# Magnetic Behaviour of Iron Oxide/Dextran Nanoparticles in a Keratin Matrix
## 3 Results  
To start, an analysis of the iron oxide/dextran MNPs alone was performed. When purchased, they come in a small container dispersed in a water solution. It is therefore necessary to extract a portion of them in order to carry out an investigation with a PPMS-VSM. The specimen can be obtained through the complete evaporation of water from a few drops of the pristine keratin solution placed on a piece of silicon wafer at a concentration of 20%. The result is a brown flake that can be detached, reduced into powder, and then placed in the PPMS-VSM sample holder. Figure 2 presents a graph showing the magnetization (M) versus the temperature (T). This type of measurement is used to perform a preliminary characterization of the magnetic properties of a sample of MNPs. In particular, the temperature associated with the maximum value (TMAX) of the ZFC branch is closely correlated to the so-called blocking temperature (T\({}_{\text{B}}\)) [8; 24; 25]. Usually, if the size distribution is narrow, TMAX represents a good approximation of the mean T\({}_{\text{B}}\); otherwise, it may indicate the value above which the majority of MNPs activate [24; 25]. The graph reported in Figure 2 does not show a maximum for the ZFC branch. The curve monotonically increases from 5 up to 250 K and then reaches a plateau. On the contrary, the FC branch increases slightly from 300 to 150 K, and remains constant down to 5 K.  
In Figure 3, two graphs are shown of M versus the magnetic field (H), recorded at 300 and 5 K. These graphs can provide further information on the magnetic behavior of the MNPs. It is possible to notice that the branches, recorded for increasing and decreasing H, are fully superposed at room T with a coercive field (H\({}_{\text{C}}\)) that is almost null. This represents a clear indication that the sample behaves as a superparamagnet. However, the branches do not superpose at 5 K with H at around 200 Oe, indicating that the magnetic momenta associated with the MNPs are completely blocked (see inset). The films spin-coated at various MNP concentrations on silicon can be characterized by techniques such as Atomic Force Microscopy (AFM), Environmental Scanning Electron Microscopy (ESEM), and the spatially resolved Magneto-Optic Kerr Effect (NanoMOKE). One can thus determine and correlate the morphology, the chemical composition, and the local magnetic behavior of all samples as a function of MNP density. The AFM images of all samples are presented in Figure 4. From them, one can derive the 3D morphology, i.e., the height values as a function of the lateral coordinates. The surface is flat for the concentration of 2.5%, with some fractal features due to keratin crystallization that are a few nm thick [20]. At a concentration of 5%, some isolated clusters start to appear. The presence of clusters further increases at concentrations of 10 and 20%. This trend in morphology may indicate that the MNPs are separated, mainly scattered, and isolated at concentrations below 5%. On the other hand, above 5%, they tend to organize in  
Figure 3: M versus H graph of an MNPs/keratin sample, recorded at different T with PPMS-VSM. Figure 2: M versus T graph of an MNPs/keratin sample as obtained with a PPMS-VSM. clusters of various dimensions. This does not exclude the presence of single nanoparticles deeply embedded in the flat regions. In Figure 5, some ESEM data obtained on a sample with a 20% concentration are reported. The morphology is very similar to the one observed with AFM: flat regions alternate with some clusters and fractal features. Whereas AFM provides the vertical dimension, ESEM allows one to probe the chemical composition correlated with morphology. The energy dispersive X-ray (EDX) analysis shows that the clusters contain both iron and oxygen. On the contrary, the flat areas present no evidence of the presence of iron or oxygen. The same results have been obtained for a sample with a concentration of 10%, whereas below 5% it was not possible to detect neither iron nor oxygen.