Supramolecular Nanopumps with Chiral Recognition for Moving

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Functional Nanostructured Materials (including low-D carbon)

Supramolecular Nanopumps with Chiral Recognition for Moving Organic Pollutants from Water Sihan Bao, Shanshan Wu, Liping Huang, Xin Xu, Rui Xu, Yongguang Li, Yongri Liang, Minyong Yang, Dong Ki Yoon, Myongsoo Lee, and Zhegang Huang ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.9b11286 • Publication Date (Web): 05 Aug 2019 Downloaded from pubs.acs.org on August 10, 2019

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ACS Applied Materials & Interfaces

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Supramolecular Nanopumps with Chiral Recognition for Moving Organic Pollutants from Water Sihan Bao1, ‡, Shanshan Wu1, ‡, Liping Huang1, ‡, Xin Xu1, Rui Xu1, Yongguang Li1, Yongri Liang2, Minyong Yang3, Dong Ki Yoon3, Myongsoo Lee4, Zhegang Huang1,*

1PCFM,

LIFM and GD HPPC Lab, School of Chemistry, Sun Yat-sen University, Guangzhou

510275, PR China. 2College

of Materials Science and Engineering, Beijing Institute of Petrochemical Technology,

Beijing 102617, PR China. 3Graduate

School of Nanoscience and Technology and KINC KAIST, Daejeon 34141, Republic

of Korea. 4State

Key Laboratory for Supramolecular Structure and Materials, College of Chemistry, Jilin

University, Changchun 130012, PR China. ‡These

authors contributed equally to this work.

*Corresponding author: [email protected]

ACS Paragon Plus Environment

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KEYWORDS: supramolecular nanopumps, dynamic assembly, pH-responsive nanodevices, supramolecular absorbents, chiral recognition

ABSTRACT Since organic pollutants in water resource have raised concerns on aquatic ecosystems and human health, mechanical machine such as nanopump for rapid and efficient removal of pollutants from water with regeneration properties remains a challenge. Here, a pH-responsive artificial pump from left-handed porous tubules into right-handed solid fibers was presented by the self-assembly of bent-shaped aromatic amphiphiles. The bent-shaped amphiphile with pHsensitive segment was demonstrated into aromatic hexameric macrocycles which could contract into dimeric discs. Such switchable aromatic pore with super-hydrophobicity was well-suitable for efficient removal and controlled release of organic pollutants from water through pulsating motion. The removal efficiency is found to be 78% for ethinyl oestradiol (EO) and 82% for bisphenol (BPA). Additionally, the pumping accompanied by chiral inversion was endowed with rapid removal and convenient regenerable ability. The inflation from right-handed solid fibers into left-handed tubules for efficient removal pollutants was remarkably promoted by (-)-acidic enantiomer of malic acid, whereas the contraction with full desorption of pollutants was incisively responsive to alkaline with (+)-conformation. The kinetically regulable porous device with chiral recognition will provide a promising platform for the construction of rapid responsible machine for sewage treatment.


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INTRODUCTION In nature, hollow organisms are capable of self-inflation and contraction in response to environmental changes to achieve vital activities.1,2 To mimic this fantastic motion from nature, artificial nano-scale pumps have been constructed for transport of signal,3 ion4 and hydrophilic drug5. Although these objects can be successfully delivered using the special devices, seldom of them are utilized for removal of hydrophobic organic pollutants. Removal by pumping device fully depends on the expansion or contraction of cavity. Up to now, meso-porous materials like porous silica,6~8 carbon,9,10 polymers,11~14 and metal-organic frameworks (MOFs)15~17 with desired size and properties are actively investigated as advanced sorbents for pollutant extraction and preconcentration. These systems exhibited high capacity as well as rapid adsorption but the desorption remains a challenge due to their stable interaction with hydrophobic pollutants.18,19 In general, the adsorbents can be regenerated through mild washing using solvents. However, desorption by washing is usually accompanied by secondary contamination.20 Dynamic pumping of adsorbents by external trigger (light, pH, heat, etc) is a promising solution for this challenging problem.21~23 Alternatively, spontaneous assembly of small molecular modules through non-covalent interactions is a key to creating stimuli-responsive dynamic devices.24~27 Among them, selfassembly of aromatic building blocks provides a facile means to construct very ordered porous structures, which are well suitable for sorption of organic pollutants.28 Compared with the traditional rigid pores that are not compatible with dynamic characteristic without bond breaking, the supramolecular pores can be easily manipulated by dynamic variation of molecular shape and external environment due to their reversible formation.29,30 For example, the water cluster as molecular glue triggered flexible noncovalent aromatic macrocycles through mutual sliding of


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adjacent molecules constructed by self-assembly of bent-shaped aromatic amphiphiles.31 The glue in dynamic assembly allowed them to adapt the changes of porous surroundings. As hinted from the adaptation by dynamic assembly, herein we have developed a pH-responsive supramolecular pump that undergoes reversible changing between porous tubules and solid fibers. The tunable walls of the pump were composed of the self-stacking from paired bentshaped aromatic segments which were able to readily inflate into hollow hexameric macrocycles by protonation (Figure 1). The discs from paired aromatic segments rotated counterclockwise to form right-handed solid fibers, while the hollow macrocycles spontaneously rotated clockwise to form left-handed tubules. We found that the porous tubules with aromatic interior could efficiently remove organic pollutants from waste water and then subsequent pumping triggered the pollutants to be desorbed through tubular contraction. Importantly, the dynamic pumping fully depended on the chirality of alkaline or acid initiator. The (+)-alkaline enantiomer remarkably promoted the contraction of hollow tubules, while the (-)-acidic enantiomer was preferable for the inflation of solid fibers due to the unique chiral recognition.


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confirmed by proton nuclear magnetic resonance (1H-NMR).32,33 1H-NMR measurements of 1a showed that most of protons in aromatic segments were downfield-shifted by titration of TFA, indicating the formation of hydrogen bonds in pyridine units (Supporting Information Figure S2). Nevertheless, the proton ortho to pyridine of benzene ring, which faced to nitrogen atom in neutral state became shielded by acidification, suggesting a rotation around the CPy-CPh bond to move away from the NH+ group (Figure 2). OR

OR

a

3py

O

m1

TFA

o N

m2

H

o

Br

3py

O

N

Br

Br

O

O

m1

m2

1a Neutral State

Br

H+

1a + Acidic State

b 3.0

2.4

nTFA (equiv)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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1.8

1.2

0.6

0.0

Ho 7.80

7.70

7.60

7.50

7.40

H3py

Hm2

7.30

7.20

Hm1 7.10

f1(ppm)

Figure 2. pH-responsive properties of aromatic block from model 1a. (a) Representation of the influence of protonation on pyridine ring and benzene ring. (b) 1H-NMR of 1a in CDCl3 with TFA titration. We envisioned that an increasement of the aromatic segment would self-assemble into an aggregation due to its amphiphilic characteristics.34 Indeed, the bent-shaped molecule 2a with an


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internal angle of 120o from the extension of pH-sensitive object 1a can self-assemble into solid fibers from the stacking of dimeric aromatic discs. The solid fibers of 2a were confirmed by transmission electron microscopy (TEM) from 0.02 wt% aqueous solution which was negatively stained with uranyl acetate. The TEM image showed thin fibers with a uniform diameter of ~5 nm and several hundred nanometer lengths (Figure 3a). To further understand the onedimensional aggregation, the atomic force microscopy (AFM) and two-dimensional X-ray diffraction (2D XRD) experiments were performed with thin film of 2a. A kind of vertically equivalent diffraction was observed at equator and meridian from 2D XRD (Figure 3c). The equidistance of 4.5 nm from equatorial diffractions corresponded to inter columnar ordering, which was well-matched with the diameter of fibers determined from TEM experiments. According to interlayer distance about 4 Å, the number of molecules in one column unit could be calculated as 2.1, indicating that the discs stacking into solid fibers consisted of paired bentshaped aromatic segments (Supporting Information Figure S5). Meanwhile, some periodic reflections with equidistance of 4.0 nm were observed along the column axis. Taking into account the structures and dimensions of dimeric discs, this diffraction pattern is attributable to a pitch of fibrous helix. As expected, when the fibrous solution of 2a was subjected to circular dichroism (CD), an obvious Cotton effect in the spectral range of the aromatic units was observed (Figure 4a), indicating that the fibers adopted a one-handed helical structure. Indeed, AFM of 2a on the mica surface revealed the formation of right-handed helical structures with a pitch of 4.2 nm (Figure 4b). These observations indicated that 2a self-assembled into dimeric discs which stacked on top of each other with mutual rotation in a single direction to allow the chiral amplification from the side groups to the aromatic segments, leading to right-handed solid fibers.


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0.8

a

c 0.6

5.0nm 0.4

0.2

5.0nm

100nm b

0.0

0.0

d

7.6nm 0.2

0.4

2.3nm

100nm

0.6

Figure 3. Dynamic diameter of supramolecular pump. TEM images of 2a obtained from 0.02 wt% aqueous solution without TFA (a) and with 1.5 equiv TFA (b) (Inset in a and b are corresponding STEM). 2D XRD patterns of 2a obtained from 0.01 wt% aqueous solution without TFA (c) and with 1.5 equiv TFA (d) (Y axis in c and d is q (Å-1) ). However, we noticed that the addition of TFA could trigger an inversion of CD signals (Figure 4a). Upon addition of TFA, the Cotton effect at 362 nm from the adsorption of aromatic units was gradually inverted from negative minimum to positive maximum up to 1.6 equiv addition of TFA, demonstrating that the solid helixes were dynamically responsive to proton.35 To prove the CD inversion was not from artifacts, the pH-responsive experiments were also performed with the enantiomer 2b. The enantiomer 2b displayed an opposite positive Cotton


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effect at 362 nm, which was inverted to negative upon addition of TFA, demonstrating that the chirality inversion was from dynamic molecular assembly. The variation of Cotton effect from both molecules was the same as molecular protonation of 2a and 2b by optical experiments in acetonitrile (Supporting Information Figure S4), suggesting that the formation of optical active states depended on the molecular protonation. Remarkably, the inversion of CD by proton was accompanied with the inflation of solid fibers. The images of 2D XRD from protonated solution of 2a and 2b clearly revealed the existent of helical structures with a pitch of 3.6 nm (Figure 3d). However, the periodic diffractions from equator showed an increased diameter close to 7.0 nm. From inter and intracolumn distance, the molecular number in one unit cell of expanded column was calculated as 5.9 (Supporting Information Figure S5). The expansion of 2a and 2b was further confirmed by TEM experiment. When the samples were cast on TEM grids from the 1.5 equiv TFA-containing solution (0.02 wt%), the images showed expanded one-dimensional tubules with an external diameter of 7.6 nm and a hollow interior. The hollow objects were further checked by scanning transmission electron microscope (STEM) with a probe aberration corrector, showing the inner diameter of tubules as 2.3 nm (Figure 3b). These results indicated that the expanded helical tubules were based on the stacks of noncovalent hexameric macrocycles formed by protonation.


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with an upward orientation for S-oligoether dendrons (Supporting Information Figure S6). Based on the calculations from 2D XRD, the porous tubules with left- and right- handedness were constructed by mutual rotations of 6.7o from 9 layered hexameric macrocycles, whereas 10 layered dimeric discs twisted 18o to give right- or left-handed solid fibers (Supporting Information Figure S7). The minimization revealed that the chiral inversion was attributed to an orientation change of the dendritic chains on the aromatic exterior. Due to the chirality of oligoethers, there was some steric crowding on the right side of dimeric aromatic vertex. To minimize steric hindrance without sacrificing J!J interaction between aromatic segments, the upper-neighbouring dimeric planes rotated counterclockwise to form right-handed solid fibers. However, in the hexameric plane, the rotation of methoxyl from outside into inner space induced marginal spaces on the left edges of aromatic cycles that the dendritic chains occupied, resulting in larger steric repulsion on the left side in each hexameric vertex. This spatial requirement leaded the upper-neighbouring macrocycles rotated clockwise to form left-handed tubules (Figure 4e). The meso-tubular formation from stacking of aromatic macrocycles is well suitable as scaffolds for adsorption of organic contaminants from water.26 Thus, a compared uptake for organic micro-pollutants such as ehinyl oestradiol (EO), bisphenol A (BPA), methyl orange (MO) and roxithromycin (ROX) was performed with porous tubules and solid fibers respectively. In contrast to solid fibers, the porous tubules from the assembly of protonated macrocycles removed most of pollutants except high soluble MO and removal efficiencies for EO, BPA, and ROX were observed up to 78 %, 82 % and 75 % respectively (Supporting Information Figure S10c). Nonetheless, the difference of porous tubules for three pollutants appeared on the equilibrium uptake. The removal capacity for EO and BPA was determined as


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31.2 and 65.6 mg g-1, which was clearly higher than 9.6 mg g-1 for ROX (Supporting Information Figure S10d), indicating that the adsorption depended on the size and solubility of pollutants. The regulable expansion and contraction from right-handed solid fibers and left-handed porous tubules motivated us to explore kinetic uptake and release experiments with chiral acidic or alkaline reagents. Thus, the contracted fibrous adsorbent of 2a was dissolved into BPA solution and then, acidified by (+)-malic acid, (-)-malic acid, and (±)-malic acid to weak acidic solution (pH~5). In particular, the adsorbent treated by (-)-malic acids gave the fastest removal rate. In contrast to (+)- and (±)-malic acid required 120 min to reach 92 % of removal equilibrium, (-)conformation needed 85 min only (Figure 5a). Subsequently, the expanded tubules with uptake BPA were additionally transferred into alkaline solution of (+)-2-amino-1-butanol, (-)-2-amino1-butanol, and (±)-2-amino-1-butanol respectively. Figure 5b showed the desorption triggered by (+)-2-amino-1-butanol was observed faster than others. The time for 95 % of desorption by (+)2-amino-1-butanol was 20 h, while treatment by (±)-2-amino-1-butanol and (-)-2-amino-1butanol required 27 h and 30 h respectively. The unique kinetic behavior for adsorbing and releasing pollutants can be explained by chiral recognition of right- or left-handed adsorbents, which were proved by rapid response from Cotton effect and molecular dynamics (MD) calculations (Supporting Information Figure S8). When the pyridines of 2a from right-handed solid fibers were acidified by (-)-malic acid, both hydroxy and carbonyl methylene were far away from upper dimeric discs. In contrast, the large hydroxy and carbonyl methylene of (+)enantiomer were close to the upper discs, inducing larger space crowding within the chiral interfaces, which reflected in higher energy minimization from MD calculation (Figure 5c). The distinct binding behavior of dimeric 2a resulted in selective expansion for (-)-malic acid. However, as the amino group of the butanol with (+)-conformation interacted with the proton


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(III) solution. (c) Schematic representation of right-handed fibers interacting with (+)-malic acid and (-)-malic acid. (d) Schematic representation of left-handed tubules interacting with (+)-2amino-1-butanol and (-)-2-amino-1-butanol by MD simulations.

CONCLUSIONS As a result, mimicking the pulsating motion of nature, a pH-responsive porous adsorbent with super-hydrophobicity has been constructed through the dynamic assembly of hexameric macrocycles from dimeric substructures. Notably, the dimeric discs stacked on top of each other counterclockwise to form right-handed solid fibers while hexameric macrocycles stacked clockwise on top of each other to form left-handed porous tubules. The mechanical switch of porous materials from hollow tubules into solid fibers was utilized for efficient removal and controlled release of organic micropollutants such as EO, BPA, and ROX from polluted water through pulsating motion. Particularly, the adsorption of pollutants rapidly responded to (-)malic acid for the expansion of right-handed solid absorbent and the desorption of pollutants from adsorbent was incisively responsive to (+)-2-amino-1-butanol owing to the spatial recognition of left-handed tubular aperture. The special uptake and regenerable behavior with chiral recognition was starkly contrast to both conventional porous adsorbents that hardly recovered the initial capacity without bond breaking and dynamic tunable supramolecular absorbents. 18,19,28,36 Although the preparation of dynamic porous device in a large scale remains challenging, we believe that our strategy for kinetically regulable porous device with chiral recognition will provide a promising platform for the construction of rapid responsible machine for sewage treatment.


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EXPERIMENTAL SECTION Materials. 4-bromo-2-methoxyphenylboronic acid, 4-(trimethylsilyl)phenylboronic acid, 4formylphenylboronic acid and iodine monochloride (1.0 M solution in dichloromethane) from Aldrich, ethyl L-(-)-lactate, tetrakis (triphenylphosphine) palladium (0) (99 %) from TCI, and 3chloro-2-chloromethyl-1-propene (96 %) from Acros were used as received. Unless otherwise indicated, all starting materials were obtained from commercial suppliers and were used without purification. Tetrahydrofuran (THF) was dried by distillation from sodium–benzophenone immediately prior to use. Dichloromethane (DCM) was dried by distillation from CaH2. Distilled water was polished by ion exchange and filtration. Visualization was accomplished with UV light, iodine vapor. Flash chromatography was carried out with Silica Gel 60 (230-400 mesh) from EM Science. Compounds 3 was synthesized according to the same procedures as described previously. Synthetic scheme of bent-shaped amphiphiles:

O

Br

OH B OH

Br

OR

OR O

O N

N Br

OH B OH

TMS

Br

Pd/K2CO3 Br

ICl NaS2O3 .5H2O

1a, 1b

3a, 3b

OR OR O

OHC O

N I

O

O N

Pd/K2CO3

4a, 4b

a: R=

OH B OH

I

O O

O O O O O O O O O O O O O O O O

2a, 2b

OHC

b: R=

O O

CHO

O O O O O O O O O O O O O O O O


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TEM experiments. To investigate aggregated structures of 2a and 2b in neutral and acidic aqueous solution, a drop of the solution from water (0.02 wt%) and acidic solution (0.02 wt%) with 1.5 equiv of TFA was placed on carbon–coated copper grids, respectively. The solutions were allowed to evaporate under ambient conditions and then negatively stained with uranyl acetate. The dried specimens were observed by using a JEM-2010HR machine operated at 200 kV to determine the aggregation. AFM experiments. For the AFM measurement of the assembled structures, the aqueous solutions of 2a and 2b (0.02 wt%) with and without 1.5 equiv addition of TFA were prepared and then 25 O of the samples were casted on mica separately. After slow evaporation under air at room temperature, the AFM experiments were performed with tapping model. The typical settings of the AFM for the high-magnification observations were as follows: a free amplitude of the oscillation frequency of ca. 1.0–1.5 V, a set-point amplitude of 0.9-1.4 V, and a scan rate of 1.0 Hz. Computation. Initial Model Construction: For constructing solid fibers and hollow tubules, the potential energy with both dimeric disc and hexameric macrocycle was minimized until the root-mean square derivative was 1.0 (Kcal/mol)/Å or less. For the chirality of the ethylene oxide dendron, most of oxygens in dendritic chain adopted upward orientation toward both aromatic plane. Then, according to the calculations from 2D XRD, two kinds of helixes with and without porosity could be constructed. The porous tubules with left- and right-handedness were constructed by mutual rotations of 6.7o from 9 layered minimized macrocycles. On the other hand, 10 layered dimeric discs twisted 18o to construct right- and left-handed solid fibers. For the comparison, face to face stacked non-chiral fibers and tubules were also constructed.


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Energy Calculations: The energy with four chiral models in neutral and acidic condition and two kind of non-chiral comparable objects was calculated using Material Studio 8.0 program. The calculations were performed by Forcite Tools based on molecular mechanics theory and molecular dynamics theory under the following parameters: task: energy; quality: ultrafine; forcefield: compass; charge: forcefield assigned; electrostatic: atom based; van der waals: atom based; job description: 5000; run in parallel on: 1 of 8 cores. The calculation showed that the dextral dimeric fibers revealed the minimum energies while the left-handed tubules were more energetically favorable than the right-handed. Based on above methods, the potential energy of right-handed dimeric fiber with (+) and (-)-malic acid was calculated. Energy minimization gave that the dextral dimeric fibers acidified by (-)-malic acid were more stable. In addition, the potential energy of left-handed tubules with (+) and (-)-2-amino-1-butanol was also calculated. The results showed that the proton interacted with (+)-form alkaline within left-handed tubules is more favorable than (-)-form enantiomer. Organic Pollutants Adsorption. The adsorption of EO, BPA, MO, ROX was performed by 2a (1 mg) with 0.01 mg EO,0.0114 mg BPA, 0.05mg MO and 0.005mg ROX solution (1 ml, respectively). After sonicating for 15 min at room temperature, the solution was filtered through an ultrafiltration spin column. The residual concentration of the pollutant in each filtrate was determined by UV-vis spectroscopy under 278 nm (EO), 272 nm (BPA), 506 nm (MO) and 212 nm (ROX). The efficiency of pollutant removal (in %) by the sorbent and the amount of pollutant bound to the sorbent were determined by the following equations:

pollutant removal efficiency =

C0 Ce 100 C0 ,


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qt

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C0 Ce M W m

where C0 (mmol-1) and Ce (mmol-1) are the initial and equilibrium concentration of the pollutants respectively, and qt (mg gR ) is the capacity of pollutant adsorbed per g of sorbent at equilibrium. m (g) is the mass of sorbent used in the study. MW (g molR ) is the molar mass of the pollutant. Kinetic Adsorption and Release of Pollutants and Regeneration of Supramolecular Adsorbent. The solid fibers of 2a with BPA solution were dissolved in CH3COOH/CH3COONa (pH=5.0) buffer solution, (±)-malic acid, (-)-malic acid, and (+)-malic acid solution respectively. In order to determine the quantity of absorbed pollutants, 0.2 mL of solution was taken out at predetermined time intervals and centrifuged for 20 min under 6000rpm. The filtrate was observed by UV-vis spectroscopy. The hollow tubules of 2a with absorbed BPA were dialyzed using Na2CO3/NaHCO3 buffer solution (pH=11.0), (±)-2-amino-1-butanol solution, (-)-2-amino1-butanol, and (+)-2-amino-1-butanol solution respectively. After taking out 0.2 mL of solution at different time, the solution was centrifuged under above methods and then checked by UV-vis. After all pollutants were released, the tubular adsorbents were recovered by acidification using CH3COOH/CH3COONa buffer solution. The regenerated adsorbents showed initial removal efficiency for BPA up to 5 cycles. Dynamic Pumps with Chiral Recognition. A kind of aqueous solutions of 2a (0.02 wt%) were prepared. And then 10 equiv of (±)-malic acid, (-)-malic acid, and (+)-malic acid were added and then stirred at room temperature to investigate the variation of CD Spectra. Meanwhile, a kind of tubular solutions (0.02 wt%) were also prepared by acidified 2a. To the stock of tubular solution, 10 equiv of (±)-2-amino-1-butanol, (-)-2-amino-1-butanol, and (+)-2-


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amino-1-butanol were added and then stirred at room temperature to investigate the variation of CD Spectra.

ASSOCIATED CONTENT Supporting Information. The Supporting Information is available free of charge on the ACS Publications website. General, Synthesis of Compounds 1a, 2a and 2b, Supporting Figures S1~S11.

AUTHOR INFORMATION Corresponding Author * E-mail: [email protected] Author Contributions ‡These

authors contributed equally. S.B., S.W. and L.H. performed the most of the experiments

and analyzed data. Y.L. and M.L. performed TEM experiment. X.X. and R.X. performed AFM experiment. Y.L., M.Y., and D.K.Y. performed XRD experiment. S.B., S.W., L.H. and Z.H. wrote the paper. Notes The authors declare no competing financial interest.


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ACKNOWLEDGMENT We thank Prof. J. Wang for help with NMR measurements. This work was supported by Sun Yat-sen University, the NSFC (21871299), Guangzhou Science and Technology Program (201707010248), Natural Science Foundation of Guangdong Province (2017A030313086), the Key Laboratory of Functional Inorganic Material Chemistry (Heilongjiang University), Ministry of Education, the Open Project of State Key Laboratory of Supramolecular Structure and Materials (Jilin University).

REFERENCES 1. Tanaka, S.; Sawaya, M. R.; Yeates, T. O. Structure and Mechanisms of a Protein-based Organelle in Escherichia Coli. Science 2010, 327, 81-84. 2. Wu, Z.; Lin, X.; Si, T.; He, Q. Recent Progress on Bioinspired Self-Propelled Micro/Nanomotors via Controlled Molecular Self-Assembly. Small 2016, 12, 3080-3093. 3. Bhosale, S.; Sisson, A. L.; Talukdar, P.; Fürstenberg, A.; Banerji, N.; Vauthey, E.; Bollot, G.; Mareda, J.; Röger, C.; Würthner, F.; Sakai, N.; Matile, S. Photoproduction of Proton Gradients with J!

:

Fluorophore Scaffolds in Lipid Bilayers. Science 2006, 313, 84-86.

4. Gouaux, E.; Mackinnon, R. Principles of Selective Ion Transport in Channels and Pumps. Science 2005, 310, 1461-1465. 5. Tu, Y.; Peng, F.; White, P. B.; Wilson, D. A. Redox-Sensitive Stomatocyte Nanomotors: Destruction and Drug Release in the Presence of Glutathione. Angew. Chem. Int. Ed. 2017, 56, 7620-7624.


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