Fe3O4@SiO2/PAM/Glycerol photonic crystal film as a long-term effective sensor for ambient humidity

•Miniemulsion process makes particle size tunable.•Thermal aggregation of Fe3O4 realizes rapid preparation of colloidal nanoparticles.•Magnetic field force drives the assembly of photonic crystals instantaneously.•The photonic crystal film sensor detects air humidity at a range from 11% to 93%.•The colorimetric sensor exhibits good repeatability, stability and accuracy.