## 4 Conclusions  
The results of the present work show that primary magnetite NPs prepared in the presence citrate during the co-precipitation have a significantly smaller size than those prepared by adsorbing citrate post-synthesis. The latter type of NPs have a size which is not significantly different from that of bare NPs. Since all the three kinds of sample have almost \begin{table}
\begin{tabular}{c c c c c} \hline \hline
**Sample** & **B\({}_{0}\) (Gauss)** & **g** & **AB (Gauss)** & **A** \\ \hline A & 2932 & 2.41 \(\pm\) 0.01 & 989 & 1.25 \\ B & 2939 & 2.41 \(\pm\) 0.01 & 1160 & 1.27 \\ C & 3417 & 2.06 \(\pm\) 0.01 & 555 & 1.10 \\ \hline \hline \end{tabular}
\end{table} 
Table 6: Summary of the parameters derived from the EPR spectra. B\({}_{0}\) induction magnetic field corresponding the resonance peak. g factor, AB width of the peak. The asymmetry parameter A is defined as the ratio of the height of maximum to the depth of the minimum of the derivative of the peak. Figure 8: X-band EPR spectra measured for samples A, B and C at RT. the same relative standard deviations of the PSD, we can conclude that the presence of citrate during the co-precipitation does not influence the level of polydispersion of the NPs. The size of the Fe\({}_{3}\)O\({}_{4}\) NPs determined by analysing the XRD measurements with different approaches are in good agreement with the magnetometric results. This result confirms the soundness of the hypotheses at the basis of the analysis of the magnetization curves. In particular that the magnetic dipole interactions among the NPs are relatively weak so that the magnetization vs. magnetic field intensity can be modelled by the Langevin equation. The size of the particles in aqueous solutions as determined by DLS are much larger than those of individual NPs indicating the formation of aggregates. In situ coating with citrate does not substantially limit the aggregation of the NPs. Magnetic NPs co-precipitated in the presence of citrate attach one another probably sharing faces of the nanocrystals not covered by citrate anions. Aggregates of citrate-coated NPs, prepared both in situ and ex situ, are smaller than those of bare magnetite NPs. Moreover, their dispersions are colloidally stable due to their negative surface charge imparted by citrate anions which prevents the further growth and sedimentation of the aggregates. Magnetization vs. magnetic field intensity and FC/ZFC curves clearly show that all the three types of sample have a superparamagnetic behaviour, with no residual magnetization at RT. The values of the blocking temperatures and the EPR spectra indicate the existence of magnetic interactions among the individual NPs inside the aggregates. Nonetheless, the magnetic properties of these assemblies are dominated by those of the individual NPs which have a size below that required to be superparamagnetic. This study shows that by adding citrate during the co-precipitation of magnetite NPs it is possible to control the size of the primary NPs and prevent, at least in part, their aggregation. Since the size of individual NPs and their aggregation have a large impact on their magnetic properties and dispersion stability, the results of this study which concern basic aspects of particle growth and aggregation are relevant for the vast range of applications of these NPs. The following are available online at [[https://www.mdpi.com/article/10.3390/app11156974/s1](https://www.mdpi.com/article/10.3390/app11156974/s1)]([https://www.mdpi.com/article/10.3390/app11156974/s1](https://www.mdpi.com/article/10.3390/app11156974/s1)), File with Figures S1-S6 and details about the determination of the domain size by analyzing the magnetization vs. magnetic field intensity curves. Draft preparation, writing, review and editing, A.A., F.F.M. and M.C.B. XRD measurements, A.S. DLS measurements, A.A. and F.F.M. EPR measurements, M.C.B. All authors have read and agreed to the published version of the manuscript.