RESUMO
The efficacy of surfactant mixtures of Pluronic® F127 and Tween 80 at overall concentration in the micromolar range and molar ratio 1:1, 1:10, and 10:1 in inhibiting aggregation of the photosensitizer meso-tetraphenyl chlorin disulphonate (TPCS2a) was investigated in aqueous media at pH 2.9 by means of steady-state absorption and fluorescence emission spectroscopy as well as time-resolved fluorescence analysis. Corresponding experiments were performed at pH 7.4 in the absence of surfactants to determine the spectroscopic properties of a monomeric sample. Aggregation resulted in a red shift of the Soret absorption band and in substantial fluorescence quenching. The fluorescence lifetime of TPCS2a was a particularly sensitive indicator of the aggregation state, as the monomer at pH 7.4 decayed with a â¼ 10 ns time constant, while aggregation resulted in subnanosecond decay. The critical micelle concentration (CMC) of the surfactant mixtures was determined spectrophotometrically in the presence of TPCS2a. The ability of the surfactant mixtures to prevent aggregation at acidic pH was evaluated at overall surfactant concentration below and above CMC. Solubilization of TPCS2a in Pluronic® F127/Tween 80 mixtures prevented aggregation of the photosensitizer at overall surfactant concentrations much lower than those needed for both pure Pluronic® F127 and pure Tween 80.