How do lipid nanocarriers – Cubosomes affect electrochemical properties of DMPC bilayers deposited on gold (111) electrodes?

Highlights

• Au(1 1 1)-supported DMPC bilayer is used as a lipid membrane model.

• Cubosomes, which are potential drug carriers, desintegrate in contact with a lipid membrane.

• Electrical properties of lipid membranes change upon contact with cubosomes.

• Lipid bilayer exposed to cubosomes becomes more porous.

• Well organized lipid layers become more blocking after exposure to cubosomes.

Abstract

Cubosome nanocarriers are promising biomimetic drug delivery systems used in particular for highly toxic drugs in cases where decreasing unwanted side effects is especially important. The properties of electrode supported lipid bilayer prepared by the combined Langmuir-Blodgett and Langmuir-Schaefer techniques were studied using electrochemical techniques following exposure of the film – covered electrode to a solution containing phytantriol - based cubosomes. The inclusion of the carrier in the model membrane under different experimental conditions was probed and the modifications induced in the lipid organization were for the first time inferred by quantitative analysis of the responses of cyclic voltammetry (CV), AC voltammetry and Electrochemical Impedance Spectroscopy (EIS) as well as blocking assays using a redox probe in the solution. Exposure of a preformed DMPC bilayer to cubosome solution resulted in the improved barrier properties of the film reflecting disintegration of cubosomes and formation of additional phytantriol/Pluronic F-108 polymer layer on the top of the DMPC bilayer. On the other hand, formation of the layer in the presence of cubosomes in the subphase lead to an increased capacitance of the film since penetration of the lipid layers by the cubosomal phytantriol increased the porosity of the film.

Introduction

Nanocarriers (NCs) are used to improve drug solubility and sustainability, decrease the toxic side effects of drugs and deliver them across biological membranes into cells, addressing the carriers to the appropriate cells [1], [2], [3]. The incorporation of an NC into the cell is controlled by several endocytosis processes that involve the enveloping and internalizing of the external NCs. Membrane fusion can be a successful strategy to perform the internalization step of an NC. In this process, after the contacting of two discrete lipid bilayers, fusion takes place, creating phospholipid domains and mixing internal components. Cubosomes have been proposed as excellent fusogenic NCs due to their unique properties [4] i.e. thermodynamic stability, bioadhesion, and the ability to encapsulate hydrophilic, hydrophobic and amphiphilic substances. The potential for controlled release of the drug through functionalization, the delivery of a large drug load, and providing sustained release are some of the advantages connected with using cubosomes as drug carriers [5].

Previous studies concerning phospholipid monolayer in the air-water interphase have demonstrated that the surface adsorption of cubosomes depends on the surface philicity as well as the internal structure of the cubosome material [6], [7], [8], [9]. However, these lipid membranes are single layers of a bilayer placed at the air –water interface. Another approach involves supported lipid bilayer (SLBs). Neutron reflectivity [10], [11], quartz crystal microbalance (QCM) [11] and ellipsometry [12] have been used to investigate changes in lipid bilayers in the presence of cubosome material. Recently, Dyett et al. have observed the total fusion of lipid cubosomes with lipid supported bilayers under flow conditions [13]. Moreover, the fusion was facilitated in the cases of charged surfaces and also depended on the composition of the lipid layer [3]. In a recent study, we used polarization modulation infrared reflection absorption spectroscopy (PM-IRRAS) to demonstrate the effect of phytantriol-based cubosomes on the structure and lipid orientation of DMPC bilayers supported on Au(111) surfaces [14]. We demonstrated that following the exposure to cubosomes the acyl chains of the DMPC molecules in the bilayer adopt a more perpendicular orientation with respect to the surface. Moreover, the cone-shape structure of both molecules, DMPC and phytantriol, inverted in the case of phytantriol, allowed us to explain the tilt angle changes and lead us to propose the organization of the lipid membrane after exposure to cubosomes.

In the present study we investigate the effect of phytantriol-based cubosomes (PT- cubosomes) on the properties of DMPC bilayers supported on Au(111) electrode using electrochemical methods. A combination of Langmuir-Blodgett and Langmuir-Schaefer techniques was employed to prepare the supported lipid bilayers using the same approaches as reported in our recent paper [14]. Three electrochemical techniques, AC voltammetry, cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS), as well as blocking assays using a redox probe were employed to evaluate the changes in the structure and electrical properties of the supported DMPC bilayers under the influence of cubosomes carriers. Exposure of a preformed bilayer to cubosome solution results in an increased resistance of the film reflecting spreading of cubosomes and formation of additional phytantriol/Pluronic F-108 layer on the top of the DMPC bilayer and improved barrier properties of the film. On the other hand, the formation of the bilayer in the presence of cubosomes in the solution results in the increased capacitance of the film reflecting penetration of the lipid layers by the phytantriol component of the cubosomes, and in result, larger porosity of the film.