Use of Nitroxides as Topological Monitors of the Interaction of Silica-Based Particles with Components of the Biological Environment

Embedding cyclic nitrone in mesoporous silica particles for EPR spin trapping of superoxide and other radicals

Mesoporous silica functionalised with a cyclic spin trap enabled the identification of a wide range of radicals in organic and aqueous media, including superoxide radical anion.

Electron paramagnetic resonance and transmission electron microscopy study of the interactions between asbestiform zeolite fibers and model membranes

Different asbestiform zeolite fibers of the erionite (termed GF1 and MD8, demonstrated carcinogenic) and offretite (termed BV12, suspected carcinogenic) families were investigated by analyzing the electron paramagnetic resonance (EPR) spectra of selected surfactant spin probes and transmission electron microscopy (TEM) images in the presence of model membranes-cetyltrimethylammonium (CTAB) micelles, egg-lecithin liposomes, and dimyristoylphosphatidylcholine (DMPC) liposomes. This was undertaken to obtain information on interactions occurring at a molecular level between fibers and membranes which correlate with entrance of fibers into the membrane model or location of the fibers at the external or internal membrane interfaces. For CTAB micelles, all fibers were able to enter the micelles, but the hair-like structure and chemical surface characteristics of GF1 modified the micelle structure toward a bilayer-like organization, while MD8 and BV12, being shorter fibers and with a high density of surface interacting groups, partially destroyed the micelles. For liposomes, GF1 fibers partially penetrated the core solution, but DMPC liposomes showed increasing rigidity and organization of the bilayer. Conversely, for MD8 and BV12, the fibers did not cross the membrane demonstrating a smaller membrane structure perturbation. Scolecite fibers (termed SC1), used for comparison, presented poor interactions with the model membranes. The carcinogenicity of the zeolites, as postulated in the series SC1<BV12<MD8<GF1, may be related to the structural modifications of the model membranes when interacting with these zeolite fibers.

Adsorption of Pure and Mixed Solvent Solutions of Spin Probes onto Stationary Phases

Water, methanol (MeOH), acetonitrile (ACN), and binary MeOH-water and ACN-water solutions of different spin probes (nitroxides), selected to mimic the behavior of different pollutants, were adsorbed onto stationary phases usually used in reversed-phase high-performance liquid chromatography (RP-HPLC). These stationary phases are constituted by porous silica and differ from each other regarding the surface area, the pore size, the particle size, the surface functions (NH2, C8, and C18), and the percentage of functionalization. The electron paramagnetic resonance (EPR) spectra of the probe solutions adsorbed into the pores were analyzed by computer-aided computation of the spectral line shape, which provided structural and dynamical parameters of the probes and their environments. These parameters provided information on the surface properties of the stationary phases, such as alkyl chain density, solvent penetration, stationary-phase ordering, and residual silanol effects, which modify the retention times in HPLC. A different availability of polar surface groups in the pure and mixed solvents was found for the different stationary phases depending on (1) the different functionalization degree, (2) the surface-chain length, (3) the particle size, and (4) the polarity of both the probe and the solvent. The C8 functionalization rendered the surface more hydrophobic with respect to C18. The endcapping process of the residual silanols strongly enhanced the surface hydrophobicity tested by the probes. At the highest water content, the adsorption of the polar or charged probes onto the hydrophobic surface is the lowest and self-aggregation occurs. When the probes bear both hydrophilic and hydrophobic moieties, the adsorption is enhanced by a synergy between hydrophilic and hydrophobic bonds with the surface. A balance between the hydrophilic and hydrophobic forces leads to high adsorption and partial insertion of the surfactant probes in an ordered C18 chain layer at the solid surface which forms in the binary mixtures; this layer is ascertained between 40% and 70% of the less hydrophilic solvent, depending on the type of both the solid and the probe. This insertion and the response on the formation of the ordered layer were favored in ACN-water with respect to MeOH-water.

Surface Properties of Vitreous Fibers

The surface properties of various vitreous fibers, suspected to be toxic to humans and animals, were investigated by means of paramagnetic labels covalently linked to the surface. Computer-aided analysis of the electron paramagnetic resonance (EPR) spectra provided structural and dynamic information on the label and its environment. Calorimetric measurements provided information on the hydration mechanism. The results were analyzed in terms of (a) different polarity and interaction abilities of surface regions, (b) presence of ions at the surface, (c) silica contents, (d) vicinity of the interacting sites, (e) fiber dimension and morphology of the surfaces, and (f) water hydration. The mobility of the labels decreased due to interaction of the fibers with ions or ionic and polar groups at the surface. Close interacting sites were identified on the basis of spin-spin effects and were distinguished and quantified in strongly and weakly interacting sites. The spin-labeling technique indicated decreased ability of the surface to interact with decreased silicon concentration and in the presence of contaminants at the surface. The interaction with water revealed in all cases a substantial heterogeneity in hydrophilicity of surface sites. The labels were not easily hydrated. Vitreous fibers of various compositions adsorbed much more water than crystalline or amorphous silica; water coordinated to surface cations played a major role in the overall adsorption. The surface reaction mechanisms were the same on fibers of different compositions, but the surface composition affected the extent of adsorption. Glass wool exhibited a much higher adsorption capacity than rock wool under the same experimental conditions. In conclusion, the combination of EPR and calorimetric measurements provided insight into the surface properties of silica-based fibers. Copyright 2000 Academic Press.

Relationship between surface properties and cellular responses to crystalline silica: studies with heat-treated cristobalite

A fibrogenic sample of cristobalite dust, CRIS (crystalline silica of mineral origin), was heated to 1300 degrees C (CRIS-1300) to relate induced physicochemical modifications to cytotoxicity. Heating did not affect dust micromorphology and crystallinity, except for limited sintering and decreased surface area of CRIS-1300. Thermal treatments deeply affected surface properties. Electron paramagnetic resonance showed surface radicals progressively annealed by heating, mostly disappearing at >/=800 degrees C. Surface hydrophilicity or hydrophobicity, evaluated with water vapor adsorption, still showed some hydrophilic patches in CRIS-800, but CRIS-1300 was fully hydrophobic. Heating modified the biological activity of cristobalite. Cytotoxicity, tested on proliferating cells of the mouse monocyte macrophage cell line J774, showed that CRIS was cytotoxic and CRIS-800 was still cytotoxic, but CRIS-1300 was substantially inert. Cytotoxicity of CRIS to the rat lung alveolar epithelial cell line, AE6, as measured by colony forming efficiency, was greatly reduced for CRIS-800 and eliminated for CRIS-1300. The rate of lactate dehydrogenase release by rat alveolar macrophages was lowered for CRIS-800, and release was completely inactivated for CRIS-1300. The absence of surface radicals and the onset of hydrophobicity may both account for the loss of cytotoxicity upon heating. Differences observed between CRIS-800 and CRIS-1300, both fully deprived of surface radicals, indicate that hydrophobicity is at least one of the surface properties determining the cytotoxic potential of a dust.