Nearly free surface silanols are the critical molecular moieties that initiate the toxicity of silica particles.

Inhalation of silica particles can induce inflammatory lung reactions that lead to silicosis and/or lung cancer when the particles are biopersistent. This toxic activity of silica dusts is extremely variable depending on their source and preparation methods. The exact molecular moiety that explains and predicts this variable toxicity of silica remains elusive. Here, we have identified a unique subfamily of silanols as the major determinant of silica particle toxicity. This population of "nearly free silanols" (NFS) appears on the surface of quartz particles upon fracture and can be modulated by thermal treatments. Density functional theory calculations indicates that NFS locate at an intersilanol distance of 4.00 to 6.00 Å and form weak mutual interactions. Thus, NFS could act as an energetically favorable moiety at the surface of silica for establishing interactions with cell membrane components to initiate toxicity. With ad hoc prepared model quartz particles enriched or depleted in NFS, we demonstrate that NFS drive toxicity, including membranolysis, in vitro proinflammatory activity, and lung inflammation. The toxic activity of NFS is confirmed with pyrogenic and vitreous amorphous silica particles, and industrial quartz samples with noncontrolled surfaces. Our results identify the missing key molecular moieties of the silica surface that initiate interactions with cell membranes, leading to pathological outcomes. NFS may explain other important interfacial processes involving silica particles.

Physico-Chemical Approaches to Investigate Surface Hydroxyls as Determinants of Molecular Initiating Events in Oxide Particle Toxicity.

The study of molecular recognition patterns is crucial for understanding the interactions between inorganic (nano)particles and biomolecules. In this review we focus on hydroxyls (OH) exposed at the surface of oxide particles (OxPs) which can play a key role in molecular initiating events leading to OxPs toxicity. We discuss here the main analytical methods available to characterize surface OH from a quantitative and qualitative point of view, covering thermogravimetry, titration, ζ potential measurements, and spectroscopic approaches (NMR, XPS). The importance of modelling techniques (MD, DFT) for an atomistic description of the interactions between membranes/proteins and OxPs surfaces is also discussed. From this background, we distilled a new approach methodology (NAM) based on the combination of IR spectroscopy and bioanalytical assays to investigate the molecular interactions of OxPs with biomolecules and membranes. This NAM has been already successfully applied to SiO2 particles to identify the OH patterns responsible for the OxPs' toxicity and can be conceivably extended to other surface-hydroxylated oxides.

From gaseous HCN to nucleobases at the cosmic silicate dust surface: an experimental insight into the onset of prebiotic chemistry in space.

HCN in the gas form is considered as a primary nitrogen source for the synthesis of prebiotic molecules in extraterrestrial environments. Nevertheless, the research mainly focused on the reactivity of HCN and its derivatives in aqueous systems, often using external high-energy supply in the form of cosmic rays or high energy photons. Very few studies have been devoted to the chemistry of HCN in the gas phase or at the gas/solid interphase, although they represent the more common scenarios in the outer space. In this paper we report about the reactivity of highly pure HCN in the 150-300 K range at the surface of amorphous and crystalline Mg2SiO4 (forsterite olivine), i.e. of solids among the constituents of the core of cosmic dust particles, comets, and meteorites. Amorphous silica and MgO were also studied as model representatives of Mg2SiO4 structural building blocks. IR spectroscopic results and the HR-MS analysis of the reaction products revealed Mg2+O2- acid/base pairs at the surface of Mg2SiO4 and MgO to be key in promoting the formation of HCN oligomers along with imidazole and purine compounds, already under very mild temperature and HCN pressure conditions, i.e. in the absence of external energetic triggers. Products include adenine nucleobase, a result which supports the hypothesis that prebiotic molecular building blocks can be easily formed through surface catalytic processes in the absence of high-energy supply.

Probing Molybdenum Active Sites during In Situ Photoreduction of the Mo6+/SiO2 Catalyst.

The photoreduction of the Mo6+/SiO2 system with CO was investigated in situ, employing a recently developed experimental setup allowing for the acquisition of transmission FT-IR spectra under simultaneous UV irradiation. Carbon monoxide, besides acting as a reducing agent in such processes, is also a useful probe molecule able to detect coordinatively unsaturated sites exposed on the surface. The unprecedented quality of the spectroscopic data, obtained as a function of the reduction time, allowed us to better rationalize the different mechanisms previously proposed for the photoreduction process. These results, coupled with UV-Vis spectroscopic data, shed light on the oxidation state and surface structure of supported molybdenum species, which are key active sites for several important reactions, such as selective oxidation, polymerization, hydrodesulfurization, epoxidation and olefin metathesis.

Morphology, Surface Structure and Water Adsorption Properties of TiO2 Nanoparticles: A Comparison of Different Commercial Samples.

Water is a molecule always present in the reaction environment in photocatalytic and biomedical applications of TiO2 and a better understanding of its interaction with the surface of TiO2 nanoparticles is crucial to develop materials with improved performance. In this contribution, we first studied the nature and the surface structure of the exposed facets of three commercial TiO2 samples (i.e., TiO2 P25, SX001, and PC105) by electron microscopy and IR spectroscopy of adsorbed CO. The morphological information was then correlated with the water adsorption properties, investigated at the molecular level, moving from multilayers of adsorbed H2O to the monolayer, combining medium- and near-IR spectroscopies. Finally, we assessed in a quantitative way the surface hydration state at different water equilibrium pressures by microgravimetric measurements.