Abstract
Graphene oxide (GO), a novel 2−dimensional carbon based nanomaterial, has shown potential applications in biomedical and biological field, including drug and gene delivery, sensing, and bioimaging. However, one critical question needs to be addressed before any actual application: how does GO interact with biological molecules, such as amino acids, peptides, proteins, and biomembranes? In this study, spectroscopy, microscopy, and surface chemistry were applied to fundamentally understand the nature of interactions between these molecules and GO. GO was found to interact with amino acids, peptides, and proteins by fluorescence quenching. The main quenching mechanism between GO and Trp or Tyr was determined as static quenching, slightly combined with dynamic quenching. Both electrostatic interaction and hydrophobic interaction contributed to the interactions between GO and Trp or Tyr. Particularly, strong electrostatic interaction between GO and lysozyme was demonstrated and confirmed using fluorescence quenching, zeta potential, dynamic light scattering, and atomic force microscopy. This interaction was so strong that one was able to subsequently eliminate and separate lysozyme using GO. The strong electrostatic interaction also rendered the selective adsorption of lysozyme on GO from a mixture of proteins. As polymer Pluronic F127 (PF127) was used to disperse GO and block the hydrophobic interaction between GO and Trp or Tyr, the interaction and behavior between GO and PF127 were also characterized using Langmuir monolayer technique at the air–water interface. To study the nature and orientation of interaction between GO and lipid models, Langmuir monolayer technique was applied at the air−water/aqueous interface. Five lipids with the same 18–carbon alkyl chain but different head groups of charges were chosen to rationalize the possible interactions. Experimental results showed that the interaction was governed by electrostatic interaction between the polar head groups and GO. GO could 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 was injected to the subphase underneath the monolayer of positively charged lipid DODAB and DSEPC, different behaviors of surface pressure were observed. An orientation model of GO was proposed to explain the different adsorption of GO. Another topic of this thesis is on protein fibrillation. Pathological conditions of human neurodegenerative diseases are now believed to be commonly associated with protein misfolding processes. Human insulin and human islet amyloid polypeptide (hIAPP) are two major hormones involved in diabetes. Fibrillation of insulin at various interfaces was summarized. The conformation and self−assembly of the hIAPP were studied at the air−aqueous interface using the Langmuir monolayer technique. Experimental results showed that hIAPP Langmuir monolayer was relatively stable and did not form aggregates when compressed. However, ongoing experiments showed there existed interaction between hIAPP and insulin, favoring the fibrillation between the two.