Investigation of methylene blue photoinduced properties in bile salts self-assemblies

Investigation of methylene blue photoinduced properties in bile salts self-assemblies

• Autor
• Victor Hladkyi Toledo
• Co-autores
• Iseli Lourenço Nantes , Cornelia Bohne
• Resumo

• Methylene blue (MB) is a cationic phenothiazine dye, that has been applied in different fields such as in medicine and biology due its low toxicity. The monomeric form of MB has a high quantum yield for triplet excited state formation, leading to the efficient formation of singlet oxygen, which is a reactive intermediate important is processes, such as photodynamic therapy. One drawback of using MB is the formation of dimers, trimers and higher aggregates in aqueous solutions, which changes the photophysics and photochemistry of this class of molecules. Phenothiazine dimeric species usually promote charge separation in the excited state leading the formation of radicals that in the case of MB are the neutral radical and the dication radical. The formation of these radicals may have potential applications as electron donors for semiconductors or redox agents in catalysis. However, the formation of MB aggregates leads to the presence of several photoactive species, which is undesirable for the development of applications.  The objective of this work is to use a supramolecular approach where bile salt (BS) self-assemblies with different strucutres are used to control the equilibrium between MB monomers and dimers and stabilize transient species selectively. BS is a class of amphiphilic molecules containing a steroidal moiety with planar polarity and a charged polar head. In aqueous solutions these molecules form small primary aggregates at low concentrations which continuously self-assemble into larger secondary aggregates as the bile salt concentration is raised. The binding to the primary and secondary aggregates is dictated by the polarity, size and rigidity of the guest [1]. We used laser flash photolysis to study how the presence of different binding sites in the bile salt aggregates affects the equilibria between the monomeric and aggregated forms of MB and the transient species yield and lifetime. The formation of transient species is diagnostic of the monomer/dimer equilibrium since the MB triplet excited state is formed from monomer, while the radicals are formed predominantly from the dimer. These experiments show that the equilibrium is displaced toward the monomeric form of MB and that MB is in a low polarity environment. In addition, the use of different bile salt structures led to differences in the fluorescence enhancement trend of MB in function of BS concentration. These results show that the photochemistry of MB can be altered using self-assembled bile salts and this strategy leads to an understanding on how the MB reactivity is influenced by the presence of negatively charged amphiphilic species, such as encountered in biological systems.

   

  [1] Waissbluth’, O. L., Morales, M. C. & Bohne, C. Influence of Planarity and Size on Guest Binding with Sodium Cholate Aggregates. Photochem Photobiol 82, 1030–1031 (2006).

   
• Palavras-chave
• metylene blue, bile salt, self-asembly, photophysics, triplet, laser flash photolysis
• Modalidade
• Comunicação oral
• Área Temática
• Nanociência e Nanotecnologia