Abstract
Carbon Nanotubes (CNTs) are considered alternative materials for the design of advanced drug and gene delivery vectors. However, the mechanism responsible for the cellular membrane intake of CNTs is not well understood. In the present study, we show how multi-walled carbon nanotubes (MWCNTs) owning different surface properties, interact with giant unilamellar vesicles (GUVs), a simple model system for cellular membranes. In particular, we want to address the hydrophilic/hydrophobic interactions between MWCNTs and lipid membranes and the subsequent mechanical properties changes of the systems. In order to elucidate this interaction, we made the following chemical modifications on MWCNTs: oxidized MWCNTs (ox-MWCNTs) displaying reduced hydrophobic surface character, pristine MWCNTs (p-MWCNTs), and alkyl functionalized MWCNTs (alk-MWCNTs) exhibiting enhanced hydrophobic surface properties, were put in contact with GUVs and observed by confocal microscopy. Our observations revealed that the interaction between the CNTs and GUVs depends on the type of chemical functionalization: ox-MWCNTs remain at the membrane interacting with the polar head of the phospholipids, p-MWCNTs internalize GUVs spontaneously, and alk-MWCNTs persist inside the membrane. The mechanical properties of MWCNTs@GUVs systems were measured using the electrodeformation method, which shows an increased bending stiffness (κ) of the GUVs as MWCNTs concentration increases. High concentrations of p-MWCNTs and alk-MWCNTs induced vesicle adhesion; p-MWCNTs produced a considerable reduction in the average size of the GUVs, while alk-MWCNTs form complex stable structures inside the membrane. The statistical analyses of the experimental results are compared with available computer simulations. The picture emerging from our results is that the interaction between GUVs and MWCNTs is due mainly to hydrophobicity.
Highlights
The simplification of cellular systems is an outstanding tool to explore the biophysical bases that govern the interactions of low dimensional materials with cells[9]
Illustrative contrast images of the electrodeformation of the ox-multi-walled carbon nanotubes (MWCNTs)@giant unilamellar vesicles (GUVs) are shown in Fig. 2b,c. ox-MWCNTs@GUVs diameter histogram plotted in Fig. 2d displays a minor modification in the size of ox-MWCNTs@GUVs when compare with pure GUVs (Fig. 1c)
This rearrangement of phospholipids on the surface of MWCNTs shortens the distance between the lattice structure of MWCNTs and the polar head groups of phospholipids causing the quenching of rhodamine. 3D image reconstruction, Fig. 2g,h clearly shows that ox-MWCNTs are completely covering the GUVs
Summary
The simplification of cellular systems is an outstanding tool to explore the biophysical bases that govern the interactions of low dimensional materials with cells[9]. We used giant unilamellar vesicles (GUVs) as a model for cellular membrane systems[11], interacting with multi-walled carbon nanotubes (MWCNTs) owning different surface properties at different concentrations: oxidized MWCNTs (ox-MWCNTs) carrying hydrophilic oxidized carbon atoms; pristine MWCNTs (p-MWCNTs); and alkyl functionalized MWCNTs (alk-MWCNTs) having hydrophobic alkyl carbon chains covalently attached to its carbon skeleton[12]. AC electric fields were applied to the MWCNTs@GUVs systems, deforming the spheroidal vesicle. The roughness of the systems is the same, only changing the chemical composition of the CNTs. The chemical structure of alk-MWCNTs@GUVs with the carbon chains (8 C atoms) covalently attached to the carbon frame induces a molecular interaction with the hydrophobic alkyl tails of the lipids (16–18 C atoms). From the slope of the curve in the bending regime, the κ value is computed
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