Potentiometric evaluation of solvent polymeric carbonate-selective membranes based on molecular tweezer-type neutral carriers

Potentiometric properties of the ion-selective electrodes) based on highly plasticized PVC membranes doped with the carbonate-selective cholic acid (CA) derivatives have been measured. The carbonate-selective neutral carriers have been prepared by coupling one to three trifluoroacetobenzoyl (TFAB) groups to a cholic acid derivative which has three hydroxyl linkers lining on the C3, C7, and C12 positions of its rigid steroidal ring structure. The membranes based on cholic acid derivatives with two TFABs [3,7-bis(TFAB)CA, 3,12-bis(TFAB)CA, and 7,12-bis(T-FAB)CA] exhibited remarkably improved carbonate selectivity, indicating that the bis(FAB)CAs behave like molecular tweezers for the carbonate ion. For example, 3,12-bis(TFAB)CA resulted in 10-300-fold-enhanced carbonate selectivity over other anions (e.g., salicylate, ClO4-, SCN-, (HPO4)2-, NO3-, NO2-, Br-, and Cl-) compared to that of the neutral carriers with a single TFAB group. The distances between the carbonate binding centers of bis(TFAB)CAs, i.e., the carbonyl carbons of the two TFAB groups, are in the 7.3-7.9-A range at the AM1 level semiempirical calculation, which is too far for the carbonate ion to form direct covalent bonding. The fast atom bombardment mass spectra of bis(TFAB)CAs show that significant fractions of the compounds are either mono- or dihydrated before complexing the carbonate ion. These findings seem to suggest that bis(TFAB)CAs recognize the incoming carbonate ion by forming both covalent and hydrogen bonding between the hydrated and unhydrated TFAB groups. The analytical utility of the carbonate-selective electrode based on 3,12-bis(TFAB)-CA has been demonstrated by measuring the total carbon dioxide in human serum in the presence of lipophilic anion interferents, e.g., salicylate.     

Studying the orientation of bio-objects by nematic liquid crystals

We have studied the ability of a liquid-crystal (LC) matrix to visualize and orient DNA molecules. It is established that the relief of the interface between the LC mesophase and conducting contact can be improved without using an additional high-ohmic polymer layer. Spectroscopic and ellipsometric techniques revealed changes in the refractive properties and structure of composites. The obtained results can be used in creating devices for rapid DNA testing with retained form of biostructures.     

Quantification of ion trapping effect of carbon nanomaterials in liquid crystals

We have determined the ion trapping coefficients of carbon nanomaterials in liquid crystals through the analysis of capacitance-voltage response. The ion trapping coefficient is defined as the ability to trap impurity ions for each carbon nanomaterial and can be used as a referable value to select proper carbon nanomaterials as dopants in liquid crystals. The characterized nanomaterials include multi-wall carbon nanotubes, carbon nanofibers, carbon nanocoils, single-wall carbon nanotubes and fullerenes, respectively. It is found that the multi-wall carbon nanotubes with a tube length of 5-15 μm have the largest ion trapping coefficient and hence serve as the best dopant to suppress the liquid crystal hysteresis.     

Phase agile reflectarray cells based on liquid crystals

Experimental results at X-band are used to compare the electromagnetic scattering from a printed reflectarray cell, which is constructed on 500 mum thick layers of three different nematic state liquid crystals. It is shown that a small voltage can be used to vary the permittivity of the tunable substrate and thereby control the phase of the reflected signals. Numerical results using Ansoft HFSS are compared with the measured phase, resonant frequencies and signal attenuation for two orientations of the liquid crystal molecules. Data fitting is employed to quantify the loss tangent and the permittivity values of the three anisotropic specimens. The performance trade-offs that are imposed by the use of commercially available materials are discussed, and the computer model is used to specify the electrical properties of a liquid crystal mixture, which can provide a signal loss of < 1 dB and a dynamic phase range of 300deg from the patch elements at 10 GHz.     

Intracellular magnesium ion selective microelectrode based on a neutral carrier

A magnesium ion selective microelectrode based on a synthetic neutral carrier is presented. The selectivity of Mg2+ over Na+, K+, H+, and Ca2+ is sufficient for assays of intracellular magnesium ion activities. The microelectrodes with an optimized membrane composition have a resistance of about 5 x 10(10) omega and a 90% response time of less than or equal to 3 s for a tip diameter around 1 microns. The lifetime of the microelectrode cell assembly is longer than 1 week and the emf drift after equilibration is less than or equal to 0.3 mV/h.     

Structural ion selectivity of thia crown ether compounds with a bulky block subunit and their application as an ion-sensing component for an ion-selective electrode

Several 14-membered thia crown ether ionophores having a bulky block subunit were synthesized, and their chemical structures and ion selectivities were examined in detail when these compounds were used as an ion-sensing component of an ion-selective electrode. The ionophores of both cyclic and noncyclic thia ethers exhibited a high selectivity for silver ion (Ag(+)), in which the sulfur atom in the ionophore molecule plays a role as the effective coordination donor site for the silver ion. The best Ag(+)-selective electrode was prepared with the 14-membered thia crown ether having one sulfur atom, three oxygen atoms, and a bulky pinan subunit. The ion selectivity of this electrode for Ag(+) was over 10(4) times that for other metal cations. In the case where the sulfide in the thia ether ionophore was changed to sulfoxide by oxidation, ion selectivity for mercury ion became higher; therefore, the sulfoxide was found to be an effective coordination site for the mercury ion. The ion selectivity features of noncyclic sulfide, sulfoxide, and sulfone were also examined and compared with the results of the cyclic and noncyclic thia ethers.     

Control of molecular weight selectivity in electrode modifications based on phase-inversion cellulose acetate membranes

Modified electrodes with size selectivity are required for electrochemical blosensors and for analysis of biological samples. The phase-inversion process for making reverse osmosis membranes is used to make modified electrodes with the goals of decreasing thickness, improving response time, and understanding the influence of both casting solution composition and casting conditions on the resultant membrane's porosity. Poly(ethylene glycol) esters of ferrocene-carboxylic acid are used for the determination of the membrane's response time, selectivity, and structure. It is shown that there is an optimum composition range of greater than 95% acetone, less than 2% cellulose acetate, and less than 5% aqueous Mg(CIO4)2. This composition range gives the minimum response time (greater than 90% of maximum signal is obtained in less than 7 min), maximum reproducibility (less than 20% relative error), and best size selectivity in the few thousand daltons range.     

Modeling of divalent/monovalent ion selectivity of ion-exchanger-based solvent polymeric membranes doped with coexchanger

It is shown that the addition of a coexchanger does not always improve the selectivity of an ion-selective electrode. As a model case, the selectivity toward divalent primary ions I2+ in the presence of monovalent interference J+ is numerically simulated for membranes based on ion-exchanger R- and doped with a coexchanger (ionic additive) S+. It is, for the first time, demonstrated that the addition of a coexchanger, together with a high mobility of ion-exchanger sites, may be either beneficial or harmful to selectivity. Accordingly, two distinctive representatives of systems, i.e., membrane/solution are discussed. The theory is illustrated with experimental data obtained from PVC membranes based on calcium bis(2ethylhexyl)-phosphate plasticized with tris(2ethylhexyl)phosphate and doped with tetradecylammonium. Electrodes with these membranes exhibit both types of selectivity behavior. Interference from lithium with a calcium response may be sufficiently decreased by the addition of tetradecylammonium, while the interference from potassium and cesium was increased in the doped membranes.     

Resolution in optically addressed spatial light modulators based on dye-doped liquid crystals

Dye-doped nematic liquid crystals (LCs) are studied as materials for single-layer optically addressed spatial light modulators. The dopant is 2,5-azo-substituted anthraquinone (ASAQ) dye. The resolution in the ASAQ-doped LC systems does not depend on the device thickness (in the 5-125 microm range). The efficiency increases with the increase of the thickness and begins to saturate in devices thicker than 40 microm. The limiting resolution in the thick devices is 400 line pairs per millimeter. The limitations of performance (efficiency and resolution) in the studied systems are discussed.     

Orientations of liquid crystals on mechanically rubbed films of bovine serum albumin: a possible substrate for biomolecular assays based on liquid crystals

We report the uniform planar anchoring of thermotropic liquid crystals on films of bovine serum albumin (BSA) covalently immobilized on the surface of glass microscope slides and mechanically rubbed using a cloth. The azimuthal orientations of the liquid crystals were measured to be parallel to the direction of rubbing. Following immersion and removal of these rubbed films of BSA from aqueous solutions containing either BSA, fibrinogen, lysozyme, anti-FITC immunoglobulin G (IgG), or antistreptavidin IgG, we measured liquid crystals placed onto these surfaces to largely retain their uniform alignment. In contrast, following immersion of a rubbed film of BSA into an aqueous solution of anti-BSA IgG, we observed liquid crystals on these surfaces to assume nonuniform orientations. We conclude that specific binding of anti-BSA IgG to the film of rubbed BSA erased anisotropy induced within the film of BSA by rubbing. This result suggests that the spatial scale of anisotropy within the rubbed film of BSA is comparable to or smaller than the size of the IgG molecule. Because the anisotropy within a rubbed film of a protein can be erased by specific binding of a second protein, we believe these types of substrates (rubbed films of proteins) have the potential to be useful in a variety of label-free biomolecular assays where specific binding of a target species to its ligand can be imaged through observation of the optical appearance of liquid crystal placed onto the surface.     

All-optical switchable holographic Fresnel lens based on azo-dye-doped polymer-dispersed liquid crystals

Fabrication of an all-optical switchable holographic liquid crystal (LC) Fresnel lens based on azo-dye-doped polymer-dispersed LCs is reported using a Michelson interferometer. It is found that, upon circularly polarized photoirradiation, the diffraction efficiency of the fabricated Fresnel lens was increased significantly in a reversible manner. We believe this is due to the anisotropy induced by reorientation of the LC molecules coupled with azo-dye molecule orientation due to trans-cis-trans photoisomerization, which modulates the refractive index of the LC-rich regions. We also studied the effect of azo dye on the polarization dependency of the fabricated lens.     

Role of metal ion incorporation in ion exchange resin on the selectivity of fluoride

Indion FR 10 resin has sulphonic acid functional group (H(+) form) possesses appreciable defluoridation capacity (DC) and its DC has been enhanced by chemical modification into Na(+) and Al(3+) forms by loading respective metal ions in H(+) form of resin. The DCs of Na(+) and Al(3+) forms were found to be 445 and 478 mg F(-)/kg, respectively, whereas the DC of H(+) form is 265 mg F(-)/kg at 10 mg/L initial fluoride concentration. The nature and morphology of sorbents are characterized using FTIR and SEM analysis. The fluoride sorption was explained using the Freundlich, Langmuir and Redlich-Peterson isotherms and kinetic models. The calculated thermodynamic parameters such as DeltaG degrees, DeltaH degrees, DeltaS degrees and sticking probability (S(*)) explains the nature of sorption. Comparison was also made by the elution capacity of these resins in order to select a cost effective material. A field trial was carried out to test the suitability of the resins with fluoride water collected from a nearby fluoride-endemic area.     

Low-frequency dielectric relaxation in 5TsB liquid crystals controlled by the ion adsorption-desorption processes

Stable liquid-crystalline states with various ε′ and ρ values were obtained in planar-oriented 5TsB liquid crystals by exposure to constant and alternating electric fields. The initial portion of the low-frequency dispersion of ε′ and ε″ obeys the Debye equation for both strictly planar and nonplanar (“oblique”) orientation of molecules.