  Virtually all clinical applications of magnetic nanoparticles (MNPs) require
the formulation of biocompatible, water-based magnetic colloids. For magnetic
hyperthermia, the requirements also include a high colloidal stability against
precipitation and agglomeration of the constituent MNPs, in order to keep the
heating efficiency of the ferrofluid in the long term. The specific power
absorption (SPA) of single-domain MNPs depends critically on the average
particle size and size distribution width, therefore first-rate reproducibility
among different batches regarding these parameters are also needed. We have
studied the evolution of the SPA of highly reproducible and stable water-based
colloid composed of polymer coated $Fe_{3}O_{4}$ magnetic nanoparticles. By
measuring the specific power absorption (SPA) values along one year as a
function of field amplitude and frequencies ($H \leq 24 kA/m$; $260 \leq f \leq
830 kHz$), we demonstrated that SPA in these samples can be made reproducible
between successive synthesis, and stable along several months, due to the in
situ polymer coating that provides colloidal stability and keeps dipolar
interactions negligible.
