Effect of Cellulose Nanofiber Length on Magnetic Separation Performance and Biotin–Streptavidin Conjugation Efficiency of CNF/Fe ₃ O ₄ Composite Particles Prepared via Spray Synthesis

Spray-dried TEMPO-oxidized cellulose nanofiber (TOCN)-Fe₃O₄ (TF) composite particles were fabricated using TOCNs of distinct fiber lengths, where the long TOCNs were approximately 3 times longer (around 1 μm) than the short TOCNs (approximately 350 nm). The objective was to elucidate how nanofiber aspect ratio governs particle morphology, magnetic performance, and bioaffinity. SEM and TEM analyses revealed that both TOCN types formed spherical particles (approximately 2-3 μm) with fibrous surface textures, yet the Fe₃O₄ distribution varied significantly. Short TOCNs promoted dense fiber entanglement within droplets, effectively entrapping Fe₃O₄ nanoparticles inside the particle core, whereas long TOCNs facilitated Fe₃O₄ migration toward the particle surface. Despite similar ζ-potentials (-44 to -49 mV) and superparamagnetic hysteresis behavior, surface Fe₃O₄ accessibility strongly influenced biofunctional response without altering magnetization. These findings provide a scalable strategy for designing bioactive magnetic cellulose composites with customizable surface reactivity for biosensing, biocatalysis, and magnetic separation in biomedical field.