Stable dispersions of silver nanoparticles in carbon dioxide with fluorine-free ligands

Iso-stearic acid, a short, stubby compound with branched, methylated tails has been shown to have high solubility in carbon dioxide. Tail solvation by carbon dioxide makes iso-stearic acid a good choice for use as a ligand to sterically stabilize metallic nanoparticles. Iso-stearic acid coated silver nanoparticles have been stably dispersed in carbon dioxide with hexane cosolvent. Neat carbon dioxide has successfully dispersed iso-stearic acid coated silver nanoparticles that had been deposited on either quartz or polystyrene surfaces. These results are the first reports of sterically stabilized nanoparticles in carbon dioxide without the use of any fluorinated compounds.     

Structure, stability, and quantum conductivity of small diameter silicon nanowires

Structures and energetics of various types of silicon nanowires have been investigated using both quantum and classical molecular dynamics simulations to determine the most stable forms. The tetrahedral type nanowires have been found to be the most stable and, surprisingly, the polycrystalline forms of nanowires, while having the smallest surface to bulk ratio, are found to be the least stable. We also show that the cagelike nanowires have greater thermal stability than the tetrahedral nanowires. Furthermore, their electrical conducting properties are found to be better than those of tetrahedral nanowires, suggesting useful molecular electronic applications.     

Tunable solvation effects on the size-selective fractionation of metal nanoparticles in CO2 gas-expanded solvents

This paper presents an environmentally friendly, inexpensive, rapid, and efficient process for size-selective fractionation of polydisperse metal nanoparticle dispersions into multiple narrow size populations. The dispersibility of ligand-stabilized silver and gold nanoparticles is controlled by altering the ligand tails-solvent interaction (solvation) by the addition of carbon dioxide (CO2) gas as an antisolvent, thereby tailoring the bulk solvent strength. This is accomplished by adjusting the CO2 pressure over the liquid, resulting in a simple means to tune the nanoparticle precipitation by size. This study also details the influence of various factors on the size-separation process, such as the types of metal, ligand, and solvent, as well as the use of recursive fractionation and the time allowed for settling during each fractionation step. The pressure range required for the precipitation process is the same for both the silver and gold particles capped with dodecanethiol ligands. A change in ligand or solvent length has an effect on the interaction between the solvent and the ligand tails and therefore the pressure range required for precipitation. Stronger interactions between solvent and ligand tails require greater CO2 pressure to precipitate the particles. Temperature is another variable that impacts the dispersibility of the nanoparticles through changes in the density and the mole fraction of CO2 in the gas-expanded liquids. Recursive fractionation for a given system within a particular pressure range (solvent strength) further reduces the polydispersity of the fraction obtained within that pressure range. Specifically, this work utilizes the highly tunable solvent properties of organic/CO2 solvent mixtures to selectively size-separate dispersions of polydisperse nanoparticles (2 to 12 nm) into more monodisperse fractions (+/-2 nm). In addition to providing efficient separation of the particles, this process also allows all of the solvent and antisolvent to be recovered, thereby rendering it a green solvent process.     

Cyclotomic numbers and primitive idempotents in the ring GF(q)[x]/(x−1)

Let q be an odd prime power and p be an odd prime with gcd(p,q)=1. Let order of q modulo p be f, and q^f=1+pλ. Here expressions for all the primitive idempotents in the ring R_pⁿ=GF(q)[x]/(x^pn−1), for any positive integer n, are obtained in terms of cyclotomic numbers, provided p does not divide λ if n2. The dimension, generating polynomials and minimum distances of minimal cyclic codes of length pⁿ over GF(q) are also discussed.     

Cogwheel phase cycling

A new method for constructing phase cycles is described. The new schemes apply to experiments involving several consecutive coherence transfer steps. The radiofrequency phases of two or more irradiation blocks are incremented simultaneously, as opposed to the traditional "nested" scheme, in which the block phases are incremented independently. In many cases, the "cogwheel" phase cycles achieve the same selectivity as traditional phase cycles, using fewer steps. Significant time savings are achievable in a wide range of NMR experiments.     

Towards disposable lab-on-a-chip: poly(methylmethacrylate) microchip electrophoresis device with electrochemical detection

A fully disposable microanalytical device based on combination of poly(methylmethacrylate) (PMMA) capillary electrophoresis microchips and thick-film electrochemical detector strips is described. Variables influencing the separation efficiency and amperometric response, including separation voltage or detection potential are assessed and optimized. The versatility, simplicity and low-cost advantages of the new design are coupled to an attractive analytical performance, with good precision (relative standard deviation RSD = 1.68% for n = 10). Applicability for assays of mixtures of hydrazine, phenolic compounds, and catecholamines is demonstrated. Such coupling of low-cost PMMA-based microchips with thick-film electrochemical detectors holds great promise for mass production of single-use micrototal analytical systems.     

The effect of magnetic fields on low frequency oscillating natural convection with pressure gradient

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Design and characterization of hydrogen bonded ferroelectric liquid crystals: A study of light modulation in nematic and smectic orderings

A novel series of hydrogen bonded ferroelectric liquid crystals (HBFLC) have been isolated. Phase transition temperatures obtained by differential scanning calorimetry (DSC) and textural observations elicited by polarizing optical microscope (POM) studies are discussed. Phase diagram is constructed from the above data. Salient feature of the present work is observation of a light modulation due to field induced transitions (FiT) in nematic and smectic C* phases. Tilt angle measurements in various FiT of smectic C* phase are discussed. One of the mesogens in nematic phase behaves like an optical modulator; helical pitch measurements support the above observation.