We present the preparation and characterization of methylene blue-containing silica-coated magnetic particles. The entrapment of methylene blue (MB), a photodynamic therapy drug under study in our group, in the silica matrix took place during the growth of a silica layer over a magnetic core composed of magnetite nanoparticles. The resulting material was characterized by transmission electron microscopy (TEM), light scattering, and X-ray diffraction. It is composed of approximately 30 nm silica spheres containing magnetic particles of 11 +/- 2 nm and methylene blue entrapped in the silica matrix. The immobilized drug can generate singlet oxygen, which was detected by its characteristic phosphorescence decay curve in the near-infrared and by a chemical method using 1,3-diphenylisobenzofuran to trap singlet oxygen. The lifetime of singlet oxygen was determined to be 52 micros (in acetonitrile) and 3 micros (in water), with both values being in good agreement with those in the literature. The release of singlet oxygen (etaDelta) was affected by the encapsulation of MB in the silica matrix, which caused a reduction to 6% of the quantum yield of MB free in solution. The magnetization curve confirmed the superparamagnetic behavior with a reduced saturation magnetization in respect to uncoated magnetic nanoparticles, which is consistent with the presence of a diamagnetic component over the magnetite surface. The result is a single particle platform that combines therapy (photosensitizer) and diagnostic (MRI contrast agent) possibilities at the same time, as well as drug targeting.

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The use of liposomes to encapsulate materials has received widespread attention for drug delivery, transfection, diagnostic reagent, and as immunoadjuvants. Phospholipid polymers form a new class of biomaterials with many potential applications in medicine and research. Of interest are polymeric phospholipids containing a diacetylene moiety along their acyl chain since these kinds of lipids can be polymerized by Ultra-Violet (UV) irradiation to form chains of covalently linked lipids in the bilayer. In particular the diacetylenic phosphatidylcholine 1,2-bis(10,12-tricosadiynoyl)-sn-glycero-3-phosphocholine (DC8,9PC) can form intermolecular cross-linking through the diacetylenic group to produce a conjugated polymer within the hydrocarbon region of the bilayer. As knowledge of liposome structures is certainly fundamental for system design improvement for new and better applications, this work focuses on the structural properties of polymerized DC8,9PC:1,2-dimyristoyl-sn-glycero-3-phosphocholine (DMPC) liposomes. Liposomes containing mixtures of DC8,9PC and DMPC, at different molar ratios, and exposed to different polymerization cycles, were studied through the analysis of the electron spin resonance (ESR) spectra of a spin label incorporated into the bilayer, and the calorimetric data obtained from differential scanning calorimetry (DSC) studies. Upon irradiation, if all lipids had been polymerized, no gel-fluid transition would be expected. However, even samples that went through 20 cycles of UV irradiation presented a DSC band, showing that around 80% of the DC8,9PC molecules were not polymerized. Both DSC and ESR indicated that the two different lipids scarcely mix at low temperatures, however few molecules of DMPC are present in DC8,9PC rich domains and vice versa. UV irradiation was found to affect the gel-fluid transition of both DMPC and DC8,9PC rich regions, indicating the presence of polymeric units of DC8,9PC in both areas. A model explaining lipids rearrangement is proposed for this partially polymerized system.

In this work, we investigated the properties of a fusogenic cationic lipid, diC14-amidine, and show that this lipid possesses per se the capacity to adopt either an interdigitated structure (below and around its transition temperature) or a lamellar structure (above the transition temperature). To provide experimental evidence of this lipid bilayer organization, phospholipids spin-labeled at different positions of the hydrocarbon chain were incorporated into the membrane and their electron spin resonance (ESR) spectra were recorded at different temperatures. For comparison, similar experiments were performed with dimyristoyl phosphatidylcholine, a zwitterionic lipid (DMPC) which adopts a bilayer organization over a broad temperature range. Lipid mixing between diC14-amidine and asolectin liposomes was more efficient below (10-15 °C) than above the transition temperature (above 25 °C). This temperature-dependent "fusogenic" activity of diC14-amidine liposomes is opposite to what has been observed so far for peptides or virus-induced fusion. Altogether, our data suggest that interdigitation is a highly fusogenic state and that interdigitation-mediated fusion occurs via an unusual temperature-dependent mechanism that remains to be deciphered.

The present work reports on the synthesis and properties of magnetic nanoparticles based on magnetite coated by a silica layer.A series of conditions was tested to allow the synthesis of the nanoparticles, usually performed in an aqueous medium, and could be efficiently coupled with the silica sol-gel process. The resulting particles were characterized by means of X-ray diffraction, transmission electron microscopy associated with electron energy loss spectra and electron spectroscopy imaging, Fourier transform IR, Mo¨ssbauer and magnetization measurements. Initially, magnetite particles with an average crystalline grain size of about 100 Å and a polydispersion of 30%, as revealed by X-ray diffraction and transmission electron microscopy analyses, respectively, were obtained with a mixture of FeCl 2 and FeCl 3 in aqueous and acid solution. The magnetization measurements, at room temperature, show that the particles are in the superparamagnetic regime. Magnetite was also synthesized in acid solutions in an alcoholic environment (25% methanol-to-base ratio), a medium that allows us to proceed with silicacoating by the sol-gel process in a one pot reaction. The resulting particles present size dispersion ranging from around 15 Å to about 200-300 Å as evidenced by electron micrographs. The superparamagnetic behavior is preserved, although its saturation magnetization value decreases from about 92 to about 50 emu g)1 , probably owing to the contribution of the smallest particles as well as to the surface spin disorder induced by addition of methanol to the synthesis medium. For higher values of the alcohol-to-base ratio, the resulting particles are amorphous, becoming crystalline under thermal treatment. When tetraethylorthosilicate is added to a solution containing 25% of methanol to base, iron oxides are SiO 2 -coated at room temperature, as evidenced by electron spectroscopy imaging and Fourier transform IR spectroscopy. The magnetization results are dependent on the Si-to-Fe volume ratio, in such a way that the values decrease as the SiO 2 amount increases, reflecting the nanoparticle coating.

Cationic bilayers have been used as models to study membrane fusion, templates for polymerization and deposition of materials, carriers of nucleic acids and hydrophobic drugs, microbicidal agents and vaccine adjuvants. The versatility of these membranes depends on their structure. Electron spin resonance (ESR) spectroscopy is a powerful technique that employs hydrophobic spin labels to probe membrane structure and packing. The focus of this review is the extensive structural characterization of cationic membranes prepared with dioctadecyldimethylammonium bromide or diC14-amidine to illustrate how ESR spectroscopy can provide important structural information on bilayer thermotropic behavior, gel and fluid phases, phase coexistence, presence of bilayer interdigitation, membrane fusion and interactions with other biologically relevant molecules.

Six-line ferrihydrite(FH) nanoparticles have been synthesized in the core of reverse micelles, used as nanoreactors to obtain average particle sizes < d > ≈ 2 to 4 nm. The blocking temperatures T m B extracted from magnetization data increased from ≈ 10 to 20 K for increasing particle size. Lowtemperature Mössbauer measurements allowed to observe the onset of differentiated contributions from particle core and surface as the particle size increases. The magnetic properties measured in the liquid state of the original emulsion showed that the ferrihydrite phase is not present in the liquid precursor, but precipitates in the micelle cores after the free water is freeze-dried. Systematic susceptibility χac(f, T ) measurements showed the dependence of the effective magnetic anisotropy energies Ea with particle volume, and yielded an effective anisotropy value of K ef f = 312 ± 10 kJ/m 3 .