Rose Bengal Immobilization in and Release from Calcium Alginate Gels for Singlet-Oxygen-Generating Photosensitizing Systems

Calcium alginate (CaA) xerogels and aerogels were evaluated as carriers for Rose Bengal (RB) in singlet-oxygen-generating systems intended for photodynamic applications. Xerogels were obtained by air-drying hydrogels, xerogels(A) by replacing water in the hydrogels with isopropanol followed by air drying, and aerogels by drying in supercritical carbon dioxide the corresponding alcogels, obtained by replacing water in the hydrogels with isopropanol. Hybrid CaA/polyvinylpyrrolidone (PVP) matrices were also prepared. Nitrogen sorption showed that the CaA xerogels(A) were mesoporous, with a specific surface area of SBET~225 m2/g and a mean pore diameter of approximately 9.3 nm; the corresponding SBET values were approximately 300 m2/g for the aerogels and below 1 m2/g for the xerogels. RB introduced during xerogel formation from water was retained in the matrix and was not released into phosphate-buffered saline (PBS, pH 7.2). By contrast, RB loaded from isopropanol was released from the xerogels(A) and aerogels, with the fastest release observed for CaA aerogels. RB incorporated into xerogels(A) and aerogels retained singlet-oxygen-generating activity. In air, the highest luminescence response was observed for RB immobilized in hybrid CaA/PVP aerogel films containing approximately 10 wt% PVP.