## 2 Structures, Synthesis, and Magnetic Properties of Fe\({}_{3}\)O\({}_{4}\) Nanoparticles
### Fe\({}_{3}\)O\({}_{4}\) Cubic Nanoparticles (CNPs)
utilized Fe(acac)\({}_{3}\) as a precursor together with 1,2-hexadecandiol, oleic acid, and oleylamine at a high temperature (290 \({}^{\circ}\)C) [49]. By increasing the heating rate from 5 to 35 \({}^{\circ}\)C/min and shortening the reaction time from 3 h to 20 min, monodisperse nanocubes with controllable sizes ranging from 6.5 to 30 nm were prepared. [PERSON] and colleagues reported a simple route using only two reagents, Fe(acac)\({}_{3}\) (precursors) and oleic acid (surfactant) [50]. This mixture with a precise 1:2 molar ratio of Fe(acac)\({}_{3}\) and oleic acid was degassed and then refluxed at 290 \({}^{\circ}\)C in benzyl ether with a fast ramping rate of 20 \({}^{\circ}\)C/min. By controlling the concentration of reagents and reaction time, Fe\({}_{3}\)O\({}_{4}\) nanocubes with edge lengths of 79 and 160 nm were obtained. Surprisingly, 1.6 g of magnetite nanocubes with an edge length of 49 nm can be synthesized by scaling up the recipe for the 79 nm nanocubes 10 times. Biphenylcarboxylic acid can be introduced in combination with oleic acid to direct the growth of 22 nm Fe\({}_{3}\)O\({}_{4}\) nanocubes as a method to reduce the nanocube size. [PERSON] and colleagues applied this synthetic approach to prepare Fe\({}_{3}\)O\({}_{4}\) nanocubes with sizes ranging from 78 to 130 nm and explored their biosensing potential [42]. The authors also observed significantly higher magnetic properties of the Fe\({}_{3}\)O\({}_{4}\) nanocubes compared to the nanospheres with a similar volume or comparable diameter/body diagonal dimensions. Using a different fatty acid in the thermal decomposition of Fe(acac)\({}_{3}\), [PERSON] et al. remarkably controlled the sizes of magnetite cubic particles in a broad range from 13 to 180 nm by adjusting the heating rate [72]. Specifically, the mixture of decanoic acid and iron(III) acetylacetonate with a 4-to-1 molar ratio was first heated to 60 \({}^{\circ}\)C for degassing, subsequently ramped to 200 \({}^{\circ}\)C, and finally refluxed at 290 \({}^{\circ}\)C with a slow ramping rate from 0.8 to 5.2 \({}^{\circ}\)C/min. [PERSON] and colleagues applied this recipe with modifications to synthesize iron oxide nanocubes with sizes ranging from 12 to 38 nm and investigated their performance in cancer hyperthermia treatments [52]. In addition, solvent mixtures of squalene and benzyl ether were used to overcome the unstable temperature problem of a pure benzyl ether solvent during the reaction [44]. This binary solvent system allowed more precise control of the reaction temperature and improved reproducibility. [PERSON] and collaborators reported the synthesis of magnetite nanocubes with sizes of 22, 36, and 57 nm at a lower refluxing temperature (200 \({}^{\circ}\)C) using an alkaline metal acetate to assist with the reduction process [58]. The amount of sodium oleate surfactant was found to play an important role in controlling the size of nanocubes. Recently, significant progress in understanding the mechanism and size-controllable synthesis was reported by [PERSON], [PERSON], and coworkers. They used a binary surfactant system containing oleic