Multiple-Responsive Hierarchical Self-Assemblies of a Smart

Feb 3, 2018 - At a low pH, only micelles are formed, which can transform into vesicles spontaneously when the pH increases to 11.8. Vesicles can gradu...
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Multiple-Responsive Hierarchical Self-Assemblies of a Smart Supramolecular Complex: Regulation of Non-covalent Interactions Panpan Sun, Aoli Wu, Na Sun, Xuanxuan Qiao, Lijuan Shi, and Li-Qiang Zheng Langmuir, Just Accepted Manuscript • DOI: 10.1021/acs.langmuir.7b03900 • Publication Date (Web): 03 Feb 2018 Downloaded from http://pubs.acs.org on February 9, 2018

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Multiple-Responsive Hierarchical Self-Assemblies of a Smart Supramolecular Complex: Regulation of Non-covalent Interactions Panpan Sun,a Aoli Wu,a Na Sun,a Xuanxuan Qiao,a Lijuan Shi*b and Liqiang Zheng*a a

Key Laboratory of Colloid and Interface Chemistry, Shandong University, Ministry of Education, Jinan 250100, China

b

Key Laboratory of Coal Science and Technology of Ministry of Education and Shanxi Province, Taiyuan University of Technology, Taiyuan 030024, China

------------------------------------------------------------------------------------------------------Corresponding author: Dr. Liqiang Zheng E-mail address: [email protected] Key Laboratory of Colloid and Interface Chemistry, Shandong University, Ministry of Education, Jinan 250100, China Phone number: +86-531-88366062 Fax number: +86-531-88564750

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Abstract We herein report a smart amphiphilic supramolecular complex ([MimA-EDA-MimA]@[DBS]2) with stimuli-responsive self-assembly, constructed

by

3-(3-formyl-4-hydroxybenzyl)-1-methylimidazolium

chloride (MimACl), sodium dodecyl benzene sulfonate (SDBS) and ethylenediamine

(EDA).

The

self-assembly

of

[MimA-EDA-MimA]@[DBS]2 shows triple-sensitivities in response to pH, concentration, and salt. At low pH, only micelles are formed, which can transform into vesicles spontaneously when pH increases to 11.8. Vesicles can gradually fuse into vesicle clusters and elongated assemblies with

increasing

concentration

of

[MimA-EDA-MimA]@[DBS]2.

Chain-like aggregates, ring-like aggregates or giant vesicles can be formed by adding inorganic salts (i.e. NaCl, NaNO3), which could be derived from the membrane fusion of vesicles. The non-covalent interactions, including π-π stacking, hydrogen bonding and electrostatic interactions, were found to be responsible for the topology evolution of assemblies. Thus, it provides an opportunity to construct smart materials through regulating the role of non-covalent interactions in self-assembly. Keywords: Self-assembly, Multiple-response, Vesicles, Membrane Fusion, Non-covalent Interaction

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Introduction Self-assembly is a ubiquitous process in living organism, such as the fabrication of DNA, RNA and biological membrane, offering a bottom-up approach to construct functional materials on multiple scales.1 It is a fascinating process, in which the scattered building blocks can hierarchically

assemble

into

supramolecular

complex,

ordered

supramolecular assemblies and then smart materials with finely controlled functions through non-covalent intermolecular interactions, such as π-π stacking, host-guest interaction, coulombic interaction, charge-transfer interaction, hydrogen bonding and hydrophobic effect.2-10 Various structurally well-defined supramolecular assemblies, such as micelles, wormlike micelles, vesicles, nanorods, nanotubes, and nanosheets, have been fabricated

through tuning the role of

non-covalent interactions.11-15 In addition, the dynamic nature of non-covalent interactions endows supramolecular assemblies with superior ability to respond to extra-stimuli. Thus, how to precisely regulate the role of non-covalent interactions is the key to construct stimuli-responsive supramolecular assemblies and smart materials.16 Vesicles have been widely investigated due to their broad applications not only in catalysis,17 micro-reactors,18-19 template of nanomaterials,20 and biosensing,21 but also in cellular membrane mimics.22-23 Lately, stimuli-sensitive vesicles in response to photo,24 electricity,25 therm,26 pH

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and/or enzyme,28 have been developed. Amongst these smart vesicles,

the pH-responsive vesicles have attracted increasing attentions due to their promising application for targeted drug release. For instance, Wang’s group successfully created pH-responsive vesicles in the system of

[Cnmim]X

(n

=

12

and

14,

X

=

[C6H4COOKCOO],

[C6H3OHCOOSO3Na], and [C6H4COOSO3Na]), and realized their reversibility through changing the pH of solutions.29 Cao et.al developed pH-responsive supramolecular vesicles based on host-guest interaction between pillar [6] arene and ferrocene derivative, and realized the controlled release of drug loaded in supramolecular vesicles via pH variation.30 This showed us that it is of great potential to regulate the assembly and disassembly of vesicles by tuning the environmental pH, whereby pH-controlled drug delivery system could be fabricated. Membrane fusion is a critical biological process controlled by lipids and proteins, and triggering vesicle fusion on target membranes is invaluable for advancing targeted drug delivery system.31-35 Diverse hierarchical topologies, such as vesicle clusters,36-37 giant vesicles,38-40 nanotubes,41-42 fibers43 and vesicle networks,44 have been constructed via vesicle

aggregation.

For

instance,

Ravoo

et.al

fabricated

the

metal-ion-responsive vesicle clusters by switching the conformation of non-covalent

linked

molecules

with

a

flexible

N,N’-bis(3-aminopropyl)ethylenediamine spacer.37 Hao et.al reported a

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controlled multiple morphology transformation based on a single entity (CN, containing a naphthalene-1,8-dicarboximide and a cholestrol moiety), caused by the variation of hydrophilcity/hydrophobicity and π-π stacking through changing solvent polarity and concentration.45 In addition, our group fabricated a controllable vesicle fusion system through tuning the π-π stacking of oligomeric supra-amphiphiles ([M-n-M]2+@2[DBS]—, n = 2, 6).7 Obviously, the vesicle/membrane fusion can be controlled by tuning the role of non-covalent interactions in the assembly process, which may provide new approaches to achieve smart functional materials. Herein,

we

successfully

constructed

a

stimuli-responsive

supramolecular complex ([MimA-EDA-MimA]@[DBS]2), and realized diverse assembly transformation in response to pH, concentration and salt (Scheme 1). Micelles can transform into vesicles with increasing pH. Vesicles can gradually fuse into vesicle clusters and elongated assemblies with

increasing

concentration

of

[MimA-EDA-MimA]@[DBS]2.

Chain-like aggregates, ring-like tubes or giant vesicles can further be formed by adding inorganic salts. It was demonstrated that π-π stacking is the major driving force for the formation of vesicles, and hydrogen bonding triggers the vesicle fusion.

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Scheme 1. Schematic representation of the molecular structure and morphological evolution process of [MimA-EDA-MimA]@[DBS]2 system controlled by pH, salts and concentration. Experimental section Materials. N-Methylimidazole (99%), salicylaldehyde (99%), D2O (99.9%), sodium dodecyl benzene sulfonate (SDBS) (95%) and sodium dodecyl sulfate (SDS) (99%) were purchased from J&K Scientific Ltd. Ethylenediamine (EDA) and concentrated HCl were obtained from Sinopharm Chemical Reagent Co., Ltd. All the materials above were used without further purification. Triple distilled water was used throughout the whole experiment. Synthesis chloride

of

3-(3-formyl-4-hydroxybenzyl)-1-methylimidazolium (MimACl).46-47

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3-(3-formyl-4-hydroxybenzyl)-1-methylimidazolium

chloride

was

synthesized according to the procedures previously reported.48-49 Firstly, the synthesis procedure of 5-chloromethyl-2-hydroxybenzaldehyde is listed as follows. Concentrated HCl (30 mL) and formaldehyde (37 wt % in H2O, 2.5 mL) and salicylaldehyde (3.5 mL, 32.99 mmol) was mixed, and the reaction was stirred at room temperature for 24 h. After the reaction, a baby pink powder was obtained, and then purified by simple washing with triple distilled water twice. Then, the solution of 1-methylimidazole (5 mL, 50.12 mmol) in acetonitrile (15 mL) at room temperature

was

added

5-chloromethyl-2-hydroxybenzaldehyde

the

solution

(8.57 g, 50.12 mmol)

of in

acetonitrile (25 mL) under nitrogen atmosphere. The solid separated out was washed with ether (3 x 10 mL) and dried to give pale yellow solid product. 1H NMR spectrum (δ ppm D2O, 400 MHz), 9.92 (s, 1H), 8.74 (s, 1H), 7.75 (s, 1H), 7.60 (s, 1H), 7.44-7.46 (m, 2H), 7.06 (s, 1H), 5.40 (s, 1H), 3.87 (s, 1H). Characterization. Dynamic light scattering (DLS). The vesicle size distributions of the vesicles were determined by DLS using a Nanotrac Particle Size Analyzer (Nanotrac NPA 250) and the microtrac FLEX application software program. All measurements were carried out using a laser diode (780 nm wavelength, 3 mW nominal, Class IIIB at the scattering angle of 180).

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The temperature was controlled using a thermostat (F31C, Julabo) with an accuracy of 25 ± 0.1 oC. Cryogenic

transmission

electron

microscopy

(Cryo-TEM)

observations. The samples were prepared in a controlled environment vitrification system (CEVS) at 25 oC under 95% relative humidity. A micropipette was used to load 5 µL solutions onto a TEM copper grid, which was blotted with two pieces of filter paper, resulting in the formation of thin films suspended on the mesh holes. After waiting for about 10 s, the samples were quickly plunged into a reservoir of liquid ethane (cooled by nitrogen) at -165 oC. The vitrified samples were then stored in liquid nitrogen until they were transferred to a cryogenic sample holder (Gatan 626) and examined under a JEOL JEM-1400 transmission electron microscope (120 kV) at about -174 oC. The phase contrast was enhanced by underfocus. The images were recorded on a GatanMultiscan CCD and processed using a Digital Micrograph. Surface tension measurements. Surface tension measurements were carried out on a model JYW-200B tensiometer (Chengde Dahua Instrument Co., Ltd., accuracy ± 0.1 mN m-1) using the ring method. The temperature was controlled by a thermostatic bath with an accuracy of 25 ± 0.1 oC. Each sample was equilibrated for 15 min and all measurements were repeated at least three times until the values were reproducible. Turbidity measurements. The turbidity measurement for the solutions of

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[MimA-EDA-MimA]@[DBS]2 at different pH, reported as absorbance, was determined by using an UV−vis spectrometer (Hitachi U-4100, Japan). The absorbance at 480 nm was chosen as the turbidity. A cuvette with 1 cm pathway was used. All the measurements were conducted at 25 ± 0.1 °C. Freeze-fracture

transmission

electron

microscopy

(FF-TEM)

observations. A small amount of the sample solution was placed on a 0.1 mm thick copper disk covered with a second copper disk. Then the copper sandwich containing the sample was plunged into liquid propane cooled by liquid nitrogen. Fracturing and replication were carried out on Balzers BAF-400D equipment at -150 oC. Pt/C was deposited at an angle of 45o. The replicas were examined on a JEOL JEM-1400 transmission electron microscope operated at 120 kV. The images were recorded on a GatanMultiscan CCD and processed using a digital micrograph. 1

H NMR measurements. 1H NMR spectra were measured on a Bruker

Advance 400 spectrometer equipped with a pulse field gradient module (Z-axis) using a 5 mm BBO probe. The instrument was operated at a frequency of 400.13 MHz at 25 ± 0.1 °C. All the samples were dissolved in D2O, and chemical shifts were referred to the center of the HDO signal (4.700 ppm). Results and discussion Vesicle

formation

by

supramolecular

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[MimA-EDA-MimA]@[DBS]2. We first fabricated a supramolecular complex [MimA-EDA-MimA]@[DBS]2 by mixing a pH-responsive imidazolium cation (MimA), a small organic diamine (EDA) and dodecyl benzene sulfonate anion (DBS). The self-assembly studies of the mixed solutions were initially carried out at pH 11.8. It was assumed that the supramolecular-spacer [MimA-EDA-MimA]2+ was constructed by MimA and EDA at a 2 : 1 stoichiometry via hydrogen bonding, as confirmed by MS spectra (ESI Figure S1†). Fixing the molar ratio of MimA / EDA at 2 : 1, the state of the aqueous solution can be further affected through tuning the molar ratio of [MimA-EDA-MimA]2+ / [DBS]—. When the [MimA-EDA-MimA]2+ / [DBS]— molar ratio was 1 : 1, the solution was transparent with no birefringence, but some oil drops precipitated out within a few days (Figure 1, top). As the [MimA-EDA-MimA]2+ / [DBS]— molar ratio decreased to 1 : 2, the solution became homogeneous with a birefringent texture (Figure 1, middle). Further decreasing the [MimA-EDA-MimA]2+ / [DBS]— molar ratio to 1 : 3, the solution turned turbid, and meanwhile, the birefringence disappeared and some amorphous solid precipitated out (Figure 1, bottom). Moreover, a typical Tyndall effect was observed from Figure 1d (middle), indicating the existence of abundant assemblies. Based on the discussed phenomenon above, we mainly focused on the self-assembly behavior of the homogeneous solution with [MimA-EDA-MimA]2+ / [DBS]— molar ratio

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at 1: 2 in this work. A binding number of 1.8 for DBS / MimA-EDA-MimA was obtained by ITC experiment (ESI Figure S2†). The

calculated

proportion

of

[MimA-EDA-MimA]@[DBS]2 and

[MimA-EDA-MimA]@[DBS]Br are 80% and 20% respectively through the analysis of the binding number. Thus, it is possibly that the supramolecular

complexes

of

[MimA-EDA-MimA]@[DBS]2

and

[MimA-EDA-MimA]@[DBS]Br are coexisted in aqueous solutions, and [MimA-EDA-MimA]@[DBS]2 plays an important role in inducing the formation of abundant assemblies. The characterizations via TEM (Figure 2a) and DLS (Figure 2b) proved the supramolecular assemblies were spherical vesicles with diameters of 80 - 200 nm (C[DBS]- = 35 mM ).

Figure 1. Optical photographs of solutions with different molar ratios of [MimA-EDA-MimA]2+ / [DBS]— observed without (a, b) and with (c) crossed polarizers; (d) The reflection of Tyndall effect. (a-d) pH = 11.8.

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Figure 2. (a) TEM image and (b) size distribution of the [MimA-EDA-MimA]@[DBS]2 solutions (C[DBS]— = 35 mM); (c) size distributions of supramolecular complex of [MimA-EDA-MimA]@[DS]2 with different concentrations; (d) optimized structures of [MimA-DBS] and [MimA-DS] by DFT calculations; (a-c) the molar ratio of [MimA-EDA-MimA]2+ / [DBS] or [DS] is 1 : 2; pH is 11.8. —



The non-covalent interactions accounting for the formation of vesicle were examined. [DB]— (dodecyl sulfate) was employed as a counterion of supramolecular complex ([MimA-EDA-MimA]@[DS]2) to probe the role of π-π stacking in vesicle generation. As can be seen from Figure 2c, only monodisperse assemblies with diameters of ca. 10 nm appeared in the [MimA-EDA-MimA]@[DS]2 aqueous system and the sizes of assemblies ACS Paragon Plus Environment

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hardly increases with increasing concentration, consistent with the obtained diameters from TEM image (Figure S3). The existence of π-π stacking was further identified by the optimized results of DFT calculations

(taking

single-chained

supramolecular

complex

of

MimA-DBS (DS) as examples), which used a hybrid functional B3LYP with the basis 6-31G(d,p) of the Gaussian 09 package (Figure 2d).50 Compared to [DS]—, the [DBS]— counterion is easier to reduce the interaction energy of complex (E[DBS]—
12.0, nearly all of the EDA+ cations are turned into EDA, and only hydrogen bonding interaction remains between EDA and aldehyde upon MimACl. Thus, the electrostatic repulsion between the anionic phenolate groups cannot be effectively shielded and a loose packing of the hydrocarbon chains would be adopted, leading to the transition of vesicles into spherical assemblies.29 Based on the pH-responsiveness of supramolecular complex, the reversibility of assembly and disassembly processes were further investigated through circularly adding acid or base to switch pH. The

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reproducibility over several cycles is shown in Figure 5c. The birefringent, turbid and viscous solutions at pH=11.8 were obtained as references. Upon adding acid (pH=11.3), a flocculent and insoluble substance appeared and the birefringent of solution disappeared. After tuning the pH back to 11.8 by adding base, the insoluble substance disappeared and the turbidity of the solution returned back. After 3 cycles of adding acid and base, the turbidity of the solution was slightly less than its initial value. Similarly, as can be seen in Figure 5d, the size of aggregates is nearly invariable after 2 cycles, but tends to increase after 3 cycles. It is assumed that the added salts during the cycles may promote the growth of vesicles. Subsequently, the morphology of assemblies with diameters of ca. 100 nm was verified by FF-TEM and Cryo-TEM (Figure 5e and 5f), according with the vesicle size range before circulation.

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Figure 6. Size and size distributions of the aggregations formed by [MimA--EDA-MimA]@[DBS]2 (50 mM [DBS]—) at pH =11.8 with adding salts (a) NaCl, (b) NaNO3 at 25 oC. The TEM images of the [MimA-EDA-MimA]@[DBS]2 system after adding 500 mM (c) NaCl and (d) NaNO3. Salt-induced vesicle fusion to elongated assemblies and giant vesicles. To illustrate the effect of salts on vesicle fusion, we added two kinds of salts (NaCl, NaNO3) into the [MimA-EDA-MimA]@[DBS]2 aqueous solution (C[DBS]— =50 mM, pH =11.8). As shown in Figure 6 (a, b), the alteration of vesicles sizes is minor and it can be neglected after adding a certain amount of salts. While the concentration of salts approaches to

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500 mM, the morphology evolution of vesicles is promoted. DLS results show that aggregates with sizes up to micrometer scale are formed. TEM images show that the fused elongated assemblies and giant vesicles are formed after adding NaCl and adding NaNO3 respectively (Figure 6c, 6d). It is probably that salts can reduce the surface electrostatic repulsion of vesicles, which favors the membrane fusion. Conclusion In summary, we successfully constructed a novel supramolecular complex ([MimA-EDA-MimA]@[DBS]2) with triple-sensitivities in response to pH, concentration, and salt. The assemblies can spontaneously transform from micelles to vesicles and then to vesicle clusters through tuning the pH. With increasing concentration of [MimA-EDA-MimA]@[DBS]2,

the

vesicles

could

undergo

an

aggregation-fusion pathway to generate vesicle cluster, elongated assemblies even to vesicle network. 1H NMR demonstrated the hydrogen bonding and π-π stacking account for the evolution of morphologies. Adding salts may reduce the surface electrostatic repulsion of vesicles, promoting the membrane fusion of vesicles. Hence, this work may advance a better understanding of triggered biological membrane fusion by tuning non-covalent interactions, laying a foundation for constructed smart materials to realize targeted drug release.

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Acknowledgements This work was supported by the National Natural Science Foundation of China (No. 21573132, No. 21403151, No. 21773141), the National Basic Research Program (No. 2013CB834505) and the Natural Science Foundation for Young Scientists of Shanxi Province (No. 2016021045). Reference (1)

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Graphical Abstract

A triple-sensitive supramolecular complex was constructed, which can hierarchically self-assembly into vesicle, vesicle cluster, giant vesicle and elongated assemblies through tuning non-covalent interaction.

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