Self-Assembly of Hybrid Lipid Membranes Doped with Hydrophobic Organic Molecules at the Water/Air Interface

Summary

We report a protocol for producing a hybrid lipid membrane at the water/air interface by doping the lipid bilayer with copper (II) 2,9,16,23-tetra-tert-butyl-29H,31H-phthalocyanine (CuPc) molecules. The resulting hybrid lipid membrane has a lipid/CuPc/lipid sandwich structure. This protocol can also be applied to the formation of other functional nanomaterials.

Abstract

Because of their unique properties, including an ultrathin thickness (3-4 nm), ultrahigh resistivity, fluidity and self-assembly ability, lipid bilayers can be readily functionalized and have been used in various applications such as bio-sensors and bio-devices. In this study, we introduced a planar organic molecule: copper (II) 2,9,16,23-tetra-tert-butyl-29H,31H-phthalocyanine (CuPc) to dope lipid membranes. The CuPc/lipid hybrid membrane forms at the water/air interface by self-assembly. In this membrane, the hydrophobic CuPc molecules are located between the hydrophobic tails of lipid molecules, forming a lipid/CuPc/lipid sandwich structure. Interestingly, an air-stable hybrid lipid bilayer can be readily formed by transferring the hybrid membrane onto a Si substrate. We report a straightforward method for incorporating nanomaterials into a lipid bilayer system, which represents a new methodology for the fabrication of biosensors and biodevices.

Introduction

As essential frameworks of cell membranes, the interior of cells are separated from the exterior by a lipid bilayer system. This system consists of amphiphilic phospholipids, which are composed of hydrophilic phosphoric ester “heads” and hydrophobic fatty acids “tails”. Due to remarkable fluidity and self-assembly ability of lipid bilayers in aqueous environment^1,2, artificial lipid bilayers can be formed using simple methods^3,4. Various types of membrane proteins, such as ion channels, membrane receptors and enzymes, have been ....

Protocol

1. Preparation of a hybrid solution

1. Wash four 4 mL disposable glass vials and screw caps (with PTFE coated seals) in an ultrasonic bath for 10 min in distilled water (purified with a filtration system), followed by ethanol and chloroform, respectively. Dry the glass vials and caps in a stream of nitrogen gas.
2. In an anaerobic glove box, prepare a CuPc stock solution (10 mg/mL) in a washed glass vial by dissolving powdered CuPc in chloroform.
3. Filter the CuPc solution through a 0.2 µm polytetrafluoroethylene (PTFE) membrane.
4. Store the filtered solution in a washed glass vial filled with nitrogen, and seal the vial with paraf....

Results

The as-formed membrane has a uniform light blue color due to the presence of CuPc molecules. The area of the colored membrane is normally several square centimeters. In Figure 1A and Figure 1B, we show a microscopic image and an atomic force microscope (AFM) image (including a height profile) of the hybrid lipid membrane on a Si substrate. In the AFM image, the membrane in the upper left is thick, with a thickness of 79.4 nm and that at the lower right is thin, .......

Discussion

In the precursor solution of the hybrid membrane, a mixed organic solvent (chloroform and hexane) rather than pure chloroform is used to dissolve lipids and CuPc. If pure chloroform is used, the density of the precursor solution would be higher than water. Therefore, it is highly likely that the solution would sink to the bottom of water rather than spread on water surface. Adding hexane, a low density solvent, to the precursor solution, ensures that the solution will float on the water surface and form a uniform hybrid .......

Materials

List of materials used in this article

Name	Company	Catalog Number	Comments
Chloroform	Wako Chemicals	033-08631
CuPc	Sigma-Aldrich	423165
DPhPc	Avanti Polar Lipids	850356C
Glass vials with screw cap	Nichiden-Rike Glass Co., Ltd	6-29801
Hexane	Wako Chemicals	084-03421
Membrane filters	Merck Millipore Ltd.	R8CA42836
Micro-syringe	Hamilton	80530
Peristaltic pump	Tokyo Rikakikai Co., Ltd.	11914199
Vortex mixer	Scientific Industries, Inc.	SI-0286