Antimicrobial photodynamic therapy (PDT) is used for the eradication of pathogenic microbial cells and involves the light excitation of dyes in the presence of O2, yielding reactive oxygen species including the hydroxyl radical (•OH) and singlet oxygen (1O2). In order to chemically enhance PDT by the formation of longer-lived radical species, we asked whether thiocyanate (SCN−) could potentiate the methylene blue (MB) and light-mediated killing of the gram-positive Staphylococcus aureus and the gram-negative Escherichia coli. SCN− enhanced PDT (10 μM MB, 5J/cm2 660 nm hv) killing in a concentration-dependent manner of S. aureus by 2.5 log10 to a maximum of 4.2 log10 at 10 mM (P < 0.001) and increased killing of E. coli by 3.6 log10 to a maximum of 5.0 log10 at 10 mM (P < 0.01). We determined that SCN− rapidly depleted O2 from an irradiated MB system, reacting exclusively with 1O2, without quenching the MB excited triplet state. SCN− reacted with 1O2, producing a sulfur trioxide radical anion (a sulfur-centered radical demonstrated by EPR spin trapping). We found that MB-PDT of SCN− in solution produced both sulfite and cyanide anions, and that addition of each of these salts separately enhanced MB-PDT killing of bacteria. We were unable to detect EPR signals of •OH, which, together with kinetic data, strongly suggests that MB, known to produce •OH and 1O2, may, under the conditions used, preferentially form 1O2.

Aerobic photoactivation of photosensitizing dye molecules can lead to the formation of oxygen radicals, singlet oxygen and other partially reduced oxygen species, collectively called reactive oxygen species (ROS), which are responsible for photodynamic damage and the accompanying cytotoxicity. This review briefly describes basic photophysical phenomena involved in the formation of electronically excited states and photochemical processes that play a key role in the generation of ROS. Physicochemical properties of the excited states of the photosensitizing dye molecules and of ROS, particularly their chemical reactivity with selected substrate molecules, as well as major spectroscopic and analytical methods used for the detection and characterization of reactive intermediates involved in photodynamic phenomena, are critically discussed in this paper.

We previously showed that antimicrobial photodynamic inactivation (aPDI) of Gram-positive and Gram-negative bacteria mediated by the phenothiazinium dye, methylene blue (MB), was potentiated by the addition of potassium thiocyanate (10 mM). The mechanism was suggested to involve a singlet oxygen-mediated reaction with SCN to form sulfite and cyanide and then to produce sulfur trioxide radical anion. We now report that potassium selenocyanate (concentrations up to 100 mM) can also potentiate (up to 6 logs of killing) aPDI mediated by a number of different photosensitizers (PS): MB, rose bengal and 5,10,15,20-tetrakis(4-sulfonatophenyl)porphyrin dihydrochloride (as low as 200 nM). When a mixture of selenocyanate with these PS in solution was illuminated and then bacteria were added after the light, there was up to 6 logs of killing (Gram-negative > Gram-positive) but the antibacterial species decayed rapidly (by 20 minutes). Our hypothesis to explain this antibacterial activity is the formation of selenocyanogen (SeCN) by reaction with singlet oxygen ( O ) as shown by quenching of O by SeCN and increased photoconsumption of oxygen. The fact that lead tetraacetate reacted with SeCN (literature preparation of (SeCN) ) also produced a short-lived antibacterial species supports this hypothesis.

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Photochemical properties of a new class of inorganic nanoparticles, namely a cationic C60 fullerene substituted with three quaternary pyrrolidinium groups (BB6) and a surface‐modified nanocrystalline TiO2 with bromopyrogallol red (Brp@TiO2) were examined for their effectiveness in photogenerating singlet oxygen and free radicals. In particular, their ability to photosensitize peroxidation of unsaturated lipids was analyzed in POPC:cholesterol liposomes and B16 mouse melanoma cells employing a range of spectroscopic and analytical methods. Because melanoma cells typically are pigmented, we examined the effect of melanin on the photosensitized peroxidation of lipids in liposomes and B16 melanoma cells, mediated by BB6 and Brp@TiO2 nanoparticles. The obtained results suggest that peroxidation of unsaturated lipids, photosensitized by BB6 occurs mainly, although not exclusively, via Type II mechanism involving singlet oxygen. On the other hand, if surface‐modified TiO2 is used as a photosensitizer, Type I mechanism of lipid peroxidation dominates, as indicated by the predominant formation of the free radical‐dependent cholesterol oxidation products. The protective effect of melanin was particularly evident when BB6 was used as a photosensitizer, suggesting that melanin could efficiently interfere with Type II processes.