Letter Cite This: ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX
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Polymer-Encapsulated Lanthanide-Containing Clusters as Platforms for Fabricating Magnetic Soft Materials Qun He,† Huanting Huang,† Xiu-Ying Zheng,‡ Jie Xiao,† Bingran Yu,*,§ Xiang-Jian Kong,*,‡ and Weifeng Bu*,† †
Key Laboratory of Nonferrous Metals Chemistry and Resources Utilization of Gansu Province, State Key Laboratory of Applied Organic Chemistry, and College of Chemistry and Chemical Engineering, Lanzhou University, Lanzhou, Gansu 730000, China ‡ State Key Laboratory of Physical Chemistry of Solid Surface and Department of Chemistry, College of Chemistry and Chemical Engineering, Xiamen University, Xiamen 361005, China § State Key Laboratory of Chemical Resource Engineering, College of Materials Science and Engineering, Beijing University of Chemical Technology, Beijing 100029, China S Supporting Information *
ABSTRACT: Although many high-nuclearity lanthanide-containing clusters with aesthetical topological nanoarchitectures and unique magnetic properties have been synthesized, there was a big time lag to develop functional soft materials and devices on the basis of these clusters, because of their stability and processability. Herein, we report a universal strategy to fabricate lanthanide cluster based magnetic soft materials under ambient conditions. The prototypical cluster [Gd52Ni56(IDA)48(OH)154(H2O)38]18+ was encapsulated as an inorganic core by polymeric shells of sulfonate end functionalized poly(ethylene glycol) monoalkyl ethers through electrostatic interaction. The thickness of the shell was readily controlled by precisely tuning length of the polymer chain, leading to controllably reduced antiferromagnetic interactions between the clusters. The encapsulated hybrids can self-assemble to form vesicles in solution and can be used as an excellent agent for in vivo magnetic resonance imaging. KEYWORDS: lanthanide-containing clusters, self-assembly, magnetic properties, vesicles, encapsulation
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nide-containing clusters have been enwrapped into silica matrices through one-pot microemulsion method, leading to monodisperse core−shell nanostructures.4,5 Such protection and isolation not only enhanced the chemical stability of the clusters but also reduced the magnetic interactions occurring between the original clusters. However, these cluster@SiO2 hybrids are still in the category of inorganic materials and not ready for the aforementioned solution-processable applications. On the other hand, we16,17 and Wu18 have prepared polyoxometalate-based supramolecular star polymers, in which the polyoxometalate anion is closely encapsulated by quaternary ammonium terminated polystyrenes. In such core− shell nanostructures, the shell thickness is really controlled by the molecular weight of the polystyrene arm. This, together with aforementioned core−shell nanoparticles of cluster@ SiO2,4,5 further prompts us to prepare polymer-encapsulated high-nuclearity lanthanide-containing clusters (PECs) and recognize the influence of the length of the polymer arms on the magnetic properties. Herein, we present, to the best of our
anthanide-containing clusters exhibit aesthetical topological nanoarchitectures and fascinating magnetic properties, representing a unique class of magnetic compounds.1−12 They are proposed to be potentially applied in the fields of magnetic cooling, quantum computing technologies, and information storage devices. Of particular note, recent synthetic strategies involving an efficient combination of metalloligand or ligandcontrolled lanthanide hydrolysis and anion templates have generated many cationic high-nuclearity lanthanide-containing clusters that show high magnetic density and large magnetocaloric effect.1−12 However, further processing and developing these clusters into magnetic functional soft materials and devices, which is directly related with the aforementioned applications, is technically challenging as a result of their high lattice energies associated with crystallization and sensitive coordination modes under external stimuli. A major literature survey showed that the processing of magnetic clusters has exclusively focused on ligand-exchanged reactions of Mn12 and Fe4, but usually with functionalized small-molecule ligands.13−15 Furthermore, this strategy shows serious limitations to the nanostructuring of lanthanidecontaining clusters because of their sensitive synthetic conditions.1−12 In our recent work, high-nuclearity lantha© XXXX American Chemical Society
Received: April 11, 2018 Accepted: May 9, 2018 Published: May 9, 2018 A
DOI: 10.1021/acsami.8b05816 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX
Letter
ACS Applied Materials & Interfaces
Figure 1. Cationic cluster of Gd52Ni56 was encapsulated by sulfonate terminated PEG monoalkyl ethers (S-1, S-2, and S-3) through electrostatic interactions, leading to polymer-encapsulated high-nuclearity lanthanide-containing clusters (PEC-1, PEC-2, and PEC-3). They self-assembled to form vesicular nanostructures in dichloromethane.
knowledge, the first examples of PECs (PEC-1, PEC-2, and PEC-3) by electrostatic association of [Gd 5 2 Ni 5 6 (IDA) 4 8 (OH) 1 5 4 (H 2 O) 3 8 ]·(NO 3 ) 1 8 ·164H 2 O (Gd52Ni56, IDA = iminodiacetate)5 with sulfonate end functionalized poly(ethylene glycol) monoalkyl ethers, respectively (S-1, S-2, and S-3; poly(ethylene glycol): PEG, Figure 1). The resulting PECs show a gradual increase in size with increasing the length of the polymer sulfonate and can selfassemble to form vesicular nanostructures in solution. The moderate length of the polymer chain in PEC-1 only partially reduced the antiferromagnetic interaction between individual clusters, while the intercluster interactions are completely shielded in the cases of PEC-2 and PEC-3 with the longer macromolecular chains. Furthermore, PEC-3 is highly watersoluble and can be used as an excellent agent for in vivo magnetic resonance imaging (MRI). Sulfonate end functionalized PEG monoalkyl ethers of S-2 and S-3 were synthesized by treating their methanesulfonate precursors with Na2SO3 in water, respectively (Scheme S1, Figures S1−S4). PEG monoalkyl ether encapsulated magnetic clusters, PEC-1, PEC-2, and PEC-3 were prepared by mixing the sulfonate terminated PEG monoalkyl ethers, S-1, S-2, and S-3 with Gd52Ni56 in water, respectively. PEC-1 and PEC-2 were directly obtained as green precipitate forms. Of difference was that the third mixture was dialyzed against water to remove the ionic exchange side product of NaNO3, due to the high aqueous solubility of PEC-3. The UV−vis absorption spectra of these PECs showed characteristic bands of the Gd52Ni56 cluster at 386 and 632 nm (Figure S5). In their Fourier transformation infrared spectra (Figure S6), bands at 1593 and 1416 cm−1 were assigned to COO− antisymmetric and symmetric stretching modes, respectively, which were associated with the ligand of iminodiacetate in Gd52Ni56. The strong absorption bands at 2887 and 1113 cm−1 were characteristic of CH2 and C−O−C symmetric stretching modes, respectively, in the PEG components,19 while the typical bands associated with the alkyl chains appeared at 2955, 2918, and 2852 cm−1.20 In addition, the characteristic bands related with SO3− appeared at 1148 and 1040 cm−1. In contrast, the characteristic band of NO3− at 1385 cm−1 in Gd52Ni56 was no longer visible in the cases of PECs. These spectroscopic data confirmed the presence of both the Gd52Ni56 cation and polymer sulfonates in PECs. PEC-1, PEC-2, and PEC-3 were further examined by elemental analyses, from which their compositions were established with 17, 19, and 20 sulfonate anions grafted onto the Gd52Ni56 cation, respectively. Such nonstoichiometric pictures were commonly observed in the case of ionic self-
assembly for the synthesis of supramolecular modular materials.20,21 As determined by the crystal structure,5 the diameter of the cluster cation was 2.8 nm, corresponding to a surface area of 24.6 nm2 by assuming a spherical cluster of Gd52Ni56. Therefore, the sulfonate anions occupied areas of 1.45, 1.30, and 1.23 nm2 on the surface of the Gd52Ni56 cation for PEC-1, PEC-2, and PEC-3, respectively. These values were much larger than the head areas of sodium dodecylsulfonate (0.56 nm2)22 and Brij-based surfactants (0.29−1.01 nm2) at the air/water interface,23 suggesting that the macromolecular chains were loosely attached to the surface of the cluster. All of the hybrids were highly soluble in dichloromethane, which was consistent with the encapsulated clusters. The resulting solutions were further subjected to dynamic light scattering measurements (DLS, 0.2 mg/mL). The DLS plots displayed that two diffusive dynamic modes emerged at 8.2 and 122 nm for PEC-1, 12 and 164 nm PEC-2, and 20 and 255 nm for PEC-3, respectively (Figure 2). In contrast, the DLS signals
Figure 2. DLS plots of Gd52Ni56, PEC-1, PEC-2, and PEC-3. Gd52Ni56 was dissolved in water, whereas PECs were in dichloromethane. All of the solution concentrations were 0.2 mg/mL.
of Gd52Ni56 in water (0.2 mg/mL) appeared at 3.2 and 74 nm (Figure 2). The small peak was only slightly larger than the diameter of the Gd52Ni56 cation (2.8 nm) and assigned to the hydrated cluster that bound weakly with the counteranion, NO3−. The latter peak at 74 nm was due to the formation of supramolecular aggregates in the aqueous solution. With this assignment in mind, the fast modes of PEC-1 (8.2 nm), PEC-2 (12 nm), and PEC-3 (20 nm) corresponded to their respective monomeric forms in dichloromethane. The increase in size was consistent with the increasing repeat units of the sulfonateterminated arms (Figure 1). Meanwhile, the slow modes also showed a similar increase in size and were accordingly B
DOI: 10.1021/acsami.8b05816 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX
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ACS Applied Materials & Interfaces
(1:2:3) showed that PEC-2 self-assembled to form a lamellar structure with a layer distance of 7.76 nm. Similarly, lamellar structures were also observed in the cases of PEC-1 and PEC-3 with interlayer spacings of 5.98 and 24.1 nm, respectively. The spacing order of PEC-1 < PEC-2 < PEC-3 conformed to the increasing hydrodynamic diameter of single PECs in dichloromethane (vide supra) and was due to the increased molecular weight of the sulfonate-terminated PEG monoalkyl ether (Figure 1). The WAXS pattern of PEC-3 showed that the strong Bragg peaks appeared at 19.0° (4.67 Å) and 23.2° (3.83 Å), indicative of crystalline PEG chains onto the surface of the cluster cation (Figure S8b). In contrast, PEC-1 and PEC-2 did not exhibit any recognizable peaks from 10 to 40°, suggestive of amorphous PEG chains there. The variable-temperature magnetic susceptibilities of Gd52Ni56, PEC-1, PEC-2, and PEC-3 were measured under an external field of 1000 Oe in the temperature ranges of 5− 300 K. As shown in Figure 4, at 300 K, the χMT values for
attributed to the presence of supramolecular aggregates in dichloromethane. To achieve the fundamental insight on the bimodal nanostructures, we further cast all of these solutions onto carbon-coated copper grids for bright-field transmission electron microscopy (BF-TEM) and high-angle annular darkfield scanning transmission electron microscopy (HAADFSTEM) observations. The latter imaging technique demonstrates that the components with heavy elements appear bright, whereas the carbon background is dark. The resulting TEM images showed that PEC-1 (Figure 3a, b), PEC-2 (Figure S7a,
Figure 4. Experiment plots of χMT against T of Gd52Ni56, PEC-1, PEC-2, and PEC-3 under an external field of 1000 Oe. Figure 3. (a−c) BF-TEM and (d) HAADF-STEM images of PEC-1 obtained from a 0.2 mmol/L dichloromethane solution.
Gd52Ni56, PEC-1, PEC-2, and PEC-3 were 455.45, 457.32, 459.07, and 458.99 cm3 mol−1 K, respectively, which were all closed to the calculated value of 465.43 cm3 mol−1 K based on 52 uncorrelated Gd3+ ions (S = 7/2, g = 2) and 56 uncorrelated Ni2+ ions (S = 1, g = 2). With decreasing temperature, the χMT value for Gd52Ni56 decreased gradually and then dropped to the minimum value of 414.85 cm3 mol−1 K at 5 K, indicating the presence of antiferromagnetic interaction and zero-field splitting effect in Gd52Ni56.5 For PEC-1, up cooling from 300 to 150 K, the χMT value remained a constant, and then decreased gradually to a minimum value of 437.77 cm3 mol−1 K at 5 K. However, the χMT values of 150 to 5 K for PEC-1 with a moderate length of the attached chain were larger than the corresponding values for the naked cluster of Gd52Ni56, suggesting that the introduction of the macromolecular chain of S-1 has partly removed the intercluster antiferromagnetic interaction. Similarly, in the cases of PEC-2 and PEC-3, where the longer macromolecular chains were tethered onto the Gd52Ni56 cation, the χMT values kept a constant in the temperature ranges of 300−150 K and decreased gradually from 150 to 5 K to the minimum values. More importantly, their magnetic susceptibility curves and values were almost overlapped and were larger than the χMT values of PEC-1 in the temperature range of 150 to 10 K. This phenomenon indicated that the χMT value saturated in the case of PEC-2 and would not show any recognizable increase even with additionally increasing length of the tethered chain, for example, in the case of PEC-3. At this stage, the intercluster antiferromagnetic
b), and PEC-3 (Figure S7d, e) self-assembled to form unilamellar vesicles in the dichloromethane solutions. As highlighted by the white arrows in the magnified TEM images (Figure 3b and Figure S7b), the vesicular wall was clearly observed and the thickness of the wall was determined to be 2.8 nm. This value was consistent with the diameter of the Gd52Ni56 cation. Further magnification and scrutiny of these images revealed that single Gd52Ni56 clusters emerged with a diameter of 2.8 nm (Figure 3c, d, and Figure S7c). This suggested that single PECs coexisted with vesicular nanostructures in dichloromethane, which agreed well with the bimodal patterns in the aforementioned DLS plots. And correspondingly, the slow modes were assigned to the vesicles. Unexpectedly, a vesicle structure was also observed in the aqueous solution of Gd52Ni56 (Figure S7f), which coincided with the large-sized aggregate found in the DLS measurement (Figure 2). The self-assembled behaviors of PECs and Gd52Ni56 were similar to those found in the organic solutions of surfactant-encapsulated polyoxometalate clusters24,25 and in the aqueous solutions of pure polyoxometalates,26,27 respectively. Furthermore, the solid powders of PEC-1, PEC-2, and PEC3 were measured by using SAXS and WAXS (Figure S8). In the SAXS curve of PEC-2 (Figure S8a), three peaks were clearly recognized at 0.81, 1.52, and 2.39 nm−1, corresponding to the spacings of 7.76, 4.13, and 2.63 nm, respectively. Their ratio C
DOI: 10.1021/acsami.8b05816 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX
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ACS Applied Materials & Interfaces
Figure 5. In vivo MR images (a) before and (b) after the intratumor injection of PEC-3 and (c) their data comparison with DTPA-Gd.
cases of PEC-2 and PEC-3 with the longer macromolecular chains, the intercluster antiferromagnetic interaction is completely shielded, whereas for PEC-1, it is only partially reduced because of the limited length of the attached chain. Furthermore, PEC-3 is useful as an excellent agent for in vivo MRI. The present concept of modifying the surface properties of high-nuclearity lanthanide-containing clusters with anion terminated polymers represents a universal strategy for developing cluster-based magnetic soft materials.
interaction can be completely shielded, thus achieving the study of the single molecule magnetic properties. This picture was consistent with the previously reported Mn12 clusters that were isolated in MOFs28,29 and mesoporous silica.30 However, PEC1 was demonstrated to show a partially reduced intercluster interaction. Such an intermediate state was not addressed previously. It should be highlighted here that before the stage that the intercluster interaction was completely eliminated, it might be possible to finely tune magnetic behaviors of highnuclearity lanthanide-containing clusters, by precisely controlling the length of the macromolecular chain attached on the periphery of the cluster. As mentioned above, PEC-3 was highly soluble in water (Figure S9), while PEC-1 and PEC-2 showed limited solubility in water. Therefore, longitudinal proton relaxation of PEC-3 with different Gd3+ concentrations was detected in phosphate buffer saline (PBS) and 4T1 cells using T1-weighted mapping sequence, respectively. The resulting signal intensity of inverse T1 was positively correlated with the Gd3+ concentration in PBS, but not in cells (Figure S10). The probable reason was that PEC-3, like the commercial contrast agent (DPTA-Gd), did not show any recognizable cellular uptake. The R1 value of PEC-3 in PBS was determined to be 3.82 mM−1 s−1, which was consistent with that of DTPA-Gd (3.91 mM−1 s−1). PEC-3 was further utilized as an MRI agent for in vivo MR imaging because of its biologically relevant PEG shell. The MRI signals were scanned using T1-weighted mapping sequence. Although the relaxitivity of PEC-3 was similar to that of the commercial MRI agent DTPA-Gd, some differences were observed in the tumor MR imaging. As shown in Figure 5 and S11, no obvious signals for both DTPA-Gd and PEC-3 were detected in the tumor before injection. After the intratumor injection, PEC-3 showed a prominent enhancement in the tumor with a factor of 1.66, far more than DTPA-Gd did (1.20). The biologically compatible PEG shell, stable encapsulated structure, and effective nanometric size of PEC-3 contributed to the administration in the tumor for enhanced permeability and retention effect. Comparing to the low MRI ability in cells, PEC-3 remained substantially in the vascular space rather than being rapidly metabolized. Therefore, PEC-3 could be regarded as an in vivo MRI agent with excellent performance. Although Gd-polyoxometalate polymeric assemblies have been utilized as MRI contrast agents,31 this is the first example involving the MRI application of high-nuclearity lanthanide-containing clusters. In summary, we have prepared a series of PECs by harnessing electrostatic self-assembly of sulfonate end functionalized PEG monoalkyl ethers with Gd52Ni56 under ambient conditions. The size of PECs increases with increasing the length of the PEG sulfonate. Both the naked and encapsulated clusters are demonstrated to form vesicles in solution. In the
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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/acsami.8b05816. Synthesis and characterization details, spectral patterns, TEM images, and in vitro and in vivo MRI data (PDF)
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AUTHOR INFORMATION
Corresponding Authors
*E-mail:
[email protected]. *E-mail:
[email protected]. *E-mail:
[email protected]. ORCID
Bingran Yu: 0000-0003-4912-5632 Xiang-Jian Kong: 0000-0003-0676-6923 Weifeng Bu: 0000-0002-6213-2928 Author Contributions
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 is supported by the NSFC (21504036, 21674044, and 21474044), the Fundamental Research Funds for the Central Universities (lzujbky-2016-42 and lzujbky-2017-k08) and the Open Project of State Key Laboratory of Supramolecular Structure and Materials of Jilin University (sklssm201801). The project was supported by Open Research Fund of State Key Laboratory of Polymer Physics and Chemistry, Changchun Institute of Applied Chemistry, Chinese Academy of Sciences (2017-08).
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DOI: 10.1021/acsami.8b05816 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX
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DOI: 10.1021/acsami.8b05816 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX
