Powder Technology 301 (2016) 1112–1118 Contents lists available at ScienceDirect Powder Technology journal homepage: www.elsevier.com/locate/powtec Controlled synthesis of monodisperse magnetite nanoparticles for hyperthermia-based treatments Dung The Nguyen 1, Kyo–Seon Kim ⁎ Department of Chemical Engineering, Kangwon National University, Chuncheon, Kangwon-Do 200-701, Republic of Korea a r t i c l e i n f o Article history: Received January 2016 Received in revised form 23 June 2016 Accepted 22 July 2016 Available online 25 July 2016 Keywords: Magnetite nanoparticles Porous/hollow structures Controlled synthesis Magnetic heating Temperature control Hyperthermia a b s t r a c t Monodisperse magnetite nanospheres with hollow interior and porous shell structure were synthesized through one-pot solvothermal process The chemical conversions of the Fe (III) compounds to generate Fe3O4 simultaneously coupled with the Ostwald ripening process within the magnetite spheres were considered as underlying mechanism for evolution of the Fe3O4 porous/hollow nanostructure The morphology of Fe3O4 nanoparticles could be controlled by adjusting the conditions of process variables We investigated their potential in hyperthermia-based treatments, using an alternative magnetic field Our study revealed that higher applied frequency resulted in the higher heat generation and thus faster temperature growth The hyperthermia efficiency of the Fe3O4 nanoparticles generally depended on particle structures and magnetic properties The Fe3O4 porous/hollow nanoparticles also exhibited an excellent heat generation for several continuous cycles of applied field for a long time © 2016 Elsevier B.V All rights reserved Introduction Magnetite Fe3O4 nanoparticles are of great interest in biomedical applications due to their magnetic behaviors under an applied magnetic fields The Fe3O4 nanoparticles have been widely investigated for targeted drug delivery, hyperthermia treatment of cancer cells, enzymatic assays and activity agent for medical diagnostics [1–3] With the possibility to convert magnetic energy into thermal energy, the application of magnetic materials for hyperthermia-based treatments has been intensively developed In hyperthermia treatment, the local temperature of a tumor could rise to between 42 °C and 46 °C due to heat generation from magnetic materials and consequently cause the change of the physiology of disease cells and eventually the apoptosis [4–6] Magnetic particles in hyperthermia treatment have been focused on the magnetite Fe3O4 and also on the nanoparticles related with cobalt, nickel, or other substitutions in a size range from several nanometers to a few tenths of micron due to their low toxicity, good biocompatibility and ease of synthesis and surface functionalization as reviewed [7,8] The hyperthermia efficiency depended on various factors, including the intrinsic factors of material such as particle structure and magnetic properties as well as the external factors such as amplitude and frequency of applied magnetic field [9–12] The effect of each significant ⁎ Corresponding author E-mail address: kkyoseon@kangwon.ac.kr (K.–S Kim) Current address: Department of Chemical Technology, Faculty of Chemistry, VNU University of Science, Vietnam National University, Hanoi, 19 Le Thanh Tong, Hoan Kiem, Hanoi, Vietnam http://dx.doi.org/10.1016/j.powtec.2016.07.052 0032-5910/© 2016 Elsevier B.V All rights reserved parameter has been independently investigated from study to study for particles obtained from different synthesis methods and for different magnetically-induced heating systems For instance, Gonzales-Weimuller et al [13] demonstrated that the highest heat generation was measured for 11.2 nm particles under an applied field of 400 kHz frequency and 24.5 kA m−1 amplitude, while Gonzalez-Fernandez et al [14] showed that the highest heat generation was measured for particles of around 30 nm diameter by applying an alternative magnetic field of 260 kHz frequency and 100 Oe (about kA m−1) amplitude to induce heat generation Recently, monodisperse magnetite nanoparticles with porous/hollow structures have emerged as an ideal candidate for biomedical applications [15] Considerable effort has been devoted to synthesize magnetite nanoparticles with porous/hollow structures in order to integrate their unique magnetic properties with the valuable characteristics of porous/hollow structure including low density, high surface-to-volume ratio and, specifically, high capacity for encapsulating various chemicals such as drugs, proteins, and genetic materials [16–18] According to the synthesis of magnetic nanoparticles with porous/hollow structures, a one-pot and template-free method based on the oriented attachment of many primary nanoparticles and subsequent Ostwald ripening has been developed [19,20] This method involves the formation of aggregates from many primary nanoparticles and the gradual outward migration of inner primary nanoparticles through a dissolution-relocation process The porous/hollow magnetic nanostructures are expected to be utilized in the storage and delivery of various molecules such as fluorescent dyes and drugs as well as nucleotides and proteins and thus to extend their applications in biomedical field D.T Nguyen, K.–S Kim / Powder Technology 301 (2016) 1112–1118 1113 Even though some extensive researches have been carried out on the preparation of magnetic nanoparticles as well as their hyperthermia application, a limited number of researches have been focused on the synthesis and application of structured Fe3O4 nanoparticles to electromagnetic heating In this study, the Fe3O nanoparticles with tunable porous/hollow structures were controllably synthesized by altering the chemical reactions of Fe precursor during the structure evolution process We then investigated the heating characteristics of the Fe 3O porous/hollow nanoparticles under an applied magnetic field Remarkably, we demonstrated that it is obtainable to control the temperature of magnetic nanoparticles within a certain temperature range by the electromagnetic-induced heating, which is utmost important to sustain pulsatile hyperthermia treatment for a long period Experimental 2.1 Synthesis of Fe3O4 porous/hollow nanoparticles The Fe3 O porous/hollow nanoparticles were synthesized through a one-pot solvothermal reaction which was previously reported [21] In a typical condition, a solution of ethylene glycol (Deajung, ≥ 99%) containing FeCl3 ·6H2 O with concentration of 0.1 mol/L (Sigma-Aldrich, ≥ 98%) and ammonium acetate of mol/L (NH4Ac, Sigma, ≥ 98%) was well mixed (the molar ratio of Fe precursor to NH4Ac was 1:10) and then transferred to a Teflon-lined autoclave cell and was kept inside an oven of 200 °C for 12 h The particles were collected and washed for several times with ethanol and water and then were dried in a vacuum oven at 60 °C for h before characterization In order to control particle morphology, other sets of experiment were carried out for different molar ratios of Fe precursor to NH 4Ac and for addition of water to the precursor solution The product particles were characterized with SEM and TEM measurements, using Hitachi S-4800 Ultra high resolution scanning electron microscope and JEOL JEM-2011 transmission electron microscope, respectively Powder XRD patterns were recorded with a Philips X'Pert PRO MPD X–ray diffractometer using Cu Kα radiation (λ = 1.54060 Å, 40 kV, 30 mA, scanning step of 0.017° in the 2θ range of 10°–80°) X-ray photoelectron spectra (XPS) were obtained using a K Alpha + (Thermo Scientific) The magnetic property was measured with a Lake Shore #7300 vibrating sample magnetometer 2.2 Magnetically induced heating of Fe3O4 porous/hollow nanoparticles A lab-made device was used to induce magnetic heating as shown in Fig The magnetically induced heating device consists of the RLC (resistor, inductor, and capacitor) circuit causing the induction of an alternative current field with a theoretical frequency of 220 kHz The alternating electromagnetic field is generated by a working coil with turns around the sample tube and can be adjusted by changing power supply The sample temperature is measured with a fluorooptic fiber thermometer (Osensa Corp., Canada) and automatically recorded as a function of time Based on the measured temperature of sample solution, a proportional-integral-derivative (PID) controller system will automatically adjust the applied field to maintain the sample temperature at a desired value To characterize the magnetic heating efficiency of Fe3O4 nanoparticles, the magnetic nanoparticles were dispersed in deionized water to make a colloidal solution with concentration of approximately mg/mL A sample volume of 0.3 mL was placed in the center of the coil Because the measurements were carried out in nonadiabatic conditions, the initial linear rise in temperature versus time dependence, dT/dt, was measured The magnetic heating efficiency of Fig Experimental set up for magnetic heating and temperature control Fe3O4 nanoparticles was quantified by estimating the specific loss power (SLP) by the following equation: SLP ẳ dT C m dt 1ị where m is the amount of magnetic material used in the experiment; C represents the sample specific heat capacity (C = 4185 J L− K− 1); and (dT/dt) is the initial linear rise in temperature versus time dependence In another investigation, the applied magnetic field was automatically adjusted to maintain the sample temperature at 42 °C which was generally considered as the required therapeutic threshold for cancer hyperthermia [4–6] Results and discussion The morphology of Fe3O4 nanoparticles for typical experimental conditions is shown in Fig As indicated in Fig 2a, a large quantity of spheres with an average diameter of 300 nm was obtained, indicating the formation of uniform, regular spheres Some spheres with holes on their surface could also be observed in the SEM analysis, showing the formation of product particles with hollow core and porous shell The surface morphology of a typical particle is shown in the inset of Fig 2a, reveals that many smaller grains assembled together to form a stable shell wall The SEM measurement of cross-sectional structure of a typical Fe3O4 sphere was obtained to visualize the hollow structure of product particles as shown in Fig 2b To observe the cross-sectional structures of the magnetite particles, the particles obtained were dispersed in epoxy mold and then the epoxy mold was polished until exposing the particle cross-sectional structures The hollow structure was also confirmed by TEM measurement of a typical sphere as shown in Fig 2c An intensive contrast between the black outer area and the bright central area of particles obviously indicated the existence of hollow structure in the Fe3O4 spheres Fig 3a shows the XRD patterns of product particles, indicating the formation of cubic structure of Fe3O4 nanoparticles (Reference code: 01-088-0315) No peak due to other impurities was detected The chemical composition of the products could be further identified by XPS spectroscopy as shown in Fig 3b The Fe2p high-resolution XPS spectra revealed that Fe2p has binding energy values of 711.6 and 724.3 eV which are very close to the values reported for Fe3O4 [16] Regarding the formation of Fe3O4 from a sole Fe (III) precursor, a series of chemical conversion should be involved such as the formation of 1114 D.T Nguyen, K.–S Kim / Powder Technology 301 (2016) 1112–1118 Fig (a) Representative SEM image of Fe3O4 porous/hollow nanoparticles prepared through typical condition Inset shows surface morphology of a representative particle composed of many smaller grains (b) SEM image for cross-section of a typical Fe3O4 porous/hollow nanoparticles and (c) TEM image of a typical Fe3O4 porous/hollow nanoparticles iron-acetate complexes, the partially reductive reaction of the Fe (III) compounds and the subsequent hydrolysis and dehydrolysis reactions of the Fe (III) and Fe (II) compounds to generate Fe3O4 [21] It has been reported that the chemical conversion of the Fe (III) compounds to generate Fe3O4 took place simultaneously with the formation of numerous tiny grains, the spherical assembly of those grains to form larger-sized spheres and subsequent evolution of the porous/hollow structures for those spheres The chemical conversions of solid material caused a little shrinkage of the grain which built the larger-sized spheres The reduced grains around the exterior surface of spheres quickly became compact and were transformed to be a solid and stable surface shell of spheres while the inner grains of spheres would be subjected to the Ostwald ripening process and thus would migrate and deposit onto the interior wall of the stable shell Additionally, the dissolution-relocation of smaller grains could be also rationalized by considering the existence of bubbles such as ammonia and acetic acid which were trapped inside the aggregates and caused the formation of spheres with loosely packed structures [22] Since the driving force for the Ostwald ripening could be attributed to the intrinsic density variations inside the solid aggregates, the presence of bubbles could initiate the Ostwald ripening as well as accelerate the structure evolution process The hollow structure was well developed with the complete conversions of core grains Fig shows the morphology of Fe3O4 spheres prepared for different molar ratios of Fe precursor to NH4Ac, while the initial concentration of Fe precursor was 0.1 M Fig 4a and b as well as their insets indicate that the Fe3O4 spheres with higher porosity could be obtained with increasing the concentration of NH4Ac The molar ratio of Fe precursor to NH4Ac was adjusted to 1:20 and 1:30, respectively TEM measurements of a typical sphere obtained for the molar ratio of Fe precursor to NH4Ac of 1:30 confirmed the formation of more porous/hollow structure of the Fe3O4 spheres as shown in Fig 4c and d The particle diameter also increased up to around 350 nm with the increase of the molar Fig (a) XRD pattern and (b) Fe2p high-resolution XPS spectrum of Fe3O4 porous/hollow nanoparticles prepared for the molar ratios of Fe precursor to NH4Ac of 1:10 concentration of NH4Ac By increasing the molar concentration of NH4Ac, more bubbles of ammonia and acetic acid could be trapped within the spheres During the ripening process, the inner spaces occupied by bubbles were released and became part of the interior void As a result, the structure evolution was accelerated which could result in formation of more porous shell structure Fig 5a and b show the morphology of Fe3O4 nanoparticles prepared with addition of water to the precursor solution The amount of water was set for 10% of total volume The Fe3O4 nanoparticles with particle size b 100 nm were obtained As shown in Fig 5a, the Fe3O4 nanoparticles composed of numerous smaller particles were obtained However, hollow structure was not clearly observed by TEM analysis as shown in Fig 5b Addition of water was expected to facilitate the hydrolysis of Fe precursor compounds and thus accelerate the particle nucleation and assembly of final Fe3O4 products As a result, the hollowing process was inhibited, leading to the formation of Fe3O4 nanoparticles with compact structure and smaller particle size Fig shows the magnetic properties of the Fe3O4 porous/hollow nanoparticles at room temperature All the particles obtained for different molar ratios of Fe precursor to NH4Ac exhibited similar ferromagnetic behaviors with the magnetization saturation and coercivity values of around 80 emu/g and 200 Oe, respectively The assembly and merging of many small grains into a larger sphere might affect the ferromagnetic behavior of Fe3O4 porous/hollow nanoparticles by turning a single domain structure of each individual grain into a multidomain structure of the whole sphere For Fe3O4 nanoparticles D.T Nguyen, K.–S Kim / Powder Technology 301 (2016) 1112–1118 1115 Fig Typical (a) SEM and (b) TEM images of Fe3O4 nanoparticles prepared with addition of water temperature under the magnetic heating and the SLP values of Fe3O4 nanoparticles generally increased with increasing field strength For the applied field in a range of 6.6 kA/m to 9.3 kA/m, the SLP values for Fe3O4 porous/hollow nanoparticles obtained for molar ratio of Fe precursor to NH4Ac of 1:10 increased from 40 W/g to 146 W/g A relatively slower heating rate with smaller SLP values in the range of 37 W/g to 140 W/g were observed for the Fe3O4 nanoparticles prepared for the molar ratio of Fe precursor to NH4Ac of 1:30, while significant increases of the heating rate and SLP values were clearly observed for the Fe3O4 nanoparticles obtained with addition of water The highest SLP value of 480 W/g was recorded for the magnetic heating of Fe3O4 Fig (a, b) Representative SEM images of Fe3O4 porous/hollow nanoparticles prepared for the molar ratios of Fe precursor to NH4Ac of 1:20 and 1:30, respectively Insets show SEM image of a typical particle (c, d) TEM images of typical Fe3O4 porous/hollow nanoparticles prepared for the molar ratio of Fe precursor to NH4Ac of 1:20 and 1:30, respectively prepared with addition of water, the structure of smaller Fe3O4 grains which assembled to build the final compact Fe3O4 nanoparticles was more preserved As a result, the Fe3O4 nanoparticles still exhibited high magnetization saturation of about 78 emu/g but with negligible coercivity of about Oe (Fig 6b) The high magnetization saturation value enabled the nanoparticles to be applicable in biomedical field The magnetization of Fe3O4 porous/hollow nanoparticles can be used to achieve magnetically induced heat generation for the thermal treatment of cancer and other diseases The specific ideal conditions for biomedical heating applications imply that one wishes to achieve fast heating effects with as small amount of magnetic particles as possible Fig shows the typical temperature increase as a function of time with the prepared colloidal Fe3O4 nanoparticles for various applied fields Eq (1) was used to calculate the SLP values of Fe3O4 nanoparticles for different applied fields Experimental data show that the raise of Fig Room-temperature magnetization curve as a function of applied magnetic field for (a) Fe3O4 porous/hollow nanoparticles prepared for the molar ratio of Fe precursor to NH4Ac of 1:10 and (b) Fe3O4 nanoparticles prepared with addition of water 1116 D.T Nguyen, K.–S Kim / Powder Technology 301 (2016) 1112–1118 Fig Typical temperature increase as a function of time of the colloidal Fe3O4 nanoparticles for various applied fields (a) Fe3O4 porous/hollow nanoparticles prepared for the molar ratio of Fe precursor to NH4Ac of 1:10, (b) Fe3O4 porous/hollow nanoparticles prepared for the molar ratio of Fe precursor to NH4Ac of 1:30 and (c) Fe3O4 nanoparticles prepared with addition of water nanoparticles obtained with addition of water under the field of 9.3 kA/ m Such SLP values at relatively low applied field can make those Fe3O4 nanoparticles to be considered as potential materials for magnetic hyperthermia treatment It is well-known that the magnetic induction heating of the Fe3O4 nanoparticles is mainly caused by relaxation of the magnetic moments to their equilibrium orientations as well as by rotational Brownian motion of the whole particles within dispersed media due to the torque exerted on the magnetic moments When the magnetic moments rotated, while the particle itself remained fixed, the particle would undergo hysteresis loss or Néel relaxation and thermal energy was dissipated by the rearrangement of atomic dipole moments within the magnetic domain The hysteresis loss mechanism dominated heat generation of ferromagnetic materials whose sizes exceeded the domain wall width, while the Néel relaxation dominated heat generation of superparamagnetic materials whose sizes maintained the single domain structure Both the hysteresis loss and the Néel relaxation relatively corresponded to the particle size of the magnetic materials When the rotation of the magnetic moments caused the rotation of the particle itself, the particle would undergo Brownian relaxation and thermal energy was delivered through shear stress in the surrounding fluid The Brownian rotation ability also corresponded to the global hydrodynamic diameter of the particle In practice, the primitive heating mechanism through the magnetic moment rotation and through the Brownian rotation would occur simultaneously and the relative contributions could be determined by the time scales at which each mechanism occurred As indicated in Fig 7, for a certain applied frequency, the higher applied amplitude would provide higher energy for the magnetic moment oscillation which would result in the higher heat generation and thus faster temperature growth The Fe3O4 nanoparticles obtained for different molar ratios of Fe precursor to NH4Ac exhibited very similar temperature increase profiles because these particles exhibited similar ferromagnetic behaviors with high magnetization saturation and coercivity and thus they would generate heat through a same fashion by the hysteresis loss It would be noticed that the Fe3O4 porous/hollow nanoparticles prepared for the molar ratio of Fe precursor to NH4Ac of 1:30 showed relatively smaller amounts of heat generation as well as D.T Nguyen, K.–S Kim / Powder Technology 301 (2016) 1112–1118 SLP values for different applied fields than the particles prepared for the molar ratio of Fe precursor to NH4Ac of 1:10 This could be attributed to the relatively larger particle size for nanoparticles prepared for the molar ratio of Fe precursor to NH4Ac of 1:30 since the larger-sized nanoparticles were not favorable for Brownian motion Additionally, the Fe3O4 porous/hollow nanoparticles prepared for the molar ratio of Fe precursor to NH4Ac of 1:30 showed relatively larger shell thickness which would hinder the conduction of thermal energy generated by the inner material to surrounding solution The Fe3O4 nanoparticles obtained with the addition of water exhibited similar magnetization saturation with those particles prepared without addition of water but with negligible coercivity as well as smaller particle size These factors would allow the Fe3O4 nanoparticles prepared with the addition of water to obtain faster relaxation of the magnetic moments and more freely and easily rotation of the whole 1117 particles Consequently, the magnetic heat generation from the Fe3O4 nanoparticles obtained with the addition of water was higher than those from the Fe3O4 porous/hollow nanoparticles Another investigation was conducted by heating a colloidal solution of Fe3O4 porous/hollow nanoparticles for four continuous cycles as shown in Fig For each cycle, an applied amplitude field of 6.6 kA/m was initially supplied Based on the measured temperature of sample solution, the applied magnetic field was automatically controlled through a proportional-integral-derivative (PID) controller system to maintain the solution temperature at 42 °C After reaching the desired temperature of 42 °C for a certain time, the magnetic field was turned off and the solution temperature decreased until it reached the room temperature This “on-off” cycle was repeated several times Obviously, the temperature profile exhibited that the solution temperature was precisely controlled at 42 °C with oscillation of solution temperature Fig (a) Temperature growth of the Fe3O4 porous/hollow nanoparticles heated for four continuous cycles and (b) oscillation of temperature around setting value of 42 °C during magnetic heating process 1118 D.T Nguyen, K.–S Kim / Powder Technology 301 (2016) 1112–1118 around 42 °C as indicated in Fig 8b The oscillation of temperature observed around 42 °C could be attributed to the time lags of temperature measurement by fiber optic probe and thermal conduction from the heated nanoparticles to surrounding solution as well as the oscillation of applied magnetic field to control the sample temperature by the PID controller system During the magnetic heating, the Fe3O4 nanoparticles tended to align themselves along the applied field rather than to be well dispersed in the solution and thus the oscillation of temperature became more obvious for the later cycles comparing to the first cycle However, Fig 8b also indicates that the oscillation of temperature was negligible as the solution temperature was precisely controlled at 42 °C ± 0.2 °C for all cycles The Fe3O4 porous/hollow nanoparticles exhibited an excellent heat generation for several continuous cycles of applied field which could reflect the ability of particles to sustain pulsatile hyperthermia treatment for a long period It could be supposed that, by increasing the initial value of the applied amplitude field, the sample temperature could be increased faster in the initial stage but the temperature control by the PID control system during operation would be obtained in the same fashion Additionally, the technology to control solution temperature should be further developed to induce and maintain the temperature of a defined target volume within a desired temperature range for practical hyperthermia applications Conclusions In summary, the magnetite Fe3O4 porous/hollow nanoparticles were successfully synthesized through one-pot solvothermal process without any surfactant and template The formation mechanism of the porous/ hollow nanoparticles comprised simultaneous chemical and physical processes The chemical conversion caused the shrinkage of tiny grains which built the spherical assemblies and enhanced the empty spaces within the spheres, subsequently induced the outward migration and relocation of the core grains toward the outer layer, leading to the formation and expansion of the hollow core structure The morphology of Fe3O4 nanoparticles could be controlled easily by changing the conditions of process variables The use of magnetic nanoparticles for hyperthermia exhibited great promise in the field of nanobiomedicine The hyperthermia efficiency of the Fe3O4 nanoparticles generally increased with increasing magnetic field strength and depended on particle structures and magnetic properties The Fe3O4 porous/hollow nanoparticles also exhibited an excellent heat generation for several continuous cycles of applied field which could reflect their ability to sustain pulsatile hyperthermia treatment for a long time Acknowledgment This work was supported under the framework of international cooperation program managed by National Research Foundation of Korea (NRF-2015K2A1A2070442) This study was also supported by 2015 Research Grant from Kangwon National University (No 520150079) Instrumental analysis was supported from the central laboratory of Kangwon National University For this 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