Evaluating physical changes of iron oxide nanoparticles due to surface modification with oleic acid

Abstract

The physical characterization of a colloidal system of superficially modified magnetic nanoparticles (MNPs) is presented. The system consists of oleic acid-coated iron oxide nanoparticles (OAMNP) suspended in water. A structural analysis is carried out by using standard physical techniques to determine the diameter and shape of the MNPs and also the width of the coating shell. The colloidal stability and the polydispersity index of this ferrofluid are determined by using Zeta potential measurements. Additionally, the magnetic characterization is conducted by obtaining the DC magnetization loops, and the blocking temperatures are determined according to the ZFC–FC protocol. Finally, the values of power absorption density P of the ferrofluid are estimated by using a magneto-calorimetric procedure in a wide range of magnetic field amplitude H and frequency f. The experimental results exhibit spherical-like shape of OAMNP with (20 ± 4) nm in diameter. Due to the use of coating process, the parameters of the magnetization loops and the blocking temperatures are significantly modified. Hence, while the uncoated MNPs show a blocking state of the magnetization, the OAMNP are superparamagnetic above room temperature (300 K). Furthermore, the reached dependence P versus f and P versus H of the ferrofluid with coated MNPs are clearly fitted to linear and quadratic correlations, respectively, showing their accordance with the linear response theory.

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“…Then, the parameter P increases at 0.078 ± 0.002 W/cm 3 per kHz, and χ 2 = 0.91 is the reached quality factor of the data fit. This statistical value indicates a good agreement of P with the theoretical linear response model of Rosensweig Equation (1); additionally, the reached slope is nearly 1.7 times higher that obtained using a ferrofluid of oleic acid-coated MNPs irradiated with identical magnetic fields [32]. Similar calorimetric experimental procedures and data processing are followed to determine the dependencies of T vs. t (data not shown) and then P vs. H. For this purpose, f = 329.8 kHz is a fixed parameter and H (in kA/m units) covers the interval 8.33 kA/m < H < 25 kA/m, with 4.17 kA/m increments.…”

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“…This is an important property because the negative charge of the SPIONs promotes their endocytosis and, at the same time, the ferrofluid maintains colloidal stability in biological experiments, where the pH of the system has to be very close to pH = 7.4. A measurement of the polydispersity index PDI when pH ≈ 6.8 reaches PDI ≈ 0.22, which is a relatively low homogeneity (according to the size distribution of Figure 1c), but it is considered sufficient for magneto-calorimeter tests and magnetic hyperthermia applications [20,23,28,29]. The magnetic saturation of the SPIONs (red line) was 50 emu/g approximately 30% lower than the one of pure magnetite (black line) (Figure 3a).…”

“…The ferrofluid with magnetite nanoparticles was prepared using the coprecipitation method with FeCl3 and FeSO4 × 7H2O as precursor salts and NH4OH as the precipitating agent as described in [20,23]. The nanoparticles were coated by adding 126 mg folate (Sigma-Aldrich, 97% purity, Milwaukee, WI, USA) to 100 mL ferrofluid (4 mg/mL), which then was sonicated (30 kHz at 39 °C) for 45 min.…”