Synthesis and characterization of oxazine-doped silica nanoparticles for their potential use as stable fluorescent reagents.

The synthesis process to obtain silica nanoparticles (NPs) doped with two oxazine dyes, nile blue and cresyl violet, has been investigated using a modification of the reverse micelle microemulsion method and a procedure based on the Stöber method. A micellar medium provided by the non-ionic surfactant Triton X-100 in a hexanol:water mixture and an ethanol:water mixture, have been used to provide the synthesis medium in each case. Tetraethoxysilane has been used as the initiator of the polymerization and condensation reactions after its hydrolysis in basic medium using ammonium hydroxide. Dye-silane precursor NPs have been also synthesized in order to compare their potential advantages against the NPs obtained by the direct encapsulation of the oxazine dyes. Size distribution and fluorescence of the synthesized NPs, which were monitored using Transmision Electron Microscopy (TEM) and a microplate reader, respectively, depend on the molar ratio and total concentration of the reagents involved in the synthesis. NPs obtained using the developed synthesis procedures had sizes below 400 nm in most instances and the best luminescent properties were observed for NPs with sizes ranging from 100 to 300 nm. Lower sizes result in a decrease in the fluorescence intensities of these nanomaterials. Parameters related with the luminescence features of these NPs were calculated in order to compare the feasibility of both synthesis approaches. The repeatability of the reverse-micelle microemulsion procedure performed in different days gave a relative standard deviation of 10% for the fluorescence intensity values.