The halogen bond occurs when there is evidence of a net attractive interaction between an electrophilic region associated with a halogen atom in a molecular entity and a nucleophilic region in another, or the same, molecular entity. In this fairly extensive review, after a brief history of the interaction, we will provide the reader with a snapshot of where the research on the halogen bond is now, and, perhaps, where it is going. The specific advantages brought up by a design based on the use of the halogen bond will be demonstrated in quite different fields spanning from material sciences to biomolecular recognition and drug design.

The Halogen Bond

Chemistries that Facilitate Nanotechnology Kim E. Sapsford,† W. Russ Algar, Lorenzo Berti, Kelly Boeneman Gemmill,‡ Brendan J. Casey,† Eunkeu Oh, Michael H. Stewart, and Igor L. Medintz*,‡ †Division of Biology, Department of Chemistry and Materials Science, Office of Science and Engineering Laboratories, U.S. Food and Drug Administration, Silver Spring, Maryland 20993, United States ‡Center for Bio/Molecular Science and Engineering Code 6900 and Division of Optical Sciences Code 5611, U.S. Naval Research Laboratory, Washington, D.C. 20375, United States College of Science, George Mason University, 4400 University Drive, Fairfax, Virginia 22030, United States Department of Biochemistry and Molecular Medicine, University of California, Davis, School of Medicine, Sacramento, California 95817, United States Sotera Defense Solutions, Crofton, Maryland 21114, United States

Functionalizing nanoparticles with biological molecules: developing chemistries that facilitate nanotechnology.

This Review discusses stimuli-responsive liquid crystalline polymer networks and elastomers as materials with programmable mechanics for use in functional devices. Liquid crystals are the basis of a pervasive technology of the modern era. Yet, as the display market becomes commoditized, researchers in industry, government and academia are increasingly examining liquid crystalline materials in a variety of polymeric forms and discovering their fascinating and useful properties. In this Review, we detail the historical development of liquid crystalline polymeric materials, with emphasis on the thermally and photogenerated macroscale mechanical responses — such as bending, twisting and buckling — and on local-feature development (primarily related to topographical control). Within this framework, we elucidate the benefits of liquid crystallinity and contrast them with other stimuli-induced mechanical responses reported for other materials. We end with an outlook of existing challenges and near-term application opportunities.

Programmable and adaptive mechanics with liquid crystal polymer networks and elastomers

Polymeric microactuators are potentially useful in micromechanical systems and lab-on-a-chip systems. However, manufacturing of miniature polymeric actuators has been complicated owing to the necessity of including electrodes for actuation or using lithographic techniques for patterning. Here, we demonstrate that all-polymer microdevices can be fabricated using inkjet printing technology in combination with self-organizing liquid-crystal network actuators. We exploit the self-assembling properties of the liquid crystal to create large strain gradients, and light-driven actuation is chosen to allow simple and remote addressing. By using multiple inks, microactuators with different subunits are created that can be selectively addressed by changing the wavelength of the light. The actuators mimic the motion of natural cilia. These artificial cilia have the potential to create flow and mixing in wet environments such as lab-on-a-chip applications. The process is easily adapted for roll-to-roll fabrication, allowing for large-scale and low-cost production of miniaturized active polymer systems.

Printed artificial cilia from liquid-crystal network actuators modularly driven by light.

Physical vapor deposition under conditions of obliquely incident flux and limited adatom diffusion results in a film with a columnar microstructure. These columns will be oriented toward the vapor source and substrate rotation can be used to sculpt the columns into various morphologies. This is the basis for glancing angle deposition (GLAD), a technique for fabricating porous thin films with engineered structures. The origin of the columnar structure characteristic of GLAD films is discussed in terms of nucleation processes and structure zone models. As deposition continues, the columnar structures are influenced by atomic-scale ballistic shadowing and surface diffusion. Competitive growth is observed where the tallest columns grow at the expense of smaller features. The column shape evolves during growth, and power-law scaling behavior is observed as shown in both experimental results and theoretical simulations. Due to the porous nature of the films and the increased surface area, a variety of chemical applications and sensor device architectures are possible. Because the GLAD process provides precise nanoscale control over the film structure, characteristics such as the mechanical, magnetic, and optical properties of the deposited film may be engineered for various applications. Depositing onto prepatterned substrates forces the columns to adopt a planar ordering, an important requirement for photonic crystal applications.

Glancing angle deposition: Fabrication, properties, and applications of micro- and nanostructured thin films

With the maturation of the information display field, liquid-crystal materials research is undergoing a modern-day renaissance. Devices and configurations based on liquid-crystal materials are being developed for spectroscopy, imaging and microscopy, leading to new techniques for optically probing biological systems. Biosensors fabricated with liquid-crystal materials can allow label-free observations of biological phenomena. Liquid-crystal polymers are starting to be used in biomimicking colour-producing structures, lenses and muscle-like actuators. New areas of application in the realms of biology and medicine are stimulating innovation in basic and applied research into these materials.

Liquid-crystal materials find a new order in biomedical applications.

Introduction to Liquid Crystals Basic Optics of Liquid Crystals Display Technologies Medical Displays Liquid Crystals in Spectroscopy, Microscopy and Hyperspectral Imaging Liquid Crystal Biosensors Biomimicking with Liquid Crystals Actuators and Delivery Systems Liquid Crystal Lasers.

Liquid crystals : frontiers in biomedical applications

By controlling the morphology of holographic polymer-dispersed ferroelectric liquid crystals (FLCs), highly aligned FLC domains are obtained for diffractive optical applications. Rapid, thresholdless switching is observed for various grating pitch sizes between ∼3 and ∼12 μm. A simple phenomenological model is presented encompassing a distribution of domain sizes and an effective field that stabilizes the FLC domains to reflect the observed thresholdless switching and optical behavior.

Holographic diffraction gratings using polymer-dispersed ferroelectric liquid crystals.

A projection apparatus includes a shutter mechanism to prevent light from reaching an image plane during calibration of light sources. The shutter mechanism may include liquid crystal material that exhibits an effective index of refraction that varies with applied voltage. During calibration, a light beam is shuttered, light sources are driven by calibration data, and optical power is measured.

Projector with shutter

Glancing angle deposition is used to create porous liquid crystal alignment layers with high aspect-ratio, three-dimensional features. We use solid-state deuterium nuclear magnetic resonance to determine the alignment of alpha deuterated 4′-pentyl-4-cyanobiphenyl in a 3.2μm thick SiO2 film comprised of independent, helical structures. Comparing measured spectra with models, we determine that the liquid crystalline molecules exhibit chiral ordering dictated by the features of the template films. In the plane of deposition the liquid crystalline molecules tilt away from the substrate normal at an average angle of 51°, which is commensurate with the helical features of the chiral template.

Scott J. Woltman

Papers

Liquid-crystal materials find a new order in biomedical applications.

Liquid crystals : frontiers in biomedical applications

Holographic diffraction gratings using polymer-dispersed ferroelectric liquid crystals.

Projector with shutter

Template induced chiral ordering in nematic liquid crystalline materials : A deuterium nuclear magnetic resonance study