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Letter Cite This: ACS Macro Lett. 2018, 7, 1278−1282
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Efficient Fabrication of Pure, Single-Chain Janus Particles through Their Exclusive Self-Assembly in Mixtures with Their Analogues Li Jiang, Mingxiu Xie, Jinkang Dou, Haodong Li, Xiayun Huang, and Daoyong Chen* The State Key Laboratory of Molecular Engineering of Polymers and Department of Macromolecular Science Fudan University, 2005 Songhu Road, Shanghai 200438, People’s Republic of China
ACS Macro Lett. Downloaded from pubs.acs.org by 95.181.183.34 on 10/17/18. For personal use only.
S Supporting Information *
ABSTRACT: We report the first example of the fabrication of pure, single-chain Janus particles (SCJPs). The SCJPs were prepared by double-cross-linking an A-b-B diblock copolymer in a common solvent. Inevitably, the double-cross-linking led to a mixture containing not only SCJPs but also multichain particles and irregular single-chain particles. Under well-controlled conditions, the SCJPs in the mixture selfassemble with high exclusivity to form regularly structured macroscopic assemblies (MAs) with a crystal-like appearance that precipitate from the suspension. Pure SCJPs that are uniform in size, shape and Janus structure were efficiently prepared by collection and dissociation of the MAs. Block copolymers with different structural parameters were successfully used for the exclusive self-assembly (ESA), and pure SCJPs with varied structural parameters were produced, confirming the reliability of the ESA method.
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since the particles are similar in size, composition and solution properties (as explained below). In addition, the existing methods for preparing Janus particles cannot produce polymeric Janus particles of such small sizes. Therefore, the preparation of pure SCJPs remains a major challenge. In the present study, we double-cross-linked an A-b-B diblock copolymer by cross-linking the A and B blocks, respectively, in a common solvent, leading to a mixture containing MCPs, SCJPs, and ISCPs. Under well-controlled conditions, SCJPs in the mixture exclusively self-assembled into regularly structured macroscopic assemblies (MAs) with a crystal-like appearance, and they precipitated from the suspension; the MCPs and ISCPs self-assembled into nanoaggregates and remained dispersed in the suspension (Scheme 1b). The exclusive self-assembly (ESA) of the SCJPs has some similarities to the recrystallization of small molecules, including the exclusivity of the aggregation and the crystal-like appearance and the macroscopic size of the assemblies. By collecting and then dissociating of the MAs, pure SCJPs that are several nanometers in size and uniform in shape and Janus structure were efficiently prepared (Scheme 1d). The preparation of pure SCJPs and the recrystallization-like ESA of the synthetic polymer are very significant and have never been reported. Furthermore, diblock copolymers with different structural parameters were successfully used for the ESA, efficiently producing pure SCJPs with various structural parameters, confirming the reliability of the processes.
ollapse of a single linear polymer chain into a single-chain polymeric nanoparticle, which is reminiscent of the folding of proteins, has been studied in great detail.1−3 By covalent or noncovalent intrachain cross-linking, spherical,4−6 polymer-chain tethered,7−9 dumbbell-like10,11 and Janus-like12 single-chain nanoparticles (SCNPs) can be generated. SCNPs can be as small as a few nanometers. The small size of SCNPs gives them promising applications in catalysis,13−16 drug delivery,17 bioimaging,18 and fabrication of ultrathin film on the surfaces of proteins.19 However, due to the statistical conformation of polymer chains and the kinetically controlled cross-linking process, there are large morphological and structural deviations among different SCNPs produced in the same cross-linking system.20,21 Even in a system where intrachain cross-linking is dominant, mixtures of SCNPs of various sizes, morphologies, and structures are produced,4,5,22 which result in SCNPs with complex and unpredictable properties and behaviors. On the other hand, Janus particles, one of the most common anisotropic particles have been extensively studied.23,24 It is possible that the ideally structured product of the cross-linking of A and B blocks of an A-b-B diblock copolymer, respectively, in a common solvent could be a single-chain Janus particle (SCJP). As depicted in Scheme 1, an SCJP is composed of A and B, which are two regular SCNPs that are closely connected but completely phase separated. However, double-cross-linking inevitably produces a large proportion of irregular single-chain particles (ISCPs) along with SCJPs; when the polymer concentration is not sufficiently low, double-cross-linking also produces a considerable amount of multichain particles (MCPs) due to interchain cross-linking. Notably, existing methods cannot separate SCJPs from the coexisting ISCPs © XXXX American Chemical Society
Received: July 4, 2018 Accepted: September 27, 2018
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DOI: 10.1021/acsmacrolett.8b00503 ACS Macro Lett. 2018, 7, 1278−1282
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ACS Macro Letters Scheme 1. Schematic of the Exclusive Self-Assembly (ESA) of SCJPsa
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(a) Mixture resulting from cross-linking the A and B blocks, respectively, and this solution contains SCJPs, MCPs, and ISCPs. (b) ESA: Formation of MAs (which can dissociate into SCJPs (d)) that precipitate from the suspension, and the nanoaggregates (which can dissociate into ISCPs and MCPs (c)) dispersed in the suspension. (e, f) Schematic showing that placing an SCJP/ISCP in the MA under the marginal conditions is an energetically favored (e)/unfavored (f) process.
The diblock copolymer poly(2-(methacryloyloxy)ethyl pent4-ynoate)-r-poly(hydroxyethyl methacrylate))-b-poly(2-(dimethylamino) ethyl methacrylate ((PMAEP-r-PHEMA)-bPDMAEMA; the alkyne groups are randomly distributed in the PMAEP-r-PHEMA block) was prepared according to the processes described in S2 of the Supporting Information. Seven kinds of (PMAEP-r-PHEMA)-b-PDMAEMA with different structural parameters were synthesized (S3 in the Supporting Information). For each of the diblock copolymer, the two blocks were double-cross-linked in methanol as the common solvent (S4 and S5 in the Supporting Information) by first cross-linking the PMAEP-r-PHEMA block through a Glaser coupling of the pendent alkyne groups and then cross-linking the PDMAEMA block through a quaternization reaction. We started this study from (PMAEP107-r-PHEMA227)-b-PDMAEMA296 (P1) in which the two blocks have similar lengths. First, we crosslinked the PMAEP107-r-PHEMA227 block in methanol at a concentration of 1.0 mg/mL; the final cross-linking degree was 29% (S4 in the Supporting Information). The mixture resulting from this reaction is denoted as P1−29. Second, we concentrated the P1−29 solution and cross-linked the PDMAEMA296 block at a concentration of 8.0 mg/mL, and the cross-linking degree of the PDMAEMA block was 28%. The final mixture of the double-cross-linking is denoted as P1−29−28. (29 and 28 represent the cross-linking degrees of the two blocks.) For SCNPs formed by intrachain crosslinking, the SCNPs have been reported to develop a substantially collapsed structure when the cross-linking degree is greater than 25% (S6 in the Supporting Information).25 Because the two cross-linking reactions were conducted at relatively high concentrations of 1.0 and 8.0 mg/mL, respectively, interchain cross-linking is inevitable; P1−29−28 should be a mixture containing both MCPs and SCNPs. The GPC curve of P1−29−28 (blue curve in Figure 1a) shows two
Figure 1. GPC (a) and DLS (b) curves of P1, P1−29, P1−29−28, SCJPs-1 (recovered by dissociation of MAs-1 precipitating from the suspension), and the excluded polymers (recovered by dissociation of the nanoaggregates dispersed in the suspension). (c) SAXS pattern of the MAs-1 particles. (d) Photograph of the suspension of selfassembled P1−29−28 in a water/methanol (5/1, v/v) solvent mixture taken after 4 weeks of incubation. (e) FE-SEM image of MAs-1. (f−i) Element mapping images of C, O, N, and I of the MAs-1 particle in (e).
peaks: the right peak at a retention time of 12.4 min (longer than that of P1 (12.2 min)) is assigned to the SCNPs (peakSCNPs) and the left peak at 11.9 min is assigned to the MCPs (peakMCPs). The weight fractions of the SCNPs and 1279
DOI: 10.1021/acsmacrolett.8b00503 ACS Macro Lett. 2018, 7, 1278−1282
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ACS Macro Letters MCPs were calculated to be 58.6% and 41.4%, respectively (S7 in the Supporting Information). The coexistence of MCPs and SCNPs in P1−29−28 was also confirmed by DLS measurements (blue curve in Figure 1b). The self-assembly of P1−29−28 was conducted in a water/ methanol (5/1, v/v) solvent mixture (S2 in the Supporting Information); the polymer concentration in the final suspension was 0.33 mg/mL. Macroscopic assemblies (denoted MAs-1) were finally formed in the system (Figure 1d; S8 in the Supporting Information). In the FE-SEM images (Figure 1e; S9 in the Supporting Information), each of the MAs-1 particles has a flat surface, sharp edges, and a size on the order of hundreds of micrometers; the MAs-1 particles exhibit a crystal-like appearance and are macroscopic in size. Elemental mapping analyses confirmed that the MAs-1 particles were formed by the double-cross-linked P1 because the distributions of C, O, N, and I in the polymer are consistent with the morphology of MAs-1 (Figure 1f−i). The sharp edges and flat surface strongly suggest that the MAs-1 particles have a regular inner structure. As shown in the SAXS pattern (Figure 1c), two distinct peaks appear with a q ratio of 1:2, indicating a lamellar structure with a d-spacing of 4.21 nm. Notably, except for a few recently reported cases where block co-oligomers were used,26 the formation of a lamellar structure with a period less than 5 nm is significant and has seldom been achieved from the self-assembly of synthetic block polymers. MAs-1 particles dissociated immediately in methanol into small nanoparticles, as confirmed by DLS analysis; the dissociation in DMF was confirmed by GPC measurements (orange curves in Figure 1a,b). The small nanoparticles recovered by dissociation of MAs-1 are the SCNPs according to the size measured by GPC and DLS (indicated by the dashed lines in Figure 1a,b; S3 in the Supporting Information). Notably, the recovered SCNPs are uniform in size, and the polydispersity index (PDI) measured by GPC was 1.06, which is much smaller than the PDIs of P1−29−28 (1.87) and the polymer precursor P1 (1.35). The monodispersity of the size of the recovered SCNPs indicates that the MCP content in MAs-1 should be negligible, and unbiased TEM observations confirmed that no MCPs were present in the TEM images of the recovered SCNPs (Figure 2a). As shown in Figure 2a, the recovered SCNPs (stained by RuO4) are regular particles with an aspect ratio of ∼2. Additionally, these particles are uniform in size (Lw/Ln = 1.010, where Lw and Ln are the weight- and number-averaged lengths of the particles, respectively;27 see S10 in the Supporting Information for the statistical results). In the high-magnification TEM images, the recovered SCNPs contain two distinct segments divided by a depression area surrounding the “waist” of the particles (Figure 2c). The twosegment shape of the recovered SCNPs was also observed by AFM (Figure 2e). Comparing the TEM images of RuO4stained P1−29 (Figure 2b,d) with those of the identically stained recovered SCNPs revealed that one P1−29 particle is similar to half (one segment) of one of the recovered SCNPs. This result not only strongly supports the two-segment shape of the recovered SCNPs but also reveals that the two segments are the cross-linked PMAEP107-r-PHEMA227 block and the cross-linked PDMAEMA296 block. Because the molecular weights of the two blocks are similar, in the sufficiently collapsed state the two segments have similar sizes, which is consistent with the TEM and AFM observations. The two segments are relatively regular and are closely connected but completely phase-separated. Therefore, the recovered SCNPs
Figure 2. TEM images of SCJPs-1 (a) and P1−29 (b). Magnified TEM images of a representative SCJP-1 (c) and a P1−29 particle (d). TEM samples were stained with RuO4. (e) An AFM image and the plot profile of an SCJP-1.
are close to ideally structured SCJPs and are thus denoted SCJPs-1. After the separation of the MAs-1, the nanoaggregates dispersed in the suspension were isolated, characterized and dissociated in methanol into the primary building blocks (S11 in the Supporting Information). Based on a combination of TEM observations (S11 in the Supporting Information) and GPC and DLS analyses (purple curves in Figure 1a,b), the primary building blocks of the nanoaggregates dispersed in the suspension are mainly MCPs and ISCPs. As previously mentioned, no MCPs coexisted with the SCJPs-1 recovered from the dissociation of the MAs-1. Furthermore, unbiased TEM observations revealed no considerable ISCPs coexisting with the SCJPs-1. Therefore, the MAs-1 are formed from pure SCJPs-1. Importantly, during the self-assembly, the pure SCJPs-1 exclusively self-assembled into the macroscopic assemblies that then precipitated from the suspension, whereas the MCPs and ISCPs self-assembled separately into nanoaggregates that remained dispersed in the suspension. These observations reveal that the self-assembly of SCJPs-1 excludes both MCPs and ISCPs. To further confirm this conclusion, P1−27−27 (the cross-linking degrees of the two blocks of P1 are 27%) was prepared by cross-linking the two blocks of P1 at a low polymer concentration of 0.5 mg/mL (S4 in the Supporting Information). The GPC and DLS analyses demonstrated that P1−27−27 is mainly composed of SCNPs (Figure 3a,b). The self-assembly of P1−27−27 was conducted under the same conditions and through the same processes used for the self-assembly of P1−29−28. Macroscopic assemblies (MAs-2) formed and precipitated from the suspension. Each of the MAs-2 particles has a flat surface, sharp edges and a size on the order of hundreds of micrometers; the MAs-2 particles also have a crystal-like appearance and are macroscopic in size (Figure 3d). Elemental mapping images confirm that MAs-2 1280
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Figure 3. GPC (a) and DLS (b) curves of P1−27−27, SCJPs-2 (recovered by dissociation of the MAs-2 that precipitated from the suspension), and the excluded polymers (recovered by dissociation of the nanoaggregates dispersed in the suspension). (c) SAXS pattern of MAs-2. (d) FE-SEM images of typical MAs-2 and the corresponding element mapping images of C, O, N, and I of MAs-2. (e−g) TEM images of SCJPs-2 (e), a magnified representative SCJP-2 (f), and the corresponding excluded polymers (g).
were formed by P1−27−27 (Figure 3d). Additionally, based on the SAXS pattern, each MAs-2 has a highly regular lamellar structure with a d-spacing of 4.27 nm (Figure 3c), which is similar to that of MAs-1 (4.21 nm) because both MAs-1 and MAs-2 were derived from the same polymer precursor, P1. In methanol, MAs-2 particles dissociated into the SCNPs, denoted recovered SCNPs-2. As previously mentioned, P1− 27−27 is mainly composed of SCNPs. Based on the DLS measurements, Rh (the Z-averaged hydrodynamic radius) of recovered SCNPs-2 is 4.1 nm, which is close to that of P1− 27−27 (4.3 nm). Nevertheless, compared to the GPC and DLS curves of P1−27−27, those of the recovered SCNPs-2 are narrower and more symmetric (Figure 3a,b); the PDI measured by GPC for the recovered SCNPs-2 is 1.06, whereas that for P1−27−27 is 1.16. A much greater difference between the recovered SCNPs-2 and P1−27−27 was observed by TEM. As shown in Figure 3e, the recovered SCNPs-2 are SCJPs (denoted SCJPs-2), while P1−27−27 are a mixture containing both SCJPs-2 and ISCPs (S12 in the Supporting Information). Moreover, the unbiased TEM observations demonstrate that the SCNPs recovered by dissociation of the nanoaggregates dispersed in the selfassembly suspension are mainly ISCPs (Figure 3g). Obviously, MAs-2 particles were formed from pure SCJPs-2; although SCJPs-2 and ISCPs coexisted in the system, SCJPs-2 exclusively self-assembled into MAs-2, which precipitated from the suspension; by contrast, the ISCPs self-assembled into nanoaggregates that remained dispersed in the suspension. Based on the aforementioned results concerning the ESA of P1−27−27 and that of P1−29−28, we concluded that the selfassembly of SCJPs not only excludes MCPs but also the ISCPs. Notably, the separation of SCJPs and the corresponding ISCPs in the mixtures should be very difficult, if not impossible, using existing methods as explained in S13 in the Supporting Information.
In addition, block copolymers with different structural parameters were successfully used for ESA (S14 in the Supporting Information). Five kinds of pure SCJPs with different structural parameters were efficiently prepared by this ESA approach (S10 in the Supporting Information). Each kind of pure SCJP has a narrow size distribution (several nanometers), uniform shape, and Janus structure. Based on the structural parameters of the MAs and SCJPs, we can conclude that the SCJPs self-assemble in an interdigitated manner into lamellar-structured MAs (S15 and 16 in the Supporting Information). The phenomenon that the length of a certain kind of SCJP is apparently larger than the d-spacing of the corresponding MAs sample can be easily explained by the fact that, in the MAs sample, an SCJP is attracted by the surrounding SCJPs in the same layer (S16 in the Supporting Information). The mechanism of the ESA is clear. During the selfassembly, under marginal conditions, an SCJP in the MA structure can be efficiently stabilized because its size, shape and surface structure match those of the surrounding SCJPs. In contrast, for an ISCP or MCP at the same position, mismatches in the size, shape, and surface structure are substantial and lead to a remarkably higher free energy that cannot be stabilized by the weak attractive interactions (or even repulsive interactions caused by the mismatches) with the surrounding SCJPs (Scheme 1e,f; further explanation of the mechanism is given in S17 in the Supporting Information). As a result, ISCPs and MCPs are excluded from the MAs. In summary, a series of A-b-B diblock copolymers were double-collapsed by cross-linking the two blocks in a common solvent. During the cross-linking of the polymer chains at a relatively high concentration in a common solvent, interchain cross-linking reactions are inevitable and result in a considerable amount of MCPs. Furthermore, the present study reveals that at either a low or a high polymer concentration, ISCPs are unavoidably produced. Nevertheless, 1281
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(10) Roy, R. K.; Lutz, J. F. Compartmentalization of single polymer chains by stepwise intramolecular cross-linking of sequence-controlled macromolecules. J. Am. Chem. Soc. 2014, 136, 12888−12891. (11) Cui, Z.; Huang, L.; Ding, Y.; Zhu, X.; Lu, X.; Cai, Y. Compartmentalization and unidirectional cross-domain molecule shuttling of organometallic single-chain nanoparticles. ACS Macro Lett. 2018, 7, 572−575. (12) Matsumoto, M.; Terashima, T.; Matsumoto, K.; Takenaka, M.; Sawamoto, M. Compartmentalization technologies via self-assembly and cross-linking of amphiphilic random block copolymers in water. J. Am. Chem. Soc. 2017, 139, 7164−7167. (13) Perez-Baena, I.; Barroso-Bujans, F.; Gasser, U.; Arbe, A.; Moreno, A. J.; Colmenero, J.; Pomposo, J. A. Endowing single-chain polymer nanoparticles with enzyme-mimetic activity. ACS Macro Lett. 2013, 2, 775−779. (14) Thanneeru, S.; Duay, S. S.; Jin, L.; Fu, Y.; Angeles-Boza, A. M.; He, J. Single chain polymeric nanoparticles to promote selective hydroxylation reactions of phenol catalyzed by copper. ACS Macro Lett. 2017, 6, 652−656. (15) Liu, Y.; Pujals, S.; Stals, P. J. M.; Paulohrl, T.; Presolski, S. I.; Meijer, E. W.; Albertazzi, L.; Palmans, A. R. A. Catalytically active single-chain polymeric nanoparticles: Exploring their functions in complex biological media. J. Am. Chem. Soc. 2018, 140, 3423−3433. (16) Rothfuss, H.; Knofel, N. D.; Roesky, P. W.; Barner-Kowollik, C. Single-chain nanoparticles as catalytic nanoreactors. J. Am. Chem. Soc. 2018, 140, 5875−5881. (17) Sanchez-Sanchez, A.; Akbari, S.; Etxeberria, A.; Arbe, A.; Gasser, U.; Moreno, A. J.; Colmenero, J.; Pomposo, J. A. Michael” nanocarriers mimicking transient-binding disordered proteins. ACS Macro Lett. 2013, 2, 491−495. (18) Wuest, K. N. R.; Lu, H.; Thomas, D. S.; Goldmann, A. S.; Stenzel, M. H.; Barner-Kowollik, C. Fluorescent glyco single-chain nanoparticle-decorated nanodiamonds. ACS Macro Lett. 2017, 6, 1168−1174. (19) Mahon, C. S.; McGurk, C. J.; Watson, S. M. D.; Fascione, M. A.; Sakonsinsiri, C.; Turnbull, W. B.; Fulton, D. A. Molecular recognition-mediated transformation of single-chain polymer nanoparticles into crosslinked polymer films. Angew. Chem., Int. Ed. 2017, 56, 12913−12918. (20) Groschel, A. H.; Schacher, F. H.; Schmalz, H.; Borisov, O. V.; Zhulina, E. B.; Walther, A.; Muller, A. H. Precise hierarchical selfassembly of multicompartment micelles. Nat. Commun. 2012, 3, 710. (21) Pomposo, J. A. Bioinspired single-chain polymer nanoparticles. Polym. Int. 2014, 63, 589−592. (22) Hosono, N.; Gillissen, M. A.; Li, Y.; Sheiko, S. S.; Palmans, A. R.; Meijer, E. W. Orthogonal self-assembly in folding block copolymers. J. Am. Chem. Soc. 2013, 135, 501−510. (23) Walther, A.; Muller, A. H. Janus particles: synthesis, selfassembly, physical properties, and applications. Chem. Rev. 2013, 113, 5194−5261. (24) Zhang, Z.; Zhou, C.; Dong, H.; Chen, D. Solution-based fabrication of narrow-disperse ABC three-segment and theta-shaped nanoparticles. Angew. Chem., Int. Ed. 2016, 55, 6182−6186. (25) Pomposo, J. A.; Perez-Baena, I.; Lo Verso, F.; Moreno, A. J.; Arbe, A.; Colmenero, J. How far are single-chain polymer nanoparticles in solution from the globular state? ACS Macro Lett. 2014, 3, 767−772. (26) van Genabeek, B.; Lamers, B. A. G.; de Waal, B. F. M.; van Son, M. H. C.; Palmans, A. R. A.; Meijer, E. W. Amplifying (im)perfection: The impact of crystallinity in discrete and disperse block cooligomers. J. Am. Chem. Soc. 2017, 139, 14869−14872. (27) Hudson, Z. M.; Boott, C. E.; Robinson, M. E.; Rupar, P. A.; Winnik, M. A.; Manners, I. Tailored hierarchical micelle architectures using living crystallization-driven self-assembly in two dimensions. Nat. Chem. 2014, 6, 893−898.
due to the ESA, pure SCJPs were efficiently prepared, and the separation between the SCJPs and the coexisting ISCPs was easily achieved. Additionally, the ESA of SCJPs that excludes not only MCPs but also ISCPs to form macroscopic assemblies with a regular inner structure is similar to the recrystallization of small molecules. The recrystallization-like self-assembly is also unprecedented for synthetic polymers.
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ASSOCIATED CONTENT
S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsmacrolett.8b00503.
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Materials, instruments and additional data, and discussion (PDF).
AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. ORCID
Xiayun Huang: 0000-0003-3053-7354 Daoyong Chen: 0000-0001-6776-6332 Author Contributions
The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This work was supported by the programs of NSFC (51721002, 21334001, 21574025 and 21871057), MOST (2016YFA0203302), and STCSM (16JC1400702).
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REFERENCES
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