Polyphenol‐Mediated Synthesis of Superparamagnetic Magnetite Nanoclusters for Highly Stable Magnetically Responsive Photonic Crystals

Abstract Achieving high stability and excellent optical performance in complex environments is crucial for practical applications of magnetically responsive photonic crystals (MRPCs). It, however, remains a great challenge. This study demonstrates a polyphenol‐mediated strategy for synthesizing size‐controllable superparamagnetic magnetite (Fe₃O₄) colloid nanocrystal clusters (CNCs) that can be stably dispersed in various polar solvents to form MRPCs with brilliant structural colors for a long term. As tannic acid (TA) functions as a linker to robustly bind polyvinylpyrrolidone (PVP) chains to Fe₃O₄ surfaces, the MRPCs can maintain nearly constant diffraction wavelength and high reflectance for up to 4 years. The strong coordination between TA and Fe³⁺ inhibits crystal growth, ensuring the small primary crystal size and superparamagnetism of Fe₃O₄@TA‐PVP CNCs. Partial oxidation of TA accelerates the crystal nucleation and growth, reducing the overall CNC particle size, which can be utilized for controlling the particle size. Additionally, enhancing the dissolution of PVP before the solvothermal reaction improves the size monodispersity of the products, making the as‐constructed MRPCs ideal for practical applications in color display, sensors, anti‐counterfeiting, and camouflage. The Fe₃O₄@TA‐PVP CNCs with high stability and versatility for surface‐functionalization are also promising for magnetic resonance imaging, targeting drug delivery, recyclable catalysis, and magnetic nanomotors.