The development of new safe inorganic UV filters to effectively protect the skin from ultraviolet (UV) radiation effects is an emerging issue. Bismuth titanate-based UV filters embedded into mesoporous silica nanoparticles (MSN) represent a new class of inorganic sunscreens, with excellent UVA and UVB shielding properties. In addition, the presence of bismuth ions promotes a self-sealing process, allowing (i) the entrapment of the active phases in the deepest core of the system and (ii) the formation of an external glassy silica layer with a consequent suppression of the photocatalytic activity. In this work, aimed at studying in detail the self-sealing mechanism and accessing the role of bismuth ions in the formation of the system, a series of samples impregnated with a different amount of bismuth were investigated. The self-sealing process already occurs at the lowest content of bismuth and the mechanism is demonstrated to be triggered by the ability of Bi to work as a low-melting point agent for silica. Finally, a sunscreen formulation containing the new UV filter was prepared and the Sun Protection Factor (SPF), the pH and the viscosity were measured, demonstrating the potential of the proposed material for large-scale applications. (C) 2019 Elsevier Inc. All rights reserved.