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The effect of dye concentration on the fluorescence,Φ, and singlet molecular oxygen,Φ, quantum yields of rose bengal loaded poly(2-hydroxyethyl methacrylate) thin films (∼200 nm thick) was investigated, with the aim of understanding the effect of molecular interactions on the photophysical properties of dyes in crowded constrained environments. Films were characterized by absorption and fluorescence spectroscopy, singlet molecular oxygen (O) production was quantified using a chemical monitor, and the triplet decay was determined by laser flash-photolysis. For the monomeric dilute dye, Φ = 0.05 ± 0.01 and Φ = 0.76 ± 0.14. The effect of humidity and the photostability of the dye were also investigated. Spectral changes in absorption and fluorescence in excess of 0.05 M and concentration self-quenching after 0.01 M are interpreted in the context of a quenching radius model. Calculations of energy migration and trapping rates were performed assuming random distribution of the dye. Best fits of fluorescence quantum yields with concentration are obtained in the whole concentration range with a quenching radius r = 1.5 nm, in the order of molecular dimensions. Agreement is obtained only if dimeric traps are considered photoactive, with an observed fluorescence quantum yield ratio Φ/Φ ≈ 0.35. Fluorescent traps are capable of yielding triplet states and O. Results show that the excited state generation efficiency, calculated as the product between the absorption factor and the fluorescence quantum yield, is maximized at around 0.15 M, a very high concentration for random dye distributions. Relevant information for the design of photoactive dyed coatings is provided.

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The synthesis and photochemical properties of two novel selenium zinc(II) phthalocyanines, a lipid-soluble 2,9(10),16(17),23(24)-tetrakis[(2-dimethylamino)ethylselanyl]phthalocyaninato zinc(II) (4) and its water-soluble quaternized derivative 2,9(10),16(17),23(24)-tetrakis[(2trimethylammonium)ethylselanyl]phthalocyaninato zinc(II) tetraiodide (5), were investigated. Maximum absorption values were 689 nm and 684 nm for 4 and 5 in DMF, respectively. In addition, phthalocyanines were revealed to be very efficient singlet oxygen generators with high values of Φ ∆ 0.74 and 0.84 for 4 and 5 in DMF, and they were photostable over the irradiation times studied. The photodynamic effect were evaluated on CT26 colon carcinoma cells. After light exposure, 4 and 5 were found to be cytotoxic, and IC 50 values were 0.5 ± 0.1 µM and 2.3 ± 0.6 µM, respectively. The production of a greater amount of reactive oxygen species after phthalocyanines irradiation would be responsible for its potent phototoxic action on CT26 colon carcinoma cells.

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Quinoline is one of the most important heterocyclic systems in life sciences. Some derivatives are normal metabolites, and others are used as antibacterial, antimalarial, and anticancer agents. In this work, we describe the synthesis, physicochemical properties, and fluorescence features of a new 4-quinolinone fluorophore, 3-hydroxykynurenic acid (3-HOKA). 3-HOKA was obtained by alkoxide-induced rearrangement of ethyl isatinacetate followed by acidification and then alkaline hydrolysis. The fluorescent compound was characterized by NMR, MS, IR, and UV-Vis spectra. 3-HOKA can exist under a keto-enol equilibrium, but the 4-quinolinone form is the predominant tautomer. In PBS (pH = 7.4), the anionic keto form of 3-HOKA showed a maximum absorption at 368 nm, a fluorescence peak at 474 nm, and a fluorescence quantum yield (Φ F): 0.73. 3-HOKA is photostable and is a moderately weak oxygen generator. Viability assays on HeLa cells indicated that 3-HOKA did not induce significative cytotoxic effects. Under UV excitation, a bright blue fluorescence was selectively found in a singular body within the cytoplasm, a labeling pattern that suggests the possible localization of the probe in the centriole or related structures. Therefore, this novel fluorophore represents a promising prototype compound owing to its biocompatibility and potential biological applications.

Natural and synthetic phenazines are widely used in biomedical sciences. In dehydrogenase histochemistry, phenazine methosulfate (PMS) is applied as a redox reagent for coupling reduced coenzymes to the reduction of tetrazolium salts into colored formazans. PMS is also currently used for cytotoxicity and viability assays of cell cultures using sulfonated tetrazoliums. Under UV (340 nm) excitation, aqueous solutions of the cationic PMS show green fluorescence (λem: 526 nm), whereas the reduced hydrophobic derivative (methyl-phenazine, MPH) shows blue fluorescence (λem: 465 nm). Under UV (365 nm) excitation, cultured cells (LM2, IGROV-1, BGC-1, and 3T3-L1 adipocytes) treated with PMS (5 µg/mL, 30 min) showed cytoplasmic granules with bright blue fluorescence, which correspond to lipid droplets labeled by the lipophilic methylphenazine. After formaldehyde fixation blue-fluorescing droplets could be stained with oil red O. Interestingly, PMS-treated 3T3-L1 adipocytes observed under UV excitation 24 h after labeling showed large lipid droplets with a weak green emission within a diffuse pale blue-fluorescing cytoplasm, whereas a strong green emission was observed in small lipid droplets. This fluorescence change from blue to green indicates that reoxidation of methyl-phenazine to PMS can occur. Regarding cell uptake and labeling mechanisms, QSAR models predict that the hydrophilic PMS is not significantly membrane-permeant, so most PMS reduction is expected to be extracellular and associated with a plasma membrane NAD(P)H reductase. Once formed, the lipophilic and blue-fluorescing methyl-phenazine enters live cells and mainly accumulates in lipid droplets. Overall, the results reported here indicate that PMS is an excellent fluorescent probe to investigate labeling and redox dynamics of lipid droplets in cultured cells.

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