Article pubs.acs.org/Langmuir
Sensitivity of Nanostructure in Charged Cubosomes to Phase Changes Triggered by Ionic Species in Solution Qingtao Liu,† Yao-Da Dong,† Tracey L. Hanley,‡ and Ben J. Boyd*,† †
Drug Delivery, Disposition and Dynamics - Monash Institute of Pharmaceutical Sciences, Monash University (Parkville Campus), 381 Royal Parade, Parkville, Victoria 3052, Australia ‡ Australian Nuclear Science and Technology Organization, Menai, New South Wales 2234, Australia S Supporting Information *
ABSTRACT: The phase behavior of dispersions comprising mixed ionic surfactant and phytantriol was precisely controlled by varying the ionic surfactant content in the mixed lipid and the ionic strength in the system. Two important trends in the phase transition of the mixed lipid systems were identified: (1) An increase in the ionic surfactant content increased the curvature of the self-assembled system toward the hydrophobic region, resulting in the phase transition from cubic phase to lamellar phase. (2) An increase in ionic strength decreased repulsion between the headgroups of the ionic surfactant, resulting in a phase transition from lamellar phase to cubic phase. The phase transitions were confirmed using small-angle X-ray scattering and cryo-TEM and were strongly correlated with the visual turbidity of the dispersions. The lipid mixture with anionic surfactant showed high sensitivity to multivalent cations for triggering the phase transition, which may be a potential strategy to develop a detection/treatment system for toxic multivalent metallic cations such as chromium.
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INTRODUCTION Various self-assembled nanostructures of amphiphilic lipids in aqueous environments have been attracting a lot of attention due to their attractive potential applications in cosmetics,1,2 drug delivery,3,4 and diagnostics.5 In particular, lipid selfassembled liquid crystalline structures including lamellar (Lα), hexagonal (H2), and cubic (V2) phases have been of recent interest because of the potential to trigger reversible transitions between them.6,7 Intelligent liquid-crystal systems that respond to environmental changes through phase transitions have been desired because of their potential to significantly improve drug delivery efficiency or be utilized to develop novel analytical methods with low cost for environment protection.8,9 The phase transition from slow release (e.g., lamellar phase) to relatively faster release (e.g., cubic phase) is particularly important in the drug delivery field.4,10,11 The inverse bicontinuous cubic phase has most often been formed using phytantriol (PHYT) or glyceride-based lipids, such as glyceryl monooleate (GMO) and monolinolein (MLO) in excess water.12,13 Dispersions of cubic phase, so-called cubosomes, retain the internal order of the cubic phase while being a more readily handled form. PHYT has been increasingly used to prepare cubic phase and cubosomes due to its chemical stability.14 The self-assembled nanostructure of PHYT in water can be controlled by addition of lipidic additives into PHYT. For example, Dong et al. mixed vitamin E acetate (VitEA) with PHYT to form hexagonal phase in water at room temperature.14 An extension of this concept therefore is stimuli © 2013 American Chemical Society
responsive matrices that undergo transitions through environmental changes, not just composition. For example, lightsensitive lipid self-assembled systems that can change their phase structure under irradiation of light including UV and NIR have been reported.6 Bulk cubic phase matrices have also been responsive to pH for drug delivery applications.7 Yamazaki et al. developed a pH-sensitive dispersion system in which the multilamellar vesicles of dioleoylphosphatidylserine (DOPS)/ monoolein (MO) mixture transitions to the inverse bicontinuous double-diamond cubic phase at low pH.8 Recently, Yamazaki et al. also used Ca2+ to induce the same transitions in monoolein (MO) and dioleoylphosphatidylglycerol (DOPG) mixtures.15−18 Muir et al. developed a salt-sensitive lipiddispersion system containing PHYT and didodecyldimethylammonium bromide (DDAB) that undergoes a phase transition from lamellar (Lα) phase to the cubic phase with increasing salt concentration.19 The focus of the work by Muir et al. was on utilizing the transition as a low-energy method of manufacture of cubosomes. However, the possible application of the transition in sensing and detoxification of ionic species in solution was not discussed. To this end we have sought to extend this concept to develop ion-selective PHYT-based liquid-crystal systems, based on the sensitivity of ionic surfactants to ionic strength and specific ions in solution, to induce transitions between Received: June 27, 2013 Revised: September 10, 2013 Published: October 10, 2013 14265
dx.doi.org/10.1021/la402426y | Langmuir 2013, 29, 14265−14273
Langmuir liposomes and cubosomes. The anionic bis(2-ethylhexyl)sulfosuccinate (AOT), surfactant, didodecyldimethylammonium shown in Figure 1, were added to PHYT
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prior to particle size measurement for optimal measurement sensitivity, measured using automated settings in low-volume cuvettes. The internal structure of mixed lipid dispersions was investigated by small-angle X-ray scattering (SAXS) and cryo-transmission electron microscopy (cryo-TEM) at 36−48 h after sample preparation. SAXS measurements were conducted on the SAXS/WAXS beamline at the Australian Synchrotron (Clayton, VIC) to define phase identity and phase boundaries at the various composition and ionic strengths to generate phase diagrams of the ionic surfactant−PHYT mixed systems. For the phase and phase boundary identification, samples of the dispersions (200 μL) were added to the wells of a clear 96-well microplate (PerkinElmer), which was mounted vertically in the beam path. The 2D SAXS patterns were collected using a Pilatus 1 M (170 mm × 170 mm) detector which was located 650 mm from the sample position, with a 2 s exposure and X-ray wavelength of 1.0322 Å. For cryo-TEM imaging, a drop of lipid dispersion was placed on a TEM grid (ProSciTech, Qld, Australia), blotted with filter paper, and then the grid with sample was rapidly plunged into liquid ethane. The sample was transferred to a TEM (FEI Tecnai 30) to operate at 300 keV while maintaining the sample at −170 °C.
surfactant, sodium and the cationic bromide (DDAB) to control the self-
Figure 1. Chemical structures of phytantriol and ionic surfactants (AOT and DDAB).
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assembled nanostructure formed by PHYT in water. Both surfactants were hypothesized to modify the lipid packing to favor formation of the lamellar phase. The impact of ionic strength on charge screening and transit into the cubic phase was investigated using small-angle X-ray scattering (SAXS), and relative responsiveness to specific ions was evaluated.
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RESULTS AND DISCUSSION The lipid dispersions with low content of DDAB or AOT in water appeared as milky emulsions whereas with high DDAB or AOT content the dispersion was colorless, transparent, and viscous. In addition, the dispersion of lipid with high DDAB content was gel-like. There was a steady increase in zeta potential for the dispersions as the content of DDAB or AOT was increased, positively or negatively in magnitude, respectively, indicating clear insertion in the lipid regions and even distribution in the dispersions (see Supporting Information Figure S1). 1. Effect of Composition of Lipid and Aqueous Phase on Properties of Ionic Surfactant and PHYT Mixed Dispersions. Phytantriol (PHYT) specifically forms cubosomes with the internal “double-diamond”-type Pn3m cubic (V2) phase due to its specific amphiphilic property when stabilized using F127 in water. Addition of either DDAB or AOT ionic surfactants to PHYT changed the structure of the PHYT dispersion in water. As shown in Figure 2, when the content of ionic surfactant was below 5% w/w of the total lipid, the mixed lipid dispersions appeared as milky emulsions when dispersed in water, similar to cubosome dispersions of PHYT. On addition of greater proportion of ionic surfactants, the mixed lipid dispersions in water transformed from milky to transparent. Coincident with change in visual appearance, the
EXPERIMENTAL METHODS
Materials. Phytantriol (98%) was a gift from DSM Nutritional Products. Sodium bis(2-ethylhexyl)sulfosuccinate (AOT) was purchased from Merck. Pluronic F127 (poly(ethylene oxide)−poly(propylene oxide)−poly(ethylene oxide)) and didodecyldimethylammonium bromide (DDAB) were purchased from Sigma-Aldrich, Sydney, Australia. Inorganic salts including sodium chloride, potassium chloride, disodium hydrogen phosphate, potassium dihydrogen phosphate, calcium chloride, aluminum chloride, and chromium chloride were purchased from Sigma-Aldrich. Preparation of Mixed Lipid Dispersions. Unless stated otherwise, 300 mg of PHYT and ionic surfactant (DDAB or AOT) were weighed into a 5 mL glass test tube. Milli-Q water (3 mL) containing F127 (1% w/w) was added immediately prior to dispersion by ultrasonication (Covaris S220X, Covaris Pty. Ltd. Woburn, MA) for 60 cycles (90 s for every cycle) at average power of 41 mW. Dispersion Characterization. Particle size and polydispersity were measured using a Malvern Zetasizer Nano ZS (Malvern Instruments, Malvern, UK) at 25 °C assuming a viscosity of pure water and presented as an average of three separate determinations. The dispersion samples were diluted 20-fold (v/v) with Milli-Q water
Figure 2. Optical images of ionic surfactant−PHYT dispersions with different compositions in phosphate buffer at increasing concentration in 96well plates. (a) AOT−PHYT dispersion system; (b) DDAB−PHYT dispersion system. For both the AOT and DDAB systems the concentration of the ionic surfactant was from left to right 1, 3, 5, 7, 9, 13, 17, 20, 23, 29, and 33.3% (w/w). In both systems, the concentrations of phosphate buffer were increased gradually from 0 to 1 (full strength) from top to bottom. 14266
dx.doi.org/10.1021/la402426y | Langmuir 2013, 29, 14265−14273
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Figure 3. Mean particle diameter (a) and polydispersity index (PDI) (b) of the ionic surfactant−PHYT dispersions in water and phosphate buffered saline (PBS) with increasing surfactant concentration. The gray dotted lines in (a) and (b) represent the particle diameter and PDI of cubosomes dispersions with PHYT alone, respectively. In both cases the actual surfactant content of the dispersions was 1, 3, 5, 7, 9, 13, 17, 20, 23, 29, and 33.3% (w/w) in total lipid.
Figure 4. SAXS profiles for AOT−PHYT dispersions with increasing AOT content in water (a), DDAB−PHYT dispersions with increasing DDAB content in water (b), and changes in lattice constant a with varying AOT (c) and DDAB (d) content. In panels a and b, the compositions from bottom to top are 1, 3, 5, 7, 9, 13, 17, 20, 23, 29, and 33.3% (w/w) in total lipid. The individual profiles in panels a and b have been offset on the intensity axis for clarity.
In contrast, the particle size of the AOT−PHYT dispersions in water did not change substantially with the increase in AOT content, and the size distribution was smaller than for cubosome dispersions with PHYT alone. The polydispersity index for the AOT−PHYT dispersion in water was also low (
