The Interactions of Graphene Oxide with Biological and Artificial Cell Membranes
Thesis or dissertation
University of Exeter
Graphene is an atomically thin carbon nanomaterial, with a honeycomb structure. It has generated enormous scientific interest since its discovery in 2004. Graphene has a set of unique properties, which has made it a highly suitable material for the potential use in electronic devices, photonics and bioapplications, including the use in a new generation of biosensors, over the past few years. Graphene has already demonstrated high sensitivity to various biological molecules which come into contact with its surface. However, there have been virtually no studies of the mechanisms of the interactions between graphene and biological materials, particularly the cell membrane, the biological component which will be in direct contact with graphene. Understanding these interactions is central to the use of graphene-based materials in drug delivery systems and its biological safety and is also relevant to the development of many types of biosensors. The work presented in this thesis provides an insight into the fundamental interactions between graphene oxide (GO), and graphene-based materials, with the cell membrane. By using model membrane systems, the aim is to reveal how GO interacts with cell membranes, the extent to which it affects the structure of the membrane, with the potential to further research the biological processes that occur at the membrane. Studies on Langmuir monolayers, using lipid mixtures representative of the composition of the inner and outer leaflets of a typical cell membrane, found that the nature of GO interaction with biological membranes may be dependent on the lipid composition, with results suggesting significant repulsive forces between GO and negatively charged phospholipids. The second model membrane system investigated were lipid bilayer vesicles. Two preparations of vesicles were used, one containing only neutral phosphatidylcholine lipids and the other containing two types of neutral lipid, as well as cholesterol and a stabilising molecule. GO was shown to permeate both vesicles without causing complete lysis. This suggests GO interacts specifically with at least one type of neutral lipid molecule within the vesicle bilayer. This is further supported through an investigation into GO interactions with a real cell membrane. In red blood cells (RBC’s) the interaction of GO increases membrane stiffness, but does not cause haemolysis, suggesting that it does not penetrate the biological membrane. This suggests GO is interacting with the outer lipid of the membrane, but it is possible the negatively charged lipids in the inner leaflet of the membrane prevent the GO from entering the cell. The increase in membrane stiffness suggests GO is interacting with the lipids altering the properties of the membrane.
PhD in Physics