Modulation of methylene blue photochemical properties based on adsorption at aqueous micelle interfaces Helena C. Junqueira, Divinomar Severino, Luis G. Dias, Marcos S. Gugliotti and Mauricio S. Baptista* Departamento de Bioquı ´mica, IQ-USP, Av. Prof. Lineu Prestes, 748, Sa ˜ o Paulo, SP, Brazil 05513-970. E-mail: baptista@iq.usp.br Received 25th October 2001, Accepted 12th March 2002 First published as an Advance Article on the web 26th April 2002 Methylene Blue (MB + ) is a sensitizer that has been used for a variety of applications including energy conversion and photodynamic therapy (PDT). Although its photochemical properties in isotropic solution are well established, its effect in vivo and in restricted reaction environments is somewhat erratic. In order to understand its photochemical behavior when it interacts with biomolecules, in particular with membranes, MB + properties were studied in sodium dodecyl sulfate (SDS) and cetyl trimethylammonium bromide (CTAB) solutions. Because of an electrostatic attraction, SDS and MB + form complexes, changing the properties of both the micelles and the MB + solutions. Surface tension measurements show that the c.m.c. of SDS decreases from 7 mM to 70 mM when the MB + concentration increases from 0 to 45 mM. Above the c.m.c., binding of MB + in the micelle pseudo-phase causes the formation of aggregates (mostly dimers) as attested by the increase in the absorption at 580 nm and the decrease in fluorescence emission. The extent of dimer formation is dependent on the relative concentrations of MB + and SDS. In the presence of excess of SDS, MB + is mainly in the monomer form and at low SDS concentration dimers are favored. Such effect, which was not observed in CTAB micelles, was modeled qualitatively by considering that MB + molecules partition to the micelle pseudo- phase which favors or disfavors dimers as a function of its volume. MB + transient species were characterized by laser flash photolysis and NIR emission showing the presence of triplets and subsequently singlet oxygen at high SDS concentration and semi-reduced and semi-oxidized MB + radicals at low SDS concentration. Therefore it was shown that, depending on the ground state MB + monomer/dimer equilibrium, induced by the micelles, the photochemical properties of MB + can be shifted from a Type II (energy transfer to oxygen forming singlet oxygen) to a Type I mechanism (electron transfer forming the semi-reduced and the semi-oxidized radicals of MB + ). Introduction Sensitization processes are extremely important in several areas including biology, chemistry and medicine. 1 Fundamen- tal processes of living organisms such as photosynthesis depend on light absorption and subsequent photophysical and photochemical processes. 2 The application of sensitization to medicine has become extremely important, specially in the new method of cancer treatment known as photodynamic ther- apy (PDT), where photoinduced in situ generation of reactive species has been used to induce tumor regression. 3 In both mentioned cases, processes of energy and/or electron transfer are involved and the knowledge of how these processes are affected/modulated by biological environments (membranes, proteins) is important. 1–4 The mechanisms of tumoral destruction involve direct oxi- dation (mechanisms Type I) of biological targets (membranes, proteins and DNA), as well as oxidation mediated by singlet oxygen ( 1 O 2 ) (mechanisms Type II), which is mainly formed through energy transfer from triplets to molecular oxygen. 5 Binding of molecules in membranes and interfaces can cause several alterations in their ground state properties (dimeriza- tion and/or ion-pair equilibria) and consequently in the photo- physical and photochemical processes in which they are involved. 6,7 The details of these processes need to be further investigated for the case of sensitizers used in photodynamic therapy, in which the photodynamic effect will happen in cells or tissues, where the photosensitizers will be interacting with macromolecules or membranes. 7,8 Adsorption of dyes into oppositely charged interfaces, for instance, micelles or polyelectrolytes, has been extensively stu- died. 9–13 Depending on the charge ratio between the dyes and the charged groups in the colloid interface, dye aggregates are favored. 9,10 This field of dye–surfactant interaction is of great relevance to the dyeing and photography industry. 11 However, there has been little effort towards proposing equilibrium schemes to model dimerization in micelle solution. 10 The effect of the dye on the micellization properties of surfactants has also been studied to a lesser extent. 12,13 Guo and coworkers have used long-chain dyes which themselves act as surfactants. Their results were interpreted in terms of the Lange and Beck theory that proposes an ideal mixture between surfactants. Deviations from ideality were observed when the long-chain dyes and surfactants have opposite charges. 12 Microheterogeneous systems are known to promote differen- tial distribution of substrates, intermediates and products, in the organic, aqueous or interfacial regions, which can usually be explained by a combination of electrostatic interaction and hydrophobic effect. 1,2,14 Due to these effects, interesting changes in photochemical processes have been observed in micelles, reversed micelles, microemulsions and vesicle systems, including yield enhancements, 14,15 variation of regioselectivity and stereoselectivity, 16,17 change in mechanisms, 18–20 enhanced charge separation, 21,22 magnetic effects 23 and increased 2320 Phys. Chem. Chem. Phys., 2002, 4, 2320–2328 DOI: 10.1039/b109753a This journal is # The Owner Societies 2002 PCCP