Linda Reven

Preparation and characterization of polyelectrolyte-coated gold nanoparticles.

Gold nanoparticles of 5 nm diameter, stabilized by 4-(dimethylamino)pyridine (DMAP), were coated with poly(sodium 4-styrene sulfonate) (PSS) via electrostatic self-assembly. The suspension stability, monitored by the gold surface plasmon band (SPB), was studied by varying the pH, the PSS chain length, and PSS concentration. Enhanced stability is obtained at pH 10 (above the pKa of DMAP) when the polymer chain length matches or exceeds the particle circumference. Solid state 13C NMR was used to determine the presence of DMAP and polymers after subsequent deposition of weak and strong polycations: poly(allylamine hydrochloride) (PAH) and poly(diallyldimethylammonium chloride) (PDADMAC). At pH 10, DMAP remains associated with the nanoparticle after the first PSS layer has been formed. When PAH or PDADMAC are subsequently added at pH 4.5, DMAP is expelled, the suspensions remain stable, and zeta potential values indicate complete charge reversal. In the case of PDADMAC, however, the first layer of PSS is not fully retained. When PDADMAC is added at pH 10, DMAP and the first PSS layer are retained but lower zeta potentials and a higher SPB shift indicate a degraded stability. For PAH addition at pH 9.5, both DMAP and PSS are expelled and the suspension becomes unstable. These differences in stability of the multilayer components and the nanoparticle suspension are rationalized in terms of chain flexibility, polymer charge density, and the ability of the polymer functional groups to directly interact with the gold surface.

Reversible long range network formation in gold nanoparticle - nematic liquid crystal composites

Nanoparticles (NPs) are dispersed into liquid crystals (LCs) to create ordered NP assemblies and thereby modify the LC and NP properties. Although low NP concentrations are normally used to avoid aggregation, high concentrations can lead to new organization through coupling of the interparticle attractive forces with the LC elastic properties. Gold nanoparticles (AuNPs) with mesogenic coatings, tailored to be highly miscible in the liquid phase of n-alkyl-cyanobiphenyl LCs, form reversible micron-scale networks on cooling at the clearing point by enrichment of the NPs at the nematic-isotropic liquid interfaces. The network topology and LC director field orientation are controlled by the cooling rate, surface alignment, film thickness, AuNP concentration and ligand shell composition. Thin film networks consisted of branches and circular areas of LC enriched in AuNPs. Nucleating nematic droplets evolve into homeotropic alignment of the host nematic matrix, accompanied by birefringent disclination lines and loops. Thick film AuNP networks in LCs form complex structures with stable radial director configurations in small domains and Schlieren domains elsewhere. Controlled formation of networks via the use of LC phase transitions offers an additional approach to produce quasi-periodic NP assemblies that are both long range and reversible in nature.

Tuning the miscibility of gold nanoparticles dispersed in liquid crystals via the thiol-for-DMAP reaction

Ligand exchange reactions using 4–5 nm 4-(N,N-dimethylamino)pyridine (DxMAP)-capped gold nanoparticles (AuNPs) formed the basis for synthesizing a family of liquid crystal (LC)-capped NPs for a rationalized miscibility in liquid crystal matrices. NPs with ligand capping layers composed of CH3(CH2)mSH (m = 5, 11) or 4′-(n-mercaptoalkyloxy)biphenyl-4-carbonitriles (CBO(CH2)nSH, n = 8, 12, 16) and their binary mixtures were prepared. The miscibility of the NPs in liquid crystals is found to be sensitive to the ligand chain length and the density of the LC ligands within the capping layers. Polarized optical microscopy and UV-vis data show that the NPs with only CH3(CH2)mSH ligands are either immiscible or only partially disperse in the isotropic phases of 4-n-pentyl-4′-cyanobiphenyl (5CB) and 4-n-octyl-4′-cyanobiphenyl (8CB). NPs with CBO(CH2)nSH (n = 8, 12, 16) ligands or mixed CH3(CH2)5SH/CBO(CH2)12SH ligand shells containing 28% or 70% CBO(CH2)12SH ligand content partly disperse. However, NPs with a 1 : 1 CH3(CH2)5SH/CBO(CH2)12SH ratio are completely miscible in isotropic 5CB up to at least 25 wt% Au. In general, the derivatization methodology developed here for mesogenic ligands provides in a complementary approach to thiol-for-thiol exchange for designing bifunctional AuNPs, offering the advantages of high reproducibility, access to a wide composition range and no need for large excesses of valuable functionalized ligand.

The Effect of Terminal Hydrogen Bonding on the Structure and Dynamics of Nanoparticle Self-Assembled Monolayers (SAMs): An NMR Dynamics Study.

conditions. The sample shown in Figure 1a was prepared by heating the solution for 3 h at 80 C and casting the hot solution on a carbon-covered microscopy grid. Gray disks 5 nm in diameter were seen, which were identified as TiO2 particles. The particles agglomerated into chains consisting of approximately 20 individual disks. Apparently the heating procedure had two effects involved in the film formation. First TiO2 particles were formed in each block copolymer micelle. In a second step, the increased temperature caused the excess HCl to evaporate. The excess HCl is known, however, to be essential for stabilizing inverse diblock copolymer micelles during film formation. [10,11] As the HCl evaporates, the PEO block loses its ionic character and the kinetic as well as the thermodynamic stability of the diblock copolymer micelles is reduced because of the increased compatibility of the constituent blocks. Casting of films by simply allowing a drop of the solution to evaporate on a carbon-coated copper grid enabled the diblock copolymer micelles to react to the changing concentration and to transform from globular to worm-like micelles.

Reversible long-range patterning of gold nanoparticles by smectic liquid crystals

4.7 nm Gold nanoparticles (AuNPs) capped with mesogenic ligands, dispersed in homeotropically aligned smectic A liquid crystal (LC), reversibly form arrays of aggregates with micron-scale periodicities over macroscopic areas at the nematic to smectic A phase transition (TN-SmA). AuNPs with a mixed capping layer of 4′-(n-mercaptododecyloxy)biphenyl-4-carbonitrile and hexanethiol in a 1 : 1 ratio were dispersed in 4′-octyl-4-cyanobiphenyl (8CB) and subjected to boundary conditions known to produce periodic edge dislocations. In a cavity cell with a meniscus, the arrays consisted of concentric circles. Linear arrays were produced by wedge cells and control over the periodicity was achieved through variation of the wedge angle. Based on the dependence of the geometry and periodicity of the arrays on boundary conditions, we propose that the AuNPs concentrate at edge dislocation defects of the smectic phase.

Phase equilibrium and structure formation in gold nanoparticles—nematic liquid crystal composites: experiments and theory

A theoretical and experimental study of mixtures of a calamitic nematic liquid crystal and gold nanoparticles capped with mixed monolayers (alkylic + mesogenic ligands) is presented. The effect of the ligand monolayer composition and nanoparticle concentration on the solubility in the isotropic phase and on the isotropic–nematic phase transition is studied. Mixed monolayers show the highest miscibility and lead to the formation of well-defined cellular networks. This behaviour is explained in terms of a mean-field thermodynamic model, in combination with a phenomenological expression for the isotropic interaction parameter that accounts for entropic and enthalpic effects of the mixed ligand monolayer. The final structure of the material is shown to be determined essentially by the phase equilibrium behaviour.

Reversible Nanoparticle Cubic Lattices in Blue Phase Liquid Crystals

Blue phases (BPs), a distinct class of liquid crystals (LCs) with 3D periodic ordering of double twist cylinders involving orthogonal helical director twists, have been theoretically studied as potential templates for tunable colloidal crystals. Here, we report the spontaneous formation of thermally reversible, cubic crystal nanoparticle (NP) assemblies in BPs. Gold NPs, functionalized to be highly miscible in cyanobiphenyl-based LCs, were dispersed in BP mixtures and characterized by polarized optical microscopy and synchrotron small-angle X-ray scattering (SAXS). The NPs assemble by selectively migrating to periodic strong trapping sites in the BP disclination lines. The NP lattice, remarkably robust given the small particle size (4.5 nm diameter), is commensurate with that of the BP matrix. At the BP I to BP II phase transition, the NP lattice reversibly switches between two different cubic structures. The simultaneous presence of two different symmetries in a single material presents an interesting opportunity to develop novel dynamic optical materials.

Alloy hardening of a smectic A liquid crystal doped with gold nanoparticles

We measure the mobility of edge dislocations in a smectic A liquid crystal doped with gold nanoparticles. The mobility is found to decrease when the concentration of nanoparticles increases, which leads to a hardening of the smectic phase. It is found that the closer the temperature to the nematic phase, the weaker the hardening. The critical behavior of the mobility near the smectic A-to-nematic transition temperature is also investigated and compared to theoretical predictions.

Thermodynamic Modelling of Phase Equilibrium in Nanoparticles – Nematic Liquid Crystals Composites

In this work, a theoretical study of phase equilibrium in mixtures of a calamitic nematic liquid crystal and hard spherical nanoparticles is presented. A mean-field thermodynamic model is used, where the interactions are considered to be proportional to the number of contacts, which in turn are proportional to the areas and area fractions of each component. It is shown that, as the radius of the particle is increased, the effect of the particles on the isotropic-nematic transition is less pronounced, and that for a large radius the miscibility increases as the particle radius increases.

Textures in polygonal arrangements of square nanoparticles in nematic liquid crystal matrices.

A systematic analysis of defect textures in faceted nanoparticles with polygonal configurations embedded in a nematic matrix is performed using the Landau-de Gennes model, homeotropic strong anchoring in a square domain with uniform alignment in the outer boundaries. Defect and textures are analyzed as functions of temperature T, polygon size R, and polygon number N. For nematic nanocomposites, the texture satisfies a defect charge balance equation between bulk and surface (particle corner) charges. Upon decreasing the temperature, the central bulk defects split and together with other -1/2 bulk defects are absorbed by the nanoparticles corners. Increasing the lattice size decreases confinement and eliminates bulk defects. Increasing the polygon number increases the central defect charge at high temperature and the number of surface defects at lower temperatures. The excess energy per particle is lower in even than in odd polygons, and it is minimized for a square particle arrangement. These discrete modeling results show for first time that, even under strong anchoring, defects are attached to particles as corner defects, leaving behind a low energy homogeneous orientation field that favors nanoparticle ordering in nematic matrices. These new insights are consistent with recent thermodynamic approaches to nematic nanocomposites that predict the existence of novel nematic/crystal phases and can be used to design nanocomposites with orientational and positional order.