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Chain Length Dependent Alkane/β Soft Matter View Article Online PAPER View Journal | View Issue Chain length dependent alkane/b-cyclodextrin nonamphiphilic supramolecular building blocks† Cite this: Soft Matter, 2016, 12,1579 Chengcheng Zhou,ab Jianbin Huang*a and Yun Yan*a In this work we report the chain length dependent behavior of the nonamphiphilic supramolecular building blocks based on the host–guest inclusion complexes of alkanes and b-cyclodextrins (b-CD). 1H NMR, ESI-MS, and SAXS measurements verified that upon increasing the chain length of alkanes, the Received 2nd November 2015, building blocks for vesicle formation changed from channel type 2alkane@2b-CD via channel type Accepted 19th November 2015 alkane@2b-CD to non-channel type 2alkane@2b-CD. FT-IR and TGA experiments indicated that hydrogen DOI: 10.1039/c5sm02698a bonding is the extensive driving force for vesicle formation. It revealed that water molecules are involved in vesicle formation in the form of structural water. Upon changing the chain length, the average number www.rsc.org/softmatter of water molecules associated with per building block is about 16–21, depending on the chain length. Introduction fabrication of proper nonamphiphilic building blocks for the formation of vesicles and other self-assembled structures still Molecular self-assembly, a ubiquitous process in chemistry, remains challenging. biology, and materials science, provides the foundation for Recently, nonamphiphilic building blocks based on the building a variety of nanostructures.1–6 Among the various host–guest inclusion complexes of cyclodextrins (CDs) and structures, vesicles have always been considered as one of the surfactants have been developed in our lab.23,26–31 CDs are most important types owing to their promising applications in doughnut-like oligosaccharides with a hydrophilic exterior and fields such as drug7 and gene delivery8 and nanoreactors.9 To a hydrophobic cavity.32 The hydrophobic cavity is an ideal harbor meet the different application demands, extensive efforts have in which poorly water-soluble molecules can shelter their hydro- been made to design various building blocks to fabricate phobic parts.33 Therefore, upon addition of surfactants into the functional vesicles.9–13 Amphiphiles have been verified to be a aqueous solution of CD, nonamphiphilic building blocks can be large family of building blocks to form vesicles, such as surfac- formed if the whole hydrophobic tail of surfactants was included Published on 20 November 2015. Downloaded by Beijing University 25/11/2016 14:37:32. tants,14 lipid molecules,15 polymers8,16 and aromatic organic com- into the cavity of CD. Our group revealed that when b-CD pounds.17,18 In the last two decades, nonamphiphiles such as and surfactants, such as sodium dodecyl sulphate (SDS)23,29 or polyoxometalate macroions19–22 have also been found to be effec- Tween 20,30 complex at the molar ratio of 2 : 1, the resultant tive building blocks for vesicle formation. The amphiphilic mole- surfactant@2b-CD inclusion complexes can self-assemble into cules self-assemble into vesicles driven by the hydrophobic effect vesicles, tubes, and lamellar structures simply with increasing and mediated by a hydrophobic–hydrophilic balance.23 In con- concentration. These self-assemblies are predominantly driven trast, the driving forces for nonamphiphilic vesicles can be electro- by hydrogen bonding between b-CDs,29,30 which demonstrates static interactions, van der Waals interactions and hydrogen that the nonamphiphilic building block surfactant@2b-CD has bonding etc.19,24 With superiorities in some aspects over their been fabricated. Most surprisingly, our latest work shows that amphiphilic counterparts, such as better environmental adap- other than surfactants, dodecane can also form a 2 : 1 inclusion tiveness, versatile functionalities, enhanced biocompatibility, complex with b-CD, which further self-assembles into vesicles or etc., nonamphiphiles as a new class of building blocks are bricks depending on concentrations.34 Meanwhile, Hao et al. attracting considerable attention.19,24,25 However, so far the revealed that nonamphiphilic vesicles can be constructed by the 1 : 1 host–guest complexes formed with b-CD and small aromatic 35 a Beijing National Laboratory for Molecular Sciences (BNLMS), State Key Laboratory molecules. These results strongly indicate that different for Structural Chemistry of Unstable and Stable Species, College of Chemistry and alkanes may produce different nonamphiphilic building blocks Molecular Engineering, Peking University, Beijing 100871, P. R. China. with b-CD. Therefore, it is highly desired to carry out a systematic E-mail: [email protected], [email protected] study to verify this conclusion. b Institute of Chemistry, Chinese Academy of Sciences, Beijing, China † Electronic supplementary information (ESI) available: 1H NMR, ESI-MS results Herein we show that the chain length of alkanes has a great and the effect of urea on the self-assembly of alkane/b-CD. See DOI: 10.1039/ influence on the structure of nonamphiphilic building blocks c5sm02698a formed with b-CD. In the case of short chained alkanes with This journal is © The Royal Society of Chemistry 2016 Soft Matter, 2016, 12, 1579--1585 | 1579 View Article Online Paper Soft Matter 5 to 10 carbons, the building block is channel type 2alkane@2b-CD, Atomic force microscopy (AFM) which can be looked on as the dimer of alkane@b-CD. Upon AFM measurements in tapping mode under ambient condi- increasing the chain length to 12–14 carbons, the building blocks tions were conducted on a D3100 AFM (VEECO, USA). One drop take the form of alkane@2b-CD. Interestingly, upon further increas- of the alkane/b-CD solution was spin-coated on a mica surface, ing the chain length to 16 and 18 carbons, the building block and then placed at room temperature to dry before AFM changes back to the form of 2alkane@2b-CD but with non- observation. channel type arrangement of b-CD. All these nonamphiphilic building blocks self-assemble into unilamellar and bilamellar Fourier transform infrared (FT-IR) vesicles in dilute solutions. Our results revealed for the first time that variation of the chain length has significant effects FT-IR measurements were performed on a Nicolet Magna IR on the fundamental structure of the nonamphiphilic building 750 equipped with an infrared microspectrograph (Thermo blocks, which brings new perspectives in designing self- Scientific Co., USA). The vesicle samples were frozen in liquid assembled structures. nitrogen and subsequently lyophilized for 48 h before FT-IR measurements. 1 Experimental section H NMR measurements The lyophilized alkane/b-CD precipitates were dissolved into Materials 1 DMSO-d6 for H NMR measurements to determine the actual n-Pentane and n-hexane were purchased from Beijing Chemical ratio of alkane to b-CD. The measurements were performed on Works. n-Tetradecane, n-hexadecane and n-octadecane were an AVANCE III 500 MHz NMR (Bruker, Switzerland). All the purchased from Alfa Aesar. n-Octane, n-decane and b-cyclodextrin proton signals were calibrated with the TMS at 0.00 ppm. By (b-CD) with a water content of 14% were purchased from Sino- comparing the integration between b-CD (H1 protons, d 4.83 ppm) pharm Chemical Reagent Co. Ultrapure water was used throughout and the total proton integration of alkanes, the alkane/b-CD the experiment. complex ratio can be determined. Sample preparation ESI-MS measurements For the precipitated samples, alkane was added into the b-CD ESI-MS measurements were carried out on an APEX IV FT-MS solutions of 16 mM and then the samples were vortex mixed (Bruker, USA). The operating conditions of the ESI source: sufficiently to create the white precipitates. After removing the positive ion mode; spray voltage À3300 V; capillary voltage supernatant, the alkane/b-CD precipitates were lyophilized for À3800 V, capillary temperature 230 1C; skimmer1 33.0 V, further measurements. Since the solubility of alkane/b-CD skimmer2 28.0 V; sheath gas nitrogen pressure 0.3 bar. The complexes is about 2 mM, vesicular samples were prepared 1 mM alkane/b-CD vesicle samples were diluted 10 times by by weighting desired amounts of lyophilized alkane/b-CD pre- methanol and introduced via direct infusion at a flow rate of cipitates into test tubes followed by addition of water to give the 3.00 mL minÀ1. Published on 20 November 2015. Downloaded by Beijing University 25/11/2016 14:37:32. alkane/b-CD concentration of 2 mM. The resultant solutions were thermostatically incubated at 25 1C (for at least 24 h) to SAXS measurements allow self-assembly formation. SAXS measurement of the b-CD powder and lyophilized alkane/ b-CD precipitates was conducted on a high-flux small-angle Dynamic light scattering (DLS) X-ray scattering instrument (SAXSess, Anton Paar) equipped Dynamic light scattering (DLS) measurements were conducted with a Kratky blockcollimation system. A Philips PW3830 on an ALV/DLS/SLS5022F light scattering-apparatus, equipped sealed-tube X-ray generator (Cu Ka) was employed to simulta- with a 22 mW He–Ne laser (632.8 nm wavelength) with a neously measure the small-angle and wide-angle X-ray scat- refractive index matching bath of filtered toluene surrounding tering (WAXS and SAXS) of the samples. The scanning the cylindrical scattering cell. The samples were filtered by patterns extend to the high-angle range (the q range covered 450 nm filters. The scattering angle was set from 301 to 901. by the imaging plate is from 0.0 to 27.2 nmÀ1, q =4p sin y/l, where l is the wavelength of 0.1542 nm and 2y is the TEM observation scattering angle). The scattering angles were calibrated with The morphology of the self-assemblies was observed using a aluminum. JEM-100CX II transmission electron microscope (JEOL, Japan). A drop of the sample was placed on 230 mesh copper grids TGA and DSC measurements coated with Formvar film.
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