*2.4. Synthesis of Superparamagnetic Iron Oxide Nanoparticles (SPIOs)*

The synthesis of SPIOs (6~8 nm) were prepared according to Sun et al [25]. In brief, Fe(acac)3 (2 mmol), 1,2–hexadecanediod (10 mmol), oleic acid (6 mmol), and olecylamine (6 mmol) were mixed in benzyl ether (20 mL) in three-necked bottle. Under nitrogen atmosphere, the mixture was refluxed at 100 ◦C for 30 min, and then sequentially heated to 200 ◦C for 1 h and 300 ◦C for 30 min. After cooling to room temperature, the product was collected by centrifugation at 6000 rpm for 5 min and washed with ethanol three times. The black-brown SPIOs were stored in ethanol at 4 ◦C.
