Studies of Adsorption of Organic Macromolecules on Oxide and Perfluorinated Surfaces

Abstract

Humic-based organic compounds containing phenol or benzoic acid groups strongly compete with phosphates for specific binding sites on the surface of these colloidal particles. To study the interactions between phenol groups and the surface binding sites of unmodified or modified colloidal particles, chemical force spectrometry (CFS) was used as a tool to measure the adhesion force between an atomic force microscopy (AFM) tip terminated with a phenol self-assembled monolayer and colloidal particles under varying pH conditions. Two modification methods, co-precipitation and post-precipitation, were used to simulate the naturally-occurring phosphate and humic-acid adsorption process. The pH dependence of adhesion forces between phenol-terminated tip and colloidal particles could be explained by an interplay of electrostatic forces, the surface loading of the modifying phosphate or humic acid species and ionic hydrogen bonding. Polydimethylsiloxane (PDMS) is a widely-used polymer in microfluidic devices. PDMS surfaces are commonly modified to make it suitable for specific microfluidic devices. We studied the surface modification of PDMS using four perfluoroalkyl-triethoxysilane molecules of differing length of perfluorinated alkyl chain. The results show that the length of fluorinated alkyl chain has important effects on the density of surface modifying molecules, surface topography and surface zeta potential. The perfluorinated overlayer makes PDMS more efficient at supporting electroosmotic flow, which has potential applications in microfluidic devices. The kinetic study of RNase A, lysozyme C, α-lactalbumin and myoglobin at different concentrations adsorbed on the self-assembled monolayers of 1-octanethiol (OT-Au) and 1H, 1H, 2H, 2H-perfluorooctyl-1-thiol (FOT-Au) has been carried out. The results show a positive relationship between the lower protein concentration and the increased adsorption rate constant (ka) on both surfaces. At low concentrations, the protein adsorption on an OT-Au surface has greater ka than it on a FOT-Au surface. Comparing ka values for four proteins on OT-Au and FOT-Au surface demonstrates that hard proteins (lysozyme and RNase A) have larger ka than soft proteins (α-lactalbumin and myoglobin) on both surfaces. The discussion is based on the hydrophobicity of OT-Au and FOT-Au surfaces, as well as average superficial hydrophobicity, flexibility, size, stability, and surface induced conformation change of proteins.