Violeta Toader

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.

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.

Redox-Induced Ion Pairing of Anionic Surfactants with Ferrocene-Terminated Self-Assembled Monolayers: Faradaic Electrochemistry and Surfactant Aggregation at the Monolayer/Liquid Interface

Oxidoreduction of self-assembled monolayers (SAMs) of ferrocenyldodecanethiolate on gold in aqueous solutions of surface-active sodium n-alkyl sulfates (NaCnSO4) of 6, 8, 10, and 12 carbons is investigated by cyclic voltammetry and surface plasmon resonance. The effects of surfactant micellization and alkyl chain length on the redox response of the surface-tethered ferrocenes are examined. The SAM redox electrochemistry is sensitive to the surfactant aggregation state in solution. The nonideal behavior of the sodium alkyl sulfates at concentrations above the critical micelle concentration leads to a non-Nernstian variation of the SAM redox potential with concentration. The presence of micelles in solution results in decreased anodic-to-cathodic peak separations and anodic peak full widths at half-maximum. A longer alkyl chain length results in an increased ability of the alkyl sulfate anion to ion pair with the SAM-bound ferrocenium, resulting in oxidation of the ferrocene at lower potential. A comparison of the SAM redox potential at a fixed surfactant concentration of ideal behavior suggests a 4.5 × 10(4) difference in the ion-pairing abilities of the shorter-chain C6SO4(-) and longer-chain C12SO4(-). One-half of the available SAM-bound ferrocenes are oxidized in the NaCnSO4 electrolyte. Surfactant anions adsorb and assemble onto the SAM surface by specific ion-pairing interactions between the sulfate headgroups and oxidized ferrocenium species, forming an interdigitated monolayer in which the surfactant anions alternate between a heads-down and heads-up orientation with respect to the SAM. The work presented points to applications of ferrocenylalkanethiolate SAMs as anion-selective membranes, probes of micelle formation, and surfaces for the electrochemically switchable assembly of organosulfates.

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.

Switching the Z/E Selectivity in the Palladium(II)-Catalyzed Decarboxylative Heck Arylations of trans-Cinnamaldehydes by Solvent

The Z/E selectivity of Pd(II)-catalyzed decarboxylative Heck-type arylations of trans-cinnamaldehydes can be controlled readily by switching the reaction solvent. Depending on the type of solvent used, each of the two isomeric products can be obtained with good to excellent Z/E ratio. In THF, Z-isomers were formed preferentially, whereas DMF provided the E-isomers predominantly.

Microcantilevers Bend to the Pressure of Clustered Redox Centers

The redox-activated deflection of microcantilevers has attracted interest for nanoactuation and chemical sensing. Microcantilever sensors are devices that transduce (bio)chemical reactions into a quantifiable nanomechanical motion via surface stress changes. Despite promising applications in analytical science, poor signal-to-noise ratios and a limited understanding of the molecular origins of the surface stress changes that cause the observed deflections remain obstacles to cantilever-based sensing becoming an established (bio)detection method, such as surface plasmon resonance and electrochemistry. We use phase-separated, binary self-assembled monolayers (SAMs) of ferrocenyldodecanethiolate and n-undecanethiolate as a model system to study the effect of the steric crowding of the redox centers on the surface stress change and cantilever deflection produced by the electrochemical oxidation of the surface-tethered ferrocene to ferrocenium. We correlate the measured surface stress change to the fraction of the clustered ferrocenyldodecanethiolate phase in the binary SAMs. The pairing of anions with the sterically crowded clustered ferroceniums induces a collective molecular reorientation which drives the cantilever deflection. The results provide fundamental insights into the response mechanism of microcantilever-based actuating and sensing technologies.

Modulation of charge transport across double-stranded DNA by the site-specific incorporation of copper bis-phenanthroline complexes.

The site-specific incorporation of transition-metal complexes within DNA duplexes, followed by their immobilization on a gold surface, was studied by electrochemistry to characterize their ability to mediate charge. Cyclic voltammetry, square-wave voltammetry, and control experiments were carried out on fully matched and mismatched DNA strands that are mono- or bis-labeled with transition-metal complexes. These experiments are all consistent with the ability of the metal centers to act as a redox probe that is well coupled to the DNA π-stack, allowing DNA-mediated charge transport.

Hydrogen-Bonded Liquid Crystal Nanocomposites.

Nanoparticle-liquid crystal (NP-LC) composites based on hydrogen bonding were explored using a model system. The ligand shells of 3 nm diameter zirconium dioxide nanoparticles (ZrO2 NPs) were varied to control their interaction with 4-n-hexylbenzoic acid (6BA). The miscibility and effect of the NPs on the nematic order as a function of particle concentration was characterized by polarized optical microscopy (POM), fluorescence microscopy and (2)H NMR spectroscopy. Nonfunctionalized ZrO2 NPs have the lowest miscibility and strongest effect on the LC matrix due to irreversible binding of 6BA to the NPs via a strong zirconium carboxylate bond. The ZrO2 NPs were functionalized with 6-phosphonohexanoic acid (6PHA) or 4-(6-phosphonohexyloxy)benzoic acid (6BPHA) which selectively bind to the ZrO2 NP surface via the phosphonic acid groups. The miscibility was increased by controlling the concentration of the pendant CO2H groups by adding hexylphosphonic acid (HPA) to act as a spacer group. Fluorescence microscopy of lanthanide doped ZrO2 NPs showed no aggregates in the nematic phase below the NP concentration where aggregates are observed in the isotropic phase. The functionalized NPs preferably concentrate into LC defects and any remaining isotropic liquid but are still present throughout the nematic liquid at a lower concentration.

Mechanochemical nanoparticle functionalization for liquid crystal nanocomposites based on COOH-pyridine heterosynthons

Nanoparticle/liquid crystal (NP/LC) composites can be stabilized by hydrogen bonding, a relatively strong, yet reversible interaction allowing for the annealing of defects. Previously, nanocomposites based on 4-hexylbenzoic acid (6BA) and ZrO2 NPs with pendant carboxylic acid groups were investigated, leading to problems of particle aggregation due to intra- and inter-particle hydrogen bonding. Here we report the synthesis of NP–LC composites based on different hydrogen bond acceptor and donor groups, promoting NP–LC coupling and reducing aggregation due to NP–NP interactions. Specifically, we developed a mechanochemical, solvent-free approach for efficient functionalization of ZrO2 NPs with (3-(pyridin-4-yl)propyl)phosphonic acid (3-PPA), leading to NPs with pendant pyridine groups that act as hydrogen bond acceptors only. This is the first example of using hydrogen bonded heterosynthons to tune the LC–NP interactions. The miscibility of the pyridine-functionalized NPs with 4-hexylbenzoic acid (6BA), a strong hydrogen bond donor, versus trans-4-butylcyclohexanecarboxylic acid (4-BCHA), a weaker hydrogen bond donor, was characterized by polarized optical and fluorescence microscopies. The specificity of the NP/LC acceptor/donor hydrogen bonds improves the miscibility over the NP/LC dispersions based on COOH dimer interactions only. The different effect of the NPs on the 4-BCHA properties as compared to 6BA can be related to the relative strengths of the COOH-pyridine hydrogen bonds.