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Dansyl-labeled anionic amphiphile with a hexadecanoic carbon chain: Synthesis and detection for shape transitions in organized molecular assemblies
Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 139 (2015) 222–228
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Dansyl-labeled anionic amphiphile with a hexadecanoic carbon chain: Synthesis and detection for shape transitions in organized molecular assemblies Lining Gao ⇑, Huiyun Xia, Xiaoman Wang, Li Li, Huaxin Chen Engineering Research Center of Transportation Materials (Ministry of Education), School of Materials Science and Engineering, Chang’an University, Xi’an 710064, PR China
h i g h l i g h t s
g r a p h i c a l a b s t r a c t
A dansyl-labeled anionic amphiphile
with a hexadecanoic carbon chain was synthesized. The synthesized amphiphile can be used to differentiate micelle and vesicle. The fluorescence anisotropy can detect the micellar growth efficiently. The synthesized amphiphile is a better fluorescence probe than pyrene and DPH.
a r t i c l e
i n f o
Article history: Received 18 September 2014 Received in revised form 22 November 2014 Accepted 25 November 2014 Available online 25 December 2014 Keywords: Fluorescence probe Dansyl Aggregates transition Micellar growth
a b s t r a c t The probing properties of a new fluorophore-labeled anionic surfactant, sodium 16-(N-dansyl)aminocetylate (16-DAN-ACA) were investigated systematically in molecular assemblies, especially in the transitions between micelles and vesicles. 16-DAN-ACA can efficiently differentiate the two different aggregate types in mixed cationic and anionic surfactant systems. The fluorescence anisotropy of 16-DAN-ACA was found to be sensitive for directly detecting the micellar growth in micelles containing oppositely charged surfactants; both cationic cetyltrimethylammonium bromide (CTAB) systems and anionic sodium dodecyl sulfate (SDS) systems were studied. The results indicated that the 16-DAN-ACA is a good fluorescent probe for differentiating the different aggregates, and even more can be used to detect the micellar growth. Ó 2014 Elsevier B.V. All rights reserved.
Introduction Recently, different organized molecular assemblies have attracted more attention, such as spherical and rod-like/worm-like micelles, vesicles and lamellar phases, due to their special properties and applications [1–11]. Therefore, identifying how to characterize the transitions of different aggregates is becoming more and more important. Because of their ability of providing the straightforward images and detailed size information about the aggregates, electron microscopy and light scattering become two of ⇑ Corresponding author. Tel.: +86 29 82337254. E-mail address: [email protected] (L. Gao). http://dx.doi.org/10.1016/j.saa.2014.11.113 1386-1425/Ó 2014 Elsevier B.V. All rights reserved.
the most popular characterizing strategies. However, in most cases, these techniques can only give limited information about the aggregates at the molecular level. New techniques need to be developed and used to solve the new problems. Fluorescence spectroscopy is well-known to provide more detailed information at the molecular level. For example, fluorescence probes are highly sensitive to their local environment such as micropolarity, and they have multiple useful photophysical parameters like fluorescence intensity, lifetime and anisotropy, excimer/exciplex formation, etc. Therefore, fluorescence probe techniques have been extensively employed to investigate the formation and transition of aggregates [11–15]. In order to ideally probe organized molecular assemblies, the desired fluorophores
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should have attractive features like being sensitive to its molecular packing. Pyrene [16] and 1,6-diphenyl-1,3,5-hexatriene (DPH) [17,18] are two commonly used fluorescence probes due to their high efficiency in the measurement of micropolarity and microviscosity, respectively. However, the location of the kind of fluorescence probe in various aggregates is removable and unknown. Therefore, the variation of their related fluorescence parameters is usually the combined contribution from both the transition of aggregates and the change of the location of probes. This problem may be expected to be solved using fluorophore-labeled amphiphile. This kind of fluorescence probe, when used as part of the surfactant, can take part in the formation of aggregates so as to provide more reliable information. In fact, some fluorophorelabeled amphiphiles have been used to detect the related physical properties of various aggregates [19–29]. However, most of these studies focused on micelle [19–23] or membrane [24–28] systems. In our previous work, a fluorescent group, dansyl, which shows representative character of intramolecular charge transfer (ICT), has been attached to the tail of an anion surfactant (12-aminolauric acid) to form a surfactant-fluorescence probe sodium 12-(Ndansyl)aminododecanate (12-DAN-ADA) [30,31]. We found that 12-DAN-ADA can efficiently differentiate two different kinds of aggregates (micelles and vesicles) in the mixed cationic and anionic surfactant systems, as well as in the double-chain cationic surfactant systems based on the polarity- and viscosity-dependence of the dansyl group. Further studies demonstrated that 12-DAN-ADA is a more efficient probe of transitions between micelles and vesicles than those commonly used fluorescence probes, such as pyrene and DPH [30]. In order to investigate the effect of the solubility of the surfactant-fluorescence probes on the efficiency of probing the transitions between micelles and vesicles, in the present work, we extended the alkyl chain of 12-DAN-ADA to prepare another surfactant-fluorescence probe with longer hydrophobic chain, sodium 16-(N-dansyl)aminocetylate (16-DAN-ACA), and measured its corresponding fluorescence parameters in the transitions of micelles and vesicles. Although, the chemical structures of 12-DAN-ADA and 16-DANACA are very similar, the procedure for obtaining the 16-DANACA is laborious. As expected, the corresponding fluorescence parameters (fluorescence anisotropy and emission maximum) of 16-DAN-ACA can also reflect efficiently the transitions between micelles and vesicles. All the results demonstrated that the solubility of the surfactant-fluorescence probes did have an effect on the efficiency of probing the transitions between micelles and vesicles.
Experimental section Chemicals 5-Cyclohexadecen-1-one (>97.0%) was purchased from TCI. Dansyl chloride (99%), pyrene (98%), 1,6-diphenyl-1,3,5-hexatriene (DPH, 98%), and sodium dodecyl sulfate (SDS, 99%) were purchased from Acros. Triton X-100 was from E. Merck Co. (Darmstadt). Branched-chain sodium dodecylbenzenesulfonate (SDBS), hydroxylamine hydrochloride (NH2OHHCl), and Pd–C catalyst were obtained from Beijing Chemical Co. All the above regents were used as received. Deionized water was treated with KMnO4 over 24 h and distilled before use. Dodecyltrimethylammonium bromide (DTAB), dodecyltriethylammonium bromide (DEAB), and cetyltrimethylammonium bromide (CTAB) were synthesized from n-alkyl bromide and corresponding trialkylamine [32]. The three products were recrystallized five times from acetone, acetone/diethyl ether, and acetone/ethanol, respectively. The purity of all the synthesized cationic surfactants was examined, and no surface tension mini-
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mum was found in the surface tension curve. All other reagents were of analytical grade at least. General instruments 1
H NMR spectra were recorded on a Mercury Plus 300 M (USA) or a ARX-400 M (Swiss) instrument, and chemical shifts are reported in parts per million relative to TMS in proton spectra. Element analyses of C, H, and N were conducted on an Elementar Vario EL elemental analyzer (Germany). MS results were conducted on a ZAB-HS magnetic mass spectrometer (Micromass, UK). Fluorescence measurements All of the fluorescence measurements were performed on a time-correlated single-photon-counting FLS 920 fluorescence spectrometer (Edinburgh) at room temperature (23–25 °C) except where noted in the text. Samples were excited at 337 nm. The stock solutions (1.0 10 4 M) of 16-DAN-ACA were prepared in ethanol. A certain amount of stock solution was added to a tube and heated slightly to remove some solvent. Then the final concentration of the probes was adjusted by adding an appropriate amount of the analyte solutions. A FLS 920 fluorescence spectrometer equipped with filter polarizers that use the L-format configuration using a 1-cm quartz cuvette was used for fluorescence depolarization measurements. An average of three fluorescence anisotropy values was recorded. Synthesis and characterization of 16-DAN-ACA The synthetic route of 16-DAN-ACA was shown in Scheme 1, in which 16-aminohexadecanoic acid (compound 6) was synthesized by adopting a modified literature method [33]. Synthesis of cyclohexadecanone (compound 2) 5-Cyclohexadecen-1-one (compound 1, 5 g, 21.2 mmol) was dissolved in ethanol (45 mL). Pd–C catalyst (1%, w%) was added to the solution of compound 1 while stirring vigorously at room temperature under a H2 atmosphere. Stirring was continued for a total of 1.5 h. The resulting solution was filtered and the solvent was removed to give compound 2 as a white solid. 1H NMR (CDCl3/ Me4Si, 400 MHz) d (ppm): 1.29 (s, 22H, –(CH2)11–), 1.62 (m, 4H, – CH2CH2COCH2CH2–), 2.40 (t, 4H, –CH2COCH2–). Synthesis of cyclohexadecanone oxime (compound 3) Compound 2 (5.8 g, 24.4 mmol) was dissolved in methanol (140 mL). NH2OHHCl (3.5 g, 50.4 mmol) and NaHCO3 (3.5 g, 41.7 mmol) were added to the stirred methanol solution. The mixture was stirred and refluxed for 5 h. After that, the mixture was brought to room temperature and stirred overnight. The methanol phase was add in water (30 mL) and extracted with CHCl3 (3 80 mL). The CHCl3 phase was dried by anhydrous sodium sulfate overnight and filtered. The filtrate was evaporated under reduced pressure and the residue was dried in vacuum to give compound 3 as a white solid. 1H NMR (CDCl3/Me4Si, 400 MHz) d (ppm): 1.30 (s, 22H, –(CH2)11–), 1.51 (m, 4H, –CH2CH2CNOHCH2CH2–), 2.18 (t, 2H, –CH2CNOH), 2.36 (t, 2H, –CH2CNOH). Synthesis of cyclohexadecanone-oxime p-toluenesulfonate (compound 4) Compound 3 (5.9 g, 23.3 mmol) was dissolved in the mixture of CH2Cl2 (240 mL) and pyridine (40 mL). TsCl (7.6 g, 39.9 mmol), dissolved in CH2Cl2 (1200 mL), was added drop-wise to the above solution while stirring vigorously at 0 °C. After addition was completed, the mixture was brought to room temperature and stirred overnight. The CH2Cl2 layer was extracted with hydrochloric acid
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Scheme 1. Synthetic route of 16-DAN-ACA.
(3.5 M, 60 mL) and 5% aqueous NaHCO3 (3 60 mL), dried over anhydrous sodium sulfate, and filtered. The filtrate was evaporated and dried in vacuum to give compound 4 as a yellow–brown liquid. Synthesis of 2-azacycloheptadecanone (compound 5) Compound 4 (9.5 g, 23.3 mmol) was dissolved in THF (650 mL). 30 mmol aqueous K2CO3 (650 mL) was added to the above solution and stirred overnight at room temperature. The mixture was evaporated under reduced pressure to remove THF, and the resulting aqueous phase was extracted with CH2Cl2. The CH2Cl2 layer was combined, dried over anhydrous sodium sulfate and filtered. The filtrate was evaporated and dried in vacuum to give compound 5 as a wax-like yellow solid. 1H NMR (CDCl3/Me4Si, 400 MHz) d (ppm): 1.30 (s, 22H, –(CH2)11–), 1.51 (m, 2H, –CH2CH2NH–), 1.65 (m, 2H, –CH2CH2CO), 2.18 (t, 2H, –CH2CO), 3.30 (q, 2H, –CH2NH– ), 5.45 (q, 1H,–NH–).
upon addition of concentrated hydrochloric acid to the above solution. The solid was filtered, washed with deionized water, and was treated as above several times to give the carboxylic-acid type of compound 7 as a light yellow solid. The mass measurement of compound 7 was conducted using its carboxylic-acid type due to the instability of compound 7 while measuring MS. MS for C28H44N2SO4: MW = 504. Found: MW = 504. Surface property of 16-DAN-ACA (compound 7) As an anionic surfactant, the critical micelle concentration (CMC) of 16-DAN-ACA was investigated using fluorescence method. Results and discussion Comparison of the solubility in water
Synthesis of 16-aminohexadecanoic acid (compound 6) Compound 5 (1 g, 3.9 mmol) was added to hydrochloric acid (6 M, 40 mL). The mixture was stirred and refluxed for 20 h. White crystal was precipitated after the mixture brought to room temperature. Then, a moderate amount of ethanol was added to the mixture to dissolve the impurity. The resulting mixture was filtered to give white crystal. The white crystal was washed with ethanol, CH2Cl2, and then dissolved in boiling water. The pure white crystal was precipitated upon cooling to room temperature. Compound 6 as white crystal was filtered and vacuum-dried. Synthesis of 16-DAN-ACA (compound 7) Dansyl chloride and compound 6 (mol:mol = 1:3) were combined in aqueous NaOH (2 M) and stirred in dark condition at room temperature for 24 h. The mixture was extracted with ethyl acetate three times. The organic phase was combined, dried over anhydrous sodium sulfate, and filtered. The filtrate was evaporated and dried in vacuum to give yellow solid. The solid was added to aqueous NaOH (2 M) and heated to dissolve. The aqueous mixture was filtered to give clean yellow solution. Compound 7 as light yellow solid was precipitated upon the solution cooling to room temperature. 1H NMR (DMSO-d6/Me4Si, 400 MHz) d (ppm): 8.45 (d, 1H, Ar–H), 8.33 (d, 1H, Ar–H), 8.10 (d, 1H, Ar–H), 7.60 (m, 2H, Ar–H), 7.25 (d, 1H, Ar–H), 2.82 (s, 6H, –N(CH3)2), 2.76 (t, 2H, COONa– CH2–), 2.67 (m, 2H, –NH–CH2–), 1.80 (t, 2H, COONa–CH2–CH2–), 1.39 (m, 2H, –NH–CH2–CH2–), 1.31–0.96 (m, 22H, –NH–CH2– CH2–(CH2)11–). EA (%) calcd for C28H43N2SO4Na2H2O: C, 59.79; H, 8.36; N, 4.98. Found: C, 60.05; H, 7.69; N, 4.74. To further purify compound 7, the light yellow solid was dissolved in hot aqueous NaOH (2 M). White solid was precipitated
Since 16-DAN-ACA is in free state when its concentration is lower than its CMC, or in micellar state when its concentration is higher than the CMC, there must be a significant change in the polarity of their microenvironment during the transition between its free state and the micellar state. Specifically, when the concentration is higher than its CMC, 16-DAN-ACA molecules will form micelles, thus the polar-water environment of 16-DAN-ACA molecules will transfer gradually to the non-polar hydrophobic environment inside of the micelles, which will make the rotation of 16DAN-ACA molecules difficult. In this case, the fluorescence emission maximum of 16-DAN-ACA hypochromatically shift. Fig. 1 shows the emission maxima and fluorescence anisotropy of 16DAN-ACA at different concentration in aqueous Na2B4O710H2O solutions (1.0 10 2 M, pH = 9.2). We can find that when the concentration is around 10 5 M, the hypochromatic shift of its emission maximum and the change in the value of anisotropy are remarkable. Thus the CMC of 16-DAN-ACA is about 10 5 M, which is much lower than the CMC of 12-DAN-ADA (about 2.2 10 3 M) [30], indicating the lower solubility of 16-DAN-ACA in water than that of 12-DAN-ADA. Determination of the polarity and viscosity dependence of 16-DANACA As an ICT compound, dansyl is very sensitive to the polarity and viscosity of its microenvironment. Hence it is expected that this polarity and viscosity dependence of dansyl can be restored when it is labeled to 16-aminolauric acid to form a novel anionic surfactant, 16-DAN-ACA. Based on this consideration, the
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fluorescence probe to detect the formation and transition of aggregates, since 16-DAN-ACA can take part in the formation of the aggregates. Thus, the variation of related fluorescence parameters in the formation of aggregates and the corresponding transition between different aggregates were examined. Probing the transition between micelles and vesicles in mixed cationic and anionic surfactant systems
Fig. 1. Emission maxima and fluorescence anisotropy for different concentration of 16-DAN-ACA in 1.0 10 2 M Na2B4O710H2O (pH = 9.2) aqueous solutions.
fluorescence emission spectra of 16-DAN-ACA in 1,4-dioxane/ water mixtures [34] and various solvents with different polarities were measured. As expected, the emission maxima of 16-DANACA bathochromically shifted considerably, along with the increasing solvent polarity. Fig. 2a shows the emission maxima of 16DAN-ACA in 1,4-dioxane/water mixtures with different water components. The emission maximum shifted bathochromically about 70 nm, from 480 to 550 nm, while the solvent changed from neat 1,4-dioxane to neat water. The normalized fluorescence emission spectra of 16-DAN-ACA in five different solvents are shown in Fig. 2b (with increasing polarity according to ET(30) values from Ref. [35]). Except the difference in polarity, the cohesive energy density, mainly reflect the interaction of molecules, also increased from c.a. 20 to 50 MPa1/2 (the data from Ref. [36]). Along with the increasing of cohesive energy density of solvent, the interaction between solvent molecule and 16-DAN-ACA was enhanced, which results in the stronger intermolecular charge transfer of dansyl group and the bathochromical shift of emission maximum. Meanwhile, the fluorescence anisotropy of 16-DAN-ACA in the binary solvents of glycerine/water (with different viscosities) [37] was also examined (cf. Fig. 3). The curve in Fig. 3 suggests that the fluorescence anisotropy of 16-DAN-ACA increases along with the increasing viscosity. These observations demonstrate that the properties of polarity and viscosity dependence of dansyl are well reserved in 16-DAN-ACA. Based on the consideration of its solubility, polarity and viscosity dependence, it may be suitable to use 16-DAN-ACA as a
Considering the rich aggregation behaviors and the challenging characterization in mixed cationic and anionic surfactant systems, the transition between micelles and vesicles in these systems was studied systematically. In the present work, the mixed cationic and anionic surfactant systems, SDS/DEAB and SDBS/DTAB were selected to study the transitions. It is well known that the molecular arrangements become more packed and more orderly [38,39], and the polarity of the hydrophobic region decreases considerably when the aggregate transforms from micelles to vesicles. Therefore, the variation of the emission maximum and fluorescence anisotropy will respond to the change of the aggregates. 16-DAN-ACA was introduced into the micelle region, the vesicle region, and their mixed region of the SDS/DEAB system [11] as a fluorescence probe. The total concentration of SDS and DEAB was 1.0 10 2 M in the following experiments. The emission maxima and fluorescence anisotropy of 16-DAN-ACA in the SDS/DEAB system are shown in Fig. 4. When the organized assemblies changed from micelles to vesicles, the emission maximum of 16-DAN-ACA shifted hypochromatically from 538 to 518 nm while the fluorescence anisotropy increased sharply along with this transition. In contrast, the emission maximum of 16-DAN-ACA bathochromically shifted back to 527 nm. In addition, the fluorescence anisotropy decreased gradually while the vesicles transformed back to micelles along with increasing the molar ratio of DEAB in the SDS/DEAB system. It should be noted that the emission maximum of 16-DAN-ACA in the DEAB-rich micelle region is lower than that in the SDS-rich micelle region. Correspondingly, the fluorescence anisotropy in the DEAB-rich micelle region is higher than that in the SDS-rich region. These observations may be due to the electrostatic interaction between DEAB and 16-DAN-ACA. Given the obvious change in the polarity and viscosity of the system during the transition between micelle and vesicle, 16-DAN-ACA could be considered as a similarly ideal fluorescence probe as 12-DAN-ADA [30] in probing the transition between micelle and vesicle in mixed cationic and anionic surfactant systems. Further investigation was also performed in another mixed cationic and anionic surfactant system, SDBS/DTAB [7]. Fig. 5 shows
Fig. 2. Emission maxima of 16-DAN-ACA in 1,4-dioxane/water mixtures (a) and normalized fluorescence emission spectra of 16-DAN-ACA in different solvents (the used solvents are 1,4-dioxane, ethyl acetate, dichloromethane, ethylene glycol and water as the arrow noted) (b) ([16-DAN-ACA] = 1.0 10 5 M).
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Fig. 5. Emission maxima and fluorescence anisotropy of 16-DAN-ACA in the SDBS/ DTAB system. [SDBS] = 5.0 10 2 M, [16-DAN-ACA] = 1.0 10 6 M. M: micelles; V: vesicles. Fig. 3. Fluorescence anisotropy of 16-DAN-ACA in glycerine/water mixtures ([16DAN-ACA] = 1.0 10 5 M). The viscosity values are obtained from Ref. [37].
Fig. 4. Emission maxima and fluorescence anisotropy of 16-DAN-ACA in the SDS/ DEAB system. [T] = 1.0 10 2 M, [16-DAN-ACA] = 1.0 10 6 M. P: precipitate; M: micelles; V: vesicles.
the change in the emission maximum and fluorescence anisotropy of 16-DAN-ACA in the SDS/DTAB system. It is found that similar results were obtained in these systems, showing that the variation of the fluorescence parameters was also consistent with that of the corresponding aggregate forms. As the control experiment, the fluorescence anisotropy of DPH and the I1/I3 value of pyrene were conducted in the SDS/DEAB system to examine the fluorescence responses of DPH and pyrene to different aggregate forms, respectively, and the results were shown in Fig. 6. Different from the observation of 16-DAN-ACA, there was no obvious variation in all the fluorescence signals over the whole tested concentration range in Fig. 6. This is consistent with the comparison of 12-DAN-ADA, DPH and pyrene [30], suggesting that 16-DAN-ACA has advantage over traditional fluorescence probes, such as DPH and pyrene, in probing the transition of micelles and vesicles in mixed cationic and anionic surfactant systems.
Probing the micellar growth A typical micellar system, SDS/NaBr, which is known in the literature because of its full spectrum of micellar shapes, was also adopted to study the morphological transition of different micelles. At a given surfactant concentration, the morphology of the micelle
will transform from sphere, ellipsoid, rod to worm at proper conditions. 16-DAN-ACA was applied to probe the micellar transition in the anionic surfactant SDS system. It is known that the micelles change from spheres to rods, then to worms when increasing the concentration of NaBr from 0 to 1 M in 35 mM or in 175 mM SDS system at 40 °C (the result from SANS studies) [40]. In this process, the SDS molecules are supposed to pack more compactly, and the local environment in the micelles changes accordingly. These changes can be examined using 16-DAN-ACA as the probe. Fig. 7 shows the fluorescence anisotropy and emission maxima of 16DAN-ACA in the SDS/NaBr system. The concentration of SDS was 35 mM and 175 mM, respectively. The fluorescence anisotropy increased slowly from 0.010 to 0.018 while the emission maximum hypochromatically shifted slightly from 532 to 529 nm upon increasing the concentration of NaBr from 0 to 1 M. The significant change in the fluorescence anisotropy indicates the change in the viscosity of the microenvironment of 16-DAN-ACA, which is consistent with the growth of the micelle. The fluorescence maximum, however, did not change remarkably (3 nm), which is inconsistent with the 20 nm blue shift in the process of micelle–vesicle transition (Figs. 4–6). These results might suggest that the polarity in the micellar core does not change considerably in the process of micellar growth. Compared with 12-DAN-ADA in the SDS/NaBr system [31], the difference between the change in the fluorescence anisotropy of the 12-DAN-ADA system and the 16-DAN-ACA system is relatively small. The emission maxima of 16-DAN-ACA in the SDS/NaBr system, however, were more hypochromatical than those of 12-DANADA in the SDS/NaBr system (see Fig. S1 in the Supporting Information). This might be due to the location of 16-DAN-ACA in the aggregates or the smaller solubility of 16-DAN-ACA in water. According to the literature [42,43], the micellar growth could also be realized by the addition of 0 to 0.6 M KBr in 10 mM CTAB aqueous solution at 30 °C. The fluorescence anisotropy and the emission maxima of 16-DAN-ACA in the CTAB/KBr system were examined and the results are shown in Fig. 8. The fluorescence anisotropy increased from 0.018 to 0.037 as the concentration of KBr increased from 0 to 0.6 M. The change in the anisotropy of 16-DAN-ACA in the CTAB/KBr system is more significant than that in the SDS/NaBr system, indicating that the electrostatic interaction between 16-DAN-ACA and CTAB indeed helps to promote the sensitivity of 16-DAN-ACA [31]. In other words, the electrostatic interaction makes 16-DAN-ACA enrich in the micelles, so that just a small amount of 16-DAN-ACA molecules can exist in the bulk solution [30]. As a result, the fluorescence contributed by free 16-DAN-ACA molecules in the bulk solution was reduced
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Fig. 6. Fluorescence anisotropy of 16-DAN-ACA and DPH (a) and emission maxima of 16-DAN-ACA and I1/I3 values of pyrene (b) in the SDS/DEAB system. ([16-DANACA] = 1.0 10 6 M, [pyrene] = 1.0 10 7 M, [DPH] = 1.0 10 6 M).
Fig. 7. Fluorescence anisotropy and emission maximum of 16-DAN-ACA in different SDS/NaBr system at 40 °C. [SDS] = 35 mM (a) and [SDS] = 175 mM (b), [16-DANACA] = 1.0 10 6 M. The related data of three points in the figure are from the results of SANS [40,41] for [SDS] = 35 mM and [SDS] = 175 mM systems respectively and indicate the different micelle type.
Conclusions
Fig. 8. Fluorescence anisotropy and emission maximum of 16-DAN-ACA in the CTAB/KBr system at 30 °C. [CTAB] = 10 mM, [16-DAN-ACA] = 1.0 10 6 M.
in the CTAB systems. On the one hand, the apparent fluorescence anisotropy for this system became more significant due to the smearing of the fluorescence from the free 16-DAN-ACA. On the other hand, the emission maximum changed slightly (about 5 nm), which is consistent with the change in SDS/NaBr system (see Fig. 7a). These results suggest that the polarity in the micellar core is not considerably changed in the process of micellar growth.
The new fluorophore-labeled surfactant 16-DAN-ACA was synthesized and was used to differentiate the two different aggregate types in mixed cationic and anionic surfactant systems and detect the growth of micelles formed in cationic and anionic surfactant systems successfully. Along with the aggregates transforming from micelles to vesicles in mixed cationic and anionic surfactant systems, the emission maxima of 16-DAN-ACA blue-shifted clearly. The fluorescence anisotropy increased sharply, which could be attributed to the closer molecular arrangement and decreasing polarity of the microenvironment. Moreover, the fluorescence anisotropy and emission maxima of 16-DAN-ACA were sensitive to the micellar growth in micelles in the SDS/NaBr and CTAB/KBr system. The control experiments showed that 16-DAN-ACA is a better fluorescence probe in studying the transition between micelles and vesicles than the two popular fluorescent probes pyrene and DPH. The results indicated that the 16-DAN-ACA is a good fluorescent probe for differentiate the different aggregates, and even more can be used to detect the micellar growth. Acknowledgments We thank the Natural Science Foundation of China (NSFC 20903015, 51202016), Program for New Century Excellent Talents in University (NCET-10-0277), China Scholarship Council, the
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National Training Programs of Innovation and Entrepreneurship for Undergraduates (201210710130), and the Fundamental Research Funds for the Central Universities (CHD2010ZD004, CHD2013G3312019 and 0009-2014G1311086) for the financial support of the research.
Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.saa.2014.11.113.
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Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy journal homepage: www.elsevier.com/locate/saa
Dansyl-labeled anionic amphiphile with a hexadecanoic carbon chain: Synthesis and detection for shape transitions in organized molecular assemblies Lining Gao ⇑, Huiyun Xia, Xiaoman Wang, Li Li, Huaxin Chen Engineering Research Center of Transportation Materials (Ministry of Education), School of Materials Science and Engineering, Chang’an University, Xi’an 710064, PR China
h i g h l i g h t s
g r a p h i c a l a b s t r a c t
A dansyl-labeled anionic amphiphile
with a hexadecanoic carbon chain was synthesized. The synthesized amphiphile can be used to differentiate micelle and vesicle. The fluorescence anisotropy can detect the micellar growth efficiently. The synthesized amphiphile is a better fluorescence probe than pyrene and DPH.
a r t i c l e
i n f o
Article history: Received 18 September 2014 Received in revised form 22 November 2014 Accepted 25 November 2014 Available online 25 December 2014 Keywords: Fluorescence probe Dansyl Aggregates transition Micellar growth
a b s t r a c t The probing properties of a new fluorophore-labeled anionic surfactant, sodium 16-(N-dansyl)aminocetylate (16-DAN-ACA) were investigated systematically in molecular assemblies, especially in the transitions between micelles and vesicles. 16-DAN-ACA can efficiently differentiate the two different aggregate types in mixed cationic and anionic surfactant systems. The fluorescence anisotropy of 16-DAN-ACA was found to be sensitive for directly detecting the micellar growth in micelles containing oppositely charged surfactants; both cationic cetyltrimethylammonium bromide (CTAB) systems and anionic sodium dodecyl sulfate (SDS) systems were studied. The results indicated that the 16-DAN-ACA is a good fluorescent probe for differentiating the different aggregates, and even more can be used to detect the micellar growth. Ó 2014 Elsevier B.V. All rights reserved.
Introduction Recently, different organized molecular assemblies have attracted more attention, such as spherical and rod-like/worm-like micelles, vesicles and lamellar phases, due to their special properties and applications [1–11]. Therefore, identifying how to characterize the transitions of different aggregates is becoming more and more important. Because of their ability of providing the straightforward images and detailed size information about the aggregates, electron microscopy and light scattering become two of ⇑ Corresponding author. Tel.: +86 29 82337254. E-mail address: [email protected] (L. Gao). http://dx.doi.org/10.1016/j.saa.2014.11.113 1386-1425/Ó 2014 Elsevier B.V. All rights reserved.
the most popular characterizing strategies. However, in most cases, these techniques can only give limited information about the aggregates at the molecular level. New techniques need to be developed and used to solve the new problems. Fluorescence spectroscopy is well-known to provide more detailed information at the molecular level. For example, fluorescence probes are highly sensitive to their local environment such as micropolarity, and they have multiple useful photophysical parameters like fluorescence intensity, lifetime and anisotropy, excimer/exciplex formation, etc. Therefore, fluorescence probe techniques have been extensively employed to investigate the formation and transition of aggregates [11–15]. In order to ideally probe organized molecular assemblies, the desired fluorophores
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should have attractive features like being sensitive to its molecular packing. Pyrene [16] and 1,6-diphenyl-1,3,5-hexatriene (DPH) [17,18] are two commonly used fluorescence probes due to their high efficiency in the measurement of micropolarity and microviscosity, respectively. However, the location of the kind of fluorescence probe in various aggregates is removable and unknown. Therefore, the variation of their related fluorescence parameters is usually the combined contribution from both the transition of aggregates and the change of the location of probes. This problem may be expected to be solved using fluorophore-labeled amphiphile. This kind of fluorescence probe, when used as part of the surfactant, can take part in the formation of aggregates so as to provide more reliable information. In fact, some fluorophorelabeled amphiphiles have been used to detect the related physical properties of various aggregates [19–29]. However, most of these studies focused on micelle [19–23] or membrane [24–28] systems. In our previous work, a fluorescent group, dansyl, which shows representative character of intramolecular charge transfer (ICT), has been attached to the tail of an anion surfactant (12-aminolauric acid) to form a surfactant-fluorescence probe sodium 12-(Ndansyl)aminododecanate (12-DAN-ADA) [30,31]. We found that 12-DAN-ADA can efficiently differentiate two different kinds of aggregates (micelles and vesicles) in the mixed cationic and anionic surfactant systems, as well as in the double-chain cationic surfactant systems based on the polarity- and viscosity-dependence of the dansyl group. Further studies demonstrated that 12-DAN-ADA is a more efficient probe of transitions between micelles and vesicles than those commonly used fluorescence probes, such as pyrene and DPH [30]. In order to investigate the effect of the solubility of the surfactant-fluorescence probes on the efficiency of probing the transitions between micelles and vesicles, in the present work, we extended the alkyl chain of 12-DAN-ADA to prepare another surfactant-fluorescence probe with longer hydrophobic chain, sodium 16-(N-dansyl)aminocetylate (16-DAN-ACA), and measured its corresponding fluorescence parameters in the transitions of micelles and vesicles. Although, the chemical structures of 12-DAN-ADA and 16-DANACA are very similar, the procedure for obtaining the 16-DANACA is laborious. As expected, the corresponding fluorescence parameters (fluorescence anisotropy and emission maximum) of 16-DAN-ACA can also reflect efficiently the transitions between micelles and vesicles. All the results demonstrated that the solubility of the surfactant-fluorescence probes did have an effect on the efficiency of probing the transitions between micelles and vesicles.
Experimental section Chemicals 5-Cyclohexadecen-1-one (>97.0%) was purchased from TCI. Dansyl chloride (99%), pyrene (98%), 1,6-diphenyl-1,3,5-hexatriene (DPH, 98%), and sodium dodecyl sulfate (SDS, 99%) were purchased from Acros. Triton X-100 was from E. Merck Co. (Darmstadt). Branched-chain sodium dodecylbenzenesulfonate (SDBS), hydroxylamine hydrochloride (NH2OHHCl), and Pd–C catalyst were obtained from Beijing Chemical Co. All the above regents were used as received. Deionized water was treated with KMnO4 over 24 h and distilled before use. Dodecyltrimethylammonium bromide (DTAB), dodecyltriethylammonium bromide (DEAB), and cetyltrimethylammonium bromide (CTAB) were synthesized from n-alkyl bromide and corresponding trialkylamine [32]. The three products were recrystallized five times from acetone, acetone/diethyl ether, and acetone/ethanol, respectively. The purity of all the synthesized cationic surfactants was examined, and no surface tension mini-
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mum was found in the surface tension curve. All other reagents were of analytical grade at least. General instruments 1
H NMR spectra were recorded on a Mercury Plus 300 M (USA) or a ARX-400 M (Swiss) instrument, and chemical shifts are reported in parts per million relative to TMS in proton spectra. Element analyses of C, H, and N were conducted on an Elementar Vario EL elemental analyzer (Germany). MS results were conducted on a ZAB-HS magnetic mass spectrometer (Micromass, UK). Fluorescence measurements All of the fluorescence measurements were performed on a time-correlated single-photon-counting FLS 920 fluorescence spectrometer (Edinburgh) at room temperature (23–25 °C) except where noted in the text. Samples were excited at 337 nm. The stock solutions (1.0 10 4 M) of 16-DAN-ACA were prepared in ethanol. A certain amount of stock solution was added to a tube and heated slightly to remove some solvent. Then the final concentration of the probes was adjusted by adding an appropriate amount of the analyte solutions. A FLS 920 fluorescence spectrometer equipped with filter polarizers that use the L-format configuration using a 1-cm quartz cuvette was used for fluorescence depolarization measurements. An average of three fluorescence anisotropy values was recorded. Synthesis and characterization of 16-DAN-ACA The synthetic route of 16-DAN-ACA was shown in Scheme 1, in which 16-aminohexadecanoic acid (compound 6) was synthesized by adopting a modified literature method [33]. Synthesis of cyclohexadecanone (compound 2) 5-Cyclohexadecen-1-one (compound 1, 5 g, 21.2 mmol) was dissolved in ethanol (45 mL). Pd–C catalyst (1%, w%) was added to the solution of compound 1 while stirring vigorously at room temperature under a H2 atmosphere. Stirring was continued for a total of 1.5 h. The resulting solution was filtered and the solvent was removed to give compound 2 as a white solid. 1H NMR (CDCl3/ Me4Si, 400 MHz) d (ppm): 1.29 (s, 22H, –(CH2)11–), 1.62 (m, 4H, – CH2CH2COCH2CH2–), 2.40 (t, 4H, –CH2COCH2–). Synthesis of cyclohexadecanone oxime (compound 3) Compound 2 (5.8 g, 24.4 mmol) was dissolved in methanol (140 mL). NH2OHHCl (3.5 g, 50.4 mmol) and NaHCO3 (3.5 g, 41.7 mmol) were added to the stirred methanol solution. The mixture was stirred and refluxed for 5 h. After that, the mixture was brought to room temperature and stirred overnight. The methanol phase was add in water (30 mL) and extracted with CHCl3 (3 80 mL). The CHCl3 phase was dried by anhydrous sodium sulfate overnight and filtered. The filtrate was evaporated under reduced pressure and the residue was dried in vacuum to give compound 3 as a white solid. 1H NMR (CDCl3/Me4Si, 400 MHz) d (ppm): 1.30 (s, 22H, –(CH2)11–), 1.51 (m, 4H, –CH2CH2CNOHCH2CH2–), 2.18 (t, 2H, –CH2CNOH), 2.36 (t, 2H, –CH2CNOH). Synthesis of cyclohexadecanone-oxime p-toluenesulfonate (compound 4) Compound 3 (5.9 g, 23.3 mmol) was dissolved in the mixture of CH2Cl2 (240 mL) and pyridine (40 mL). TsCl (7.6 g, 39.9 mmol), dissolved in CH2Cl2 (1200 mL), was added drop-wise to the above solution while stirring vigorously at 0 °C. After addition was completed, the mixture was brought to room temperature and stirred overnight. The CH2Cl2 layer was extracted with hydrochloric acid
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Scheme 1. Synthetic route of 16-DAN-ACA.
(3.5 M, 60 mL) and 5% aqueous NaHCO3 (3 60 mL), dried over anhydrous sodium sulfate, and filtered. The filtrate was evaporated and dried in vacuum to give compound 4 as a yellow–brown liquid. Synthesis of 2-azacycloheptadecanone (compound 5) Compound 4 (9.5 g, 23.3 mmol) was dissolved in THF (650 mL). 30 mmol aqueous K2CO3 (650 mL) was added to the above solution and stirred overnight at room temperature. The mixture was evaporated under reduced pressure to remove THF, and the resulting aqueous phase was extracted with CH2Cl2. The CH2Cl2 layer was combined, dried over anhydrous sodium sulfate and filtered. The filtrate was evaporated and dried in vacuum to give compound 5 as a wax-like yellow solid. 1H NMR (CDCl3/Me4Si, 400 MHz) d (ppm): 1.30 (s, 22H, –(CH2)11–), 1.51 (m, 2H, –CH2CH2NH–), 1.65 (m, 2H, –CH2CH2CO), 2.18 (t, 2H, –CH2CO), 3.30 (q, 2H, –CH2NH– ), 5.45 (q, 1H,–NH–).
upon addition of concentrated hydrochloric acid to the above solution. The solid was filtered, washed with deionized water, and was treated as above several times to give the carboxylic-acid type of compound 7 as a light yellow solid. The mass measurement of compound 7 was conducted using its carboxylic-acid type due to the instability of compound 7 while measuring MS. MS for C28H44N2SO4: MW = 504. Found: MW = 504. Surface property of 16-DAN-ACA (compound 7) As an anionic surfactant, the critical micelle concentration (CMC) of 16-DAN-ACA was investigated using fluorescence method. Results and discussion Comparison of the solubility in water
Synthesis of 16-aminohexadecanoic acid (compound 6) Compound 5 (1 g, 3.9 mmol) was added to hydrochloric acid (6 M, 40 mL). The mixture was stirred and refluxed for 20 h. White crystal was precipitated after the mixture brought to room temperature. Then, a moderate amount of ethanol was added to the mixture to dissolve the impurity. The resulting mixture was filtered to give white crystal. The white crystal was washed with ethanol, CH2Cl2, and then dissolved in boiling water. The pure white crystal was precipitated upon cooling to room temperature. Compound 6 as white crystal was filtered and vacuum-dried. Synthesis of 16-DAN-ACA (compound 7) Dansyl chloride and compound 6 (mol:mol = 1:3) were combined in aqueous NaOH (2 M) and stirred in dark condition at room temperature for 24 h. The mixture was extracted with ethyl acetate three times. The organic phase was combined, dried over anhydrous sodium sulfate, and filtered. The filtrate was evaporated and dried in vacuum to give yellow solid. The solid was added to aqueous NaOH (2 M) and heated to dissolve. The aqueous mixture was filtered to give clean yellow solution. Compound 7 as light yellow solid was precipitated upon the solution cooling to room temperature. 1H NMR (DMSO-d6/Me4Si, 400 MHz) d (ppm): 8.45 (d, 1H, Ar–H), 8.33 (d, 1H, Ar–H), 8.10 (d, 1H, Ar–H), 7.60 (m, 2H, Ar–H), 7.25 (d, 1H, Ar–H), 2.82 (s, 6H, –N(CH3)2), 2.76 (t, 2H, COONa– CH2–), 2.67 (m, 2H, –NH–CH2–), 1.80 (t, 2H, COONa–CH2–CH2–), 1.39 (m, 2H, –NH–CH2–CH2–), 1.31–0.96 (m, 22H, –NH–CH2– CH2–(CH2)11–). EA (%) calcd for C28H43N2SO4Na2H2O: C, 59.79; H, 8.36; N, 4.98. Found: C, 60.05; H, 7.69; N, 4.74. To further purify compound 7, the light yellow solid was dissolved in hot aqueous NaOH (2 M). White solid was precipitated
Since 16-DAN-ACA is in free state when its concentration is lower than its CMC, or in micellar state when its concentration is higher than the CMC, there must be a significant change in the polarity of their microenvironment during the transition between its free state and the micellar state. Specifically, when the concentration is higher than its CMC, 16-DAN-ACA molecules will form micelles, thus the polar-water environment of 16-DAN-ACA molecules will transfer gradually to the non-polar hydrophobic environment inside of the micelles, which will make the rotation of 16DAN-ACA molecules difficult. In this case, the fluorescence emission maximum of 16-DAN-ACA hypochromatically shift. Fig. 1 shows the emission maxima and fluorescence anisotropy of 16DAN-ACA at different concentration in aqueous Na2B4O710H2O solutions (1.0 10 2 M, pH = 9.2). We can find that when the concentration is around 10 5 M, the hypochromatic shift of its emission maximum and the change in the value of anisotropy are remarkable. Thus the CMC of 16-DAN-ACA is about 10 5 M, which is much lower than the CMC of 12-DAN-ADA (about 2.2 10 3 M) [30], indicating the lower solubility of 16-DAN-ACA in water than that of 12-DAN-ADA. Determination of the polarity and viscosity dependence of 16-DANACA As an ICT compound, dansyl is very sensitive to the polarity and viscosity of its microenvironment. Hence it is expected that this polarity and viscosity dependence of dansyl can be restored when it is labeled to 16-aminolauric acid to form a novel anionic surfactant, 16-DAN-ACA. Based on this consideration, the
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fluorescence probe to detect the formation and transition of aggregates, since 16-DAN-ACA can take part in the formation of the aggregates. Thus, the variation of related fluorescence parameters in the formation of aggregates and the corresponding transition between different aggregates were examined. Probing the transition between micelles and vesicles in mixed cationic and anionic surfactant systems
Fig. 1. Emission maxima and fluorescence anisotropy for different concentration of 16-DAN-ACA in 1.0 10 2 M Na2B4O710H2O (pH = 9.2) aqueous solutions.
fluorescence emission spectra of 16-DAN-ACA in 1,4-dioxane/ water mixtures [34] and various solvents with different polarities were measured. As expected, the emission maxima of 16-DANACA bathochromically shifted considerably, along with the increasing solvent polarity. Fig. 2a shows the emission maxima of 16DAN-ACA in 1,4-dioxane/water mixtures with different water components. The emission maximum shifted bathochromically about 70 nm, from 480 to 550 nm, while the solvent changed from neat 1,4-dioxane to neat water. The normalized fluorescence emission spectra of 16-DAN-ACA in five different solvents are shown in Fig. 2b (with increasing polarity according to ET(30) values from Ref. [35]). Except the difference in polarity, the cohesive energy density, mainly reflect the interaction of molecules, also increased from c.a. 20 to 50 MPa1/2 (the data from Ref. [36]). Along with the increasing of cohesive energy density of solvent, the interaction between solvent molecule and 16-DAN-ACA was enhanced, which results in the stronger intermolecular charge transfer of dansyl group and the bathochromical shift of emission maximum. Meanwhile, the fluorescence anisotropy of 16-DAN-ACA in the binary solvents of glycerine/water (with different viscosities) [37] was also examined (cf. Fig. 3). The curve in Fig. 3 suggests that the fluorescence anisotropy of 16-DAN-ACA increases along with the increasing viscosity. These observations demonstrate that the properties of polarity and viscosity dependence of dansyl are well reserved in 16-DAN-ACA. Based on the consideration of its solubility, polarity and viscosity dependence, it may be suitable to use 16-DAN-ACA as a
Considering the rich aggregation behaviors and the challenging characterization in mixed cationic and anionic surfactant systems, the transition between micelles and vesicles in these systems was studied systematically. In the present work, the mixed cationic and anionic surfactant systems, SDS/DEAB and SDBS/DTAB were selected to study the transitions. It is well known that the molecular arrangements become more packed and more orderly [38,39], and the polarity of the hydrophobic region decreases considerably when the aggregate transforms from micelles to vesicles. Therefore, the variation of the emission maximum and fluorescence anisotropy will respond to the change of the aggregates. 16-DAN-ACA was introduced into the micelle region, the vesicle region, and their mixed region of the SDS/DEAB system [11] as a fluorescence probe. The total concentration of SDS and DEAB was 1.0 10 2 M in the following experiments. The emission maxima and fluorescence anisotropy of 16-DAN-ACA in the SDS/DEAB system are shown in Fig. 4. When the organized assemblies changed from micelles to vesicles, the emission maximum of 16-DAN-ACA shifted hypochromatically from 538 to 518 nm while the fluorescence anisotropy increased sharply along with this transition. In contrast, the emission maximum of 16-DAN-ACA bathochromically shifted back to 527 nm. In addition, the fluorescence anisotropy decreased gradually while the vesicles transformed back to micelles along with increasing the molar ratio of DEAB in the SDS/DEAB system. It should be noted that the emission maximum of 16-DAN-ACA in the DEAB-rich micelle region is lower than that in the SDS-rich micelle region. Correspondingly, the fluorescence anisotropy in the DEAB-rich micelle region is higher than that in the SDS-rich region. These observations may be due to the electrostatic interaction between DEAB and 16-DAN-ACA. Given the obvious change in the polarity and viscosity of the system during the transition between micelle and vesicle, 16-DAN-ACA could be considered as a similarly ideal fluorescence probe as 12-DAN-ADA [30] in probing the transition between micelle and vesicle in mixed cationic and anionic surfactant systems. Further investigation was also performed in another mixed cationic and anionic surfactant system, SDBS/DTAB [7]. Fig. 5 shows
Fig. 2. Emission maxima of 16-DAN-ACA in 1,4-dioxane/water mixtures (a) and normalized fluorescence emission spectra of 16-DAN-ACA in different solvents (the used solvents are 1,4-dioxane, ethyl acetate, dichloromethane, ethylene glycol and water as the arrow noted) (b) ([16-DAN-ACA] = 1.0 10 5 M).
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Fig. 5. Emission maxima and fluorescence anisotropy of 16-DAN-ACA in the SDBS/ DTAB system. [SDBS] = 5.0 10 2 M, [16-DAN-ACA] = 1.0 10 6 M. M: micelles; V: vesicles. Fig. 3. Fluorescence anisotropy of 16-DAN-ACA in glycerine/water mixtures ([16DAN-ACA] = 1.0 10 5 M). The viscosity values are obtained from Ref. [37].
Fig. 4. Emission maxima and fluorescence anisotropy of 16-DAN-ACA in the SDS/ DEAB system. [T] = 1.0 10 2 M, [16-DAN-ACA] = 1.0 10 6 M. P: precipitate; M: micelles; V: vesicles.
the change in the emission maximum and fluorescence anisotropy of 16-DAN-ACA in the SDS/DTAB system. It is found that similar results were obtained in these systems, showing that the variation of the fluorescence parameters was also consistent with that of the corresponding aggregate forms. As the control experiment, the fluorescence anisotropy of DPH and the I1/I3 value of pyrene were conducted in the SDS/DEAB system to examine the fluorescence responses of DPH and pyrene to different aggregate forms, respectively, and the results were shown in Fig. 6. Different from the observation of 16-DAN-ACA, there was no obvious variation in all the fluorescence signals over the whole tested concentration range in Fig. 6. This is consistent with the comparison of 12-DAN-ADA, DPH and pyrene [30], suggesting that 16-DAN-ACA has advantage over traditional fluorescence probes, such as DPH and pyrene, in probing the transition of micelles and vesicles in mixed cationic and anionic surfactant systems.
Probing the micellar growth A typical micellar system, SDS/NaBr, which is known in the literature because of its full spectrum of micellar shapes, was also adopted to study the morphological transition of different micelles. At a given surfactant concentration, the morphology of the micelle
will transform from sphere, ellipsoid, rod to worm at proper conditions. 16-DAN-ACA was applied to probe the micellar transition in the anionic surfactant SDS system. It is known that the micelles change from spheres to rods, then to worms when increasing the concentration of NaBr from 0 to 1 M in 35 mM or in 175 mM SDS system at 40 °C (the result from SANS studies) [40]. In this process, the SDS molecules are supposed to pack more compactly, and the local environment in the micelles changes accordingly. These changes can be examined using 16-DAN-ACA as the probe. Fig. 7 shows the fluorescence anisotropy and emission maxima of 16DAN-ACA in the SDS/NaBr system. The concentration of SDS was 35 mM and 175 mM, respectively. The fluorescence anisotropy increased slowly from 0.010 to 0.018 while the emission maximum hypochromatically shifted slightly from 532 to 529 nm upon increasing the concentration of NaBr from 0 to 1 M. The significant change in the fluorescence anisotropy indicates the change in the viscosity of the microenvironment of 16-DAN-ACA, which is consistent with the growth of the micelle. The fluorescence maximum, however, did not change remarkably (3 nm), which is inconsistent with the 20 nm blue shift in the process of micelle–vesicle transition (Figs. 4–6). These results might suggest that the polarity in the micellar core does not change considerably in the process of micellar growth. Compared with 12-DAN-ADA in the SDS/NaBr system [31], the difference between the change in the fluorescence anisotropy of the 12-DAN-ADA system and the 16-DAN-ACA system is relatively small. The emission maxima of 16-DAN-ACA in the SDS/NaBr system, however, were more hypochromatical than those of 12-DANADA in the SDS/NaBr system (see Fig. S1 in the Supporting Information). This might be due to the location of 16-DAN-ACA in the aggregates or the smaller solubility of 16-DAN-ACA in water. According to the literature [42,43], the micellar growth could also be realized by the addition of 0 to 0.6 M KBr in 10 mM CTAB aqueous solution at 30 °C. The fluorescence anisotropy and the emission maxima of 16-DAN-ACA in the CTAB/KBr system were examined and the results are shown in Fig. 8. The fluorescence anisotropy increased from 0.018 to 0.037 as the concentration of KBr increased from 0 to 0.6 M. The change in the anisotropy of 16-DAN-ACA in the CTAB/KBr system is more significant than that in the SDS/NaBr system, indicating that the electrostatic interaction between 16-DAN-ACA and CTAB indeed helps to promote the sensitivity of 16-DAN-ACA [31]. In other words, the electrostatic interaction makes 16-DAN-ACA enrich in the micelles, so that just a small amount of 16-DAN-ACA molecules can exist in the bulk solution [30]. As a result, the fluorescence contributed by free 16-DAN-ACA molecules in the bulk solution was reduced
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Fig. 6. Fluorescence anisotropy of 16-DAN-ACA and DPH (a) and emission maxima of 16-DAN-ACA and I1/I3 values of pyrene (b) in the SDS/DEAB system. ([16-DANACA] = 1.0 10 6 M, [pyrene] = 1.0 10 7 M, [DPH] = 1.0 10 6 M).
Fig. 7. Fluorescence anisotropy and emission maximum of 16-DAN-ACA in different SDS/NaBr system at 40 °C. [SDS] = 35 mM (a) and [SDS] = 175 mM (b), [16-DANACA] = 1.0 10 6 M. The related data of three points in the figure are from the results of SANS [40,41] for [SDS] = 35 mM and [SDS] = 175 mM systems respectively and indicate the different micelle type.
Conclusions
Fig. 8. Fluorescence anisotropy and emission maximum of 16-DAN-ACA in the CTAB/KBr system at 30 °C. [CTAB] = 10 mM, [16-DAN-ACA] = 1.0 10 6 M.
in the CTAB systems. On the one hand, the apparent fluorescence anisotropy for this system became more significant due to the smearing of the fluorescence from the free 16-DAN-ACA. On the other hand, the emission maximum changed slightly (about 5 nm), which is consistent with the change in SDS/NaBr system (see Fig. 7a). These results suggest that the polarity in the micellar core is not considerably changed in the process of micellar growth.
The new fluorophore-labeled surfactant 16-DAN-ACA was synthesized and was used to differentiate the two different aggregate types in mixed cationic and anionic surfactant systems and detect the growth of micelles formed in cationic and anionic surfactant systems successfully. Along with the aggregates transforming from micelles to vesicles in mixed cationic and anionic surfactant systems, the emission maxima of 16-DAN-ACA blue-shifted clearly. The fluorescence anisotropy increased sharply, which could be attributed to the closer molecular arrangement and decreasing polarity of the microenvironment. Moreover, the fluorescence anisotropy and emission maxima of 16-DAN-ACA were sensitive to the micellar growth in micelles in the SDS/NaBr and CTAB/KBr system. The control experiments showed that 16-DAN-ACA is a better fluorescence probe in studying the transition between micelles and vesicles than the two popular fluorescent probes pyrene and DPH. The results indicated that the 16-DAN-ACA is a good fluorescent probe for differentiate the different aggregates, and even more can be used to detect the micellar growth. Acknowledgments We thank the Natural Science Foundation of China (NSFC 20903015, 51202016), Program for New Century Excellent Talents in University (NCET-10-0277), China Scholarship Council, the
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National Training Programs of Innovation and Entrepreneurship for Undergraduates (201210710130), and the Fundamental Research Funds for the Central Universities (CHD2010ZD004, CHD2013G3312019 and 0009-2014G1311086) for the financial support of the research.
Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.saa.2014.11.113.
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