- Email: [email protected]
A novel spectrofluorometric method for the determination of methiocarb using an amphiphilic p-sulfonatocalix[4]arene
Spectrochimica Acta Part A 81 (2011) 44–47
Contents lists available at ScienceDirect
Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy journal homepage: www.elsevier.com/locate/saa
A novel spectrofluorometric method for the determination of methiocarb using an amphiphilic p-sulfonatocalix[4]arene Xue-Ping Ding, Dong-Bao Tang, Tao Li, Su-Fan Wang, Yun-You Zhou ∗ Anhui Key Laboratory of Chemo-Biosensing, College of Chemistry and Materials Science, Anhui Normal University, Wuhu 241000, PR China
a r t i c l e
i n f o
Article history: Received 31 January 2011 Received in revised form 23 April 2011 Accepted 16 May 2011 Keywords: p-Sulfonatocalix[4]arene Methiocarb Fluorescence Inclusion interaction
a b s t r a c t The characteristics of host–guest complexation between tetrabutyl ether derivatives of psulfonatocalix[4]arene (SC4Bu) and methiocarb [3,5-dimethyl-4-(methylthio) phenyl methylcarbamate] were investigated by fluorescence spectrometry. Upon addition of methiocarb, the fluorescence intensity of SC4Bu was quenched regularly and a slight red shift was observed for the maximum emission peak. These results indicated that the SC4Bu-methiocarb complex was formed a 1:1 mole ratio. An association constant of 1.67 × 104 L mol−1 was calculated by applying a deduced equation. The interaction mechanism of the inclusion complex was discussed. Based on the results, a novel spectrofluorimetric method was described for the determination of methiocarb with a detection limit at 0.05 g mL−1 . This method is very simple and shows high sensitivity and selectivity. Moreover, the proposed method was successfully applied to the determination of methiocarb in water samples. © 2011 Elsevier B.V. All rights reserved.
1. Introduction Methiocarb [3,5-dimethyl-4-(methylthio) phenyl methylcarbamate] (Scheme 1),one of the mostly important N-methylcarbamate pesticide, is used worldwide in agriculture and health programes [1]. Recent years, methiocarb has been attracting increasing attention because its potential toxicity for animals and humans [1–4]. Common methodologies used for methiocarb detection are mainly based on chromatographic methods [5,6], which are sensitive and specific but require expensive instrumentation, complex sample preparation and purification procedures and not suitable for monitor of a large number of samples [7]. Thus it is very necessary to develop a simple and rapid method for the efficient detection of methiocarb. Fluorescence detection would be the best choice due to its simplicity, high sensitivity and selectivity features. Calixarenes, as the third generation of host molecules [8], have attracted considerable attentions in the host–guest chemistry due to their excellent recognition ability [9,10]. Among those, water-soluble calix[n]arenesulfonates have been studied on their binding abilities to several dye molecules [11,12], including native amino acids [13,14], several specific drugs and their intermediates [15–17]. On the other hand, due to the weak fluorescence of this type of molecule in aqueous solution, there are few reports about their optical properties in particular (especially) the fluorescence properties [18].
∗ Corresponding author. Tel.: +86 553 3869303; fax: +86 553 3869303. E-mail address: [email protected] (Y.-Y. Zhou). 1386-1425/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.saa.2011.05.038
Previously, we have synthesized a new fluorogenic tetrabutyl ether derivatived of p-sulfonatocalix[4]arene (Scheme 1), in this paper, we reported the application of this molecule in the spectrofluorometric titrations method as an efficient detection agent for methiocarb in a mixed solution system. Fluorescence studies indicate that SC4Bu and methiocarb forms a complex (1:1 mole ratio), with a binding constant of 1.67 × 104 L mol−1 . The temperaturedependent inclusion constants were calculated, from which H and S values were calculated. 2. Experimental 2.1. Apparatus Fluorescence spectra and relative fluorescence intensities were measured on a model F-4600 fluorescence spectrophotometer (Hitachi, Japan) using a conventional 1 × 1 cm2 quartz cell. Excitation and emission bandwidths were set to 5.0 nm and 10.0 nm, respectively. All measurements were carried out at desired temperature using a thermostatic cell holder. 1 H NMR spectra were measured using an Avance Bruker-300 MHz spectrometer. Quantum-mechanical molecular model was implemented in the Gaussian 03 program package. 2.2. Reagents All reagents were of analytical-reagent grade or the best grade commercially available. SC4Bu was synthesized according to the modified literature procedure [12,19] and was characterized by
X.-P. Ding et al. / Spectrochimica Acta Part A 81 (2011) 44–47
45
Scheme 1. Chemical structure of SC4Bu (1) and methiocarb (2).
IR and 1 H NMR. Methiocarb was obtained from aladdin-reagent, China. Stock solutions (1.00 × 10−3 mol L−1 ) of SC4Bu and methiocarb were prepared as in a mixture solvent of water and DMSO (90:10, v/v). Doubly distillated water was used thoroughly. 2.3. Procedure
Fig. 2. Fluorescence spectra of SC4Bu (5.00 × 10−6 mol L−1 ) with different concentrations of methiocarb at 23.0 ◦ C, from (1) to (12) of methiocarb were 0.00, 0.25, 0.50, 0.75, 1.00, 1.5, 2.00, 2.50, 3.00, 3.50, 4.00 and 4.50 × 10−5 mol L−1 .
2.3.1. Inclusion procedure To a 10 mL volumetric flask were added a 0.5 mL of 1.00 × 10−4 mol L−1 solution of SC4Bu, and an appropriate amount of 1.00 × 10 −4 mol L−1 methiocarb. The mixture was diluted to the final volume using the water–DMSO (90:10, v/v) binary solvent, mixed thoroughly and was incubated at the desired temperature. The fluorescence intensities were studied 30 min later at each temperature. 2.3.2. Determination of methiocarb To a 10 mL volumetric flask were placed 0.8 mL of 1.00 × 10−4 mol L−1 SC4Bu solution, and an appropriate amount of either sample or the working solution. The mixture was diluted to the final volume with the water–DMSO (90:10, v/v) binary solvent and mixed thoroughly. 3. Results and discussion
Fig. 3. Plot of fluorescence intensity quenching of SC4Bu at 323.6 nm vs. the concentration of methiocarb.
3.1. The characteristics of the fluorescence spectra When excited at 250 nm, SC4Bu gave a strong fluorescence emission at around 324 nm in the water–DMSO (90:10, v/v) binary solvent, as shown in Fig. 1. Upon addition of an appropriate amount of 1.00 × 10−4 mol L−1 methiocarb, the regular quenching of the fluorescence of SC4Bu was observed as shown in Fig. 2, which indicated the formation of the inclusion complexes.
The calibration curve was constructed under the optimal conditions. It was found that the fluorescence quenching intensity (F) was proportional to that of the concentration of methiocarb (C) in the range 0.23 ∼4.50 g mL−1 as shown in Fig. 3. The linear regression equation is: F = 1428.42 − 136.34C (g mL−1 ), with a correlation coefficient of 0.9991 (SD = 8.03, n = 13). The limit of detection (LOD) is 0.05 g mL−1 , which is given by the equation LOD = KS/N. Here K is a numerical factor chosen according to the confidence level desired (K = 3 for our system), S is the standard deviation of the blank measurement (n = 8) and N is the slope of the calibration curve. For comparative purpose, the analytical performance of several selected methods for methiocarb detection is summarized in Table 1. In comparison with previous results, the proposed method possesses comparable or superior detection limit and linear range, as shown in Table 1.
Table 1 Comparison of the linear range and detection limit of different methods for determination of methiocarb.
Fig. 1. Fluorescence excitation (a) and emission (b) spectra (8.00 × 10−6 mol L−1 ) in the water–DMSO (90:10, v/v) binary solvent.
of
SC4Bu
Methods
Linear range (g mL−1 )
LOD (g mL−1 )
Reference
GC-FID HPLC-CL HPLC-F Spectrofluorometric
1–20 0.0583–1.65 0.4–36.0 0.23–4.50
0.18 0.0583 0.003 0.05
[20] [21] [22] This work
46
X.-P. Ding et al. / Spectrochimica Acta Part A 81 (2011) 44–47
Fig. 4. Influence ionic strength of NaCl on the fluorescence intensity of SC4Bu–methiocarb system; [SC4Bu] = 8.00 × 10−6 mol L−1 , [NaCl] = 0.01 mol L−1 .
Fig. 6. A plot of 1/(F − F0 ) versus 1/[methiocarb] for SC4Bu–methiocarb complex.
3.3. Stoichiometry and inclusion constant 3.2. Discussion of the interaction mechanism Calixarene derivatives have been known to be able to form non-covalent inclusion complexes with various guest molecules through many interactions, such as the electrostatic interaction, cation– interactions, hydrogen bonding, Vander Waals and hydrophobic interactions [8]. The main driving force for the inclusion of methiocarb by SC4Bu was investigated. The electrostatic effect was studied by varying the ionic strength of NaCl, and the results were shown in Fig. 4. The presence of NaCl caused no significant changes of the fluorescence intensity of the SC4Bu–methiocarb system in comparison with in the absence of NaCl. Thus the ionic strength of NaCl has little effect on this inclusion process, which indicates the electrostatic effect is not the main driving force. Based on the calculation result, we supposed that the complexation was an “external” inclusion process and the hydrogen bonding between H atom of methylic on methiocarb and the sulfonate of SC4Bu facilitates the formation of this SC4Bu–methiocarb host–guest complex, as shown in Fig. 5. The lowest energy was 0 kcal/mol. The result was implemented in the Gaussian 03 program package.
Fig. 5. The proposed inclusion pattern of the SC4Bu-Methiocarb complex.
Assuming that SC4Bu and methiocarb forms a 1:1 ratio complex, the following expression can be written as H + G HG
(1)
The formation constant of the complex (K) is given by K=
[HG] [H][G]
(2)
An equation of inclusion constant K of the complex with one guest–one host was used to calculate the inclusion constant [10]: 1 1 1 1 = + K˛ G ˛ If
(3)
In which, If = Ifh−g − Ifg − Ifh , where Ifh−g , Ify and Ifh are the fluorescence intensity of the host–guest complex, the guest molecule and the host molecule, respectively. [G] is the original concentration of the methiocarb and ˛ is a constant. The excellent linear relationship (R = 0.9990) between 1/Ij and 1/[methiocarb] in the double reciprocal plot was shown in Fig. 6, which indicated the formation of a 1:1 host:guest complexation with a binding constant at K = 1.67 × 104 L M−1 . The formation of 1:1 SC4Bu–methiocarb complex was also confirmed by the continuous variation method (Job’s plot) using the fluorescence spectroscopy. The solutions of SC4Bu and methiocarb were mixed in a different mole ratio while keeping the
Fig. 7. Job’s plot of SC4Bu–methiocarb 1.00 × 10−6 mol L−1 , 23.0 ◦ C.
system,
[SC4Bu] + [methiocarb] =
X.-P. Ding et al. / Spectrochimica Acta Part A 81 (2011) 44–47 Table 2 Inclusion constants of complexes at different temperatures. Temperature (K) 4
Inclusion constant (×10 ) R
47
4. Conclusion
296
306
316
326
1.67 ± 0.06 0.9990
1.96 ± 0.13 0.9985
2.78 ± 0.17 0.9996
4.38 ± 0.09 0.9992
Table 3 The results of determination of the samples. Pesticide Sample
Methiocarb added (g mL−1 )
Methiocarb found (g mL−1 )
R.S.D. (%)
Recovery (%) (n = 4)
1 2 3 4
0.90 1.35 3.15 3.83
0.94 1.33 3.18 3.78
5.64 1.58 0.78 0.78
104.40 98.52 101.27 98.69
± ± ± ±
1.15 0.97 1.32 0.81
total concentrations of these two components to be a constant. The maximal relative fluorescence intensity was observed when [SC4Bu]/([SC4Bu] + [methiocarb]) = 0.5 as shown in Fig. 7. 3.4. Thermodynamic parameter value The binding constants at various temperatures were investigated by measuring the fluorescence spectra of SC4Bu in the presence of methiocarb, and the results were summarized in Table 1. By plotting ln K against 1/T (the Van’t Hoff method) [23], the corresponding enthalpy (H = 25.99 kJ mol−1 ) and entropy (S = 167.86 J mol−1 K−1 ) were obtained from the slope and intercept of the plot (with 10 K intervals). These results indicate that the entropic driving force facilitates the formation of SC4Bu–methiocarb complex. The relevant free energy change for this system is G = −24.03 kJ mol−1 at 298 K. This indicates that this inclusion is an energetically favored process. 3.5. Sample determinations In order to check the validity of the proposed methods, the standard addition method was applied by adding methiocarb to the previously analyzed solution. The recovery of each pesticide was calculated by comparing the concentration obtained from the spiked mixture with those of the pure pesticides. Table 2 shows the results of analysis of synthetic samples by the standard addition method Table 3.
In this work, we have developed a simple efficient method for the fluorescence detection of methiocarb, and have successfully applied it for the detection of methiocarb in synthetic samples. In our method, spectrofluorometric titrations were performed to investigate the inclusion behavior between SC4Bu and methiocarb in a mixture of water and DMSO (90:10, v/v) system, and the regular quenching of the fluorescence intensity of SC4Bu was observed upon addition of methiocarb. This study would provide useful information for applying calixarenes in pesticides detection. Acknowledgement The authors thank the financial supports of the National Natural Science Foundation of PR China (20875002). References [1] S. Ozden, B. Catalgol, S. Gezginci-Oktayoglu, P. Arda-Pirincci, S. Bolkent, B. Alpertunga, Food Chem. Toxicol. 47 (2009) 1676. [2] K. Sitarek, Teratog. Carcinog. Mutagen. 21 (2001) 335. [3] M. Tomizawa, D.L. Lee, J.E. Casida, J. Agric. Food Chem. 48 (2000) 6016. [4] R.W. Sayre, L. Clark, J. Wildl. Manag. 65 (2001) 461. [5] Y. Ito, T. Goto, S. Yamada, T. Ohno, H. Matsumoto, H. Oka, Y. Ito, J. Chromatogr. A 1187 (2008) 53. [6] G. Sagratini, J. Mãnes, D. Giardina, P. Damiani, Y. Pico, J. Chromatogr. A 1147 (2007) 135. [7] J.W. Sun, T.T. Dong, Y. Zhang, Sh. Wang, Anal. Chim. Acta 666 (2010) 76–82. [8] S. Shinkai, Tetrahedron 49 (1993) 8933. [9] C.D. Gutsche, Calixarenes Revisited: Monographs in Supramolecular Chemistry, Royal Society of Chemistry, Cambridge, U.K., 1998. [10] J.H. Yang, F. Huang, M. Wang, X. Wu, C.X. Sun, Spectrochim. Acta Part A 58 (2002) 1775. [11] Y. Liu, B. Han, Y. Chen, J. Org. Chem. 65 (2000) 6227. [12] S. Shinkai, S. Mori, H. Koreishi, T. Tsubaki, O. Manabe, J. Am. Chem. Soc. 108 (1986) 2409. [13] G. Arena, A. Contino, F.G. Gulino, A. Magri, F. Sansone, D. Sciotto, R. Ungaro, Tetrahedron Lett. 40 (1999) 1597. [14] F. Sansone, S. Barboso, A. Casnati, D. Sciotto, R. Ungaro, Tetrahedron Lett. 40 (1999) 4741. [15] J.S. Millership, J. Inclusion Phenom. 39 (2001) 327. [16] Y.Y. Zhou, Q. Lu, C. Liu, S.K. She, L. Wang, Spectrochim. Acta A 63 (2006) 423. [17] Y.Y. Zhou, Q. Lu, C. Liu, S.K. She, L. Wang, Anal. Chim. Acta 552 (2005) 152. [18] S. Kunsagi-Mate, K. Szabo, B. Lemli, I. Bitter, G. Nagy, L. Kollar, Tetrahedron Lett. 45 (2004) 1387. [19] T. Jin, F. Fujii, H. Sakata, M. Tamur, M. Kinjo, Chem. Commun. (2005) 4300. [20] M.J.S. Delgado, S.R. Barroso, G.T. Fernandez-Tostado, L.M. Polo-Dıez, J. Chromatogr. A 921 (2001) 287. [21] J.F. Huertas-Pérez, A.M. García-Campana, Anal. Chim. Acta 630 (2008) 194. [22] H.P. Li, J.H. Li, G.C. Li, J.F. Jen, Talanta 63 (2004) 547. [23] Y. Liu, B.H. Han, Y.T. Chen, J. Phys. Chem. B 106 (2002) 4678.
Contents lists available at ScienceDirect
Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy journal homepage: www.elsevier.com/locate/saa
A novel spectrofluorometric method for the determination of methiocarb using an amphiphilic p-sulfonatocalix[4]arene Xue-Ping Ding, Dong-Bao Tang, Tao Li, Su-Fan Wang, Yun-You Zhou ∗ Anhui Key Laboratory of Chemo-Biosensing, College of Chemistry and Materials Science, Anhui Normal University, Wuhu 241000, PR China
a r t i c l e
i n f o
Article history: Received 31 January 2011 Received in revised form 23 April 2011 Accepted 16 May 2011 Keywords: p-Sulfonatocalix[4]arene Methiocarb Fluorescence Inclusion interaction
a b s t r a c t The characteristics of host–guest complexation between tetrabutyl ether derivatives of psulfonatocalix[4]arene (SC4Bu) and methiocarb [3,5-dimethyl-4-(methylthio) phenyl methylcarbamate] were investigated by fluorescence spectrometry. Upon addition of methiocarb, the fluorescence intensity of SC4Bu was quenched regularly and a slight red shift was observed for the maximum emission peak. These results indicated that the SC4Bu-methiocarb complex was formed a 1:1 mole ratio. An association constant of 1.67 × 104 L mol−1 was calculated by applying a deduced equation. The interaction mechanism of the inclusion complex was discussed. Based on the results, a novel spectrofluorimetric method was described for the determination of methiocarb with a detection limit at 0.05 g mL−1 . This method is very simple and shows high sensitivity and selectivity. Moreover, the proposed method was successfully applied to the determination of methiocarb in water samples. © 2011 Elsevier B.V. All rights reserved.
1. Introduction Methiocarb [3,5-dimethyl-4-(methylthio) phenyl methylcarbamate] (Scheme 1),one of the mostly important N-methylcarbamate pesticide, is used worldwide in agriculture and health programes [1]. Recent years, methiocarb has been attracting increasing attention because its potential toxicity for animals and humans [1–4]. Common methodologies used for methiocarb detection are mainly based on chromatographic methods [5,6], which are sensitive and specific but require expensive instrumentation, complex sample preparation and purification procedures and not suitable for monitor of a large number of samples [7]. Thus it is very necessary to develop a simple and rapid method for the efficient detection of methiocarb. Fluorescence detection would be the best choice due to its simplicity, high sensitivity and selectivity features. Calixarenes, as the third generation of host molecules [8], have attracted considerable attentions in the host–guest chemistry due to their excellent recognition ability [9,10]. Among those, water-soluble calix[n]arenesulfonates have been studied on their binding abilities to several dye molecules [11,12], including native amino acids [13,14], several specific drugs and their intermediates [15–17]. On the other hand, due to the weak fluorescence of this type of molecule in aqueous solution, there are few reports about their optical properties in particular (especially) the fluorescence properties [18].
∗ Corresponding author. Tel.: +86 553 3869303; fax: +86 553 3869303. E-mail address: [email protected] (Y.-Y. Zhou). 1386-1425/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.saa.2011.05.038
Previously, we have synthesized a new fluorogenic tetrabutyl ether derivatived of p-sulfonatocalix[4]arene (Scheme 1), in this paper, we reported the application of this molecule in the spectrofluorometric titrations method as an efficient detection agent for methiocarb in a mixed solution system. Fluorescence studies indicate that SC4Bu and methiocarb forms a complex (1:1 mole ratio), with a binding constant of 1.67 × 104 L mol−1 . The temperaturedependent inclusion constants were calculated, from which H and S values were calculated. 2. Experimental 2.1. Apparatus Fluorescence spectra and relative fluorescence intensities were measured on a model F-4600 fluorescence spectrophotometer (Hitachi, Japan) using a conventional 1 × 1 cm2 quartz cell. Excitation and emission bandwidths were set to 5.0 nm and 10.0 nm, respectively. All measurements were carried out at desired temperature using a thermostatic cell holder. 1 H NMR spectra were measured using an Avance Bruker-300 MHz spectrometer. Quantum-mechanical molecular model was implemented in the Gaussian 03 program package. 2.2. Reagents All reagents were of analytical-reagent grade or the best grade commercially available. SC4Bu was synthesized according to the modified literature procedure [12,19] and was characterized by
X.-P. Ding et al. / Spectrochimica Acta Part A 81 (2011) 44–47
45
Scheme 1. Chemical structure of SC4Bu (1) and methiocarb (2).
IR and 1 H NMR. Methiocarb was obtained from aladdin-reagent, China. Stock solutions (1.00 × 10−3 mol L−1 ) of SC4Bu and methiocarb were prepared as in a mixture solvent of water and DMSO (90:10, v/v). Doubly distillated water was used thoroughly. 2.3. Procedure
Fig. 2. Fluorescence spectra of SC4Bu (5.00 × 10−6 mol L−1 ) with different concentrations of methiocarb at 23.0 ◦ C, from (1) to (12) of methiocarb were 0.00, 0.25, 0.50, 0.75, 1.00, 1.5, 2.00, 2.50, 3.00, 3.50, 4.00 and 4.50 × 10−5 mol L−1 .
2.3.1. Inclusion procedure To a 10 mL volumetric flask were added a 0.5 mL of 1.00 × 10−4 mol L−1 solution of SC4Bu, and an appropriate amount of 1.00 × 10 −4 mol L−1 methiocarb. The mixture was diluted to the final volume using the water–DMSO (90:10, v/v) binary solvent, mixed thoroughly and was incubated at the desired temperature. The fluorescence intensities were studied 30 min later at each temperature. 2.3.2. Determination of methiocarb To a 10 mL volumetric flask were placed 0.8 mL of 1.00 × 10−4 mol L−1 SC4Bu solution, and an appropriate amount of either sample or the working solution. The mixture was diluted to the final volume with the water–DMSO (90:10, v/v) binary solvent and mixed thoroughly. 3. Results and discussion
Fig. 3. Plot of fluorescence intensity quenching of SC4Bu at 323.6 nm vs. the concentration of methiocarb.
3.1. The characteristics of the fluorescence spectra When excited at 250 nm, SC4Bu gave a strong fluorescence emission at around 324 nm in the water–DMSO (90:10, v/v) binary solvent, as shown in Fig. 1. Upon addition of an appropriate amount of 1.00 × 10−4 mol L−1 methiocarb, the regular quenching of the fluorescence of SC4Bu was observed as shown in Fig. 2, which indicated the formation of the inclusion complexes.
The calibration curve was constructed under the optimal conditions. It was found that the fluorescence quenching intensity (F) was proportional to that of the concentration of methiocarb (C) in the range 0.23 ∼4.50 g mL−1 as shown in Fig. 3. The linear regression equation is: F = 1428.42 − 136.34C (g mL−1 ), with a correlation coefficient of 0.9991 (SD = 8.03, n = 13). The limit of detection (LOD) is 0.05 g mL−1 , which is given by the equation LOD = KS/N. Here K is a numerical factor chosen according to the confidence level desired (K = 3 for our system), S is the standard deviation of the blank measurement (n = 8) and N is the slope of the calibration curve. For comparative purpose, the analytical performance of several selected methods for methiocarb detection is summarized in Table 1. In comparison with previous results, the proposed method possesses comparable or superior detection limit and linear range, as shown in Table 1.
Table 1 Comparison of the linear range and detection limit of different methods for determination of methiocarb.
Fig. 1. Fluorescence excitation (a) and emission (b) spectra (8.00 × 10−6 mol L−1 ) in the water–DMSO (90:10, v/v) binary solvent.
of
SC4Bu
Methods
Linear range (g mL−1 )
LOD (g mL−1 )
Reference
GC-FID HPLC-CL HPLC-F Spectrofluorometric
1–20 0.0583–1.65 0.4–36.0 0.23–4.50
0.18 0.0583 0.003 0.05
[20] [21] [22] This work
46
X.-P. Ding et al. / Spectrochimica Acta Part A 81 (2011) 44–47
Fig. 4. Influence ionic strength of NaCl on the fluorescence intensity of SC4Bu–methiocarb system; [SC4Bu] = 8.00 × 10−6 mol L−1 , [NaCl] = 0.01 mol L−1 .
Fig. 6. A plot of 1/(F − F0 ) versus 1/[methiocarb] for SC4Bu–methiocarb complex.
3.3. Stoichiometry and inclusion constant 3.2. Discussion of the interaction mechanism Calixarene derivatives have been known to be able to form non-covalent inclusion complexes with various guest molecules through many interactions, such as the electrostatic interaction, cation– interactions, hydrogen bonding, Vander Waals and hydrophobic interactions [8]. The main driving force for the inclusion of methiocarb by SC4Bu was investigated. The electrostatic effect was studied by varying the ionic strength of NaCl, and the results were shown in Fig. 4. The presence of NaCl caused no significant changes of the fluorescence intensity of the SC4Bu–methiocarb system in comparison with in the absence of NaCl. Thus the ionic strength of NaCl has little effect on this inclusion process, which indicates the electrostatic effect is not the main driving force. Based on the calculation result, we supposed that the complexation was an “external” inclusion process and the hydrogen bonding between H atom of methylic on methiocarb and the sulfonate of SC4Bu facilitates the formation of this SC4Bu–methiocarb host–guest complex, as shown in Fig. 5. The lowest energy was 0 kcal/mol. The result was implemented in the Gaussian 03 program package.
Fig. 5. The proposed inclusion pattern of the SC4Bu-Methiocarb complex.
Assuming that SC4Bu and methiocarb forms a 1:1 ratio complex, the following expression can be written as H + G HG
(1)
The formation constant of the complex (K) is given by K=
[HG] [H][G]
(2)
An equation of inclusion constant K of the complex with one guest–one host was used to calculate the inclusion constant [10]: 1 1 1 1 = + K˛ G ˛ If
(3)
In which, If = Ifh−g − Ifg − Ifh , where Ifh−g , Ify and Ifh are the fluorescence intensity of the host–guest complex, the guest molecule and the host molecule, respectively. [G] is the original concentration of the methiocarb and ˛ is a constant. The excellent linear relationship (R = 0.9990) between 1/Ij and 1/[methiocarb] in the double reciprocal plot was shown in Fig. 6, which indicated the formation of a 1:1 host:guest complexation with a binding constant at K = 1.67 × 104 L M−1 . The formation of 1:1 SC4Bu–methiocarb complex was also confirmed by the continuous variation method (Job’s plot) using the fluorescence spectroscopy. The solutions of SC4Bu and methiocarb were mixed in a different mole ratio while keeping the
Fig. 7. Job’s plot of SC4Bu–methiocarb 1.00 × 10−6 mol L−1 , 23.0 ◦ C.
system,
[SC4Bu] + [methiocarb] =
X.-P. Ding et al. / Spectrochimica Acta Part A 81 (2011) 44–47 Table 2 Inclusion constants of complexes at different temperatures. Temperature (K) 4
Inclusion constant (×10 ) R
47
4. Conclusion
296
306
316
326
1.67 ± 0.06 0.9990
1.96 ± 0.13 0.9985
2.78 ± 0.17 0.9996
4.38 ± 0.09 0.9992
Table 3 The results of determination of the samples. Pesticide Sample
Methiocarb added (g mL−1 )
Methiocarb found (g mL−1 )
R.S.D. (%)
Recovery (%) (n = 4)
1 2 3 4
0.90 1.35 3.15 3.83
0.94 1.33 3.18 3.78
5.64 1.58 0.78 0.78
104.40 98.52 101.27 98.69
± ± ± ±
1.15 0.97 1.32 0.81
total concentrations of these two components to be a constant. The maximal relative fluorescence intensity was observed when [SC4Bu]/([SC4Bu] + [methiocarb]) = 0.5 as shown in Fig. 7. 3.4. Thermodynamic parameter value The binding constants at various temperatures were investigated by measuring the fluorescence spectra of SC4Bu in the presence of methiocarb, and the results were summarized in Table 1. By plotting ln K against 1/T (the Van’t Hoff method) [23], the corresponding enthalpy (H = 25.99 kJ mol−1 ) and entropy (S = 167.86 J mol−1 K−1 ) were obtained from the slope and intercept of the plot (with 10 K intervals). These results indicate that the entropic driving force facilitates the formation of SC4Bu–methiocarb complex. The relevant free energy change for this system is G = −24.03 kJ mol−1 at 298 K. This indicates that this inclusion is an energetically favored process. 3.5. Sample determinations In order to check the validity of the proposed methods, the standard addition method was applied by adding methiocarb to the previously analyzed solution. The recovery of each pesticide was calculated by comparing the concentration obtained from the spiked mixture with those of the pure pesticides. Table 2 shows the results of analysis of synthetic samples by the standard addition method Table 3.
In this work, we have developed a simple efficient method for the fluorescence detection of methiocarb, and have successfully applied it for the detection of methiocarb in synthetic samples. In our method, spectrofluorometric titrations were performed to investigate the inclusion behavior between SC4Bu and methiocarb in a mixture of water and DMSO (90:10, v/v) system, and the regular quenching of the fluorescence intensity of SC4Bu was observed upon addition of methiocarb. This study would provide useful information for applying calixarenes in pesticides detection. Acknowledgement The authors thank the financial supports of the National Natural Science Foundation of PR China (20875002). References [1] S. Ozden, B. Catalgol, S. Gezginci-Oktayoglu, P. Arda-Pirincci, S. Bolkent, B. Alpertunga, Food Chem. Toxicol. 47 (2009) 1676. [2] K. Sitarek, Teratog. Carcinog. Mutagen. 21 (2001) 335. [3] M. Tomizawa, D.L. Lee, J.E. Casida, J. Agric. Food Chem. 48 (2000) 6016. [4] R.W. Sayre, L. Clark, J. Wildl. Manag. 65 (2001) 461. [5] Y. Ito, T. Goto, S. Yamada, T. Ohno, H. Matsumoto, H. Oka, Y. Ito, J. Chromatogr. A 1187 (2008) 53. [6] G. Sagratini, J. Mãnes, D. Giardina, P. Damiani, Y. Pico, J. Chromatogr. A 1147 (2007) 135. [7] J.W. Sun, T.T. Dong, Y. Zhang, Sh. Wang, Anal. Chim. Acta 666 (2010) 76–82. [8] S. Shinkai, Tetrahedron 49 (1993) 8933. [9] C.D. Gutsche, Calixarenes Revisited: Monographs in Supramolecular Chemistry, Royal Society of Chemistry, Cambridge, U.K., 1998. [10] J.H. Yang, F. Huang, M. Wang, X. Wu, C.X. Sun, Spectrochim. Acta Part A 58 (2002) 1775. [11] Y. Liu, B. Han, Y. Chen, J. Org. Chem. 65 (2000) 6227. [12] S. Shinkai, S. Mori, H. Koreishi, T. Tsubaki, O. Manabe, J. Am. Chem. Soc. 108 (1986) 2409. [13] G. Arena, A. Contino, F.G. Gulino, A. Magri, F. Sansone, D. Sciotto, R. Ungaro, Tetrahedron Lett. 40 (1999) 1597. [14] F. Sansone, S. Barboso, A. Casnati, D. Sciotto, R. Ungaro, Tetrahedron Lett. 40 (1999) 4741. [15] J.S. Millership, J. Inclusion Phenom. 39 (2001) 327. [16] Y.Y. Zhou, Q. Lu, C. Liu, S.K. She, L. Wang, Spectrochim. Acta A 63 (2006) 423. [17] Y.Y. Zhou, Q. Lu, C. Liu, S.K. She, L. Wang, Anal. Chim. Acta 552 (2005) 152. [18] S. Kunsagi-Mate, K. Szabo, B. Lemli, I. Bitter, G. Nagy, L. Kollar, Tetrahedron Lett. 45 (2004) 1387. [19] T. Jin, F. Fujii, H. Sakata, M. Tamur, M. Kinjo, Chem. Commun. (2005) 4300. [20] M.J.S. Delgado, S.R. Barroso, G.T. Fernandez-Tostado, L.M. Polo-Dıez, J. Chromatogr. A 921 (2001) 287. [21] J.F. Huertas-Pérez, A.M. García-Campana, Anal. Chim. Acta 630 (2008) 194. [22] H.P. Li, J.H. Li, G.C. Li, J.F. Jen, Talanta 63 (2004) 547. [23] Y. Liu, B.H. Han, Y.T. Chen, J. Phys. Chem. B 106 (2002) 4678.