A Magnetically Controlled Molecular Nanocontainer as a Drug Delivery System: The Effects of Carbon Nanotube and Magnetic Nanoparticle Parameters from Monte Carlo Simulations

Monte Carlo simulations are used to study a carbon nanotube with a magnetic nanoparticle attached to each of its ends with a single -CH2- group. This system constitutes a switchable nanocontainer that can be used as a drug delivery system. The simulation force field comprises the magnetic dipole interactions (pairwise and with the external magnetic field) and the Hamaker potential for modeling the interactions of large nanoparticles. In the absence of an external magnetic field, the most favorable configurations are where the nanoparticles cap each end of the nanotube. Less favorable configurations occur when at least one of the nanoparticles detaches from the end of the nanotubc and is instead located in the vicinity of the sidewall; this is the uncapped state. It is shown that the capped and uncapped states differ in total energy and that the transition from one to the other is accompanied by an activation barrier. Analysis of the activation barriers and total energies leads to the conclusion that the capped state is preferred and that spontaneous uncapping should be insignificant. These characteristics are favored by large Hamaker constants, by large particle dimensions, and when the diameters of the nanotube and the nanoparticles are similar. The uncapping of the nanocontainer can be promoted by the interaction of ferromagnetic nanoparticles with an external magnetic field. The magnetic energy induced by the magnetic field may overcome the uncapping activation barriers provided that the nanoparticle diameters are large enough. It is shown that nanoparticles with diameters greater than 50 angstrom are effective as magnetically controlled caps operating under magnetic fields in the range 3-12 T; smaller nanoparticles would require much larger magnetic fields. The role of the mutual alignment of the magnetic moments associated with the nanoparticles is also discussed. In the case of larger nanoparticles (90 angstrom in diameter) the alignment of the magnetic moments is not critical; even with randomly assigned magnetic moments, a functional nanocontainer can be synthesized with a probability of around 75%.