Supercoiled Fibers Research Articles

Self-Assembled Extended π-Systems for Sensing and Security Applications.

Molecules and materials derived from self-assembled extended π-systems have strong and reversible optical properties, which can be modulated with external stimuli such as temperature, mechanical stress, ions, the polarity of the medium, and so on. In many cases, absorption and emission responses of self-assembled supramolecular π-systems are manifested several times higher when compared with the individual molecular building blocks. These properties of molecular assemblies encourage scientists to have a deeper understanding of their design to explore them for suitable optoelectronic applications. Therefore, it is important to bring in highly responsive optical features in π-systems, for which it is necessary to modify their structures by varying the conjugation length and by introducing donor-acceptor functional groups. Using noncovalent forces, π-systems can be put together to form assemblies of different shapes and sizes with varied optical band gaps through controlling intermolecular electronic interactions. In addition, using directional forces, it is possible to bring anisotropy to the self-assembled nanostructures, facilitating efficient exciton migration, resulting in the modulation of optical and electron-transport properties. In this Account, we mainly summarize our findings with optically tunable self-assemblies of extended π-systems such as p-phenylenevinylenes (PVs), p-phenyleneethynylenes (PEs), and diketopyrrolopyrroles (DPPs) as different stimuli-responsive platforms to develop sensors and security materials. We start with how PV self-assemblies and their coassemblies with appropriate electron-deficient systems can be used for the sensing of analytes in contact mode or in the vapor phase. For example, whereas the PV having electron-deficient terminal groups has high sensitivity toward trinitrotoluene (TNT) in contact mode, the supercoiled fibers formed by the coassembly of self-sorted stacks of C3-symmetrical PV and C3-symmetrical electron-deficient perylene bisimide are capable of sensing vapors of nitrobenzene and o-toluidine. The power of different functional groups in combination with PVs has been further illustrated by attaching CO2-sensitive tertiary amine moieties to a cyano-substituted PV, which allowed the bimodal detection of CO2 using fluorescence and Raman spectroscopy. Interestingly, the functionalization of PVs with terminal amide groups and chiral alkoxy side chains provided a mechanochromic system that allows self-erasable imaging. Whereas PVs exhibit quenching of fluorescence in most cases during self-assembly, PE derivatives exhibit aggregation-induced emission. This property of PEs has been exploited for the development of stimuli-responsive security materials, especially for currency and documents. For instance, the blue fluorescence of a PE attached to hydrophilic oxyethylene side chains coated on a filter paper upon contact with water changes to cyan emission due to the change in the molecular packing. Interestingly, the molecular packing of a Bodipy-attached PE-based gelator allowed a stress-induced change in the emission behavior, resulting in strong near-infrared (NIR) emission upon the application of mechanical stress or gelation. Finally, the use of DPP-based π-systems for the development of NIR transparent optical filters that block UV-vis light and their security- and forensic-related applications are described. These selected examples of the π-system self-assemblies provide an idea of the current status and future opportunities for scientists interested in this field of self-assembly and soft materials research.

Torsional refrigeration by twisted, coiled, and supercoiled fibers.

Higher-efficiency, lower-cost refrigeration is needed for both large- and small-scale cooling. Refrigerators using entropy changes during cycles of stretching or hydrostatic compression of a solid are possible alternatives to the vapor-compression fridges found in homes. We show that high cooling results from twist changes for twisted, coiled, or supercoiled fibers, including those of natural rubber, nickel titanium, and polyethylene fishing line. Using opposite chiralities of twist and coiling produces supercoiled natural rubber fibers and coiled fishing line fibers that cool when stretched. A demonstrated twist-based device for cooling flowing water provides high cooling energy and device efficiency. Mechanical calculations describe the axial and spring-index dependencies of twist-enhanced cooling and its origin in a phase transformation for polyethylene fibers.

Architecture of the Chinese hamster metaphase chromosome.

The development of procedures for the isolation of unfixed metaphase chromosomes has made feasible a direct analysis of their morphology. Wholemount stereo electron microscopy was used to examine intact and partially disrupted chromosomes produced by physical shearing and extraction with salt and urea solutions. A model of chromosome architecture was developed to accommodate evidence from studies using both light and electron microscopy. In the proposed model the chromatid (anaphase chromosome) consists of two half-chromatids; each half-chromatid contains two deoxyribonucleoprotein ribbons wound into a single fiber (termed the core), with many loops of chromatin (termed epichromatin) attached along its length. The core ribbons are each about 50 A thick by 4000 A wide and are composed of many parallel deoxyribonucleoprotein strands. The epichromatin loops appear to be 250 A supercoiled fibers containing about 75 per cent of the chromosomal DNA. The epichromatin can be selectively removed from the core fibers by extraction with 2.0 M NaCl or 6.0 M urea solutions.