Jiyu Fang

Polarization-independent liquid crystal phase modulator using a thin polymer-separated double-layered structure.

A polarization-independent phase-only liquid crystal (LC) phase modulator using a double-layered structure is demonstrated. Two orthogonal LC layers are separated by two ultra-thin anisotropic polymer films. The anisotropic polymeric films not only separate the LC layers but also provide good molecular alignment. As a result, a polarization-independent phase modulator with 2pi phase shift is achieved at 9Vrms and 8.1pi at 40Vrms using a 12-microm-thick E7 LC layers. This operating voltage is ~10X lower than that using a conventional 0.3-mm-thick glass separator.

Increased resonance energy transfer between fluorophores bound to DNA in proximity to metallic silver particles

We examined the effects of metallic silver particles on resonance energy transfer (RET) between fluorophores covalently bound to DNA. A coumarin donor and a Cy3 acceptor were positioned at opposite ends of a 23-bp double helical DNA oligomer. In the absence of silver particles the extent of RET is near 9%, consistent with a Forster distance R(0) near 50 A and a donor to acceptor distance near 75 A. The transfer efficiency increased when the solution of AMCA-DNA-Cy3 was placed between two quartz plates coated with silver island films to near 64%, as determined by both steady-state and time-resolved measurements. The apparent R(0) in the presence of silver island films increases to about 110 A. These values of the transfer efficiency and R(0) represent weighted averages for donor-acceptor pairs near and distant from the metallic surfaces, so that the values at an optimal distance are likely to be larger. The increased energy transfer is observed only between two sandwiched silvered slides. When we replaced one silvered slide with a quartz plate the effect vanished. Also, the increased energy transfer was not observed for silvered slides separated more than a few micrometers. These results suggest the use of metal-enhanced RET in PCR, hybridization, and other DNA assays, and the possibility of controlling energy transfer by the distance between silver surfaces.

Fluorescence spectral properties of cyanine dye-labeled DNA oligomers on surfaces coated with silver particles

We examined the fluorescence spectral properties of DNA oligomers, labeled with Cy3 or Cy5, when bound to quartz surfaces coated with metallic silver particles. Prior to binding of labeled DNA the surfaces were treated with polylysine or 3-aminopropyl triethoxysilane or were coated with avidin for binding of biotinylated oligomers. The fluorescence intensities were increased an average of 8-fold on these surfaces. Despite the increased emission intensity, the photostability of the labeled DNA was the same or higher on the silver-coated surfaces than on the uncoated slides. The time-integrated intensities, that is the area under the intensity plots with continuous illumination, increased an average of 6-fold. In all cases the lifetimes were dramatically shortened on the silver particles, indicating an over 100-fold increase in the radiative decay rates. These results suggest the use of substrates containing silver particles for increased sensitivity of DNA detection on DNA arrays.

Self‐Assembly of pH‐Switchable Spiral Tubes: Supramolecular Chemical Springs

Helical structures are commonly found in nature at different length scales. Perhaps the most notable examples are simple a-helical peptides, double-helical nucleic acids, triple-helical collagens, and more complex helical microtubules and tobacco mosaic viruses. These helical structures in nature have captivated the minds of scientists over decades because they are associated with many biological events. It is known that these biological helical structures, such as collagens, microtubules, and viruses are formed by the self-assembly of basic building blocks (protein subunits). Inspired by the synthetic strategy in nature, great efforts have been made in synthesizing helical supramolecular structures with controlled morphology and helicity by the self-assembly of synthetic small molecules because of their potential applications in materials science and enantioselective catalysis. The self-assembly of chiral amphiphilic molecules in aqueous solutions is of particular interest because the chirality of individual molecules is often expressed in their supramolecular structures. For example, a number of synthetic chiral amphiphilic molecules have shown the ability of selfassembling into helical ribbons in aqueous solutions. These include carbohydrate amphiphiles, amino-acid-based amphiphiles, peptide amphiphiles, diacetylenic lipids, gemini surfactants, nucleotide-based amphiphiles, and bile acids. Interest in the self-assembly of helical ribbons is driven by not only the fundamental understanding of the relationship between molecular chirality and supramolecular chirality but also their material applications. In many cases, the self-assembled helical ribbons are metastable. The closing of the gaps of the helical ribbons can lead to the formation of rigid and straight tubes. Herein, we report the self-assembly of lithocholic acid (LCA) tubes in aqueous solution with pH 12.0 at room temperature. The self-assembled LCA tubes can coil into a spiral shape and then transit into a straight shape as the pH of solution is reduced from 12.0 to 7.4 with HCl. The shape

Photostability of Cy3 and Cy5-Labeled DNA in the Presence of Metallic Silver Particles

We examined the photostability of a double-stranded DNA oligomer, covalently labeled with Cy3 or Cy5 on one strand, in the presence of metallic silver island films. In our experimental configuration a minor fraction of the labeled DNA was close to the silver particles and the remainder was distant from the particles. Proximity of the fluorophores to silver island films resulted in increased intensity. Upon continuous illumination we found a fraction of the emission that was resistant to the photobleaching. The emission spectra of the residual fractions were identical to the initial spectra. The frequency-domain lifetime measurements of this fraction revealed greatly shortened decay times. These results are consistent with the photostable fraction being close to the silver particles. This results suggest that the number of photons detected per fluorophore, prior to photobleaching, can be increased 5-fold or more by proximity to silver particles. Localization at an optimal distance from the silver surface may result in larger enhancements.

Ordered arrays of self-assembled lipid tubules: fabrication and applications

Self-assembled lipid tubules with crystalline bilayer walls are supramolecular architectures that are attractive for a number of applications including drug releases, chemical sensors, nanofluidics, nanoreactors, and microelectronics. The ability to fabricate lipid tubules into ordered arrays on substrates is an important step in integrating them with functional devices. The ordered arrays of aligned lipid tubules may also lead to new applications in materials science. This feature article reviews recent developments in fabricating lipid tubules into ordered arrays on a variety of substrates. We also discuss the applications of ordered tubule arrays as templates in the synthesis of aligned and patterned silica cylinders and liquid-crystal fibers.

IPS-LCD using a glass substrate and an anisotropic polymer film

A lightweight in-plane-switching liquid crystal display (IPS-LCD) using a single glass substrate and an anisotropic polymeric film is demonstrated. The liquid crystal molecules are aligned by the elongated polymer grain of the film. The alignment capability of the anisotropic film is comparable to a buffed polyimide layer. Compared to the LCD using two glass substrates, our new device exhibits a comparable contrast ratio (/spl sim/514:1), driving voltage, and response time because of good LC alignment. Such an anisotropic film can also function as a phase compensation film for widening the viewing angle. This technology is particularly attractive for making single-substrate displays and also has potential for a double-layered guest-host display and a flexible display using IPS LCDs.

Polyelectrolyte-coated liquid crystal droplets for detecting charged macromolecules

Chemically engineered liquid crystal droplets with stimuli responsive director configurations are attractive for sensing applications. Here, we find that the adsorption of positively charged poly(amido amine) (PAMAM) dendrimers on negatively charged poly(styrenesulfonate sodium) (PSS)-coated liquid crystal droplets can trigger a bipolar-to-radial configuration transition of the liquid crystal in the droplets, which can be easily detected with a polarizing optical microscope. The sensitivity of the PSS-coated liquid crystal droplets for detecting PAMAM dendrimers depends on the size and number of liquid crystal droplets as well as the dendrimer generation.

Electrically tunable wettability of liquid crystal/polymer composite films

An electrically tunable wettability in a liquid crystal/ polymer composite film is demonstrated, in which liquid crystal molecules are anchored among polymer grains. The tunable wettability of the composite films originates from the reorientation of the anchored liquid-crystal molecules, which is switched by an in-plane electric field with squared pulses of voltages. These liquid crystal/polymer composite films with electrically tunable wettability have potential applications in polarizer-free displays, ink-jet printing, microfluidic devices, and lab-on-a-chip.

Synthesis and photodimerization in self-assembled monolayers of 7-(8-trimethoxysilyloctyloxy)coumarin

We report a synthesis of 7-(8-trimethoxysilyloctyloxy)coumarin that can self-assemble into optically active monolayers on substrates. Atomic force microscopy and UV–vis spectroscopy have been used to characterize the self-assembly processes and photochemical reaction of the coumarin silane on substrates. We also show that when irradiated with linearly polarized UV light, the self-assembled coumarin monolayer can induce a homogeneous alignment of nematic liquid crystals.