes with the determination of Zn(II). Herein, we report a selective fluoroionophore for Zn(II) based on N-methyltetraphenylporphyrin, which is barely affected by various metal ions except Cu(II). The N-methylporphyrins are a class of porphyrins with a nonplanar coordination site of pyrrolic nitrogens and the rate of complexation of the metal with N-methylporphyrin is much more quickly than that with non-methylated porphyrin [45,46]. The metal complexes of Nmethylporphyrin exhibit very similar visible absorption spectra [47,48]. Lavallee et al. [49] investigated the fluorometric properties of N-methyltetraphenylporphyrin and several derivatives and chose their zinc complexes as standards for determination of chlorophyll a. Tanaka et al. [50] used N-methyltetrakis(4sulfonatophenyl)porphyrin for kinetic determination of copper and zinc by UV–vis spectroscopy in serum. Making use of its relatively fast complexation rate, we studied in detail the use of N-methyl␣,,␥,␦-tetraphenylporphine (NMTPPH, Fig. 1) as a fluorescent probe for zinc ions and showed its outstanding characteristics including large Stokes shifts, nice sensitivity and selectivity for zinc over other metal ions.
A stock solution of 2 × 10−5 mol/L NMTPPH was obtained by dissolving NMTPPH in absolute ethanol. A stock solution of 1 × 10−2 mol/L Zn(II) was prepared by dissolving Zn(OAc)2 ·2H2 O in doubly distilled water. The stock solution of Zn(II) was diluted to lower concentrations of 1 × 10−3 to 1 × 10−7 mol/L stepwise. The wide pH range solutions were prepared by adjustment of 0.05 mol/L Tris–HCl solution with HCl or NaOH solution. The complex solution of Zn(II) and NMTPPH was obtained by mixing 12.5 mL of the stock solution of NMTPPH and 2.5 mL of Zn(II) solution of the different concentrations and diluting the mixture to 25 mL in a volumetric flask. In the solution thus obtained, the concentrations were 1 × 10−5 mol/L in NMTPPH and 1 × 10−4 to 1 × 10−8 mol/L in Zn(II). Blank solution of NMTPPH was prepared under the same conditions without Zn(II). 2.3. Apparatus UV–vis absorption spectra were scanned on a Shimadzu MultiSpec-1501 spectrophotometer. All fluorescence measurements were taken on a PerkinElmer LS 55 Luminescence Spectrometer with excitation slit set at 10.0 nm and emission at 15.0 nm. The measurements of pH were carried out on a MettlerToledo Delta 320 pH meter. Data processing was performed on a Pentium IV computer with software of SigmaPlot.
2. Experimental
2.4. Measurement procedures
2.1. Reagents
The fluorescence intensity was measured at the maximum of excitation wavelength of 432 nm with the emission wavelength varied over 600–750 nm. Before the fluorescence measurement, the complex solution of NMTPPH/Zn(II) was allowed to stand for a few minutes to allow complete formation of metal–ligand complex.
CF3 SO2 OCH3 was obtained from Aldrich Chemical Co. Doubly distilled water was used throughout all experiments. CH2 Cl2 was distilled from calcium hydride and stored over molecular sieves. Before being used, benzaldehyde was simply distilled from K2 CO3 . Pyrrole was distilled at atmospheric pressure from CaH2 . Unless otherwise stated, all other chemicals were of analytical reagent grade and used without further purification. ␣,,␥,␦tetraphenylporphine (TPPH2 ) was prepared from pyrrole and benzaldehyde according to previously described method [51]. NMTPPH was synthesized from tetraphenylporphyrin (TPPH2 ) and CF3 SO2 OCH3 according to documented procedures [49].
3. Results and discussion 3.1. Spectral characteristics The fluorescence emission spectra were recorded from 1 × 10−5 mol/L solution of NMTPPH at 25 ◦ C in CH3 CH2 OH–H2 O (1:1, v/v) and Tris–HCl (0.05 mol/L) buffer (pH 7.10). Fig. 2 shows
Q.-J. Ma et al. / Spectrochimica Acta Part A 71 (2009) 1683–1687
Fig. 3. Absorption spectra of NMTPPH (1.0 × 10−5 mol/L) before (—) and after (- - -) the addition of Zn(OAc)2 ·2H2 O (1.0 × 10−5 mol/L). The inset shows the corresponding spectra magnified between 500 and 700 nm. Measured at 25 ◦ C in CH3 CH2 OH–H2 O (1:1, v/v) and Tris–HCl (0.05 mol/L) buffer (pH 7.10).
the effect of Zn(II) concentration on the fluorescence emission spectra of NMTPPH. As can be seen from Fig. 2, the fluorescence emission spectra of NMTPPH are sensitive to Zn(II). In the presence of various concentration of zinc ion ranging from 5.0 × 10−7 to 1.0 × 10−5 mol/L, significant fluorescence enhancement of fluorescent probe was observed. The fluorescence intensity is enhanced with increasing Zn(II) concentration, which constitutes the basis for the recognition of Zn(II) with fluorescent probe proposed in this work. In the presence of excess of Zn(II), a 3.6-fold fluorescence enhancement at the maximum emission wavelength of 665 nm was observed. This corresponds to the complete conversion of NMTPPH into complexed state with Zn(II). One observes a blue shift of about 20 nm for the emission spectrum peak of NMTPPH from its uncomplexed state (zero Zn(II) concentration) to complexed form. The fluorescence enhancement is caused by the complexation of Zn(II) and the blue shift can be rationalized by the deprotonation of NMTPPH upon Zn(II) binding. The fluorescence quantum yield of NMTPPH in EtOH/H2 O (1:1, v/v) solution is 0.008, as determined by using TPPH2 in Benzene (˚F = 0.11) as a reference [52]. NMTPPH shows larger enhancements (3.4-fold in ˚F ) upon completely coordinating Zn(II). While the mechanism by which this chelation-enhanced fluorescence occurs is still unclear, it is possible that incorporation of Zn(II) enhances the rigidity of NMTPPH thus inhibiting some of non-radiative transitions between electronic states. In order to study the reaction of NMTPPH with Zn(II), the visible absorption spectra of NMTPPH in the absence and presence of Zn(II) were investigated (Fig. 3). From Fig. 3, it can be seen that the absorption spectrum of NMTPPH displays a single maximum in the Soret region and two prominent peaks in the 500–700 nm region and upon zinc binding the spectrum of solution shows a split Soret band and three prominent peaks in the 500–700 nm region. These spectral characteristics coincide with the spectra of NMTPPH and its Zn(II) true complex studied previously [47]. Fig. 4 confirms the 1:1 complexation of NMTPPH with Zn(II). The possible structure of 1:1 complex of NMTPPH and Zn(II) is shown in Fig. 5. The puckered form of ring due to methylation seems to allow rapid metal incorporation [46] and thus decreases the response time of NMTPPH towards Zn(II). The response time for Zn(II) concentration ≤1.0 × 10−5 mol/L is less than 5 min. The linear response of the fluorescence emission intensity toward log CZn(II) was obtained in zinc concentration range of 5.0 × 10−7 to 1.0 × 10−5 mol/L. The response can be expressed by the following
1685
Fig. 4. Plot of log[(F − F0 )/F0 ] as a function of the log CZn(II) . F0 and F are the fluorescence intensity of NMTPPH (1.0 × 10−5 mol/L) in the absence and presence of Zn(II), respectively. Measured at 25 ◦ C in CH3 CH2 OH–H2 O (1:1, v/v) and Tris–HCl (0.05 mol/L) buffer (pH 7.10), with ex = 432 nm.
equation: log
F − F 0
F0
= 1.3779 log CZn(II) + 7.3722 (R = 0.9763)
(1)
Here F is the fluorescence intensity of NMTPPH actually measured at a given metal concentration, F0 denotes the fluorescence intensity of NMTPPH in the absence of zinc metal ion and CZn(II) represents the concentration of zinc ion added. The detection limit was estimated as 1.5 × 10−7 mol/L (calculated as three times standard deviation of blank solution). 3.2. Effect of pH Fig. 6 displays the influence of pH values on the fluorescence emission intensity of fluorescent probe in the presence of Zn(II). The fluorescence intensity measurements were carried out by fixing 1.0 × 10−5 mol/L Zn(II) at various pH values. As can be seen from
Fig. 5. The assumed structure of 1:1 complex of NMTPPH and Zn(II).
1686
Q.-J. Ma et al. / Spectrochimica Acta Part A 71 (2009) 1683–1687
Fig. 7. Fluorescence intensity changes profile of NMTPPH (1.0 × 10−5 mol/L) in the presence of various cations. Cations (10 equiv. relative to NMTPPH) were added as Li2 SO4 ·H2 O, Co(CH3 COO)2 ·4H2 O, NiCl2 ·6H2 O, Mn(CH3 COO)2 ·4H2 O, NaCl, Al2 (SO4 )3 ·18H2 O, Pb(NO3 )2 , AgNO3 , KCl, Ca(CH3 COO)2 ·H2 O, and MgSO4 . Other cations were added as Zn(CH3 COO)2 ·2H2 O (1.0 × 10−5 mol/L), CdCl2 ·2.5H2 O (1.0 × 10−5 mol/L), HgNO3 ·H2 O (1.0 × 10−5 mol/L), FeCl3 ·6H2 O (5.0 × 10−5 mol/L), and CuCl2 ·2H2 O (1.0 × 10−5 mol/L). Measured at 25 ◦ C in CH3 CH2 OH–H2 O (1:1, v/v) and Tris–HCl (0.05 mol/L) buffer (pH 7.10), with ex = 432 nm.
Fig. 6. Effect of pH on the fluorescence intensity of NMTPPH (1.0 × 10−5 mol/L) in the presence of Zn(II) (the concentration of Zn(II) was fixed at 1.0 × 10−5 mol/L). Measured at 25 ◦ C in CH3 CH2 OH–H2 O (1:1, v/v).
Fig. 6, the fluorescence intensity almost did not vary with pH in the range of 6.30–10.50 in CH3 CH2 OH–H2 O (1:1, v/v) solution. When pH is lower than 6.30, the fluorescence intensity decreases with decreasing pH value. At pH higher than 10.50, the fluorescence intensity of the Zn(II)–NMTPP complex also decreases. These experiments can be conveniently explained according to the ligation process. Lower pH leads to protonation of NMTPPH and release of Zn(II) and thus at higher acidity the fluorescence intensity of Zn(II) complex decreases with a red shift of maximum emission wavelength. At higher pH values the concentration of Zn(II) complex decreases owing to the formation of the precipitation of Zn(OH)2 . Taking into consideration the sensitivity and response speed, a pH 7.10 Tris/HCl buffer solution was chosen as optimum experimental condition.
probe was significantly enhanced upon the addition of Zn(II) and was slightly quenched upon binding to Cu(II). Moreover, the addition of other cations did not affect the fluorescence intensity of the fluorescent probe. In order to further test the interference for other common cations on the determination of Zn(II), a competition experiment was performed in which the fluorescent probe was added to a solution of Zn(II) in the presence of other metal ions (Table 1). From Table 1, one can see that the relative error of common species except Cu(II) and Hg(II), such as alkali, alkaline earth, transitional and heavy metal ions, is less than ±5% which is considered as tolerated. Thus, NMTPPH exhibits nice selectivity for Zn(II) over other common cations except Cu(II).
3.3. Evaluation of selectivity To evaluate selectivity of NMTPPH, the fluorescence emission intensity of the fluorescent probe binding to other metal ions was investigated. Fig. 7 demonstrates the selectivity of the fluorescent probe for zinc over various common cations in pH 7.10 Tris–HCl solution. The concentration of Zn(II) was fixed at 1.0 × 10−5 mol/L. Cations were added as chlorides, nitrates, acetate and sulfates. As shown in Fig. 7, the fluorescence intensity of the fluorescent
4. Conclusion In conclusion, a selective fluorescent probe for Zn(II) based on NMTPPH with high selectivity has been reported. The fluorescence emission intensity of NMTPPH is remarkably enhanced upon the addition of zinc which is attributed to the formation of 1:1 complex. The method proposed in this work is simple, rapid, sensitive and
Table 1 Interference of several cations to the fluorescence determination of Zn(II) with NMTPPH Interference +
Li Co2+ Ni2+ Mn2+ Na+ Al3+ Pb2+ Cu2+ Fe3+ Hg2+ Cd2+ Ag+ K+ Ca2+ Mg2+
Concentrationa (mol/L) −4
1.0 × 10 1.0 × 10−4 1.0 × 10−4 1.0 × 10−4 1.0 × 10−4 1.0 × 10−4 1.0 × 10−4 1.0 × 10−5 5.0 × 10−5 1.0 × 10−5 1.0 × 10−5 1.0 × 10−4 1.0 × 10−4 1.0 × 10−4 1.0 × 10−4
Fluorescence (F = F − F0 )b
Relative error (%), (F/F0 ) × 100
1.1151 0.3622 1.2850 1.4911 0.6282 0.4365 1.0017 −45.1434 −2.8169 −9.5198 −0.7692 −2.2692 1.1706 0.5714 0.2833
1.33 0.43 1.20 1.78 0.75 0.52 1.20 −54.00 −3.37 −11.39 −0.92 −2.72 1.40 0.68 0.34
The concentration of Zn(II) is fixed at 1.0 × 10−5 mol/L (pH 7.10). F0 and F indicate the average fluorescence intensities of fluorescent probe NMTPPH (1.0 × 10−5 mol/L) contacting with 1.0 × 10−5 mol/L Zn(II) solution before and after adding the interference (F0 = 83.5671). a
b
Q.-J. Ma et al. / Spectrochimica Acta Part A 71 (2009) 1683–1687
selective. To improve water solubility of NMTPPH without compromising the selectivity, modification of NMTPPH could be achieved by tethering hydrophilic side chains which would allow the study in aqueous and buffer solutions. Further work in this direction is in progress. Acknowledgments This work was supported by the National Natural Science Foundation of China (Grant nos. 20505008 and 20435010), “973” National Key Basic Research Program of China (2007CB310500), Hunan Natural Science Foundation (07JJ3025), and the National 863 High Technology Research Foundation of China (2006AA06Z407). References [1] P.J. Jiang, L.Z. Chen, J. Lin, Q. Liu, J. Ding, X. Gao, Z.J. Guo, Chem. Commun. (2002) 1424. [2] J.M. Berg, Y. Shi, Science 271 (1996) 1071. [3] M.P. Cuajungco, G.J. Lees, Neurobiol. Dis. 4 (1997) 137. [4] A. Voegelin, S. Pfister, A.C. Scheinost, M.A. Marcus, R. Kretzschmar, Environ. Sci. Technol. 39 (2005) 6616. [5] J. Mertens, F. Degryse, D. Springael, E. Smolders, Environ. Sci. Technol. 41 (2007) 2992. [6] C.V. Banks, R.E. Bisque, Anal. Chem. 29 (1957) 522. [7] A.R. Fakhari, M. Shamsipur, Kh. Ghanbari, Anal. Chim. Acta 460 (2002) 177. [8] Q. Li, X.H. Zhao, Q.Z. Lv, G.G. Liu, Sep. Purif. Technol. 55 (2007) 76–81. [9] T. Budde, A. Minta, J.A. White, A.R. Kay, Neuroscience 79 (1997) 347. [10] H. Wang, Q. Gan, X. Wang, L. Xue, S. Liu, H. Jiang, Org. Lett. 9 (2007) 4995. [11] Y. Zhang, X. Guo, W. Si, L. Jia, X. Qian, Org. Lett. 10 (2008) 473. [12] E.M. Nolan, J. Jaworski, K. Okamoto, Y. Hayashi, M. Sheng, S.J. Lippard, J. Am. Chem. Soc. 127 (2005) 16812. [13] G.K. Walkup, S.C. Burdette, S.J. Lippard, R.Y. Tsien, J. Am. Chem. Soc. 122 (2000) 5644. [14] S.C. Burdette, G.K. Walkup, B. Spingler, R.Y. Tsien, S.J. Lippard, J. Am. Chem. Soc. 123 (2001) 7831. [15] T. Hirano, K. Kikuchi, Y. Urano, T. Nagano, J. Am. Chem. Soc. 124 (2002) 6555. [16] S.C. Burdette, C.J. Frederickson, W. Bu, S.J. Lippard, J. Am. Chem. Soc. 125 (2003) 1778. [17] N.C. Lim, C. Brückner, Chem. Commun. (2004) 1094. [18] C.R. Goldsmith, S.J. Lippard, Inorg. Chem. 45 (2006) 555.
1687
[19] T. Koike, T. Watanable, S. Aoki, E. Kimura, M. Shiro, J. Am. Chem. Soc. 118 (1996) 12696. [20] T. Hirano, K. Kikuchi, Y. Urano, T. Higuchi, T. Nagano, Angew. Chem. Int. Ed. 39 (2000) 1052. [21] S. Aoki, S. Kaido, H. Fujioka, E. Kimura, Inorg. Chem. 42 (2003) 1023. [22] A. Bencini, E. Berni, A. Bianchi, P. Fornasari, C. Giorgi, J.C. Lima, C. Lodeiro, M.J. Melo, J.S. Melo, A.J. Parola, F. Pina, J. Pina, B. Valtancoli, Dalton Trans. (2004) 2180. [23] N.C. Lim, J.V. Schuster, M.C. Porto, M.A. Tanudra, L. Yao, H.C. Freake, C. Brückner, Inorg. Chem. 44 (2005) 2018. [24] S. Yunus, S. Charles, F. Dubois, E.V. Donckt, J. Fluoresc. 18 (2008) 499. [25] A.E. Majzoub, C. Cadiou, I. Déchamps-Olivier, F. Chuburu, M. Aplincour, Eur. J. Inorg. Chem. 32 (2007) 5087. [26] W. Goodall, J.A.G. Williams, Chem. Commun. (2001) 2514. [27] C. Goze, G. Ulrich, L. Charbonnière, M. Cesario, T. Prangé, R. Ziessel, Chem. Eur. J. 9 (2003) 3748. [28] O. Reany, T. Gunnlaugsson, D. Parker, Chem. Commun. (2000) 473. [29] O. Reany, T. Gunnlaugsson, D. Parker, J. Chem. Soc., Perkin Trans. 2 (2000) 1819. [30] R. Parkesh, T.C. Lee, T. Gunnlaugsson, Org. Biomol. Chem. 5 (2007) 310. [31] S. Maruyama, K. Kikuchi, T. Hirano, Y. Urano, T. Nagano, J. Am. Chem. Soc. 124 (2002) 10650. [32] M.D. Shults, D.A. Pearce, B. Imperiali, J. Am. Chem. Soc. 125 (2003) 10591. [33] M. Taki, J.L. Wolford, T.V. O’Halloran, J. Am. Chem. Soc. 126 (2004) 712. [34] T. Hirano, K. Kikuchi, Y. Urano, T. Higuchi, T. Nagano, J. Am. Chem. Soc. 122 (2000) 12399. [35] J.N. Ngwendson, A. Banerjee, Tetrahedron Lett. 48 (2007) 7316. [36] Y. Chen, K. Han, Y. Liu, Bioorg. Med. Chem. 15 (2007) 4537. [37] X. Xu, H. Zhang, C. Zhang, J. Cheng, Anal. Chem. 63 (1991) 2529. [38] M. Tabata, M. Kumamoto, J. Nishimoto, Anal. Chem. 68 (1996) 758. [39] Q. Hu, G. Yang, J. Yin, Y. Yao, Talanta 57 (2002) 751. [40] C. Grant, P. Hambright, J. Am. Chem. Soc. 91 (1969) 4195. [41] M. Tabata, M. Tanaka, Inorg. Chem. 27 (1988) 203. [42] Y. Zhang, W. Xiang, R. Yang, F. Liu, K. Li, J. Photochem. Photobiol. A: Chem. 173 (2005) 264. [43] E. Bellacchio, S. Gurrieri, R. Lauceri, A. Magri, L.M. Scolaro, R. Purrello, A. Romeo, Chem. Commun. (1998) 1333. [44] R. Yang, K. Li, K. Wang, F. Zhao, N. Li, F. Liu, Anal. Chem. 75 (2003) 612. [45] B. Shah, B. Shears, P. Hambright, Inorg. Chem. 10 (1971) 1828. [46] M.J. Bain-Ackerman, D.K. Lavallee, Inorg. Chem. 18 (1979) 3358. [47] D.K. Lavallee, Bioinorg. Chem. 6 (1976) 219. [48] D.K. Lavallee, M.J. Bain-Ackerman, Bioinorg. Chem. 9 (1978) 311. [49] D.K. Lavallee, T.J. Mcdonough Jr., L. Cioffi, Appl. Spectrosc. 36 (1982) 431. [50] S. Funahashi, Y. Tto, H. Kakito, M. Inamo, Y. Hamada, M. Tanaka, Mikrochim. Acta 88 (1986) 33. [51] A.D. Adler, F.R. Longo, J.D. Finarelli, J. Org. Chem. 32 (1967) 476. [52] P.G. Seybold, M. Gouterman, J. Mol. Spectrosc. 31 (1969) 1.