  Magnetic and NMR relaxivity properties of {\gamma}-Fe2O3 nanoparticles
embedded into the walls of polyelectrolyte multilayer capsules and freely
dispersed in a sodium borate buffer solution have been investigated. The
different geometric distribution of both configurations provides the
opportunity to study the relationship of water accessibility and magnetic
properties of particles on the NMR relaxivity. Changes in their blocking
temperature and average dipolar field were modeled as a function of packing
fraction in the ensemble of free and entrapped nanoparticles. For free
nanoparticles with relatively low concentration, relaxivity values increase
with packing fraction according to an increase in the dipolar field and larger
water accessibility. However for embedded NPs in the capsule wall, packing
fractions should be limited to optimise the efficiency of this system as
magnetic resonance imaging (MRI) contrast agent.
