Multivariable Modulation of the Self-Assembly Behavior and Chiral Nematic Phase of Hydroxyapatite Nanorods

Abstract Liquid crystals (LCs) are naturally present in the bone extracellular matrix (ECM) and contribute critically to its hierarchical structure, where collagen fibers form an ordered LC template guiding hydroxyapatite (HAp) deposition. Although organic matrix LCs have been extensively mimicked, the bottom-up self-assembly of inorganic HAp into ordered LC phases remains poorly understood. Herein, utilizing a sustainable and scalable hydrothermal route mediated by eco-friendly citrate, HAp nanorods with tunable aspect ratios and surface charges were synthesized, and multivariable control over their self-assembly behavior and LC phase transition was achieved. They exhibit a broad, concentration-dependent transition from isotropic to nematic to chiral nematic phases. Moreover, this interaction-modulated and entropy-dominated phase transition is governed by hydrogen bonding and Ca2+–COO– coordination, which directs the chiral superstructure formation. Innovatively, a high aspect ratio is accompanied by enhanced entropic forces and a reduced pitch. Meanwhile, ionic strength and pH dynamically modulate LC stability via electrical double-layer effects and surface protonation. This work provides new insights into inorganic LC self-assembly and paves a green and versatile avenue for the low-carbon design of advanced biomimetic materials with bone ECM-like structures.