Paper BI1-WeM2
Effect of Metal Ions on Lipid Bilayer Formation on Semiconductor Surfaces

Wednesday, October 20, 2010, 8:20 am, Room Taos

Lipid bilayers have applications in drug delivery, bio-sensing for clinical diagnosis, and device fabrication. Just as in a living cell where a lipid bilayer separates aqueous compartments from their surroundings, a lipid membrane supported on a surface can function as a mask that allows selective mass transport via intermembrane proteins. Lipid bilayers have been used primarily to support proton channel proteins in sensors, but there are many other types of intermembrane proteins with different functions. With an aim to extend the use of biomolecules in device fabrication, the effect of heavy metal ions on bilayer formation was investigated using atomic force microscopy (AFM) and x-ray photoelectron spectroscopy (XPS). 1,2-dimyristoyl-sn-glycero-3-phosphocholine (DMPC) lipid molecules were used to form supported bilayers by the vesicle fusion method on hydroxylated Si, As₂O₃-terminated GaAs, and Al₂O₃ surfaces. Alumina was deposited on a p-Si(100) surface by atomic layer deposition (ALD) at 170°C using trimethylaluminum (TMA) and water as precursors. The bilayer was formed on an AFM stage for 6 hr and the stage was heated to 41.5±5°C, which is 17.5°C above the DMPC phase transition temperature. The height of lipid membranes measured with AFM after digging a hole was 4.9±0.5 nm on a hydroxylated Si surface and 4.0±0.6 nm on an alumina surface, which correspond to the thickness of a bilayer. The mechanical strength, uniformity, and integrity of membranes were measured after flowing copper sulfate solutions (100-3000 ppm) through a liquid flow cell over the formed bilayer on the Al₂O₃ surface. Al₂O₃ was chosen because Cu²⁺ ions are reduced to Cu⁰ on alumina, but not on SiO₂. It was found by XPS that the copper permeates through the lipid bilayer and deposits on the alumina surface as Cu⁰. Force distance measurements were made to understand copper permeation. The adhesion force of copper on Al₂O₃ was 2-3 times higher than that of lipid molecules, leading to breaking of the bilayer and deposition of copper. AFM confirmed the breakage and the bilayer thickness after copper exposure was 1.4±0.2 nm. This study shows that metal ions with a higher adhesion force than lipid molecules on an insulator surface disrupt bilayer formation, placing limitations on how bilayers can be used in device fabrication. These results also suggest an additional mechanism for the antibacterial properties of copper.