Article pubs.acs.org/Langmuir
Spontaneous Nematic Alignment of a Lipid Nanotube in Aqueous Solutions Wuxiao Ding, Hiroyuki Minamikawa,* Naohiro Kameta, Momoyo Wada, Mitsutoshi Masuda, and Toshimi Shimizu Nanosystem Research Institute, National Institute of Advanced Industrial Science and Technology (AIST), Tsukuba Central 5, 1-1-1 Higashi, Tsukuba, Ibaraki 305-8565, Japan S Supporting Information *
ABSTRACT: The dispersibility and liquid crystal formation of a self-assembled lipid nanotube (LNT) was investigated in a variety of aqueous solutions. As the lipid component, we chose a bipolar lipid with glucose and tetraglycine headgroups, which self-assembled into an LNT with a small outer diameter of 16 to 17 nm and a high axial ratio of more than 310. The LNT gave a stable colloidal dispersion in its dilute solutions and showed spontaneous liquid crystal (LC) alignment at relatively low concentrations and in a pH region including neutral pH. The LNT samples with shorter length distributions were prepared by sonication, and the relationship between the LNT axial ratio and the minimum LC formation concentration was examined. The robustness of the LNT made the liquid crystal stable in mixed solvents of water/ethanol, water/acetone, and water/tetrahydrofuran (1:1 by volume) and at a temperature of up to 90 °C in water. The observed colloidal behavior of the LNT was compared to those of similar 1D nanostructures such as a phospholipid tubule.
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INTRODUCTION In nanoscience and nanotechnology, one-dimensional (1D) nanostructures have attracted great interest because of their distinguished chemical, optical, electrical, mechanical, and biological properties.1−3 Toward these applications, various methods for 1D nanostructure alignment have been developed.4,5 Recently, intensive studies have been done on the alignment methodologies of lipid nanotubes (LNTs), which show defined morphology and the capability of encapsulating various functional materials in the inner nanospaces.6−10 The fabrication of the LNT ordered arrays 9 has potential applications such as chemical sensors, biochips, nanofluidic systems, electronic devices, and anisotropic or micropatterned surface modification for cell growth and adsorption. For a potential approach it is noteworthy that high-aspectratio 1D nanostructures can themselves exhibit spontaneous alignment if the excluded volume effect acts.11 The formation of nematic liquid crystals (LCs) in aqueous media has been observed for rod-shaped viruses,12−15 cellulose fibers,16,17 peptide nanofibers,18 a phospholipid tubule,19 and bile salt nanotubes.20,21 A phospholipid tubule19 (a nanotube with a relatively large diameter) of approximately 30 μm in length and 0.5 μm in outer diameter aligns one another at 10 mg/mL below pH 3.5.22 Only in strongly acidic media does the phosphocholine headgroup become cationic, and thus electrostatic repulsion effectively overcomes the strong van der Waals attraction between the thick tubules. The lithocholate salt in aqueous NaOH20 and ammonia21 forms self-assembled nanotubes with monodisperse sections. The bile salt nanotubes © 2014 American Chemical Society
showed a well-aligned orientation above 0.8 wt % in 0.18 M NaOH (as mentioned in ref 21) and at 2.5 wt % in 7.9 wt % ammonia (as evidenced by SAXS results), but the nanotubes were not stable in neutral media. In the present work, we propose an approach to the spontaneous alignment of LNT dispersions in neutral media. Previously we demonstrated that bipolar lipids with glucose and oligoglycine headgroups self-assemble into LNTs with a small outer diameter (15−20 nm), a high aspect ratio (AR), and high morphological robustness at high temperatures.23,24 We expect that this type of LNT can improve its colloidal dispersibility in aqueous media, LC formation at low concentrations, and LC phase stability under a wide range of conditions. For these purposes, we select a bipolar lipid with glucose and tetraglycine headgroups (Figure 1A) as the lipid component. Owing to the multiple intermolecular hydrogen bonds between tetraglycine groups, the LNT from this particular lipid shows sufficient morphological robustness in mixed solvents of water and some organic solvents. Employing the LNT, we examine its dispersibility and LC formation under various conditions of concentration, temperature, and solvents. Briefly, we also discuss these properties in comparison to those of rodlike nanostructures such as the phospholipid tubule.22 Received: October 30, 2014 Revised: December 19, 2014 Published: December 29, 2014 1150
DOI: 10.1021/la5042772 Langmuir 2015, 31, 1150−1154
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kV, 30 mA), and a flat camera with an imaging plate (Rigaku R-Axis IV). The powder LNT sample was flame-sealed in a quartz capillary (1.5 mm outer diameter, 10 μm wall thickness) and measured at room temperature and 170 °C using a Mettler Toledo FP82 hotstage. The exposure time was 30 min with a 150 mm sample-to-camera length. Zeta Potential Measurement. The zeta potentials of LNTs at different pH values were measured on a Malvern Zetasizer Nano ZS system.
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RESULTS AND DISCUSSION Thermal Stability of the LNT. STEM observation (Figure 1B) evidenced that the LNT in water possesses a homogeneous nanotube morphology (outer diameter ∼16 nm, length > 5 μm, AR > 310). It was also noted that the LNT was randomly deposited on the carbon grid at a sampling concentration of 0.05 mg/mL. The thermal stability of the LNT was investigated by DSC, XRD (Figure 2), and STEM (Figure S1 in Supporting Figure 1. (A) Bipolar lipid used for the construction of a highly stable LNT. (B) STEM image of the dried LNT from a dilute dispersion (0.05 mg/mL) on a carbon grid. Hollow cylinders of the nanotubes with a random orientation were observed.
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EXPERIMENTAL SECTION
Construction of LNT. A bipolar lipid with glucose and tetraglycine headgroups linked by an eicosanedioic acid chain was synthesized as reported previously (Figure 1A).23 The lipid was self-assembled in dimethyl sulfoxide by a heating−cooling procedure to form a nanotube gel. After 5× dilution with deionized water, the LNTs were isolated by filtration (Omnipore membrane filters, 100 nm JG) and redispersed in deionized water to give an LNT dispersion (10 mg/mL, pH 6.3). The length of the as-prepared LNT was estimated to exceed 5 μm [aspect ratio (AR) greater than 310] as shown by the scanning transmission electron microscopy (STEM) observation (Hitachi S-4800). Liquid Crystal Observation. One milliliter of the LNT dispersion (10 mg/mL) was put into quartz cuvettes (width, 1 cm; depth, 1 cm) and diluted to 5, 3, 2, and 1 mg/mL for photography with crossed polarizers or without crossed polarizers. The formation of the LC domain at a concentration of 5 mg/mL was also confirmed by polarized microscopy (via an Olympus BX51 optical microscope). To shorten the LNT lengths physically, 2 mL of the LNT dispersions was treated with a probe-type sonicater (a Branson 250DA sonifier with a 1/8 in. Microtip, 20 W) for 5, 10, 20, and 30 s. The samples in cuvettes were observed under crossed polarizers. Deionized water was gradually added to dilute these shortened LNT dispersions until the LC phenomenon disappeared. For each LNT sample, the concentration before the LC disappearance was taken as the minimum LC concentration. For the length distribution plots (length vs number for the samples), the lengths of approximately 100 LNTs were measured in the STEM images. The LC formation behavior at different pH values and in different solvents was also observed under crossed polarizers. STEM Observation. A 2 μL drop of the diluted LNT dispersion (0.05 or 2 mg/mL) was dried on a carbon grid. The grid was then negatively stained with 2% phosphotungstate and dried in vacuum before STEM observation at room temperature. To check the thermal stability of the LNT, we held the LNT-deposited grid on a hot plate at 170 °C for 10 min and then negatively stained it with 2% phosphotungstate at room temperature. Differential Scanning Calorimetry (DSC) Measurement. Twenty microliters of a concentrated LNT dispersion (about 20 mg/mL) or 5 mg of a dry LNT sample was hermetically sealed into a silver capsule. DSC curves were recorded at a heating rate of 2 °C/min on an EXSTAR DSC 6100 calorimeter (Seiko Instruments Inc.). Powder X-ray Diffraction (XRD). The XRD patterns of the freeze-dried LNT were measured with a Rigaku type 4037 diffractometer using graded d-space elliptical side-by-side multilayer optics, monochromated Cu Kα radiation (wavelength 0.1542 nm, 40
Figure 2. DSC curves of (A) the LNT in a water dispersion and (B) the dry LNT. (C) XRD patterns of the dry LNT at room temperature (RT) and 170 °C.
Information). The heating thermogram of the LNT dispersion showed neither an endothermic nor exothermic peak below 100 °C in water (Figure 2A), proving that the LNT in aqueous dispersions was stable at least up to 100 °C for a short period. After the DSC run we confirmed no weight loss of the sample in the sealed DSC capsule. In the DSC heating curve of dry LNT (Figure 2B), the sharp endothermic peak at 220 °C is ascribed to the decomposition of the lipid because the inside of the silver capsule turned black after the heating run (data not shown). A small, broad exothermic peak was observed at around 150−160 °C. The DSC peak appears to be associated with the dissociation of bound water from the LNT because the morphology of LNT was maintained when the LNT-deposited TEM grid was heated to 170 °C (Figure S1 in SI). Therefore, the LNT is proven to retain the nanotube morphology up to 100 °C in water and 170 °C in the dry state. The XRD data also supported the fact that the dry LNT was stable up to 170 °C 1151
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Langmuir because the d-spacing value at 170 °C (Figure 2C) was practically the same as that at room temperature. By comparing the thermal stability of a glycolipid-based LNT,25 the observed stability of the present LNT at these temperatures is attributable to a strong polyglycine II-type hydrogen bond network23,24 and the hydrophobic interaction of the long alkyl chain as illustrated in Figure 1A. Liquid Crystal Phase Formation. The LNT showed good colloidal stability and concentration-dependent LC formation in water (Figure 3A,B). The dispersion behavior of the LNT in
mL showed substantially aligned LNTs in the observed image (Figure 3D) and over a relative large area (Figure S2 in SI). Although the observations were made in dry states, it is likely that the LNTs dispersed at high concentrations can take a sideby-side alignment in the ribbon-shaped domains. Length-Dependent Minimum LC Concentration. The as-prepared LNT (aspect ratio AR > 310) showed a minimum LC formation concentration of 2 mg/mL, which is 5-fold and 11-fold lower than those of phospholipid tubules (10 mg/mL, AR > 60)22 and the tobacco mosaic virus (TMV, 23 mg/mL, AR ≈ 17).13 For these particular rod-shaped colloids, the minimum LC formation concentration is roughly inversely proportional to the AR values. The AR value primarily determines the minimum LC formation concentration, instead of different diameters, stiffnesses, and surfaces of these 1D nanostructures. We further examined the relationship between AR values and the minimum LC formation concentration by controlling the length of LNT (Figure 4). The as-prepared LNT showed a
Figure 3. Concentration-dependent LC formation of the LNT. Photographs of the LNT dispersions at a series of concentrations (A) without crossed polarizers and (B) with crossed polarizers. (C) Polarized microscopy image of an aqueous LNT dispersion (5 mg/ mL) on a glass slide. (D) STEM image of the dried LNT from a 2 mg/ mL dispersion on a carbon grid. The inset magnification shows the hollow cylinder of the nanotubes.
water was examined at 10, 5, 3, 2, and 1 mg/mL. At 1 mg/mL, the as-prepared LNT gave a clear isotropic dispersion, and the colloidal dispersion was stable for more than 1 year. It may be because of relatively weak van der Waals attraction between the present LNTs with its small diameter (∼16 nm). At 2 mg/mL, optical anisotropy was observed under crossed polarizers. A further increase in the concentration to 3, 5, and 10 mg/mL showed more noticeable birefringence and imposed interesting colors on the LNT dispersions. We concluded that the asprepared LNT can exhibit a lyotropic LC phase above 2 mg/ mL. In relation to efficient LC formation, we noted that the present LC dispersion shows obvious ribbon-shaped domains at 5 mg/mL via polarizing microscopy (Figure 3C). The domains were relatively long (>130 μm) and wide (>10 μm). It has been reported that some rod- or needle-shaped inorganic colloid dispersions exhibit spindle-shaped nematic droplets or domains (tactoids) where the objects are directed toward the spindle apexes on average.26 In the present LNT-LC system, the observed ribbon-shaped domains are supposed to have a tactoid-like orientation order and be elongated in the longitudinal direction because of the high LNT aspect ratio of >310. As shown in Figure 1B, the STEM observation by sampling at 0.05 mg/mL gave an image of randomly orientated LNTs in its dry state. In contrast, the sampling concentration of 2 mg/
Figure 4. (A) Length distributions of sonicated LNTs (sonication times of 5, 10, 20, and 30 s). The lengths were measured from the STEM images. (B) Relationship between the AR and minimum LC concentration. The AR values were calculated from the peak lengths in the distributions and the LNT diameter (∼16 nm).
length of >5 μm. Sonication effectively shortened the lengths of the LNT but did not affect its outer diameter (Figure S3 in SI). The length distribution plots (Figure 4A) indicated that the peak lengths in the distributions are 2.6, 1.4, 1.0, and 0.6 μm after 5, 10, 20, and 30 s of sonication, respectively. For each LNT sample, the LC formation was tested at different concentrations. In Figure 4B the minimum LC formation concentrations are plotted against 1/AR values, where the AR values were calculated from the peak lengths and a diameter of ∼16 nm. With the higher AR, the LC phase appears above the lower concentration (the minimum LC formation concentration becomes the lower concentration). The plot shows that the minimum LC formation concentration is roughly inversely 1152
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electrostatic repulsion between the LNTs as well as the relatively weak van der Waals attraction. At pH 6.3 and 9.4, the zeta potential showed distributions with mean values of −50 and −45 mV. The highly negative zeta potential is reasonable because the water/glucose interface can effectively adsorb hydroxyl ions (OH−) at pH 6.5 to 10.29 At pH 3.3 and 10.6 the distributions shifted in the positive potential direction (−25 and −39 mV), but the estimated mean values were not sufficiently reliable for further discussion because the samples contained a substantial aggregation of the LNT. We further observed that the present LNT can maintain the nematic phase in hot water and in several mixed solvents (Figure 6). In Onsager theory,11 the nematic LC exhibited by a
proportional to the AR values; this observation in the present system is in moderate agreement with the theory of nematic phase formation in rod-shaped colloid systems.11 It was also found that the shortened LNT (after 10 s of sonication) showed relatively smaller LC domains in its optical microscopy observation (Figure S4 in SI), in comparison to the large ribbon-shaped domains observed for the long LNT (Figure 3C). In these experiments on the concentration dependence, we assume that the monomolecularly dispersed lipid does not practically affect the LNT length because the monomolecular lipid concentration in coexistence with the self-assembled LNT can be negligibly low in comparison to the experimental concentrations (2 to 10 mg/mL, about 10−3 to 10−2 M). We here estimate the monomolecular lipid concentration by extrapolation. The CMCs of n-alkyl-β-D-glucosides in water at 25 °C are 2.5 × 10−2 M for the C8-chained glucoside, 2.2 × 10−3 M for the C10-chained glucoside, and 1.9 × 10−4 M for the C12-chained glucoside.27 An increase by two methylene units in an alkyl chain reduces the CMC by about 1/10.27,28 The hydrophobic attraction by the long eicosanedioic acid chain can reduce the concentration to the order of 10−7 M. Inside the solid self-assembled LNT (Figure 1A), the five amide groups afford multiple interlipid hydrogen bonding rather than act as hydrophilic groups. The strong attraction can further lower the concentration (by several orders of magnitude). It is acceptable that the monomolecular lipid concentration in water is very low in comparison to the experimental concentrations. LC Stability under Different Conditions of pH, Temperature, and Organic Solvents. The dispersion stability of the LNT and the LNT-LC phase stability are critical to the processes for aligning LNTs. Here we evaluated the LC formation in a water dispersion with different pH values, in hot water, and in mixed solvents. The pH of the asprepared LNT dispersion (pH 6.3) was adjusted to acidic or alkaline pH with dilute hydrochloric acid or sodium hydroxide solution. It was noted that the LNT-LC is stable in a pH region including neutral conditions (Figure 5). The nematic phase
Figure 6. LNT-LC in hot water and mixed solvents (observation under crossed polarizers). The LNT (5 mg/mL) retained the optically anisotropic phase in (A) hot water (90 °C), (B) water/ethanol = 1/1 (v/v), (C) water/acetone = 1/1 (v/v), and (D) water/tetrahydrofuran = 1/1 (v/v).
concentrated rod-shaped object is insensitive to temperature (“athermal”). Actually the thermal stability of the LC phase is dependent on the thermal or material stability of the individual rod-shaped object. For example, the phospholipid tubule has a melting temperature of approximately 43 °C,30 and some bacteriophage LC systems are stable only below 60 °C.14,15 In the present LNT system in water, the LC phase was stable at least up to 90 °C (Figure 6A). The observed wider temperature range of the LNT-LC phase is partially attributable to the thermal morphological robustness of the solid self-assembled LNT. Figure 6B−D demonstrated that in mixed solvents of water/ethanol, water/acetone, and water/tetrahydrofuran the LNT dispersions at 5 mg/mL showed birefringency. The STEM observations also indicated that the nanotube morphology is stable in these mixed solvents (Figure S5 in SI). The remarkable robustness and dispersion stability observed in the present LNT system may offer a wide range of potential applications of the LNT-LCs for various processes and purposes. As demonstrated in Figure S6 in the SI, the LNT dispersed in water/ethanol might be aligned simply in an evaporation process such as the “tears of wine” phenomenon (a kind of Marangoni effect).
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CONCLUSIONS The present self-assembled LNT spontaneously aligns side by side in aqueous solutions to form LC phases above relatively low concentrations. Its small outer diameter, high aspect ratio, and strong electrostatic repulsion contribute to the good dispersibility and the LC formation at low concentrations in aqueous media including neutral ones. The remarkable robustness of the LNT morphology made the LC stable over the pH range including neutral pH, in several mixture solvents and over a wide temperature range. For potential applications of the LNT array fabrication in nanotechnology,9 the present system may be useful because the developed method is easy and applicable under various conditions. These LNTs can be modified on their outer surfaces and can encapsulate functional materials such as
Figure 5. pH-dependent LC formation of long LNT at 5 mg/mL. Each zeta potential of the LNT was measured by light scattering. Substantial aggregation of the LNT was observed at pH 3.3 and 10.6.
remained at pH 6.3 and 9.4, and the birefringency disappeared and the LNT aggregated at pH 3.3 and 10.6. In the pH range of 6.3 to 9.4, the present LNT system retains the dispersibility and LC formation. This is in marked contrast to the case where the phospholipid tubule LC is stable under limited medium conditions such as below pH 3.5.22 The observed dispersibility of the present LNT at pH 6.3 to 9.4 is attributable to effective 1153
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(12) Bawden, F. C.; Pirie, N. W.; Bernal, J. D.; Fankuchen, I. Liquid Crystalline Substances from Virus-Infected Plants. Nature 1936, 138, 1051−1052. (13) Fraden, S.; Maret, G.; Caspar, D. L. Angular Correlations and the Isotropic-Nematic Phase Transition in Suspensions of Tobacco Mosaic Virus. Phys. Rev. E 1993, 48, 2816−2837. (14) Barrientos, L. G.; Louis, J. M.; Gronenborn, A. M. Characterization of the Cholesteric Phase of Filamentous Bacteriophage fd for Molecular Alignment. J. Magn. Reson. 2001, 149, 154−158. (15) Zweckstetter, M.; Bax, A. Characterization of Molecular Alignment in Aqueous Suspensions of Pf1 Bacteriophage. J. Biomol. NMR 2001, 20, 365−377. (16) Dong, X. M.; Revol, J. F.; Gray, D. G. Effect of Microcrystallite Preparation Conditions on the Formation of Colloid Crystals of Cellulose. Cellulose 1998, 5, 19−32. (17) Araki, J.; Wada, M.; Kuga, S.; Okano, T. Birefringent Glassy Phase of a Cellulose Microcrystal Suspension. Langmuir 2000, 16, 2413−2415. (18) Pomerantz, W. C.; Yuwono, V. M.; Pizzey, C. L.; Hartgerink, J. D.; Abbott, N. L.; Gellman, S. H. Nanofibers and Lyotropic Liquid Crystals from a Class of Self-Assembling Beta-Peptides. Angew. Chem., Int. Ed. 2008, 47, 1241−1244. (19) Lu, M. H.; Rosenblatt, C. Observation of a Nematic Phase in an Aqueous Suspension of Phospholipid Tubules. Mol. Cryst. Liq. Cryst. 1992, 210, 169−177. (20) Yager, P.; Schoen, P. E. Formation of Tubules by a Polymerizable Surfactant. Mol. Cryst. Liq. Cryst. 1984, 106, 371−381. (21) Jean, B.; Oss-Ronen, L.; Terech, P.; Talmon, Y. Monodisperse Bile-Salt Nanotubes in Water: Kinetics of Formation. Adv. Mater. 2005, 17, 728−731. (22) Terech, P.; Jean, B.; Ne, F. Hexagonally Ordered Ammonium Lithocholate Self-Assembled Nanotubes with Highly Monodisperse Sections. Adv. Mater. 2006, 18, 1571−1574. (23) Ding, W.; Kameta, N.; Minamikawa, H.; Wada, M.; Shimizu, T.; Masuda, M. Hybrid Organic Nanotubes with Dual Functionalities Localized on Cylindrical Nanochannels Control the Release of Doxorubicin. Adv. Healthcare Mater. 2012, 1, 699−706. (24) Kameta, N.; Yoshida, K.; Masuda, M.; Shimizu, T. Supramolecular Nanotube Hydrogels: Remarkable Resistance Effect of Confined Proteins to Denaturants. Chem. Mater. 2009, 21, 5892− 5898. (25) John, G.; Minamikawa, H.; Masuda, M.; Shimizu, T. Liquid Crystalline Cardanyl β-D-Glucopyranosides. Liq. Cryst. 2003, 30, 747− 749. (26) Sonin, A. S. Inorganic Lyotropic Liquid Crystals. J. Mater. Chem. 1998, 8, 2557−2574. (27) Shinoda, K.; Yamaguchi, T.; Hori, R. The Surface Tension and the Critical Micelle Concentration in Aqueous Solution of β-D-Alkyl Glucosides and their Mixtures. Bull. Chem. Soc. Jpn. 1961, 34, 237− 241. (28) Rosen, M. J. Surfactants and Interfacial Phenomena, 3rd ed.; John Wiley & Sons: Hoboken, NJ, 2004. (29) Baba, T.; Zheng, L. Q.; Minamikawa, H.; Hato, M. Interglycolipid Membrane Interactions: pH-Dependent Aggregation of Nonionic Synthetic Glycolipid Vesicles. J. Colloid Interface Sci. 2000, 223, 235−243. (30) Yavlovich, A.; Singh, A.; Tarasov, S.; Capala, J.; Blumenthal, R.; Puri, A. Design of Liposomes Containing Photopolymerizable Phospholipids for Triggered Release of Contents. J. Therm. Anal. Calorim. 2009, 98, 97−104.
organic materials (dyes, low-molecular-weight liquid crystals, medicines, etc.), biomolecules (proteins and DNAs), and inorganic nanoparticles into the hollow-interior LNTs.7,9 Therefore, the materials appended in the LNTs appear to be arrayed and positioned in some nanodevices such as chemical sensors, biochips, and nanofluidic systems. Additionally the method may be used to prepare anisotropic surfaces for the control of cell adsorption and growth.
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ASSOCIATED CONTENT
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AUTHOR INFORMATION
S Supporting Information *
STEM images of LNT treated at 170 °C. Low-magnification STEM image of aligned LNT. STEM images of shortened LNT. Polarized microscopy of short LNT dispersions. STEM images of LNT in mixed solvents. Spontaneous alignment of LNT by the evaporation of ethanol. This material is available free of charge via the Internet at http://pubs.acs.org. Corresponding Author
*Tel: +81-29-861-9386. Fax: +81-29-861-4545. E-mail:
[email protected]. Author Contributions
The manuscript was written through the contributions of all authors. All authors have given approval to the final version of the manuscript. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS We are grateful to Dr. Takahito Inoue at AIST for helpful discussions. REFERENCES
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