Release behavior of methylene blue dimers from silica-methylene blue@octacalcium phosphate powders in phosphate-buffered saline and lysosome-like buffer

Methylene blue-loaded silica (silica-MB) was coated by octacalcium phosphate (silica-MB@OCP) through a facile sol-gel method. Both MB monomers and dimers were released from silica-MB, while release of MB dimers from silica-MB@OCP was predominant in phos- phate-buffered saline and lysosome-like buffer as an indicative of the influence of the molecular weight of polyethylene glycol (PEG) on the interaction density of shell network consisted of PEG, citric acid, ethylenediaminetetraacetic acid and OCP. The ratio evo- lution of absorbance of dimers over that of monomers was caused by the radical distribution of MB molecules in silica-MB and silica-MB@OCP. The investigation of MB dimers/carrier system would shed light on the investigation of MB dimers in photodynamic therapy for the specific treatment to outgrowing solid tumors with the dispropor- tion between oxygen supply and consumption. Graphical Abstract