Detection of singlet oxygen by EPR: The instability of the nitroxyl radicals.

The use of spin traps and redox probes coupled with electron paramagnetic resonance (EPR) is a method frequently applied in the evaluation of the efficiency of photosensitizers and photocatalysts in phototherapeutic and photocatalytic processes that involve reactive oxygen species. In this way, the method helps to clarify the mechanism behind photo-induced reactions. Hydroxy-TEMP is a very specific redox probe for selectively identifying and quantifying singlet oxygen (1O2). In this work, the kinetics of radical generated by the oxidation products of the Hydroxy-TEMP redox probe was analyzed from EPR spectra in aqueous solutions of several water-soluble porphyrins ([H2T4MPyP](OTs)4, Na4[H2T4SPP], [H2T2MPyP](OTs)4, [ZnT4MyPyP](OTs)4, [MnT4MyPyP](OTs)5, H2T4CPP, and [H2T4TriMAPP](OTs)4) under white light illumination. Different factors such as the concentration of the redox probe, pH of the medium, and photostability of the porphyrins were evaluated. A systematic study was carried out to reveal the factors associated with stable radical degradation (TEMPOL) by illumination in the visible spectral region in systems containing photosensitizer (porphyrin) and redox probe (Hydroxy-TEMP). With the aid of EPR and gas chromatography coupled with mass spectroscopy (GC-MS) techniques, the mechanism of the radical degradation and the photobleaching of porphyrins were investigated. After successive interactions with the porphyrin in its excited state, in alkaline aqueous solution (pH > 10), the free radical TEMPOL is transformed into TEMPONE until the final diamagnetic product Phorone. A protocol was elaborated to identify and quantify the generation of 1O2 by Hydroxy-TEMP reliably, to avoid possible errors in the interpretation of efficiency of photosensitizers.

Hybrid systems based on gold nanostructures and porphyrins as promising photosensitizers for photodynamic therapy.

Gold nanostructures of two different shapes (spheres and rods) were synthesized to form a colloidal hybrid system with 5,10,15,20-tetrakis(N-methylpyridinium-4-yl)porphyrin tosylate salt (H2TM4PyP(OTs)4) (POR) for applications in photodynamic therapy (PDT) using light in the visible spectral range. Electron paramagnetic resonance (EPR) experiments in combination with spin trapping were used for the detection of reactive oxygen species (ROS) and evaluation of the efficiency of these novel hybrid systems as photosensitizers. It is shown that the hybrid system consisting of gold nanorods (AuNR) and porphyrin (POR) is by far more efficient than its isolated components. This enhanced efficiency is explained by a synergetic effect between the AuNR and the porphyrin, wherein a rapid energy transfer from the former to the latter produces a large amount of singlet oxygen followed by its conversion into hydroxyl radicals. The mechanism was investigated using different spin traps and different ROS inhibitors. On the other hand, spherical gold nanoparticles (AuNP) do not show this synergetic effect. The synergetic effect for gold nanorods/POR hybrid is attributed to a larger field enhancement close to the gold nanorod surface in addition to the electrostatic attraction between the components of the hybrid system.

Unravelling the mechanisms of reactive oxygen species formation in nanohybrid systems of porphyrins and enriched (6,5) single-walled carbon nanotubes for photosensitization.

Nanohybrids of enriched (6,5) single-walled semiconducting carbon nanotubes (E-SWCNTs) and porphyrin can be used synergistically as photodynamic therapy (PDT) agents. The efficiency of different porphyrins within the nanohybrids was investigated and compared with results obtained from porphyrins in previous studies. Reactive oxygen species singlet oxygen (1)O2 and for the first time hydroxyl radical ˙OH generation by the nanohybrids under illumination were detected by electron paramagnetic resonance using spin trapping molecules TEMP and PBN. Based on the analysis, we improve the modelling of charge transport within the nanohybrids, which is also detected by Raman scattering. It is shown that the 5,10,15,20-tetrakis(4-trimethylammoniumphenyl)porphyrin [H2TTMAPP(OTs)4] and E-SWCNT form very efficient nanohybrids for PDT applications in the visible spectral range.

Nanocomposite treatment reduces disease and lethality in a murine model of acute graft-versus-host disease and preserves anti-tumor effects.

Graft versus host disease (GVHD) is an immunological disorder triggered by bone marrow transplantation that affects several organs, including the gastrointestinal tract and liver. Fullerenes and their soluble forms, fullerols, are nanocomposites with a closed symmetrical structure with anti-inflammatory and anti-oxidant properties. The present study evaluated the effects of treatment with the fullerol (C60(OH)18-20) in the development and pathogenesis of GVHD in a murine model. Mice with experimental GVHD that were treated with the fullerol showed reduced clinical signs of disease and mortality compared with untreated mice. Treatment with the fullerol decreased the hepatic damage associated with reduced hepatic levels of reactive oxygen species, pro-inflammatory cytokines and chemokines (IFN-γ TNF-α, CCL2, CCL3 and CCL5) and reduced leukocyte accumulation. The amelioration of GVHD after treatment with the fullerol was also associated with reduced intestinal lesions and consequent bacterial translocation to the blood, liver and peritoneal cavity. Moreover, the fullerol treatment alleviated the GVHD while preserving effects of the graft against a leukemia cell line (GFP+P815). In summary, the fullerol was effective in reducing the GVHD inflammatory response in mice and may suggest novel ways to treat this disease.

Cobalt lawsone complexes: searching for new valence tautomers.

Bi-stable molecular systems presenting valence tautomerism are associated with the development of new functional materials, which can be used for applications in organic electric conductors, optoelectronic and molecular magnetic devices. The properties of these materials can be adjusted with slight chemical changes and can be induced by external stimuli. Typical examples of valence tautomer compounds are coordination complexes of Co and o-dioxolene ligands, notably quinone like ones. In the search for a new class of cobalt complexes presenting valence tautomerism we report herein the synthesis and characterization of five new coordination compounds of cobalt and 2-hydroxy-1,4-naphthoquinone (lawsone or shortly Law). Complexes [Co(Law)2(im)2]·C6H5CH3 (1), [Co(Law)2(py)2]·CH3OH (2), [Co(Law)2(phen)]·(C4H8O)2 (3), [Co(Law)2(2,2-bpy)]·C6H5CH3 (4) and [Co(Law)2(2,2-bpy)] (5) were synthesized and fully characterized by X-ray diffraction and EPR techniques in a wide range of temperatures and under illumination. Despite presenting similar molecular and geometry packing of the valence tautomer complexes of cobalt and o-dioxolenes, neither structural nor electronic evidence of valence tautomerism could be found in the Co and lawsone complexes.

Structural and photophysical properties of peptide micro/nanotubes functionalized with hypericin.

Hypericin is a photosensitizer with promising applications in photodynamic therapy (PDT) for cancer and infectious diseases treatments. Herein, we present a basic research study of L-diphenylalanine micro/nanotubes (FF-NTs) functionalized with hypericin. The system has special properties according to the hypericin concentration, with direct consequences on both morphological and photophysical behaviors. A clear dependence between the size of the tubes and the concentration of hypericin is revealed. The generation of reactive oxygen species (ROS) is found to be improved by ∼57% in the presence of FF-NTs, as indirectly measured from the absorbance profile of 1,3-diphenylisobenzofuran (DPBF). In addition, when hypericin appears conjugated with FF-NTs, the characteristic fluorescence lifetime is significantly boosted, demonstrating the role of FF-NTs to enhance the photophysical properties and stabilizing the fluorophore in excited states. Electron paramagnetic resonance allows the proposition of a mechanism for the generation of ROS. Molecular dynamics simulations bring new insights into the interaction between hypericin and peptide assemblies, suggesting the spatial organization of the fluorophore onto the surface of the supramolecular structures as a key element to improve the photophysical properties reported here.

Quantification of fullerene nanoparticles suspensions in water based on optical scattering.

Fullerenes, and in particular the C60, have been intensively investigated in the last decades mainly because of their vast range of potential applications in biomedicine and materials science. These molecules are inherently hydrophobic, tending thus to form clusters and aggregates in polar solvents resulting in colloidal suspensions. In this work was developed a quantification method for C60 nanoparticles (nano-C60) in colloidal aqueous suspensions based on optical light scattering (nephelometry). This method can be done in a conventional spectrofluorimeter either on the excitation wavelengths or on the second-order satellite lines that arise from the diffraction gratings. The detection limit of the proposed method was about (0.0090+/-0.0008) mg L(-1), in a linear concentration range from 0.007 to 0.360 mg L(-1). A comparison of this scattering technique with spectrophotometry based on molecular absorption shows that for the former, even at the second-order lines, the threshold concentrations detected are about 20 times lower that the latter.