Silica microparticles as a solid support for gadolinium phosphonate magnetic resonance imaging contrast agents

Particle-based magnetic resonance imaging (MRI) contrast agents have been the focus of recent studies, primarily due to the possibility of preparing multimodal particles capable of simultaneously targeting, imaging, and treating specific biological tissues in vivo. In addition, particle-based MRI contrast agents often have greater sensitivity than commercially available, soluble agents due to decreased molecular tumbling rates following surface immobilization, leading to increased relaxivities. Mesoporous silica particles are particularly attractive substrates due to their large internal surface areas. In this study, we immobilized a unique phosphonate-containing ligand onto mesoporous silica particles with a range of pore diameters, pore volumes, and surface areas, and Gd(III) ions were then chelated to the particles. Per-Gd(III) ionic relaxivities ranged from ～2 to 10 mM(-1) s(-1) (37 °C, 60 MHz), compared to 3.0-3.5 mM(-1) s(-1) for commercial agents. The large surface areas allowed many Gd(III) ions to be chelated, leading to per-particle relaxivities of 3.3 × 10(7) mM(-1) s(-1), which is the largest value measured for a biologically suitable particle.     

On an efficient implementation and mass boundedness conditions for a discrete Dirichlet problem associated with a nonlinear system of singular partial differential equations

In this work, we propose an efficient implementation of a finite-difference method employed to approximate the solutions of a system of partial differential equations that appears in the investigation of the growth of biological films. The associated homogeneous Dirichlet problem is discretized using a linear approach. This discretization yields a positivity- and boundedness-preserving implicit technique which is represented in vector form through the multiplication by a sparse matrix. A straightforward implementation of this methodology would require a substantial amount of computer memory and time, but the problem is conveniently coded using a continual reduction of the zero sub-matrices of the representing matrix. In addition to the conditions that guarantee the positivity and the boundedness of the numerical approximations, we establish some parametric constraints that assure that the same properties for the discrete total mass at each point of the mesh-grid and each discrete time are actually satisfied. Some simulations are provided in order to illustrate both the performance of the implementation, and the preservation of the positivity and the boundedness of the numerical approximations.     

Convenient Pd‐Catalyzed Synthesis of Large Unimolecular Star Polyethylene Nanoparticles

A simplistic convenient “arm‐first” catalytic synthesis method is demonstrated to render soft unimolecular star polyethylene nanoparticles. Low‐dispersity polyethylene arms of controllable length and topology are first synthesized via Pd‐catalyzed “living” ethylene poly­merization. The subsequent addition of norbornadiene as a unique cross‐linker renders the block polymer containing a short polynorbornadiene (PNBD) sequence. Efficient and rapid catalytic cross‐linking of the PNBD sequences occurs in the polymer precipitation and drying steps to give rise to star polyethylene nanoparticles. The star polymers are featured with tunable arm length and topology, high molecular weight (as high as 1770 kg mol⁻¹), high arm numbers (as high as 88), and desirable average nano­particle size (29−72 nm).

     

Potassium hexacyanoruthenate(II) as an analytical reagent for iron

Fe(III) undergoes a reaction with colourless Ru(CN)(4-)(6) to produce an intensely violet-blue complex that absorbs at 550 nm and obeys Beer's law over the iron concentration range 0.04-2 mug/ml in acidic medium. Some common cations and anions are tolerable at low concentrations. The procedure is applicable for determination of total iron in potable water. Destruction of organic matter is required for contaminated surface waters or soil samples.     

Characterization of solvation properties of lipid bilayer membranes in liposome electrokinetic chromatography

The nature of solute interactions with biomembrane-like liposomes, made of naturally occurring phospholipids and cholesterol, was characterized using electrokinetic chromatography (EKC). Liposomes were used as a pseudo-stationary phase in EKC that provided sites of interactions for uncharged solutes. The retention factors of uncharged solutes in liposome EKC are directly proportional to their liposome-water partition coefficients. Linear solvation energy relationship (LSER) models were developed to unravel the contributions from various types of interactions for solute partitioning into liposomes. Size and hydrogen bond acceptor strength of solutes are the main factors that determine partitioning into lipid bilayers. This falls within the general behavior of solute partitioning from an aqueous into organic phases such as octanol and micelles. However, there exist subtle differences in the solvation properties of liposomes as compared to those of octanol and various micellar pseudo-phases such as aggregates of sodium dodecyl sulfate (SDS), sodium cholate (SC), and tetradecylammonium bromide (TTAB). Among these phases, the SDS micelles are the least similar to the liposomes, while octanol, SC, and TTAB micelles exhibit closer solvation properties. Subsequently, higher correlations are observed between partitioning into liposomes and the latter three phases than that into SDS.     

Novel UV absorbing eluents for speciation and ion chromatographic determination of anions

2–3 and 2–4 pyridine dicarboxylic acids can be used as mobile phases for ion chromatographic analyses of anions in simple matrices. By selecting a specific reaction scheme or matrix elimination before the chromatographic step their use can be extended to more complex samples.     

Miscibility predictions for poly(phenylene sulfide): Heats of mixing of model compounds

Calorimetry was used to gain insight into the thermodynamics of mixing of unlike species, designed to represent organic polymers. These results are to be useful in predicting miscibility between various polymers and poly(arylene sulfides). Phenyl sulfide (PS) and phenyl disulfide (PSS) were used as models for poly(phenylene sulfide) (PPS) and were mixed with model compounds that are representative of other polymer classes. The enthalpy of mixing was measured for a large number of combinations. The results are interpreted in terms of a summation of the effects of specific interactions and dispersive forces. It is seen from these data that the presence of aromatic side groups enhances the miscibility of the species with the poly(phenylene sulfide) model compound. Furthermore, exothermic heats of mixing were observed between the PPS model and compound. Furthermore, exothermic heats of mixing were observed between the PPS model and compounds that contain carbonyl, amide, or amine moieties. Finally, the sulfur atom in PPS seems to play only a minor role in determining its interactions with other species. Replacing sulfur with oxygen (or even an alkyl group) is seen to have very little effect on the heats of mixing results. © 1994 John Wiley & Sons, Inc.