RESUMEN

Nitrogen doped tin(iv) oxide (SnO2) materials in the form of nanometric powders have been prepared by precipitation with ammonia. Their properties have been compared with those of undoped materials obtained in a similar way using various physical techniques such as photoelectron spectroscopies (XPS and UPS), UV-Vis-NIR spectroscopy and electron paramagnetic resonance (EPR). Nitrogen doping leads to the formation of various nitrogen containing species, the more relevant of which is a nitride-type ionic species, based on the substitution of a lattice oxygen atom with a nitrogen atom. This species exists in two forms, paramagnetic (hole centre, formally N(2-)) and diamagnetic (N(3-)). The mutual ratio of the two species varies according to the oxidation state of the material. The doped solid, like most of the semiconducting oxides, tends to lose oxygen forming oxygen vacancies upon annealing under vacuum and leaving an excess of electrons in the solid. The stoichiometry of the solid can thus be markedly changed depending on the external conditions. Excess electrons are present both as itinerant electrons in the conduction band and as Sn(ii) states lying close to the valence band maximum. The presence of nitride-type centres, which are low energy states located below the top of the valence band, decreases the energy cost for the formation of oxygen vacancies by O2 release from the lattice. This particular feature of the doped system represents a severe limit to the preparation of a p-type SnO2via nitrogen doping.

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Nitrogen boron co-doped TiO(2) prepared via sol-gel synthesis and active under visible light, contains two types of paramagnetic extrinsic defects, both exhibiting a well resolved EPR spectrum. The first center is the well characterized [N(i)O]˙ species (i = interstitial) also present in N-doped TiO(2), while the second one involves both N and B. This latter center (labeled [NOB]˙) exhibits well resolved EPR spectra obtained using either (14)N or (15)N which show a high spin density in a N 2p orbital. The structure of the [NOB]˙ species is different from that previously proposed in the literature and is actually based on the presence of interstitial N and B atoms both bound to the same lattice oxygen ion. The interstitial B is also linked to two other lattice oxygen ions reproducing the trigonal planar structure typical of boron compounds. The energy level of the [NOB]˙ center lies near the edge of the valence band of TiO(2) and, as such, does not contribute to the visible light absorption. However, [NOB]˙ can easily trap one electron generating the [NOB](-) diamagnetic center which introduces a gap state at about 0.4 eV above the top of the valence band. This latter species can contribute to the visible light activity.

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The interaction of H2O2 with TS-1 has been investigated by the ESR technique. A well manufactured TS-1 has been considered, where the presence of extra-structural titanium can be excluded. The two main signals observed are attributed to oxygen radical species co-ordinated on structural titanium sites, and the relevant effect of water presence in the paramagnetic complex is discussed. A comparison is performed with O2-/Ti species in TS-1 obtained by reduction/re-oxidation procedures of Ti(IV). Moreover, some possible structural configurations of the paramagnetic sites are proposed, for two of which ab initio calculation yield g(z)-values quite close to the experimental ones.

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The interactions between iron and neuromelanin (NM) have been studied by means of EPR spectroscopy. The variable temperature EPR spectral features of a specimen of NM extracted from normal human midbrains clearly indicate that iron is present as polynuclear oxy-hydroxy ferric aggregates as well as isolated Fe(III) centres. Ferric oxy-hydroxy phases are typical of the iron storage proteins ferritin and hemosiderin, but the comparison of the variable temperature EPR spectra of ferritin and NM highlights significant differences between the two iron(III)oxy-hydroxy domains. Moreover, further investigations on melanin models synthesised in the presence of either ferritin or a ferric salt as iron sources suggest that the same pathway of formation and inclusion of the polynuclear iron oxide is operating in NM and in the model systems, whatever is the source of iron.

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The potential for free radical release has been measured by means of the spin trapping technique on three kinds of iron containing particulate: two asbestos fibers (chrysotile and crocidolite); an iron-exchanged zeolite and two iron oxides (magnetite and haematite). DMPO (5,5'-dimethyl-1-pirroline-N-oxide), used as spin trap in aqueous suspensions of the solids, reveals the presence of the hydroxyl and carboxylate radicals giving rise respectively to the two adducts [DMPO-OH] and [DMPO-CO2], each characterized by a well-defined EPR spectrum. Two target molecules have been considered: the formate ion to evidence potential for hydrogen abstraction in any biological compartment and hydrogen peroxide, always present in the phagosome during phagocytosis. The kinetics of decomposition of hydrogen peroxide has also been measured on all solids. Ferrozine and desferrioxamine, specific chelators of Fe(II) and Fe(III) respectively, have been used to remove selectively iron ions. Iron is implicated in free radical release but the amount of iron at the surface is unrelated to the amount of radicals formed. Only few surface ions in a particular redox and coordination state are active. Three different kinds of sites have been evidenced: one acting as H abstracter, the other as a heterogeneous catalyst for hydroxyl radical release, the third one related to catalysis of hydrogen peroxide disproportionation. In both mechanisms of free radical release, the Fe-exchanged zeolite mimics the behaviour of asbestos whereas the two oxides are mostly inert. Conversely magnetite turns out to be an excellent catalyst for hydrogen peroxide disproportionation while haematite is inactive also in this reaction. The results agree with the implication of a radicalic mechanism in the in vitro DNA damage and in the in vivo toxicity of asbestos.

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Reactive radicals at the surface of quartz or other SiO2 polymorphs have been studied by EPR in relation to their possible role in pathogenicity. All the examined dusts bear the characteristic radicals of silica ground in air: Si, SiO., SiO.2 (peroxyradical) and O2.- (superoxide ion), but some also show additional spectral lines belonging to other radical forms. Comparison of standard quartz dusts (DQ-12, Min-u-sil 5) with a natural quartz and with what obtained by grinding a very pure quartz crystal indicates that to a higher purity corresponds a higher radical population. Cristobalite and vitreous silica exhibit similar spectra, with larger proportion by respect to quartz, of partially reduced oxygen forms. The reactivity of the silica surface towards O2 and NaClO aqueous solutions are investigated by examining the modification in the EPR spectra induced by these treatments. A possible mechanism for fibrogenicity is proposed whereby, within the activated macrophage, a catalytic reaction occurs between surface functionalities and macrophage oxygen metabolites. This reaction would trigger the abnormal production of fibroblast stimulating factors, ending up with silicosis.

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