Heterogeneous singlet oxygen generation: in-operando visible light EPR spectroscopy

The use of photosensitizers immobilized on mesoporous materials to produce singlet oxygen (¹O₂) has opened a new way to synthetic and environmental applications due to the fast development of flow photochemistry and continuous-flow microreactors. ¹O₂-based photosensitized processes can be employed for the degradation of organic pollutants in an aqueous medium and the photosensitizer can be covalently attached to the support and separated from the effluent reducing the environmental impact. The aim of the present paper is to evaluate the ¹O₂ generation of Rose Bengal (RB) in homogeneous and heterogeneous systems using in-operando evaluation. Mesoporous SiO₂ nanoparticles (MSNs) were successfully conjugated with RB (MSN-RB) and electron paramagnetic resonance (EPR) spectroscopy in combination with the spin trap TEMP was employed to obtain paramagnetic TEMPO via generated ¹O₂ when RB or MSN-RB are exposed to visible light. Additionally, EPR/DMPO was used to exclude the possible generation of other reactive oxygen species (ROS) by the functionalized nanoparticles. We found that in situ ¹O₂ generation was enhanced when the same amount of RB is immobilized inside of mesoporous SiO₂.