Nanoscience & Technology Open Access

Super paramagnetic maghemite γ-Fe₂O₃ nanoparticles (average diameter of ~10 nm) are applied for the selective binding of radioactive strontium-85 using 4,4'(5')- di-(t-butyldicyclo-hexano)- 18-crown-6 as a chelating agent. These magnetic nanoparticles offer easy separation once strontium is complexed with the crown ether. Binding test results demonstrate a 65% uptake of Sr²⁺ by gamma radiation measurement.

Application of crown ether for selective extraction and determination of radio isotopes of strontium is well established. In the existing methods, the extraction is carried out either by liquid-liquid extraction (adding the crown ether in an organic solvent) or by extraction chromatography (where crown ether is impregnated onto an inert polymeric support). This research utilizes Magnetic Nanoparticles (MNPs) as the primary separation tool for strontium ions in aqueous sample. Magnetic separation has been found to be effective when handling particles on a Nanosize scale (1-10 nm) [1-3]. Magnetic separation is an affordable and efficient alternative to centrifugation and filtration. It overcomes problems such as blocking of filters and allows the handling of large samples. Magnetic separation also allows for accelerated sedimentation of particles when an external field is applied, therefore making separation and purification steps experimentally simple.

Maghemite has the empirical formula of Fe₂O₃ and is generally made in single crystals smaller than 1 μm. The crystal structure of γ- Fe₂O₃ is classified as isometric tetartoidal and is described as a spinel structure with systematic defects in the octahedral cation [4]. MNPs have a wide range of applications in chemistry and medicine. They are used in waste water treatment, [5] magnetic resonance imaging, [6] hyperthermia tumor treatment, [7] DNA separation, [8] and site specific drug delivery [9]. The γ-Fe₂O₃

MNPs are super paramagnetic and do not have a magnetic memory to aggregate after the external magnetic field is withdrawn. These crystallites offer better stability and biocompatibility compared to higher magnetization materials [10]. Their small size allows them to remain in aqueous suspension for prolonged periods.

Given the tremendous analytical potential of MNPs, herein we demonstrate one of its applications in radioactive hazard determination using selective binding of strontium to 18-crown-6 ether. Stable strontium occurs naturally as Sr-88, 87, 86 and 84. Radioactive strontium- 90 (Sr- 90) is a byproduct of nuclear fission of U-235 or Pu-239, which is a highly hazardous isotope with a long half-life and serious human health concerns [11]. Detection of radioactive strontium can be used as an indication of nuclear contamination. To develop the applications of MNPs for Sr- 90, we used Sr- 85 as a radioactive strontium isotope in our work. Sr-85 is much less hazardous, has a shorter half-life of 65 days and emits gamma radiation.

In this work we describe a novel method of synthesizing hybrid MNPs containing 18-crown-6 ether for selective strontium separation. In order to synthesize this hybrid material, a polymer matrix was needed; the monomer used was 2-acrylamido-2- methylpropanesulfonic acid (AMPS). Divinylbenzene (DVB) was the cross-linker that forms a 3-dimensional matrix with AMPS. This polymer matrix was selected to assist in Sr2+ binding onto crown ether. The novelty of our work is that, after the extraction, the radio-strontium can be conveniently isolated from the aqueous phase by application of an external magnetic field. This technology is very promising for automated radioactive waste disposal/ and clean up, remotely, using robotic arms where the amount of radioactivity will be too high to handle by workers in close proximity using the traditional techniques.

All chemical reagents were purchased from commercial sources. Centrifugation was performed using a micro centrifuge (Revolutionary Science RS- 102), and sonication was done using a sonication apparatus (Branson 2510). Temperatures were regulated using a digital water bath (VWR 89032-214). IR spectra were acquired using a spectrometer (Varian 100 FTIR). Radiation measurements were made using a radiation tester (Triathler multilabel tester) in gamma counting mode for the detection of radiation from Sr-85 in a sample using a NaI detector. Particle size measurements were done using a Brookhaven Instruments Nano DLS analyzer. Nano imaging analysis was performed by scanning electron microscopy (SEM) on a Tescan Vega-II XMU VPSEM system and by electron transmission microscopy (TEM) on a FEI Tecnai G2 F20 TEM system.

_{Synthesis of magnetic nanoparticles}

Nanoparticles of ferromagnetic iron oxide were synthesized from co-precipitation of ferrous (Fe²⁺) and ferric (Fe³⁺) ions in sodium hydroxide solution [12]. A molar ratio of Fe²⁺/Fe³⁺ = 0.5 was prepared by dissolving 1.625 g of FeCl₃ and 1.0 g of FeC^l2.4H²O in 15 mL of aqueous HCl (12.5 mL DDW (distilled de ionized water) and 2.5 mL of 1 M HCl). This solution was added drop wise into 25 mL of 1 M NaOH under vigorous magnetic stirring. The solution turned black upon addition of Fe²⁺/Fe³⁺ to the NaOH solution. The reaction was carried out under a gentle stream of He. The solution was stirred for 30 min once all the Fe solution was added. The resultant colloidal solution was centrifuged at 10,000 rpm and the supernatant was removed. The MNPs were washed with DDW five times or more until the pH was 6.0. They were stored in 125 mL of DDW at 5°C for further coatings.

Synthesized MNPs in water were separated by applying an external magnetic field and decanting the supernatant. MNPs were then suspended in 15mL of methanol under mechanical stirring. The following compounds were added in order: 0.0590 g of 2-acrylamido-2-methylpropane sulfonic acid (AMPS), 0.450 mL of divinylbenzene (DVB) and 0.0082 g of 2, 2'-azobisisobutyronitrile (AIBN). The contents were sonicated for 10 min (40 kHz, 5.71 L, 103 Watts) at room temperature followed by bubbling of N₂ gas for 15 min. The reaction flask was placed in a 60°C water bath for 24 hrs. This resulted in a dark suspension of MNPs. In order to confirm successful coating of the MNPs by the polymer, FTIR analysis was done.
The crown ether, 4, 4' (5')-di-(t-butyldicyclo-hexano)-18- crown-6 (DtBuCH18C6), was used as a chelating agent for selective uptake of Sr₂₊ [11], as illustrated in Figure 1. This crown ether was impregnated onto the P (AMPS/DVB)-coated MNPs following Horwitz's procedure [13] by mixing 20 mg of crown ether (1 M crown ether solution in 1-octanol) and 15 mg of coated MNPs. The contents were sonicated for 30 min and the solvent was evaporated. FTIR analysis was used to confirm successful impregnation of crown ether onto the polymer-coated MNPs.

Strontium binding tests were done on the crown etherimpregnated polymer-coated MNPs as well as the nonimpregnated ones. The dried particles were suspended in a 50:50 MeOH: DDW mixture, and 0.5 mL of this solution was mixed with

0.4 mL DDW and 0.1mL of stable Sr (II) stock solution (0.5 ppm), giving a total sample volume of 1 mL [Table 1] shows a summary of sample preparation for Sr (II) measurement using Hidex Triathler multilabel radiation tester.

Transmission Electron Microscopy (TEM) was used to characterize the MNPs on a FEI Tecnai G2 F20 microscope operating at 200 kV. It offered a point resolution of 0.27 nm and a magnification ranging from 21 x to 700,000 x.

The magnetic-field-assisted separation efficiency and transferability of the synthesized MNPs was determined using an automated magnetic particle transfer workstation.Transmission electron microscopy in Figure 2 showed spherical nanoparticles that averaged 10 nm in diameter [14].

Successful synthesis of MNPs (γ- Fe₂O₃) was confirmed by FTIR analysis. Figure 3 shows the IR spectrum of MNPs. The broad OH peak at 3300-3500 cm-1 indicates presence of surface bound hydroxyl groups coming from residual solvent, as suggested in literature [15]. The broad peak at 575 cm-1 indicates that the iron oxide phase is maghemite [16]. Upon visual inspection of the synthesized MNPs, they appeared pitch black, indicating they were not oxidized to rust.

Next, MNPs were coated with poly-2-acrylamido-2- methylpropanesulfonic acid-co-divinylbenzene, P (AMPS/DVB). This polymer showed successful incorporation of 4, 4', (5')-di-(t-

butyl dicyclohexano)-18-crown-6. The FTIR spectrum in [Figure 4A] confirms the polymerization of P (AMPS/DVB). The presence of maghemite Fe-O stretches at 632-586 cm-1 is observed in both spectra. The presence of AMPS is confirmed by the O-H stretch at 3200-3600 cm-1, C=O stretch at 1701 cm^-1 and N-H bend at 1636 cm-1. The presence of DVB is confirmed by the C=C aromatic stretches in the 1400-1600 cm-1 region [17].

Figure 4B is the FTIR spectrum of 18-crown-6 impregnated P (AMPS/DVB)-coated MNPs. The binding of crown ether to the polymer is suggested to be electrostatic and hydrophobic in nature. The C-O stretches at 1365 cm-1 and 1098 cm-1 confirm the presence of 18-crown-6. The 18-crown-6 impregnated P (AMPS/DVB)-coated MNPs had no magnetic memory. When an external magnetic field was applied, the particles in suspension would aggregate towards the magnet. Once the magnetic field was removed, the particles would readily disperse to form a suspension again.

Upon confirmation of successful crown ether complexation, the Sr²⁺ binding tests were carried out by measuring the characteristic gamma emission peak of Sr-85 [Table 2] using

a Hidex Triathler multi-label tester. The measurements made were in counts per minute (cpm) which were then converted to counts per second (cps). The calculation for activity was done using a conversion factor which was predetermined by standard calibrations. Strontium uptake is measured in terms of % Sr uptake, calculated by taking the percent difference between the sample activity after binding and the control activity before binding.

Although the binding tests had previously been done in the absence of MNPs [18], the major significance of MNP was in separating the Sr2+- bound particles efficiently. This was done by placing the sample container in a magnetic field with separation time being 1-2 min. This separation was necessary for measuring the activity of supernatant solution to determine the % Sr²⁺ uptake accurately. As shown in Table 2, the % Sr2+ uptake for the 18-crown-6-impregnated P (AMPS/ DVB)-coated MNPs is 66% (± 15%) where as for the non-impregnated P (AMPS/ DVB)- coated MNPs it is 23% (± 10%). Although 81% and 50% bindings seem like a large spread of the individual results, it is regarded as reasonable considering the very low gamma activity of 1-2 Bq remaining in the samples. This statistically significant difference,

between 66% (± 15%) and 23% (± 10%), demonstrates the effectiveness of the crown ether used in binding Sr²⁺. It must be noted that DtBuCH18C6 is able to weakly coordinate cations such as K⁺, Na⁺, Cs⁺, Y3⁺ and Pd²⁺ as well as Sr²⁺ [19]. Further optimization of sorption conditions is needed to improve the observed sorption efficiency. The uptakes of Sr2+ by non-crown ether impregnated polymeric MNPs can be explained by the fact that the polymer is anionic (with the sulfonic acid functionality) and can electro statically bind the cation.

Our findings demonstrate the efficiency of DtBuCH18C6 in binding Sr²⁺ and the use of magnetism for rapid separation of the hybrid MNPs. Magnetic susceptibility measurements using either Superconducting Quantum Interference Device (SQUID) or a vibrating sample magnetometer can be done to determine the magnetic strength of synthesized MNPs and the subsequent loss (if any) after polymer coatings and crown ether impregnation.

This work demonstrated the significance of magnetic separation, rather than centrifugation/ filtration, as a rapid, efficient, and energy saving method for sample treatment in analytical applications. The polymeric crown ether hybrid material synthesis was completed successfully as evident from the FTIR spectra. Strontium binding tests successfully showed 66% (± 15%) uptake by the crown ether-bound MNPs, which was superior to only 23% (± 10%) by the non-bound particles. The MNP core made separation (after the binding tests) simple and efficient, requiring only the presence of an external magnetic field. A magnetic separation time of 1-2 min is demonstrably superior to all existing methods of liquid-liquid extraction and extraction chromatography. Further research work would be conducted in developing an automated system for radioactive Sr2+ determination based on MNPs. A number of works have reported the recent advancement of imprinting methods for crown ethers; [20] they can be readily adapted to expand the applicability of magnetic separation in this rapidly growing field of research.

This work is part of the CRTI06-230RD project: Rapid Methods for Emergency Radio Bioassay. Partial financial support by the Natural Sciences and Engineering Research Council (NSERC) Canada is also acknowledged.

1. Kang YS, Risbud S, Rabolt JF, Stroeve P. Synthesis and characterization of nanometer-size Fe3O4 and γ-Fe2O3 particles. Chemistry of Materials. 1996; (8): 2209-2211. doi:10.1021/cm960157j.
2. Liu ZL, Wang HB, Lu QH, Dua GH, Penga L, Dua YQ, et al. Synthesis and characterization of ultrafine well-dispersed magnetic nanoparticles. Journal of Magnetism and Magnetic Materials. 2004; (283): 258-262. doi: 10.1016/j.jmmm.2004.05.031.
3. Yavuz CT, Mayo JT, Yu WW, Prakash A, Falkner JC, Yean S, et al. Lowfield magnetic separation of monodisperses Fe3O4 nanocrystals. Science. 2006; 314: 964-967. doi: 10.1126/science.1131475.
4. Pecharroman C, Gonzalez-Carreno T, Iglesias JE. The infrared dielectric properties of maghemite, γ-Fe2O3, from reflectance measurement on pressed powders. Physics and Chemistry of Minerals. 1995; (22): 21-29. doi: 10.1007/BF00202677.
5. Liang X, Xi B, Xiong S, Zhu Y, Xue F, Qian Y. Porous soft magnetic material: the maghemite microsphere with hierarchical nanoarchitecture and its application in water purification. Materials Research Bulletin. 2009; (44): 2233-2239. doi:10.1016/j.materresbull.2009.08.003.
6. Briggs RW, Wu Z, Mladinich RJ, Stoupic C, Gauger J, Liebig T, et al. In vivo animal tests of an artifact-free contrast agent for gastrointestinal MRI. Magnetic Resonance Imaging. 1997; 15: 559-566. doi: 10.1016/S0730- 725X(97)00020-9.
7. Pardoe H, Clark PR, Pierre TG, Moroz P, Lones S. A magnetic resonance imaging based method for measurement of tissue iron concentration in liver arterially embolized with ferromagnetic particles designed for magnetic hyperthermia treatment of tumors. Magnetic Resonance Imaging. 2003; 21: 483-488. doi: 10.1016/S0730-725X(03)00072-9.
8. Uhlen M. Magnetic separation of DNA: Nature. 1989; 340: 733-734. doi: 10.1038/340733a0.
9. Scherer F, Anton M, Schillinger U, Henke J, Bergemann C, Krüger A, et al. Magnetofection: enhancing and targeting gene delivery by magnetic force in vitro and in vivo. Gene Therapy. 2002; 9(2): 102-109. doi: 10.1038/ sj.gt.3301624.
10. Harris LA, Goff JD, Carmichael AY, Riffle J S, Harburn JJ, St. Pierre TG, et al. Magnetite nanoparticle dispersions stabilized with triblock copolymers. Chemistry of Materials. 2003; 15: 1367-1377. doi: 10.1021/cm020994n.
11. Mohapatra PK, Lakshmi DS, Mohan D, Manchanda VK, Diluent effect on Sr(II) extraction using di-tert-butyl cyclohexano 18-crown-6 as the extractant and its correlation with transport data obtained from supported liquid membrane studies. Desalination. 2006; 198: 166-172. doi: 10.1016/j. desal.2006.03.516.
12. Horwitz EP, Dietz ML, Chiarizia R. The application of novel extraction chromatographic materials to the characterization of radioactive waste solutions. Journal of Radioanalytical and Nuclear Chemistry.1992; 161: 575-583. doi: 10.1007/BF02040504.
13. Hrdina A, Lai EPC, Li C, Sadi B, Kramer G. A comparative study of magnetic transferability of superpara-mgnetic nanoparticles: Journal of Magnetism and Magnetic Materials. 2010; 322(17): 2622-2627. doi:10.1016/j.jmmm.2010.03.031.
14. Frinak EK, Wermeille SJ, Mashburn CD, Tolbert MA, Pursell CJ. Heterogeneous reaction of gaseous nitric acid on γ-phase iron (III) oxide. Journal of Physical Chemistry A. 2004; 108(9): 1560-1566. doi: 10.1021/ jp030807o.
15. Yu S, Chow GM. Carboxyl group functionalized ferrimagnetic iron oxide nanoparticles for potential bio-applications. Chemistry of Materials. 2004; 14: 2781-2786. doi: 10.1039/B404964K.
16. Pavia DL, Lampman GM, Kriz GS, Vyvyan Introduction to Spectroscopy, 3rd edition, Thomson Learning, USA, 2001. p. 26.
17. Howritz EP, Dietz ML, Fisher DE. Extraction of stoontium from nitric acid solutions using dicyclohexano-18-crown-5 and its derivatives. Solvent Extraction and Ion Exchange. 1990; 8(4,5): 557-572.
18. Zhang A, Xiao C, Kuraoka E, Kumagai M. Molecular modification of a novel macroporous silica-based impregnated polymeric composite by trin- butyl phosphate and its application in the adsorption for some metals contained in a typical simulated HLLW. Journal of Hazardous Materials. 2007; 147(1,2): 601-609. doi: 10.1016/j.jhazmat.2007.01.056.
19. Shamsipur M, Rajabi HR. Flame photometric determination of cesium ion after its preconcentration with nanoparticles imprinted with the cesiumdibenzo- 24-crown-8 complex. MicrochimicaActa. 2013; 180: 243-252. doi:10.1007/ s00604-012-0927-x.
20. Rajabi HR, Shamsipur M, Pourmortazavi SM. Preparation of a novel potassium ion imprinted polymeric nanoparticles based on dicyclohexyl 18C6 for selective determination of K+ ion in different water samples: Materials Science & Engineering C. 2013; 33: 3374-3381. doi:10.1016/ j.msec.2013.04.022.