Encapsulation of metallic iron magnetic nanoparticles by polyacrylamide in water suspensions 158 Shankar A., Safronov A., Beketov I. Chimica Techno Acta. 2017. Vol. 4, No. 3. P. 158–166. ISSN 2409–5613 D O I: 1 0. 15 82 6/ ch im te ch /2 01 7. 4. 3. 01 Ajay Shankar1, Alexander Safronov2,3, Igor Beketov3 1Indira Gandhi National Tribal University, Amarkantak, Madhya Pradesh, 484886, India e-mail: ajayshankar0@gmail.com 2Ural Federal University, 19 Mira St., Ekaterinburg, 620002, Russian Federation e-mail: safronov@iep.uran.ru 3Institute of Electrophysics UB RAS, 106 Amundsen St., Ekaterinburg, 620016, Russian Federation e-mail: beketov@iep.uran.ru Encapsulation of metallic iron magnetic nanoparticles by polyacrylamide in water suspensions Theoretical consideration of the factors of the stability of metallic iron mag- netic nanoparticles (MNPs) in water suspensions was done using extended DLVO (Derjaguin-Landau-Verwey-Overbeek) approach based on the balance among Van der Waals, electrostatic, magnetic and steric interactions. Magnetic and steric interactions dominate over other in suspensions of Fe MNPs. To test the theory Fe MNPs with average diameter 84 nm were synthesized by electrical explosion of wire and encapsulated by polyacrylamide in water suspension to provide steric repulsion. It was shown that encapsulation resulted in the ef- ficient diminishing of the aggregation of metallic iron MNPs in water. Keywords: Iron nanoparticles; encapsulation; polyacrylamide. Received: 28.06.2017; accepted: 24.07.2017; published: 20.10.2017. © Shankar A., Safronov A., Beketov I., 2017 Introduction Magnetic nanoparticles (MNPs) are the subject of intensive research due to the special properties required for tech- nological and biomedical applications such as magnetic fluids, catalysis, mag- netic resonance imaging, data storage, and environmental remediation [1–3]. In these applications magnetic material is to be dispersed in a solid phase giving a composite material, or in a liquid giving a ferrofluid or a suspension. Among oth- ers iron-based MNPs attract special atten- tion due to their relatively low cost and comparatively low toxicity for the living systems, which is of the major importance for the biotechnological and biomedical applications. In this respect iron oxide MNPs are mostly studied. There are a lot of different methods for their synthesis, a  variety of actual and prospective appli- cations [3], and numerous studies of their compatibility [4]. At the same time metal- lic iron MNPs are less studied. Meanwhile, from the viewpoint of their magnetic properties metallic iron has indisputable advantages over its oxides. Its saturation 159 magnetization is at least three times high- er than that of magnetite. If used in mag- netic sensors, actuators, contrast agents for MRS iron MNPs might provide higher sensitivity, better response, and lower de- tectable doses. However, there are several unresolved problems of the application of metallic iron MNPs in suspensions. The major one is strong aggregation of metal- lic iron MNPs. It is known, that the aggregation of nanoparticles is thermodynamically favo- rable process. The surface between coex- isting phases carries on excess free energy, which might be very high for the nano- system as its specific surface is also high due to small dimensions. The aggregation of MNPs diminishes the surface of the di- rect contact among the phases and leads to the minimization of the free surface energy [5]. In order to overcome the ther- modynamic force for the aggregation and to provide the stability of disperse sys- tems with nanoparticles, such approaches as electrostatic or steric stabilization are used. Unfortunately, the variety of stabi- lizers, which proved to be successful for the stabilization of iron oxide suspen- sions, are not such for the suspensions of metallic iron particles in water. It is the re- sult of the enhanced magnetic properties of iron, which dominate over other forces in colloid suspensions. The objective of the present study was to examine the problem of the stability of magnetic iron nanoparticles from theo- retical point of view and experimentally test the possibility of the stabilization of the suspension of spherical iron MNPs in water, using their encapsulation by water- soluble polymer – polyacrylamide. Experimental Materials Metallic iron magnetic nanoparticles (MNP) were synthesized by the method of electric explosion of wire (EEW). The detailed description of EEW equipment designed at the Institute of Electrophy- sics of RAS (Ekaterinburg, Russia) is giv- en elsewhere [6–8]. The method is based on the evaporation of a portion of metal wire by the electric high power pulse in the explosion chamber filled with the in- ert atmosphere. Further condensation of the expanding metal vapors resulted in the formation of spherical MNPs. The ap- plied voltage was 30 kV and the length of the exploded portion of wire was 70 mm. The wire was continuously fed to the ex- plosion chamber by the feeding device, the high voltage source was concurrently recharged after each explosion, and the process was repeated in the pulsed man- ner resulted in rapid production of MNPs (200 g/h). The reaction chamber was filled with a circulating mixture of 70 % of Ar and 30 % of N2 providing the working gas pressure of 0.12 MPa. Polyacrylamide (PAAm) was synthe- sized by the radical polymerization reac- tion of acrylamide (AAm) (AppliChem, Darmstadt) in 1.6 M water solution at 80  °C. Ammonium persulfate (PSA) in 5  mM concentration was used as an ini- tiator. The reaction mixture was kept at 80  °C for 1  h. The obtained PAAm solu- tion was then diluted with distilled water down to 5 % concentration by weight. The resulted solution was then used as a stock for the encapsulation of iron MNPs. The molar weight of PAAm determined by viscometry was M = 1.46*105 g/mol. 160 Methods The powder X-ray diffraction (XRD) patterns were recorded using Bruker D8 Discover with Cu Kα radiation (λ = 1.542  Å) with graphite monochromator. The Rietveld refinement of XRD patterns were performed using Topas-3 software. The morphology of MNPs was exami- ned using JEOL JEM2100 transmission electron microscope (TEM) operating at 200 kV. The specific surface area of MNPs was measured by the low-temperature adsorption of nitrogen (Brunauer-Emett- Teller (BET) approach) using Micromeri- tics TriStar3000 analyzer. The magnetic measurements were carried out using (Cryogenics Ltd. VSM) vibrating sample magnetometer (VSM) at room tempera- ture for powder samples placed in a gela- tine capsule. The magnetization values in a field of 1.8 T were designated as the satu- ration magnetization values (Ms). Ther- mal analysis was done using NETZSH STA409 thermal analyzer operated in li- near heating mode from 40 to 1000 °C at 10 K/min in the air. Dynamic light scat- tering (DLS) and electrophoretic light scattering (ELS) measurements were performed using Brookhaven ZetaPlus particle size analyzer: 5 and 3 runs were recorded for hydrodynamic size and zeta- potential measurements, respectively. Results and their discussion Theory The aggregation features of iron MNPs in water suspension can be qualitatively modeled by the extended DLVO approach. In classical DLVO theory, the attractive and repulsive interactions are modeled for van der Waals and electrostatic interac- tions only. In case of the magnetic colloidal dispersions where both steric and magne- tic interactions are also present, they must also be taken into account. The modified approach to consider all these interactions is known as xDLVO approach [9]. This theory was elaborated to study the stability of iron MNPs in water suspension. The van der Waals interaction energy (VvdW) between MNPs with radius r at a distance s was calculated as [10]: V A r r s r s r r s s r s r s vdW = − +( ) +     + + + + +     ( ) ( ) ln ( ) ( ) 6 2 4 2 2 4 2 2 2 2 2      , (1) where A(r) = 1.77 × 10–19 + 1.60 × 10–19e–r/3.05 + + 6.35 × 10–20e–r/10.75 + 2.05 × 10–20 e–r/52.18 [J]. The electrostatic repulsions under constant charge boundary condition were taken as [11]: V ee o o s= +( )−2 12π ε ε ψ κr r ln , (2) where κ ε ε = ∑       − k T q N z c B r A i i 0 2 2 1 2/ . Here εr is the relative dielectric constant of water, ε0 is the permittivity of free space, ψ0 is the surface potential, q is the elemen- tary charge, zi is the charge of simple ions, ci is their molar concentration, NA, kB, and T have their usual meanings. We used the value of electrokinetic (zeta) potential of –16 mV for iron MNPs in water (by ELS) as an approximation for the surface po- tential. The steric repulsion was taken into ac- count through a hard core combined with a soft tail potential, as modeled previously under self-consistent field (SCF) theory. 161 This originating overall steric term for two identical stabilized MNPs was taken as [12]: where, δ is thickness of adsorbed LPAAm layer, σp is surface density of adsorbed chains, Np is number of free segments and l is the length of one free segment. The maximum magnetic attraction energy (VM) between MNPs was taken as [10]: V M r s r M o= − +      8 9 2 2 3πµ (4) The total energy of interaction be- tween iron MNPs was calculated as a combination of equations (1)–(4). V(s) = VvdW(s) + Ve(s) + Vst(s) + VM(s) (5) Considering contributions from dif- ferent terms in equation (5) it was found out that steric (equation (3)) and mag- netic (equation (4)) terms are dominat- ing for iron MNPs. These two terms in turn strongly depend on such parameters of MNPs as the radius, the thickness of the steric protective layer, and the mag- netization of particle. Fig. 1 presents the dependence of the energy of interpar- ticle interaction at different combinations of these parameters. The parameters are taken close to that characteristic for the MNPs studied below. It is noticeable that each curve in Fig. 1 has a minimum, which is the result of the balance among attractive magnetic force and repulsive steric interaction. It is con- Vst = ∞ for < 0s r kT N l sp p π δ σ δ 3 3 212 2       − ln      − −       + −             − −       9 5 1 2 1 3 1 2 1 30 1 2 3 s s s δ δ δ 66 0               for 0 < < 2 s δ for >s 2 δ        (3) Fig. 1. Energy of interaction as a function of the distance between iron MNPs: A – The influence of the diameter of MNPs at constant thickness of steric layer (30 nm) and constant magnetization (100 kA/m); B – The influence of the thickness of steric layer of MNPs at constant diameter (84 nm) and constant magnetization (100 kA/m); C – The influence of the magnetization of MNPs at constant diameter (84 nm) and constant thickness of steric layer (30 nm) 162 ventionally accepted that the aggregates can be disrupted by the thermal motion if the corresponding minimum is less than 20 kBT, as statistically only a few particles will cross barrier in this case [13]. Thus, the depth of the minimum indicates the tendency of the ensemble of MNPs to ag- gregation. It is obvious that the depth of the minimum increases with the increase of particle radius, with the increase in magnetization, and the diminishing of the thickness of protective layer. Based on these results, we analyzed the possibility of de-aggregation of Fe MNPs by their encapsulation by polyacrylamide. Characterization of metallic iron MNPs Fig. 2 presents TEM image of metallic iron MNPs synthesized by EEW. They are spherical in shape and non-aglomerated. The spherical shape of MNPs is the re- sult of the EEW conditions. The electrical pulse, which passes the portion of wire, provides its overheating to ca 104 K and complete vaporization. Then iron MNPs are condensed in a vapor phase under the thermodynamic condition for the minimization of free energy. The sphere has a minimal surface among other pos- sible geometrical figures with a constant volume. Hence, the obtained iron MNPs condense in a shape of spheres. The den- sity of vaporized metal in the EEW ex- plosion chamber is kept low by constant circulation of inert working gas; it mini- mizes the probability of collisions among condensing MNPs and prevents their coa- lescence in liquid phase. The particle size distribution (PSD) (Fig. 2, Inset), which was obtained by the graphical analysis of more than 2000 images of MNPs, fits well the following lognormal equation: PSD d d e d ( ) (ln ln( . )) * .= − − 10.70 83 9 2 2 0 4022 (6) The specific surface area of MNPs (Ssp) measured by the low-temperature adsorption of nitrogen was 9.0 m2/g. The surface average diameter of MNPs, calcu- lated from this value using the equation ds = 6/(ρSsp) (ρ = 7.87 g/cm 3 being iron ox- ide density) was 84.7 nm. It was in a good agreement with the median diameter 83.9 nm in PSD described by Equation (6) (the latter value was used a basic level in the theoretical calculations given above). Fig. 3 shows XRD patterns of iron particles. MNPs contain 93 % of α-Fe with а  = 2.867(2) Å and coherent length 82(5) nm and 7 % of cubic phase of γ-Fe with а  = 3.591(3) Å and coherent length 27(2) nm. The coherent length of α-Fe phase perfectly correlates with the aver- age diameter of Fe MNPs obtained both by the analysis of TEM images and by the calculation based on BET sorption re- sults. It means that each singular MNP is a monocrystalline particle. The coherent length of γ-Fe phase is much lower. Most Fig. 2. TEM image of metallic iron magnetic nanoparticles synthesized by electric explosion of wire. Inset: histogram – calculation of particle weight fraction from the image analysis, line – fitting of PSD by equation (6) 163 likely it means that γ-Fe phase predomi- nantly corresponds to the smallest MNPs in the ensemble. Magnetic hysteresis loops of iron MNPs (see Fig. 4) are typical for the mag- netically soft materials. The low field be- havior (inset in Fig. 4) reveals the exist- ence of magnetic hysteresis and coercivity. It can be understood taking into account that although in the ensemble of spherical iron MNPs with average diameter of about 82 nm the majority of them are in multi- domain state, one cannot exclude the pres- ence of a small fraction of single domain MNPs contributing to non-zero coercivi- ty. The value of the saturation mag- netization in bulk state for pure iron is Ms = 1710 (kA/m) [14] for room tempera- ture. The obtained value for Ms for MNPs is about 30 % lower. Most likely this differ- ence stems from two reasons. First, there is a thin oxide layer on the surface of iron MNPs, which appear inevitably if the ac- tive surface of MNPs is exposed to the air. The layer 5 nm in thickness can not be de- tected by XRD but as the magnetization of iron oxide is lower, it certainly contributes to the diminishing of Ms values for MNPs. Another possibility is the disturbance of the crystalline structure of iron in several layers adjacent to the surface of a spherical nanoparticle. These layers are not contrib- uting to the ferromagnetic response due to the insufficient number of the nearest neighbours [15]. Both processes are con- tributing to Ms reduction but it is difficult to make more precise analysis first of all due to the existence of the MNPs size dis- tribution. Encapsulation of Fe MNPs by PAAm Encapsulation of iron MNPs was per- formed by grinding in an agate mortar with 5 % water solution of PAAm at 25 °C. Then the slurry was diluted by the excess of distilled water. The supernatant was decanted and the precipitant was washed several times by distilled water; after that it was collected and tested. The total amount of PAAm, which ad- sorbed onto Fe MNPs was determined by TG/DSC thermal analysis. Fig. 5 presents Fig. 3. XRD diffractogram of iron MNPs synthesized by EEW Fig. 4. Magnetic hysteresis loop of Fe MNPs at 25 °C. Inset – enlarged view of hysteresis loop in low fields Fig. 5. Thermograms of the heating of pristine MNPs (1) and encapsulated MNPs (2) in the air 164 the thermograms for the initial Fe MNPs and MNPs encapsulated by PAAm. Both pristine MNPs and encapsulated MNPs exhibit weight gain (Fig. 5A) in the process of heating. It is the result of the oxidation of iron by the atmospheric oxy- gen. There is clear difference in the total weight gain of these two samples. It stems from the decomposition of PAAm deposit on the surface of MNPs, which effectively decreases the weight gain. The difference of the weight gain is ca 14 %. This value corresponds to the PAAm deposit on the surface of MNPs. The thickness of this layer can be estimated using the residual amount of LPAAm on the surface of MNPs determined by thermal analysis. Corre- sponding conversion into volume fraction taking into account the difference in den- sities of PAAm and Fe core gives 40 % of polymer. The calculation of the thickness of a layer at the surface of the spherical particle with the diameter 90 nm gives ca 8 nm for the layer. Meanwhile, this value corresponds to the dry layer of polymer on the surface. If the MNPs are dispersed in water the polymeric layer swells and its thickness increases. If we assume that the conformation of PAAm macromolecules in the layer is a random Gaussian coil, the volume fraction of a polymer in a coil is given by the following relation: ϕG N = 6 8 3 2/ (7) N is the number of monomeric seg- ments in the chain. The number of Kuhn segments for the molecular weight of PAAm (143.6 kDa) is N = 500, which in turn gives φG = 0.08. It is a reasonable esti- mation for the volume fraction of PAAm in a swollen Gaussian coil. Thus, in water the volume of PAAm layer increases by a factor of 1/0.08 = 12.5. Then, the thick- ness of a layer increases up to ca. 30 nm. (This value was used as a basic level in the theoretical calculations given above) Fig. 6 presents multimodal distribu- tion of particles/aggregates in water sus- pension of iron MNPs encapsulated by PAAm. PSD of iron MNPs in water sus- pension comprises two peaks. The first one is positioned at 160–200 nm. This peak most likely corresponds to individu- al Fe MNPs in suspension. The estimation of the characteristic dimensions of encap- sulated particle, which comprises 84 nm Fe core and 30 nm PAAm steric protective layer on the surface gives ca 144 nm for the diameter. It is rather close to the posi- tion of the first peak at the PSD plot. The second mode is positioned at ca 1000 nm. This peal obviously stans for the aggre- gates of MNPs. The relative number frac- tions of these two peals are 90 % for the individual MNPs and 10 % for large ag- gregates. It means that individual MNPs dominate over aggregates in iron MNPs suspension. Qualitatively, this result is in agreement with the conclusions made based on the theoretical consideration, which favoured the possibility of de-ag- gregation of iron MNPs if sterically stabi- lized by protective layers. Fig. 6. Multimodal PSD of iron MNPs in water suspension by DLS 165 However, full de-aggregation was not achieved. The fraction of aggregates is still substantial. Most likely it is due to high polydispersity of MNPs. As it was shown in Theory section the energy of interac- tion among MNPs strongly depends on their radius. If the ensemble of MNPs is polydisperse, then there is a large fraction of particles with enhanced interaction. This fraction obviously provides aggrega- tion which can not be prevented by 30 nm PAAm layers. Conclusions The factors of aggregation of Fe mag- netic nanoparticles (mean diameter 84  nm) in water suspension were ana- lyzed using extended DLVO (Derjaguin- Landau-Verwey-Overbeek) approach. It is based on the balance among Van der Waals, electrostatic, magnetic and steric interactions. It was shown that attractive magnetic and repulsive steric interactions dominate over other in suspensions of Fe MNPs. As a result of their superposing the dependence of the energy of interaction between MNPs exhibits minimum, which corresponds to the formation of aggre- gates of MNPs. If the depth of the mini- mum is less than 20 kT, the aggregates can be disrupted by the thermal motion. The depth of the minimum is very sensitive to the size of MNP, to its magnetization, and to the thickness of the layer on its surface. It was shown that for Fe MNPs 84 nm in diameter and magnetization 100 kA/m the threshold of the stability corresponds to the protective layer 30 nm. To test the theory Fe MNPs synthesized by electri- cal explosion of wire were encapsulated by polyacrylamide in water suspension to provide steric repulsion. It was shown that the fraction of PAAm in the protective layer is around 14 % and it resulted in the efficient diminishing of the aggregation of metallic iron MNPs in water. Acknowledgement Authors thank Dr. K. Balymov for performing the magnetic characterization of MNPs, Dr. A. I. Medvedev and Dr. A. M. Murzakayev for special support. References 1. Huber DL. Synthesis, Properties, and Applications of Iron Nanoparticles. Small. 2005;1(5):482-501. DOI:10.1002/smll.200500006. 2. Llandro J, Palfreyman JJ, Ionescu A, Barnes CHW. Magnetic biosensor technolo- gies for medical applications: a review. Med Biol Eng Comput. 2010;48(10):977–98. DOI:10.1007/s11517-010-0649-3. 3. Lu AH, Salabas EL, Schuth F. Magnetic nanoparticles: synthesis, protection, func- tionalization, and application. Angew Chem Int Ed Engl. 2007;46(8):1222–44. 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Nanoscaling Laws of Magnetic Nanoparticles and Their Applicabilities in Biomedical Sciences. Acc Chem Res. 2008;41(2):179–89. DOI:10.1021/ar700121f. Cite this article as: Shankar A, Safronov A, Beketov I. Encapsulation of metallic iron magnetic nanoparti- cles by polyacrylamide in water suspensions. Chimica Techno Acta. 2017;4(3):158–66. DOI:10.15826/chimtech/2017.4.3.01.