|Abstract:||This dissertation describes research in two different areas relating to materials chemistry. The first is an examination of protein corona formation on charged gold nanoparticles with an emphasis on the effect that the deformability of the protein has on the corona formed. The second describes the synthesis of polyamide membrane active layers using a support-free approach and examines the effect of monomer concentration on the morphology, water sorption, longitudinal elastic constant, and thermal transport properties of the film. Although these two areas do not have significant overlap, interfaces play an important role in both systems.
When nanoparticles contact biological milieu, proteins adsorb to the surface forming a layer termed the protein corona. As interest grows in harnessing the useful properties of nanoparticles for biomedical applications, the protein corona has become an important phenomenon because the it has been shown to trigger responses in biological systems. The field is driving towards the ability to predict adsorption outcomes based on the properties of the nanoparticles and proteins in question.
Proteins can undergo conformational changes upon adsorption, so it stands to reason that the ease with which a protein can be denatured could be useful for predicting aspects of protein corona formation. One protein was chosen from each of the three classifications of acid denaturation behavior, and this metric was used to predict the relative changes to secondary structure, and the relative binding constants upon adsorbing to negatively and positively charged gold nanospheres (AuNS) capped with citrate and poly(allylamine hydrochloride) (PAH) respectively. Type II proteins (bovine serum albumin (BSA)) are the most deformable and were predicted to denature the most upon adsorption resulting in the highest binding constant as a more conformal coating is expected to form. Type I proteins (α-amylase (A-Amy)) are moderately deformable and were predicted to experience moderate denaturation and have a lower binding constant than Type II proteins. Type III proteins (β-lactoglobulin (BLG)) are the least deformable, and therefore should maintain their conformation upon adsorption, resulting in the lowest binding constant.
Circular dichroism (CD) spectroscopy of AuNs/protein conjugates confirms that BSA exhibits the greatest changes in secondary structure followed by A-Amy, and BLG which matches predictions based on their acid denaturation types. The three proteins exhibit greater secondary structure change upon adsorption to PAH AuNSs relative to citrate AuNSs. Adsorption isotherms were measured using shifts in plasmon peak position and hydrodynamic diameter. Adsorption to citrate AuNSs can be modeled with Langmuir adsorption isotherms. While BSA does have the highest binding constant as predicted, the binding constant of A-Amy is an order of magnitude less than the others because A-Amy is close to neutral charge under the experimental conditions. The enzymatic activity of citrate AuNS/A-Amy conjugates decreases relative to the native protein. All three proteins agglomerate in the presence of PAH AuNSs which can be explained by the CD results. Loss of structure can expose interior hydrophobic domains which is associated with protein agglomeration.
The performance of reverse osmosis and nanofiltration membranes depends on the properties of the polyamide active layer of the membrane. At the most fundamental level these separation processes depend on the 3-D cross-linked structure of the polyamide film which dictates everything from the charge distribution to the density. To date, characterization of these materials has focused on physicochemical properties like the thickness, roughness, and charge, but the mechanical and thermal transport properties have not been thoroughly reported despite these properties being related to the structure of the polymer network through the density, degree of cross-linking, porosity, etc. Synthesis of free-standing polyamide films enables measurements of these properties by time-domain thermal reflectance (TDTR) which was not previously possible for polyamide membranes due to complications arising from the polysulfone support.
Free-standing polyamide active layers were synthesized via interfacial polymerization of m-phenylenediamine (MPD) and trimesoyl chloride (TMC) using sacrificial support materials, and a support-free approach. The ease of the support-free approach is preferable and produces films that are smoother (RMS roughness <10 nm) and thinner (~9 – 15 nm) than conventional membranes. The thickness, roughness, and density of MPD/TMC films increase when the net concentration of the monomers is increased. A dendrimer analogue (pG1) of MPD and piperazine (Pip) were also used to synthesize free-standing films with TMC.
Water sorption by free-standing MPD/TMC films was measured by quartz crystal microbalance with respect to relative humidity. The free-standing materials uptake water in the same manner as conventional polyamide active layers with the mass of the film increasing by ~11% on average at saturation with denser films taking up a relatively smaller mass percentage.
The longitudinal speed of sound, longitudinal elastic constant, and thermal transport properties including thermal conductivity, thermal conductance, and thermal conductance of the interfaces of free-standing MPD/TMC films was measured using TDTR. The speed of sound, elastic constant, and thermal conductivity increase with increasing monomer concentration concurrently with increasing film density. The interfacial thermal conductance is low relative to spin coated polymers. Finally, examining the thermal conductivity with respect to the speed of sound in MPD/TMC, pG1/TMC, and Nomex films reveals that these properties are similar across different fully aromatic polyamide materials.