CdTe quantum dots modified polystyrene spheres with Ag nanoparticle caps: Applications both in fluorescence and in SERS

A simple but versatile technique of layer-by-layer self-assembly was used on polystyrene (PS) colloidal templates to produce CdTe quantum dots (QDs) modified PS spheres with Ag nanoparticle (NP) caps. The templates were obtained by self-assembly of monodispersed suspension of PS spheres (700 nm) on glass slides. CdTe QDs and Ag NPs were deposited on PS spheres through electrostatic force with a media layer of poly(diallyldimethyl-ammonium chloride) (PDDA) polyelectrolyte. A novel and interesting site-selective deposition effect of NPs with different sizes on PS spheres was introduced. The small size NPs of CdTe QDs (3 nm) were deposited on the outer surfaces of the PS templates. But the big size NPs of Ag NPs (45 nm) were only deposited on the top surfaces of the PS templates. This site-selective deposition effect benefited from the uniform microporous structures of the monolayer PS templates, which served as sieves to filter the NPs. Only small NPs like CdTe QDs can go through the pathways of the micropores of the PS templates. Big NPs like Ag NPs cannot go through the uniform pores due to the steric hindrance effect. Finally, CdTe QDs modified PS spheres with Ag NPs caps were obtained. It was a new class of dual-function microspheres with applications both in fluorescence and in surface-enhanced Raman scattering (SERS). The morphology and structure of the microspheres were characterized by scanning electron microscopy (SEM) and transmission electron microscopy (TEM). The fluorescence and SERS properties were obtained by confocal fluorescence microscopy and Raman spectroscopy.