Preparation of yolk–shell microspheres as temperature switch on/off catalysts

A novel kind of yolk-shell microspheres was presented in this Letter. First, 250 nm of magnetic Fe3O4 nanoparticles was prepared by solvothermal reaction. Then, the Fe3O4 particles were coated with Au nanoparticles (AuNPs), SiO2, and poly(N-isopropylacrylamide) (PNIPAM). After the removal of the SiO2 layer by etching with HF solution, the yolk-shell microspheres were prepared. Each yolk-shell microsphere had a movable core (Fe3O4 self-assembled AuNPs) and temperature-responsive shell (PNIPAM). The yolk-shell structure of these microspheres was confirmed by transmission electron microscopy and Fourier-transform infrared spectroscopy. The catalytic performances of these yolk-shell microspheres were investigated under different environmental temperatures. The results indicated that the reduction from 4-nitrophenol to 4-aminophenol by NaBH4 was slowed when the temperature was increased because of the hydrophobic and the shrinking sate of PNIPAM shell and the yolk-shell microspheres had a characteristic of temperature switch on/off when the cross-linking degree was about 20%.