Jarrod C. Williams

Broadband terahertz characterization of the refractive index and absorption of some important polymeric and organic electro-optic materials

We report broad bandwidth, 0.1–10 THz time-domain spectroscopy of linear and electro-optic polymers. The common THz optical component materials high-density polyethylene, polytetrafluoroethylene, polyimide (Kapton), and polyethylene cyclic olefin copolymer (Topas) were evaluated for broadband THz applications. Host polymers polymethyl methacrylate, polystyrene, and two types of amorphous polycarbonate were also examined for suitability as host for several important chromophores in guest-host electro-optic polymer composites for use as broadband THz emitters and sensors.

Superresolution imaging of targeted proteins in fixed and living cells using photoactivatable organic fluorophores

Superresolution imaging techniques based on sequential imaging of sparse subsets of single molecules require fluorophores whose emission can be photoactivated or photoswitched. Because typical organic fluorophores can emit significantly more photons than average fluorescent proteins, organic fluorophores have a potential advantage in super-resolution imaging schemes, but targeting to specific cellular proteins must be provided. We report the design and application of HaloTag-based target-specific azido DCDHFs, a class of photoactivatable push-pull fluorogens which produce bright fluorescent labels suitable for single-molecule superresolution imaging in live bacterial and fixed mammalian cells.

Stable amorphous blue phase of bent-core nematic liquid crystals doped with a chiral material

We report an induction of the blue phase III (BPIII) at a relatively low and wide (over 20 °C) temperature range in nematogenic achiral bent-core liquid crystals doped with a high twisting power chiral material. The pitch decreases with increasing chiral dopant ratio, and easily reaches the ultraviolet wavelength, so that completely dark texture is obtained under crossed polarizers. Electrooptical switching was achieved in a time range of a few to a few tens of milliseconds. We propose for the stabilization of BPIII that broad-temperature range smectic nano-clusters inhibit the long-range order of the double twisted helical structures, and also inhibit possible separation of chiral dopants from the mixture.

Short-Range Smectic Order in Bent-Core Nematic Liquid Crystals

Small angle X-ray diffraction from the uniaxial nematic phase of certain bent-core liquid crystals is shown to be consistent with the presence of molecular clusters possessing short-range tilted smectic (smectic-C) order. Persistence of these clusters throughout the nematic phase, and even into the isotropic state, likely accounts for the unusual macroscopic behavior previously reported in bent-core nematics, including an anomalously large flexoelectric effect (∼ 1000 times that of conventional calamitic nematics), very large orientational and flow viscosities (∼ 10–100 and ∼ 100–1000 times, respectively, typical values for calamitics), and an extraordinary flow birefringence observed in the isotropic state.

Small-Molecule Labeling of Live Cell Surfaces for Three-Dimensional Super-Resolution Microscopy

Precise imaging of the cell surface of fluorescently labeled bacteria requires super-resolution methods because the size-scale of these cells is on the order of the diffraction limit. In this work, we present a photocontrollable small-molecule rhodamine spirolactam emitter suitable for non-toxic and specific labeling of the outer surface of cells for three-dimensional (3D) super-resolution (SR) imaging. Conventional rhodamine spirolactams photoswitch to the emitting form with UV light; however, these wavelengths can damage cells. We extended photoswitching to visible wavelengths >400 nm by iterative synthesis and spectroscopic characterization to optimize the substitution on the spirolactam. Further, an N-hydroxysuccinimide-functionalized derivative enabled covalent labeling of amines on the surface of live Caulobacter crescentus cells. Resulting 3D SR reconstructions of the labeled cell surface reveal uniform and specific sampling with thousands of localizations per cell and excellent localization precision in x, y, and z. The distribution of cell stalk lengths (a sub-diffraction-sized cellular structure) was quantified for a mixed population of cells. Pulse-chase experiments identified sites of cell surface growth. Covalent labeling with the optimized rhodamine spirolactam label provides a general strategy to study the surfaces of living cells with high specificity and resolution down to 10–20 nm.

Enzymatic activation of nitro-aryl fluorogens in live bacterial cells for enzymatic turnover-activated localization microscopy

Many modern super-resolution imaging methods based on single-molecule fluorescence require the conversion of a dark fluorogen into a bright emitter to control emitter concentration. We have synthesized and characterized a nitro-aryl fluorogen which can be converted by a nitroreductase enzyme into a bright push-pull red-emitting fluorophore. Synthesis of model compounds and optical spectroscopy identify a hydroxyl-amino derivative as the product fluorophore, which is bright and detectable on the single-molecule level for fluorogens attached to a surface. Solution kinetic analysis shows Michaelis-Menten rate dependence upon both NADH and the fluorogen concentrations as expected. The generation of low concentrations of single-molecule emitters by enzymatic turnovers is used to extract subdiffraction information about localizations of both fluorophores and nitroreductase enzymes in cells. Enzymatic Turnover Activated Localization Microscopy (ETALM) is a complementary mechanism to photoactivation and blinking for controlling the emission of single molecules to image beyond the diffraction limit.

Highly Porous, Rigid-Rod Polyamide Aerogels with Superior Mechanical Properties and Unusually High Thermal Conductivity

We report here the fabrication of polyamide aerogels composed of poly-p-phenylene-terephthalamide, the same backbone chemistry as DuPonts Kevlar. The all-para-substituted polymers gel without the use of cross-linker and maintain their shape during processing-an improvement over the meta-substituted cross-linked polyamide aerogels reported previously. Solutions containing calcium chloride (CaCl2) and para-phenylenediamine (pPDA) in N-methylpyrrolidinone (NMP) at low temperature are reacted with terephthaloyl chloride (TPC). Polymerization proceeds over the course of 5 min resulting in gelation. Removal of the reaction solvent via solvent exchange followed by extraction with supercritical carbon dioxide provides aerogels with densities ranging from 0.1 to 0.3 g/cm3, depending on the concentration of calcium chloride, the formulated number of repeat units, n, and the concentration of polymer in the reaction mixture. These variables were assessed in a statistical experimental study to understand their effects on the properties of the aerogels. Aerogels made using at least 30 wt % CaCl2 had the best strength when compared to aerogels of similar density. Furthermore, aerogels made using 30 wt % CaCl2 exhibited the lowest shrinkage when aged at elevated temperatures. Notably, whereas most aerogel materials are highly insulating (thermal conductivities of 10-30 mW/m K), the polyamide aerogels produced here exhibit remarkably high thermal conductivities (50-80 mW/(m K)) at the same densities as other inorganic and polymer aerogels. These high thermal conductivities are attributed to efficient phonon transport by the rigid-rod polymer backbone. In conjunction with their low cost, ease of fabrication with respect to other polymer aerogels, low densities, and high mass-normalized strength and stiffness properties, these aerogels are uniquely valuable for applications such as lightweighting in consumer electronics, automobiles, and aerospace where weight reduction is desirable but trapping of heat may be undesirable-applications where other polymer aerogels have to date otherwise been unsuitable-creating new opportunities for commercialization of aerogels.