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Modulation of
the Methylene Blue Photochemical Properties Based on the Adsorption
at Aqueous Micelle Interfaces
Junqueira, Helena 1,
Gugliotti, Marcos1, Dias, Luis1
and Baptista, Mauricio 1
Chemistry Institute, University of São Paulo, CP 26077, 05513-9701
Abstract-
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 is somewhat erratic. In order to
understand its photochemical behavior when it interacts with biomolecules
in particular with membranes, the MB properties were studied in Sodium
Dodecil Sulfate (SDS) and Cetyl trimetylammonium 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 critical micelle concentration (cmc) of SDS
decreases from ~7mM to ~70  M
when the MB concentration increases from 0 to 45 M.
Above the cmc, adsorption of MB in the micelle interface caused the
formation of dimers as attested by the increase in the absorption at
580 nm and the decrease in the fluorescence emission. The extent of
dimer formation is dependent on the relative concentrations of MB and
SDS. In the presence of excess of SDS (~15 x cmc), MB is mainly in the
monomer form and at low SDS concentration (~3-4 x cmc) dimers are favored.
Such effect was not observed in CTAB micelles. Transient species were
characterized by laser flash photolysis and NIR emission, attesting
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 micelle interface,
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).
Keywords: methylene
blue, photochemistry, singlet oxygen, aggregation
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