Urea-Urease Reaction in Controlling Properties of Supramolecular Hydrogels: Pros and Cons

Supramolecular hydrogels are useful in many areas such as cell culturing, catalysis, sensing, tissue engineering, drug delivery, environmental remediation and optoelectronics. The gels need specific properties for each application. The properties arise from a fibrous network that forms the matrix. A common method to prepare hydrogels is to use a pH change. Most methods result in a sudden pH jump and often lead to gels that are hard to reproduce and control. The urease-urea reaction can be used to control hydrogel properties by a uniform and controlled pH increase as well as to set up pH cycles. The reaction involves hydrolysis of urea by urease and production of ammonia which increases the pH. The rate of ammonia production can be controlled which can be used to prepare gels with differing properties. Herein, we show how the urease-urea reaction can be used for the construction of next generation functional materials.     

A mild and efficient synthesis of 4-quinolones and quinolone heterocycles

The cycloacylation of aniline derivatives to 4-quinolones in the presence of Eaton's reagent is described. This high-yielding methodology is applicable to a wide variety of functionalized anilines and requires milder conditions than those traditionally employed. This cyclization protocol is used to prepare a host of heterocycles and bis-quinolones and is characterized by relatively low reaction temperature and ease of product isolation.     

ATR-IR spectroscopic study of antimonate adsorption to iron oxide

Antimonate ions adsorb to iron oxides in mining contexts, but the nature of the adsorbed antimonate species has not frequently been investigated. In this study, ATR-IR spectroscopy was used to reveal that the adsorption of Sb(OH)6- ion from aqueous solutions onto an amorphous iron oxide particle film is accompanied by changes in the Sb(OH)6- spectrum and the loss of OH stretching absorptions from iron oxide surface hydroxyl groups. These spectral changes upon adsorption imply an inner-sphere surface interaction with the formation of Sb-O-Fe bonds as well as some outer-sphere adsorption. The corresponding results from solutions of antimonate in D2O confirm that chemisorption occurs. The dependence of antimonate adsorption on pH in the range from 8 to 3 follows that expected for anions on iron oxide considering its pH-dependent surface charge, with the greatest amount of adsorbed antimonate at pH 3. The study of adsorption/desorption kinetics showed a more rapid desorption of adsorbed antimonate under alkaline conditions. This trend is expected from the pH dependence of the antimonate charge and iron oxide surface charge, but it might be partly due to the fact that high pH favors hydrolysis of antimonate oligomers formed on the iron oxide surface from adsorption under acidic conditions.     

Self-assembly of surfactant-like peptides

Inspired by recent work describing surfactant-like peptides, we have carried out a systematic study on peptides with the underlying composition of V6D2, altering the absolute sequence to determine the importance of the surfactant-like structure. All of the peptides examined here formed self-assembled structures in water. However, in contrast to other reports, we have found a surprising diversity of structures including fibers, tapes, and twisted ribbons but an absence of the vesicles and nanotubes described previously. Further investigations demonstrated that peptide purity plays a significant role in the outcome of the self-assembly. Different batches behave very differently, which can be linked to the compositions of these batches. This work shows that there is a need for not only rational design but also ease of synthesis of the building blocks for self-assembled structures.     

Electrochemical stability of self-assembled alkylphosphate monolayers on conducting metal oxides

Alkylphosphate self-assembled monolayers (SAMs) were prepared on Nb-doped SrTiO(3) (Nb-STO) conducting metal oxide substrates. Unlike thiols on gold, the alkylphosphate SAMs on Nb-STO exhibited an electrochemical stability over a wide voltage range from -2 to 2 V. Cyclic voltammetry showed that the SAM modification inhibited the electrochemical activity of the underlying conducting substrate with an efficiency dependent on the chain length. Impedance spectroscopy showed that SAM-modified Nb-STO substrates have a significantly higher resistance than bare substrates.     

Dissociation and degradation of thiol-modified DNA on gold nanoparticles in aqueous and organic solvents

Gold nanoparticles functionalized with thiol-modified DNA have been widely used in making various nanostructures, colorimetric biosensors, and drug delivery vehicles. Over the past 15 years, significant progress has been made to improve the stability of such functionalized nanoparticles. The stability of the gold-thiol bond in this system, however, has not been studied in a systematic manner. Most information on the gold-thiol bond was obtained from the study of self-assembled monolayers (SAMs). In this study, we employed two fluorophore-labeled and thiol-modified DNAs. The long-term stability of the thiol-gold bond as a function of time, salt, temperature, pH, and organic solvent has been studied. We found that the bond spontaneously dissociated under all tested conditions. The dissociation was favored at high salt, high pH, and high temperature, and little DNA degradation was observed in our system. Most organic solvents showed a moderate protection effect on the gold-thiol bond. The stability of the gold-thiol bond in the DNA system was also compared with that in SAMs. While there are many similarities, we also observed opposite trends for the salt and ethanol effect. This study suggests that the purified DNA-functionalized gold nanoparticles should be freshly prepared and used in a day or two. Long-term storage should be carried out at relatively low temperature in low salt and slightly acidic buffers.     

Salt-induced hydrogelation of functionalised-dipeptides at high pH

Addition of divalent cations to a solution of a naphthalene-diphenylalanine that forms worm-like micelles at high pH results in the formation of a rigid, self-supporting hydrogel.     

The impedance of thin dense oxide cathodes

The impedance is derived for a dense layer electrode of a mixed conducting oxide, assuming that the electronic resistance may be ignored. The influence of layer thickness, oxygen diffusion and surface exchange rate on the ‘General Finite Length Diffusion’ expression is evaluated. The thickness dependence is tested for a series of thin, dense layer electrodes of La_0.6Sr_0.4Co_0.2Fe_0.8O_3-δ (LSCF) deposited on a Ce_0.9Gd_0.1O_1.95 electrolyte by pulsed laser deposition (PLD). A minimum thickness is required to avoid the influence of contact points of the contacting Pt-gauze and sheet resistance, which is about 1 μm for the studied LSCF electrodes. LEISS surface analysis indicates that PLD deposition process easily leads to a significant Cr contamination of the LSCF surface. Electrochemical impedance spectroscopy analysis indicates that the influence on the exchange rate of this Cr-contamination is still negligible.     

Unusual Phosphorus Ligands

Abstract

A variety of novel coordination complexes with cyclo-P₃, cyclo-P₄, cyclo-P₅, P₄ ^2-, P₄ ^4- and P₄ ^6- as ligands will be described.     

A review on synthetic account of 1,2,4-oxadiazoles as anti-infective agents

Molecular Diversity - Most of the currently marketed drugs consist of heterocyclic scaffolds containing nitrogen and or oxygen as heteroatoms in their structures. Several research groups have...