The effect of biological substrates on the ultrafast excited-state dynamics of zinc phthalocyanine tetrasulfonate in solution

Zinc phthalocyanine tetrasulfonate (ZnPcS4), a potential photosensitizer for photodynamic therapy (PDT), has been studied using femtosecond laser spectroscopy, The excited-state dynamics in water have been found to be fast (<80 ps) and dominated by intermolecular aggregation. Since the proposed mechanism for PDT is energy transfer from the triplet excited state of the photosensitizer to triplet O-2 creating singlet O-2, the short lifetime is expected to be unfavorable for producing singlet O-2, This leads to the suggestion that the presence of biological substrates may have an effect on the excited-state dynamics, To test this hypothesis, the lifetimes of the excited states of ZnPcS4 have been directly measured in the presence of a model membrane, n-hexadecyltrimethylammonium bromide (CTAB). The excited-state dynamics of ZnPcS4 in buffer solutions and with human serum albumin (HSA) have also been measured, The presence of HSA and CTAB increases the excited-state lifetime significantly relative to that observed in water, The longer lifetime of ZnPcS4 in CTAB (>1 ns) indicates that the micellar surface favors monomer formation, By increasing the excited-state lifetime, the surface substantially increases the photosensitizing potential of ZnPcS4.