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Doctor of Philosophy (PhD)
Year Degree Awarded
Month Degree Awarded
Maria M. Santore
David M. Ford
Other Chemical Engineering
This thesis addresses the interactive interfacial character of large-area supported graphene in an aqueous environment near neutral pH. Studies of molecular bio-interactions with proteins and colloidal interactions with microparticles probe the role of hydrophobicity, van der Waals, and electrostatic contributions with varied ionic strength. The respective roles of the silica support and the graphene itself are identified. Results are benchmarked against other systems directly in experiments, and against published behavior with other materials, especially self-assembled monolayers. The adhesive and adsorption behavior of supported graphene is also put into context by calculations of surface and interaction potentials.
Interest in graphene is broadly driven by its unusual electrical and mechanical properties. Different applications motivate interest in different graphene-based materials: Epitaxial supported graphene, CVD supported graphene, exfoliated graphene dispersions in suspension or deposited on supports, graphene oxide, and reduced graphene oxide. As a result of its focus on the fundamental science of the aqueous interactions of the planar face of sp2 graphene, this thesis employs CVD graphene on a silica support. Findings with this system are directly relevant to epitaxial graphene and to the faces of exfoliated graphene flakes, influencing dispersion stability and driving the adsorption of macromolecules, such as stabilizing surfactants. By contrast, graphene oxide and reduced graphene oxide possess fundamentally different chemistry. The physical interactions addressed in this work are central to the processing and deposition of graphene dispersions, the performance of graphene-based biosensors, and the toxicity of graphene itself.
Motivated by bio-interactions, a major study within this thesis examines protein adsorption on graphene. Fibrinogen was chosen as a model because it is abundant in blood, integral to clot formation, and the focus of much previous scientific research. In this thesis, fibrinogen adsorption kinetics on silica-supported graphene are benchmarked against adsorption on bare silica and polycarbonate, where the latter represents a real-world plastic with potential for use in devices, possessing hydrophobicity and surface oxidation from plasma treatment. It was discovered here that fibrinogen adsorbs on supported graphene from flow in a manner consistent with its adsorption on surfaces more hydrophobic than the graphene itself. Adsorption rates were transport limited, adhesion was almost completely irreversible, and the amount of protein adsorbed was sensitive to the rate of accumulation during the transport-limited period: With conditions set to achieve greater transport limited rates, more protein was ultimately adsorbed, consistent with the role of interfacial reconfiguration in limiting the ultimate surface coverage. The extent of reconfiguration, producing increases in the protein footprint by a factor of 2, are consistent with some interfacial denaturing in addition to a relaxation from end-on to side-on conformation. The rates of the footprint growth on graphene match those of adsorption on surfaces of greater hydrophobicity, such as self-assembled monolayers. These findings argue that while the underlying support may contribute to protein adhesion to the surface, the graphene itself has substantial hydrophobicity that drives both adhesion and reconfiguration during adsorption. Thus, proteins such as fibrinogen can adsorb on graphene driven by the properties of the graphene itself, and without substantial π- π interactions such as those deriving from pyrene-graphene interactions.
Two additional studies in this thesis revealed an electrostatically charged character of the supported graphene/water interface, with and without adsorbed protein, and sensitive to ionic strength near neutral pH. In a first study, fluorescein labels on the fibrinogen (incorporated at levels sufficiently low to avoid influence on the adsorption itself), acted as probes of the near-interfacial electrostatic potential, in this case local pH. In the vicinity of the adsorbed protein on silica-supported graphene or on bare silica, there was a distinctly acidic environment. Such low interfacial pH generally occurs with negative surface potentials, as is established for silica. With silica-supported graphene, plasma-treated polycarbonate, and bare silica, the near-surface pH was sensitive to ionic strength, becoming more similar to the bulk solution at higher ionic strength. In addition to revealing an ionic strength-dependent, near-surface reduced pH, the study with protein-bound fluorescein revealed that graphene does not always quench the fluorescence of labels on adsorbing proteins as had been reported in the literature. In this thesis, the observed fluorescence levels for protein adsorbed on the graphene substrate, polycarbonate, and bare silica were similar over a range of ionic strengths, although the amount of quenching with increasing Debye length was greatest for silica, followed by polycarbonate, and then graphene.
In addition to the molecular-scale evidence for charge near silica-supported graphene, this thesis also provides independent evidence, based on colloidal scale interactions, for negative interfacial charge. In an additional study, negatively charged silica microparticles flowing in a microfluidic channel, adhered (or not) to silica-supported graphene in an ionic-strength dependent manner that paralleled silica particle capture on bare silica. Silica particle capture on supported graphene was moderately more adhesive than that on bare silica at each fixed ionic strength when the two systems were compared. Calculated sphere-flat potentials explained the well-understood interactions of silica colloids at ionic strengths sufficient to screen electrostatic repulsions and allow adhesion and capture by van der Waals forces. The same treatment applied to silica-supported graphene revealed negative electrostatic potentials in the range -10 to -15 mV, weaker than that on bare silica but sufficient to repel negative particles at low ionic strengths.
In summary, the studies in this thesis reveal that with silica-supported CVD graphene containing 2-3 graphene layers, sufficient hydrophobicity originates from the graphene to drive protein adsorption and interfacial reconfigurations. This graphene hydrophobicity is therefore expected to drive the adsorption of other surfactants and macromolecules. At the same time, supported graphene possesses negative near-interfacial charge that facilitates ionic strength-dependent stability against van der Waals attractions from the underlying silica support. Thus, we find that supported graphene should generally be considered a composite material, with the support contributing van der Waals attractions and possibility interfacial charge. These findings illustrate how bio-adhesion to graphene can proceed, at the molecular and colloidal scales, in a salt-dependent way and not requiring the π-π interactions or covalent attachment thought necessary for some applications. The ability of supported graphene to interact and adhere in these ways provides the flexibility to create a broad range of graphene-interactive materials and devices, but at the same time it may lead to fouling. This variety of interactions opens the door to economical strategies to engineer application-specific graphene materials and devices at large scale.
Chen, Aaron, "Interactions at the Aqueous Interface of Large-Area Graphene: Colloidal-Scale and Protein Adsorption" (2017). Doctoral Dissertations. 1025.