To explore the thermal response of the densely packed inner regime of poly(N-isopropylacrylamide) (PNIPAM) brushes grafted to gold nanoparticles (AuNPs), we systematically studied the thermoresponsive properties of NIPAM oligomeric brushes affected both by oligomer molar mass and Au core size. A series of NIPAM oligomers with various molar masses ranging from ca. 600 to 3400 g/mol were obtained by RAFT polymerization and fractionated with HPLC. The AuNPs stabilized with various NIPAM oligomers were prepared by a one-pot reaction and further fractionated to achieve three pairs of AuNP fractions with narrow size distributions. When decreasing the molar mass of brush chains from ca. 3300 to 700 g/mol, a significant molar mass effect on the thermal transition was found, i.e., the phase transition temperature (defined as the endothermic peak temperature T p by DSC) shifted from ca. 31 to 15 °C and the endothermic peak became broadened. As a comparison, we also studied the aqueous solutions of free NIPAM oligomers (molar mass from ca. 3400 to 600 g/mol) by turbidity measurements. They showed a completely opposite trend of the thermally induced phase transitions; i.e., the transition shifted to higher temperature with decreasing molar mass. The Au core size also affected the thermal response of NIPAM oligomer brushes, especially in the case of the shortest oligomers. Large Au cores caused the thermal transition of NIPAM oligomer brush to occur at lower temperatures compared to the small Au cores. This was attributed to the hydrophobic nature of Au nanocrystal surfaces. Enthalpy changes (∆H) associated with the thermal transitions of the oligomer brushes are indicative of strong interchain interactions in the brushes, especially on large Au cores.
A three-wavelength angular-scanning surface plasmon resonance based analysis has been utilized for characterizing optical properties of organic nanometer-thick layers with a wide range of thicknesses. The thickness and refractive index were determined for sample layers with thicknesses ranging from subnanometer to hundreds of nanometers. The analysis approach allows for simultaneous determination of both the refractive index and thickness without prior knowledge of either the refractive index or the thickness of the sample layers and without the help of other instruments, as opposed to current methods and approaches for characterizing optical properties of organic nanometer-thick layers. The applicability of the three-wavelength angular-scanning surface plasmon resonance approach for characterizing thin and thick organic layers was demonstrated by ex situ deposited mono- and multilayers of stearic acid and hydrogenated soy phosphatidylcholine and in situ layer-by-layer deposition of two different polyelectrolyte multilayer systems. In addition to the three-wavelength angular-scanning surface plasmon resonance approach, another surface plasmon resonance optical phenomenon, i.e., the surface plasmon resonance waveguide mode, was utilized to characterize organic sample layers whose thicknesses border the micrometer scale. This was demonstrated by characterizing both in situ layer-by-layer deposited polyelectrolyte multilayer systems and an ex situ deposited spin-coated polymer layer.
In this study, surface coatings were used to control the morphology of the deposited lipid layers during vesicle spreading, i.e., to control if liposomes self-assemble on a surface into a supported lipid bilayer or a supported vesicular layer. The influence of the properties of the surface coating on formation of the deposited lipid layer was studied with quartz crystal microbalance and two-wavelength multiparametric surface plasmon resonance techniques. Control of lipid self-assembly on the surface was achieved by two different types of soft substrate materials, i.e., dextran and thiolated polyethylene glycol, functionalized with hydrophobic linkers for capturing the lipid layer. The low-molecular-weight dextran-based surface promoted formation of supported lipid bilayers, while the thiolated polyethylene glycol-based surface promoted supported vesicular layer formation. A silicon dioxide surface was used as a reference surface in both measurement techniques. In addition to promoting supported lipid bilayer formation of known lipid mixtures, the dextran surface also promoted supported lipid bilayer formation of vesicles containing the cell membrane extract of human hepatoblastoma cells. The new dextran-based surface was also capable of protecting the supported lipid bilayer against dehydration when exposed to a constant flow of air. The well-established quartz crystal microbalance technique was effective in determining the morphology of the formed lipid layer, while the two-wavelength surface plasmon resonance analysis enabled further complementary characterization of the adsorbed supported lipid bilayers and supported vesicular layers.
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