Hydrothermal synthesis has emerged as a powerful method for producing high-quality magnetic nanocrystals, particularly iron oxide (Fe3O4) and cobalt ferrite (CoFe2O4), due to its ability to control crystallinity, size, and morphology under high-temperature and high-pressure aqueous conditions. This method offers advantages over coprecipitation, including better control over particle uniformity and reduced aggregation. The process involves dissolving metal precursors in water or other solvents, sealing the mixture in an autoclave, and heating it above the boiling point of water to facilitate crystal growth. Precursor selection plays a critical role in determining the final product’s properties. For Fe3O4, common precursors include ferric and ferrous salts such as FeCl3 and FeSO4, while CoFe2O4 synthesis typically requires cobalt salts like Co(NO3)2 alongside iron precursors. The molar ratio of Fe2+ to Fe3+ must be carefully balanced to avoid unwanted phases like hematite (α-Fe2O3).

Oxidation control is crucial during hydrothermal synthesis to maintain the desired magnetic phase. For Fe3O4, the presence of dissolved oxygen can lead to oxidation into maghemite (γ-Fe2O3), which has different magnetic properties. To prevent this, reducing agents such as hydrazine or sodium borohydride are often introduced, or the reaction is conducted under an inert atmosphere. In the case of CoFe2O4, cobalt’s oxidation state must be stabilized to ensure a spinel structure. The pH of the reaction medium also influences oxidation; alkaline conditions favor Fe3O4 formation, while acidic conditions may promote undesired phases.

Surface functionalization is essential for colloidal stability, especially in biomedical and environmental applications where aggregation must be minimized. Hydrothermal synthesis allows in-situ or post-synthesis modification with surfactants like oleic acid, citric acid, or polyethylene glycol (PEG). These coatings prevent particle agglomeration by providing steric or electrostatic repulsion. For example, PEG-functionalized Fe3O4 nanocrystals exhibit improved stability in physiological environments, making them suitable for intravenous administration. Silica shells are another common modification, enhancing biocompatibility and providing anchor points for further conjugation with targeting ligands.

The size and shape of magnetic nanocrystals directly influence their magnetic properties. Smaller particles (below 20 nm) typically exhibit superparamagnetism, where thermal energy overcomes magnetic anisotropy, preventing permanent magnetization in the absence of an external field. This is critical for biomedical applications like magnetic resonance imaging (MRI), where superparamagnetic particles enhance contrast without causing agglomeration. Larger particles or anisotropic shapes (e.g., nanorods, octahedrons) exhibit higher coercivity and remanence due to increased shape anisotropy. For instance, CoFe2O4 nanocrystals with controlled aspect ratios show tunable magnetic hyperthermia performance, as shape anisotropy affects heat generation under alternating magnetic fields.

In biomedical applications, hydrothermally synthesized magnetic nanocrystals are widely used as MRI contrast agents due to their ability to shorten transverse relaxation times (T2 contrast). Fe3O4 nanocrystals with diameters between 5-15 nm are optimal for this purpose, offering high relaxivity values. For magnetic hyperthermia cancer therapy, CoFe2O4 nanocrystals are preferred because of their higher coercivity and specific loss power (SLP), which translates to more efficient heat generation. Surface functionalization with targeting moieties like folic acid or antibodies further enhances tumor-specific accumulation.

Environmental applications leverage the high surface area and magnetic responsiveness of these nanocrystals for heavy metal removal. Hydrothermally synthesized Fe3O4 nanocrystals functionalized with thiol or amino groups effectively adsorb toxic ions like Pb2+, Cd2+, and As3+ from contaminated water. The magnetic properties enable easy separation using an external magnet, eliminating the need for filtration or centrifugation. CoFe2O4 nanocrystals, with their higher chemical stability, are particularly useful in acidic or high-salinity environments where Fe3O4 might dissolve.

The hydrothermal method’s versatility allows tuning of reaction parameters such as temperature, pressure, and reaction time to optimize nanocrystal properties. Higher temperatures (180-250°C) generally yield larger, more crystalline particles, while shorter reaction times produce smaller sizes. Additives like urea or hexamethylenetetramine can further modulate morphology by altering nucleation and growth kinetics.

Compared to coprecipitation, hydrothermal synthesis offers superior control over crystallinity and purity, making it the preferred method for high-performance applications. However, scalability and energy consumption remain challenges, as the process requires specialized equipment and prolonged heating. Future research may focus on continuous-flow hydrothermal systems to address these limitations while maintaining product quality.

In summary, hydrothermal synthesis provides a robust route for producing magnetic nanocrystals with tailored properties for diverse applications. Precursor selection, oxidation control, and surface functionalization are key to optimizing performance, while size and shape dictate magnetic behavior. Biomedical uses such as MRI and hyperthermia benefit from the high crystallinity and stability of hydrothermally synthesized particles, while environmental applications exploit their adsorption capabilities and magnetic recoverability. Advances in reaction engineering and functionalization chemistry will further expand their utility in these fields.