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Monitoring in real time the production of Fe-oxide nanoparticles
Author’s Accepted Manuscript Monitoring in real time the production of Fe-oxide nanoparticles M.P. Fernández-García, J.M. Teixeira, P. Machado, M. Enis Leblebici, J.C.B. Lopes, C. Freire, J.P. Araujo www.elsevier.com
PII: DOI: Reference:
S0009-2509(15)00601-6 http://dx.doi.org/10.1016/j.ces.2015.08.036 CES12563
To appear in: Chemical Engineering Science Received date: 26 May 2015 Revised date: 1 July 2015 Accepted date: 28 August 2015 Cite this article as: M.P. Fernández-García, J.M. Teixeira, P. Machado, M. Enis Leblebici, J.C.B. Lopes, C. Freire and J.P. Araujo, Monitoring in real time the production of Fe-oxide nanoparticles, Chemical Engineering Science, http://dx.doi.org/10.1016/j.ces.2015.08.036 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting galley proof before it is published in its final citable form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Monitoring in real time the production of Fe-oxide nanoparticles *
†
M.P. Fernández-García1 , J.M. Teixeira1, P. Machado1, M. Enis Leblebici2 , J.C.B. Lopes3, C. Freire4, J.P. Araujo1 1
IFIMUP and IN-Institute of Nanoscience and Nanotechnology, Department of Physics and Astronomy,
Faculty of Sciences, University of Oporto, Portugal. 2
Process Engineering for Sustainable Systems (ProcESS), Department of Chemical Engineering, KU
Leuven, Belgium 3
Laboratory of Separation and Reaction Engineering, Faculty of Engineering, University of Oporto,
Portugal 4
REQUIMTE/LAQV, Department of Chemistry and Biochemistry, Faculty of Sciences, University of
Oporto, Portugal.
We herein demonstrate an in-situ device and a real time method to observe the vital stages of Fe-oxide nanoparticles’ (NPs) preparation by coprecipitation reaction. The reaction volume is monitored by a home-made AC susceptometer to analyze the magnetic response of the growing NPs. Three different regions (nucleation, growth and diffusion) could be identified in the recorded in-phase and out-of phase curves. The morphological and magnetic characterization of the obtained Fe-oxide NPs reveal minute particle dimensions (< 3 nm), narrow polidispersity (1-7 nm), superparamagnetic behavior (null coercivity) and narrow distribution of blocking temperatures ( ~ 67 K). We show that the device and technique enable real time assessment of the mixing speed which has paramount impact on the final product’s physical and chemical properties. Keywords: Magnetic nanoparticles, coprecipitation reaction, AC susceptometer, real time monitoring, rapid mixing.
*
Corresponding author: M.P. Fernández-García; Electronic mail: [email protected]; Tel.: +351.220.402.368; Fax: +351.220.402.406 † Past address: Laboratory of Separation and Reaction Engineering, Faculty of Engineering, University of Oporto, Portugal
1
I.
INTRODUCTION Superparamagnetic nanoparticles (NPs) are widely studied materials in biomedicine,
chemistry and material science due to their excellent magnetic properties (i.e. absence of coercivity and large magnetic response) and their inexpensive manufacture [1-4]. Due to their low toxicity levels and biocompatible character Fe-oxide NPs are the most popular choice for biomedical applications [5-8]. These NPs are widely used in theragnostics [9,10], magnetic resonance imaging [11-13], targeted drug delivery [14] and magnetic hyperthermia [15-18]. Furthermore, Fe-oxide NPs also receive attention from industry and environmental engineering because they can be employed as nanoadsorbents for removing various pollutants [19], water purification [20], and as catalyst and photocatalyst on industrial chemical processes or pigments in many colorant formulations [21]. Above all, for all these purposes a detailed characterization correlating the microstructure, morphology and magnetic properties of the NPs is needed to control both their individual and collective behavior [22-26]. There are several methods to synthesize Fe-oxide NPs as for example on water-oil microemulsions,
hydrothermal
synthesis,
coprecipitation,
pyrolysis,
or
thermal
decomposition [1,8,27-30]. However, coprecipitation methods have the advantage of being easy-to-follow, inexpensive and efficient to produce massive samples. Following such method, Fe(II):Fe(III) ions precipitate in alkaline solution, such as sodium hydroxide or alkanolamines [31,32]. Coprecipitation is a fast kinetically driven reaction, so there are several operational factors that are likely to influence its outcome: temperature; concentration of iron salts and base; nature of the salts (perchlorates, chlorides, nitrates and sulfates) pH values; and, mixing rates [33-37]. All these variables affect the magnetic
2
response, crystallinity, chemical composition, mean dimension and size distribution of the final NPs. Therefore, a strict control of all these parameters and the reproducibility of the coprecipitation reaction is needed. For that purpose, a way to monitor the full course of magnetic NPs’ coprecipitation reaction in real time is highly desirable. Traditional magnetometry methods that are quite suitable for studying magnetic NPs are: vibrating sample magnetometer (VSM) and superconducting quantum interference device (SQUID) techniques. In addition, magneto-optical techniques [38] could also be employed to differentiate superparamagnetic materials [39]. Above all, magnetic measurements in AC fields are extremely important for characterizing magnetic NPs, as they yield information of the magnetization dynamics that cannot be obtained from direct DC measurements [4043]. The measured magnetic moment in AC susceptometers varies with time at the same frequency () of the exciting field or delayed () with respect to it. A complex notation for magnetic susceptibility, ' i '' , is used to describe this time-dependent behavior. Considering an AC magnetic field ( H H 0 sin wt ) the in-phase susceptibility (’) is related with the low field limit of the steepness of the hysteresis loop – the magnetic susceptibility for (super)paramagnetic materials – through:
' m H 0 cos
(Eq. 1)
whereas, the out-of-phase (’’) is associated with the coercivity (see details on [44]).
'' m H 0 sin
(Eq. 2)
Recently, Ström and co-authors have successfully studied the full course of coprecipitation reactions with an AC susceptometer [42]. They developed an external mixer to blend the jets of precursors and reactants and afterwards, transferred the mixture to a sample vial that is placed inside the AC susceptometer. Therefore, the initial stages of 3
nucleation (typically ms) could not be closely followed by their designed setup. In that sense, we have designed and reported a home-made and desktop AC susceptometer set-up, to monitor coprecipitation reactions from the exact moment that the precursors are vigorously blended until the end of the synthesis [44]. Basically, on our setup the alkali base is firstly stirred strongly with a pitched blade impeller and afterwards, a jet of Fe salts is impinged and blended with the stirred alkali base. Our integrated system allows the simultaneous monitor of the in-phase (’ ≡ X) and out-of-phase (’’ ≡ Y) signals, as well as 1 R X 2 Y 2 and, tan Y X which is the phase shift between the reference and
pickup measured signals. Our set-up provides precise information about the different regimes of NPs’ size generation [37] [burst nucleation - region I, growth - region II and, diffusion of the solutes to the surface - region III] through the identification of these stages on the real time curves. Our work put on view the importance of mixing efficiently the reactants on coprecipitation reactions. The specific way in which different streams are mixed is a main factor for an efficient operation, especially for those processes where chemical reaction occurs [45,46]. The time necessary to achieve to the state of perfect mixing have important repercussions on the industrial level: economy of reactants, higher degree of control of the chemical process and, environmental impact due to the reduction of energy consumption. The impact of fast mixing is particularly important for reactions with faster kinetics such as coprecipitation. However, the understanding of the mixing phenomena is complex and its knowledge is rather incomplete. In the particular case of magnetic NPs, it has been reported the importance of a uniform mixing rate between iron ion source and base solution for achieving particles with narrow size distribution on coprecipitation reactions [34,35].
4
Herein, we corroborate the influence of such mixing on the final properties of the Fe-oxide NPs by means of our home-made set-up. We compare two different blending schemes: i) a jet of iron precursor solution is injected into the base solution while being strongly blended with pithched blade agitator (sample A); ii) the same jet of precursors discharged into the base solution without being stirred with
the impeller (sample B). The detailed
interpretation of the real time in-phase and out-of phase signals recorded by our AC susceptometer lead us to predict that the Fe-oxide NPs’ on sample A, exhibit tiny dimensions
and
thus,
superparamagnetic
properties.
Structural
and
magnetic
characterization of samples A and B were developed to confirm our findings.
II. MATERIAL AND METHODS
A. Chemicals and reagents The Fe(II) : Fe(III) solution was prepared from commercially available analytical grade reagents. All materials were employed without further purification. Iron (II) chloride tetrahydrate and sodium hydroxide (analytical grade) were purchased from Merck and, iron (III) chloride hexahydrate (>98%) from Riedel-de Haën. Hydrochloric acid (37%, analytical grade) was supplied by Panreac. Ultrapure water (milipore with specific resistivity of 18 m·cm) was used throughout the experiments.
B. Synthesis of Fe-oxide nanoparticles The Fe-oxide NPs were prepared by the aqueous co-precipitation method under alkaline conditions using NaOH as the base [47]. Briefly, stoichiometric quantities of FeCl2·4H2O and FeCl3·6H2O were dissolved individually in deoxygenated aqueous solution of 0.5 M
5
HCl. Afterwards, the precursor iron ion solution was prepared by mixing both Fe chloride solutions. This solution was protected from ambient atmosphere conditions by Ar gas flow. For the in situ synthesis of sample A, 2 ml of deoxygenated NaOH solution (1.5 M) were stirred with an pitched blade impeller in the sample vial and afterwards, 0.2 ml of iron ion solution were discharged at 1.3 ml/min (controlled by a syringe pump). The discharge is directed instantly inside the sample vial and kept at room temperature. A similar procedure was followed for the in situ synthesis of sample B but the reactants were not agitated. It should be remarked that for the synthesis of both samples A and B, the ferrous:ferric ratio was 1:2 corresponding to magnetite, Fe3O4. C. Characterization methods In order to obtain samples A and B, the synthesized ferrofluids were centrifuged with ultrapure water until pH values reaches 9-10 and supernatant was discarded. Transmission electron microscopy (TEM) images were obtained using LEO 906E microscope operating at acceleration voltages of 100-120 kV. Samples were prepared by dispersing few drops of samples A and B in high-purity ethanol under sonication. Afterwards, several drops of this solution were deposited on carbon-coated 400 mesh copper grid and dried on air. The magnetic properties of 80 l of magnetic liquid samples were studied using a commercial Quantum Design superconducting quantum interference device (SQUID) magnetometer. For the M(T) curves, the samples were cooled in zero field (ZFC) from 300 to 10 K. Next, the magnetic field (H = 50 Oe) was applied and kept constant during the measurement of the magnetization, MZFC and MFC, at fixed temperatures (ΔT = 2 K) between 10 and 270 K and from 270 to 10 K, respectively. The hysteresis cycle, M(H)
6
curves, were measured in the range ± 30 kOe at 270 K. Temperature did not exceed 270 K, in order to maintain a frozen suspension during the magnetic measurements.
D. Mixing process with pitched blade impellers The coprecipitation growth of the magnetic NPs is monitored in situ by a home-made and desktop AC susceptometer that we have recently designed and built [44]. Summing up, our set-up comprises: i) one pair of Helmholtz coils (primary) that are fed by an AC sinusoidal electrical current to create the applied magnetic field; ii) one pair of identical pick-up sensing coils (secondary) oppositely wound and electrically connected in series. The secondary coils are positioned in the middle of the primary coils where it is assured a homogeneous magnetic field. The sample vial containing the base solution, where the chemical reaction takes place, is introduced into one of the secondary coils. The X, Y, R and
channels are only zeroed after the test vial, the Teflon thin tube (connected to the pump) and, the motorized impeller are placed inside the sensing coil. Data started to be saved at the same time that the chemical reaction started (i.e. the discharge of the iron ion solution) and, until the software is interrupted by the operator. According to the Faraday law, the magnetic ferrofluid sample will generate magnetic flux changes on the sensing coil that are converted into an output voltage. The picked up signal is detected by a lock-in amplifier that measures the in-phase and out-of-phase signals [44]. It is well established that the coprecipitation synthesis of NPs is obtained through the vigorous mixing of two or more reactant solutions, resulting in a final product with low solubility in the medium. Researchers agree that a large stirring rate, increase the mobility and homogeneous distribution of species generating narrower particle size distribution (PSD) [48]. Additionally, in order to prepare monodisperse nanocrystals, it is highly 7
desirable inducing two separate stages: i) single nucleation event and, ii) particle growth process. Otherwise, if nucleation takes place throughout the whole process, the control of the PSD would be difficult [37]. This burst-nucleation process requires homogeneous concentration of precursors throughout the medium from the moment they are blended until the end of the particle growth. Particularly, in our setup the alkaline solution is stirred vigorously by means of the pitched blade impeller that produces a confined axial flow and low shear. Our impeller has four blades with rounded leading edges at an angle of 45º from the horizontal (see Figure 1). In fact, radial flow mixers could produce two undesirable re-circulating loops (below and above it) where liquids are inefficiently blended [49]. This can be minimized by the usage of hydrofoil impellers which are more streamlined with small loop underneath (see schematic flow pattern on [44]). This can be an improvement of the current system. Since velocities and shear diminished from regions near the blades to corner areas or liquid surface, the discharge of Fe ion solution is done in our case near the blade to avoid initial coalescence, agglomeration and flocculation at low mixing energy regions [49]. At the same time, the reactants are discharged inside the base solution to reach an efficient diffusion of the precursors and also near the blades where the local energy is high (see Figure 1). As a result, the medium is efficiently blended throughout the whole volume of the fluid with an axial flow that, assures the pH homogenization throughout the entire volume. Thus, low pH zones in the reaction medium are avoided minimizing possible side reactions.
8
FIG. 1. (Color online). Enlarged view of blades placed at an angle of 45º from the horizontal. The precursors discharge is done near the blade where the local energy is large.
III. RESULTS AND DISCUSSION Figure 2 shows the real time coprecipitation reaction of Fe-oxide NPs on sample A (precursors and reactants strongly blended with an pitched blade impeller that promotes a strong and homogenous mixing) and, sample B (precursors discharged into the sample vial without such mixing). The evolution of the X- and signals of samples A and B within time reveal that, the reactions are completely different and thus, put on view the importance of the mixing process. The coprecipitation reaction of sample A is very fast in the first seconds and finishes after ~ 750 s (X-signal remains stable). In contrast, the in-phase curve of sample B increases gradually and did not stabilize even after 1750 s [see Fig. 2c)]. Additionally, the X-signal of sample B clearly shows the detection of the three stages of crystal growth: nucleation (region I), growth (region II) and diffusion (region III). In contrast for sample A, only growth and diffusion stages are detected, which is a clear
9
evidence of the mixing efficiency achieved by the impeller [see Figure 2.a)]. The in situ curves indicate that the nucleation mechanism of sample A might be faster than typical nucleation times of ms. It should be noted, that we are not limited by our set-up resolution because it is of tens of s. Therefore, it seems that nucleation takes place as a burst process due to the efficient mixing of reactants and the homogeneous concentration of precursors conquered by the impeller in the in situ synthesis of sample A. On the other hand, the direct relation between X-signal values and the NPs’ magnetic susceptibility (m) (see Eq. 1) lead us to predict that sample A will exhibit lower magnetic moment than that of sample B (further details in [44]). (a)
(b)
0.21
0.2
0.14
0.1
0.07 -3
sample A
sample A
0.0
0.00
(c)
(d)
0.3
()
X (10 V)
0.3
1.4 0.2 0.7
0.1
sample B
0.0 0
500
1000
1500
sample B 0
500
1000
0.0
1500
time (s) FIG. 2. (Color online) AC susceptometer measured signals (X and ) of sample A [(a) - (b)] and sample B [(c) - (d)]. In Fig. 2(a) and 2(c), different regions of nanoparticle’s growth are shown (see text).
Regarding the out-of-phase signal, it does not change for sample A and remains null during the formation of the NPs as illustrated in Fig. 2(b). This means that the NPs of 10
sample A oscillate with the same frequency of the applied AC field predicting their superparamagnetic nature (absence of hysteresis) [33, 44]. On the other hand, the -signal of sample B shows a slight variation ( ~ 0.3º) [see Fig. 2(d)] envisaging MNPs with small coercivity. In comparison, a powder sample of ferrimagnetic Fe-oxide NPs, that was also measured with our home-made AC susceptometer, showed double out-of-phase value (~0.6º) and Hc ~ 100 Oe (see Fig. S.1 of the Supplementary material). The analysis of our real time curves are in good agreement with the M(H) curves performed at 270 K [see Fig. 3(a)]. It should be noted that M vs. H curves have been corrected to remove the negative slope arising from the diamagnetic contribution of the sample-holder and water. Fig. 3(a) displays that both samples showed ‘Langevin-like’ curves with minute Hc values (or at least under the detection limit of SQUID magnetometer) and thus, MNPs could be roughly considered as superparamagnetic. Unfortunately, Ms values could not be estimated for samples A and B because the reduced volume of the liquid samples (80 l) avoids the accurate estimation of concentration after drying. Even tough, magnifications around low magnetic fields display small differences between both samples [see inset of Fig. 3(a)]. Firstly, Mr/Ms values are 0.007 and 0.022 for samples A and B, respectively. Secondly, as we have previously reported [44], the lowfield limit of the steepness of M(H) curves is directly proportional to the magnetic susceptibility (m) of (super)paramagnetic materials (M = m·H with m > 0). The minor steepness of sample’s A M(H) curve, is an indication of the smaller magnetic susceptibility of MNPs on sample A. It is well established that NPs’ magnetic moment is drastically diminished with the length-scale reduction. Therefore, we envisage that MNPs on sample A
11
will exhibit smaller mean dimension and/or narrower distribution than those on sample B, corroborating the analysis of -signal on Fig. 2(b) and 2(d). 1.0
sample A sample B Fit
T = 270 K
0.1
0.0
M / Mmax
M / Mmax
0.5
(a)
0.0
-0.5
sample A sample B
-0.1 -50
-25
0
25
50
H (Oe)
-1.0 -3.0
-1.5
0.0
1.5
3.0
H (kOe) (b)
1.0
FC ZFC
sample A
0.5
67 K
M / Mmax
0.0
FC
1.0
(c)
ZFC sample B
0.5
0.0 0
100
200
300
T (K)
FIG. 3. (Color online) (a) M(H) curves of ferrofluid samples A and B measured at 270 K. Solid lines correspond to the fits of experimental curves through Langevin functions (see text). The inset is a magnified view near null H values revealing the minute coercivity of the samples. M(T) curves in the ZFC and FC regime of samples A (b) and B (c), respectively.
12
From the fit of M(H) curves to Langevin functions [solid lines in Fig. 3(a)], magnetic domain dimensions dlan() = 2.7(0.5) nm and 3.4(0.6) nm have been estimated for samples A and B, respectively. On the one hand, such small dimensions indicate the single domain character of the studied MNPs. On the other hand, dLan dimensions of MNPs on sample A being smaller than those of sample B, corroborates previous analysis of Mr / Ms values, and -signals. The reversibility of the M(H) curves point out that the blocking temperatures, TB, are below 270 K for both samples. Magnetization measurements as a function of temperature, M(T) curves, in the ZFC-FC regime were performed to further characterize the samples. The MZFC(T) curve of sample A [Fig. 3(b)] exhibit well defined and narrow TB ( ~ 67 K) suggesting that homogeneous and fast nucleation with the impeller do lead to tiny PSD as we have identified on in situ curves. In the case of sample B [Fig. 3(c)] a broader distribution of TB could be clearly observed confirming the larger Hc values and not-null signal. Nevertheless, M(T) curves of both samples are reversible at 270 K in good agreement with the almost superparamagnetic behavior obtained on M(H) curves [see Fig. 3(a)]. Figures 4(a) and 4(b) show the TEM micrographs of the quasi-spherical shape NPs in samples A and B, respectively. The size distributions of the NPs [illustrated in Fig. 4(c) and 4(d)] were determined by measuring ~500 spheroids and creating the histograms necessary to model the dimensions of the NPs. In the case of sample A, a narrow size distribution (between 1- 7 nm) was obtained whereas, for sample B a broader size distribution (between 1- 20 nm) was observed. This means that NPs dimensions could be reduced up to 3 times by means of the impeller promoting the fast and uniform homogenization of reactants as
13
described above. The fits of the histograms to log-normal functions reveal mean dimensions of <()> = 2.8(0.3) nm and <()> = 5.5(0.4) nm for samples A and B, respectively, being in agreement with dLan values previously obtained and following the same relation as Hc and Mr/Ms values (see above). Elemental analysis by energy dispersive X-ray spectroscopy (EDS) developed on a scanning electron microscope displayed that the resulting material contains mainly Fe and O (see Fig. S.2 of the supplementary material). Additionally, C, Cu (from the grid) and Mo, Al (arising from the sample holder) were also detected. Furthermore, bright and individual spots were detected on dark field images of TEM suggesting the crystalline character of the NPs (see Fig. S.2). However, only techniques sensitive to the valence state of Fe such as, electron energy loss spectroscopy (EELS) on high resolution TEM [50] or Mössbauer spectroscopy combined with X-ray absorption near edge structure through synchrotron experiments [23] would help us on identifying whether magnetite, maghemite (or both) were synthesized.
14
FIG. 4. (Color online). TEM images and size distribution of Fe-oxide NPs on sample A [(a) and (c), respectively] and sample B [(b) and (d), respectively]. Fits of the histograms [red solid lines in (c) and (d)] to log-normal functions are also shown. The mean NP sizes are <()> = 2.8 (0.3) nm and <()> = 5.5 (0.4) nm for sample A and B, respectively.
Our findings evidence that the real-time monitor of coprecipitation reactions developed with the pith-blade impeller, provide Fe-oxide NPs with homogeneous and smaller dimensions (even to those reported in the literature by impinging reactants on an external mixer [33, 34]). As we have herein demonstrated, our home-made AC susceptometer is an efficient setup to monitor in situ and in real time the synthesis of magnetic NPs by coprecipitation reaction and, predict the absence of coercivity by the straightforward analysis of the in-phase and out-of phase signals. However, we are aware that for some applications, it is not only important obtaining reduced MNPs dimensions and narrow PSD but also, controllable and enhanced magnetic moments.
Therefore,
it
would
be
highly
desirable
not
only
predicting
the
superaparamagnetic character of the samples in situ and in real time, but also verifying afterwards how the different synthetic conditions have altered their magnetic moment. Therefore, future work should be devoted to a trustworthy method to obtain the magnetic moment of the MNPs after their synthesis within the AC susceptometer. We expect that the real time monitor and adequate analysis of in situ curves, could rapidly help on the selection of the best synthetic conditions (in terms of a better magnetic spin arrangement at the NPs’ surface) to rise up the magnetic moment (increase X´ values) maintaining the superparamagnetic character (X´´ ~ 0). In fact, in situ techniques have emerged recently as powerful alternatives to identify the optimal synthesis parameters on the synthesis of magnetic NPs [51, 52]. These real time techniques offer the advantage of reducing posterior 15
physicochemical characterizations and also, the risk of post processing steps affecting the results. In that sense, our work open future perspectives to optimize rapidly other relevant conditions of the synthetic procedure such as: the type of chemical base, its concentration and the agitators’ angular velocity. Additionally, our setup is not limited to the study of Fe3O4 NPs and could be easily extended to other NP ferrites, c.a. XFe2O4 with X = Co, Mn, Zn, Ni… just by changing the initial Fe(II) chloride solution into the desired X(II) one.
IV. CONCLUSIONS In summary, we have monitored in situ and in real time the mixing zone of the Fe(II) : Fe(III) reactants during the full course of coprecipitation reactions with a home-made magnetic AC susceptometer. This set-up can be particularly interesting for fast reactions like the coprecipitation of magnetic nanoparticles. Since the steps of the reaction are clearly visible on the in situ curves, an effort to elongate the particle growth step may be the key to control precisely the NP size distribution. Currently, this size control is only possible via cumbersome methods such as thermal decomposition or hydrothermal synthesis which involve high energy and time consuming steps. Our work also demonstrates the key importance of mixing the iron oxide coprecipitation reaction with a pitch blade impeller. As a consequence, Fe-oxide NPs exhibit narrow particle size distribution (1-7 nm) and superparamagnetic behavior. Our setup opens the possibility of rapidly predict in situ and in less than 15 min, the evolution of coprecipitation reactions because time dependent in-phase and out-of-phase curves are fingerprints of the polidispersity and magnetic properties of the synthesized NPs.
16
ACKNOWLEDGMENTS
This work was supported by Fundação para a Ciência e a Tecnologia (FCT) through project PTDC/QUI-QUI/105304/2008 in the framework of Program COMPETE and NORTE– 070124–FEDER–000070–MULTIFUNCTIONAL NANOMATERIALS. MPFG, JMT and PM are also indebted to FCT for grants (SFRH/BPD/87430/2012, SFRH/BPD/72329/2010 and PTDC/FIS-NAN/0533/2012, respectively). Authors are thankful to Prof. Pedro Tavares and MSc. Lisete Fernandes for allocating TEM time.
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[28] N.A. Frey, S. Peng, K. Cheng, S. Sun, Magnetic nanoparticles: synthesis, functionalization, and applications in bioimaging and magnetic energy storage, Chem. Soc. Rev. 38 (2009) 25322542. [29] M.P. Fernandez, D.S. Schmool, A.S. Silva, M. Sevilla, A.B. Fuertes, P. Gorria, J.A. Blanco, Control of crystalline phases in magnetic Fe nanoparticles inserted inside a matrix of porous carbon, J. Magn. Magn. Mater. 322(9) (2010) 1300-1303. [30] M.P. Fernandez, P. Gorria, M. Sevilla, A.B. Fuertes, J.M. Greneche, J.A. Blanco, Onion-like nanoparticles with -Fe core surrounded by -Fe/Fe-oxide double shell, J. Alloys Compd. 509 (2011) S320-S322. [31] C. Pereira, A.M. Pereira, C. Fernandes, M. Rocha, R. Mendes, M.P. Fernández-García, A. Guedes, P.B. Tavares, J.M. Greneche, J.P. Araújo, C. Freire, Superparamagnetic MFe2O4 (M = Fe, Co, Mn) nanoparticles: Tuning the particle size and magnetic properties through a novel one-step coprecipitation route, Chem. Mater. 24 (2012) 1496-1504. [32] C. Fernandes, C. Pereira, M.P. Fernández-García, A.M. Pereira, A. Guedes, R. FernándezPacheco, A. Ibarra, M.R. Ibarra, J.P. Araújo, C. Freire, Tailored design of CoxMn1xFe2O4 nanoferrites: a new route for dual control of size and magnetic properties, J. Mater. Chem. 2 (2014) 5818-5828. [33] V. Ström, R.T. Olsson, K.V. Rao, Real-time monitoring of the evolution of magnetism during precipitation of superparamagnetic nanoparticles for bioscience applications, J. Mater. Chem. 20 (2010) 4168-4175. [34] M. Fang, V. Ström, R. Olsson, L. Belova, K.V. Rao, Rapid mixing: A route to synthesize magnetite nanoparticles with high moment, Appl. Phys. Lett. 99 (2011) 222501. [35] L. Wei, M. Hervé, P. Edouard, Use of different rapid mixing devices for controlling the properties of Magnetite nanoparticles produced by precipitation, J. Crystal Growth 342 (2012) 2127. [36] C. Blanco-Andujar, D. Ortega, Q.A. Pankhurst, N.T. Kim Thanh, Elucidating the morphological and structural evolution of iron oxide nanoparticles formed by sodium carbonate in aqueous medium, J. Mater. Chem. 22 (2012) 12498-12506. [37] S. Laurent, D. Forge, M. Port, A. Roch, C. Robic, L.V. Elst, R.N. Muller, Magnetic iron oxide nanoparticles: synthesis, stabilization, vectorization, physicochemical characterizations, and biological applications, Chem. Rev. 108 (2008) 2064-2110. [38] J.M. Teixeira, R. Lusche, J. Ventura, R. Fermento, F. Carpinteiro, J.P. Araujo, J.B. Sousa, S. Cardoso P.P. Freitas, Versatile, high sensitivity, and automatized angular dependent vectorial Kerr magnetometer for the analysis of nanostructured materials, Rev. Sci. Instrum. 82 (2011) 043902. [39] S. Vandendriessche, W. Brullot, D. Slavov, V.K. Valev, T. Verbiest, Magneto-optical harmonic susceptometry of superparamagnetic materials, Appl. Phys. Lett. 102 (2013) 161903. [40] A. Krozer, C. Johansson, J. Blomgren, K. Petersson, D. Ilver, A. Prieto-Astalan, C. Jonasson, Detection device and method, U.S.2009/0085557 A1 (2009). [41] J. Tafur, A.P. Herrera, C. Rinaldi, E.J. Juan, Development and validation of a 10 kHz–1MHz magnetic susceptometer with constant excitation field, J. Appl. Phys. 111 (2012) 07E349. [42] V. Strom, R. Olsson, Real time monitored synthesis of ultra low coercivity magnetic nanoparticles with narrow size distribution, U.S.2012/0145945 A1 (2012).
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[43] L. Gutiérrez, M.P. Morales, F.J. Lázaro, Prospects for magnetic nanoparticles in systemic administration: synthesis and quantitative detection, Phys. Chem. Chem. Phys. 16 (2014) 44564464. [44] M.P. Fernández-García, J.M. Teixeira, P. Machado, M.R. Oliveira, J.M. Maia, C. Pereira, A.M. Pereira, C. Freire, J.P. Araujo, Automatized and desktop AC-susceptometer for the in situ and real time monitoring of magnetic nanoparticles’ synthesis by coprecipitation, Rev. Sci. Instrum. 86 (2015) 043904. [45] J.C.B. Lopes, P. Laranjeira, M.M. Dias, A.A. Martins, Network mixer and related mixng process, U.S.2009/0016154 A1 (2009). [46] P. Laranjeira, A.A. Martins, J.C.B. Lopes, M.M. Dias, NETmix®, a new type of static mixer: modeling, simulation, macromixing, and micromixing characterization, AIChE Journal, 55 (2009) 2226-2243. [47] R. M. Cornell, U. Schwertmann, The Iron Oxides. Structure, Properties, Reactions, Occurences and Uses, 2nd Edition, Wiley-VCH, Weinheim, 2003. [48] J.J. Ibarra-Sánchez, R. Fuentes-Ramírez, A.G. Roca, M.P. Morales, L.I. Cabrera-Lara, Key parameters for scaling up the synthesis of magnetite nanoparticles in organic media: stirring rate and growth kinetic, Ind. Eng. Chem. Res. 52 (2013) 17841-17847. [49] E.L. Paul, V.A. Atiemo-Obeng, S.M. Kresta, Handbook of industrial mixing WileyInterscience, New Jersey, 2004, pp. 366. [50] C. Wang, D.R. Baer, J.E. Amonette, M.H.Engelhard, J. Antony, Y. Qiang, Morphology and Electronic Structure of the Oxide Shell on the Surface of Iron Nanoparticles, J. Am. Chem. Soc. 131 (2009) 8824-8832. [51] H.L. Andersen, M. Christensen, In situ powder X-ray diffraction study of magnetic CoFe2O4 nanocrystalline synthesis, Nanoscale 7 (2015) 3481-3490. [52] K.M.Ø. Jensen, H.L. Andersen, C. Tyrsted, E.D. Bǿjesen, A.C. Dippel, N. Lock, S.J.L. Bilinge, B.B. Iversen, M. Christensen, Mechanisms for Iron Oxide Formation under Hydrothermal Conditions: An in Situ Total Scattering Study, ACS Nano, 8 (2014) 10704-10714.
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FIGURE CAPTIONS FIG. 1. (Color online). Enlarged view of blades placed at an angle of 45º from the horizontal. The precursors discharge is done near the blade where the local energy is large. FIG. 2. (Color online) AC susceptometer measured signals (X and ) of sample A [(a) (b)] and sample B [(c) - (d)]. In Fig. 2(a) and 2(c), different regions of nanoparticle’s growth are shown (see text). FIG. 3. (Color online) (a) M(H) curves of ferrofluid samples A () and B (Δ) measured at 270 K. Blue solid lines correspond to the fits of experimental curves through Langevin functions (see text). The inset is a magnified view near null H values revealing the minute coercivity of the samples. M(T) curves in the ZFC and FC regime of samples A and B are displayed in (b) and (c), respectively. FIG. 4. (Color online). TEM images and size distribution of Fe-oxide NPs on sample A [(a) and (c), respectively] and sample B [(b) and (d), respectively]. Fits of the histograms [red solid lines in (c) and (d)] to log-normal functions are also shown. The mean NP sizes are <()> = 2.8 (0.3) nm and <()> = 5.5 (0.4) nm for sample A and B, respectively.
21
FIGURE 1.
22
FIGURE 2.
(a)
(b)
0.21
0.2
0.14
0.1
0.07 -3
sample A
sample A
0.0
0.00
(c)
(d)
0.3
()
X (10 V)
0.3
1.4 0.2 0.7
0.1
sample B
0.0 0
500
1000
1500
sample B 0
500
1000
0.0
1500
time (s)
23
FIGURE 3.
1.0
sample A sample B Fit
T = 270 K
0.1
0.0
M / Mmax
M / Mmax
0.5
(a)
0.0
-0.5
sample A sample B
-0.1 -50
-25
0
25
50
H (Oe)
-1.0 -3.0
-1.5
0.0
1.5
3.0
H (kOe) (b)
1.0
FC ZFC
sample A
0.5
67 K
M / Mmax
0.0
FC
1.0
(c)
ZFC sample B
0.5
0.0 0
100
200
300
T (K)
24
FIGURE 4.
Highlights
Assessment to the vital stages of Fe-oxide NPs’ formation by coprecipitation AC susceptometer to monitor in real time the magnetic response of the mixing volume Nucleation, growth and diffusion identified on the in- and out-phase signals Mixing reactants with pitch blade impellers allow narrow particle size distribution
25
26
PII: DOI: Reference:
S0009-2509(15)00601-6 http://dx.doi.org/10.1016/j.ces.2015.08.036 CES12563
To appear in: Chemical Engineering Science Received date: 26 May 2015 Revised date: 1 July 2015 Accepted date: 28 August 2015 Cite this article as: M.P. Fernández-García, J.M. Teixeira, P. Machado, M. Enis Leblebici, J.C.B. Lopes, C. Freire and J.P. Araujo, Monitoring in real time the production of Fe-oxide nanoparticles, Chemical Engineering Science, http://dx.doi.org/10.1016/j.ces.2015.08.036 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting galley proof before it is published in its final citable form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Monitoring in real time the production of Fe-oxide nanoparticles *
†
M.P. Fernández-García1 , J.M. Teixeira1, P. Machado1, M. Enis Leblebici2 , J.C.B. Lopes3, C. Freire4, J.P. Araujo1 1
IFIMUP and IN-Institute of Nanoscience and Nanotechnology, Department of Physics and Astronomy,
Faculty of Sciences, University of Oporto, Portugal. 2
Process Engineering for Sustainable Systems (ProcESS), Department of Chemical Engineering, KU
Leuven, Belgium 3
Laboratory of Separation and Reaction Engineering, Faculty of Engineering, University of Oporto,
Portugal 4
REQUIMTE/LAQV, Department of Chemistry and Biochemistry, Faculty of Sciences, University of
Oporto, Portugal.
We herein demonstrate an in-situ device and a real time method to observe the vital stages of Fe-oxide nanoparticles’ (NPs) preparation by coprecipitation reaction. The reaction volume is monitored by a home-made AC susceptometer to analyze the magnetic response of the growing NPs. Three different regions (nucleation, growth and diffusion) could be identified in the recorded in-phase and out-of phase curves. The morphological and magnetic characterization of the obtained Fe-oxide NPs reveal minute particle dimensions (< 3 nm), narrow polidispersity (1-7 nm), superparamagnetic behavior (null coercivity) and narrow distribution of blocking temperatures (
*
Corresponding author: M.P. Fernández-García; Electronic mail: [email protected]; Tel.: +351.220.402.368; Fax: +351.220.402.406 † Past address: Laboratory of Separation and Reaction Engineering, Faculty of Engineering, University of Oporto, Portugal
1
I.
INTRODUCTION Superparamagnetic nanoparticles (NPs) are widely studied materials in biomedicine,
chemistry and material science due to their excellent magnetic properties (i.e. absence of coercivity and large magnetic response) and their inexpensive manufacture [1-4]. Due to their low toxicity levels and biocompatible character Fe-oxide NPs are the most popular choice for biomedical applications [5-8]. These NPs are widely used in theragnostics [9,10], magnetic resonance imaging [11-13], targeted drug delivery [14] and magnetic hyperthermia [15-18]. Furthermore, Fe-oxide NPs also receive attention from industry and environmental engineering because they can be employed as nanoadsorbents for removing various pollutants [19], water purification [20], and as catalyst and photocatalyst on industrial chemical processes or pigments in many colorant formulations [21]. Above all, for all these purposes a detailed characterization correlating the microstructure, morphology and magnetic properties of the NPs is needed to control both their individual and collective behavior [22-26]. There are several methods to synthesize Fe-oxide NPs as for example on water-oil microemulsions,
hydrothermal
synthesis,
coprecipitation,
pyrolysis,
or
thermal
decomposition [1,8,27-30]. However, coprecipitation methods have the advantage of being easy-to-follow, inexpensive and efficient to produce massive samples. Following such method, Fe(II):Fe(III) ions precipitate in alkaline solution, such as sodium hydroxide or alkanolamines [31,32]. Coprecipitation is a fast kinetically driven reaction, so there are several operational factors that are likely to influence its outcome: temperature; concentration of iron salts and base; nature of the salts (perchlorates, chlorides, nitrates and sulfates) pH values; and, mixing rates [33-37]. All these variables affect the magnetic
2
response, crystallinity, chemical composition, mean dimension and size distribution of the final NPs. Therefore, a strict control of all these parameters and the reproducibility of the coprecipitation reaction is needed. For that purpose, a way to monitor the full course of magnetic NPs’ coprecipitation reaction in real time is highly desirable. Traditional magnetometry methods that are quite suitable for studying magnetic NPs are: vibrating sample magnetometer (VSM) and superconducting quantum interference device (SQUID) techniques. In addition, magneto-optical techniques [38] could also be employed to differentiate superparamagnetic materials [39]. Above all, magnetic measurements in AC fields are extremely important for characterizing magnetic NPs, as they yield information of the magnetization dynamics that cannot be obtained from direct DC measurements [4043]. The measured magnetic moment in AC susceptometers varies with time at the same frequency () of the exciting field or delayed () with respect to it. A complex notation for magnetic susceptibility, ' i '' , is used to describe this time-dependent behavior. Considering an AC magnetic field ( H H 0 sin wt ) the in-phase susceptibility (’) is related with the low field limit of the steepness of the hysteresis loop – the magnetic susceptibility for (super)paramagnetic materials – through:
' m H 0 cos
(Eq. 1)
whereas, the out-of-phase (’’) is associated with the coercivity (see details on [44]).
'' m H 0 sin
(Eq. 2)
Recently, Ström and co-authors have successfully studied the full course of coprecipitation reactions with an AC susceptometer [42]. They developed an external mixer to blend the jets of precursors and reactants and afterwards, transferred the mixture to a sample vial that is placed inside the AC susceptometer. Therefore, the initial stages of 3
nucleation (typically ms) could not be closely followed by their designed setup. In that sense, we have designed and reported a home-made and desktop AC susceptometer set-up, to monitor coprecipitation reactions from the exact moment that the precursors are vigorously blended until the end of the synthesis [44]. Basically, on our setup the alkali base is firstly stirred strongly with a pitched blade impeller and afterwards, a jet of Fe salts is impinged and blended with the stirred alkali base. Our integrated system allows the simultaneous monitor of the in-phase (’ ≡ X) and out-of-phase (’’ ≡ Y) signals, as well as 1 R X 2 Y 2 and, tan Y X which is the phase shift between the reference and
pickup measured signals. Our set-up provides precise information about the different regimes of NPs’ size generation [37] [burst nucleation - region I, growth - region II and, diffusion of the solutes to the surface - region III] through the identification of these stages on the real time curves. Our work put on view the importance of mixing efficiently the reactants on coprecipitation reactions. The specific way in which different streams are mixed is a main factor for an efficient operation, especially for those processes where chemical reaction occurs [45,46]. The time necessary to achieve to the state of perfect mixing have important repercussions on the industrial level: economy of reactants, higher degree of control of the chemical process and, environmental impact due to the reduction of energy consumption. The impact of fast mixing is particularly important for reactions with faster kinetics such as coprecipitation. However, the understanding of the mixing phenomena is complex and its knowledge is rather incomplete. In the particular case of magnetic NPs, it has been reported the importance of a uniform mixing rate between iron ion source and base solution for achieving particles with narrow size distribution on coprecipitation reactions [34,35].
4
Herein, we corroborate the influence of such mixing on the final properties of the Fe-oxide NPs by means of our home-made set-up. We compare two different blending schemes: i) a jet of iron precursor solution is injected into the base solution while being strongly blended with pithched blade agitator (sample A); ii) the same jet of precursors discharged into the base solution without being stirred with
the impeller (sample B). The detailed
interpretation of the real time in-phase and out-of phase signals recorded by our AC susceptometer lead us to predict that the Fe-oxide NPs’ on sample A, exhibit tiny dimensions
and
thus,
superparamagnetic
properties.
Structural
and
magnetic
characterization of samples A and B were developed to confirm our findings.
II. MATERIAL AND METHODS
A. Chemicals and reagents The Fe(II) : Fe(III) solution was prepared from commercially available analytical grade reagents. All materials were employed without further purification. Iron (II) chloride tetrahydrate and sodium hydroxide (analytical grade) were purchased from Merck and, iron (III) chloride hexahydrate (>98%) from Riedel-de Haën. Hydrochloric acid (37%, analytical grade) was supplied by Panreac. Ultrapure water (milipore with specific resistivity of 18 m·cm) was used throughout the experiments.
B. Synthesis of Fe-oxide nanoparticles The Fe-oxide NPs were prepared by the aqueous co-precipitation method under alkaline conditions using NaOH as the base [47]. Briefly, stoichiometric quantities of FeCl2·4H2O and FeCl3·6H2O were dissolved individually in deoxygenated aqueous solution of 0.5 M
5
HCl. Afterwards, the precursor iron ion solution was prepared by mixing both Fe chloride solutions. This solution was protected from ambient atmosphere conditions by Ar gas flow. For the in situ synthesis of sample A, 2 ml of deoxygenated NaOH solution (1.5 M) were stirred with an pitched blade impeller in the sample vial and afterwards, 0.2 ml of iron ion solution were discharged at 1.3 ml/min (controlled by a syringe pump). The discharge is directed instantly inside the sample vial and kept at room temperature. A similar procedure was followed for the in situ synthesis of sample B but the reactants were not agitated. It should be remarked that for the synthesis of both samples A and B, the ferrous:ferric ratio was 1:2 corresponding to magnetite, Fe3O4. C. Characterization methods In order to obtain samples A and B, the synthesized ferrofluids were centrifuged with ultrapure water until pH values reaches 9-10 and supernatant was discarded. Transmission electron microscopy (TEM) images were obtained using LEO 906E microscope operating at acceleration voltages of 100-120 kV. Samples were prepared by dispersing few drops of samples A and B in high-purity ethanol under sonication. Afterwards, several drops of this solution were deposited on carbon-coated 400 mesh copper grid and dried on air. The magnetic properties of 80 l of magnetic liquid samples were studied using a commercial Quantum Design superconducting quantum interference device (SQUID) magnetometer. For the M(T) curves, the samples were cooled in zero field (ZFC) from 300 to 10 K. Next, the magnetic field (H = 50 Oe) was applied and kept constant during the measurement of the magnetization, MZFC and MFC, at fixed temperatures (ΔT = 2 K) between 10 and 270 K and from 270 to 10 K, respectively. The hysteresis cycle, M(H)
6
curves, were measured in the range ± 30 kOe at 270 K. Temperature did not exceed 270 K, in order to maintain a frozen suspension during the magnetic measurements.
D. Mixing process with pitched blade impellers The coprecipitation growth of the magnetic NPs is monitored in situ by a home-made and desktop AC susceptometer that we have recently designed and built [44]. Summing up, our set-up comprises: i) one pair of Helmholtz coils (primary) that are fed by an AC sinusoidal electrical current to create the applied magnetic field; ii) one pair of identical pick-up sensing coils (secondary) oppositely wound and electrically connected in series. The secondary coils are positioned in the middle of the primary coils where it is assured a homogeneous magnetic field. The sample vial containing the base solution, where the chemical reaction takes place, is introduced into one of the secondary coils. The X, Y, R and
channels are only zeroed after the test vial, the Teflon thin tube (connected to the pump) and, the motorized impeller are placed inside the sensing coil. Data started to be saved at the same time that the chemical reaction started (i.e. the discharge of the iron ion solution) and, until the software is interrupted by the operator. According to the Faraday law, the magnetic ferrofluid sample will generate magnetic flux changes on the sensing coil that are converted into an output voltage. The picked up signal is detected by a lock-in amplifier that measures the in-phase and out-of-phase signals [44]. It is well established that the coprecipitation synthesis of NPs is obtained through the vigorous mixing of two or more reactant solutions, resulting in a final product with low solubility in the medium. Researchers agree that a large stirring rate, increase the mobility and homogeneous distribution of species generating narrower particle size distribution (PSD) [48]. Additionally, in order to prepare monodisperse nanocrystals, it is highly 7
desirable inducing two separate stages: i) single nucleation event and, ii) particle growth process. Otherwise, if nucleation takes place throughout the whole process, the control of the PSD would be difficult [37]. This burst-nucleation process requires homogeneous concentration of precursors throughout the medium from the moment they are blended until the end of the particle growth. Particularly, in our setup the alkaline solution is stirred vigorously by means of the pitched blade impeller that produces a confined axial flow and low shear. Our impeller has four blades with rounded leading edges at an angle of 45º from the horizontal (see Figure 1). In fact, radial flow mixers could produce two undesirable re-circulating loops (below and above it) where liquids are inefficiently blended [49]. This can be minimized by the usage of hydrofoil impellers which are more streamlined with small loop underneath (see schematic flow pattern on [44]). This can be an improvement of the current system. Since velocities and shear diminished from regions near the blades to corner areas or liquid surface, the discharge of Fe ion solution is done in our case near the blade to avoid initial coalescence, agglomeration and flocculation at low mixing energy regions [49]. At the same time, the reactants are discharged inside the base solution to reach an efficient diffusion of the precursors and also near the blades where the local energy is high (see Figure 1). As a result, the medium is efficiently blended throughout the whole volume of the fluid with an axial flow that, assures the pH homogenization throughout the entire volume. Thus, low pH zones in the reaction medium are avoided minimizing possible side reactions.
8
FIG. 1. (Color online). Enlarged view of blades placed at an angle of 45º from the horizontal. The precursors discharge is done near the blade where the local energy is large.
III. RESULTS AND DISCUSSION Figure 2 shows the real time coprecipitation reaction of Fe-oxide NPs on sample A (precursors and reactants strongly blended with an pitched blade impeller that promotes a strong and homogenous mixing) and, sample B (precursors discharged into the sample vial without such mixing). The evolution of the X- and signals of samples A and B within time reveal that, the reactions are completely different and thus, put on view the importance of the mixing process. The coprecipitation reaction of sample A is very fast in the first seconds and finishes after ~ 750 s (X-signal remains stable). In contrast, the in-phase curve of sample B increases gradually and did not stabilize even after 1750 s [see Fig. 2c)]. Additionally, the X-signal of sample B clearly shows the detection of the three stages of crystal growth: nucleation (region I), growth (region II) and diffusion (region III). In contrast for sample A, only growth and diffusion stages are detected, which is a clear
9
evidence of the mixing efficiency achieved by the impeller [see Figure 2.a)]. The in situ curves indicate that the nucleation mechanism of sample A might be faster than typical nucleation times of ms. It should be noted, that we are not limited by our set-up resolution because it is of tens of s. Therefore, it seems that nucleation takes place as a burst process due to the efficient mixing of reactants and the homogeneous concentration of precursors conquered by the impeller in the in situ synthesis of sample A. On the other hand, the direct relation between X-signal values and the NPs’ magnetic susceptibility (m) (see Eq. 1) lead us to predict that sample A will exhibit lower magnetic moment than that of sample B (further details in [44]). (a)
(b)
0.21
0.2
0.14
0.1
0.07 -3
sample A
sample A
0.0
0.00
(c)
(d)
0.3
()
X (10 V)
0.3
1.4 0.2 0.7
0.1
sample B
0.0 0
500
1000
1500
sample B 0
500
1000
0.0
1500
time (s) FIG. 2. (Color online) AC susceptometer measured signals (X and ) of sample A [(a) - (b)] and sample B [(c) - (d)]. In Fig. 2(a) and 2(c), different regions of nanoparticle’s growth are shown (see text).
Regarding the out-of-phase signal, it does not change for sample A and remains null during the formation of the NPs as illustrated in Fig. 2(b). This means that the NPs of 10
sample A oscillate with the same frequency of the applied AC field predicting their superparamagnetic nature (absence of hysteresis) [33, 44]. On the other hand, the -signal of sample B shows a slight variation ( ~ 0.3º) [see Fig. 2(d)] envisaging MNPs with small coercivity. In comparison, a powder sample of ferrimagnetic Fe-oxide NPs, that was also measured with our home-made AC susceptometer, showed double out-of-phase value (~0.6º) and Hc ~ 100 Oe (see Fig. S.1 of the Supplementary material). The analysis of our real time curves are in good agreement with the M(H) curves performed at 270 K [see Fig. 3(a)]. It should be noted that M vs. H curves have been corrected to remove the negative slope arising from the diamagnetic contribution of the sample-holder and water. Fig. 3(a) displays that both samples showed ‘Langevin-like’ curves with minute Hc values (or at least under the detection limit of SQUID magnetometer) and thus, MNPs could be roughly considered as superparamagnetic. Unfortunately, Ms values could not be estimated for samples A and B because the reduced volume of the liquid samples (80 l) avoids the accurate estimation of concentration after drying. Even tough, magnifications around low magnetic fields display small differences between both samples [see inset of Fig. 3(a)]. Firstly, Mr/Ms values are 0.007 and 0.022 for samples A and B, respectively. Secondly, as we have previously reported [44], the lowfield limit of the steepness of M(H) curves is directly proportional to the magnetic susceptibility (m) of (super)paramagnetic materials (M = m·H with m > 0). The minor steepness of sample’s A M(H) curve, is an indication of the smaller magnetic susceptibility of MNPs on sample A. It is well established that NPs’ magnetic moment is drastically diminished with the length-scale reduction. Therefore, we envisage that MNPs on sample A
11
will exhibit smaller mean dimension and/or narrower distribution than those on sample B, corroborating the analysis of -signal on Fig. 2(b) and 2(d). 1.0
sample A sample B Fit
T = 270 K
0.1
0.0
M / Mmax
M / Mmax
0.5
(a)
0.0
-0.5
sample A sample B
-0.1 -50
-25
0
25
50
H (Oe)
-1.0 -3.0
-1.5
0.0
1.5
3.0
H (kOe) (b)
1.0
FC ZFC
sample A
0.5
M / Mmax
0.0
FC
1.0
(c)
ZFC sample B
0.5
0.0 0
100
200
300
T (K)
FIG. 3. (Color online) (a) M(H) curves of ferrofluid samples A and B measured at 270 K. Solid lines correspond to the fits of experimental curves through Langevin functions (see text). The inset is a magnified view near null H values revealing the minute coercivity of the samples. M(T) curves in the ZFC and FC regime of samples A (b) and B (c), respectively.
12
From the fit of M(H) curves to Langevin functions [solid lines in Fig. 3(a)], magnetic domain dimensions dlan() = 2.7(0.5) nm and 3.4(0.6) nm have been estimated for samples A and B, respectively. On the one hand, such small dimensions indicate the single domain character of the studied MNPs. On the other hand, dLan dimensions of MNPs on sample A being smaller than those of sample B, corroborates previous analysis of Mr / Ms values, and -signals. The reversibility of the M(H) curves point out that the blocking temperatures, TB, are below 270 K for both samples. Magnetization measurements as a function of temperature, M(T) curves, in the ZFC-FC regime were performed to further characterize the samples. The MZFC(T) curve of sample A [Fig. 3(b)] exhibit well defined and narrow TB (
13
described above. The fits of the histograms to log-normal functions reveal mean dimensions of <()> = 2.8(0.3) nm and <()> = 5.5(0.4) nm for samples A and B, respectively, being in agreement with dLan values previously obtained and following the same relation as Hc and Mr/Ms values (see above). Elemental analysis by energy dispersive X-ray spectroscopy (EDS) developed on a scanning electron microscope displayed that the resulting material contains mainly Fe and O (see Fig. S.2 of the supplementary material). Additionally, C, Cu (from the grid) and Mo, Al (arising from the sample holder) were also detected. Furthermore, bright and individual spots were detected on dark field images of TEM suggesting the crystalline character of the NPs (see Fig. S.2). However, only techniques sensitive to the valence state of Fe such as, electron energy loss spectroscopy (EELS) on high resolution TEM [50] or Mössbauer spectroscopy combined with X-ray absorption near edge structure through synchrotron experiments [23] would help us on identifying whether magnetite, maghemite (or both) were synthesized.
14
FIG. 4. (Color online). TEM images and size distribution of Fe-oxide NPs on sample A [(a) and (c), respectively] and sample B [(b) and (d), respectively]. Fits of the histograms [red solid lines in (c) and (d)] to log-normal functions are also shown. The mean NP sizes are <()> = 2.8 (0.3) nm and <()> = 5.5 (0.4) nm for sample A and B, respectively.
Our findings evidence that the real-time monitor of coprecipitation reactions developed with the pith-blade impeller, provide Fe-oxide NPs with homogeneous and smaller dimensions (even to those reported in the literature by impinging reactants on an external mixer [33, 34]). As we have herein demonstrated, our home-made AC susceptometer is an efficient setup to monitor in situ and in real time the synthesis of magnetic NPs by coprecipitation reaction and, predict the absence of coercivity by the straightforward analysis of the in-phase and out-of phase signals. However, we are aware that for some applications, it is not only important obtaining reduced MNPs dimensions and narrow PSD but also, controllable and enhanced magnetic moments.
Therefore,
it
would
be
highly
desirable
not
only
predicting
the
superaparamagnetic character of the samples in situ and in real time, but also verifying afterwards how the different synthetic conditions have altered their magnetic moment. Therefore, future work should be devoted to a trustworthy method to obtain the magnetic moment of the MNPs after their synthesis within the AC susceptometer. We expect that the real time monitor and adequate analysis of in situ curves, could rapidly help on the selection of the best synthetic conditions (in terms of a better magnetic spin arrangement at the NPs’ surface) to rise up the magnetic moment (increase X´ values) maintaining the superparamagnetic character (X´´ ~ 0). In fact, in situ techniques have emerged recently as powerful alternatives to identify the optimal synthesis parameters on the synthesis of magnetic NPs [51, 52]. These real time techniques offer the advantage of reducing posterior 15
physicochemical characterizations and also, the risk of post processing steps affecting the results. In that sense, our work open future perspectives to optimize rapidly other relevant conditions of the synthetic procedure such as: the type of chemical base, its concentration and the agitators’ angular velocity. Additionally, our setup is not limited to the study of Fe3O4 NPs and could be easily extended to other NP ferrites, c.a. XFe2O4 with X = Co, Mn, Zn, Ni… just by changing the initial Fe(II) chloride solution into the desired X(II) one.
IV. CONCLUSIONS In summary, we have monitored in situ and in real time the mixing zone of the Fe(II) : Fe(III) reactants during the full course of coprecipitation reactions with a home-made magnetic AC susceptometer. This set-up can be particularly interesting for fast reactions like the coprecipitation of magnetic nanoparticles. Since the steps of the reaction are clearly visible on the in situ curves, an effort to elongate the particle growth step may be the key to control precisely the NP size distribution. Currently, this size control is only possible via cumbersome methods such as thermal decomposition or hydrothermal synthesis which involve high energy and time consuming steps. Our work also demonstrates the key importance of mixing the iron oxide coprecipitation reaction with a pitch blade impeller. As a consequence, Fe-oxide NPs exhibit narrow particle size distribution (1-7 nm) and superparamagnetic behavior. Our setup opens the possibility of rapidly predict in situ and in less than 15 min, the evolution of coprecipitation reactions because time dependent in-phase and out-of-phase curves are fingerprints of the polidispersity and magnetic properties of the synthesized NPs.
16
ACKNOWLEDGMENTS
This work was supported by Fundação para a Ciência e a Tecnologia (FCT) through project PTDC/QUI-QUI/105304/2008 in the framework of Program COMPETE and NORTE– 070124–FEDER–000070–MULTIFUNCTIONAL NANOMATERIALS. MPFG, JMT and PM are also indebted to FCT for grants (SFRH/BPD/87430/2012, SFRH/BPD/72329/2010 and PTDC/FIS-NAN/0533/2012, respectively). Authors are thankful to Prof. Pedro Tavares and MSc. Lisete Fernandes for allocating TEM time.
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FIGURE CAPTIONS FIG. 1. (Color online). Enlarged view of blades placed at an angle of 45º from the horizontal. The precursors discharge is done near the blade where the local energy is large. FIG. 2. (Color online) AC susceptometer measured signals (X and ) of sample A [(a) (b)] and sample B [(c) - (d)]. In Fig. 2(a) and 2(c), different regions of nanoparticle’s growth are shown (see text). FIG. 3. (Color online) (a) M(H) curves of ferrofluid samples A () and B (Δ) measured at 270 K. Blue solid lines correspond to the fits of experimental curves through Langevin functions (see text). The inset is a magnified view near null H values revealing the minute coercivity of the samples. M(T) curves in the ZFC and FC regime of samples A and B are displayed in (b) and (c), respectively. FIG. 4. (Color online). TEM images and size distribution of Fe-oxide NPs on sample A [(a) and (c), respectively] and sample B [(b) and (d), respectively]. Fits of the histograms [red solid lines in (c) and (d)] to log-normal functions are also shown. The mean NP sizes are <()> = 2.8 (0.3) nm and <()> = 5.5 (0.4) nm for sample A and B, respectively.
21
FIGURE 1.
22
FIGURE 2.
(a)
(b)
0.21
0.2
0.14
0.1
0.07 -3
sample A
sample A
0.0
0.00
(c)
(d)
0.3
()
X (10 V)
0.3
1.4 0.2 0.7
0.1
sample B
0.0 0
500
1000
1500
sample B 0
500
1000
0.0
1500
time (s)
23
FIGURE 3.
1.0
sample A sample B Fit
T = 270 K
0.1
0.0
M / Mmax
M / Mmax
0.5
(a)
0.0
-0.5
sample A sample B
-0.1 -50
-25
0
25
50
H (Oe)
-1.0 -3.0
-1.5
0.0
1.5
3.0
H (kOe) (b)
1.0
FC ZFC
sample A
0.5
M / Mmax
0.0
FC
1.0
(c)
ZFC sample B
0.5
0.0 0
100
200
300
T (K)
24
FIGURE 4.
Highlights
Assessment to the vital stages of Fe-oxide NPs’ formation by coprecipitation AC susceptometer to monitor in real time the magnetic response of the mixing volume Nucleation, growth and diffusion identified on the in- and out-phase signals Mixing reactants with pitch blade impellers allow narrow particle size distribution
25
26