On the structural stability of dye-doped silica nanoparticles

Encapsulation of dye molecules into a silica framework promises to overcome some of the functional limitations of free organic dyes. In the present work, structural stability of tetramethyl rhodamine-dextran (TMR-dex) doped silica nanoparticles (NPs) prepared through a reverse microemulsion method is probed with respect to observable changes of NP morphology and dye leaching from the NPs during their storage and post-synthesis modification. It is found that NPs synthesized with the higher (of two samples) water concentration convert during processing into hollow-like spheres. For the NPs prepared with the lower water concentration, which show no observable morphology variations, dye leaching under "static" and "dynamic" conditions is assessed. The "static" stability is determined by measuring the percentage of dye molecules leached after shelf- storing the NPs in water, ethanol or 2-(N-morpholine)-ethane sulphonic acid (MES) buffer for two weeks. The results demonstrate that dye leaching is most severe in MES while almost negligible in ethanol, suggesting ethanol to be the most suitable medium for NP storage. Dye-doped NPs normally require surface modification prior to their use for bio-labeling or bio-imaging, with common modification procedures involving multiple centrifugation/ultrasonication steps. The "dynamic" stability is, thus, evaluated by measuring the dye loss from NPs after the modification. It is found that over 70% of dye molecules are retained by the NPs after modification.