Simple strategy for preparation of core colloids modified with metal nanoparticles

A simple method was developed for preparation of core colloids modified with Au (Ag) nanoparticles (NPs). The core colloids vary from polymer colloids (polystyrene spheres) to dielectric inorganic spheres (SiO2) and semiconductors (ZnO) particles. The composite colloids were obtained by heating the solution of the composites composed of core colloids, poly(ethylenimine) (PEI), and metal ions. Poly(ethylenimine) used here played two important roles. It links the metal ions and the core colloids, and meanwhile acted as a reductant for in situ reduction of metal ions to metal nanoparticles. There is little effect of the colloid concentration on the formation of core colloids modified with metal nanoparticles and the reaction may be easily scaled up. X-ray photoelectron spectroscopy and IR studies demonstrated that the amine groups in PEI were oxidized to carboxyl groups after the reaction. UV-visible spectra studies indicated that all the formed metal NPs were immobilized on the core surfaces and no free metal NPs formed in the solution. Based on the results, an improved surface seeding and shell growth technique was adopted to prepare core colloids with controllable metal coverage. Accordingly, optical plasmon resonance of the composite colloids could be tuned in a broader spectrum range by changing the metal coverage.