Metal ion-responsive photonic colloidal crystalline micro-beads with electrochemically tunable photonic diffraction colours

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 λ_max 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 the photonic micro-beads in 1 mM Cu²⁺ solution between 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 under ambient light. This concept of rapid electrochemical regulation of photonic diffraction colour of PCC micro-beads may be useful in reflective displays applications.