Cathy E. McNamee

Formation of Large-Scale Flexible Transparent Conductive Films Using Evaporative Migration Characteristics of Au Nanoparticles

To sustain the growing demand of transparent conductive films for wide applications, such as flat panel displays, a much more cost-effective film is required over the widely used indium tin oxide film. Here we developed a promising method to manufacture a cost-effective flexible transparent conductive film of high performance by first making grid-iron patterns of thin lines on a large scale using evaporative migration characteristics of gold nanoparticles, and then by burying the grid-iron pattern into a poly(ethylene terephthalate) film.

Interfacial forces between a silica particle and phosphatidylcholine monolayers at the air-water interface.

Interfacial forces between a silica or borosilicate particle in water and phospholipids at the air-water interface were studied using the Monolayer Particle Interaction Apparatus. This instrument allowed the forces to be measured as the colloidal probe approached the monolayer from the liquid phase. The proper working principle of this setup was demonstrated by measuring the forces between a particle and a mica plate in 0.1 mM NaCl. The effect of the alkyl chain length on the adhesion between the particle and the monolayer was investigated using four different 1,2-dialkyl-sn-glycero-3-phosphocholines (DMPC, DPPC, DSPC, and DBPC), which had 14, 16, 18, and 22 carbon atoms per alkyl chain, respectively. The adhesion force increased with the square of the particle radius. The lipids in the liquid-expanded (LE) phase showed an attraction to the particle, explained by an electrostatic attraction and/or the formation of a three-phase contact line that lead to a capillary force. All monolayers showed an adhesion in their retract force curve, which decreased with an increased chain length and surface pressure. Interfacial stiffness was generally seen to increase with the phospholipid chain length and to decrease with surface pressure, explained by an increase in the intermolecular van der Waals interaction and a decrease in the interfacial tension, respectively. The adhesion between the particle and monolayer was concluded to be controlled by the contact area between the particle and monolayer, and therefore the monolayer stiffness and the electrostatic interactions.

Interaction of a microsphere with a solid-supported liquid film.

The interaction between particles with thin liquid films on solid surfaces was studied by sintering polystyrene microspheres of 4 to 5 microm diameter to the end of atomic force microscope cantilevers. Films of three silicone oils (viscosity 4.6, 9.2, and 9700 mPa s) and water of thickness 0.2-1.8 microm were formed on glass. The interaction between a particle and the film was measured at different particle approach/retraction velocities. The interaction is dominated by capillary and hydrodynamic forces. It depends on the surface tension and the viscosity of the liquid. The film thickness can be determined from the force curves. In addition, the meniscus formation of a film wetting a particle was demonstrated experimentally by solidifying a liquid polystyrene film as it wetted glass particles.

A Straightforward Way To Form Close-Packed TiO2 Particle Monolayers at an Air/Water Interface

The aim of this study was to analyze if and how monolayers of TiO(2) particles could be directly formed at the air/water interface and if these monolayers could be transferred to a solid surface. TiO(2) particles with diameters of 300 nm, 500 nm, 1 μm, 5 μm, 10 μm, and 20 μm formed stable monolayers at pH 2. At low surface pressures, the particles formed small two-dimensional aggregates. Particles up to a radius of 5 μm displayed close packing at increased surface pressures. Particles of 10 μm radius formed a loose network, which is attributed to the strong adhesion caused by the weight-induced lateral capillary attraction. Every monolayer of particles could be transformed to a solid surface by the Langmuir-Blodgett deposition. At pH 6 or 11, the particles did not form stable monolayers at the air/water interface. They were instead dispersed in the aqueous phase and eventually sank to the bottom of the trough. At pH 11 the monolayer could, however, be stabilized by the addition of salt (0.5 M NaCl). The results are interpreted based on a changed wettability of the particles depending on pH and salt concentration.

Adsorption of quarternarised polyvinylpyridine and subsequent counterion binding of perfluorinated anionic surfactants on silica as a function of concentration and pH: a zeta potential study

Abstract The adsorption of a cationic polyelectrolyte, poly(2-vinyl-1-methyl-pyridinium bromide), P2VP, on colloidal silica (0.15 μm radius) and the subsequent counterion binding of perfluorinated anionic surfactants, CF 3 CF 2 COONa, CF 3 (CF 2 ) 2 COONa, CF 3 (CF 2 ) 6 COONa, CF 3 (CF 2 ) 7 SO 3 Li and CF 3 (CF 2 ) 9 COOLi, were studied by electrophoresis. The zeta potential ( ζ -potential) of silica changed its sign from negative to positive with an increase in P2VP concentration at pH 4.0, 6.6, and 9.2. The fractional surface coverage of P2VP on silica ( θ ) was estimated from the zeta potentials of bare silica ( ζ 1 ) and silica fully covered with P2VP ( ζ 2 ), and the Langmuir adsorption model as a function of P2VP concentration and pH. The negative ζ 1 increased with increasing pH, whereas the positive ζ 2 was constant at all pH. The higher θ values at high pH suggested that the dominant interaction of the P2VP adsorption on silica was electrostatic. Surfactant anions did not adsorb onto the bare silica surface when pH>isoelectric point, but did adsorb onto a silica surface modified to saturation with P2VP. Electrokinetic measurements in the presence of 0.1 mM lower alkyl chain-length surfactants ( n ≤3) indicated that they behaved as indifferent electrolytes, with no surfactant adsorption detectable. The adsorption increased with the chain-length for 0.1 mM surfactants with >6 carbon chains, indicating specific binding. An increase in the concentration of CF 3 (CF 2 ) 6 COONa or CF 3 (CF 2 ) 7 SO 3 Li changed the sign of the ζ -potential of the P2VP modified silica surface from positive to negative. The mechanism of charge reversal was discussed in terms of the excess adsorption of the surfactant anions.

Forces between a monolayer at an air/water interface and a particle in solution: influence of the sign of the surface charges and the subphase salt concentration

The way the rigidity, molecular packing density, and charge of an insoluble monolayer at an air/aqueous interface and the type of substrate affect the deposition of a monolayer on a substrate is still unclear. In this study, we aimed to better understand the forces involved in the deposition process, when a solid surface is brought near a monolayer. We achieved this by using the Monolayer Particle Interaction Apparatus to directly measure the interaction forces between a monolayer at an air/aqueous interface and a hard particle in solution. The monolayer surface pressure, the monolayer and particle surface charge, and the concentration of salt in the subphase were varied. An adhesion was observed between a particle and a monolayer in an unlike-charged monolayer–particle system, or when a particle with a non-zero contact angle was used in the measurement. The addition of salt changed the packing of the monolayer, increasing the stiffness of the monolayer and generally decreasing the adhesion. Increasing the surface pressure of the monolayer decreased the interfacial stiffness and generally decreased the adhesion.

Chemical Groups that Adhere to the Surfaces of Living Malignant Cells

PurposeWe determined the adhesion of particles with phenyl, carboxylic acid (COOH), amine, dialkyl phosphonate, ester, and hydroxyl groups to malignant and nonmalignant cells, in order to better design drug delivery systems (DDS) for malignant cells.MethodsLiving mouse melanoma skin (B16F10) and noncancerous mouse fibroblast (L929) cells, and an Atomic Force Microscope were used to determine the adhesion strengths.ResultsThe measurement of the particles against B16F10 cells showed that COOH had the highest average maximum adhesion force () and a large standard deviation (std), and phenyl had the lowest and a lower std. The high and std suggested that COOH was binding the strongest to malignant cells, and to groups overexpressed on malignant cells. In the case of L929 cells, of phenyl and COOH were higher and lower, respectively, than those of the B16F10 cells. Additionally, Phenyl and COOH gave a lower std than that for the B16F10 cells. These results suggest that the lower binding of COOH to the nonmalignant cells was due to the lower number of groups that were overexpressed in the malignant cells.ConclusionsOur results suggest that COOH is the best group for malignant cell targeting DDS systems.

Ionic Enhancement of Silica Surface Nanowear in Electrolyte Solutions

The nanoscale wear and friction of silica and silicon nitride surfaces in aqueous electrolyte solutions were investigated by using sharp atomic force microscope (AFM) cantilever tips coated with silicon nitride. Measurements were carried out in aqueous solutions of varying pH and in monovalent and divalent cation chloride and nitrate solutions. The silica surface was shown to wear strongly in solutions of high pH (≈11.0), as expected, but the presence of simple cations, such as Cs(+) and Ca(2+), was shown to dramatically effect the wear depth and friction force for the silica surface. In the case of monovalent cations, their hydration enthalpies correlated well with the wear and friction. The weakest hydrated cation of Cs(+) showed the most significant enhancement of wear and friction. In the case of divalent cations, a complex dependence on the type of cation was found, where the type of anion was also seen to play an important role. The CaCl(2) solution showed the anomalous enhancement of wear depth and friction force, although the solution of Ca(NO(3))(2) did not. The present results obtained with an AFM tip were also compared with previous nanotribology studies of silica surfaces in electrolyte solutions, and possible molecular mechanisms as to why cations enhance the wear and friction were also discussed.

Interaction of Cationic Hydrophobic Surfactants at Negatively Charged Surfaces Investigated by Atomic Force Microscopy

Atomic force microscopy was used to study the adsorption of the surfactant octadecyl trimethyl ammonium chloride (C18TAC) at a low concentration (0.03 mM) to negatively charged surfaces in water. Atomic force microscopy tips were functionalized with dimethyloctadecyl(3-tripropyl)ammonium chloride (C18TAC-si) or N-trimethoxysilylpropyl-N,N,N-trimethylammomium chloride (hydrophilpos-si) to facilitate imaging of the adsorbed surfactant without artifacts. Tapping mode images and force measurements revealed C18TAC patches, identified as partial surfactant bilayers or hemimicelles. The forces controlling the adsorption process of the C18TAC to a negatively charged surface were investigated by measuring the forces between a C18TAC-si or a hydrophilpos-si tip and a silica surface in the presence of varying concentrations of either NaCl or NaNO3. Screening of forces with an increasing NaCl concentration was observed for the C18TAC-si and hydrophilpos-si tips, proving an electrostatic contribution. Screening was also observed for the hydrophilpos-si tip in NaNO3, whereas a long-range attraction was observed for the C18TAC-si tip for all NaNO3 concentrations. These results indicate that screening of the forces for the C18TAC-si tip depended on the type and/or size of the anion, possibly due to a different probability of the anions to enter the silane layers. The interaction of C18TAC patches with C18TAC-si tips in the presence of NaCl and the interaction of the patches with hydrophilpos-si tips in either NaCl or NaNO3 were repulsive and independent of the number of force curves measured, indicating a stable, positively charged C18TAC patch. However, the forces measured between the patches and a C18TAC-si tip in NaNO3 depended on the number of force curves measured, indicating a change in patch structure induced by the first interaction.

Determination of the binding of non-cross-linked and cross-linked gels to living cells by atomic force microscopy.

In this study, we first proposed a method to directly measure the interaction forces between nanoparticle gels and living cells by using the atomic force microscope (AFM). This was achieved by attaching the nanoparticles to a carrier silica probe with epoxy resin and then by directly measuring the interaction force between the probe and the living cells with the AFM. We subsequently used this technique to investigate the ability of triblock copolymer nanoparticle gels to bind to living B16F10 (mouse skin melanoma) cells. We particularly studied how the copolymer composition and structure, and the introduction of chemical cross-linking affected the adhesion magnitude. We found that a gel particle was capable to bind to a living melanoma cell. The binding strength of the particle was determined by the composition of the gel particle, where a composition change appeared to affect the number and type of chemical groups on the surface of the gel that could bind to the cell. The introduction of cross-links into the gel did not decrease the adhesion ability to a cell. Instead, it was seen that the adhesion could be increased, if a cross-linker was chosen that contained chemical groups that could bind with the cell and that preferred a conformation at the surface of the particle.