(63j) Preparation of Two-Dimensional Fe3O4 Nanoparticles Transformed from a-Fe2O3 Analogues and Their Applications in Magnetic Field-Assisted Microalgal Biorefinery Process

Hematite (a-Fe₂O₃) and magnetite (Fe₃O₄) are two iron oxides with a wide range of potential technological applications due to their unique materials properties. For example, they have been explored as efficient MRI contrast agents due to their unique magnetic anisotropy in biomedical applications. They were used in water treatment applications due to their enhanced ability to adsorb heavy metals and organic pollutants. Due to their high surface area and electrochemical activity, they were also explored as potential electrode materials for lithium-ion batteries and supercapacitors. There have been reports of synthesizing a-Fe₂O₃ and Fe₃O₄ into thin plates or disks with thicknesses ranging from micro to nanometers. However, fine tuning their sizes, shapes, and morphologies has not been well demonstrated.

Herein, we report a facile hydrothermal synthesis of preparing a-Fe₂O₃ nanodisks (NDs) and nanosheets (NSs), followed by thermal transformation from a-Fe₂O₃ to Fe₃O₄ NDs and NSs, respectively. They were characterized by employing XRD, AFM, SEM, and TEM equipped with electron diffraction measurements. The average lateral size (LS) and thickness (TN) of a-Fe₂O₃ NDs and NSs were affected by the amount of Al³⁺ ions as shape-controlling additives present in the hydrothermal reaction, leading to 381± 116 nm and 8.1 ± 0.7 nm (NDs) and 1.2 ± 0.4 mm, 3.7 ± 0.4 nm (NSs), respectively. XPS, FT-IR, TGA/DTA, Raman, and magnetic hysteresis measurements confirmed their unique physicochemical and magnetic properties based on their characteristic anisotropic nanostructures. Magnetic harvesting of microalgae cell Haematococcus pluvialis (H. pluvialis) was successfully demonstrated by Fe₃O₄ NDs and NSs, under facile conditions with external magnetic fields, resulting in a harvesting efficiency of >~95% at less than 1 min incubation process. Efficient extraction of astaxanthin (ATX), a highly valuable antioxidant chemical, was also successfully demonstrated through laceration of the cell wall of H. pluvialis by Fe₃O₄ NDs and NSs, as a nanoscale scalpel, under facile ultrasonic agitations. ATX extraction efficiency of up to ~83% with < ~5 min mild ultrasonication and minimal cellular damage could be accomplished via Fe₃O₄ NSs due to a higher “nanorazor” effect than NDs. Furthermore, the feasibility of the recovery and recyclability of Fe₃O₄ NDs and NSs under external magnetic fields for future usage was assessed. Our work represents an important step toward the development of highly efficient microalgal harvesting and biorefinery processes.