Trapping of organic molecules and dyes within nanoporous matrices is of great interest for the potential creation of new materials with tailored features and, thus, different possible applications ranging from nanomedicine to material science. The understanding of the physical basis of entrapment and the spectral properties of the guest molecules within the host matrix is an essential prerequisite for the design and control of the properties of these materials. In this work, we show that a mesoporous silica xerogel can efficiently trap the dye thioflavin T (ThT, a molecule used as a marker of amyloid fibrils and with potential drug benefits), sequestering it from an aqueous solution and producing a highly fluorescent material with a ThT quantum yield 1500 times greater than that of the free molecule. The study of spectroscopical properties of this system and the comparison with fluorescence of an uncharged analogue of ThT give indications about the mechanism responsible for the fluorescence switching-on of ThT molecules during their uptaking into the glass. Diffusion and nanocapillarity are responsible for ThT absorption, whereas electrostatic interaction between positive ThT molecules and negative dangling ≡SiO groups covering the pore surfaces causes the immobilization of ThT molecules inside the pores and the enhancement of its fluorescence, in line with the molecular rotor model proposed for this dye. We also show that entrapment efficiency and kinetics can be tuned by varying the electrostatic properties of the dye and/or the matrix.