Mesoporous Silica Spheres as Microreactors for Performing CdS Nanocrystal Synthesis

CdS nanoparticles were exclusively synthesized in the pores of mesoporous silica (MS) particles that had been coated with two bilayers of poly(allylamine hydrochloride) (PAH)/poly(styrene sulfonate) (PSS) via the layer-by-layer (LbL) self-assembly technique. The PAH/PSS multilyers film could prevent generation of nanocrystals outside the MS spheres. After removal of the silica cores, CdS nanocrystal encapsulated microcapsules were obtained confirming that numerous CdS nanocrystals were successfully formed. The sizes of the CdS nanocrystals were mainly determined by the pore size of the MS spheres. After being treated with a mixed solution of three salts including NaCl, LiCl, and KNO 3, the MS spheres showed two different pore sizes of 3.2 and 9.7 nm. Thus, two differently sized (3.2 nm, 6.8 nm) CdS nanocrystals were formed in the pores of MS spheres. This result is believed to explain the photoluminescence (PL) band originating from band-to-band transition. Measurements and analysis of X-ray diffraction, transmission electron microscopy, high-resolution TEM, photoluminescence spectra and confocal laser scanning microscopy showed that the CdS nanocrystals were almost perfectly inserted into the pores of the MS spheres and had good crystallinity, which created a fluorescence emission. This approach can be used to prepare composite materials involving functional inorganic nanoparticles, which have potential application in biological immunoassay and photoelectronic fields.