Iron-Induced Cyclodextrin Self-Assembly into Size-Controllable

Oct 7, 2009 - Iron-induced self-assembly of β-cyclodextrin, β-CD, into size controllable ... of iron-embedded β-CD primary particles with disordere...
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Iron-Induced Cyclodextrin Self-Assembly into Size-Controllable Nanospheres Jae Woo Chung and Seung-Yeop Kwak* Department of Materials Science and Engineering, Seoul National University, 599 Gwanak-ro, Gwanak-gu, Seoul 151-744, Korea Received August 4, 2009. Revised Manuscript Received September 18, 2009 Iron-induced self-assembly of β-cyclodextrin, β-CD, into size controllable nanospheres with a well-defined spherical morphology and a relatively narrow size distribution was formed when acetone was added to a solution of β-CD with iron(II) acetate, Fe(OAc)2, in DMF. Thermogravimetric analysis, inductively coupled plasma atomic emission spectroscopy, and high resolution transmission electron microscopy showed that the irons were present as a welldispersed state in the β-CD nanospheres. In the 1H NMR spectrum of the β-CD/Fe(OAc)2, β/Fe, solution before adding acetone, the peaks corresponding to β-CD were broadened and their spin-spin splitting had disappeared. In particular, the β/Fe solutions were found to remain in a clean solution state at 1 week after solution preparation. These findings indicate the isolation of individual iron ions caused by the surrounding of each ion with the β-CD molecules in the solution before the addition of acetone. X-ray crystal structure analysis, morphological observations, and N2 adsorption and desorption experiments showed that the β-CD nanospheres were generated by the formation of iron-embedded β-CD primary particles with disordered cage type structure and simultaneous spherical assembly of the primary particles during the addition of acetone to the β/Fe solution with appropriate mole ratio between β-CD and Fe(OAc)2. Interestingly, the size of the β-CD nanospheres could be simply controlled by changing the speed at which acetone was added to the solution, with higher acetone addition speeds yielding smaller particles.

Introduction In the past decades, cyclic compounds have played a key role in self-assembly chemistry for the creation of ordered and functionalized assemblies.1 The cyclic oligomers of glucose known as cyclodextrins (CDs) are ideal candidates to provide various self-assembled structures and functionalities. In particular, much attention has been focused on CDs because of their potential pharmaceutical, environmental, electrochemical, biological, and catalytic applications.2 CDs are cyclic oligosaccharides with six (R-), seven (β-), or eight (γ-) glucose units linked by 1,4-R-glycosidic bonds. CDs have a shallow truncated cone shape with a hydrophobic cavity, and this cavity can act as a host for a wide variety of guest compounds, ranging from short to long molecules.3 Recently, CDs crystallized in the presence of metal ions were found to form crystalline complexes incorporating the metal cations,4 where the metal *To whom correspondence should be addressed. Tel: þ82-2-880-8365. Fax: þ82-2-885-1748. E-mail: [email protected].

(1) (a) Ghadiri, M. R.; Granfa, J. R.; Milligan, R. A.; McRee, D. E.; Kjazanovich, N. Nature 1993, 366, 324–327. (b) Harada, A.; Li, J.; Kamachi, M. Nature 1992, 356, 325–327. (c) Zhang, J.; Moore, S. J. Am. Chem. Soc. 1994, 116, 2655–2656. (d) Amabilino, D. B.; Stoddart, J. F. Chem. Rev. 1995, 95, 2725–2828. (e) Ranganathan, D.; Lakshmi, C.; Karle, I. L. J. Am. Chem. Soc. 1999, 121, 6103–6107. (2) Cyclodextrins and Their Complexes; Dodziuk, H., Ed.; Wiley-VCH Verlag GmbH & Co. KGaA: Weinheim, 2006. (3) (a) Szejtli, J. Cyclodextrins and Their Inclusion Complexes; Akademiai Kiado: Budapest, 1982. (b) Saenger, W. Angew. Chem., Int. Ed. Engl. 1980, 19, 344–362. (c) Harada, A.; Takahashi, S. J. Chem. Soc., Chem. Commun. 1984, 645–646. (d) Colquhoun, H. M.; Stoddard, J. F.; Williams, D. J. Angew. Chem., Int. Ed. Engl. 1986, 25, 487–507. (e) Harada, A.; Kamachi, M. Macromolecules 1990, 23, 2821–2823. (f) Harada, A.; Kamachi, M. J. Chem. Soc., Chem. Commun. 1990, 1322–1323. (4) (a) Nicolis, I.; Coleman, A. W.; Selkti, M.; Villain, F.; Charpin, P.; Rango, C. J. Phys. Org. Chem. 2001, 14, 35–37. (b) Nicolis, I.; Coleman, A. W.; Charpin, P.; De Rango, C. Angew. Chem., Int. Ed. Engl. 1995, 34, 2381–2383. (c) Nicolis, I.; Coleman, A. W.; Charpin, P.; De Rango, C. Acta Crystallogr., Sect. B 1996, 52, 122– 130. (d) Russell, N. R.; Mcnamara, M. J. Inclusion Phenom. Macrocyclic Chem. 1989, 7, 455–460. (e) Charpin, P.; Nicolis, I.; Villain, F.; De Rango, C.; Coleman, A. W. Acta Crystallogr., Sect. C 1991, 47, 1829–1833.

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cations are in direct contact with the hydroxyl oxygens on the CD molecules. In these systems, the metal ions are located in the intermolecular space between CD molecules and are bound to the hydroxyl groups of CD and/or water molecules.5 These features have provided a new synthetic route toward advanced functional nanomaterials.6 However, most researches on CD-metal complexes have still focused on the CD-metal binding structure or on the use of CDs as additive agents for the production of metallic nanomaterials, although many studies have sufficiently shown that CDs are useful building blocks for constructing various self-assembled structures such as lamellar, needle-to-plate, tubular, microfibril and nanorod.7 In this study, we describe the iron-induced, size controllable fabrication of CD nanospheres where irons were embedded. To the best of our knowledge, this is the first report to show that iron can induce the self-assembly of CDs into nanoparticles with a well-defined spherical morphology and relatively narrow size distribution, and that the size of the CD nanospheres can be controlled in a simple manner. (5) Harata, K. Chem. Rev. 1998, 98, 1803–1828. (6) (a) Sun, L.; Crooks, R. M.; Chechik, C. Chem. Commun. 2001, 359–360. (b) Liu, J.; Ong, W.; Roman, E.; Lynn, M. J.; Kaifer, A. E. Langmuir 2000, 16, 3000– 3002. (c) Palaniappan, K.; Xue, C.; Arumugam, G.; Hackney, S. A.; Liu, J. Chem. Mater. 2006, 18, 1275–1280. (d) Topchieva, I. N.; Spiridonov, V. V.; Kataeva, N. A.; Gubin, S. P.; Filippov, S. K.; Lezov, A. V. Colloid Polym. Sci. 2006, 284, 795–801. (e) Bonacchi, D.; Caneschi, A.; Dorignac, D.; Falqui, A.; Gatteschi, D.; Rovai, D.; Sangregorio, C.; Sessoli, R. Chem. Mater. 2004, 16, 2016–2020. (7) (a) Rusa, C. C.; Bullions, T. A.; Fox, J.; Porbeni, F. E.; Wang, X.; Tonelli, A. E. Langmuir 2002, 18, 10016–10023. (b) Liu, Y.; You, C.-C.; Zhang, H.-Y.; Kang, S.-Z.; Zhu, C. F.; Wang, C. Nano Lett. 2001, 1, 613–616. (c) Ohira, A.; Sakata, M.; Taniguchi, I.; Hirayama, C.; Kunitake, M. J. Am. Chem. Soc. 2003, 125, 5057–5065. (d) Zhu, X.; Chen, L.; Yan, D.; Chen, Q.; Yao, Y.; Xiao, Y.; Hou, J.; Li, J. Langmuir 2004, 20, 484–490. (e) Hwang, M. J.; Bae, H. S.; Kim, S. J.; Jeong, B. Macromolecules 2004, 37, 8820–8822. (f) Chung, J. W.; Kang, T. J.; Kwak, S.-Y. Macromolecules 2007, 40, 4225–4234. (g) Chung, J. W.; Kang, T. J.; Kwak, S.-Y. Langmuir 2007, 23, 12366– 12370.

Published on Web 10/07/2009

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Experimental Section Materials. β-cyclodextrin, β-CD, and iron(II) acetate, Fe(OAc)2, were purchased from Tokyo Kasei Kogyo Co., Ltd. and Aldrich Chemical Co., respectively. Dimethylformamide (DMF, 99%) and acetone (99%) were supplied from Daejung Chemicals & Metals. All materials were used as received without any additional purification.

Figure 1. (a) FE-SEM and (b) HR-TEM images of R-β/Fe.

Preparation. To fabricate the β-CD nanospheres, 150 mg (1.32  10-4 mol) of β-CD and 11.5 mg (6.6  10-5 mol) of Fe(OAc)2 were first dissolved in 5 mL of DMF for 1 h. Then, 40 mL of acetone was added to the β-CD/Fe(OAc)2, β/Fe, solution, resulting in a self-assembled colloidal suspension via the recrystallization of the β-CD with Fe(OAc)2 (Supporting Information, Figure S-1). The colloidal particles were collected by centrifugation, yielding a yellowish powder. The powder was washed with DMF/acetone (1:8 v/v) cosolvent several times to remove the free Fe(OAc)2 or the free acetato ligand separated from Fe(OAc)2, and then dried at room temperature in vacuo. This resulting material is denoted herein as R-β/Fe (i.e., recrystallized β-CD/Fe(OAc)2). Measurements. The morphologies of R-β/Fe were visualized by field-emission scanning electron microscopy (FE-SEM, SUPRA 55 VP) and high-resolution transmission electron microscopy (HR-TEM, Jeol JEM 3010CX). The FE-SEM samples were coated with a thin conductive Pt layer prior to observation. The content of iron in R-β/Fe was measured by inductively coupled plasma atomic emission spectroscopy (ICP-AES, Optima-4300 DV) with argon plasma source (6000 K), and all samples were analyzed three times and averaged. H NMR spectra were recorded at 600 MHz in DMF-d7 on a Bruker Avance 600 spectrometer. Wide-angle X-ray diffraction (WXRD) patterns were obtained at room temperature on a MAC/Science MXP 18XHF-22SRA diffractometer with a Cu KR radiation source (wavelength = 0.154 nm). The supplied voltage and current were set to 50 kV and 200 mA, respectively. Powder samples were mounted on a sample holder and scanned at a rate of 2θ = 5° min-1 between 2θ = 5 and 35°. N2 adsorption and desorption isotherms of β-CD and R-β/Fe were obtained at 77 K using BELSORP-mini II. All samples were degassed at 100 °C and 10-6 Torr for 6 h prior to measurements. The size of nanospheres were measured with a dynamic light scattering (DLS) method using a Photal DLS-7000 spectrophotometer equipped with a Photal GC1000 digital autocorrelator. In this procedure, the wavelength of the argon laser was 488 nm, and the scattering angle was 90° with respect to the incident beam. The correlation functions were analyzed with the constrained regularized CONTIN method to determine the distribution decay rates. These experiments were

Figure 2. 1H NMR (sol: DMF-d7) spectra of (a) β-CD and (b) β/Fe solutions. Langmuir 2010, 26(4), 2418–2423

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Figure 3. WXRD profiles and morphologies of (a) R-β/Fe and (b) R-R/Fe. carried out at room temperature, and each experiment was repeated two or more times.

Results and Discussion Figure 1 shows FE-SEM and HR-TEM images of the R-β/Fe particles. These images exhibit that well-defined nanospheres with a relatively narrow size distribution were produced when acetone, which is nonsolvent of β-CD and Fe(OAc)2, was added to the β/Fe solution. By contrast, no specific morphologies were observed for β-CD, R-β (the counterpart of R-β/Fe prepared by the addition of acetone to the β-CD solution without Fe(OAc)2), and Fe(OAc)2 (Supporting Information, Figure S-2). Moreover, the presence of iron in R-β/Fe particles was verified by thermogravimetric analysis (TGA) (Supporting Information, Figure S-3), and ICP-AES shows that iron atoms were present at a concentration of ca. 1.89  104 ppm in R-β/Fe. These findings clearly indicate that Fe(OAc)2 is a key material in the formation of the nanospherical assembly observed in R-β/Fe. However, no iron agglomerates were observed on the HR-TEM images of R-β/Fe. Thus, we believe that iron-entrapped β-CD nanospheres, containing well-dispersed iron, were successfully 2420 DOI: 10.1021/la9028645

produced by the recrystallization of β-CD in the presence of Fe(OAc)2 upon the addition of acetone. To elucidate the nanospherical assembly mechanism of the R-β/Fe, we investigated the structural relationships between β-CD and Fe(OAc)2 in β/Fe solution and R-β/Fe powder. From 1H NMR spectra of the solutions of β-CD and β/Fe, the spectrum of β-CD (see Figure 2a) exhibits clear signals with locations and splittings corresponding to those reported for β-CD.8 In the spectrum for β/Fe (see Figure 2b), by contrast the peaks corresponding to β-CD were broadened and their spin-spin splitting disappeared. As is typical, rapid intermolecular proton exchange of hydroxyl group often leads to peak broadening. However, please note that peak broadening of the β/Fe solution appears in all protons including the protons of hydroxyl groups in β-CD. This depicts that these peak broadenings were not caused by rapid intermolecular proton exchange of β-CD. On the other hand, it is known that Fe2þ is paramagnetic when it coordinates with water molecules and that paramagnetic Fe2þ can lead to NMR peak broadening by (8) Bender, M. L.; Komiyama, M. Cyclodextrin Chemistry; Springer-Verlag: Berlin, 1978.

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Figure 4. (a) WXRD profiles and (b) morphologies of the R-β/Fes produced by the addition of acetone to β/Fe solutions in a various mole ratio (β-CD/Fe(OAc)2 = 10:1, 2:1, 1:1, 1:2 and 1:4). 9

increasing the relaxation rate of nearby nuclei. From NMR experiment of R-β/Fe, we could find a remarkable decrease of NMR peak corresponding to CH3 of acetato ligand (Supporting Information, Figure S-4). This means the formation of Fe2þ ions ascribed to the separation and elimination of acetato ligands from Fe(OAc)2. In addition, as shown in Figure 2b, the broadening of HOD peak for β/Fe solution was observed, and it was proven by DSC measurement (Supporting Information, Figure S-5) that removal temperature of water molecules, which inherently exist in β-CD,8 for R-β/Fe increased compared to that for β-CD. This indicates the coordination between water molecules and Fe2þ to give a paramagnetic property to Fe2þ. Thus, it was considered that the NMR peak broadening for the β/Fe solution was attributed to the presence of paramagnetic Fe2þ ions in the solution and that the β-CDs were in close proximity to Fe2þ ions.10 In particular, the peaks corresponding to the secondary hydroxyl groups (2OH at 5.88 ppm and 3OH at 5.80 ppm) of β-CD in β/Fe solution exhibited considerably more peak broadening (9) (a) Cohn, M.; Hughes, R. T. J. Biol. Chem. 1962, 237, 176–181. (b) Eaton, D. R.; Phillips, W. D. In Advances in Magnetic Resonance; Waugh, J. S., Ed.; Academic Press: New York, 1965; Vol. 1. (10) Zheng, W.; Tarr, M. A. J. Phys. Chem. B 2004, 108, 10172–10176.

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Figure 5. (a) N2 adsorption-desorption isotherm plots of β-CD and R-β/Fe, and (b) highly magnified FE-SEM image (  300 000) of R-β/Fe.

and slight peak shift. This demonstrates that the secondary hydroxyl groups of β-CD in β/Fe solution were most affected by Fe2þ ions, suggesting that the Fe2þ ions mainly bind to the secondary hydroxyl groups in the rim of β-CD.6d,e,10 Actually, we found that the β/Fe solution remained in a clean solution state over 1 week after solution preparation, whereas the Fe(OAc)2 solution without β-CD showed a brownish precipitate after 1 day indicative of iron aggregation (Supporting Information, Figure S-6). The stability of the β/Fe solution clearly reveals that β-CDs surround the individual iron ions in the solution state, being in good agreement with the 1H NMR results. To analyze the crystal structure of R-β/Fe, we carried out WXRD experiments. As shown in Figure 3a, β-CD exhibited peaks characteristic of a cage-type crystalline structure.11 R-β/Fe also showed the two peaks corresponding to the main characteristic peaks of β-CD. However, compared to β-CD, the peak intensity for R-β/Fe was remarkably decreased and the peaks were broader. As is typical, decreasing and broadening of WXRD peaks can be caused by a disordering of crystal structure. Thus, it is suggested that R-β/Fe has a disordered cage type crystalline (11) (a) Harada, A.; Okada, M.; Li, J.; Kamachi, M. Macromolecules 1995, 28, 8406–8411. (b) Okumura, H.; Kawaguchi, Y.; Harada, A. Macromolecules 2001, 34, 6338–6343.

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Figure 6. Schematic illustration depicting the structure and nanospherical assembly mechanism of R-β/Fe.

Figure 7. FE-SEM images and DLS plots of R-β/Fe(f), R-β/Fe(m), and R-β/Fe(s) prepared by adding acetone to a β/Fe solution at speeds of 20, 0.2, and 0.025 mL/s, respectively.

structure owing to the irons located in the intermolecular space between CD molecules with cage type crystalline. On the other hand, R-R/Fe (see Figure 3b), which was prepared by the addition of acetone to R-CD/Fe(OAc)2, R/Fe, solution, showed three peaks corresponding to a channel type crystalline structure generally observed for inclusion complexes between R-CD and long guest molecules.12 Furthermore, it was found that R-R/Fe had bundle-like shape where the short nanorods were agglomerated instead of the spherical morphology. These results indicate that when the CDs were recrystallized in the presence of iron, the crystalline structure significantly affected their resulting morphologies. In particular, it is believed that the disordered cage type packing of the β-CDs around the iron ions induces the spherical assembly observed in R-β/Fe. To investigate the iron effect on the crystal structure and morphology of R-β/Fe in greater detail, we prepared R-β/Fes produced by the addition of acetone to β/Fe solutions in various mole ratios ( β-CD/Fe(OAc)2 = 10:1, 2:1, 1:1, (12) (a) Harada, A.; Li, J.; Kamachi, M. Macromolecules 1994, 27, 4538–4543. (b) Harada, A.; Li, J.; Kamachi, M. J. Am. Chem. Soc. 1994, 116, 3192–3196.

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1:2, and 1:4). As shown in Figure 4a, the WXRD patterns of all of the R-β/Fes contained the two peaks corresponding to the cage type crystalline peaks of β-CD, regardless of the iron content. However, the peaks became broader and weaker as the iron content increased. This evidently shows that the irons act as an impurity that can inhibit the formation of perfect cage type crystals of β-CD and that the poor diffraction of R-β/Fe resulted from the disordered cage type packing of β-CD around the iron. Interestingly, although all of the R-β/Fes had a similar disordered cage type crystalline structure, their morphologies differed depending on the iron content (see Figure 4b). In the case of R-β/Fe(2:1), a well-defined nanospherical morphology was observed. However, in the case of R-β/Fe(10:1), the intermediated phase between lamellar and sphere, shown as if the spherical assembly of β-CD was not made enough progress, was observed. R-β/Fe(1:1) and R-β/Fe(1:4) also showed an irregular phase instead of the well-defined spherical morphology. In general, an impurity can act as a nucleation center during recrystallization. Thus, it is considered that the irons act as nucleation centers for the spherical assembly of β-CD in our system during recrystallization. Langmuir 2010, 26(4), 2418–2423

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However, for [ β-CD] . [Fe(OAc)2], the nucleation centers for the β-CD nanosphere formation are insufficient, and the packing between β-CDs seems to have a greater influence on the morphology than does the packing of β-CD around the iron ions. On the other hand, for [ β-CD] e [Fe(OAc)2], the presence of excessive irons seem to disturb the well-defined spherical assembly of β-CD, even though sufficient nucleation centers are present and they had a similar crystalline structure. These results reveal that using the appropriate mole ratio of β-CD/Fe(OAc)2 is crucial to β-CD selfassembly into spherelike particles. Figure 5a shows the N2 adsorption-desorption isotherm plots of β-CD and R-β/Fe. As shown in Figure 5a, no change of plot was observed for β-CD. In the case of R-β/Fe, however, a steep increase in the adsorption amount was observed at low (P/P0 < 0.1) and high (P/P0 > 0.9) relative pressure. This may be due to the presence of micropores attributed to the intervoids between primary particles and the macropores attributed to intervoids between β-CD nanospheres in R-β/Fe. In other words, these results demonstrate that β-CD nanospheres might be formed by spherical agglomeration of the primary particles. Clear evidence of the spherical agglomeration of primary particles could be obtained from the highly magnified FE-SEM image, shown in Figure 5b. Figure 5b shows that very small primary particles are present at β-CD nanosphere surface. In particular, we could observe the internal morphology from the fragment of broken β-CD nanosphere and confirm that β-CD nanosphere has primary particles-aggregated internal morphology, which is consistent with N2 adsorption-desorption result of R-β/Fe. Consequently, considering all of the above observations, we propose the following structure and nanospherical assembly mechanism of R-β/Fe (see Figure 6). First, iron ions are surrounded by β-CD in a solution with a proper mole ratio of β-CD and Fe(OAc)2. On addition of acetone to this β/Fe solution, iron-embedded primary particles with a disordered cage type crystalline structure are formed with iron acting as nucleation centers. Simultaneously or subsequently, these primary particles spherically agglomerate, resulting in iron-induced β-CD nanospheres with a relatively narrow size distribution. At this time, the intermediated phase observed in [ β-CD] . [Fe(OAc)2] seems to be ascribed to the primary units with the crystalline structure relatively close to β-CD due to a lack of the iron acting as nuclear center, and irregular phase observed in [ β-CD] e [Fe(OAc)2] may be ascribed to the irregular agglomeration of primary particles by excessive number of irons. To the best of our knowledge, this is the first

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detailed report to investigate the effect of iron on nanosphere formation of β-CD. Interestingly, the size of the nanospheres could be simply controlled by manipulating the speed with which the acetone was added. When 40 mL of acetone was added at speeds of 20, 0.2, and 0.025 mL/s, the all resulting R-β/Fes (denoted herein as R-β/Fe(f), R-β/Fe(m), and R-β/Fe(s), respectively) had a spherical morphology with a relatively narrow size distribution, as shown in Figure 7. However, their particle size differed according to the addition speed of acetone, and the mean particle sizes for R-β/Fe(f), R-β/Fe(m), and R-β/Fe(s), determined by DLS, were ca. 158 ((51), 232 ((80), and 316 ((133) nm, respectively, which is in good agreement with the FE-SEM results. The decrease in β-CD nanosphere size with increasing acetone addition speed likely arises from a fast recrystallization rate of the β-CDs, although the driving force underlying the size variation is not fully understood.

Conclusion In this study, interesting iron-entrapped size-controllable β-CD nanospheres with a relatively narrow size distribution were successfully fabricated by the addition of acetone to β-CD solution with Fe(OAc)2. The combined results of FE-SEM, 1H NMR, WXRD, and N2 adsorption and desorption experiments showed that the β-CD nanospheres were formed by the spherical agglomeration of iron-embedded β-CD primary particles with a disordered cage-type crystalline structure due to the iron acting as the nucleation center. Furthermore, the size of the nanospheres could be simply controlled by manipulating the speed with which the acetone was added. Our results will open up the possibility of creating novel and functional self-assembled material. In particular, we are currently attempting to apply the iron-entrapped β-CD nanospheres to the biological and medical fields such as contrast agent and iron carrier, and there results will be reported in future articles. Acknowledgment. The authors would like to thank the Korea Science and Engineering Foundation (KOSEF) for sponsoring this research through the SRC/ERC Program of MOST/KOSEF (R11-2005-065). Supporting Information Available: This material is available free of charge via the Internet at http://pubs.acs.org.

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