A universal approach for the reversible phase transfer of hydrophilic nanoparticles

The ability to engineer the surface properties of magnetic nanoparticles is important for their various applications, as numerous physical and chemical properties of nanoscale materials are seriously affected by the chemical constitution of their surfaces. For some specific applications, nanoparticles need to be transferred from a polar to a nonpolar environment (or vice versa) after synthesis. In this work we have developed a universal method for the phase transfer of magnetic nanoparticles that preserves their shape and size. Octadecyltrimethoxysilane was used to cap the surfaces of the aqueous magnetic nanoparticles, thereby allowing their transfer into nonpolar solution. The resulting hydrophobic magnetic nanoparticles were transferred back into aqueous solution by subsequently covering them with an egg-PC lipid monolayer. The superparamagnetic properties of the particles were retained after the phase transfer. The maximum transfer yields are dependent on their particle size with a maximum value of 93.16 ± 4.75% for magnetic nanoparticles with a diameter of 100 nm. The lipid-modified magnetic particles were stable over 1 week, and thus they have potential applications in the field of biomedicine. This work also provides a facile strategy for the controllable engineering of the surface properties of nanoparticles.