Film Of 5CB Research Articles

Liquid crystal-based sensor for real-time detection of paraoxon pesticides based on acetylcholinesterase enzyme inhibition

A liquid crystal-based assay (LC) was developedto monitor paraoxon by incorporating a Cu2+ -coated substrate and the inhibitory effect of paraoxon with acetylcholinesterase (AChE). We observed that thiocholine (TCh), a hydrolysate of AChE and acetylthiocholine (ATCh), interfered with the alignment of 5CB films through a reaction between Cu2+ ions and the thiol moiety of TCh. The catalytic activity of AChE was inhibited in the presence of paraoxon due to the irreversible interaction between TCh and paraoxon; consequently, no TCh molecule was available to interact with Cu2+ on the surface. This resulted in a homeotropic alignment of the liquid crystal. The proposed sensor platform sensitively quantified paraoxon with a detection limit of 2.20 ± 0.11 (n = 3) nM within a range of 6 to 500nM. The specificity and reliability of the assay were verified by measuring paraoxon in the presence of various suspected interfering substances and spiked samples. As a result,the sensor based on LC can potentially be used as a screening tool for accurate evaluation of paraoxon and other organophosphorus compounds.

Influence of Substrate Surface Properties on Spin Dewetting, Texture, and Phase Transitions of 5CB Liquid-Crystal Thin Films.

We report how the texture and stability of a nematic liquid-crystal (LC) thin film of 5CB vary as a function of UV-ozone (UVO) exposure of the underlying poly(methyl methacrylate) (PMMA) substrate. UVO exposure of the PMMA substrate not only increases its surface energy, making it more wettable, but also results in the generation of oxygen-containing polar functional groups on the PMMA surface due to photolysis of ester. While the stability of the 5CB films is expectedly enhanced on UVO-exposed PMMA substrates against thermally induced dewetting, the texture of the film also changes as a function of the UV exposure time (tE). We show that the films continue to exhibit the nematic Schlieren texture for tE ≤ 20 min, although the disclination point density (|m|) gradually reduces with an increase in tE. However, the texture changes completely to a spherulite or fanlike texture in tE ≥ 20 min due to enhanced anchoring of the 5CB molecules on the substrates. In addition, enhanced wettability and stronger anchoring by the UVO-exposed PMMA substrates also suppress the tendency of spin dewetting of the 5CB films due to spontaneous rupture of the dispensed solution layer during spin coating, particularly when the solute concentration (Cn) is very low. The latter observation allows possible creation of thinner LC films, which are otherwise difficult to form by spin coating due to enhanced cohesive interactions between the anisotropic LC molecules. Finally, we show that in continuous films, the nematic-to-isotropic (N → I) and I → N phase transitions with gradual heating and cooling remain completely reversible, irrespective of the texture of the film and wettability of the substrate.

Transition from Spin Dewetting to continuous film in spin coating of Liquid Crystal 5CB

Spin dewetting refers to spontaneous rupture of the dispensed solution layer during spin coating, resulting in isolated but periodic, regular sized domains of the solute and is pre-dominant when the solute concentration (Cn) is very low. In this article we report how the morphology of liquid crystal (LC) 5CB thin films coated on flat and patterned PMMA substrate transform from spin dewetted droplets to continuous films with increase in Cn. We further show that within the spin dewetted regime, with gradual increase in the solute concentration, periodicity of the isotropic droplets (λD) as well as their mean diameter (dD), gradually decreases, till the film becomes continuous at a critical concentration (Cn*). Interestingly, the trend that λD reduces with increase in Cn is exact opposite to what is observed in thermal/solvent vapor induced dewetting of a thin film. The spin dewetted droplets exhibit transient Radial texture, in contrast to Schlieren texture observed in elongated threads and continuous films of 5CB, which remains in the Nematic phase at room temperature. Finally we show that by casting the film on a grating patterned substrate it becomes possible to align the spin dewetted droplets along the contours substrate patterns.

Understanding Atomic-Scale Behavior of Liquid Crystals at Aqueous Interfaces.

The ordered environment presented by liquid crystals at interfaces enables a range of novel functionalities that is only now beginning to be exploited in applications ranging from light focusing devices to biosensors. One key feature of liquid crystals is that molecular events occurring at an interface propagate over large distances through the bulk. In spite of their importance, our fundamental understanding of liquid crystal-water and liquid crystal-air interfaces remains limited. In this work, we present results from large-scale atomistic molecular dynamics simulations on the organization of the nematic and isotropic phases of the nitrile-containing mesogenic molecule 4-cyano-4'-pentylbiphenyl (5CB) in the vicinity of vacuum and aqueous interfaces. Hybrid boundary conditions are imposed by confining 5CB films between vacuum and an aqueous medium to examine how those two types of interfaces influence the specific structural arrangement and ordering of 5CB. Consistent with experiments, our results indicate that 5CB exhibits homeotropic anchoring at the vacuum interface, and planar alignment at aqueous interfaces. Two-dimensional molecular dynamics potential of mean force calculations and average polarization densities show that the polar nitrile group of 5CB remains hydrated near the aqueous interface, where it modulates the orientation of water molecules. Estimates of the anchoring strength reveal an oscillatory decay and a semilinear decay with distance from the interface in vacuum and water, respectively.

Predicting the Anchoring of Liquid Crystals at a Solid Surface: 5-Cyanobiphenyl on Cristobalite and Glassy Silica Surfaces of Increasing Roughness

We employ atomistic molecular dynamics simulations to predict the alignment and anchoring strength of a typical nematic liquid crystal, 4-n-pentyl-4'-cyano biphenyl (5CB), on different forms of silica. In particular, we study a thin (~20 nm) film of 5CB supported on surfaces of crystalline (cristobalite) and amorphous silica of different roughness. We find that the orientational order at the surface and the anchoring strength depend on the morphology of the silica surface and its roughness. Cristobalite yields a uniform planar orientation and increases the order at the surface with respect to the bulk whereas amorphous glass has a disordering effect. Despite the low order at the amorphous surfaces, a planar orientation is established with a persistence length into the film higher than the one obtained for cristobalite.

Influence of Specific Anions on the Orientational Ordering of Thermotropic Liquid Crystals at Aqueous Interfaces

We report that specific anions (of sodium salts) added to aqueous phases at molar concentrations can trigger rapid, orientational ordering transitions in water-immiscible, thermotropic liquid crystals (LCs; e.g., nematic phase of 4'-pentyl-4-cyanobiphenyl, 5CB) contacting the aqueous phases. Anions classified as chaotropic, specifically iodide, perchlorate, and thiocyanate, cause 5CB to undergo continuous, concentration-dependent transitions from planar to homeotropic (perpendicular) orientations at LC-aqueous interfaces within 20 s of addition of the anions. In contrast, anions classified as relatively more kosmotropic in nature (fluoride, sulfate, phosphate, acetate, chloride, nitrate, bromide, and chlorate) do not perturb the LC orientation from that observed without added salts (i.e., planar orientation). Surface pressure-area isotherms of Langmuir films of 5CB supported on aqueous salt solutions reveal ion-specific effects ranking in a manner similar to the LC ordering transitions. Specifically, chaotropic salts stabilized monolayers of 5CB to higher surface pressures and areal densities (12.6 mN/m at 27 Å(2)/molecule for NaClO(4)) and thus smaller molecular tilt angles (30° from the surface normal for NaClO(4)) than kosmotropic salts (5.0 mN/m at 38 Å(2)/molecule with a corresponding tilt angle of 53° for NaCl). These results and others reported herein suggest that anion-specific interactions with 5CB monolayers lead to bulk LC ordering transitions. Support for the proposition that these ion-specific interactions involve the nitrile group was obtained by using a second LC with nitrile groups (E7; ion-specific effects similar to 5CB were observed) and a third LC with fluorine-substituted aromatic groups (TL205; weak dipole and no ion-specific effects were measured). Finally, we also establish that anion-induced orientational transitions in micrometer-thick LC films involve a change in the easy axis of the LC. Overall, these results provide new insights into ionic phenomena occurring at LC-aqueous interfaces, and reveal that the long-range ordering of LC oils can amplify ion-specific interactions at these interfaces into macroscopic ordering transitions.

Adsorbate-Induced Ordering Transitions of Nematic Liquid Crystals on Surfaces Decorated with Aluminum Perchlorate Salts

We report an investigation of interfacial physicochemical phenomena underlying adsorbate-induced ordering transitions in nitrile-containing liquid crystals (LCs) supported on surfaces coated with metal perchlorate salts. When the mass density of Al(ClO4)3 deposited onto a surface was low (0.39 ± 0.03 ng/mm2), we measured 20 μm thick films of nematic 4-cyano-4′-n-pentyl-biphenyl (5CB) to initially exhibit perpendicular (homeotropic) ordering, consistent with the influence of coordination interactions between the nitrile groups of 5CB and Al3+ ions on the surface on the ordering of the LC. Furthermore, exposure of freshly prepared LC films to vapors of an adsorbate that coordinates strongly to Al3+ ions (dimethylmethylphosphonate, DMMP) triggered an ordering transition in the LC films, supporting our conclusion that the initial perpendicular orientation of the LC was induced by nitrile-Al3+ coordination interactions. Subsequent equilibration of the LC on the surface (hours), however, resulted in a slow, time-dependent ordering transition in the absence of DMMP that corresponded to the tilting of the LC away from the surface normal. Measurements of the solubility of Al(ClO4)3 in nematic 5CB (saturation value of 1.7 μmol/mL) supported our hypothesis that the slow ordering transition observed in the absence of DMMP was due to the loss of metal ions from the surface into the LC film (dissolution). In contrast, the solubilization capacity of a 20 μm thick film of 5CB was determined to be insufficient to dissolve 2.14 ± 0.24 ng/mm2 of the salt from a surface, and we measured the homeotropic ordering of nematic films of 5CB on these surfaces to persist for days. Equilibration of these samples, however, was accompanied by a loss of response to DMMP (perpendicular orientation of the LC before and after exposure to DMMP). Control experiments performed with a noncoordinating metal perchlorate salt (sodium perchlorate) confirmed our proposition that the loss of response to DMMP was due to formation of an electrical double layer that promoted perpendicular ordering of the LC and thus masked the effects of changes in coordination interactions induced by DMMP on the ordering of the LC (the electric field of the double layer promotes the perpendicular orientation of the 5CB). Finally, by coating surfaces with Al(ClO4)3 at loadings that were intermediate to those reported above (1.13 ± 0.09 ng/mm2), we observed (i) perpendicular ordering of the LC in the absence of DMMP, and (ii) reversible ordering transitions induced by DMMP during storage of samples over 4−5 days. These results, when combined, indicate that the ordering of the LC on the metal-salt-decorated surfaces is strongly dependent on the loading of metal salt, with key interfacial physicochemical processes being metal ion−nitrile coordination interactions, dissolution of salt into the LC, and formation of electrical double layers. The results of this study provide guidance for the design of LC films that respond to specific chemical analytes and suggest principles for passive chemical sensors.

Real-time liquid crystal-based glutaraldehyde sensor

Abstract We develop a liquid crystal (LC)-based optical sensor for detecting glutaraldehyde vapor by using real-time orientational responses of 4-cyano-4′-pentylbiphenyl (5CB) to glutaraldehyde. The sensor comprises a thin layer of 5CB (∼20 μm) supported on an amine-decorated surface. When a glutaraldehyde molecule diffuses through the layer of 5CB, it reacts with a surface amine group and forms an imine bond. Subsequently, the resulting free aldehyde group from the glutaraldehyde molecule triggers an orientational transition of 5CB, which manifests as an optical signal visible to the naked eye. Because the performance of the LC-based sensor critically depends on the diffusion of glutaraldehyde in 5CB, we also measured the solubility and diffusivity of glutaraldehyde in 5CB. Based on the diffusivity, an optimal LC sensor with a 20-μm-thick film of 5CB can respond to 207 ppmv glutaraldehyde within 20 s and shows a full response in 4 min. It also shows good specificity. Only a very weak response is observed when this sensor is exposed to acetic acid, and no response is recorded for methanol, ethanol or formaldehyde.

Chirality transfer in nematic liquid crystals doped with (S)-naproxen-functionalized gold nanoclusters: an induced circular dichroism study

In a recent study, we found that nematic liquid crystals (N-LCs) doped with chiral (S)-naproxen-functionalized dodecane thiolate protected gold nanoclusters (Au2, Au3) or non-chiral alkyl thiolate protected Au clusters (Au1) produce thin film textures with characteristic uniform stripe patterns separated by areas of homeotropic alignment. While these textures closely resemble textures commonly observed for chiral nematic (N*-)phases with large helical pitch, so-called cholesteric finger textures, they originate from local concentration differences of the nanoclusters in the N-LC solvent. While areas with higher particle content form linear particle aggregates (stripe domains) due to the surface anchoring of the N-LC molecules to the cluster surface, areas of lower particle concentration give homeotropic alignment as a result of particles residing at the glass–N-LC interfaces. To elucidate and confirm a chirality transfer from the chirally modified gold clusters to the non-chiral N-LC, despite the complex thin film textures, we here present detailed induced circular dichroism (ICD) studies of thin films of 5CB doped with the three different Au clusters. These experiments revealed that the chiral Au nanoclusters (Au2, Au3) successfully transfer chirality to the N-LC host producing a chiral nematic phase (N*) with the opposite helical sense in comparison to the pure, organic chiral dopant dispersed in the same N-LC host. Thus, these results provide the first experimental proof for the usefulness of gold nanoclusters as chiral dopants for N-LCs. In contrast, for the non-chiral Au cluster, at a macroscopic level, no relationship between the cholesteric finger-like textures and chirality was found.

Anchoring and electro-optical dynamics of thin liquid crystalline films in a polyimide cell: Experiment and theory

The surface-dependent anchoring and electro-optical (EO) dynamics of thin liquid crystalline films have been examined using Fourier transform infrared spectroscopy. A simple nematic liquid crystal, 4-n-pentyl-4'-cyanobiphenyl (5CB), is confined as 40, 50, and 390 nm thick films in nanocavities defined by gold interdigitated electrode arrays (IDEAs) patterned on polyimide-coated zinc selenide (ZnSe) substrates [Noble et al., J. Am. Chem. Soc. 124, 15020 (2002)]. New strategies for controlling the anchoring interactions and EO dynamics are explored based on coating a ZnSe surface with an organic polyimide layer in order to both planarize the substrate and induce a planar alignment of the liquid crystalline film. The polyimide layer can be further treated so as to induce a strong alignment of the nematic director along a direction parallel to the electrode digits of the IDEA. Step-scan time-resolved spectroscopy measurements were made to determine the rate constants for the electric-field-induced orientation and thermal relaxation of the 5CB films. In an alternate set of experiments, uncoated ZnSe substrates were polished unidirectionally to produce a grooved surface presenting nanometer-scale corrugations. The dynamical rate constants measured for several nanoscale film thicknesses and equilibrium organizations of the director in these planar alignments show marked sensitivities. The orientation rates are found to vary strongly with both the magnitude of the applied potential and the initial anisotropy of the alignment of the director within the IDEA. The relaxation rates do not vary in this same way. The marked variations seen in EO dynamics can be accounted for by a simple coarse-grained dynamical model.

Orientational order of 4′- n-pentyl-4-cyanobiphenyl molecules deposited on azobenzene monolayers

Using optical second harmonic generation (SHG) and polarized absorption (PA) measurements, the n-th orientational order parameters S n ( n = 1, 2, 3) of 4′- n-pentyl-4-cyanobiphenyl (5CB) films were determined whilst 5CB molecules were deposited onto silica substrate coated with a trans- or cis-form 6Az10PVA (azobenzene side-chain polymer) monolayer by evaporation method. The experimental results clearly show that the orientational order of 5CB film depends on the form of surface azobenzene monolayer, especially at the beginning of 5CB deposition. It was also found that the orientational order saturates when monolayer of 5CB film is deposited on the azobenzene monolayer.

Electric field effects in freely suspended liquid crystalline (FSLC) films of 5CB

Electric field effects on FSLC films of p-pentyl cyano biphenyl (5CB) have been optically studied. Due to the initial alignment of the molecules, initially formed transition layers grow with increasing field and merge to form a linear wall at the middle of the sample. This linear wall splits into two disclinations at relatively high fields. But a sudden application of relatively high electric field results in the formation of closed metastable walls. The wall orientation shows interesting behaviour with the field frequency. At high field, the closed walls undergo distortion and division into two or more parts.

Thermal Fluctuations of Disclination Lines in a Thin Nematic Film

Polarisation microscopy and dynamic light scattering were used to study the dynamical behaviour of disclination lines in a thin nematic film of 5CB. Our experiments show that the lines exhibit thermally excited diffusive motion with the ends of disclination lines being pinned to the surface. In the dynamic light scattering experiments a process with a broad distribution of relaxation times was observed, which was due to the diffusion of the disclination lines with different lengths and orientations. Polarisation microscopy was used to follow the time dependence of the displacement of a single disclination line, which enabled the measurements of the diffusion coefficient and the line tension of the disclination lines in the samples.

An Experimental System for Imaging the Reversible Adsorption of Amphiphiles at Aqueous−Liquid Crystal Interfaces

We report an experimental system that permits optical imaging of the reversible adsorption of amphiphiles at stable, planar interfaces formed between aqueous phases and immiscible thermotropic liquid crystals. Copper grids (hole sizes of 19−292 μm and thicknesses of 18−20 μm) supported on glass surfaces treated using octadecyltrichlorosilane were impregnated with nematic 4-cyano-4‘-pentylbiphenyl (5CB). Films of 5CB confined within the grids were stable (did not dewet the grid) to immersion into either water, aqueous solutions of sodium dodecyl sulfate (SDS), or aqueous solutions of SDS containing NaCl. Whereas the anchoring of 5CB on the copper grid dominated the appearance of the 5CB when using grids with hole sizes of 19 μm, reversible changes in the orientation of the liquid crystal (observed using polarized light) caused by adsorption of SDS at the liquid crystal−aqueous interface were readily observed when using grids with hole sizes larger than 19 μm. With increasing concentrations of SDS in the aqueous phase, a series of reproducible and distinct surface-driven distortions were observed within the 5CB confined to the grid. We also observed the effects of added NaCl on the distortions induced within the liquid crystal to be consistent with increased adsorption of SDS caused by screening of the electrostatic interactions between the SDS adsorbed at the interface. This result suggests that the orientation of the 5CB is influenced by the areal density of SDS molecules at the liquid crystal−aqueous interface (probably through steric interactions). Because the aqueous phase contacting the liquid crystal can be exchanged (thus permitting the addition and removal of reactants), this experimental system is a simple and broadly useful one for investigations in which liquid crystals are used to amplify interfacial phenomena at fluid interfaces into optical images.

Self-Assembled Monolayers of BPS Control the Molecular Aggregation of Polymer-Dispersed Liquid-Crystal Films of 5CB

Self-Assembled Monolayers of BPS Control the Molecular Aggregation of Polymer-Dispersed Liquid-Crystal Films of 5CB

Steady-State and Time-Resolved Fluorescence Analysis for a Cyanobiphenyl Mesogen in Polymer-Dispersed Liquid Crystal Films

Steady-state and time-resolved fluorescence behavior of 4-cyano-4‘-pentylbiphenyl (5CB) in polymer-dispersed liquid crystal (PDLC) films sandwiched between two quartz substrates was characterized by the surface-limited excitation (SLE) and through-film excitation (TFE) methods, which were used to discriminate the emissions from the interface layer with the substrate and from the interior domain. Although PDLC films showed both the monomer and excimer emissions of 5CB, SLE gave a substantially greater contribution of the former in the fluorescence than TFE. By contrast, a neat nematic film of 5CB showed exclusively the excimer emission independently of the excitation methods. Time-resolved analysis of the excimer emission from PDLC films revealed double-exponential decay profiles with shorter-lived (1.5−1.8 ns) and longer-lived (12−17 ns) components, in which the former is dominant for the excimer emission obtained by SLE, while the latter takes a major part for that obtained by TFE. By contrast, the excim...

Free-surface effects on the response of nematic liquid crystal films to a laser pulse

Abstract The dynamic response of 5CB films with a free surface to a laser pulse is investigated. A magnetic field above the Freedericksz transition is applied initially to induce a starting angle on the molecular orientation. A single 1·06 μm laser pulse with e −1 pulse width 0·2 ms is incident normally upon the films. When the laser pulse polarization is perpendicular to the magnetic field, only a thermal effect is involved. While it is parallel to the magnetic field, both molecular orientation and thermal effects are involved. The results from films with a free surface are compared with those from films sandwiched between two glass substrates. The free surface effect, beam size effect, and thermal effect are discussed by comparing with theoretical analysis.