• encapsulated fluorescent dyes;
  • mesoporous silica particles;
  • nanophotonics;
  • self-assembly;
  • ultrabright fluorescence

The problem of functionalization of recently reported ultrabright fluorescent mesoporous silica nanoparticles while preserving their fluorescent brightness is solved. This is a serious issue because of the open geometry of mesoporous channels and physical encapsulation of fluorescent dye inside those channels. Amine modification of mesoporous nanoparticles is described to preserve the brightness comparable to that of earlier reported ultrabright silica nanoparticles. Scaling to 40 nm sized particles, amine-functionalized nanoparticle have fluorescent brightness equivalent to the one of 630 free rhodamine 6G (R6G) dye molecules in water. To demonstrate further most challenging functionalization, which relies on using organic-solvent-based chemistry, folic acid conjugation is developed. Two different methods are used to conjugate folites to the amine functionalities. Both methods result in a decrease of fluorescence intensity, which can nonetheless still be called ultrabright. The brightness can drop to either 310 or 80 R6G dye molecules per particle of nominal diameter of 40 nm.