Magnetic Levitation To Characterize the Kinetics of Free-Radical Polymerization.

This work describes the development of magnetic levitation (MagLev) to characterize the kinetics of free-radical polymerization of water-insoluble, low-molecular-weight monomers that show a large change in density upon polymerization. Maglev measures density, and certain classes of monomers show a large change in density when monomers covalently join in polymer chains. MagLev characterized both the thermal polymerization of methacrylate-based monomers and the photopolymerization of methyl methacrylate and made it possible to determine the orders of reaction and the Arrhenius activation energy of polymerization. MagLev also made it possible to monitor polymerization in the presence of solids (aramid fibers, and carbon fibers, and glass fibers). MagLev offers a new analytical technique to materials and polymer scientists that complements other methods (even those based on density, such as dilatometry), and will be useful in investigating polymerizations, evaluating inhibition of polymerizations, and studying polymerization in the presence of included solid materials (e.g., for composite materials).

A molecular and thermodynamic view of the assembly of gold nanoparticles in nematic liquid crystal.

The molecular interactions driving the assembly of gold nanoparticles (AuNPs) in a nematic liquid crystal (LC) are directly detected by nuclear magnetic resonance (NMR) spectroscopy and thermodynamically analyzed. The orientational orders of the selectively deuterated LC matrix and AuNP ligands, each separately followed by variable temperature (2)H NMR as a function of particle concentration, were observed to be strongly correlated. The mechanism of the reversible formation of long-range, quasi-periodic nanoparticle structures is attributed to the coupling of the AuNP ligands to the LC matrix, inducing an isotropic-nematic biphasic state. Experimentally validated thermodynamic modeling shows that, in contrast to colloidal nematics that are dominated by elastic forces, nematic dispersions of nanoparticles self-organize through a subtle balance of entropic forces and excluded volume, interface-mediated mesogen and nanoparticle molecular interactions, and couplings between conserved and nonconserved order parameters. Fine-tuning of these interactions through ligand and mesogen chemistry, together with mesoscale modeling, provides a route for materials innovations by merging structured fluid physics and nanoscience.

DNMR study of hydrophilic and hydrophobic silica dispersions in EBBA liquid crystals.

Dispersions of hydrophilic (A300) and hydrophobic (R812) silica aerosils in a Schiff-base-type liquid crystal (LC), p-ethoxy(benzylidene)-p-n-butylaniline (2O.4), EBBA, were characterized by deuterium nuclear magnetic resonance (DNMR). The formation and stability of random (RAN) versus anisotropic (AAN) aerosil networks under zero- versus in-field cooling was studied as a function of aerosil density and compared to previous studies of n-alkylcyanobiphenyl (nCB) dispersions. Whereas the LC directors of the hydrophobic R812 dispersions are almost completely annealed after in-field cooling, the hydrophilic A300 silica in EBBA gives rise to a mixture of RAN and AAN. The more complete R812 AAN partially breaks under in-field sample rotation, but the partial AAN formed by the A300 silica is stable. Weakening the aerosil network to compensate for weaker LC surface anchoring results in a complete network, but a strong LC/silica surface interaction must be combined with hydrophilic aerosils to produce AANs which are both complete and stable.

Sliding-strip microfluidic device enables ELISA on paper.

This article describes a 3D microfluidic paper-based analytical device that can be used to conduct an enzyme-linked immunosorbent assay (ELISA). The device comprises two parts: a sliding strip (which contains the active sensing area) and a structure surrounding the sliding strip (which holds stored reagents-buffers, antibodies, and enzymatic substrate-and distributes fluid). Running an ELISA involves adding sample (e.g. blood) and water, moving the sliding strip at scheduled times, and analyzing the resulting color in the sensing area visually or using a flatbed scanner. We demonstrate that this device can be used to detect C-reactive protein (CRP)-a biomarker for neonatal sepsis, pelvic inflammatory disease, and inflammatory bowel diseases-at a concentration range of 1-100ng/mL in 1000-fold diluted blood (1-100µg/mL in undiluted blood). The accuracy of the device (as characterized by the area under the receiver operator characteristics curve) is 89% and 83% for cut-offs of 10ng/mL (for neonatal sepsis and pelvic inflammatory disease) and 30ng/mL (for inflammatory bowel diseases) CRP in 1000-fold diluted blood respectively. In resource-limited settings, the device can be used as a part of a kit (containing the device, a fixed-volume capillary, a pre-filled tube, a syringe, and a dropper); this kit would cost ~ $0.50 when produced in large scale (>100,000 devices/week). This kit has the technical characteristics to be employed as a pre-screening tool, when combined with other data such as patient history and clinical signs.

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.