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This work reports a metal ion-responsive photonic colloidal crystalline(PCC)microspheric material and the convenient,reversible switching of its photonic diffraction properties by the redox reaction of copper in a simple electrochemical cell.The PCC micro-beads were fabricated from the orderly three-dimensional packing of core-shell nanoparticles with hydrogel coatings laden with anionic phosphate functionalities.Electrostatic binding of metal cations by the hydrogel coating lowered its internal osmotic pressure and caused shrinkage of the core-shell nanoparticles.This brought about a blue-shift of the photonic diffraction λmax of the micro-beads.The maximum shift,from 650 to 590 nm(colour change from magenta to green)was found to be produced by Cu2+at a concentration of 1 mM.A simple electrochemical photonic device was constructed by sandwiching a suspension of photonic micro-beads in 1 mM Cu2+solution with two transparent ITO-glass electrodes.Photonic diffraction colour of the micro-beads was reversibly switched via the electrochemical reduction/oxidation of Cu2+inside the device.The colour change can be observed by naked eyes.This concept of rapid electrochemical regulation of photonic diffraction colour of PCC micro-beads has potentials in achieving electronic-paper-like reflective displays.