Magnetic Mesoporous Silica Nanoparticles for Drug Delivery Systems: Synthesis, Characterization and Application as Norfloxacin Carrier.

Mesoporous silica nanoparticles, with and without the inclusion of a magnetic core, were hydrothermally synthesized and employed as carrier of the antibiotic norfloxacin (NFX). The antibiotic-loaded materials were prepared by wet impregnation. Differences in drug content (and in further release profile) were directly related to changes in surface area, particle aggregation and hydrophobicity of the solids. The kinetics of NFX release has been studied in batch experiments. In all cases, more than 55% of the antibiotic was quickly desorbed during the first 5 min due to the localization of NFX on the external surface of the nanoparticles. The rest of the drug (situated inside the mesopores) was released through a diffusion-controlled transport and the rate was strongly dependent of the pH, reaching its minimum value at neutral pH. The calculated activation energy confirmed that the release was controlled by a diffusion process. Breaking of H-bonds and electrostatic and hydrophobic interactions appear to be responsible for NFX desorption from the solid surface. Such interactions increase, however, the thermal stability of the drug when the NFX and the carriers are combined. The antimicrobial activities of the drug loaded nanoparticles and the free antibiotic were compared and discussed.

Interaction of pesticides with natural and synthetic solids. Evaluation in dynamic and equilibrium conditions.

Interactions between pesticides (paraquat, glyphosate, 2,4-D, atrazine, and metsulfuron methyl) and soil organic and inorganic components have been studied in batch experiments by performing adsorption, dissolution, and chemical and photochemical degradation under different conditions. The obtained results confirm that the affinity of a pesticide to the solid surface depends on the nature of both and shows that each reactant strongly affects the mobility of the other one, e.g., anionic pesticides promote the dissolution of the solid humic acid but if this last is retained into the inorganic matrix enhances the adsorption of a cationic pesticide. Adsorption also seems to protect the bonded specie to be chemical degraded, such as shown in two pesticide/clay systems at constant pH. The use of mesoporous silicas could result in a good alternative for pesticide remediation. In fact, the solid shows high adsorption capacity towards paraquat and its modification with TiO2 nanoparticles increases not only the pesticide adsorption but also seems to catalyze its degradation under UV light to less-toxic metabolites. UV-VIS spectroscopy was relevant and novel in such sense. Electrostatic interactions, hydrogen and coordinative bonds formations, surface complexations and hydrophobic associations play a key role in the fate of mentioned pesticides on soil and ground/surface water environments.

Synthesis of mesoporous silicas in alkaline and acidic media using the systems cetyltrimethylammonium tosylate (CTAT)-Pluronic F127 triblock copolymer and CTAT-Pluronic F68 triblock copolymer as templates.

Mesoporous silica materials were synthesized in alkaline and acidic media, using cetyltrimethylammonium tosylate (CTAT), Pluronic triblock copolymers F127 and F68, and mixtures of CTAT with each copolymer in order to investigate the effects of pH, surfactant concentration, and CTAT/triblock copolymer molar ratios on the morphology and texture of the synthesized materials. The results show that the kind of mesoporous materials and their pore size can be tuned by changing not only the pH but also the proportion of components and the nature of the copolymer. In alkaline synthesis, microscopic bicontinuous materials are obtained, which are composed by nanoscopic plate-like particles having slit-shaped pores. In acidic synthesis, on the contrary, monolithic silicas are obtained. These materials are also composed by nanoscopic plate-like particles having slit-shaped pores, although in some cases, the microscopic structures are formed by fused spherical particles. The inclusion of the triblock copolymer in the template composition causes a transformation from a bimodal to a monomodal pore size distribution, leading to small and nearly round pores which are probably formed by copolymer or copolymer-CTAT mixed micelles. The differences between the systems synthesized by CTAT-Pluronic F127 and CTAT-Pluronic F68 are explained on the basis of the different interactions between each copolymer and CTAT.

Adsorption of paraquat on mesoporous silica modified with titania: effects of pH, ionic strength and temperature.

The adsorption of the herbicide paraquat (PQ(2+)) on the binary system titania-silica has been studied in batch experiments by performing adsorption isotherms under different conditions of pH, supporting electrolyte concentration, and temperature. Adsorption kinetic on the studied material has also been carried out and discussed. PQ(2+) adsorption is very low on the bare silica surface but important on the composed TiO(2)-SiO(2) adsorbent. In this last case, the adsorption increases by increasing pH and decreasing electrolyte concentration. There are no significant effects of temperature on the adsorption. The increase of the adsorption in TiO(2)-SiO(2) seems to be related to an increase in acid sites of the supported titania and to the homogenously dispersion of the TiO(2) nanoparticles over the silica support. The adsorption takes place by direct binding of PQ(2+) to TiO(2) leading to the formation of surface species of the type SiO(2)-TiO(2)-PQ(2+). Electrostatic interactions and charge-transfer and outer-sphere complexes formations seem to play a key role in the adsorption mechanism. The analysis of thermodynamic parameters suggests that the adsorption on TiO(2)-SiO(2) is endothermic and spontaneous in nature.

Remotion of the antibiotic tetracycline by titania and titania-silica composed materials.

Removal of the antibiotic tetracycline (TC) by TiO(2) and the mesoporous binary system TiO(2)-SiO(2) have been studied in batch experiments by performing adsorption isotherms/kinetics and photodegradation kinetics under different conditions of pH, supporting electrolyte concentration, temperature, adsorbent amount, and TiO(2)-loading. On the one hand, the adsorption of TC on the studied materials is strongly dependent on pH, increasing as pH decreases. The adsorption mechanism, controlled by diffusion processes, is strongly related to electrostatic attractions and H-bond formations mainly between amide, carbonylic and phenolic groups of the antibiotic and the functional groups of TiO(2). The adsorption capacity at constant pH increases in the order TiO(2)

Effect of humic acids on the adsorption of paraquat by goethite.

The adsorption of the herbicide paraquat (PQ(2+)) on goethite and on the binary system humic acid-goethite has been studied in batch experiments by performing adsorption isotherms under different conditions of pH, supporting electrolyte concentration and temperature. The results were completed with capillary electrophoresis (CE) in order to measure the binding isotherm between PQ(2+) and humic acid (HA) molecules in solution. PQ(2+) adsorption is negligible on the bare goethite surface but important on the HA-goethite adsorbent. In this last case, the adsorption increases by increasing pH and decreasing electrolyte concentration. There are no significant effects of temperature on the adsorption. The adsorption takes place by direct binding of PQ(2+) to adsorbed HA molecules leading to the formation of surface species of the type goethite-HA-PQ(2+). The results are consistent with a mechanism where PQ(2+) binds negatively charged groups of HA (carboxylates and phenolates) forming ionic pairs or outer-sphere complexes. Since goethite in nature usually contains adsorbed HA molecules, it may act as a good adsorbent for cationic herbicides. This will not only benefit the deactivation of the herbicides but also reduce their leaching and transport through groundwater.

On the dissolution kinetics of humic acid particles. Effect of monocarboxylic acids.

The dissolution kinetics of humic acid particles has been studied in batch experiments, and the effects of monocarboxylic (formic, acetic, and propionic) acids are reported. The dissolution rate of the particles is significantly affected by the presence of monocarboxylic acids in the pH range 4-10. At pH 7, for example, propionic acid increases 30 times this dissolution rate. The capacity of increasing the dissolution rate is in the order formic acid

Adsorption kinetics of phosphate and arsenate on goethite. A comparative study.

The adsorption kinetics of phosphate and arsenate on goethite is studied and compared. Batch adsorption experiments were performed at different adsorbate concentrations, pH, temperatures and stirring rates. For both oxoanions the adsorption rate increases by increasing adsorbate concentration, decreasing pH and increasing temperature. It does not change by changing stirring rate. The adsorption takes place in two processes: a fast one that takes place in less than 5 min and a slow one that takes place in several hours or more. The rate of the slow process does not depend directly on the concentration of phosphate or arsenate in solution, but depends linearly on the amount of phosphate or arsenate that was adsorbed during the fast process. Apparent activation energies and absence of stirring rate effects suggest that the slow process is controlled by diffusion into pores, although the evidence is not conclusive. The similarities in the adsorption kinetics of phosphate and arsenate are quantitatively shown by using a three-parameters equation that takes into account both the fast and the slow processes. These similarities are in line with the similar reactivity that phosphate and arsenate have in general and may be important for theoretical and experimental studies of the fate of these oxoanions in the environment.

Kinetics of phosphate adsorption on goethite: comparing batch adsorption and ATR-IR measurements.

The adsorption kinetics of phosphate on goethite has been studied by batch adsorption experiments and by in situ ATR-IR spectroscopy at different pH, initial phosphate concentrations and stirring rates. Batch adsorption results are very similar to those reported by several authors, and show a rather fast initial adsorption taking place in a few minutes followed by a slower process taking place in days or weeks. The adsorption kinetics could be also monitored by integrating the phosphate signals obtained in ATR-IR experiments, and a very good agreement between both techniques was found. At pH 4.5 two surface complexes, the bidentate nonprotonated (FeO)(2)PO(2) and the bidentate protonated (FeO)(2)(OH)PO complexes, are formed at the surface. There are small changes in the relative concentrations of these species as the reaction proceeds, and they seem to evolve in time rather independently. At pH 7.5 and 9 the dominating surface species is (FeO)(2)PO(2), which is accompanied by an extra unidentified species at low concentration. They also seem to evolve independently as the reaction proceeds. The results are consistent with a mechanism that involve a fast adsorption followed by a slow diffusion into pores, and are not consistent with surface precipitation of iron phosphate.