Spatial and temporal electrochemical control of singlet oxygen production and decay in photosensitized experiments

Singlet Oxygen Photophysics: From Liquid Solvents to Mammalian Cells

Molecular oxygen, O2, has long provided a cornerstone for studies in chemistry, physics, and biology. Although the triplet ground state, O2(X3Σg-), has garnered much attention, the lowest excited electronic state, O2(a1Δg), commonly called singlet oxygen, has attracted appreciable interest, principally because of its unique chemical reactivity in systems ranging from the Earth's atmosphere to biological cells. Because O2(a1Δg) can be produced and deactivated in processes that involve light, the photophysics of O2(a1Δg) are equally important. Moreover, pathways for O2(a1Δg) deactivation that regenerate O2(X3Σg-), which address fundamental principles unto themselves, kinetically compete with the chemical reactions of O2(a1Δg) and, thus, have practical significance. Due to technological advances (e.g., lasers, optical detectors, microscopes), data acquired in the past ∼20 years have increased our understanding of O2(a1Δg) photophysics appreciably and facilitated both spatial and temporal control over the behavior of O2(a1Δg). One goal of this Review is to summarize recent developments that have broad ramifications, focusing on systems in which oxygen forms a contact complex with an organic molecule M (e.g., a liquid solvent). An important concept is the role played by the M+•O2-• charge-transfer state in both the formation and deactivation of O2(a1Δg).

The effect of iodide and temperature on enhancing antibacterial properties of nanoparticles with an encapsulated photosensitizer

Aqueous dispersions of sulfonated polystyrene nanoparticles (average diameter: 30 ± 14 nm) with encapsulated 5,10,15,20-tetraphenylporphyrin (TPP) are promising candidates for antibacterial treatments due to the photogeneration of cytotoxic singlet oxygen species O₂(¹Δ_g) under physiological conditions using visible light. The antibacterial effect on gram-negative Escherichia coli was significantly enhanced after the addition of nontoxic potassium iodide (0.001-0.01 M) because photogenerated O₂(¹Δ_g) oxidized iodide to I₂/I₃^-, which is another antibacterial species. The improved antibacterial properties were predicted using luminescence measurements of O₂(¹Δ_g), transient absorption of TPP triplets and singlet oxygen-sensitized delayed fluorescence (SODF). In contrast to a solution of free photosensitizers, the aqueous dispersion of photoactive nanoparticles did not exhibit any quenching of the excited states after the addition of iodide or any tendency toward aggregation and/or I₃^--induced photo-aggregation. We also observed a decrease in the lifetime of O₂(¹Δ_g) and a significant increase in SODF intensity at higher temperatures, due to the increased oxygen diffusion coefficient in nanoparticles and aqueous surroundings. This effect corresponds with the significantly stronger antibacterial effect of nanoparticles at physiological temperature (37 °C) in comparison with that at room temperature (25 °C).

Anion exchange nanofiber materials activated by daylight with a dual antibacterial effect

Anion exchange polystyrene nanofiber materials (AE) were prepared by electrospinning followed by two-step functionalization of the nanofiber surface by chlorosulfonic acid and ethylendiamine. The photoactive character of these materials was introduced through adsorption of the tetra-anionic 5,10,15,20-tetrakis-(4-sulfonatophenyl)porphyrin photosensitizer (TPPS-AE) on the nanofiber surface or by encapsulation of the nonpolar 5,10,15,20-tetraphenylporphyrin photosensitizer (AE(TPP)) into the nanofibers. Anion exchange nanofiber materials with porphyrins are characterized by a high ion-exchange capacity, photogeneration of singlet oxygen O2((1)Δg), and singlet oxygen-sensitized delayed fluorescence. Due to the photogeneration of cytotoxic O2((1)Δg), the nanofibers exhibited oxidation of the external substrates in aqueous solution and an efficient antibacterial effect when activated by simulated daylight. Adsorption of both TPPS and I(-) on the surface of AE led to the formation of more efficient I-TPPS-AE materials. Rapid photooxidation of I(-) by O2((1)Δg), and the formation of another cytotoxic species, I3(-), on the surface of the nanofibers were responsible for the increased antibacterial properties of I-TPPS-AE and the prolonged antibacterial effect in the dark.

Photomediated Oxidation of Atomically Precise Au25(SC2H4Ph)18(-) Nanoclusters

The anionic charge of atomically precise Au25(SC2H4Ph)18(-) nanoclusters (abbreviated as Au25(-)) is thought to facilitate the adsorption and activation of molecular species. We used optical spectroscopy, nonaqueous electrochemistry, and density functional theory to study the interaction between Au25(-) and O2. Surprisingly, the oxidation of Au25(-) by O2 was not a spontaneous process. Rather, Au25(-)-O2 charge transfer was found to be a photomediated process dependent on the relative energies of the Au25(-) LUMO and the O2 electron-accepting level. Photomediated charge transfer was not restricted to one particular electron accepting molecule or solvent system, and this phenomenon likely extends to other Au25(-)-adsorbate systems with appropriate electron donor-acceptor energy levels. These findings underscore the significant and sometimes overlooked way that photophysical processes can influence the chemistry of ligand-protected clusters. In a broader sense, the identification of photochemical pathways may help develop new cluster-adsorbate models and expand the range of catalytic reactions available to these materials.