# Magnetic Behaviour of Iron Oxide/Dextran Nanoparticles in a Keratin Matrix
## 2 Materials and Methods  
The commercial magnetic nanoparticles (MNPs) employed had a diameter of 20 nm (#WHM-G053) with a polydispersity index <0.25 [22]. They contained iron oxide cores with a mean diameter of 8 nm, with the producer reporting that they have a magnetite structure (Fe\({}_{3}\)O\({}_{4}\)). No further details on the size distribution are provided by the manufacturer. These MNPs were designed with an unmodified dextran surface, thus making them soluble in water. According to the manufacturer, MNP density in the purchased solution is equal to 4.3 \(\times\) 10\({}^{15}\) particles/mL. One can calculate that there are 4300 MNPs per 1 \(\upmu\)m\({}^{3}\) of pristine solution. High molecular weight keratin powder (\(\approx\)50 kDa) was extracted from raw wool and kindly provided by Kerline Srl (Bologna, Italy) for these experiments. The MNPs/keratin blend films were prepared by spin coating a water solution in air on Si/SiO\({}_{2}\) substrates that were plasma-treated for 3 min. The basic solution contained water and 10% \(w/v\) of keratin. The content of MNPs was varied, with 2.5, 5, 10, or 20% \(w/w\) vs. keratin of the MNP solution added to the pristine solution. For a concentration > 20%, the solution becomes unstable and precipitation occurs. The spinning conditions chosen for the deposition were the following: final speed 4000 rpm, acceleration 4000 rpm/s, time duration of 60 s. Using a profilometer, the thickness of the film was found to be around 200 nm. All the samples were analyzed as prepared with no further treatment [23]. The MNP powder was characterized using a Physical Property Measurement System (PPMS, by Quantum Design, Tokyo, Japan) equipped with a Vibrating Sample Magnetometer (VSM) insert. The magnetization per unit mass (M, emu/g) has been measured as a function of the temperature M (T) and the magnetic field M (H). To perform the M (T) measurement, the sample was cooled down to 5 K in the absence of a magnetic field. After  
Figure 1: (**a**) Schematic representation of a single dextran MNP with magnetic cores embedded; the magnetic cores drawn have a diameter of 8 nm and the dextran MNP has a diameter of 20 nm. (**b**) Sketch of dextran MNPs dispersed in a keratin film of 200 nm. The thickness is not to scale. The sketch is intended to provide an indicative idea of the sample. this, a magnetic field of 1000 Oe was applied, and the sample was warmed up to 300 K to obtain the Zero Field Cooling curve (ZFC). Subsequently, it was cooled down to 5 K to obtain the Field Cooling curve (FC). For the M (H) measurements, the target temperature was fixed and the magnetic field was swept from 0 to 90 kOe, from 90 to \(-\)90 kOe, and again from \(-\)90 to 90 kOe. NanoMOKE(r) is an instrument based on the Magneto-Optic Kerr Effect (MOKE). It combines the capabilities of the Durham Magneto-Optics NanoMOKE3 with the flexibility of a closed-cycle optical cryostation. This system allows for characterization of the magnetic properties of the sample surface from room temperature down to about 8 K within a few micrometers of lateral resolution. The maximum H value that can be applied is 4000 Oe, and the frequency range can span from 0.1 to 20 Hz depending on the maximum H value. The typical values employed are 2000 Oe and 2.1 Hz. The experimental setup is based on a scanning laser microscope operating at \(\lambda\) = 660 nm whose position is managed by means of a pair of galvanometric mirrors. The measurements were performed in a polar configuration with the external magnetic field applied perpendicular to the sample surface (out-of-plane magnetization) [20]. Atomic Force Microscopy (AFM) was performed using a hybrid system that was assembled using a commercial head (SMENA, NT-MDT), home-built electronics, home-developed software, and a commercial digital lock-in amplifier (Zurich HF2 LI). The setup was operated in intermittent contact mode (ICM).