Magnetic cellulose microcrystals with tunable magneto-optical responses

Magnetically actuated liquid crystals are potential alternatives to conventional liquid crystal systems in many applications thanks to their advantages of electrodeless operation, remote control, and low cost. However, their practical uses face a major challenge, namely the strong optical absorption of the magnetic components, which are mostly iron oxide-based materials. Here, we overcome this challenge by developing a nanocomposite composed of rod-shaped cellulose microcrystals with magnetite nanoparticles attached to their surfaces. This design takes advantage of the optical transparency and birefringence of the cellulose microcrystals, as well as their anisotropic shape which allows their efficient orientational alignment when their surfaces are modified by magnetite nanoparticles. Only a minimum amount of magnetite nanoparticles is required to enable instant and reversible orientational control of the cellulose microcrystals, ensuring a high degree of transparency of the system. These unique nanocomposites can be fixed in a polymer matrix with defined orientations at different regions by combining magnetic alignment and photolithography processes, producing thin films that appear near-transparent under the illumination of normal light and display patterns with high contrast when sandwiched between cross polarizers. This work reveals the enormous potential of the magnetic assembly strategy and making it a promising candidate for anti-counterfeiting applications. (c) 2020 Elsevier Ltd. All rights reserved.