Synthetic procedures for magnetoelectric nanodiscs were reproduced across two institutions (Massachusetts Institute of Technology and Friedrich-Alexander University of Erlangen - Nuremberg). The Fe3O4 magnetic nanodiscs (MNDs) were synthesized by reducing hematite nanodiscs. Hematite nanodiscs were first produced by heating a uniform mixture of 0.273 g of FeCl3·6H2O (Fluka), 10 mL Ethanol, and 600 μL of deionized (DI) water in a sealed Teflon-lined steel vessel at 180°C for 18 hours. After washing the red hematite nanodiscs with DI water and ethanol 3–5 times, the dried hematite was dispersed in 20 mL of trioctyl-amine (Sigma-Aldrich) and 1g of oleic acid (Alfa Aesar/Thermo Fisher Scientific). For the reduction of hematite to magnetite, the mixture was transferred into a three-neck flask connected to a Schlenk line, and evacuated for 20 min at room temperature, and then heated to 370 °C (20 °C/min) in H2 (5%) and N2 (95%) atmosphere for 30 min.
The core-shell Fe3O4-CoFe2O4 nanodiscs (CFOND) were formed by nucleation and growth of a CoFe2O4 layer on the surface of MNDs. For this procedure, 120 mg of MNDs (cores) were dispersed uniformly in a precursor solution of 20 mL diphenyl ether (Aldrich), 1.90 mL oleic acid (Sigma Aldrich), 1.97 mL oleylamine (Aldrich), 257 mg cobalt acetylacetonate (Co(acac)2, Aldrich), and 706 mg iron acetyl acetonate (Fe(acac)3, Aldrich). A three-neck flask including the solution of MND cores and the shell precursors was connected to a Schlenk line. The solution was evacuated and then heated to 100 °C (7 °C/min) for 30 min in N2 atmosphere while magnetically stirring at 400 rpm. After closing the N2 line, the temperature was increased to 200 °C (7 °C/min) and maintained for 30 min, and then increased to 230 °C (7 °C/min) and maintained for 30 min. The solution was cooled to room temperature (~30 min), and the resulting CFONDs were washed with ethanol and n-hexane and subjected to centrifugation at 8000 rpm for 8 min; the washing process was repeated 2–3 times. The thickness of CoFe2O4 layer is controlled by repeating the organometallic synthesis and washing steps described above. To obtain a 5 nm CoFe2O4 layer, the synthesis was repeated three times.
The Fe3O4-CoFe2O4-BaTiO3 magnetoelectric nanodiscs (MENDs) were made by formation of BaTiO3 shell on the surface of CFOND via the sol-gel method. A mixture comprising 16 mg of CFONDs dispersed in n-hexane, 30 mL of DI water, 6 mL of ethanol, and 2g of poly(vinylpyrrolidone) (Sigma Aldrich) was sonicated for 20 min, which led to segregation of the oil phase. The oil phase and other insoluble solids were removed with a spatula. The hydrophilic CFOND dispersions were then transferred to a three-neck flask connected to a Schlenk line, and then dried in vacuum at 80 °C until amber-colored gel was formed on the bottom of the flask. The gel was re-dispersed in the BaTiO3 shell precursor solution which was prepared by mixing 0.5 g citric acid (Sigma Aldrich) and 24 μL titanium isopropoxide (Aldrich) dissolved in 15 mL of ethanol and 0.1 g citric acid and 0.0158 g barium carbonate (Aldrich) dissolved in DI water. The solution of CFONDs and BaTiO3 precursors were moved to the three-neck flask connected to the vacuum line and kept at 80 °C for 12–14 hours. The powders were then moved to a clean ceramic container and heated at 600 °C for 2 hours, 700 °C for 2 hours, then 800 °C for 1 hour, sequentially. To prevent breaking the BaTiO3 shell, the furnace door was kept closed until the temperature slowly cooled down to room temperature. The MENDs were dispersed in Tyrode and PBS before being used for in-vitro and in-vivo experiments.
The core-shell Fe3O4-CoFe2O4 nanodiscs (CFOND) were formed by nucleation and growth of a CoFe2O4 layer on the surface of MNDs. For this procedure, 120 mg of MNDs (cores) were dispersed uniformly in a precursor solution of 20 mL diphenyl ether (Aldrich), 1.90 mL oleic acid (Sigma Aldrich), 1.97 mL oleylamine (Aldrich), 257 mg cobalt acetylacetonate (Co(acac)2, Aldrich), and 706 mg iron acetyl acetonate (Fe(acac)3, Aldrich). A three-neck flask including the solution of MND cores and the shell precursors was connected to a Schlenk line. The solution was evacuated and then heated to 100 °C (7 °C/min) for 30 min in N2 atmosphere while magnetically stirring at 400 rpm. After closing the N2 line, the temperature was increased to 200 °C (7 °C/min) and maintained for 30 min, and then increased to 230 °C (7 °C/min) and maintained for 30 min. The solution was cooled to room temperature (~30 min), and the resulting CFONDs were washed with ethanol and n-hexane and subjected to centrifugation at 8000 rpm for 8 min; the washing process was repeated 2–3 times. The thickness of CoFe2O4 layer is controlled by repeating the organometallic synthesis and washing steps described above. To obtain a 5 nm CoFe2O4 layer, the synthesis was repeated three times.
The Fe3O4-CoFe2O4-BaTiO3 magnetoelectric nanodiscs (MENDs) were made by formation of BaTiO3 shell on the surface of CFOND via the sol-gel method. A mixture comprising 16 mg of CFONDs dispersed in n-hexane, 30 mL of DI water, 6 mL of ethanol, and 2g of poly(vinylpyrrolidone) (Sigma Aldrich) was sonicated for 20 min, which led to segregation of the oil phase. The oil phase and other insoluble solids were removed with a spatula. The hydrophilic CFOND dispersions were then transferred to a three-neck flask connected to a Schlenk line, and then dried in vacuum at 80 °C until amber-colored gel was formed on the bottom of the flask. The gel was re-dispersed in the BaTiO3 shell precursor solution which was prepared by mixing 0.5 g citric acid (Sigma Aldrich) and 24 μL titanium isopropoxide (Aldrich) dissolved in 15 mL of ethanol and 0.1 g citric acid and 0.0158 g barium carbonate (Aldrich) dissolved in DI water. The solution of CFONDs and BaTiO3 precursors were moved to the three-neck flask connected to the vacuum line and kept at 80 °C for 12–14 hours. The powders were then moved to a clean ceramic container and heated at 600 °C for 2 hours, 700 °C for 2 hours, then 800 °C for 1 hour, sequentially. To prevent breaking the BaTiO3 shell, the furnace door was kept closed until the temperature slowly cooled down to room temperature. The MENDs were dispersed in Tyrode and PBS before being used for in-vitro and in-vivo experiments.
