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.

Nanodiamond Effects on Cancer Cell Radiosensitivity: The Interplay between Their Chemical/Physical Characteristics and the Irradiation Energy.

Nanoparticles are being increasingly studied to enhance radiation effects. Among them, nanodiamonds (NDs) are taken into great consideration due to their low toxicity, inertness, chemical stability, and the possibility of surface functionalization. The objective of this study is to explore the influence of the chemical/physical properties of NDs on cellular radiosensitivity to combined treatments with radiation beams of different energies. DAOY, a human radioresistant medulloblastoma cell line was treated with NDs-differing for surface modifications [hydrogenated (H-NDs) and oxidized (OX-NDs)], size, and concentration-and analysed for (i) ND internalization and intracellular localization, (ii) clonogenic survival after combined treatment with different radiation beam energies and (iii) DNA damage and apoptosis, to explore the nature of ND-radiation biological interactions. Results show that chemical/physical characteristics of NDs are crucial in determining cell toxicity, with hydrogenated NDs (H-NDs) decreasing either cellular viability when administered alone, or cell survival when combined with radiation, depending on ND size and concentration, while OX-NDs do not. Also, irradiation at high energy (γ-rays at 1.25 MeV), in combination with H-NDs, is more efficient in eliciting radiosensitisation when compared to irradiation at lower energy (X-rays at 250 kVp). Finally, the molecular mechanisms of ND radiosensitisation was addressed, demonstrating that cell killing is mediated by the induction of Caspase-3-dependent apoptosis that is independent to DNA damage. Identifying the optimal combination of ND characteristics and radiation energy has the potential to offer a promising therapeutic strategy for tackling radioresistant cancers using H-NDs in conjunction with high-energy radiation.

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.

Top-Down Preparation of Nanoquartz for Toxicological Investigations.

Occupational exposure to quartz dust is associated with fatal diseases. Quartz dusts generated by mechanical fracturing are characterized by a broad range of micrometric to nanometric particles. The contribution of this nanometric fraction to the overall toxicity of quartz is still largely unexplored, primarily because of the strong electrostatic adhesion forces that prevent isolation of the nanofraction. Furthermore, fractured silica dust exhibits special surface features, namely nearly free silanols (NFS), which impart a membranolytic activity to quartz. Nanoquartz can be synthetized via bottom-up methods, but the surface chemistry of such crystals strongly differs from that of nanoparticles resulting from fracturing. Here, we report a top-down milling procedure to obtain a nanometric quartz that shares the key surface properties relevant to toxicity with fractured quartz. The ball milling was optimized by coupling the dry and wet milling steps, using water as a dispersing agent, and varying the milling times and rotational speeds. Nanoquartz with a strong tendency to form submicrometric agglomerates was obtained. The deagglomeration with surfactants or simulated body fluids was negligible. Partial lattice amorphization and a bimodal crystallite domain size were observed. A moderate membranolytic activity, which correlated with the number of NFS, signaled coherence with the previous toxicological data. A membranolytic nanoquartz for toxicological investigations was obtained.

Triggering Neurotransmitters Secretion from Single Cells by X-ray Nanobeam Irradiation.

The employment of ionizing radiation is a powerful tool in cancer therapy, but beyond targeted effects, many studies have highlighted the relevance of its off-target consequences. An exhaustive understanding of the mechanisms underlying these effects is still missing, and no real-time data about signals released by cells during irradiation are presently available. We employed a synchrotron X-ray nanobeam to perform the first real-time simultaneous measurement of both X-ray irradiation and in vitro neurotransmitter release from individual adrenal phaeochromocytoma (PC12) cells plated over a diamond-based multielectrode array. We have demonstrated that, in specific conditions, X-rays can alter cell activity by promoting dopamine exocytosis, and such an effect is potentially very attractive for a more effective treatment of tumors.

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.

Time and space resolved modelling of the heating induced by synchrotron X-ray nanobeams.

X-ray synchrotron sources, possessing high power density, nanometric spot size and short pulse duration, are extending their application frontiers up to the exploration of direct matter modification. In this field, the use of atomistic and continuum models is now becoming fundamental in the simulation of the photoinduced excitation states and eventually in the phase transition triggered by intense X-rays. In this work, the X-ray heating phenomenon is studied by coupling the Monte Carlo method (MC) with the Fourier heat equation, to first calculate the distribution of the energy absorbed by the systems and finally to predict the heating distribution and evolution. The results of the proposed model are also compared with those obtained removing the explicit definition of the energy distribution, as calculated by the MC. A good approximation of experimental thermal measurements produced irradiating a millimetric glass bead is found for both of the proposed models. A further step towards more complex systems is carried out, including in the models the different time patterns of the source, as determined by the filling modes of the synchrotron storage ring. The two models are applied in three prediction cases, in which the heating produced in Bi2Sr2CaCu2O8+δ microcrystals by means of nanopatterning experiments with intense hard X-ray nanobeams is calculated. It is demonstrated that the temperature evolution is strictly connected to the filling mode of the storage ring. By coupling the MC with the heat equation, X-ray pulses that are 48 ps long, possessing an instantaneous photon flux of ∼44 × 1013 photons s-1, were found to be able to induce a maximum temperature increase of 42 K, after a time of 350 ps. Inversely, by ignoring the energy redistribution calculated with the MC, peaks temperatures up to hundreds of degrees higher were found. These results highlight the importance of the energy redistribution operated by primary and secondary electrons in the theoretical simulation of the X-ray heating effects.

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.

Ti-Based Catalysts and Photocatalysts: Characterization and Modeling.

This perspective article aims to underline how cutting-edge synchrotron radiation spectroscopies such as extended X-ray absorption spectroscopy (EXAFS), X-ray absorption near edge structure (XANES), high resolution fluorescence detected (HRFD) XANES, X-ray emission spectroscopy (XES) and resonant inelastic X-ray scattering (RIXS) have played a key role in the structural and electronic characterization of Ti-based catalysts and photocatalysts, representing an important additional value to the outcomes of conventional laboratory spectroscopies (UV-Vis, IR, Raman, EPR, NMR etc.). Selected examples are taken from the authors research activity in the last two decades, covering both band-gap and shape engineered TiO2 materials and microporous titanosilicates (ETS-10, TS-1 and Ti-AlPO-5). The relevance of the state of the art simulation techniques as a support for experiments interpretation is underlined for all the reported examples.

Elucidating the structure and dynamics of CO ad-layers on MgO surfaces.

The combination of quantum-mechanical simulations and infrared absorption spectroscopy measurements provides a clear picture for a long standing puzzle in surface science: the actual structure and vibrational dynamics of the low-temperature ordered CO monolayer adsorbed on (001) MgO surfaces. The equilibrium structure of the commensurate (4 × 2) adsorbed phase consists of three CO molecules per primitive cell (surface coverage of 75%) located at two inequivalent sites: one molecule seats upright on top of a Mg site while two molecules, tilted off the normal to the surface, are symmetrically positioned relative to the upright one with anti-parallel projections on the surface. This configuration, long believed to be incompatible with measured polarization infrared spectra, is shown to reproduce all observed spectral features, including a new, unexpected one: the vanishing anharmonicity of CO stretching modes in the monolayer.

The Case of Formic Acid on Anatase TiO2 (101): Where is the Acid Proton?

Carboxylic-acid adsorption on anatase TiO2 is a relevant process in many technological applications. Yet, despite several decades of investigations, the acid-proton localization-either on the molecule or on the surface-is still an open issue. By modeling the adsorption of formic acid on top of anatase(101) surfaces, we highlight the formation of a short strong hydrogen bond. In the 0 K limit, the acid-proton behavior is ruled by quantum delocalization effects in a single potential well, while at ambient conditions, the proton undergoes a rapid classical shuttling in a shallow two-well free-energy profile. This picture, supported by agreement with available experiments, shows that the anatase surface acts like a protecting group for the carboxylic acid functionality. Such a new conceptual insight might help rationalize chemical processes involving carboxylic acids on oxide surfaces.

Direct-Write X-ray Nanopatterning: A Proof of Concept Josephson Device on Bi2Sr2CaCu2O8+δ Superconducting Oxide.

We describe the first use of a novel photoresist-free X-ray nanopatterning technique to fabricate an electronic device. We have produced a proof-of-concept device consisting of a few Josephson junctions by irradiating microcrystals of the Bi2Sr2CaCu2O8+δ (Bi-2212) superconducting oxide with a 17.6 keV synchrotron nanobeam. Fully functional devices have been obtained by locally turning the material into a nonsuperconducting state by means of hard X-ray exposure. Nano-XRD patterns reveal that the crystallinity is substantially preserved in the irradiated areas that there is no evidence of macroscopic crystal disruption. Indications are that O ions have been removed from the crystals, which could make this technique interesting also for other oxide materials. Direct-write X-ray nanopatterning represents a promising fabrication method exploiting material/material rather than vacuum/material interfaces, with the potential for nanometric resolution, improved mechanical stability, enhanced depth of patterning, and absence of chemical contamination with respect to traditional lithographic techniques.

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.

Doping change in the Bi-2212 superconductor directly induced by a hard X-ray nanobeam.

We describe the controlled use of a 17 keV X-ray synchrotron nanobeam to progressively change the oxygen doping level in Bi-2212 superconducting whisker-like single crystals. Our data combine structural and electrical information collected on the same crystals, showing a maximum change in the critical temperature Tc of 1.3 K and a maximum elongation of ∼1 Šin the c-axis length, compared to the as-grown conditions. Simulations of our experimental conditions by means of a finite element model exclude local heating induced by the X-ray nanobeam as a possible cause for the change in the doping level and suggest an important role of secondary electrons. These findings support the possible use of hard X-rays as a novel direct-writing, photoresist-free lithographic process for the fabrication of superconducting devices, with potential nanometric resolution and 3D capability.

The interaction of oxygen with the surface of CeO2-TiO2 mixed systems: an example of fully reversible surface-to-molecule electron transfer.

The interaction of oxygen with the surface of CeO2-TiO2 mixed oxides prepared via sol-gel was investigated by means of electron paramagnetic resonance (EPR). Upon admission of molecular oxygen onto the surface of the as prepared materials (which underwent final oxidative calcination) the formation of superoxide O2(-) ions is observed without the need for preliminary annealing in a vacuum and consequent oxygen depletion. The superoxide species is symmetrically adsorbed ("side-on" structure) on the top of a Ce(4+) ion. Surprisingly the electron transfer is fully reversible at room temperature having the typical behavior shown by molecular oxygen carriers, which, however, link to oxygen in a completely different manner ("end-on" structure). We suggest that the active sites are Ce(3+) ions present in the stoichiometric cerium titanate which forms during the synthesis. The features of these Ce(3+) ions must be different from those of the same ions formed in CeO2 by reductive treatments, which show a different reactivity to O2. The observation reported here opens up innovative perspectives in the field of heterogeneous catalysis and in that of sensors as the total reversibility of the electron transfer is observed in a significant range of oxygen pressure.

Designing functionalized nanodiamonds with hyaluronic acid-phospholipid conjugates for enhanced cancer cell targeting and fluorescence imaging capabilities.

Nanomedicine aims to develop smart approaches for treating cancer and other diseases to improve patient survival and quality of life. Novel nanoparticles as nanodiamonds (NDs) represent promising candidates to overcome current limitations. In this study, NDs were functionalized with a 200 kDa hyaluronic acid-phospholipid conjugate (HA/DMPE), enhancing the stability of the nanoparticles in water-based solutions and selectivity for cancer cells overexpressing specific HA cluster determinant 44 (CD44) receptors. These nanoparticles were characterized by diffuse reflectance Fourier-transform infrared spectroscopy, Raman spectroscopy, and photoluminescence spectroscopy, confirming the efficacy of the functionalization process. Scanning electron microscopy was employed to evaluate the size distribution of the dry particles, while dynamic light scattering and zeta potential measurements were utilized to evaluate ND behavior in a water-based medium. Furthermore, the ND biocompatibility and uptake mediated by CD44 receptors in three different models of human adenocarcinoma cells were assessed by performing cytofluorimetric assay and confocal microscopy. HA-functionalized nanodiamonds demonstrated the advantage of active targeting in the presence of cancer cells expressing CD44 on the surface, suggesting higher drug delivery to tumors over non-tumor tissues. Even CD44-poorly expressing cancers could be targeted by the NDs, thanks to their good passive diffusion within cancer cells.

Pt-TiO2 catalysts for glycerol photoreforming: comparison of anatase, brookite and rutile polymorphs.

We compared the H2 production from glycerol photoreforming for different TiO2 polymorphs, highlighting an increase of activity in the order Pt-rutile < Pt-P25 ≈ Pt-anatase < Pt-brookite with a different distribution of the reaction intermediates. We show that the highest ability to adsorb water and the different distribution of Pt active sites in brookite can positively influence its photoactivity.

Impact of metal and anion substitutions on the hydrogen storage properties of M-BTT metal-organic frameworks.

Microporous metal-organic frameworks are a class of materials being vigorously investigated for mobile hydrogen storage applications. For high-pressure storage at ambient temperatures, the M(3)[(M(4)Cl)(3)(BTT)(8)](2) (M-BTT; BTT(3-) = 1,3,5-benzenetristetrazolate) series of frameworks are of particular interest due to the high density of exposed metal cation sites on the pore surface. These sites give enhanced zero-coverage isosteric heats of adsorption (Q(st)) approaching the optimal value for ambient storage applications. However, the Q(st) parameter provides only a limited insight into the thermodynamics of the individual adsorption sites, the tuning of which is paramount for optimizing the storage performance. Here, we begin by performing variable-temperature infrared spectroscopy studies of Mn-, Fe-, and Cu-BTT, allowing the thermodynamics of H(2) adsorption to be probed experimentally. This is complemented by a detailed DFT study, in which molecular fragments representing the metal clusters within the extended solid are simulated to obtain a more thorough description of the structural and thermodynamic aspects of H(2) adsorption at the strongest binding sites. Then, the effect of substitutions at the metal cluster (metal ion and anion within the tetranuclear cluster) is discussed, showing that the configuration of this unit indeed plays an important role in determining the affinity of the framework toward H(2). Interestingly, the theoretical study has identified that the Zn-based analogs would be expected to facilitate enhanced adsorption profiles over the compounds synthesized experimentally, highlighting the importance of a combined experimental and theoretical approach to the design and synthesis of new frameworks for H(2) storage applications.

Probing the surface of nanosheet H-ZSM-5 with FTIR spectroscopy.

Herein we report FTIR in situ adsorption of molecular hydrogen, carbon monoxide, water, methanol, pyridine and 2,4,6-trimethylpyridine (collidine) on nanosheet H-ZSM-5 which was recently studied in the methanol to hydrocarbons (MTH) reaction. The nature of the hydroxyl groups and surface species are described in detail. The IR spectrum of nanosheet H-ZSM-5 is dominated by silanols, which saturate the external surfaces. The acidity of Si(OH)Al is comparable to that observed in the case of standard microcrystalline H-ZSM-5. The study of the external surface allows the recognition of Si(OH)Al species located at the channel entrance, which are mostly all accessible to hindered molecules such as collidine.

Elucidating NO x Surface Chemistry at the Anatase (101) Surface in TiO2 Nanoparticles.

Understanding NO x chemistry at titania nanoparticle surfaces is important for photocatalytic environmental remediation processes. We focus on this problem and put forward an experimental-computational approach based on vibrational spectroscopy grounds. Temperature-dependent IR experiments of NO x adsorption on shape-engineered nanoparticle (101) anatase surfaces are paired with power spectra obtained from Born-Oppenheimer trajectories. Then, the harmonic versus anharmonic vibrational frequencies of several adsorption scenarios are directly compared with the IR experiments. We conclude that molecules are adsorbed mainly by the N-end side and both the intermolecular interactions between adsorbed molecules and (NO)2 dimer formation are responsible for the main NO adsorption spectroscopic features. We also investigate the spectroscopy and the mechanism of formation on defective anatase surfaces of the long-lived greenhouse gas N2O.