Anisotropy-dependent chirality transfer from cellulose nanocrystals to β-FeOOH nanowhiskers†

Abstract

Chiral iron oxides and hydroxides have garnered considerable interest owing to the unique combination of chirality and magnetism. However, improving their g-factor, which is critical for optimizing the chiral magneto-optical response, remains elusive. We demonstrated that the g-factor of β-FeOOH could be boosted by enhancing the anisotropy of nanostructures during a biomimetic mineralization process. Cellulose nanocrystals were used as both mineralization templates and chiral ligands, driving oriented attachment of β-FeOOH nanoparticles and inducing the formation of highly aligned chiral nanowhiskers. Circular dichroism spectra and time-dependent density-functional theory proved that chirality transfer was induced from cellulose nanocrystals to β-FeOOH through ligand–metal charge transfer. Interestingly, chirality transfer was significantly enhanced during the elongation of nanowhiskers. A nearly 34-fold increase in the g-factor was observed when the aspect ratio of nanowhiskers increased from 2.6 to 4.4, reaching a g-factor of 5.7 × 10⁻³, superior to existing dispersions of chiral iron oxides and hydroxides. Semi-empirical quantum calculations revealed that such a remarkable improvement in the g-factor could be attributed to enhanced dipolar interactions. Cellulose nanocrystals exert vicinal actions on highly anisotropic β-FeOOH with a large dipole moment, increasing structural distortions in the coordination geometry. This mechanism aligns with the static coupling principle of one-electron theory, highlighting the strong interaction potential of supramolecular templates. Furthermore, paramagnetic β-FeOOH nanowhiskers alter the magnetic anisotropy of cellulose nanocrystals, leading to a reversed response of helical photonic films to magnetic fields, promising for real-time optical modulation.