Head Groups of Lipids Govern the Interaction and Orientation between Graphene Oxide and Lipids

Graphene oxide (GO) has demonstrated various promising applications in biomedical field. But information on how GO interacts with cell membrane or model system is still extremely limited. It is also unknown how GO orientates itself when interacting with lipids. In this study, the Langmuir monolayer technique is applied at the air–water/aqueous interface to study the nature and orientation of interaction between GO and lipid models. Five lipids (DODAB, DSEPC, DSPC, DSPA, and SA) with the same 18-carbon alkyl chain but different head groups of charges are purposely chosen to rationalize the possible interactions. Experimental results show that the interaction is governed by electrostatic interaction between the polar head groups and GO. GO can incorporate into the monolayer of positively charged lipids DODAB and DSEPC, but not the neutrally or negatively charged lipids (DSPC, DSPA, and SA). When GO is injected to the subphase underneath the monolayer of positively charged lipid DODAB and DSEPC, different behaviors of surface pressure are observed. An orientation model of “edge-in” instead of “face-in” is proposed to explain the adsorption of GO to the monolayer of DODAB.