The Impact of Shapes and Crystal Lattice Orientation of Iron Oxide Nanoparticles on Photothermal Effect

Purpose The purpose of this study is (1) to compare the photothermal effect induced by near infrared laser light among the different shapes of iron oxide nanoparticles (IONPs) loaded micelles and (2) to determine the factors contributing to thermal production of IONPs. Methods IONPs with different shapes were synthesized by using previously published method (Ho, Chem mater, 2011) with modification. Briefly, spherical IONPs were synthesized by thermal decomposition method and hot injection to control over the shapes. The successful synthesis of different shape IONPs was confirmed by using TEM. The crystal lattice of the IONPs was determined by using high resolution TEM and synchrotron X-ray diffraction (XRD). Cubic, semi-cubic, irregular, and spherical IONPs were subsequently encapsulated within polystyrene-b-polyethylene oxide (PS-b-PEO) amphiphilic polymer. The temperature was measured by using a thermal camera after 10 minutes of 885nm NIR laser irradiation with power of 2.5W/cm 2. Results Different shapes of IONPs are successfully synthesized. Cubic, semi-cubic, and irregular IONPs have the average edge length of 10nm measured by TEM and two sizes for the spherical IONPs, 12nm and 15nm, were also produced. After 10 minutes of NIR laser irradiation, cubic IONPs can generate higher temperatures than semi-cubic and irregular IONPs, respectively. However, there is no significant difference in the temperature between cubic and 12 nm spherical IONPs. We pick 12nm spherical IONPs because they have the same volume with 10nm cubic ones. The cubic IONPs can reach 72°C after 10 minutes of NIR treatment. The comparative study between commercial IONPs (FeRex) and our IONPs was also conducted to compare the photothermal efficacy. The data suggest that our IONPs have a better photothermal effect. We further investigated the crystal lattice of IONPs and found that our spherical IONPs produced by the thermal decomposition have preferred lattice orientation and better align in order compared to FeRex. Conclusion The cubic IONPs could potentially be used in photothermal therapy in the future as they can generate high temperature and then kill cancer cells. We also conclude that the crystal lattice alignment and the preferred plan orientation of IONPs are the major factors contributing to the photothermal effect.