Colloidal Crystals of Thermosensitive, Core/Shell Hybrid Microgels

Abstract

We present the assembly of colloidal crystals composed of various thermoresponsive core/shell microgels and their hybrid microgel counterparts that contain localized Au nanoparticles. To obtain nanostructured microgels, we conducted a three-stage polymerization. First, core microgels composed of cross-linked poly(N-isopropylacrylamide) (pNIPAm) were synthesized by precipitation polymerization. Using these microgels as cores or "seeds", a copolymer shell of pNIPAm was added to these core particles using a functional comonomer. Finally, a second shell consisting solely of cross-linked pNIPAm was added onto the preformed core/shell microgels by using this seeded polymerization technique. With use of these core/shell/shell microgels as templates, Au nanoparticles were synthesized in situ, using the cationic sites in the inner shell to nucleate particle growth. To control the optical properties of the Au nanoparticles, electroless Au plating was carried out with use of the preformed Au nanoparticles in the microgel as seeds. These hybrid microgels retained their thermoresponsive properties and possessed a high degree of size uniformity as confirmed by dynamic light scattering and were assembled into three-dimensional colloidal crystals by using thermal annealing processes. The resulting colloidal crystals were characterized by optical microscopy and reflectance spectroscopy. This synthetic design for producing distinctive, functional building blocks could be expanded to enable the creation of complex tunable optical materials containing refractive index periodicity on multiple length scales.