Article pubs.acs.org/Langmuir
Adsorption of Lipid Liquid Crystalline Nanoparticles: Effects of Particle Composition, Internal Structure, and Phase Behavior Debby P. Chang,*,† Marija Jankunec,‡ Justas Barauskas,‡,§ Fredrik Tiberg,†,∥ and Tommy Nylander† †
Physical Chemistry, Lund University, P.O. Box 124, SE-22100 Lund, Sweden Vilnius University Institute of Biochemistry, Mokslininkų 12, LT-08662 Vilnius, Lithuania § Department of Biomedical Science, Faculty of Health and Society, Malmö University, SE-20506 Malmö, Sweden ∥ Camurus AB, Ideon Science Park, Gamma Building, Sölvegatan 41, SE-22370 Lund, Sweden ‡
S Supporting Information *
ABSTRACT: Controlling the interfacial behavior and properties of lipid liquid crystalline nanoparticles (LCNPs) at surfaces is essential for their application for preparing functional surface coatings as well as understanding some aspects of their properties as drug delivery vehicles. Here we have studied a LCNP system formed by mixing soy phosphatidylcholine (SPC), forming liquid crystalline lamellar structures in excess water, and glycerol dioleate (GDO), forming reversed structures, dispersed into nanoparticle with the surfactant polysorbate 80 (P80) as stabilizer. LCNP particle properties were controlled by using different ratios of the lipid building blocks as well as different concentrations of the surfactant P80. The LCNP size, internal structure, morphology, and charge were characterized by dynamic light scattering (DLS), synchrotron smallange X-ray scattering (SAXS), cryo-transmission electron microscopy (cryo-TEM), and zeta potential measurements, respectively. With increasing SPC to GDO ratio in the interval from 35:65 to 60:40, the bulk lipid phase structure goes from reversed cubic micellar phase with Fd3m space group to reversed hexagonal phase. Adding P80 results in a successive shift toward more disorganized lamellar type of structures. This is also seen from cryo-TEM images for the LCNPs, where higher P80 ratios results in more extended lamellar layers surrounding the inner, more dense, lipid-rich particle core with nonlamellar structure. When put in contact with a solid silica surface, the LCNPs adsorb to form multilayer structures with a surface excess and thickness values that increase strongly with the content of P80 and decreases with increasing SPC:GDO ratio. This is reflected in both the adsorption rate and steady-state values, indicating that the driving force for adsorption is largely governed by attractive interactions between poly(ethylene oxide) (PEO) units of the P80 stabilizer and the silica surface. On cationic surface, i.e., silica modified with 3-aminopropltriethoxysilane (APTES), the slightly negatively charged LCNPs give rise to a very significant adsorption, which is relatively independent of LCNP composition. Finally, the dynamic thickness measurements indicate that direct adsorption of intact particles occurred on the cationic surface, while a slow buildup of the layer thickness with time is seen for the weakly interacting systems.
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INTRODUCTION
such applications, it is important to better understand the interrelation between chemical and structural properties of LCNPs, on the one hand, and their dynamic and steady-state behavior at interfaces, on the other. Moreover, for biological applications it is critical to develop new lipid formulations that are biocompatible and nontoxic at relevant physiological exposures, in particular for applications as carriers and delivery systems of nutritionals, cosmetics, and pharmaceuticals, by oral, topical, or parenteral routes of administration. There is definitely a lack of systematic studies on how the individual components of LCNPs affect the adsorption and interfacial structure formation. The aim of the present work is to study the effect of lipid composition as well as the content of
Lipid liquid crystalline nanoparticle (LCNP) has potential as delivery vehicles for bioactive components, such as drugs, pesticides, proteins, and peptides, because of their space dividing structure and morphology and the resulting ability to encapsulate both lipophilic and hydrophobic components. So far, most of the focus on the studies of such vehicles has been on their structure and morphology and their internal structure with respect to their capacity to solubilize and release the bioactive component.1−5 However, many of these applications are linked to the adsorption characteristics of the LCNPs at interfaces, such as cell membranes, plant leaf surfaces, catheter, or storage vial surfaces, which so far has been subject to few fundamental studies.6 Furthermore, LCNP has through their chemical and controllable structural features great potential for modifying interfacial properties of surfaces and thereby imposing desired surface functionalities.7 In order to realize © 2012 American Chemical Society
Received: April 18, 2012 Revised: June 21, 2012 Published: June 22, 2012 10688
dx.doi.org/10.1021/la301579g | Langmuir 2012, 28, 10688−10696
Langmuir
Article
and charge with light scattering and zeta potential measurements, respectively. These results were then related to the amount adsorption determined by ellipsometry and the lateral organization by confocal microscopy.
stabilizer on the interfacial behavior. The system we used in the present study is comprised of a phospholipid, soy phosphatidylcholine (SPC), a diglyceride, glycerol dioleate (GDO), coformulated with the surface active stabilizer polyoxyethylene sorbitan monooleate (Polysorbate 80, P80) as colloidal stabilizer. This system has been shown to have excellent biocompatibility with a minimum of hemolytic activity.8 Furthermore, it is readily dispersed into stable nanoparticles by mixing and without the requirement of high-pressure homogenization.9 Recently, we studied for the first time the adsorption properties of this system at hydrophobic, hydrophilic, and charged surfaces,6 showing the impact of surface chemistry on the interfacial structure formation and associated adsorption kinetics of LCNPs. In this paper, we built upon the previous study and for the first time systematically examine the effect of particle composition on the internal structure and subsequent adsorption property. For this purpose we have varied the ratio of the two lipids, one forming lamellar type of liquid crystalline self-assembly structure and the other reversed structures, as well as the amount of stabilizer used. The ratios of the two types of lipids allow tuning the liquid crystalline structure of the LCNPs. A crucial factor when formulating drug delivery vehicles, apart from their capacity to encapsulate and deliver the drug, is their stability. This means that the vehicles should be in a stable dispersion and the loss of material in terms of adsorption to vials, catheters, tubes and other delivery devices should be minimal. Unwanted consequences can occur if the LCNP is disintegrated by contact with an interface, for instance with hydrophobic surfaces.6,10 On the other hand, a maximum possible adsorption may be desirable, such as when the LCNP is used for enhancing drug delivery. It is clear that the ability to control the LCNP interaction with the surface to achieve selective deposition only on surfaces with certain properties is both important and desired. This requires a delicate balance in the formulation of the particles and fundamental understanding of the colloidal forces involved. The main components of the LCNP are the lipids that assemble into the nanostructure and the stabilizer that stabilizes the dispersion. The lipid system for nonlamellar LCNPs are chosen so that they can form the desired stable reversed phases (cubic, hexagonal, or sponge) in equilibrium with excess of water. To disperse the liquid crystalline phases into welldefined particulate form, it is essential to locate a suitable dispersion stabilizer for the particular lipid system. A dispersion stabilizer is needed to reduce the exposure of the hydrophobic domains to aqueous medium, i.e., reduce the interfacial tension, and to prevent aggregation of the dispersed particles, which apart from forming large aggregates can also lead to disruption of the inner liquid crystalline structure. An ideal stabilizer should not interfere with the crystalline phases and at the same time provide stability to particle formation. To satisfy these requirements, a stabilizer is typically amphiphilic with a hydrophobic region that is partly miscible with the particular lipid and has hydrophilic unit(s) at the surface of the particle to provide steric repulsion against aggregation. Previous studies by others and us have suggested that the lipid phase and its interaction with the stabilizer as well as the surface interaction of the stabilizer dictate the adsorption properties of LCNP.10−14 This hypothesis will be investigated by varying the lipid composition as well as the stabilizer concentration. We monitor the effect on of the internal structure by means of synchrotron X-ray diffraction and cryo-TEM and particle size
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MATERIALS AND METHODS
Materials. Soy phosphatidylcholine (SPC) was purchased from Lipoid GmbH (Ludwigshafen, Germany) containing as major components phosphatidylcholine (>94%), lysophosphatidylcholine (
