Microcrystalline cellulose as a carrier for hydrophobic photosensitizers in water

Samples of pheophorbide-a adsorbed on microcrystalline cellulose, which have been previously characterized in the solid state (M.G. Lagorio, E. San Roman, A. Zeug, J. Zimmermann and B. Röder, Phys. Chem. Chem. Phys., 2001, 3. 1524-1529), were washed with water, leading to stable suspensions of ultrafine particles (d < 2 microm) carrying photoactive, monomeric dye molecules. Detachment can be controlled through the particle size distribution. Suspensions are fluorescent and generate singlet molecular oxygen efficiently. A similar effect has been observed on washing samples containing hematoporphyrin IX adsorbed on the same support. Thus, using cellulose as a heterogeneous carrier, it is possible to introduce hydrophobic photosensitizers into the aqueous medium while avoiding aggregation, thus preserving their photophysical properties. At the same time, the spectroscopic properties of dyes attached to cellulose particles are compared with those in ethanol and ethanol-water mixtures and their differences are explained in terms of medium polarity and dye aggregation.