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Study on the fluorescence behavior of p-sulfonated calix[4,6]arene in cationic surfactant cetyltrimethylammonium bromide solution and its analytical application
Spectrochimica Acta Part A 66 (2007) 919–923
Study on the fluorescence behavior of p-sulfonated calix[4,6]arene in cationic surfactant cetyltrimethylammonium bromide solution and its analytical application Yunyou Zhou ∗ , Chun Liu, Hongwei Xu, Huapeng Yu, Qin Lu, Lun Wang College of Chemistry and Materials Science, Anhui Key Laboratory of Functional Molecular Solids, Anhui Normal University, Wuhu 241000, China Received 5 January 2006; accepted 28 April 2006
Abstract The fluorescence properties of p-sulfonated calix[4,6]arene (SCnA, n = 4, 6) in cationic surfactant cetyltrimethylammonium bromide (CTAB) solution were investigated. It was found that the fluorescence intensity of SCnA could be enhanced markedly by an appropriate amount of CTAB. The results indicate the formation of complex between CTAB and SCnA at a 1:1 complex stoichiometry and their association constants were calculated by applying a deduced equation. Based on the obtained results, a new fluorimetric method has been developed for rapid determination of SCnA with a good linearity in the concentration range of 2 × 10−7 to 7 × 10−6 mol L−1 . © 2006 Elsevier B.V. All rights reserved. Keywords: p-Sulfonated calix[4,6]arene; Cationic surfactant; CTAB; Complex; Fluorescence
1. Introduction Water-soluble calixarenes are a versatile family of molecules that have attracted much attention in recent years due to their ability to form host–guest arrangements either in solution or the solid state [1]. For example, some water-soluble calix[n]arenes (n = 4, 6 and 8) and resorcinarenes towards quaternary ammonium ions [2,3], trimethylammonium cations [4–6], dyes [7,8], native amino acids [9,10], and small neutral organic molecules [11] have been investigated extensively. Among these water soluble calixarenes, p-sulfonated calix[n]arenes, which have flexible and often poorly defined cavities that tend to bind positively charged species and even have been often viewed as promising water-soluble hosts, have attracted the keen interest of researchers studying different area, such as conformational flexibility [5,12], electrochemical behaviors [13–15], molecular recognitions [11,16], interacting with drugs [17,18] and determination [19]. However, there are few literatures about their natural optical properties in particular fluorescence properties owing to their weak fluorescence in the aqueous solution [20]. Surfactant molecules, which have an ionic end-group as well as a large variable hydrophobic tail, have been used extensively ∗
Corresponding author. Tel.: +86 553 3869303; fax: +86 553 3869303. E-mail address: [email protected] (Y. Zhou).
1386-1425/$ – see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.saa.2006.04.031
in chemical synthesis, fluorescence analysis and electrochemical analysis [21–23]. During the last few years, a number of studies on the interaction between surfactants and cyclodextrins were reported [24–26]. From those results it was concluded that the surfactant hydrophobic chain could enter inside the cyclodextrin hydrophobic cavity to form inclusion complexes with cyclodextrins or their derivatives. However, there are only few reports on the inclusion behavior of calixarenes and their derivatives with surfactants [27]. In this paper, the fluorescence behaviors of p-sulfonated calix[4,6]arene in cationic surfactant of CTAB have been studied. Experimental results reveal that SCnA can form 1:1 complex with cationic surfactants, which lead to the enhancement of fluorescence intensity of SCnA. Based on this, we present a new spectrofluorimetric method for determination of solution concentration of SCnA. The present study may extend the application range water soluble calixarenes in molecular recognitions. 2. Experimental 2.1. Apparatus Fluorescence spectra and relative fluorescence intensities were measured on a model F-4500 fluorescence spectrophotometer (Hitachi, Japan) using a conventional 1 cm × 1 cm
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Y. Zhou et al. / Spectrochimica Acta Part A 66 (2007) 919–923
quartz cell. Excitation and emission bandwidths were set to 5 and 10 nm, respectively. All measurements were carried out at room temperature. A model pHS-3C (Dazhong Analytical Instruments Factory, Shanghai, China) pH meter was used for accurate adjustment of pH. 2.2. Reagents All reagents used were of analytical-reagent grade or the best grade commercially. Doubly distilled water was used throughout. p-Sulfonated calix[4,6]arene were synthesized according to the literature [28] and identified by IR, 1 H NMR and element analysis. Stock solutions of SCnA were prepared as 1 × 10−3 mol L−1 . CTAB was obtained from Shanghai Chemical Reagent Co., China, and 1 × 10−3 mol L−1 stock solution was prepared in water. A 0.2 wt.% stock solution of PDDA (weight, 20 wt.% in water, Aldrich) was made. All the working solutions were prepared by diluting the stock solutions to the concentration required. Buffer solution (pH 6.7) was prepared by mixing 0.2 mol L−1 sodium monohydrogen orthophosphate solution with 0.2 mol L−1 sodium dihydrogen orthophosphate solution. 2.3. Procedure 2.3.1. Inclusion process A 0.5 mL of 1 × 10−4 mol L−1 SC4A or SC6A solution, 1 mL of phosphate buffer (pH 6.7) solution and appropriate volumes of 1 × 10−5 mol L−1 CTAB were transferred into a 10 mL volumetric flask in turn. The mixed solution was diluted to final volume with water and stirred thoroughly; the fluorescence intensities were determined after 15 min−1 at room temperature.
Fig. 1. Influence of pH value on the fluorescence intensity of SC6A–CTAB complex; CSC6A = 5 × 10−6 mol L−1 , CCTAB = 3 × 10−5 mol L−1 .
Fig. 2 shows the fluorescence excitation and emission spectra of 5 × 10−5 mol L−1 SCnA in phosphate buffer (pH 6.7). As can be seen it has weak fluorescence with excitation and emission wavelengths of 265 and 426 nm, respectively. When an appropriate amount of 1 × 10−4 mol L−1 CTAB was added into it, the fluorescence intensity increases remarkably. Meanwhile, the excitation peak has a blue shift (265 → 250.4 nm) and the emission peak exhibits a red shift (426 → 440 nm) (see Fig. 3A). Similarly, proper amount of 1 × 10−4 mol L−1 CTAB can enhance the fluorescence intensity of SC4A, too (see Fig. 3B). The excitation and emission peak also show a certain shifts, respectively. 3.2. Discussion of the fluorescence behavior
2.3.2. Determination of SCnA Into a 10 mL volumetric flask were placed in turn appropriate volumes of 1 × 10−5 mol L−1 SC4A or SC6A solution, 1 mL of phosphate buffer (pH 6.7) solution and 0.4 mL 1 × 10−4 mol L−1 CTAB. Then the same operations as mentioned above were performed.
To understand the fluorescence phenomena of p-sulfonated calix[n]arene in micellar solution, on the one hand, the inter-
3. Results and discussion 3.1. Fluorescence spectra of systems In order to study the fluorescence properties of systems, different pH values were adjusted using different buffers such as citric acid–Na2 HPO4 (pH 2–5), Na2 HPO4 –NaH2 PO4 (pH 6–8), NaB4 O7 –NaOH (pH 9–10) (Fig. 1). It can be seen that the relative fluorescence intensity increases slightly when the value of pH is above 6. Kunsagi-Mate et al. also reported that SC6A could provide excellent conditions for the investigation of its host properties between pH 6 and 8.5[29]. In this work, all the data were measured in a pH 6.7 Na2 HPO4 –NaH2 PO4 buffer medium. We also found the influence of doses of buffer on the fluorescence intensity was neglectable, so 1 mL of phosphate buffer was used in the systems.
Fig. 2. Fluorescence excitation (left) and emission (right) spectra of SC6A (a and a ) and SC4A (b and b ) (5 × 10−5 mol L−1 ) in phosphate buffer (pH 6.7) solution.
Y. Zhou et al. / Spectrochimica Acta Part A 66 (2007) 919–923
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Fig. 3. (A) Fluorescence excitation (left) and emission (right) spectra of 5 × 10−6 mol L−1 SC6A in phosphate buffer (pH 6.7) with different concentrations of CTAB: from 0 to 9: 0.0; 0.25; 0.5; 0.8; 1.0; 1.3; 1.5; 1.8; 2.0; 2.5 × 10−5 mol L−1 . (B) Fluorescence excitation (left) and emission (right) spectra of 5 × 10−6 mol L−1 SC4A in phosphate buffer (pH 6.7) with different concentrations of CTAB: from 0 to 8: 0.1; 0.2; 0.4; 0.6; 1.0; 1.5; 2.0; 3.0; 4.0 × 10−5 mol L−1 .
action between 4-phenolsulfonate (the monomer of SCnA) and CTAB has been investigated. It was found that the fluorescence spectra of 4-phenolsulfonate could be influenced hardly when various concentrations of CTAB solutions were added, although CTAB is usually used to enhance the fluorescence of some sulfuric fluorophores [30–32]. Compared 4-phenolsulfonate with SCnA, the obvious difference between them is that the latter has a cavity. The macrocyclic structure of the ligands obviously can play an important role for the complex formation. These indicate probably the formation of host–guest complexes between SCnA and CTAB would be the origin of the spectral changes described above. On the other hand, other cationic surfactants such as dodecyltrimethylammonium bromide (DTAB), cetylpyridinium bromide (CPB) and polydiallyldimethylammonium chloride (its structure is shown in Fig. 4, PDDA) were also selected to interact with SCnA under the same conditions. It was found that the fluorescence intensity of SCnA increased more evidently by addition of proper amount of PDDA and DTAB solution like CTAB, compared with the CPB for which natural fluorescence peak (460 nm) is quite close to that of SCnA (426 nm). That suggests that all of these long-alkyl trimethylammonium cationic surfactants can form complexes with SCnA and enhance their fluorescence intensities.
It is well known that p-sulfonated calix[n]arenes can form non-covalent inclusion complexes with quaternary ammonium ions [2] and trimethylammonium cations [4] with the aid of electrostatic interaction, hydrogen bonding, van der Waals, hydrophobic interaction, and so on. We proposed the probable complex manner between SCnA and CTAB: In aqueous solution, SCnA adopts a truncated cone conformation (SC4A) or an up-down double partial cone conformation (SC6A), which is suited to form bi-layer arrangement [33,34]. When CTAB were added, the ammonium cationic ion of CTAB could bind with the negatively charged sulphonyl groups of SCnA to form salt with the help of electrostatic interaction and the substituted methyl of N atom may enter (or partially enter) into the cavities of SCnA by means of hydrophobic interaction, then stabilized
Fig. 4. Structures of p-sulfonated calix[4,6]arene (1), 4-phenolsulfonate (2), polydiallyldimethylammonium chloride (PDDA, 3).
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Y. Zhou et al. / Spectrochimica Acta Part A 66 (2007) 919–923 Table 1 Parameters obtained from Fig. 5 Complexes
Complex constant, K (L mol −1 )
Complex ratio, N
R
SC4A–CATB SC6A–CTAB
2.25 × 104 1.15 × 104
1 1
0.9981 0.9997
The concentrations of SC4A and SC6A are 5 × 10−6 mol L−1 , pH 6.7 phosphate buffer.
Fig. 5. A graph of the 1/If vs. 1/[H]0 for SC4A–CTAB (a) and SC6A–CTAB (b) complexes.
by the CH– interaction between the methyl and aromatic ring [35,36]. Thus the alkyl of CTAB can but stay outside of the SCnA as the extended long chain of complexes. It is imaginable that the structure of the complexes is so similar to the one of surfactants that they may display micelle-like state in aqueous solution, which probably makes the fluorescence spectral of SCnA change. Besides, the remaining of CTAB molecules that occupy interstitial spaces among the complexes may provide a special microenvironment for SCnA. Therefore, the formations of host–guest complexes as well as microenvironment were thought to play main roles in enhancement of fluorescence intensity of SCnA. 3.3. Stoichiometry and complex constant In this paper, a similar equation of inclusion constant Kn of the complex with one guest–multiple host was used to calculate the inclusion constant: 1/If = (1/Kn α)(1/[H]n0 ) + (1/α) [37] In which, If = Ifh–g − Ifg − Ifh , where Ifh–g , Ifg , and Ifh are the fluorescence intensity of the host–guest complex, the guest molecule and the host molecule, respectively. [H]0 is the original concentration of the CTAB, n the number of host molecule(s) in a complex and α is a constant. By drawing the 1/If versus 1/[H]n0 graph with different n, the n that results a straight line can be taken as the number of host molecules, consequently, the inclusion ratio can be obtained as 1:n. A similar way to decide the inclusion ratio was also used in cyclodextrin system by Cervero and Mendicuti [38]. From the intercept and the slope of the straight line, the inclusion constant can be obtained. From Fig. 5, the inclusion constants of SC4A (Kna ) and SC6A (Knb ) can be discovered (see Table 1), and the results indicate that the composition ratio of the inclusion complex is 1:1 for SC4A or SC6A:CTAB.
Fig. 6. The effect of CTAB concentration on the fluorescence intensity of system, CSC4A or CSC6A = 5 × 10−6 mol L−1 , pH 6.7.
Fig. 6). The fluorescence intensity increases sharply with the addition of CTAB and then changes negligibly when the concentration of CTAB is beyond 3 × 10−5 mol L−1 and even up to its critical micelle concentration (9.2 × 10−4 mol L−1 , 25 ◦ C). That also reveals the enhancement of fluorescence intensity of SCnA is mainly due to the formation of complexes between SCnA and CTAB, not only simple micelle enhancement effect. Therefore, 4 × 10−5 mol L−1 CTAB was adopted in the system. Based on the obtained results, the calibration graphs and sensitivity of determining SCnA were found according to the experimental procedure mentioned above (see Fig. 7). Those equations, along with linear range and regression constant are
3.4. Calibration graphs of determining SCnA To obtain best working curves, the effect of CTAB concentration on the fluorescence intensity of system was examined (see
Fig. 7. Calibration graphs of SC4A–CTAB () and SC6A–CTAB ().
Y. Zhou et al. / Spectrochimica Acta Part A 66 (2007) 919–923 Table 2 The parameters for determination of SC[4,6]A SCnA
Linear range (mol L−1 )
Regression equation, C (mol L−1 )
R
SC4A SC6A
0.2–7.0 0.2–6.0
If = 28.47 + 191.77C If = 13.88 + 320.95C
0.9990 0.9990
CCTAB = 4 × 10−5 mol L−1 , pH 6.7.
summarized in Table 2, where If is the fluorescence increasing intensity and C is the concentration of SC4A or SC6A. The data show there is good linearity for determining SCnA. 4. Conclusion The fluorescence characteristics of p-sulfonated calix[4,6]arene in cationic surfactant of CTAB solution have been studied. The changes of the fluorescence spectra of p-sulfonated calix[4,6]arene owing to the formation of complexes with CTAB at a 1:1 complex ratio were investigated and the complex constants were also calculated by a deduced equation. In addition, according to the reported results, a novel useful spectrofluorimetric assay for the microanalysis of psulfonated calix[4,6]arene was developed, in concentration range of 2 × 10−7 to 7 × 10−6 mol L−1 . We hope this work may provide some useful information for investigating the interaction between p-sulfonated calix[n]arene and surfactants. The complexes with well fluorescence, which avoid modifying fluorophores on the upper or down rim of calixarenes by organic synthesis, are expected to be applied in molecule recognition as a new water-soluble and fluorescent probes. In order for a better understanding of the binding processes and the analytical applications further studies are in progress. Acknowledgement The authors acknowledge the generous support of this research from the NSFC (No. 20375001). References [1] Z. Asfari, V. Bohmer, Calixarenes 2001, Kluwer Academic Publications, Dordrecht, 2001. [2] G. Arena, A. Casnati, L. Mirone, D. Sciotto, R. Ungaro, Tetrahedron Lett. 38 (1997) 1999. [3] G. Arena, A. Casnati, A. Contino, F.G. Gulino, D. Sciotto, R. Ungaro, J. Chem. Soc.: Perkin Trans. II (2000) 419. [4] S. Shinkai, K. Araki, O. Manabe, J. Am. Chem. Soc. 110 (1988) 7214. [5] S. Shinkai, K. Araki, T. Matsuda, N. Nishiyama, H. Ikeda, I. Takasu, M. Iwamoto, J. Am. Chem. Soc. 112 (1990) 9053.
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[6] G. Arena, S. Gentile, F.G. Gulino, D. Sciotto, C. Sgarlata, Tetrahedron Lett. 45 (2004) 7091. [7] Y. Liu, B. Han, Y. Chen, J. Org. Chem. 65 (2000) 6227. [8] S. Shinkai, S. Mori, H. Koreishi, T. Tsubaki, O. Manabe, J. Am. Chem. Soc. 108 (1986) 2409. [9] G. Arena, A. Contino, F.G. Gulino, A. Magri, F. Sansone, D. Sciotto, R. Ungaro, Tetrahedron Lett. 40 (1999) 1597. [10] F. Sansone, S. Barboso, A. Casnati, D. Sciotto, R. Ungaro, Tetrahedron Lett. 40 (1999) 4741. [11] G. Arena, A. Contino, F.G. Gulino, A. Magri, D. Sciotto, R. Ungaro, Tetrahedron Lett. 41 (2000) 9327. [12] J. Alvarez, Y. Wang, W. Ong, A.E. Kaifer, J. Supramol. Chem. 1 (2001) 269. [13] G. Diao, Z. Wei, J. Electroanal. Chem. 567 (2004) 325. [14] A. Pailleret, N. Magan-Oliva, S. Ollivier, D.W.M. Arrigan, J. Electroanal. Chem. 508 (2001) 81. [15] A. Pailleret, D.W.M. Arrigan, Langmuir 18 (2002) 9447. [16] M. Selkti, A.W. Coleman, I. Nicolis, N. Douteau-Guevel, F. Villain, A. Tomas, C. de Rango, Chem. Commun. (2000) 161. [17] J.S. Millership, J. Incl. Phenom. Macro. Chem. 39 (2001) 327. [18] Y.Y. Zhou, Q. Lu, Ch. Liu, S.K. She, L. Wang, Anal. Chim. Acta 552 (2005) 152. [19] C.F. Rousseau, S. Cecillon, E. Da Silva, A.M. Freyria, D. Herbage, E. Leclere, A.W. Coleman, J. Incl. Phenom. Macro. Chem. 53 (2005) 9. [20] S. Kunsagi-Mate, K. Szabo, B. Lemli, I. Bitter, G. Nagy, L. Kollar, Tetrahedron Lett. 45 (2004) 1387. [21] S. Wu, D.H. Chen, J. Colloid Interf. Sci. 273 (2004) 165. [22] N.O. Mchedlov-Petrossyan, N.A. Vodolazkaya, A.O. Doroshenko, J. Fluoresc. 13 (2003) 3. [23] H. Gharibi, S. Jalili, T. Rajabi, Colloids Surf. A: Physicochem. Eng. Aspects 175 (2000) 361. [24] D.J. Jobe, R.E. Verrall, E. Junquera, E. Aicart, J. Colloid Interf. Sci. 189 (1997) 294. [25] M. Valero, A.D. Arco-Gomez, L.J. Rodriguez, J. Incl. Phenom. Macro. Chem. 42 (2002) 121. [26] E. Memisoglu, A. Bochot, M. Sen, D. Duchene, A.A. Hncal, Int. J. Pharm. 251 (2003) 143. [27] W.H. Chan, A.W.M. Lee, J.Zh. Lu, X.J. Wu, Anal. Chim. Acta 370 (1998) 259. [28] C.D. Gutsche, L.G. Lin, Tetrahedron 42 (1986) 1633. [29] S. Kunsagi-Mate, K. Szabo, B. Lemli, I. Bitter, G. Nagy, L. Kollar, Thermochim. Acta 425 (2005) 121. [30] J.D. Zhao, Y.L. He, X.G. Qian, Zh.Y. Zhao, F.P. Wang, Chin. J. Anal. Chem. 22 (1994) 1057. [31] Zh.Y. Zhao, M.F. Huang, Y.L. He, Chin. J. Anal. Chem. 12 (1997) 1464. [32] X.Y. Li, J.W. Zhao, K. Zhu, Y. Wang, Sh.Zh. Liu, Y.Zh. Guo, J. KunMin Univ. Sci. Technol. 28 (2003) 25. [33] G. Arena, A. Casnati, L. Mirone, D. Sciotto, R. Ungaro, Tetrahedron Lett. 11 (1997) 1999. [34] J.L. Atwood, D.L. Clark, R.K. Juneja, G.W. Orr, K.D. Robinson, R.L. Vincent, J. Am. Chem. Soc. 114 (1992) 7558. [35] M. Nishio, Y. Umezawa, M. Hurita, Y. Takeuchi, Tetrahedron 51 (1995) 8665. [36] J.L. Atwood, S.J. Dalgarno, M.J. Hardie, C.L. Raston, Chem. Commun. (2005) 337. [37] J.H. Yang, F. Huang, M. Wang, X. Wu, C.X. Sun, Spectrochim. Acta 58A (2002) 1775. [38] M. Cervero, F. Mendicuti, J. Phys. Chem. B 104 (2000) 1572.
Study on the fluorescence behavior of p-sulfonated calix[4,6]arene in cationic surfactant cetyltrimethylammonium bromide solution and its analytical application Yunyou Zhou ∗ , Chun Liu, Hongwei Xu, Huapeng Yu, Qin Lu, Lun Wang College of Chemistry and Materials Science, Anhui Key Laboratory of Functional Molecular Solids, Anhui Normal University, Wuhu 241000, China Received 5 January 2006; accepted 28 April 2006
Abstract The fluorescence properties of p-sulfonated calix[4,6]arene (SCnA, n = 4, 6) in cationic surfactant cetyltrimethylammonium bromide (CTAB) solution were investigated. It was found that the fluorescence intensity of SCnA could be enhanced markedly by an appropriate amount of CTAB. The results indicate the formation of complex between CTAB and SCnA at a 1:1 complex stoichiometry and their association constants were calculated by applying a deduced equation. Based on the obtained results, a new fluorimetric method has been developed for rapid determination of SCnA with a good linearity in the concentration range of 2 × 10−7 to 7 × 10−6 mol L−1 . © 2006 Elsevier B.V. All rights reserved. Keywords: p-Sulfonated calix[4,6]arene; Cationic surfactant; CTAB; Complex; Fluorescence
1. Introduction Water-soluble calixarenes are a versatile family of molecules that have attracted much attention in recent years due to their ability to form host–guest arrangements either in solution or the solid state [1]. For example, some water-soluble calix[n]arenes (n = 4, 6 and 8) and resorcinarenes towards quaternary ammonium ions [2,3], trimethylammonium cations [4–6], dyes [7,8], native amino acids [9,10], and small neutral organic molecules [11] have been investigated extensively. Among these water soluble calixarenes, p-sulfonated calix[n]arenes, which have flexible and often poorly defined cavities that tend to bind positively charged species and even have been often viewed as promising water-soluble hosts, have attracted the keen interest of researchers studying different area, such as conformational flexibility [5,12], electrochemical behaviors [13–15], molecular recognitions [11,16], interacting with drugs [17,18] and determination [19]. However, there are few literatures about their natural optical properties in particular fluorescence properties owing to their weak fluorescence in the aqueous solution [20]. Surfactant molecules, which have an ionic end-group as well as a large variable hydrophobic tail, have been used extensively ∗
Corresponding author. Tel.: +86 553 3869303; fax: +86 553 3869303. E-mail address: [email protected] (Y. Zhou).
1386-1425/$ – see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.saa.2006.04.031
in chemical synthesis, fluorescence analysis and electrochemical analysis [21–23]. During the last few years, a number of studies on the interaction between surfactants and cyclodextrins were reported [24–26]. From those results it was concluded that the surfactant hydrophobic chain could enter inside the cyclodextrin hydrophobic cavity to form inclusion complexes with cyclodextrins or their derivatives. However, there are only few reports on the inclusion behavior of calixarenes and their derivatives with surfactants [27]. In this paper, the fluorescence behaviors of p-sulfonated calix[4,6]arene in cationic surfactant of CTAB have been studied. Experimental results reveal that SCnA can form 1:1 complex with cationic surfactants, which lead to the enhancement of fluorescence intensity of SCnA. Based on this, we present a new spectrofluorimetric method for determination of solution concentration of SCnA. The present study may extend the application range water soluble calixarenes in molecular recognitions. 2. Experimental 2.1. Apparatus Fluorescence spectra and relative fluorescence intensities were measured on a model F-4500 fluorescence spectrophotometer (Hitachi, Japan) using a conventional 1 cm × 1 cm
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Y. Zhou et al. / Spectrochimica Acta Part A 66 (2007) 919–923
quartz cell. Excitation and emission bandwidths were set to 5 and 10 nm, respectively. All measurements were carried out at room temperature. A model pHS-3C (Dazhong Analytical Instruments Factory, Shanghai, China) pH meter was used for accurate adjustment of pH. 2.2. Reagents All reagents used were of analytical-reagent grade or the best grade commercially. Doubly distilled water was used throughout. p-Sulfonated calix[4,6]arene were synthesized according to the literature [28] and identified by IR, 1 H NMR and element analysis. Stock solutions of SCnA were prepared as 1 × 10−3 mol L−1 . CTAB was obtained from Shanghai Chemical Reagent Co., China, and 1 × 10−3 mol L−1 stock solution was prepared in water. A 0.2 wt.% stock solution of PDDA (weight, 20 wt.% in water, Aldrich) was made. All the working solutions were prepared by diluting the stock solutions to the concentration required. Buffer solution (pH 6.7) was prepared by mixing 0.2 mol L−1 sodium monohydrogen orthophosphate solution with 0.2 mol L−1 sodium dihydrogen orthophosphate solution. 2.3. Procedure 2.3.1. Inclusion process A 0.5 mL of 1 × 10−4 mol L−1 SC4A or SC6A solution, 1 mL of phosphate buffer (pH 6.7) solution and appropriate volumes of 1 × 10−5 mol L−1 CTAB were transferred into a 10 mL volumetric flask in turn. The mixed solution was diluted to final volume with water and stirred thoroughly; the fluorescence intensities were determined after 15 min−1 at room temperature.
Fig. 1. Influence of pH value on the fluorescence intensity of SC6A–CTAB complex; CSC6A = 5 × 10−6 mol L−1 , CCTAB = 3 × 10−5 mol L−1 .
Fig. 2 shows the fluorescence excitation and emission spectra of 5 × 10−5 mol L−1 SCnA in phosphate buffer (pH 6.7). As can be seen it has weak fluorescence with excitation and emission wavelengths of 265 and 426 nm, respectively. When an appropriate amount of 1 × 10−4 mol L−1 CTAB was added into it, the fluorescence intensity increases remarkably. Meanwhile, the excitation peak has a blue shift (265 → 250.4 nm) and the emission peak exhibits a red shift (426 → 440 nm) (see Fig. 3A). Similarly, proper amount of 1 × 10−4 mol L−1 CTAB can enhance the fluorescence intensity of SC4A, too (see Fig. 3B). The excitation and emission peak also show a certain shifts, respectively. 3.2. Discussion of the fluorescence behavior
2.3.2. Determination of SCnA Into a 10 mL volumetric flask were placed in turn appropriate volumes of 1 × 10−5 mol L−1 SC4A or SC6A solution, 1 mL of phosphate buffer (pH 6.7) solution and 0.4 mL 1 × 10−4 mol L−1 CTAB. Then the same operations as mentioned above were performed.
To understand the fluorescence phenomena of p-sulfonated calix[n]arene in micellar solution, on the one hand, the inter-
3. Results and discussion 3.1. Fluorescence spectra of systems In order to study the fluorescence properties of systems, different pH values were adjusted using different buffers such as citric acid–Na2 HPO4 (pH 2–5), Na2 HPO4 –NaH2 PO4 (pH 6–8), NaB4 O7 –NaOH (pH 9–10) (Fig. 1). It can be seen that the relative fluorescence intensity increases slightly when the value of pH is above 6. Kunsagi-Mate et al. also reported that SC6A could provide excellent conditions for the investigation of its host properties between pH 6 and 8.5[29]. In this work, all the data were measured in a pH 6.7 Na2 HPO4 –NaH2 PO4 buffer medium. We also found the influence of doses of buffer on the fluorescence intensity was neglectable, so 1 mL of phosphate buffer was used in the systems.
Fig. 2. Fluorescence excitation (left) and emission (right) spectra of SC6A (a and a ) and SC4A (b and b ) (5 × 10−5 mol L−1 ) in phosphate buffer (pH 6.7) solution.
Y. Zhou et al. / Spectrochimica Acta Part A 66 (2007) 919–923
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Fig. 3. (A) Fluorescence excitation (left) and emission (right) spectra of 5 × 10−6 mol L−1 SC6A in phosphate buffer (pH 6.7) with different concentrations of CTAB: from 0 to 9: 0.0; 0.25; 0.5; 0.8; 1.0; 1.3; 1.5; 1.8; 2.0; 2.5 × 10−5 mol L−1 . (B) Fluorescence excitation (left) and emission (right) spectra of 5 × 10−6 mol L−1 SC4A in phosphate buffer (pH 6.7) with different concentrations of CTAB: from 0 to 8: 0.1; 0.2; 0.4; 0.6; 1.0; 1.5; 2.0; 3.0; 4.0 × 10−5 mol L−1 .
action between 4-phenolsulfonate (the monomer of SCnA) and CTAB has been investigated. It was found that the fluorescence spectra of 4-phenolsulfonate could be influenced hardly when various concentrations of CTAB solutions were added, although CTAB is usually used to enhance the fluorescence of some sulfuric fluorophores [30–32]. Compared 4-phenolsulfonate with SCnA, the obvious difference between them is that the latter has a cavity. The macrocyclic structure of the ligands obviously can play an important role for the complex formation. These indicate probably the formation of host–guest complexes between SCnA and CTAB would be the origin of the spectral changes described above. On the other hand, other cationic surfactants such as dodecyltrimethylammonium bromide (DTAB), cetylpyridinium bromide (CPB) and polydiallyldimethylammonium chloride (its structure is shown in Fig. 4, PDDA) were also selected to interact with SCnA under the same conditions. It was found that the fluorescence intensity of SCnA increased more evidently by addition of proper amount of PDDA and DTAB solution like CTAB, compared with the CPB for which natural fluorescence peak (460 nm) is quite close to that of SCnA (426 nm). That suggests that all of these long-alkyl trimethylammonium cationic surfactants can form complexes with SCnA and enhance their fluorescence intensities.
It is well known that p-sulfonated calix[n]arenes can form non-covalent inclusion complexes with quaternary ammonium ions [2] and trimethylammonium cations [4] with the aid of electrostatic interaction, hydrogen bonding, van der Waals, hydrophobic interaction, and so on. We proposed the probable complex manner between SCnA and CTAB: In aqueous solution, SCnA adopts a truncated cone conformation (SC4A) or an up-down double partial cone conformation (SC6A), which is suited to form bi-layer arrangement [33,34]. When CTAB were added, the ammonium cationic ion of CTAB could bind with the negatively charged sulphonyl groups of SCnA to form salt with the help of electrostatic interaction and the substituted methyl of N atom may enter (or partially enter) into the cavities of SCnA by means of hydrophobic interaction, then stabilized
Fig. 4. Structures of p-sulfonated calix[4,6]arene (1), 4-phenolsulfonate (2), polydiallyldimethylammonium chloride (PDDA, 3).
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Y. Zhou et al. / Spectrochimica Acta Part A 66 (2007) 919–923 Table 1 Parameters obtained from Fig. 5 Complexes
Complex constant, K (L mol −1 )
Complex ratio, N
R
SC4A–CATB SC6A–CTAB
2.25 × 104 1.15 × 104
1 1
0.9981 0.9997
The concentrations of SC4A and SC6A are 5 × 10−6 mol L−1 , pH 6.7 phosphate buffer.
Fig. 5. A graph of the 1/If vs. 1/[H]0 for SC4A–CTAB (a) and SC6A–CTAB (b) complexes.
by the CH– interaction between the methyl and aromatic ring [35,36]. Thus the alkyl of CTAB can but stay outside of the SCnA as the extended long chain of complexes. It is imaginable that the structure of the complexes is so similar to the one of surfactants that they may display micelle-like state in aqueous solution, which probably makes the fluorescence spectral of SCnA change. Besides, the remaining of CTAB molecules that occupy interstitial spaces among the complexes may provide a special microenvironment for SCnA. Therefore, the formations of host–guest complexes as well as microenvironment were thought to play main roles in enhancement of fluorescence intensity of SCnA. 3.3. Stoichiometry and complex constant In this paper, a similar equation of inclusion constant Kn of the complex with one guest–multiple host was used to calculate the inclusion constant: 1/If = (1/Kn α)(1/[H]n0 ) + (1/α) [37] In which, If = Ifh–g − Ifg − Ifh , where Ifh–g , Ifg , and Ifh are the fluorescence intensity of the host–guest complex, the guest molecule and the host molecule, respectively. [H]0 is the original concentration of the CTAB, n the number of host molecule(s) in a complex and α is a constant. By drawing the 1/If versus 1/[H]n0 graph with different n, the n that results a straight line can be taken as the number of host molecules, consequently, the inclusion ratio can be obtained as 1:n. A similar way to decide the inclusion ratio was also used in cyclodextrin system by Cervero and Mendicuti [38]. From the intercept and the slope of the straight line, the inclusion constant can be obtained. From Fig. 5, the inclusion constants of SC4A (Kna ) and SC6A (Knb ) can be discovered (see Table 1), and the results indicate that the composition ratio of the inclusion complex is 1:1 for SC4A or SC6A:CTAB.
Fig. 6. The effect of CTAB concentration on the fluorescence intensity of system, CSC4A or CSC6A = 5 × 10−6 mol L−1 , pH 6.7.
Fig. 6). The fluorescence intensity increases sharply with the addition of CTAB and then changes negligibly when the concentration of CTAB is beyond 3 × 10−5 mol L−1 and even up to its critical micelle concentration (9.2 × 10−4 mol L−1 , 25 ◦ C). That also reveals the enhancement of fluorescence intensity of SCnA is mainly due to the formation of complexes between SCnA and CTAB, not only simple micelle enhancement effect. Therefore, 4 × 10−5 mol L−1 CTAB was adopted in the system. Based on the obtained results, the calibration graphs and sensitivity of determining SCnA were found according to the experimental procedure mentioned above (see Fig. 7). Those equations, along with linear range and regression constant are
3.4. Calibration graphs of determining SCnA To obtain best working curves, the effect of CTAB concentration on the fluorescence intensity of system was examined (see
Fig. 7. Calibration graphs of SC4A–CTAB () and SC6A–CTAB ().
Y. Zhou et al. / Spectrochimica Acta Part A 66 (2007) 919–923 Table 2 The parameters for determination of SC[4,6]A SCnA
Linear range (mol L−1 )
Regression equation, C (mol L−1 )
R
SC4A SC6A
0.2–7.0 0.2–6.0
If = 28.47 + 191.77C If = 13.88 + 320.95C
0.9990 0.9990
CCTAB = 4 × 10−5 mol L−1 , pH 6.7.
summarized in Table 2, where If is the fluorescence increasing intensity and C is the concentration of SC4A or SC6A. The data show there is good linearity for determining SCnA. 4. Conclusion The fluorescence characteristics of p-sulfonated calix[4,6]arene in cationic surfactant of CTAB solution have been studied. The changes of the fluorescence spectra of p-sulfonated calix[4,6]arene owing to the formation of complexes with CTAB at a 1:1 complex ratio were investigated and the complex constants were also calculated by a deduced equation. In addition, according to the reported results, a novel useful spectrofluorimetric assay for the microanalysis of psulfonated calix[4,6]arene was developed, in concentration range of 2 × 10−7 to 7 × 10−6 mol L−1 . We hope this work may provide some useful information for investigating the interaction between p-sulfonated calix[n]arene and surfactants. The complexes with well fluorescence, which avoid modifying fluorophores on the upper or down rim of calixarenes by organic synthesis, are expected to be applied in molecule recognition as a new water-soluble and fluorescent probes. In order for a better understanding of the binding processes and the analytical applications further studies are in progress. Acknowledgement The authors acknowledge the generous support of this research from the NSFC (No. 20375001). References [1] Z. Asfari, V. Bohmer, Calixarenes 2001, Kluwer Academic Publications, Dordrecht, 2001. [2] G. Arena, A. Casnati, L. Mirone, D. Sciotto, R. Ungaro, Tetrahedron Lett. 38 (1997) 1999. [3] G. Arena, A. Casnati, A. Contino, F.G. Gulino, D. Sciotto, R. Ungaro, J. Chem. Soc.: Perkin Trans. II (2000) 419. [4] S. Shinkai, K. Araki, O. Manabe, J. Am. Chem. Soc. 110 (1988) 7214. [5] S. Shinkai, K. Araki, T. Matsuda, N. Nishiyama, H. Ikeda, I. Takasu, M. Iwamoto, J. Am. Chem. Soc. 112 (1990) 9053.
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