| Description |
In this dissertation, the surface coating method/characteristics and a novel synthesis method for perfluorocarbon (PFC) emulsions were studied. These materials have relevance in a wide range of industrial and biomedical areas. Electrostatic coating of emulsions with poly-L-lysine (PLL) and chitosan (CS) was used to enhance the stability of the emulsions against phase separation. The ionic surfactant, 1,2-dioleoyl-sn-glycero-3-phosphate, DOPA, was used as an additive to the nonionic surfactant lecithin in order to increase the surface charge of the emulsion surface and facilitate accumulation of coating materials on the emulsion surface. Analysis of zeta potential versus pH was used to establish the best conditions for the coating process. The particle size and zeta potential was used to follow the coating progress. To characterize the final product, colorimetric determination was used to measure the bound/unbound fraction of PLL and CS and this was compared with a model-based analysis of zeta potential as a function of coating. Through this comparison and accounting for experimental error, a discrepancy in the effective particle number was revealed that was interpreted in terms of the compression and expansion of coating molecules on the surface as it assembles. Fluorescence quenching measurements using pyrene and fluorescein-doped emulsions supported the compression/expansion concept, resulting in more quenching when expansion of the coating took place. Thermodynamic analysis also supported these conformation changes and indicated a lowering of surface free energy for expanded coating. The second part of the dissertation reports a novel cosolvent method to synthesize PFOB emulsion with high yield production, in contrast to traditional extrusion methods that generate a large amount of water-filled liposomes as a side product. It was found that the selection of best cosolvent is related to its polarity relative to that of perfluorooctyl bromide (PFOB). Fourier transform infrared spectroscopy (FTIR) analyses were used to measure the retention of PFOB and revealed that nonpolar hexane has the best PFOB retention ability compared to methanol, ethanol, and chloroform. Moreover, phase transition temperatures (PTT) of the hexane/PFOB/lecithin system were observed at 13~19 °C and 22~24 °C by monitoring both the change in transparency by UV-Vis spectrophotometry and PFOB retention by FTIR. If emulsions are produced above the PTT (at 30 oC), PFOB is not retained. Quantitative measurement using FTIR for the best conditions gave 72% PFOB retention using the cosolvent method. Fluorescent and density analysis by centrifugation indicated that the traditional method for emulsion synthesis (directly emulsified) produced significantly more low-density water-filled liposomes than the cosolvent method. Higher density PFOB emulsion made with the cosolvent method can be easily separated and concentrated by centrifugation. The image of emulsions made by the traditional and cosolvent method could be viewed by cryogenic transmission electron microscopy. |
