UV-vis spectra and reduction of 4-NP, linear relationship, and reusability of the microspheres. (A) UV-vis spectrum of 4-NP before and after adding NaBH4 solution. (B) The reduction of 4-NP in aqueous solution recorded every 3 min using the porous γ-Fe2O3/Au/SiO2 microspheres as a catalyst. (C) The relationship between ln(Ct/C0) and reaction time (t). (D) The reusability of the porous γ-Fe2O3/Au/mSiO2 microspheres as a catalyst for the reduction of 4-NP with NaBH4.