The disclosure describes systems and methods for separating a plurality of molecular entities with differing densities. The system includes: a pair of magnetic poles of like polarity to provide a magnetic field; and a container holding the plurality of molecular entities in a fluid medium comprising nanoparticles that substantially change a magnetic susceptibility of the fluid medium such that, when the container is placed inside the magnetic field, sufficient gradients in an effective density of the fluid medium are generated inside the container to levitate the plurality of molecular entities to respective layers within the container, each respective layer corresponding to a respective density.