No abstract

Gold nanoparticles (GNPs) are often used as colloidal carriers in numerous applications owing to their low-cost and size-controlled preparation as well as their straightforward surface functionalization with thiol containing molecules forming self-assembling monolayers (SAM). The quantification of the ligand density of such modified GNPs is technically challenging, yet of utmost importance for quality control in many applications. In this contribution, a new method for the determination of the surface coverage of GNPs with thiol containing ligands is proposed. It makes use of the measurement of the gold-to-sulfur (Au/S) ratio by inductively coupled plasma mass spectrometry (ICP–MS) and its dependence on the nanoparticle diameter. The simultaneous ICP–MS measurement of gold and sulfur was carefully validated and found to be a robust method with a relative standard uncertainty of lower than 10%. A major advantage of this method is the independence from sample preparation; for example, sample loss during the washing steps is not affecting the results. To demonstrate the utility of the straightforward method, GNPs of different diameters were synthesized and derivatized on the surface with bifunctional (lipophilic) ω-mercapto-alkanoic acids and (hydrophilic) mercapto-poly(ethylene glycol) (PEG)n-carboxylic acids, respectively, by self-assembling monolayer (SAM) formation. Thereby, a size-independent but ligand-chain length-dependent ligand density was found. The surface coverage increases from 4.3 to 6.3 molecules nm–2 with a decrease of ligand chain length from 3.52 to 0.68 nm. Furthermore, no significant difference between the surface coverage of hydrophilic and lipophilic ligands with approximately the same ligand length was found, indicating that sterical hindrance is of more importance than, for example, intermolecular strand interactions of Van der Waals forces as claimed in other studies.

No abstract

In an attempt to overcome the limited applicability scope of earlier proposed Cinchona alkaloid-based chiral weak anion exchangers (WAX) and recently reported aminosulfonic acid-based chiral strong cation exchangers (SCX), which are conceptionally restricted to oppositely charged solutes, their individual chiral selector (SO) subunits have been fused in a combinatorial synthesis approach into single, now zwitterionic, chiral SO motifs. The corresponding zwitterionic ion-exchange-type chiral stationary phases (CSPs) in fact combined the applicability spectra of the parent chiral ion exchangers allowing for enantioseparations of chiral acids and amine-type solutes in liquid chromatography using polar organic mode with largely rivaling separation factors as compared to the parent WAX and SCX CSPs. Furthermore, the application spectrum could be remarkably expanded to various zwitterionic analytes such as alpha- and beta-amino acids and peptides. A set of structurally related yet different CSPs consisting of either a quinine or quinidine alkaloid moiety as anion-exchange subunit and various chiral or achiral amino acids as cation-exchange subunits enabled us to derive structure-enantioselectivity relationships, which clearly provided strong unequivocal evidence for synergistic effects of the two oppositely charged ion-exchange subunits being involved in molecular recognition of zwitterionic analytes by zwitterionic SOs driven by double ionic coordination.

Monolithic columns for chiral capillary electrochromatography have been prepared within the confines of untreated fused-silica capillaries in a single step by a simple copolymerization of mixtures of O-[2-(methacryloyloxy)ethylcarbamoyl]-10,11-dihydroquinidine , ethylene dimethacrylate, and glycidyl methacrylate or 2-hydroxyethyl methacrylate in the presence of mixture of cyclohexanol and 1-dodecanol as a porogenic solvent. The porous properties of the monolithic columns can easily be controlled through changes in the composition of the binary porogenic solvent. Although both thermal- and UV light-initiated polymerizations afford useful capillary columns, monoliths prepared using the former approach exhibit better chromatographic properties. The ability to control pore size independently of the polymerization mixture composition enables the preparation of monoliths with varying percentages of the chiral monomer and cross-linker, as well as the optimization of their separation properties. Very good separations of model racemate (R,S)-N-3,5-dinitrobenzoylleucine were achieved using an optimized monolithic CEC column, with high efficiencies of up to 74000 plates/m for the retained peaks.

An altered platelet lipidome might be associated with thrombotic disposition in CAD, a mechanism potentially regulated by CXCL12-CXCR4-CXCR7 axis.

The most recent literature dealing with enantioselective separations and stereoselective analyses of chiral entities including especially pharmaceuticals, phytochemicals, biochemicals, agrochemicals, fine chemicals and specific test compounds by electromigration techniques such as CE, MEKC, MEEKC, CEC and microchip CE is reviewed. The review covers literature from 2007 until mid-2008, i.e. studies that were published after the appearance of the latest review article on that topic in Electrophoresis by Gübitz and Schmid (see Electrophoresis 2007, 28, 114). Particular attention is given to the description of new chiral selector systems, studies on separation mechanisms and applications in the above-specified electromigration techniques.

The effect of chromatographic conditions on the performance of chiral monolithic poly(O-[2-(methacryloyloxy)-ethylcarbamoyl]-10,11-dihydroqui nidine-co-ethylene dimethacrylate-co-2-hydroxyethyl methacrylate) columns in the capillary electrochromatography of enantiomers has been studied. The flow velocity was found to be proportional to the pore size of the monolith and both the pH and the composition of the mobile phase. The length of both open and monolithic segments of the capillary column was found to exert a substantial effect on the run times. The use of monoliths as short as 8.5 cm and the "short-end" injection technique enabled the separations to be achieved in approximately 5 min despite the high retentitivity of the quinidine selector. Very high column efficiencies of close to 250000 plates/m and good selectivities were achieved for the separations of numerous enantiomers using the chiral monolithic capillaries with the optimized chromatographic conditions.