Field-responsive structural color and fluorescence in Fe₃O₄/PPy/ZnS nanocomposites for smart coating systems

We develop a novel Fe₃O₄/PPy/ZnS core-shell nanocomposite with magnetically tunable optical and fluorescent properties. A stepwise coating strategy is employed, starting with the formation of hollow Fe₃O₄ microspheres through time-controlled hydrothermal treatment (3, 6, and 12 h). These are subsequently coated with polypyrrole (PPy) via in-situ oxidative polymerization, followed by ZnS shell deposition. Structural and compositional analyses (XRD, FT-IR, Raman, UV-Vis, TEM, XPS, and TGA) confirm the successful formation of the core-shell architecture. Magnetic measurements reveal strong field responsiveness and superparamagnetic behavior. Through selective etching, hollow PPy shells are obtained, and the pyrrole monomer concentration is tuned to control the PPy layer thickness. The nanocomposites display unique magnetochromatic responses under external magnetic fields up to 0.390 T, attributed to field-induced self-assembly that modifies light scattering and reflection to produce visible color shifts. Notably, under applied magnetic fields of 0, 35.5, 40.2, 55.8, 72.5, and 90.2 G and an applied voltage range of 0-2.5 V, the Fe₃O₄/PPy/ZnS film exhibits a visible blue shift, measured using a short focal length imaging spectrophotometer. The composites also show magnetic-field-dependent fluorescence, suggesting strong excitation-field interactions. These results highlight the promise of Fe₃O₄/PPy/ZnS nanocomposites in magnetic-field-responsive sensors.