Supermolecular theranostic capsules for pH-sensitive magnetic resonance imaging and multi-responsive drug delivery

Magnetite (Fe₃O₄) microcapsules prepared by layer-by-layer self-assembly are investigated as multi-functional magnetic resonance imaging contrast agents and drug carriers. They are produced by host-guest interactions and Coulombic force from different supramolecular polymers. Drug molecules are released controllably from the microcapsules by non-invasive ultra-violet light induced photo-isomerization of the azobenzene molecule and pH sensitive Schiff's base. In addition, by encapsulation of the superparamagnetic iron oxide nanoparticles (SPION) in the nearby layers, magnetic field targeting and MRI contrast are achieved. Under tumor-like acidic conditions (pH = 5.6), the r2 relaxivity of the microcapsules is 126 mM^-1 s^-1 which is 37% higher than that in a neutral environment (92 mM^-1 s^-1). As a result of the low pH enhanced MRI contrast agent, the tumor structure can be observed clearly in vivo confirming the high efficacy as a negative MRI agent in T2-weighted imaging. The materials as combined carriers have great potential in clinical applications as drug delivery agents and contrast agents in MRI.