Facile Stimuli-Responsive Transformation of Vesicle to Nanofiber to

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Facilely stimuli-responsive transformation of vesicle to nanofiber to supramolecular gel via #-amino acid-based dynamic covalent chemistry Yajie Wang, Pengyao Xing, Shangyang Li, Mingfang Ma, Minmin Yang, Yimeng Zhang, Bo Wang, and Aiyou Hao Langmuir, Just Accepted Manuscript • DOI: 10.1021/acs.langmuir.6b02478 • Publication Date (Web): 30 Sep 2016 Downloaded from http://pubs.acs.org on October 4, 2016

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Facilely stimuli-responsive transformation of vesicle to nanofiber to supramolecular gel via ω-amino acid-based dynamic covalent chemistry Yajie Wang, † Pengyao Xing, † Shangyang Li, ‡ Mingfang Ma, † Minmin Yang, † Yimeng Zhang, † Bo Wang † and Aiyou Hao*† †Key Laboratory of Colloid and Interface Chemistry of Ministry of Education and School of Chemistry and Chemical Engineering, Shandong University, Jinan 250100, PR China. ‡Department of chemistry, College of Science, Agricultural University of Hebei, Bao ding 071001, PR China.

KEYWORDS: self-assembly, dynamic covalent bonds, pH reversibility, gel, nanofiber, vesicle, amino acid.

ABSTRACT This paper reports an interesting type of self-assembly systems based on dynamic covalent bonds. The systems are pH-responsible and reversible, which could be utilized for controlling the morphology transformation of the assemblies. In alkaline

ACS Paragon Plus Environment

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condition, the amine group of 11-aminoundecanoic acid (AUA) can connect with the aldehyde group of benzaldehyde (BA) or 1-naphthaledhyde (NA) by dynamic covalent bond to form a small organic building block accompanied with the morphological transformation from vesicles to fibers. When pH down to neutral value, the dynamic covalent bonds (imine bonds) can be hydrolyzed, leading to the dissociation of fibers and appearance of spherical aggregates. The transformation was confirmed reversible as fibers appeared again when the pH was changed back to alkaline value. In addition, a reversibly controlled gel was designed based on the nanofiber formation. NaCl, which is capable of greatly enhance the nanofiber density and crosslinking, was used to induce the growth of free-standing gel from freeflowing nanofiber system, and the resultant gel was proven to be pH-reversible.

Introduction Intelligent or stimuli-responsive materials

1-3

have been persisted over many decades,

and a great deal of work has been dedicated to developing sensitive self-assembled systems that can be crafted into novel smart materials. There are numerous methods to realize stimulating response. Dynamic covalent bonds are protruding for its advantages and gain much attention in the field of supramolecular science,

4-6

as they

are similar to noncovalent interactions due to the dynamic nature. Compared with the unstability of noncovalent bonds, dynamic covalent bonds are kind of relatively stable covalent bonds with dynamic feature, which are desirable for constructing structural controllable materials and have already been used to control the formation of functional self-assemblies.

7-22

On account of the reversible nature of dynamic

covalent bonds, the structure and morphology of self-assembly aggregates are easier


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to be modulated reversibly. Stimuli-responsive self-assemblies exhibit diversely controllable aggregates with different morphologies such as vesicles, micelles, fibers and gel nanofiber networks.

23-31

The morphological modulation and evolution could

be realized facilely through dynamic covalent bond chemistry. The control over morphology is important in the design of intelligent materials due to the structureproperty relationship. 32 Among various dynamic covalent bonds, 33 the benzoic imine bond 34, 35 is especially attractive due to its dynamic equilibrium in water. It can be fabricated in alkaline environment and hydrolyzed under neutral condition. The unique nature of this dynamic covalent bond has been widely employed to fabricate reversibly pHresponsive assemblies. Giuseppone and co-workers employed the dynamic imine bond on polymeric building blocks to fabricate self-assembled aggregates for the first time in 2009. 14 The formation and dissociation of aggregates can be controlled by the dynamic imine bond. van Esch and co-workers extended the similar concept to fabricate different types of amphiphilic building blocks which can self-assemble into vesicles, micelles and gels, showing controllable characteristics by forming and rupturing of dynamic imine bonds. 13, 36, 37 Herein we report a type of reversible controlled pH-responsive self-assembled system by utilizing small organic molecules based on dynamic imine bonds to realize triple transformation of vesicle to nanofiber to supramolecular gel (Scheme 1). In alkaline conditions, AUA connects with BA or NA by a dynamic imine bond to form a type of self-assembled building block. The AUA can self-assemble into vesicles in alkaline solution. When BA or NA is added, fibers are observed with the formation of dynamic imine bond. When pH is changed to neutral values, the dynamic imine bond is hydrolyzed, leading to the transformation of morphology from fibers to spherical


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aggregates. The transformation is confirmed reversible since the fibers appeare again with pH changed back to alkaline value. Based on the reversible nature of dynamic imine bond, a reversible controllable gel has been designed. When NaCl is added in BA/AUA system in alkaline condition, a white gel is obtained. The gel collapse when the pH is changed to neutral value and then reform again with the recovering of the pH value.

Scheme 1. Reversibly controlled amphiphiles based on dynamic covalent bonds.

Experimental Section Materials AUA, BA and NA were purchased from Aladdin Chemical Reagent Co. Ltd, Shanghai, China. All the other reagents are of analytical reagent (AR) grade and were purchased from country Medicine Reagent Co. Ltd., Shanghai, China. All chemicals were used as received without further purifications. Preparation of samples AUA and BA (both 40mM) were mixed together in 10 ml H2O with a molar ratio of 1:1, and proper amounts of NaOH were added to tune the pH value to 12.8. After


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being stirred for 30 minutes under ultrasound, a clear solution was obtained. Same method was adopted for AUA/NA system. The concentration of NA and AUA was 20mM, and the pH value of the system was changed to 11.4. All systems were aged for at least 8 h before characterization. Characterization Proton nuclear magnetic resonance (1H NMR) spectra of BUA and NYUA systems were measured on a Bruker AM-400 spectrometer at room temperature with D2O as the solvent and TMS as the reference. The fiber samples were dried in vacuum for 12 h until become dried powders at room temperature. The dried fiber samples of BUA and NYUA, solid samples of AUA and liquid samples of BA and NA were measured by an Avatar 370 Fourier-transform infrared (FT-IR) Spectrometer. KBr was used as the sample disks. The samples for transmission electron microscope (TEM) were measured on a JEM-100CX II electron microscope (100 kV). Field-emission scanning electronic microscopy (SEM, Hitachi S-4800) was used to study the microstructures of the samples. Dynamic light scattering (DLS) measurements were carried out with a Wyatt QELS Technology Dawn Heleos instrument set at constant room temperature by using a 12-angle replaced detector in a scintillation vial and a 50 mW solid state laser (k = 658.0nm). All solutions for DLS were filtered through 0.80 µm filters before detection. XRD patterns of dried fibers and AUA solid samples were performed on a German Bruker/D8 ADVANCE diffractometer with Cu Ka radiation (λ = 0.15406 nm, 40 KV, 40 mA). The rheology of the hydrogels was directly tested by a Thermo Scientific HAAKE RheoStress 6000.

Results and discussion


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The formation of imine bond. To confirm the formation of the imine bond, 1H NMR and FTIR spectroscopy was utilized. In comparison with the 1H NMR spectra of BA and AUA, the spectrum of their equimolar mixture under pH 12.8 reveals visible differences (Figure 1). The signal of the aldehyde group (9.8 ppm) weakens obviously, and in the meantime, a new signal peak at 8.2 ppm appears, indicating the formation of the imine bond. Same situation appears when we talk about the 1H NMR spectra of NA, AUA and their equimolar mixture under pH 11.4 (Figure 2). They all demonstrate the formation of the imine bonds in both systems.

Figure 1. The 1H NMR of BA (DMSO-d6, 40 mM), AUA (D2O, 40 mM), and a mixture of BA and AUA in a molar ratio of 1:1 (D2O, 40 mM BA, 40 mM AUA, pH = 12.8).


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Figure 2. The 1H NMR of NA (DMSO-d6, 20 mM), AUA (D2O, 20 mM), and a mixture of NA and AUA in a molar ratio of 1:1 (D2O, 20 mM BA, 20 mM AUA, pH = 11.4) FTIR spectroscopy can further confirm the formation of imine bonds. (Figure S1) By comparing the infrared spectroscopy of BA, NA, AUA and the two complexes BUA and NYUA, it is obvious that the characteristic IR bands of the imine at 1650 cm-1 appear in the spectra of the BUA and NYUA complexes. The self-assembly of the complexes. To investigate the self-assembled behaviors of BA/AUA and NA/AUA systems, TEM and SEM were performed. TEM and SEM images (Figure 3a and b, Figure S11) show that, AUA forms vesicles with an average diameter of 400 ± 50 nm in aqueous media at a pH value of 12.8. DLS was used to further investigate the diameter of the vesicles (Figure S2). The


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results show that the mean hydrodynamic diameter is about 500 nm, slightly larger than the observation of TEM image due to the hydration. When the pH of system was changed down to neutral value, spherical aggregates were found. (Figure S3). DLS was also used to investigate the hydrodynamic diameters of the spherical aggregates. With decreasing of the pH, the hydrodynamic diameters were reduced. (Figure S4) In contrast to AUA, BUA and NYUA in self-assembled state exist as rigid fibers and ribbons with width of (400 ± 50) and (1200 ± 50) nm, respectively (Figure 3c-f), as confirmed by the TEM and SEM studies.


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Figure 3. TEM images of the aggregates of (a) AUA (40 mM, pH=12.8), (c) BUA (40 mM, pH=12.8) and (e) NYUA (20 mM, pH=11.4) in water. SEM images of the aggregates of (b) AUA (40 mM, pH=12.8), (d) BUA (40 mM, pH=12.8) and (f) NYUA (20 mM, pH=11.4) in water. XRD technique was performed to explore the molecular arrangements of BA/AUA and NA/AUA assemblies. As shown in Figure 4, the self-assembled aggregates of BA/AUA and NA/AUA complexes have different diffraction peaks compared with the original AUA solid power. Typically, three scattering peaks (2θ = 2.22°, 4.39° and 6.54°) with d spacings of 3.95, 2.01 and 1.30 nm, appear in the BA/AUA complexes scattering pattern with a ratio of 1:0.5:0.33, which correspond to the 001, 002 and 003 planes of lamellar structure. 38

31,

The layer spacing of 3.95 nm, obtained from the XRD pattern, is in good agreement

with twice the length of a BUA molecule (ca. 1.98 nm, obtained from optimized molecular model based on Material Studio 5.5). Thus, these results imply that the selfassembly of BUA is composed of a bilayer structure. (Figure 5) π-π stacking interactions between benzene groups, as well as the electrostatic interaction, may drive BUA molecules into well-defined arrangements. The XRD patterns of BUA also remain the peak of AUA at about 5.13°, due to that the reaction between BA and AUA is not completed. The scattering peaks of NYUA aggregates also show the ratio (1:0.5:0.33) of lamellar structure with d spacings of 4.08, 1.99 and 1.33 nm, indicating the existence of lamellar structure. (Figure S5) In contrast to BUA system, NYUA XRD pattern barely display the pristine peak of AUA, this may due to the product of NA/AUA system is more stable compared to the product of BA/AUA system. Hence, the reaction efficiency of NA/AUA system is higher than BA/AUA system. Moreover, though the molar ratios of the BA/AUA and NA/AUA system are same in the reaction


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system, the dosage of reactants in two systems is different since the solubility of NA is poor than BA.

Figure 4. XRD patterns of AUA reagent, BA/AUA and NA/AUA complexes. The complexes were dried in vacuum at room temperature.


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Figure 5. Schematic representation of the molecular arrangements in BUA fibers pH-responsiveness. BA/AUA and NA/AUA systems showed reversible pH-responsiveness probed by 1H NMR. As shown in Figure 6, when pH is changed to 7.4, the chemical shift at 8.2 ppm corresponding to the imine bond disappears, accompanied by the enhancement of the signal at 9.8 ppm, which corresponds to the aldehyde proton. When the pH is changed back to 12.8, the imine signal recovers and aldehyde signal becomes weaker again. When it came to the NA/AUA system, the same situation occurs (Figure S6). The phenomenon confirms that the formation and decomposition of the imine bond is reversible in both systems of BA/AUA and NA/AUA.


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Figure 6. 1H NMR spectra of BA/AUA system at different pH values. In view of the reversible pH-responsiveness of imine bond shown above, we considered that the transformation of morphology between AUA and BA/AUA, NA/AUA aggregates can be also reversibly controlled by pH. As it has been found that, BUA can self-assemble into fibers. When the pH is decreased to 7.4, significant changes of morphology occurs. The fibers disassemble and spherical aggregates appear (Figure S7a). When the pH value is raised back to 12.8, the fibers reform again (Figure S7b). To the NA/AUA system, the similar phenomenon appears (Figure S7cd). SEM images also confirm the reversible transformation of morphology for both BA/AUA and NA/AUA systems. (Figure S8) Salt-responsiveness and formation of gel. Interestingly, when appropriate amount of NaCl was mixed with the BA/AUA system, gel phase was observed immediately at room temperature (Figure 7). Similar


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experiment about NA/AUA system was also carried on ever. None gel and other obvious phenomenon appeared for the poor solubility of NA in the system. We further investigated that salt-responsiveness of this system. In the basic environment, deprotonation occurs to BUA molecules, attributing to its anionic selfassemblies. It has been proven in some previous reports that NaCl has a profound impact on the self-assembly of some anionic assemblies like sodium laurate.

39

In

most cases, NaCl enhances the self-assembly of anionic building blocks. Sodium ion has an electrostatic attraction to carboxylate, leading to a crystallization of nanofibers. The process has great resemblance to crystal salt-out. Besides, NaCl with high concentration could be solvated, which shall induce the de-solvation of the free BUA molecules, enabling the increase of aggregation number to generate more nanofibers. According to the experimental phenomena and discussion above, we speculate that Na+ plays an important role in this system. Therefore, control experiments were carried out to verify whether other mental ions influences and four metal chlorides were tested. In order to avoid the interference of Na+, their hydroxides were used to adjust the pH of system. As we expected, no other metal compound except for NaCl could induce BA/AUA aqueous solution into gels. Table 1 gives the phase behavior of BA/AUA system mixed with different metal chlorides at room temperature.


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Figure 7. The phase transition process of BA/AUA system: homogeneous solution (a) of BA/AUA system, and a white gel (b) after adding NaCl at room temperature. Salt (250 mM)

NaCl

KCl

LiCl

BaCl2

CaCl2

Alkali

NaOH

KOH

LiOH

Ba(OH)2

Ca(OH)2

BUA (40 mM)

Gel

P

P

P

P

Table 1. Typical phase behavior of a 40 mM BA/AUA solution induced by different metal chlorides and the pH of BA/AUA solution was adjusted by respective metal hydroxides. The morphology of the gel induced by NaCl was investigated by TEM and SEM (Figure 8). Compared to the self-assembly aggregates of BUA (Figure 3c and d), the micro-morphology of the gel induced by NaCl shows a tighter fibrous network, with approximate 500 nm in width. Those fibers entangle with each other to construct a strong three-dimensional networks, immobilizing solvent molecules from flowing macroscopically, leading to the formation of gel.


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Figure 8. TEM (a), (b) and SEM (c), (d) images of the gel induced by NaCl. Rheological properties of the gel induced by NaCl are also investigated. Dynamic oscillatory stress sweep and frequency sweep of the gel are shown in Figure 9. The strain response to an applied stress can be described as two dynamic moduli, one of which is elastic modulus (G’: solid-like behavior), and the other is viscous modulus (G’’: liquid-like behavior).

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Solid character of the gel can be revealed from the

linear viscoelastic region where the storage moduli (G’) are higher than the loss moduli (G’’) (Figure 9a). Beyond the linear viscosity region, G’ falls lower than G’’, and the gel transforms into a liquid-like appearance. Therefore, when high oscillatory stress over 119.3 Pa is applied, the solid-like gel begins to flow. Frequency sweep manifests all G’ values of the gel are much higher than that of G’’ within the entire of frequency range (Figure 9b). Both G’ and G’’ values of the gel are slightly frequencydependent, which means some weak matrixes exist in the gel.


41

In addition, the G’

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values in frequency sweep reflect the mechanical strength. The magnitude of G’ is relatively high which proves good mechanical strength of the gel. Through analysis of the rheological studies, we can learn that the gel induced by NaCl has a certain degree of viscoelasticity which is the nature of gels.

Figure 9. Dynamic oscillatory stress sweep (a) and frequency sweep (b) of the gel induced by NaCl. As we have proved above that formation and decomposition of the imine bonds could be reversibly controlled by pH value. The dynamical variation of the imine bonds can also make a difference on the morphology of system. Thus, we speculated that formation and collapsing of the gel induced by NaCl might be reversibly controlled by regulating pH value. As we expected, the gel was found dilapidated and precipitates appeared when the pH value was changed from 12.8 to 7.4 where the imine bond was cracked. The precipitates were also investigated by TEM and SEM. As shown in Figure S9, fibers disappeared accompanied by the appearance of irregular particles. With the recovery of pH from 7.4 to 12.8, the gel was found regenerated. (Figure 10)


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Figure 10. Phase transition process of the reversible controlled gel. To explore properties of the regenerated gel, rheological studies of the regenerated gel are investigated. The linear viscoelastic region where the storage moduli (G’) are higher than the loss moduli (G’’) implies solid character of the regenerated gel. (Figure S10a) When stronger oscillatory stress over 99.1 Pa is applied, the linear viscoelastic region disappears with G’ falls lower than G’’. The regenerated gel transforms into a liquid-like appearance, revealing the classic characteristic of gel. Frequency sweep indicates all G’ values of the regenerated gel are much higher than that of G’’ within the entire of frequency range, and both G’ and G’’ values are slightly frequency-dependent (Figure S10b). The results confirm the gel state (solidlike appearance) and suggest the matrices of gel have good tolerance to external shear force. Compared with rheological properties of the initial gel, the regenerated gel shows slightly narrow linear viscoelastic region and more frequency-dependent, which means the regenerated gel is less stable than those of former gel. Most of that cause we owe to the redundant water from neutralization of base and acid which are applied to adjust pH value.

Conclusion


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In conclusion, we fabricated a reversible and controllable pH-responsive selfassembled system by using two small organic building blocks based on the dynamic imine covalent bond. A reversible morphological transformation between the vesicles and fibers was also observed along with the dynamic chemical reaction which was controlled by pH value. The dynamic process of the reversible morphology transition was monitored by TEM, SEM and XRD. Moreover, the reversible gel formed from the fibers solution after adding NaCl into the BA/AUA system at pH 12.8. As predicted, the gel collapsed after changing pH value to 7.4 where the dynamic covalent bond was proved broken. And then a gel reformed when the pH changed back to 12.8. TEM and SEM were also used to investigate the morphologies of the gel and precipitate. Rheological studies were utilized to explore the mechanical properties of the gel and the reformed gel. Furthermore, this new pH-sensitive supramolecular assembled system based on a dynamic imine bond showed dynamic nature to form various kinds of assemblies. The unique dynamic behavior provides a convenient method to design diverse assemblies with different morphologies, which maybe widely used for intelligent materials.

ASSOCIATED CONTENT Supporting Information 1H NMR of NA/AUA systems, FTIR spectroscopy information, DLS, molecular arrangements of NYUA, TEM and SEM are included in the supporting information. This material is available free of charge via the Internet at http://pubs.acs.org.

AUTHOR INFORMATION

Corresponding Author


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* E-mail: [email protected]

Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript.

ACKNOWLEDGMENT This work is supported by the research foundation, Department of Science & Technology of Shandong province, China.

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Table of Contents Graphic


Facile Stimuli-Responsive Transformation of Vesicle to Nanofiber to

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