Evidence for rigid binding of rhodamine 6G to silica surfaces in aqueous solution based on fluorescence anisotropy decay analysis

Strong ionic binding of the cationic probe rhodamine 6G (R6G) to the anionic surface of silica particles in water provides a convenient labeling procedure to study both particle growth kinetics and surface modification by time-resolved fluorescence anisotropy (TRFA). The decays for R6G dispersed in diluted Ludox silica sols usually fit to a sum of picosecond and nanosecond decay components, along with a significant residual anisotropy component. The origin of the nanosecond decay component (phi2) is not fully understood, and has been ascribed to wobbling of the probe on the silica surface, the presence of a subpopulation of small nanoparticles in the Ludox sol, or rapid exchange between free and bound R6G. To elucidate the physical meaning of phi2, measurements were performed in various silica-based colloidal systems using different concentrations of silica. We found that the fraction of phi2 was generally higher in Ludox than in aqueous sodium silicate and decreased with increasing silica concentration; phi2 vanished upon gelation of sodium silicate at pH 7 leading to a total loss of R6G depolarization (r(t) = const). These results rule out the presence of local R6G wobbling when bound ionically to colloidal silica and support the rigid sphere model to describe the TRFA decays for R6G-Ludox. This conclusion is entirely supported by steady-state anisotropy data and structural considerations for the R6G molecule and the silica surface.     

Fluorescence anisotropy in studies of solute interactions with covalently modified colloidal silica nanoparticles

Time-resolved anisotropy decays of a fluorescent cationic solute, rhodamine 6G (R6G), in Ludox sols were measured to characterize the extent of the ionic binding of the probe to silica particles after modification of the surface with neutral or cationic silane coupling agents. The anisotropy decays provided direct evidence for distribution of the dye between the aqueous solution (picosecond decay component) and silica particles (nanosecond decay component and residual anisotropy component, which were attributed to the wobbling motion of dye on the silica surface and to the ionically bound probe, respectively). The dye was strongly adsorbed to unmodified silica nanoparticles, to the extent that less than 1% of the dye was present in the surrounding aqueous solution. Significant decreases in the degree of probe adsorption were obtained upon covalent modification of the silica with neutral or cationic silanes, with up to 80% of the probe being present in the aqueous solution in cases where the surface was coated with (3-aminopropyl)triethoxysilane. The addition of such agents also altered the fractional distribution between the nanosecond decay component and the residual anisotropy component in favor of the nanosecond component, indicative of weaker interactions between the dye and the modified surface (i.e., more wobbling motion). The data clearly show the power of time-resolved fluorescence anisotropy decay measurements for probing the modification of silica surfaces and should prove useful in characterization of new chromatographic stationary phases.     

A new perspective on the coarse-grained dynamics of fluids

A computational methodology is presented that is designed to model, at a coarse-grained level, the mesoscale dynamics of fluids and potentially other forms of soft matter. Within a molecular dynamics simulation, "ghost" particles of a specific size, corresponding to the fundamental length-scale of coarse-graining, are used as micro-probes designed to respond to local mesoscale fluid flows and stress gradients. A subsequent coarse-grained model is then developed that incorporates both the coarse-grained mesoscale dynamics and isothermal compressibility of the original microscopic system. The method is applied to water and methanol. A contrast with dissipative particle dynamics (DPD) is also presented.     

Electrochemical charging and electrocatalysis at hybrid films of polymer-interconnected polyoxometallate-stabilized carbon submicroparticles

Using the layer-by-layer technique, carbon submicroparticles, that have been modified and stabilized with monolayers of Keggin-type phosphododecamolybdate (PMo₁₂O₄₀³⁻), can be dispersed in multilayer films of organic polymers, poly(3,4-ethylenedioxythiophene), i.e., PEDOT, or poly(diallyldimethylammonium) chloride, i.e., PDDA, deposited on glassy carbon or indium-tin oxide conductive glass electrodes. The approach involves alternate treatments in the colloidal suspension of PMo₁₂O₄₀³⁻-covered carbon submicroparticles in the solution of monomer, 3,4-ethylenedioxythiophene or in solution of PDDA polymer. Electrostatic attractive interactions between anionic phosphomolybdate-modified carbon submicroparticles and cationic polymer layers permit not only uniform and controlled growth of the hybrid organic–inorganic film but also contribute to its overall stability. The system composed of PMo₁₂O₄₀³⁻-covered carbon submicroparticles dispersed in PEDOT is characterized by fast dynamics of charge transport and has been used to construct symmetric microelectrochemical redox capacitor. The PDDA-based system has occurred to be attractive for electrocatalytic reduction of hydrogen peroxide.     

Analysis of flunarizine in the presence of some of its degradation products using micellar liquid chromatography (MLC) or microemulsion liquid chromatography (MELC)--application to dosage forms

The separation of flunarizine hydrochloride (FLZ) and five of its degradation products--1-[bis(4-fluorophenyl)methyl]-4-(3-phenyl-2-propenyl)piperazine, 4-oxide (A), bis(4-fluorophenyl)methanone (B), bis(4-fluorophenyl)methanol (C), 1-(3-phenyl-2-propenyl)piperazine(D), and 1-[bis-4-fluorophenyl) methyl] piperazine (E)--could be accomplished by reversed phase liquid chromatography using either micellar or microemulsion mobile phases. Cyanopropyl-bonded stationary phase has been used with UV detection at 254 nm. Microemulsion mobile phase consisting of 0.15 M SDS, 10% n-propanol, 1% n-octanol, and 0.3% triethylamine in 0.02 M phosphoric acid of pH 7.0, has been used for the separation of FLZ and its degradation products (B, C, D, and E). Micellar mobile phases consisting of 0.15 M sodium dodecyl sulphate (SDS), 10% n-propanol, 0.3% triethylamine (TEA) in 0.02 M phosphoric acid of pH values either 4.0 or 6.8 have been used for the separation of FLZ from its degradation products, i.e. either from (B, C, D, and E) or from (A, B, C, and D), respectively. Micellar liquid chromatography (MLC) was applied to the determination of FLZ in pure form as well as in dosage forms; the calibration graph was linear over the concentration range of 0.15-50 microg/mL with detection limit of 0.02 microg/mL (4.19 x 10(-8)M).     

Synthesis of new molecular scaffolds: 3-aza-7,9-dioxa-bicyclo[4.2.1]nonane (8-exo BTKa) and 3-aza-8,10-dioxa-bicyclo[5.2.1]decane (9-exo BTKa) carboxylic acids

Two classes of enantiopure molecular scaffolds were prepared, whose lactam structure formally derives from the coupling between tartaric acid and β- or γ-ketoamines. We labelled these compounds as 8-exo and 9-exo BTKa, indicating the lactam size (8- and 9-membered ring, respectively). Starting from β- and γ-nitroketones, the synthesis involves the ketal formation by (R,R)-dimethyl tartrate. The subsequent amide bond formation occurs during the hydrogenation of the nitro group over Raney-Ni and no expected open chain amine was observed.     

Poly(2-oxazoline)-Based Polyplexes as a PEG-Free Plasmid DNA Delivery Platform

The present study expands the versatility of cationic poly(2-oxazoline) (POx) copolymers as a polyethylene glycol (PEG)-free platform for gene delivery to immune cells, such as monocytes and macrophages. Several block copolymers are developed by varying nonionic hydrophilic blocks (poly(2-methyl-2-oxazoline) (pMeOx) or poly(2-ethyl-2-oxazoline) (pEtOx), cationic blocks, and an optional hydrophobic block (poly(2-isopropyl-2-oxazoline) (iPrOx). The cationic blocks are produced by side chain modification of 2-methoxy-carboxyethyl-2-oxazoline (MestOx) block precursor with diethylenetriamine (DET) or tris(2-aminoethyl)amine (TREN). For the attachment of a targeting ligand, mannose, azide-alkyne cycloaddition click chemistry methods are employed. Of the two cationic side chains, polyplexes made with DET-containing copolymers transfect macrophages significantly better than those made with TREN-based copolymer. Likewise, nontargeted pEtOx-based diblock copolymer is more active in cell transfection than pMeOx-based copolymer. The triblock copolymer with hydrophobic block iPrOx performs poorly compared to the diblock copolymer which lacks this additional block. Surprisingly, attachment of a mannose ligand to either copolymer is inhibitory for transfection. Despite similarities in size and design, mannosylated polyplexes result in lower cell internalization compared to nonmannosylated polyplexes. Thus, PEG-free, nontargeted DET-, and pEtOx-based diblock copolymer outperforms other studied structures in the transfection of macrophages and displays transfection levels comparable to GeneJuice, a commercial nonlipid transfection reagent.     

An expedient synthesis of mellitic triimides

Heating of the solid ammonium salts obtained from treatment of mellitic acid with 3 equiv of a primary amine yields trisubstituted mellitic triimides via dehydration and imide ring closure. This surprisingly simple synthetic approach is amenable to incorporation of alkyl, aryl, and amino acid ester substituents, thereby opening broad access to a family of C(3)-symmetric organic electron acceptors.