Synthesis of magnetic hollow nanotubes based on the kirkendall effect for MR contrast agent and colorimetric hydrogen peroxide sensor

We developed a simple solvothermal approach to synthesize hollow Mn ferrite nanostructures. A mixture of ferric stearate (Fe(SA)₃) and manganese stearate (Mn(SA)₂) reacted with [Fe³⁺]/[M²⁺] ratio = 1:1 in 1-octanol solvent at 240 °C. No additional capping agent was necessary in this reaction. Transmission electron microscopy (TEM) and X-ray diffractometry (XRD) showed that solid tetragonal-structured hausmannite nanorods were primary formed at ∼1 h and then followed by a hollowing process to form core-free spinel-type Mn ferrite nanotubes at ∼12 h. The as-obtained Mn ferrite nanotubes showed a non-stoichiometric composition, which has an average ratio of Fe and Mn elements of ∼1 based on the measurement of an inductively coupled plasma atomic emission spectrometer (ICP-AES). High resolution TEM (HRTEM) analysis was carried out to understand the hollowing process which is suspected to have a crystallographic relationship of facet orientation and epitaxial growth via a Kirkendall effect pathway. Superconducting quantum interference device (SQUID) measurements determined that the paramagnetic behavior of hausmannite nanorods with low mass magnetization converted to superparamagnetism of Mn ferrite nanotubes with high mass magnetization. Magnetic resonance imaging (MRI) contrast signal was significantly enhanced by using high magnetic Mn ferrite nanotubes. Both solid hausmannite nanorods and hollow Mn ferrite nanotubes performed the potential application in peroxidase-like catalytic activity. Furthermore, this hollowing strategy using solvothermal method could be easily used to prepare stoichiometric composition of hollow Mn ferrite nanospheres by adjusting the [Fe³⁺]/[M²⁺] ratio (2:1) of metal precursors through either a one-step or step-by-step syntheses.