Stable Encapsulation of Methylene Blue in Polysulfide Organosilica Colloids for Fluorescent Tracking of Nanoparticle Uptake in Cells

[Image: see text] Methylene blue (MB), a century-old drug and a fluorescent dye, has a long history of diverse applications, both in drug therapy and as a tissue-staining agent. However, MB is inherently unstable when exposed to light and reducing agents. In this study, we aim to prepare and characterize polysulfide-based organosilica colloidal particles for efficient, stable, and protective encapsulation of MB. Disulfide- and tetrasulfide-containing organosilane congeners were used as organosilica precursors for direct synthesis of organosilica colloids based on the silica ouzo effect. MB was spontaneously entrapped into the colloidal particles during the particle formation process. The following properties of the colloidal MB were evaluated: particle size, surface charge, atomic distribution, encapsulation efficiency, MB release, photodynamic activity, thiol and ascorbate reactivity, and cytotoxicity. The DLS measurements show that the size of colloidal MB is tunable in a range of 100 nm to 1 μm. SEM images reveal spherical particles with composition-dependent particle sizes of 70–120 nm (coefficient of variation: 15–18%). MB was encapsulated in the colloidal particles with a maximal efficiency of 95%. The release of MB from the colloids was <1% at 4 h and <3.5% at 48 h. The colloidal particles show much reduced photodynamic activity, low reactivity toward reducing agents, and low cytotoxicity. Accordingly, the colloidal MB was proposed and further investigated as a fluorescent particle tracer for the study of cell–nanoparticle interactions. In conclusion, MB can be efficiently and stably loaded into polysulfide organosilica colloidal particles using a simple and convenient physical route.