H-Shaped Supra-Amphiphiles Based on a Dynamic Covalent Bond

Sep 17, 2012 - Micellar aggregates formed by the self-assembly of the H-shaped supra- amphiphiles are observed. When pH is tuned down from basic to ...
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H‑Shaped Supra-Amphiphiles Based on a Dynamic Covalent Bond Guangtong Wang, Chao Wang, Zhiqiang Wang, and Xi Zhang* Key Lab of Organic Optoelectronics and Molecular Engineering, Department of Chemistry, Tsinghua University, Beijing 100084, PR China S Supporting Information *

ABSTRACT: The imine bond, a kind of dynamic covalent bond, is used to bind two bolaform amphiphiles together with spacers, yielding H-shaped supra-amphiphiles. Micellar aggregates formed by the self-assembly of the H-shaped supraamphiphiles are observed. When pH is tuned down from basic to slightly acidic, the benzoic imine bond can be hydrolyzed, leading to the dissociation of H-shaped supra-amphiphiles. Moreover, H-shaped supra-amphiphiles have a lower critical micelle concentration than their building blocks, which is very helpful in enhancing the stability of the benzoic imine bond being hydrolyzed by acid. The surface tension isotherms of the H-shaped supra-amphiphiles with different spacers indicate their twisty conformation at a gas−water interface. The study of H-shaped supra-amphiphiles can enrich the family of amphiphiles, and moreover, the pH-responsiveness may make them apply to controlled or targetable drug delivery in a biological environment.

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of benzoic imine bonds have been prepared since 2009.33−38 It is also found that the aggregation of these amphiphiles is very helpful in decreasing the pH at which benzoic imine starts hydrolyzing. The reason is probably due to the fact that the pHsensitive imine bonds are buried in the hydrophobic core of the aggregates, preventing the acid from entering the aggregates and influencing the hydrolysis of the imine bonds.34,38 In other words, the supra-amphiphile with a lower critical micelle concentration (cmc) has a lower critical pH of imine hydrolyzation. Nevertheless, benzoic imine bonds in these supra-amphiphiles, especially in low-molecular-weight ones, still need to be stabilized by high pH and high concentration. A bolaform38−44 supra-amphiphile can be fabricated by linking two hydrophilic headgroups with a hydrophobic chain using a noncovalent interaction or a dynamic covalent bond.38,45 Herein we have successfully applied two benzoic imine bonds to connect two bolaform amphiphiles together to form a series of supra-amphiphiles with an “H” shape46,47 (Scheme 1). The H-shaped supra-amphiphiles can aggregate to form micelles at even lower concentrations. As a result, the imine bonds in these H-shaped supra-amphiphiles are more

INTRODUCTION Supra-Amphiphiles or supramolecular amphiphiles1−3 refer to amphiphiles that are fabricated by noncovalent interactions or dynamic covalent bonds. In contrast to the amphiphiles formed by ordinary covalent bonds, the formation and aggregation of supra-amphiphiles are easier to modulate because of the reversible and dynamic nature of the noncovalent interactions as well as dynamic covalent bonds. Therefore, the study of supra-amphiphiles provides a powerful tool for controlled selfassembly and disassembly, leading to the fabrication of stimuliresponsive surfaces and nanocontainers.4−9 Various noncovalent interactions have been used to create supraamphiphiles, such as hydrogen bonds, electrostatic interactions, host−guest interactions, and charge-transfer interactions.10−13 Dynamic covalent bonds14−19 (DCBs) are a kind of covalent bond that can be formed and broken reversibly and quickly under equilibrium control. Because of their dynamic nature, DCBs have already been used in the fabrication of supramolecular polymers,20,21 stimuli-responsive gels,22,23 selfadapting materials,24 and stimuli-responsive surfaces25,26 and nanocarriers.27−29 Among all of the DCBs, the benzoic imine bond30−32 is very suitable for fabricating supra-amphiphiles because of its simple and well-defined structure and pH responsiveness in water. Several supra-amphiphiles with various topologies on the basis © 2012 American Chemical Society

Received: August 13, 2012 Revised: September 15, 2012 Published: September 17, 2012 14567

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Scheme 1. Schematic Illustration of the H-Shaped Supra-Amphiphile and Its Behavior in Water at the Water−Air Interfacea

a

Two parts of H-shaped supra-amphiphiles, “spacer” and “arm”, are defined in the scheme. modulated to 12.4. The diameters of the aggregates were measured with a Malvern Instruments Ltd. Nano ZS90. After that, the solutions were immediately used to prepare the TEM samples. The solutions were drop cast on a carbon-coated copper grid and negatively stained with a 0.2% phosphotungstic acid solution. TEM experiments were performed with a Hitachi H-7650B electron microscope. Measurement of cmc Using a Nile Red Fluorescent Probe. For cmc measurements, a series of imine-2n-p, imine-10-pasy, aldehyde-2n, and aldehyde-10asy solutions at different concentrations were prepared in phosphate buffers at the same pH (12.4) and concentration. Nile red (2 mM, dissolved in THF) was added to each solution under sonification to ensure that Nile red was loaded into the aggregates. The solutions were allowed to stand for 4 to 5 h, and their fluorescence was recorded using a Hitachi F-7000 fluorescence spectrophotometer. Surface Tension Measurement. The surface tension was measured using a Data Physics DCAT21 surface tension meter by the Wilhelmy plate method. Solutions (9.0 mM) of the H-shaped supra-amphiphiles were prepared, and the pH of the solutions was tuned to 12.4 by adding Na3PO4. For each supra-amphiphile, the 9.0 mM solution was added gradually to a 20.0 mL Na3PO4 aqueous solution with the same concentration (about 60 mM) so that the surface tension and the concentration of the sample could be recorded at the same time and the surface tension isotherm of each supraamphiphile could be obtained.

stable than conventional amphiphiles formed by imine bonds. Their behaviors as controllable surfactants are also investigated by measuring their surface tension isotherms (γ−log C curves), indicating the twisty conformation of the H-shaped supraamphiphiles at the water−air interface.

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EXPERIMENTAL SECTION

Material Preparation. A series of bola-amphiphiles, as shown in Figure 1, were prepared as the building blocks of the H-shaped supraamphiphiles. Their synthesis routes are well documented in the Supporting Information. The other building blocks with two amino groups, such as 1,4-phenylenedimethanamine, pentane-1,5-diamine, and nonane-1,9-diamine, also shown in Figure 1, were purchased from Alfa Aesar. The H-shaped supra-amphiphiles were prepared by mixing the corresponding building blocks in methanol (1:1 aldehyde groups/ amino groups). The solution was allowed to stand for 30 min, and then the methanol was removed by vacuum evaporation. 1 H NMR Characterization at Different pH Values. For the NMR test, the solvent was D2O, which was buffered by DCl and Na3PO4. A series of samples with different pH values were prepared; a part of each solution was taken to perform 1H NMR analysis using a JEOL JNM-ECX400 spectrometer, and the rest was used to measure the final pH using a Mettler Toledo Delta 320 pH meter. Dynamic Light Scattering Study (DLS) and Transmission Electron Microscopy (TEM). The self-assembled structures of Hshaped supra-amphiphiles were revealed using DLS and TEM. For DLS measurement, 3 mM imine-10-p, imine-10-C5, and imine-10-C9 solutions were prepared, and the pH values of the solutions were 14568

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Figure 1. Building blocks and H-shaped supra-amphiphiles (2n = 6, 8, 10). The number following “imine” refers to the number of CH2 groups in the arm of the H-shaped supra-amphiphile. p, C5, and C9 at the ends of names refer to the spacers. The subscript “asy” refers to the asymmetrical substitute positions of the two alkoxy groups on the benzene ring. All of the bola-amphiphiles are named in a similar way.

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RESULT AND DISCUSSION Formation and Self-Assembly of H-Shaped SupraAmphiphiles. Herein, imine-10-p is chosen as an example to investigate the formation and self-assembly of H-shaped supraamphiphiles. Its formation is first confirmed by 1H NMR, as shown in Figure 2. In the spectrum of aldehyde-10, the peak of

of imine-10-p in CD3OD and in D2O, the peaks in D2O become broader, suggesting the aggregation of imine-10-p in water. cmc measurements by the Nile red (NR) probe show the difference between the aggregation of imine-10-p and aldehyde10. The cmc of imine-10-p is as low as 1.5 × 10−6 M, indicating that its aggregating ability is better than its bolaform building block (Figure 3a). The reason may be rationalized by the fact that the motion of the bolaform building blocks is restricted when they are linked by a dynamic spacer.48 The fluorescence emission of NR is sensitive to the polarity of the solvent. There is a blue shift when the NR is dissolved in a less-polar solvent.49 It is easily found that the wavelength of the fluorescence emission of NR in imine-10-p aggregates is shorter than that of aldehyde-10, indicating that the cores of imine-10-p aggregates are more hydrophobic. Aggregates formed by the self-assembly of imine-10-p are investigated using TEM and DLS. As shown in Figure 3b, spherical micellar aggregates can be clearly observed in TEM. However, because of the effect of the staining agent, the diameter measured by TEM may not correspond to their real sizes. Therefore, besides TEM observation, the DLS experiment was performed to confirm the diameter of the micelles. The DLS result shows that the average diameter is 4.8 nm, which can reflect the size of the micellar aggregates formed by imine10-p molecules (Figure 3c). pH Responsiveness. To investigate the pH responsiveness, a series of aqueous solutions of imine-10-p, imine-8-p, imine-6p, and imine-10-pasy with the same concentration (3 mM) but different pH values were prepared, respectively, and studied by

Figure 2. Formation of the imine bond. 1H NMR spectra of (a) aldehyde-10 (3 mM, in D2O, pH 12.4), (b) imine-10-p (3 mM, in CD3OD), and (c) imine-10-p (3 mM, in D2O, pH 12.4).

the aldehyde group appears at 9.6 ppm. The spectrum of imine10-p prepared in CD3OD shows that the peak of the aldehyde group totally disappears and a new peak at around 8.4 ppm appears, indicating almost 100% formation of the imine bond. When the solvent is changed to D2O (pH 12.4), the peak of the imine bond appears at 7.7 ppm. To compare the NMR spectra 14569

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carbon chain has a lower cmc, leading to a lower critical hydrolyzation pH of imine. It is also notable that the critical pH of imine hydrolyzation of imine-10-p, which is measured as being about 7.5, is quite low among the examples of lowmolecular-weight supra-amphiphile based on dynamic imine bonds. Its low critical pH of hydrolyzation could attribute to its special topology and long carbon chain. However, in imine-6-p, in which the carbon chain is too short to aggregate, the imine conversion is so low that very few imine-6-p molecules can form at pH 12.4 and 3 mM. The substituted position of alkoxyl groups on the phenyl aldehyde also has an influence on the property of H-shaped supra-amphiphiles. Compared to those for imine-10-p, both the cmc and the critical pH of the imine hydrolyzation of imine-10-pasy are higher. Behavior at the Gas−Water Interface. The H-shaped supra-amphiphiles can certainly be considered to be controllable surfactants. The surface-tension isotherms of imine-10-p and aldehyde-10 were measured under both acidic (pH 2.4) and alkaline (pH 12.4) conditions, as shown in Figure S3. It is easy to find that their surface-tension isotherms are quite similar when the pH is 2.4 because imine-10-p is totally hydrolyzed under this condition. However, a great difference appears when the pH is 12.4, which indicates the poorer surface activity of imine-10-p compared to that of aldehyde-10. The minimum surface area (Amin) of imine-10-p at the gas−water interface can be calculated to be 4.8 nm2 per molecule from the surface-tension isotherm (as shown in Figure 5) using the

Figure 3. Self-assembly of imine-10-p in water (pH 12.4). (a) cmc measurement by the NR fluorescent probe. (b) TEM image of the micelles formed by imine-10-p. (c) DLS data of the micelles. 1

H NMR. The intensity of the aldehyde signal decreases or even disappears with the increase in pH. Meanwhile, the signal of the imine group intensifies significantly. The ratio Aimine/ (Aimine + Aaldehyde), in which Aaldehyde is the integral value of the aldehyde proton and Aimine is the integral value of the imine proton, can be used to estimate the conversion of the aldehyde groups, as shown in Figure 4. By comparing the pH-conversion

Figure 5. Surface-tension isotherms of the H-shaped supraamphiphiles with different spacers at 298 K. Figure 4. pH-conversion curves of imine-10-p, imine-8-p, imine-6-p, and imine-10-pasy (3 mM in D2O).

Gibbs−Duhem equation. It suggests that the conformation of imine-10-p should be twisted with all four hydrophilic headgroups immersed in water, and their packing at the water−air interface should not be very compact as shown in Scheme 1. The surface-tension isotherms of imine-10-C5 and imine-10-C9, of which the spacers are different from those in imine-10-p, are also measured for comparison. It can be seen that their minimum surface areas are quite different (Table 2). The difference should be caused by the spacers. 1,4-Phenylenedimethanamine and pentane-1,5-diamine are approximately equal in length; however, the flexibility of the alkyl chain in pentane-1,5-diamine makes imine-10-C5 occupy less area at the gas−water interface. Nonane-1,9-diamine, which is much longer than the other two molecules, makes imine-10-C9 have the largest Amin. Additionally, the sizes of the micelles formed by these supra-amphiphiles reveal the same trend with Amin. In other words, the minimum surface area is smaller when the diameter of the micelles in the bulky phase decreases.

curves of imine-10-p, imine-8-p, and imine-6-p, of which the imine bonds are the same, it can be found that the cmc and the hydrolyzation of the imine bonds is relevant to the length of the carbon chain (Table 1). The supra-amphiphile with longer Table 1. cmc Values of H-Shaped Supra-Amphiphiles and Their Bolaform Building Blocksa amphiphiles aldehyde-10 aldehyde-8 aldehyde-6 aldehyde-10asy

1

/2 cmc (mol/L) 5.1 5.6 4.1 1.2

× × × ×

10−5 10−4 10−3 10−4

amphiphiles imine-10-p imine-8-p imine-6-p imine-10-pasy

cmc (mol/L) 1.5 9.4 1.0 7.8

× × × ×

10−5 10−5 10−3 10−5

a1

/2cmc of the bolaform building blocks is given in order to compare conveniently. 14570

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Table 2. cmc, γcmc, Minimum Surface Area, and Diameter of the Micelles in the Bulk Phase of the H-Shaped SupraAmphiphiles with Different Spacers name imine10-p imine10-C5 imine10-C9

cmc (μM)

γcmc (mN/ m)

Amin (nm2)

average diameter of the micelles in the bulky phase (nm)

12.0

61.0

4.8

4.8 ± 0.1

4.8

62.2

2.7

4.5 ± 0.1

2.4

57.9

7.7

6.2 ± 0.2

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CONCLUSIONS We have prepared a series of new H-shaped supra-amphiphiles by linking two bolaform amphiphiles together with a spacer using dynamic imine bonds. Their properties in water and at the water−air interface are investigated, respectively. It is found that these H-shaped supra-amphiphiles have a stronger aggregating ability than their building blocks of bolaform amphiphile, which is responsible for their low critical pH values of imine hydrolyzation. In addition, H-shaped supra-amphiphiles adopt twisty conformations at the gas−water interface. It is anticipated that this line of research can enrich the traditional field of colloids and interface science by introducing a new family of amphiphiles. Moreover, the micellar aggregates formed by these supra-amphiphiles are endowed with stimuliresponsive properties.

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ASSOCIATED CONTENT

S Supporting Information *

Synthesis details. 1H NMR spectra of pH responsiveness. Surface tension isotherms of imine-10-p and aldehyde-10 at pH 12.4 and 2.4. Statistics of the diameters in the TEM image. This material is available free of charge via the Internet at http:// pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Tel: +86-10-62796283. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was financially supported by the National Basic Research Program of China (2013CB834502), the NSFC (50973051 and 20974059), and the Foundation for Innovative Research Groups of the NSFC (211210004). We thank Guanglu Wu, Kai Liu, Fei Li, and Yu Yi for helpful discussions.

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