Functionalized nanomaterials for an efficient photogeneration of 1O2 in microreactors

In this work, we have applied heterogeneous photocatalysis in flow chemistry for the selective oxygenation of organic molecules by photoactivation of molecular oxygen (O2) into singlet oxygen (1O2) using visible light. The main goal was to associate nanotechnology with flow photochemistry to develop a new heterogeneous system allowing 1O2 photogeneration in continuous-flow microreactors. Some obstacles to the use of 1O2, such as the degradation of the photosensitizer and the low photonic efficiency due to the low lifetime of 1O2, have been solved through (i) the formation of 1O2 using the chemically supported Rose Bengal (RB) in porous SiO2 nanospheres and (ii) the synthesis of plasmon core-shell nanoparticles for enhanced 1O2 generation.
As a preliminary study, different parameters involved in the homogeneous 1O2 photooxygenation of the methionine amino acid were evaluated in batch in order to facilitate the transition to continuous-flow microreactors. The use of this recent technology has shown the possibilities to perform 1O2 photosensitized reactions efficiently. However, the degradation of the Rose Bengal photosensitizer in the homogeneous system was high (70%).
This photosensitizer has then been immobilized inside porous SiO2 nanoparticles (NPs). The system has been completely characterized and a stable covalent bond has been detected by NMR and IR. This immobilization has allowed to reduce the photodegradation to 17% and to keep a high yield. Moreover, the functionalized NPs have been tested in different continuous-flow microreactor setups to oxidize six organic model molecules. The ease of separation and reutilization of the heterogeneous system have made the process much more attractive.
Additionally, Au bipyramid nanostructures with a 12 nm SiO2 shell have been also synthesized to maximize the plasmon coupling between the photosensitizer and the metallic surface in order to achieve a metal-enhanced photosensitization of Rose Bengal in presence of the synthesized core-shell NPs. These obtained nanostructures were characterized by different techniques. Finally, the metal-enhanced 1O2 generation was measured by three different methods, demonstrating that the synthesized nanostructures are able to increase the efficiency of the photochemical process and to decrease the photodegradation of the photosensitizer.