Blanc SIMI 8 - Sciences de l'information, de la matière et de l'ingénierie : Chimie du solide, colloïdes, physicochimie

MEcanism of Photochemistry at Interfaces : Study of Transient reactive Oxygen species – MEPHISTO

Atmospheric oxygen: shine a light to make it more reactive

Reactive Oxygen Species (ROS), generated by irradiation of inorganic or organic photocatalysts, are able to oxidize a great number of molecules. They were produced by different types of materials and analyzed for a better understanding of oxidation mechanisms.

Materials to activate oxygen with light and understanding oxydation mechanisms

Photo-induced activation of oxygen in ambient air may contribute to chemical or bacteriological decontamination of various environmental media, particularly the air. Besides the well known combination of titanium dioxide and UV light, a strategy working in the visible region of the spectrum is to fix organic photo-sensitizers on inert substrates. The MEPHISTO project therefore examines the generation and reactivity of reactive oxygen species at gas-solid interfaces. The aims are the preparation of transparent, thin films or monoliths showing activity in the UV/visible spectrum, detection of the transient species and measurement of reactive oxygen species produced in the presence of air and water. Our improved understanding of the mechanisms of activation of oxygen will further rational design of photo-active materials with applications in several fields: anti-germ films, textiles and surfaces, air depollution, photodynamic cancer therapy,....

Synthesis and photophysical characterisation of both commercial and novel photosensitizers was completed, allowing production of transparent bulk hybrid materials based on silica. Thin pure or doped mesoporous TiO2 films were prepared and their photocatalytic performances established in situ, by ellipso-porometry. Stationary and time resolved spectroscopy was applied to analyse the formation of reactive transient species (such as singlet oxygen) at the gas-solid interface of bulk and thin film materials undergoing irradiation. We determined how the substrate influences the properties of the dyes and applied fluorescence microscopy to determine the location, physical state and mobility of the dyes in the materials. Several tests of the photocatalytic activity were performed: oxidation of gaseous acetone (solid-gas), decomposition of stearic acid (solid-solid), oxidation of specific probes in several solvents (solid-liquid).

New dyes for photosensitisation of singlet oxygen in the graftable cyano-anthracene series were synthesised and characterised. Techniques for detecting reactivity at gas-solid interfaces were improved. The lifetime and reactivity of singlet oxygen generated in absence of solvents show this species diffuses and reacts fast in the gas phase. These results led to new international collaborations on new strategies for decontamination by photochemically activated materials.

The project favored the rational design strategy for applied photoactive materials in the frame of new projects. Especially, a joined supervision of a PhD thesis with Bilbao University aims at studying new BODIPYS sensitizers and at grafting them on nanoparticles for Photodynamic Therapy. Preliminary encouraging results were also obtained for the prepapartion of bulk silica materials containing fluorescent dyes for lasing materials.
The controlled production of singlet oxygen for bactericidal effects or for the synthesis of high added value products is currently used in the frame of new applications in 2015 calls. New international collaborations (New-Orleans or Antwerpen University) will develop the understanding of mechanisms at the solid-gas and solid-liquid interface, specially for water tretament. From its expertise in gas-solid reactions, the IPREM team now masters the use and analysis of Volatile Organic Compounds, which could initiate a promising collaboration on the impact of atmospheric pollutants on cells. At last, thanks to the ROS detection, the better understanding of the oxidative photo-induced mechanisms will be extended to water treatment and to the photoreactivity of organic micropollutants (ANR CophotoFe).
After the extensive characterization of the specificity of TiO2 mesoporous films, new ones were prepared and will be analyzed by fluorescence microscopy with single molecule probes. The skills developed during the project also led to a new fruitful collaboration on the microscopic study of ctristalline growth at the liquid-liquid interface and a new project with a big company is currently under construction.

Three PhD's were defended, of which one was entirely financed by the ANR. Three post-doctoral researchers worked on the project. Eight papers were published in international journals. Five of them involved more than one of the partner teams. A chapter was contributed to a book on reference photosensitisers of singlet oxygen. Three invited lectures and ten talks were presented at international conferences.

Despite numerous commercial applications of photocatalytic and photosensitised oxidation reaction in the absence of solvent, such as self-cleaning surfaces, batericidal materials or materials for passive or active decontamination of air, basic research on these processes at the gas-solid interface is still today the subject of topical interest.
The mechanisms of these reactions in solution are well known. The photochemical activity of TiO2 in aqueous solution involves both the oxidising properties of photogenerated holes (h+) and the formation of hydroxyl radicals HO•, or the less reactive superoxide anion O2•-. Similarly, the quantum yields of formation and lifetime of singlet oxygen (1O2), produced by photosensitisation in various solvents are well known.
MEPHISTO is a basic research project, aimed at a deeper understanding of the generation and behaviour of reactive oxygen species (ROS) at solid-gas interfaces encountered in such applications as photocatalysis for air treatment or sensitized photo-oxidation reactions. A detailed knowledge of the processes involved is the first step to development of original materials, particularly modified TiO2 or hybrid organic-inorganic materials, with specific and optimized photo-oxidizing properties.
Therefore, MEPHISTO will:
- prepare various TiO2-based reference materials, to be activated under visible light, via doping with rare earths or transition metals, via partial replacement of oxygen by other anions (e.g. nitrogen), or via photosensitisation with organics. These materials will be prepared as thin films or as monoliths, with variable matrix fractions of anatase TiO2.
- prepare organic-inorganic hybrid materials based on photochemically inert matrices (SiO2, ZrO2) or hybrid TiO2/ZrO2 with various hydrophilic/hydrophobic properties, grafted with organic sensitizers (covalent bonding to the surface via silyl or phosphonate derivatives or iono-covalent bonding). These hybrid materials will present various meso-scale organisations (2D hexagonal, 3D hexagonal, cubic), with different pore sizes, using templates such as cationic surfactants, or non-ionic templates. They will be prepared as thin films or monoliths.
- apply different spectroscopic methods (steady-sate or time resolved) to study the production of singlet oxygen and, in the case of transparent monoliths containing PS’s, transient species such as radical cations and anions, triplet states,..., wherever possible directly at the gas-solid interface, under different conditions (oxygen and water content).
- apply fluorescence microscopy to locating the photosensitizers (in hybrid materials), or to monitoring production and diffusion of reactive oxygen species in inorganic materials, including state of the art single molecule detection techniques to monitor molecular orientation, mobility, activity and quenching by atmospheric oxygen, water or electron donors.
-correlate experimental results with models, to improve our understanding of the structure and dynamics of the photoactive materials, as well as the diffusion of reactive species inside such matrices by molecular modelling.

Project coordination

Sylvie LACOMBE (CENTRE NATIONAL DE LA RECHERCHE SCIENTIFIQUE - DELEGATION AQUITAINE LIMOUSIN) – sylvie.lacombe@univ-pau.fr

The author of this summary is the project coordinator, who is responsible for the content of this summary. The ANR declines any responsibility as for its contents.

Partner

CNRS UMR 7574 LCMCP UNIVERSITE PARIS VI [PIERRE ET MARIE CURIE]
CNRS UMR 5623 IMRCP CENTRE NATIONAL DE LA RECHERCHE SCIENTIFIQUE - DELEGATION REGIONALE MIDI-PYRENEES
CNRS UMR 6505 LPMM CENTRE NATIONAL DE LA RECHERCHE SCIENTIFIQUE - DELEGATION REGIONALE RHONE-AUVERGNE
CNRS UMR 5254 IPREM CENTRE NATIONAL DE LA RECHERCHE SCIENTIFIQUE - DELEGATION AQUITAINE LIMOUSIN

Help of the ANR 720,000 euros
Beginning and duration of the scientific project: - 48 Months

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