AbstractGraphene has shown its potential in bio-sensing. However, the cytotoxicity of graphene could restrict its applications in biotechnology. To improve the biocompatibility, one feasible solution is to functionalise graphene with biomolecules, such as lipids, to prevent direct contact of graphene from cell membranes. Therefore, understanding the structure of graphene in aqueous media the adsorption behaviour of lipids on graphene becomes important.
In this study, synchrotron X-ray reflectivity (XRR) has been applied to investigate the surface structure of graphene in air and in aqueous media, and also the interaction between graphene and liposomes of 1,2-dioleoyl-sn-glycero-3-phosphocholine (DOPC) or 1,2- dipalmitoyl-sn-glycero-3-phosphocholine (DPPC). X-ray photoelectron spectroscopy, photoemission electron microscopy, ellipsometry, atomic force microscopy, and contact angle measurement have been performed as complementary techniques to evaluate the composition, morphology, and the surface chemistry of the graphene substrates. The influences of temperature, the presence of ions (provided by phosphate buffered saline), and overnight water submergence on the surface structure of graphene and the graphene/lipid interaction have been examined.
The characterisation of bare graphene indicates the samples consisted of 3 - 4 layers of graphene, which should be accurately described as few layer graphene (FLG). In addition, a “contaminant” layer, comprising polymethylmethacrylate and graphene multilayers, was found present atop FLG. In aqueous systems, a diffuse layer of air bubbles was found immediately adjacent to FLG, which diminished after soaking. Tentative results showing the effects of temperature on the FLG thickness and affinity to ions have also been reported.
Both DOPC and DPPC were observed to form monolayer on the un-soaked FLG, but bilayer on the soaked FLG, due to the enhanced FLG hydrophilicity after soaking. The morphologies of lipid membranes were found to be temperature dependent. Our results have provided a new understanding of the morphology of lipid membranes on graphene and also demonstrated the capability of XRR – as a rigorous and quantitative method - to probe the surface and interfacial structures of graphene coated with biomolecules.
|Date of Award||23 Jan 2019|
|Supervisor||Wuge H Briscoe (Supervisor)|