Long term stability and reproducibility of magnetic colloids are key   issues for steady values of Specific Power Absorption through time
  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. 
B. Sanz, M.P. Calatayud, N. Cassinelli, M.R. Ibarra and G. F. Goya
physics.chem-ph cond-mat.soft