Stimulated raman generation of aqueous singlet oxygen without photosensitizers

Authors: MARCANO OLAIZOLA, ARISTIDES - Delaware State University; KUIS, ROBINSON - University Of Maryland; JOHNSON, ANTHONY - University Of Maryland; Kingsley, David

Submitted to: Photochemistry and Photobiology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 9/1/2022
Publication Date: 9/8/2022
Citation: Marcano Olaizola, A., Kuis, R., Johnson, A., Kingsley, D.H. 2022. Stimulated raman generation of aqueous singlet oxygen without photosensitizers. Photochemistry and Photobiology. 235. https://doi.org/10.1016/j.jphotobiol.2022.112562.
DOI: https://doi.org/10.1016/j.jphotobiol.2022.112562

Interpretive Summary: Singlet oxygen is a reactive short-lived excited oxygen molecule that can act as nature’s disinfectant reducing bacteria, viruses, and fungi and even killing cancer cells. As part of previous USDA research evaluating the potential of visible blue light to inactivate viral pathogens, it became clear that inactivation observed was oxygen-dependent and presumably singlet oxygen. The accepted procedure to generate singlet oxygen consists of using a photosensitizer, also termed an enhancer. Here we report a unique finding that visible blue light can produce singlet oxygen without requiring photosensitizers in water. Furthermore, we characterize the mechanism of production as a stimulated Raman generation process involving oxygen dissolved in water and heavy water. Singlet oxygen is evidenced by detecting red photons that disappear when using an oxygen quencher. We note that energy difference between input and output photons is precisely the energy needed to excite O2. We confirm the presence of singlet oxygen in these experiments by measuring the destruction of diluted solutions of uric acid, a method generally used for singlet oxygen detection. While more of a fundamental discovery, it is anticipated that this finding may open new avenues for basic research and applied investigation.

Technical Abstract: Singlet oxygen is traditionally produced via enhancer molecules such as methylene blue, which function as catalysts. Here we investigate stimulated Raman generation of singlet oxygen from dissolved oxygen in both water (H2O) and heavy water (D2O) using nanosecond-pulsed visible blue laser light in the 400 - 440 nm spectral region without singlet oxygen enhancers. We report an oxygen-dependent Stokes peak in the red spectrum (600 - 670 nm) that is identical when produced in H2O and D2O. These red Stokes photons are not detected when an oxygen quencher is present. Temporal photodepletion of the uric acid absorbance peak at 294 nm confirms singlet oxygen generation. We postulate that a two-photon stimulated Raman process produces singlet oxygen from O2 dissolved within the water and heavy water and note that the energy difference between input and output photons of 0.97 eV is precisely the energy needed to excite O2 to its singlet state.