Fabrication of Smart pH-Responsive Fluorescent Solid-like Giant

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Fabrication of Smart pH-responsive Fluorescent Solidlike Giant Vesicles by Ionic Self-Assembly Strategy Jinglin Shen, Xia Xin, Guokui Liu, Jinyu Pang, Zhaohua Song, Guiying Xu, and Shiling Yuan J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.6b08140 • Publication Date (Web): 17 Nov 2016 Downloaded from http://pubs.acs.org on November 26, 2016

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The Journal of Physical Chemistry

Fabrication of Smart pH-responsive Fluorescent Solid-like Giant Vesicles by Ionic Self-Assembly Strategy Jinglin Shen, a Xia Xin, a, b * Guokui Liu, a Jinyu Pang, c Zhaohua Song, a Guiying Xu, a, b Shiling Yuan a *

a

Key Laboratory of Colloid and Interface Chemistry (Shandong University), Ministry of Education, Shanda nanlu No. 27, Jinan, 250100, P. R. China

b

National Engineering Technology Research Center for Colloidal Materials, Shandong University, Shanda nanlu No. 27, Jinan, 250100, P. R. China c

*

Shanxi Transportation Research Institute, Taiyuan 030006

Author to whom correspondence should be addressed, E-mail: [email protected].

Phone: +86-531-88365896. Fax: +86-531-88564750

ACS Paragon Plus Environment


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ABSTRACT: Fluorescent solid-like giant vesicle (SGV) was prepared by using an anionic dye methyl orange (MO) and an oppositely charged surfactant 1-tetradecyl-3-methylimidazolium bromide (C14mimBr) on the basis of ionic self-assembly (ISA) strategy. The properties of MO/C14mimBr complexes were comprehensively characterized. The results indicated that the giant vesicle was formed by the fusion of small vesicles and could keep its original structure during the evaporation of solvent. Besides, the giant vesicles perform luminescent property owing to the break of intermolecular π-π stacking of MO, which achieves the transformation from aggregation-caused quenching to aggregation-induced emission by noncovalent interaction. Moreover, MO/C14mimBr complexes also perform smart pH-responsive characteristics and abundant thermic phase behaviour. That is, various fluorescent structures (polyhedron, giant vesicle, chrysanthemum, peony-like structure) were obtained when pH≥4, while simple nonfluorescent structure (microflake) was obtained when pH=2 due to the changes of MO configuration. Thus, the fluorescence behavior can be predicted with the color change directly visible to the naked eye by changing the pH. It is expected that the facile and innovative design of supramolecular material by ISA strategy could be used as pH detection probes, microreactor.


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INTRODUCTION

Supramolecular self-assembly by bottom-up strategy have been widely used to construct hierarchical structure with complex unctions on multiple length scales.

1-5

Various materials ranging from

nanoparticles, vesicles, nanotubes to micelles can be easily construct in this way and those materials have potential application in gene therapy, sensor, biomimetic models and drug delivery.

6-8

Noncovalent interactions such as electrostatic interaction, hydrogen-bond interaction, π-π stacking, hydrophobic interaction are crucial for the self-assembly of nanomaterials with a high degree of structural complexity.

9, 10

Moreover, the dynamic and reversible nature of the noncovalent

interactions endows the resultant supramolecular architectures with fascinating stimuli-responsive features (pH, temperature, light and ionic strength), which allows us to manipulate the systems for desired functions.

11-14

Especially, fluorescent switch of supramolecular materials have attracted

widespread attention owing to its promising application in optoelectronic devices.

14,15

For example,

Nuthanakanti et al. reported a new family of supramolecular nucleolipid synthons by combining the properties of environmentally-sensitive fluorescent nucleoside analogs with fatty acids.

15

These

fluorescent nucleolipids could form organogels via a hierarchical self-assembly. What’s more, the nucleolipid organogels exhibited aggregation-enhanced emission, but also the gelation behavior could be reversibly switched by the application of multiple external stimuli such as temperature, ultrasound and chemicals. Among various strategies of realizing supramolecular self-assembly through non-covalent



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interactions, ionic self-assembly (ISA) on the base of electrostatic interaction, with the aid of other noncovalent interaction (π-π stacking, hydrogen-bond interaction, hydrophobic effect and geometric factor) provides a simple and feasible way to build functional materials. 16-18 For example, Faul et al. have reported lots of works about building nanostructures by using charged dyes molecules (orange G, ponceau 2R, crystal scarlet, amaranth) and oppositely charged surfactants (C10TAB, C12TAB, C14TAB, C16TAB, C18TAB). 19-23 Those complexes could be used as new liquid-crystalline materials with special optical and electronic properties. The dye molecules have excellent properties for building blocks owing to their defined and regular shape (extended π system). Then, several followup works were reported in regard to the self-assembly of dye-surfactant molecules.

24-30

Zhao et al.

reported the construction of highly ordered supramolecular microfibers through ISA strategy using N-tetradecyl-N-methylpyrrolidinium bromide (C14MPB) and methyl orange (MO) dye molecule. The self-assembled microfibers were observed to be about 1 to 5 µm in width and their length is from tens of micrometers to almost a millimeter and these microfibers perform strong fluorescent properties.

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However, all of the structures of supramolecular materials constructed by dye-

surfactant have been limited to 1D and 2D structures, such as fiber and flake morphology and mostly have no fluorescent property, which greatly restricts the application of the material. 3D vesicle structure built via ISA strategy has rarely been reported although vesicle is more useful for cargo encapsulation and controlled release. 31, 32 Herein, anionic dye methyl orange (MO), (possesses multiple functional groups, a defined and


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regular shape (extended π system), along with easy availablity and tunable structure on adjustment of pH) and oppositely charged surfactant 1-tetradecyl-3-methylimidazolium bromide (C14mimBr) , were choesn to study the self-assembly on the basis of ISA. Fluorescent solid-like giant vesicle (SGV), which could keep its original morphology with the evaporation of solvent was obtained. The observation of the assembling dynamics of SGV was implemented using TEM and DLS. It could be confirmed that the giant vesicles were formed by the fusion of small vesicles. More interestingly, various morphologies such as microflake structure, polyhedron-like structure, chrysanthemum-like structure, peony-like structure were obtained by changing the pH. The system displays color conversion and fluorescence switch between “on” and “off” states upon modulating of pH. Besides, the supramolecular complexes also show abundant phase behavior transitions, including a thermotropic liquid crystalline phases. EXPERIMENTAL SECTION

Chemicals and Materials. MO (>99%) was purchased from Tokyo Chemical Industry Co., Ltd. (Japan). C14mimBr, C12mimBr and C8mimBr (>99%) were purchased from Lanzhou institute of chemical physics (China). The structures of dye molecule MO and surfactant C14mimBr are shown in Figure 1. HCl and NaOH were purchased from Shanghai Chemical Co. (China). Water used in the experiments was triply distilled by a quartz water purification system.

O O S Na+

O

N

N

N

N

N

Br



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Figure 1. The structure of MO (left) and C14mimBr (right).

Preparation of Complexes. MO, C14mimBr, C12mimBr and C8mimBr, were weighed and dissolved in test tubes to obtain stock solutions. Complex formed by MO and oppositely charged surfactant was prepared by mixing these stock solutions. The mixed solutions were stirred vigorously to achieve uniformity. All the samples were kept at T = 20 ± 0.1 °C for at least one week before testing. Trace amounts of NaOH and HCl aqueous solution was added to adjusting the pH of system. Methods and Characterizations. Optical microscopy (OM) and polarized optical microscopy (POM) observations were performed by AXIOSKOP 40/40 FL (ZEISS, Germany) microscope. The sample (25µL) was put on the glass slide, and another glass slide was putted above to avoid solvent evaporation. Transmission electron microscopy (TEM) observation was carried out by JEM-100CX II (JEOL) with an accelerating voltage of 80 kV. A small volume of sample was placed on carboncoated copper grid, and copper grids were freeze-drying in a vacuum extractor at -60 °C for 24 h. Field-emission scanning electron microscopy (FE-SEM) image was observed on a JEOL JSM-6700F at 5.0 kV. An amount of sample was put on a silica wafer, excess sample was suck by filter paper to form a thin film. The silica wafers were freeze-drying in a vacuum extractor at -60 °C for 24 h. Confocal laser scan microscopy (CLSM) observation was performed using an inverted microscope (model IX81, Olympus, Tokyo, Japan) equipped with a high-numerical-aperture 60 oil-immersed objective lens, a UV-mercury lamp (OSRAM, HBO, 103w/2, Germany), a mirror unit consisting of a 330–385 nm excitation filter, a 535-565 nm emission filter, and a 16 bit thermoelectrically cooled


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EMCCD (Cascade512B, Tucson, AZ, USA). The EMCCD was used for collecting the fluorescent images. UV-vis spectrum was carried out on Hitachi UV-vis 4100 spectrophotometer with 0.1 mm path length quartz cell. Fourier transform infrared (FT-IR) spectrum was recorded from 4000 cm-1 to 400 cm-1 on a VERTEX-70/70v spectrometer (Bruker Optics, Germany). The fluorescence spectrum was performed on a LS-55 spectrofluorometer (PerkinElmer, Waltham, MA, USA) with a quartz cell (1×1 cm). Thermogravimetric analysis (TGA) was performed using a Universal V3.6 TA Thermal Analysis Q5000 system. Samples were heated from room temperature to 800 °C with 10 °C/min in a flowing nitrogen atmosphere. Differential scanning calorimetry (DSC) was performed on DSC8500 (PerkinElmer, Waltham, MA, USA) with a 10 °C/min heating rate from -20 to 250 °C under nitrogen atmosphere. As for computational simulation, the geometry optimization was performed at CAMB3LYP/6-31+G(d, p) level with Gaussian 09 program. Frequency calculations at the same theoretical level were performed to identify all the optimized structures as minima. 33

RESULTS AND DISCUSSION

First, the morphologies of supramolecular materials formed by equal molar quantity of 0.5 mmol L-1 MO/0.5 mmol L-1 C14mimBr were observed by SEM (Figure 2A-C) and optical microscope (Figure 2D). From optical microscope observation, microspheres could be observed with the diameter of 5~20 µm. In SEM observation, solid-like microspheres were observed and it could be seen that the



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microsphere is hollow with the diameter of 5~20 µm and the wall thickness is about 150 nm by

calculate the cross-sectional SEM images. The thickness of wall is much larger in value than the length of MO and C14mimBr (Figure S2), meaning that the microsphere is multi-lamellar structure. The supramolecular material exhibited the morphology similar to the vesicles, whereas its original structure was remained without collapse during the evaporation of solvent, which exhibited a feature of “solid-like giant vesicle” (SGV).

[32]

In Figure 2C, it could also be observed that small

vesicles were embedded into larger vesicle, which is rarely reported via ISA strategy. It has been demonstrated that synthetic vesicles are ideal model membranes for mimicking the dynamic and structural features of cellular processes. However, most of the work has been limited to the aggregation with submicroscopic sizes, and the scale of the reported vesicle aggregates is smaller than 1 µm. However, in nature, cells are generally micrometer sized and giant vesicle possessing cell-like size (1~200 µm) is promising candidate formimicking cellular process but it has hardly been performed. Herein, in our system, a facile ISA strategy to construct giant vesicle was managed to fulfill this subject.


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Figure 2. Morphology of solid-like giant vesicle formed by 0.5 mmol L-1 MO/0.5 mmol L-1 C14mimBr: (A), (B), (C), SEM images; (D) optical microscope; (E) CLSM image. (F) Fluorescence spectrum of 0.5 mmol L-1 MO and 0.5 mmol L-1 MO/0.5 mmol L-1 C14mimBr. (scale bar = 20 µm)

Luminescent materials were used in a vast range of areas such as organic light-emitting diodes (OLEDs), fluorescence probe, organic field-effect transistors (OFETs).

34,35

While, the phenomenon

of aggregation-caused quenching (ACQ) limited the application of this category of materials. 36 As is shown in Figure 2F, for pure 0.5 mmol L-1 MO solution, there is almost no fluorescence because of the strong intermolecular π-π stacking interactions between MO molecules, while there appears a strong emission peak at 545 nm (λex=310 nm) after it self-assembly with C14mimBr. In CLSM observation, strong red fluorescent spherical aggregates with bright red periphery and the dark inner cavity can be observed which is consistent with the hollow structures observed in SEM and optical microscope (Figure 2E). The formation of fluorescent complex through ISA could be attributed to



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the restrictive effect of C14mimBr on the π-π stacking interaction between MO molecules which consequently enhanced the luminescence of the material. It is interesting to find that the MO/C14mimBr and MO/C12mimBr system displays excellent fluorescence but not in MO/C8mimBr system (Figure 3A). And the morphology of MO/C12mimBr system is giant vesicle but small vesicle in MO/C8mimBr system (Figure S1). Computational studies were carried out to get a better insight into the assembly, the optimized the geometries of MO/C14mimBr are shown in Figure 3B. It could be found that the imidazole group of C14mimBr act with sulfonic group of MO through electrostatic interaction and the hydrophobic section of two molecules close to each other by hydrophobic effect. Because the alkyl chain of C8mimBr is shorter than the length of MO (Figure 3C), the C8mimBr can not break intermolecular π−π stacking of MO and the excited states of MO aggregates relax back to the ground state via non-radiative channels, resulting in the emission quenching of the luminophores. It can be indicated that the hydrophobic chains of surfactants have significant impact on the selfassembly behavior and only surfactant with longer alkyl chain could achieve the transformation from aggregation caused quenching (ACQ) to aggregation induced emission (AIE).


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Figure 3. (A) Fluorescence spectrum of MO/C14mimBr, MO/C12mimBr, MO/C8mimBr. The optimized the geometries of MO/C14mimBr (B) and MO/C8mimBr (C).

In order to detect the structure of the self-assemblies fabricated by MO and C14mimBr, small angle XRD (SAXRD) measurements were carried out. It can be seen that three diffraction peaks were detected for complex of MO/C14mimBr and q1: q2: q3:=1:2:3 (Figure 4A), indexing to a lamellar structure. The spacing of the lamellae can be calculated (d = λ/2 sin θ) is 3.46 nm. The d value (3.46 nm) is smaller than twice the extended alkyl chain length of C14mimBr, but larger than the length of a single molecule of C14mimBr (Figure S2). Hence, the complex of MO/C14mimBr maybe present bilayer structures with interdigitated hydrocarbon tails. 37 Moreover, the ζ potential of self-assembly system is 33.9 mV (Figure S3), meaning the C14mimBr arranged in the periphery of the vesicles, which is proved with the optimized structure of computational studies that the polar head of C14minBr arranged at the outside of complex. 1HNMR was used to study the stoichiometry of MO/C14mimBr integrated in the giant vesicle (Figure 4B). It was detected that the molar ratio of



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MO/C14mimBr is 1:1 (6/6 : 3/3) by comparing the peak integral intensities indicating MO act with C14mimBr through the formation of ionpairs between imidazolium cations and surfactant anions on the basis of electrostatic interaction.

Figure 4. (A) SAXRD patterns of 0.5 mmol L-1 MO/0.5 mmol L-1 C14mimBr. (B) 1H NMR spectra of complex of C14mimBr/MO in DMSO-D6.

To study the interaction between MO and C14mimBr at molecular level, FT-IR was employed. As is shown in Figure 5A, 1040 cm-1 and 1572 cm-1 was attributed to the symmetric stretching vibration of sulfonic group of MO and the C=N group of C14mimBr, respectively. After the formation of giant vesicle, those bands shift to 1028 cm-1and 1582 cm-1, respectively, indicating electrostatic interaction acts a crucial role in the formation of giant vesicle. Moreover, the spectra of pure C14mimBr displays CH2 symmetric stretching vibrations at 2851 cm-1 (νsym) and antisymmetric stretching at 2917 cm-1 (νasym), implying that C14mimBr adopted an all-trans conformation with higher order. But 2917 cm-1 blue-shifted to 2923 cm-1 and 2851 cm-1 blue-shifted to 2854 cm-1 after


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the formation of the giant vesicles, respectively, meaning that the gauche conformation ratio of C14mimBr system increases and the alkyl chains are in a disordered state in the SGV of 38

MO/C14mimBr.

The aggregation behavior of MO/C14mimBr was also studied by UV-vis

spectroscopy (Figure 5B). The absorption at 462 nm of MO attributed to n–π* transition. The peak blue-shifted to 370 nm and its intensity decreased sharply with the formation of giant vesicle, meaning that MO and C14mimBr molecules could self-assemble through parallel plane-to-plane stacking to form a sandwich-type array, which is referred to H-type aggregation. 39,40

A

B1.5

MO/C14mimBr

MO C14mimBr

462

MO/C14mimBr

2923

2854 MO

1582

1028 1.0 370

Abs

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1040 C14mimBr 2851

2917 3500

3000

0.5

1572 1500

-1

1000

0.0 200

300

Wavenumber/cm

400

Wavelength/nm

500

600

Figure 5. (A) FT-IR spectrum of MO, C14mimBr and 0.5 mmol L-1 MO/0.5 mmol L-1 C14mimBr complex. (B) UV-vis spectrum of 0.5 mmol L-1 MO, 0.5 mmol L-1 C14mimBr, 0.5 mmol L-1 MO/0.5 mmol L-1 C14mimBr.

Next, the investigation of the assembling dynamics of C14mimBr/MO was carried out by performing TEM, UV-vis spectrum and DLS measurements. In a representative ISA process, upon equivalent injection of C14mimBr into MO solution, the mixture was stirred vigorously to realize



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uniform mixing. After10 minutes, vesicles with the diameter of about 300 nm were formed (Figure 6A), as revealed by TEM. Subsequently, much bigger vesicles with diameter of about 1.5 µm were obtained after 2 days (Figure 6B). Interestingly, it can be observed that the vesicles could adhere to each other as time goes on, which demonstrated bigger vesicle was fabricated by the fusion of smaller vesicles.

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At last, precipitates formed at the bottom of test tube were separated out after 4

days and giant vesicles with diameter of about 10 µm were observed in TEM (Figure 6C). The timedependent DLS was also carried out to capture the evolution of sizes of vesicles. As shown in Figure 6D, with the time goes on, the Rh of aggregate increases. For example, the Rh of aggregate is 150 nm after mixing C14mimBr into MO solutions 1 min later. Then, the Rh increased to 400 nm after 200 mins and 1.5 µm after 2 days, which is consistent with the result of TEM. To gain more additional insight into the self-assembly process, the self-assembly arrangement at molecular level was also traced by UV-vis spectrum. The color of MO solution changes from orange to yellow rapidly with the addition of C14mimBr. As is shown in Figure S4, the absorption peak of MO at 462 nm blue-shift to 370 nm emerged and reduced significantly, meaning the formation of vesicle. With further time aging, no obvious changes in the absorption spectra were observed, but only slight increase of the peak intensity at 462 nm along with decrease and red shift of the peak at 370 nm, suggesting that the variation of the molecular arrangement is not dramatic in the transformation of small vesicles to bigger vesicles. This can also proves that there is no enormous change of aggregates arrangement. In XRD spectra, the sharp peak at 2θ=20° indicated the highly


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ordered packing of alkyl chains for the precipitates of C14mimBr/MO. (Figure S5). Thus, It can be speculate that the giant vesicles are possibly formed under a crystallization-driven self-assembly process.

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In addition to the formation of giant vesicles in the precipitates at the bottom, small

vesicles with diameter of about 150 nm was observed at the upper-phase after 4 days of storage (Figure S6), which provide the evidence that the solid-like giant vesicles were formed by the fusion of small vesicles. In a word, the small vesicles are kinetically trapped, and the fusion of small vesicles is thermodynamically favored. This is because that C14mimBr and MO carry opposite charge, the electrostatic interaction is reduced in the mixed assembly.

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Moreover, C14mimBr has a long

alkyl chain which possesses strong hydrophobic effect, inducing the self-assembly grow into large scales and finally precipitate out of the aqueous media. The self assembly process was summarized in Scheme 1.



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Figure 6. Time-dependant TEM images of 0.5 mmol L-1 MO/C14mimBr: (A) 10 minutes; (B) 2 days; (C) 4 days. (D) Time-dependant DLS result of 0.5 mmol L-1 MO/C14mimBr. (scale bar = 2 µm)

Moreover, the ISA of MO/C14mimBr system also shows an interesting pH chromism phenomenon. The pH-responsive of MO/C14mimBr system attributed to MO moieties since only MO can cause color changes in the system.

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MO takes a more conjugated coplanar state bridged by

N=N under alkaline condition (state a, Figure 7I), while with the decreasing of pH, MO takes quinoid structure crossed by N-N (state b, Figure 7I) and the conjugated configuration break. Thus, the MO/C14mimBr system displays abundant aggregate morphologies with the changing of pH. As revealed by SEM, solid-like giant vesicles were formed at pH=7 (Figure 2A). Decreasing pH to 4,


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the solid-like giant vesicle transforms into hollow polyhedron-like structure with the diameter of about 10 µm (Figure 7B). Futher decreasing pH to 2, the hollow structure break up into microflake with 200~350 nm in thickness and 10 µm in width (Figure 7A). However, when increasing the pH to 11, chrysanthemum-like structure with the diameter of 1 µm was observed, which was hierarchically self-assembled by nanorods (Figure 7C). Keep increasing pH up to 14, peony-like structure with the diameter of 1 µm composed of nanosheet was formed (Figure 7D). Similar structures were also confirmed by CLSM (Figure 7F, G, H). Fluorescent property of MO/C14mimBr system was also used to study the molecular assignment at different pH. As shown in Figure 7E, it can be seen that when pH≥4, there exists luminescence while no luminescence appears when pH=2. This is attributes to the changes of MO configuration. MO takes a conjugated coplanar state at high pH. While, the conjugated configuration breaks up at pH=2 inducing the disappearance of luminescence. The fluorescent properties of MO/C14mimBr at different pH is consistent with the morphological observation which suggests that conjugated structure of MO is crucial for building complex hierarchical self-assembly aggregates. Besides, in basic condition, the hydrogen-bond interaction between iminazole groups were destroyed, indicating the hydrogen-bond interaction is also important for the formation of solid-like giant vesicle. Moreover, we can also catch that the color of system at pH=2 is distinctly different with those at other pH values both by naked eyes and UV-vis spectra (Figure 7J, S7). This is agree with the results of fluorescence spectra that there is non luminescence at pH=2 but have luminescence at pH≥4, thus,



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the fluorescence behavior could be predicated by naked eyes which is challenging to construct in previous studies while it can be easy achieved using a simple ISA strategy here.

Figure 7. SEM images of 0.5 mmol L-1 MO/C14mimBr at different PH: (A) pH=2; (B) pH=4; (C) pH=11; (D) pH=14; (E) Fluorescence spectrum of 0.5 mmol L-1 MO/C14mimBr at different pH. CLSM images of 0.5 mmol L-1 MO/C14mimBr at different PH: (F) pH=4; (G) pH=11; (H) pH=14. (I) PH-dependant mechanism of MO molecule. (J) The digital photos of solution at different pH. (scale

bar = 2 µm)

The reversibility of MO/C14mimBr self-assembly behavior dependent on pH was also studied. In SEM, it could be found that microflake structure formed at pH=2 (Figure 8A) transformed into giant vesicle at pH=7 (Figure 8B). Decreasing pH to 2, microflake structure was reformed again as is shown in Figure 8A. More importantly, the reversible transformation between the two states could be


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repeated several times (Figure 8C). When adjusting the pH from 2 to 7, the color of system changes from red to yellow and the fluorescence intensity increases significantly (Figure 8D).

Figure 8. The morphologies of MO/C14mimBr self-assembly system at pH=2 (A) and pH=7 (B). The fluorescence intensity (C) and fluorescence spectra (D) at different pHs. Insets of C are the digital photos of solution under hand-held ultraviolet lamp at pH=7 (top) and pH=2 (below). (scale bar = 5

µm)



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Scheme 1. Schematic demonstration of formation of fluorescent giant vesicle and its pH responsivity.

At last, the MO/C14mimBr complexes were also subjected to thermal analysis to determine their stability and phase behaviors. TGA curves showed that the complexes are stable up to 283 ℃(Figure 9A). The phase behavior of the complexes was investigated in detail by DSC and POM. Firstly, Heating the complexes to 150 ℃, the isotropic phase formed (Figure 9C), then slowly cooled the sample (1 ℃ min-1) from isotropic phase to room temperature. During the cooling process, many spherulitic textures with a typical Maltese extinction cross weredispersed in 101~99 ℃ (Figure 9D), which corresponded to the typical nematic droplet and indicated the presence of the thermotropic nematic-type liquid crystalline (LC) state.

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Further keep cooling the sample to room temperature,

another texture appeared, meaning transition from liquid crystalline state to solid state. DSC was also used to study the thermotropic behavior of complexes. It was noting that the complexes show a


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remarkable super-cooling effect: in the DSC thermogram, the endothermic peak (118 ℃) observed during the heating process is much higher than exothermic peaks (80 ℃) detected during the cooling process (Figure 9B). Analogous phenomena have been reported, and it was attributed to a “stretchedbent shape” transition of the soft alkyl chain section along with the changes in the packing of the azobenzene units which effecting the packing of complex.

45,46

Surprisingly, there is only one peak

occurred in DSC thermogram, this is maybe attributes that the first-order thermodynamic transition temperature (solid state to LC state) is very closely to the clearing temperature (LC state to isotropic phase). Here, we presented a simple and facile method of preparing highly ordered thermotropic LC materials by ISA strategy instead of complicated covalent chemistry. The dye and surfactant were chosen as the building blocks making the complexes are potential candidates for organic conducting materials.



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Figure 9. (A) TGA curves of MO, C14mimBr, and complexes of MO/C14mimBr. (B) DSC curve from the second heating–cooling cycles of complexes of MO/C14mimBr. POM images of complexes of MO/C14mimBr at different temperature (C, D, E). (scale bar = 20 µm)

CONCLUSION

In conclusion, 3D SGVs were successfully prepared by an anionic dye MO and an oppositely charged surfactant C14mimBr which is rarely reported via ISA strategy. The formation process of SGVs is that kinetically trapped small vesicles were formed firstly, and fused into thermodynamically stable SGVs. The giant vesicle performs excellent fluorescence properties meaning the self-assembly process could achieve the transformation of ACQ into AIE by noncovalent interaction. The self-assembly performs pH responsive not only in morphology, but also in optical property. Besides, the fluorescence behavior could be predicted with the color change directly visible to the naked eye. Moreover, the solid-like giant vesicle performs abundant thermic phase behavior, including thermotropic LC phases. It can be expected that our MO/C14mimBr complexes prepared via ISA strategy are potential candidates for fluorescent molecule switch, cargo carrier and pH detection probes.

Supporting Information. Further characterizations (TEM, UV/vis spectra, zeta potential and XRD) are reported.


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Acknowledgements We gratefully acknowledge the financial support from the National Natural Science Foundation of China (21573130, 21173128), and Young Scholars Program of Shandong University (2016WLJH20).

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TOC GRAPHIC


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Fabrication of Smart pH-Responsive Fluorescent Solid-like Giant

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