Optimization of Brownian ratchets for the manipulation of charged components within supported lipid bilayers

In probability theory, there is a counter-intuitive result that it is possible to construct a winning strategy from two individually losing (or at most breaking-even) "games" by alternating between them. The work presented here demonstrates the application of this principle to supported lipid bilayers (SLBs) in order to create directed motion of charged lipid components in the membrane, which was achieved through the use of "Brownian ratchets" in patterned SLBs. Both a finite element analysis model and an experimental setup have been used to investigate the role of key parameters for the operation of these ratchets: (1) the asymmetry of the ratchet teeth and (2) the relation of the ratchet height to the period of the applied electric field. Importantly, we find that the efficiency of the ratchet for a given charged species is dependent on the diffusion coefficient. This opens the possibility for separation of membrane species according to their size or viscous drag coefficient within the membrane

New poly(amino acid methacrylate) brush supports the formation of well-defined lipid membranes

A novel poly(amino acid methacrylate) brush comprising zwitterionic cysteine groups (PCysMA) was utilized as a support for lipid bilayers. The polymer brush provides a 12-nm-thick cushion between the underlying hard support and the aqueous phase. At neutral pH, the zeta potential of the PCysMA brush was ∼-10 mV. Cationic vesicles containing >25% DOTAP were found to form a homogeneous lipid bilayer, as determined by a combination of surface analytical techniques. The lipid mobility as measured by FRAP (fluorescence recovery after photobleaching) gave diffusion coefficients of ∼1.5 μm2 s-1, which are comparable to those observed for lipid bilayers on glass substrates

A Stokes polarimetric light microscopy view of liquid crystal droplets

The optical characteristics of materials, such as their magnetooptical effects, birefringence, optical activities, linear and circular dichroism, are probed via the polarisation states of light transmitted through or reflected from the specimens. As such, the measurements of the polarisation states play an important role in many research disciplines. Experimentally, Stokes parameters provide a full description of the polarisation states of light. We report the implementation of a dual- photoelastic modulator based polarimeter in a light microscope, enabling the determination of Stokes parameters at each pixel. As a case study, polarimetric images of liquid crystal droplets of different internal structures are obtained, showing their distinct polarisation characteristics. We demonstrate that the prototype Stokes polarimetric microscope allows the quantitative determination of the polarisation characteristics of light at the object plane and enables the access of the information of full polarisation states as compared to a conventional cross polariser microscope. This work shows that Stokes polarimetric microscopy may find potential applications in a wide range of research fields

Efficient and long-lived quantum memory with cold atoms inside a ring cavity

Quantum memories are regarded as one of the fundamental building blocks of linear-optical quantum computation and long-distance quantum communication. A long standing goal to realize scalable quantum information processing is to build a long-lived and efficient quantum memory. There have been significant efforts distributed towards this goal. However, either efficient but short-lived or long-lived but inefficient quantum memories have been demonstrated so far. Here we report a high-performance quantum memory in which long lifetime and high retrieval efficiency meet for the first time. By placing a ring cavity around an atomic ensemble, employing a pair of clock states, creating a long-wavelength spin wave, and arranging the setup in the gravitational direction, we realize a quantum memory with an intrinsic spin wave to photon conversion efficiency of 73(2)% together with a storage lifetime of 3.2(1) ms. This realization provides an essential tool towards scalable linear-optical quantum information processing.Comment: 6 pages, 4 figure

Production of Giant Unilamellar Vesicles and Encapsulation of Nematic Lyotropic Liquid Crystals

We describe a modified microfluidic method for making Giant Unilamellar Vesicles (GUVs)viawater/octanol-lipid/water double emulsion droplets. At a high enough lipid concentration we show that thede-wetting of the octanol from these droplets occurs spontaneously (off-chip) without the need to useshear to aid the de-wetting process. The resultant mixture of octanol droplets and GUVs can beseparated by making use of the buoyancy of the octanol. A simpler microfluidic device and pumpsystem can be employed and, because of the higher flow-rates and much higher rate of formation ofthe double emulsion droplets (B1500 s 1compared to up toB75 s 1), it is easier to make largernumbers of GUVs and larger volumes of solution. Because of the potential for using GUVs thatincorporate lyotropic nematic liquid crystals in biosensors we have used this method to make GUVs thatincorporate the nematic phases of sunset yellow and disodium chromoglycate. However, the phasebehaviour of these lyotropic liquid crystals is quite sensitive to concentration and we found that there isan unexpected spread in the concentration of the contents of the GUVs obtained

Textures of Nematic Liquid Crystal Cylindric-Section Droplets Confined by Chemically Patterned Surfaces

The director fields adopted by nematic liquid crystals (LCs) that are confined by the surface to form long, thin droplets are investigated using polarising optical microscopy. Samples are produced by de-wetting of the LC on a surface patterned with alternating high-surface energy and low-surface energy stripes of 10–30 μm width. The droplets obtained are expected to adopt a profile which is that of a longitudinal section of a cylinder and, as this suggests, the director fields observed are variants in the case where the LC is constrained in a cylindrical capillary or fibre. Hence, when there is normal anchoring at the air interface, the textures observed are related to the well-known escaped radial texture (for the nematic LC mixture E7) or plane polar texture (for the LC mixture MLC6609). More surprising is the observation that the nematic LC mixture MLC7023, which is anchored in a planar or tilted manner at the air interface, also gives what appears to be an escaped radial director field. As an exploration of the possibility of using these systems in creating sensors, the effects of adding a chiral dopant and of adding water to the substrates are also investigated

The Control of Director Fields in Phospholipid-Coated Liquid Crystal Droplets

In liquid crystal (LC) droplets, small changes in surface anchoring energy can produce large changes in the director field which result in readily detectable optical effects. This makes them attractive for use as biosensors. Coating LC droplets with a phospholipid monolayer provides a bridge between the hydrophobic world of LCs and the water-based world of biology and makes it possible to incorporate naturally occurring biosensor systems. However, phos-pholipids promote strong perpendicular (homeotropic) anchoring that can inhibit switching of the director field. We show that the tendency for phospholipid layers to promote perpendicular anchoring can be suppressed by using syn-thetic phospholipids in which the acyl chains are terminated with bulky tert-butyl or ferrocenyl groups; the larger these end-group(s), the less likely the system is to be perpendicular/radial. Additionally, the droplet director field is found to be dependent on the nature of the LC, particularly its intrinsic surface properties; but not (apparently) on the sign of the dielectric anisotropy, the proximity to the melting/isotropic phase transition, the surface tension (in air) or the values of the Frank elastic constants

Application of Graphene within Optoelectronic Devices and Transistors

Scientists are always yearning for new and exciting ways to unlock graphene's true potential. However, recent reports suggest this two-dimensional material may harbor some unique properties, making it a viable candidate for use in optoelectronic and semiconducting devices. Whereas on one hand, graphene is highly transparent due to its atomic thickness, the material does exhibit a strong interaction with photons. This has clear advantages over existing materials used in photonic devices such as Indium-based compounds. Moreover, the material can be used to 'trap' light and alter the incident wavelength, forming the basis of the plasmonic devices. We also highlight upon graphene's nonlinear optical response to an applied electric field, and the phenomenon of saturable absorption. Within the context of logical devices, graphene has no discernible band-gap. Therefore, generating one will be of utmost importance. Amongst many others, some existing methods to open this band-gap include chemical doping, deformation of the honeycomb structure, or the use of carbon nanotubes (CNTs). We shall also discuss various designs of transistors, including those which incorporate CNTs, and others which exploit the idea of quantum tunneling. A key advantage of the CNT transistor is that ballistic transport occurs throughout the CNT channel, with short channel effects being minimized. We shall also discuss recent developments of the graphene tunneling transistor, with emphasis being placed upon its operational mechanism. Finally, we provide perspective for incorporating graphene within high frequency devices, which do not require a pre-defined band-gap.Comment: Due to be published in "Current Topics in Applied Spectroscopy and the Science of Nanomaterials" - Springer (Fall 2014). (17 pages, 19 figures

Plasmonic band-edge modulated surface-enhanced Raman scattering

The band structure of surface plasmon polaritons (SPPs) on the Ag surface in the presence of gratings and SPP-based surface-enhanced Raman scattering (SERS) are investigated theoretically and experimentally. The SPP bandgap position can be tuned by geometric parameters. The SPP band edge dominates the SERS behavior. The template stripping process is introduced to reduce SPP propagation losses, improving SERS sensitivity by ˜40. Apart from flexibility and a moderate SERS enhancement factor of the order of 10⁵–10⁶, the SPP band structure is highly reproducible with a relative standard deviation of 10.9%. Our results open opportunities for SPP band structures to serve as SERS substrates

The stellar and sub-stellar IMF of simple and composite populations

The current knowledge on the stellar IMF is documented. It appears to become top-heavy when the star-formation rate density surpasses about 0.1Msun/(yr pc^3) on a pc scale and it may become increasingly bottom-heavy with increasing metallicity and in increasingly massive early-type galaxies. It declines quite steeply below about 0.07Msun with brown dwarfs (BDs) and very low mass stars having their own IMF. The most massive star of mass mmax formed in an embedded cluster with stellar mass Mecl correlates strongly with Mecl being a result of gravitation-driven but resource-limited growth and fragmentation induced starvation. There is no convincing evidence whatsoever that massive stars do form in isolation. Various methods of discretising a stellar population are introduced: optimal sampling leads to a mass distribution that perfectly represents the exact form of the desired IMF and the mmax-to-Mecl relation, while random sampling results in statistical variations of the shape of the IMF. The observed mmax-to-Mecl correlation and the small spread of IMF power-law indices together suggest that optimally sampling the IMF may be the more realistic description of star formation than random sampling from a universal IMF with a constant upper mass limit. Composite populations on galaxy scales, which are formed from many pc scale star formation events, need to be described by the integrated galactic IMF. This IGIMF varies systematically from top-light to top-heavy in dependence of galaxy type and star formation rate, with dramatic implications for theories of galaxy formation and evolution.Comment: 167 pages, 37 figures, 3 tables, published in Stellar Systems and Galactic Structure, Vol.5, Springer. This revised version is consistent with the published version and includes additional references and minor additions to the text as well as a recomputed Table 1. ISBN 978-90-481-8817-