Stefano Livraghi

Non-UV-Induced Radical Reactions at the Surface of TiO2 Nanoparticles That May Trigger Toxic Responses

Kept in the dark: The non-photocatalytic generation of free radicals from fine and ultrafine TiO(2) particles has been studied by means of a spin-trapping/ESR spectroscopy technique (see figure). The amount and kind of free radicals generated depends on the crystalline structure, but not on the particle dimensions.Titania is generally considered to be an inert and safe material. Several studies, however, have reported that nanosized TiO(2) may elicit toxic effects. In some cases the observed adverse effects have been related to free radicals. Although new studies mainly concern irradiated titania, the role and the mechanisms of the generation of free radicals by TiO(2) in the absence of UV irradiation are not well known. The purpose of the present study is to investigate the free-radical-generation mechanisms by nano- and micronsized anatase or rutile powders under normal laboratory illumination or in the dark by means of a spin-trapping/ESR spectroscopy technique. This technique is used to identify the nature and the amount of free radicals released in solution, and in the solid-state to characterise the paramagnetic centres at the surface of particles that may participate in the reactions. The following radical-generating mechanisms have been considered: 1) the generation of oxygenated free radicals (HO(2) (.), O(2) (.-), HO(.)) following the reaction of TiO(2) with oxygen, water or H(2)O(2) and 2) the generation of carbon-centred radicals following the cleavage of the C--H bond in a model molecule. Although no free radicals were detected in a simply buffered solution, anatase and rutile generated O(2) (.-) and HO(.), respectively, in the presence of H(2)O(2). Both polymorphs were also active in the cleavage of the C--H bond. Although the formation of O(2) (.-) appears to be related to exposure to sunlight, the generation of HO(.) and carbon-centred free radicals also occurs in the dark. When samples of equal surface area were tested, micron- and nanosized anatase was found to react in the same way indicating that a reduction in diameter does not generate new kinds of reactive sites. The data presented herein may have implications in the assessment of the health risk associated with the exposure to TiO(2) nanoparticles and in the ecotoxicological impact following their possible leakage into the environment.

Charge trapping in TiO2 polymorphs as seen by Electron Paramagnetic Resonance spectroscopy

Electron Paramagnetic Resonance (EPR) techniques have been employed to investigate charge carrier trapping in the two main TiO2 polymorphs, anatase and rutile, with particular attention to the features of electron trapping sites (formally Ti(3+) ions). The classic CW-EPR technique in this case provides signals based on the g tensor only. Nevertheless a systematic analysis of the signals obtained in the various cases (anatase and rutile, surface and bulk centers, regular and defective sites) has been performed providing useful guidelines on a field affected by some confusion. The problem of the localization of the electron spin density has been tackled by means of Pulse-EPR hyperfine techniques on samples appositely enriched with (17)O. This approach has led to evidence of a substantial difference, in terms of wavefunction localization between anatase (electrons trapped in regular lattice sites exhibiting delocalized electron density) and rutile (interstitial sites showing localized electron density).

The nature of paramagnetic species in nitrogen doped TiO2 active in visible light photocatalysis.

Nitrogen doped TiO2, a novel photocatalyst active in the decomposition of organic pollutants using visible light, contains two different types of paramagnetic centres (neutral NO radicals and NO2(2-) type radical ions respectively) which are likely related to specific properties of the solid.

The nitrogen–boron paramagnetic center in visible light sensitized N–B co-doped TiO2. Experimental and theoretical characterization

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.

Probing the Local Environment of Ti3+ Ions in TiO2 (Rutile) by 17O HYSCORE

Reduced states in TiO(2) : (17)O hyperfine sublevel correlation spectroscopy was used to monitor the local environment of stable Ti(3+) ions generated in a (17)O-enriched polycrystalline TiO(2) (rutile) sample. A hyperfine interaction of about 8 MHz is found, which is analogous to that observed for molecular Ti(3+) aqua complex cations and suggests a localized nature of the unpaired electron wave function for these centers at 4 K.

Hydration structure of the Ti(III) cation as revealed by pulse EPR and DFT studies: new insights into a textbook case.

The (17)O and (1)H hyperfine interactions of water ligands in the Ti(III) aquo complex in a frozen solution were determined using Hyperfine Sublevel Correlation (HYSCORE) and Pulse Electron Nuclear Double Resonance (ENDOR) spectroscopies at 9.5 GHz. The isotropic hyperfine interaction (hfi) constant of the water ligand (17)O was found to be about 7.5 MHz. (1)H Single Matched Resonance Transfer (SMART) HYSCORE spectra allowed resolution of the hfi interactions of the two inequivalent water ligand protons and the relative orientations of their hfi tensors. The magnetic and geometrical parameters extracted from the experiments were compared with the results of DFT computations for different geometrical arrangements of the water ligands around the cation. The theoretical observable properties (g tensor (1)H and (17)O hfi tensors and their orientations) of the [Ti(H(2)O)(6)](3+) complex are in quantitative agreement with the experiments for two slightly different geometrical arrangements associated with D(3d) and C(i) symmetries.

Trapped molecular species in N-doped TiO2

Nitrogen-doped TiO2, a novel photocatalyst active in the decomposition of organic pollutants using visible light, contains several different types of paramagnetic centers. These are molecular species, such as NO and NO2 radicals and other species, deeply interacting with the TiO2 structure. All or part of these species is related to specific properties of the solid. Electron paramagnetic resonance has been employed to characterize the N-containing paramagnetic species present in N-doped anatase TiO2 powders obtained via sol-gel synthesis. In the present work attention is focused on molecular species generated during the synthesis process and segregated in cavities of the TiO2 structure.

Inactivation of TiO2 nano-powders for the preparation of photo-stable sunscreens via carbon-based surface modification

A new protocol based upon the modification of the TiO2 surface by thermal decomposition of ethylene glycol to reduce the ability to generate free radicals leaving unaltered the UV filtering activity has been recently reported by some of us (S. Livraghi et al., Chem. Commun., 2010, 46, 8478). Here we explore the efficacy of three other organic modifiers i.e. ethanol, glycolic acid and citric acid in comparison to ethylene glycol. The ability of the modified powders to generate free radicals was evaluated by means of EPR/spin trapping and probing techniques and the filtering efficacy by UV-Vis diffuse reflectance. The mechanism of inactivation was investigated by EPR spectroscopy and thermogravimetric analysis (TGA) coupled with FT-IR spectroscopy. The results indicate that the treatment with organic modifiers having oxygenated functionalities (hydroxyl or carboxyl groups) in a vicinal position inhibits both reductive and oxidative photocatalytic activity of TiO2 but not the generation of singlet oxygen. The effect may be relatable to the presence of both carbonaceous residues, probably acting as scavengers of free radicals, and carboxylate/carbonate species adsorbed at the surface acting as a protective coating.

Inhibition of the ROS-mediated cytotoxicity and genotoxicity of nano-TiO2 toward human keratinocyte cells by iron doping

Nano-TiO2 powders are widely used in sunscreen lotions as UV filters in combination with other substances. The activation of TiO2 by UV rays leads to the release of reactive oxygen species (ROS, e.g., hydroxyl radicals and singlet oxygen) which are potentially harmful. For this reason the TiO2 particles are generally coated with inert materials (e.g., silica or alumina) that inhibit such reactivity. Alternatively, the release of ROS may be inhibited by introducing in the TiO2 lattice doping elements. In the present study we report a new modification consisting in a wet impregnation of TiO2 with iron salts followed by a thermal treatment that results in an inhibition of the surface reactivity. The insertion of iron ions also gradually reduces the ability of photo-activated TiO2 to cleave DNA and proteins. At the same time, a clear inhibition of cyto- and geno-toxicity toward human (HaCaT) keratinocytes was observed. The data presented herein suggest the insertion of Fe3+ ions at the surface of nano-TiO2 as a promising strategy to reduce the photo-induced toxicity of nano-TiO2 powders.

On the Redox Mechanism Operating along C2H2 Self-Assembly at the Surface of TiO2

The interaction of acetylene with the TiO2 surface at room temperature entails a complex set of self-assembly reactions with the formation of products having relatively high molecular weight. In a previous paper by some of us (Jain, S. M.; et al. J. Mater. Chem. A 2014, 2, 12247-12254), the C2H2-TiO2 reaction has been monitored, essentially by Fourier transform infrared spectroscopy, at the surface of P25 (a mixture of anatase and rutile, typical benchmark material in the field of photocatalysis) in order to elucidate the nature of the products of this surface reaction. In the present paper, the same process was followed, for the first time, using electron paramagnetic resonance (EPR) and monitoring by the thermogravimetric analysis the weight loss of the material upon heating in order to further investigate the complex mechanism of the surface reaction. This was done using pure anatase and comparing the EPR results with those concerning both rutile and P25. The self-assembly mechanism occurring at the interface is accompanied by the formation of EPR visible Ti(3+) centers due to electrons injection in the TiO2 substrate. This finding clarifies that at least one of the reaction channels of this complex process (namely, the formation of polycyclic aromatic hydrocarbons) is based on the heterolytic dissociative chemisorption of acetylene, followed by a redox interaction between the adsorbate and the solid, which allows the creation of the building blocks necessary to assemble polyaromatic molecules.