This is an Accepted Manuscript for the Microscopy and Microanalysis 2020 Proceedings. This version may be subject to change during the production process. DOI: 10.1017/S1431927620022709 1082 – Synthesis and Characterization of Monodisperse Magnetic Nanoparticles by a Scanning Susceptometer Jefferson Araujo 1 , Frederico Gutierrez 1 , Elder Yokoyama 2 , Geronimo Perez 3 and Guillermo Solórzano 1 1 PUC-Rio, Rio de Janeiro, Rio de Janeiro, Brazil, 2 UNB, Brasilia, Distrito Federal, Brazil, 3 National Institute of Metrology, Rio de Janeiro, Rio de Janeiro, Brazil In this article, monodisperse magnetic nanoparticles (MNPs) of magnetite synthesized by co- precipitation were characterized by a magnetic method especially appropriate for low mass samples in the order of tens of micrograms. This method is based on scanning magnetic microscopy (SMM) and has been an important research tool for obtaining the magnetic properties of different materials and for their applications in areas such as geology, biomedicine, science and technology [1-3]. In this study, it was used a susceptometer composed by a reading system based on a gradiometric configuration using two Hall effect sensors made of GaIn with a magnetic detection diameter of 300 µm. This equipment can generate magnetic maps under an applied magnetic field of up to 500 mT. The magnetic moment sensitivity reached a value of the order of 10 -12 Am 2 . As an application example, an average diameter of monodisperse magnetic nanoparticles of magnetite, synthesized by co-precipitation, was estimated. The results were compared with those obtained by other techniques, such as transmission electron microscopy (TEM) and dynamic light scattering (DLS). The reagents ferric chloride hexahydrate (FeCl 3 .6H 2 O), ferrous chloride tetrahydrate (FeCl 2 .4H 2 O), ammonium hydroxide (NH 4 OH, 30%), and hydrochloride acid (HCl, 38%) were used in the synthesis process, which can be summarized as follow: (1) Iron salts preparation, which consisted of preparing two solutions, one by dissolving 6.8 g of FeCl 3 .6H 2 O in 25 mL of distilled water, and another by dissolving 3.95 g of FeCl 2 .4H 2 O in 10 mL of aqueous HCl solution (5.49 M). (2) The iron solutions were mixed together in a proportion of 24 mL of iron (III) to 6 mL of iron (II), and the obtained mixture was added to an aqueous NH 4 OH solution (1.30 M), heated to 80°C for 10 minutes, with vigorous stirring. The iron oxide precipitation was instantly observed, as the solution turned black [4]. After making the nanoparticles, they were characterized using the SMM technique in the susceptometer. This technique is based on the measurement of the dispersed induced magnetic field generated by the samples. In this way, magnetic maps can be obtained for each external magnetic field applied by the equipment, making it possible to obtain the magnetization curve necessary to magnetically characterize the samples (FIG. 1.a and 1.b). In the case of nanoparticles, the magnetization curve shows a superparamagnetic behavior with no remanence and coercivity field (FIG. 1.b). In addition to characterizing the behavior of MNPs (superparamagnetic behavior) at room temperature, the average diameter (around 12 nm) was estimated based on the magnetization curve [3]. The average diameter of MNPs is usually obtained by using techniques such as TEM (FIG. 2.a and 2.b), which, in our study, obtained an average diameter of 14 nm (FIG. 2.c). For the TEM measurements, we used a Tecnai G2 Spirit TWIN FEI micro-scope, USA, operating at 120 kV with a LaB 6 (lanthanum hexaboride) filament. The particle size distribution of magnetic nanoparticles was also estimated by DLS. We performed DLS using a Nano SZ-100 HORIBA-Japan Scientific nanoparticle analyzer, with an incident beam at a wavelength of 532 nm. The particle has a diameter of 207 nm (FIG. 2.d). As expected, the diameter of nanoparticles obtained by DLS was larger than those obtained from other https://doi.org/10.1017/S1431927620022709 Downloaded from https://www.cambridge.org/core. IP address: 54.83.178.125, on 17 Nov 2020 at 06:03:45, subject to the Cambridge Core terms of use, available at https://www.cambridge.org/core/terms.