Chelation Competition Induced Polymerization (CCIP): A Binding

Jul 18, 2017 - Here, we report a method that enables synthesizing nonspherical polymer nanocontainers with different morphologies and properties by us...
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Chelation Competition Induced Polymerization (CCIP): A Binding Energy Based Strategy for Nonspherical Polymer Nanocontainers’ Fabrication Siyuan Xiang,† Hu-Jun Qian,† Yixin Chen,† Kai Zhang,*,† Yanhong Shi,‡ Wendong Liu,† Haizhu Sun,‡ Hongchen Sun,§ and Bai Yang† †

State Key Laboratory of Supramolecular Structure and Materials, College of Chemistry, Jilin University, 2699 Qianjin Avenue, Changchun 130012, P. R. China ‡ Faculty of Chemistry, Northeast Normal University, Changchun 130024, China § Department of Oral Pathology, School and Hospital of Stomatology, Jilin University, Changchun 130021, P. R. China S Supporting Information *

ABSTRACT: Polymer based nanocontainers have attracted a myriad of interests due to their functional groups, mechanical stability, combining with inner voids and large specific surface area. Preparation of polymer nanocontainers with specific nonspherical geometric morphology, precisely controllable structure, composition, and functionalities still remains a significant challenge. Here, we report a method that enables synthesizing nonspherical polymer nanocontainers with different morphologies and properties by using anisotropic metal−organic coordination compounds as templates. The preparation process is based on the stronger monomer’s binding ability to metal cations than that to the organic linkers. The correlation between the binding energy and the formation of the hollow structure is calculated theoretically. Polymer nanocontainers with multiple morphologies (such as octahedron, dodecahedron, etc.) are constructed by expanding template materials. The as-prepared polymer nanocontainers exhibit active properties (biomedical and energy storage) that derive from the intrinsic nature as well as further functionalization, suggesting promising candidates in thermo-chemo cancer eradication, Li-ion batteries, and so on.

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etc.18,19 In spite of the evolutions in the templating method that are achieved by developing different mechanisms, such as galvanic replacement, ion-exchange, and Ostwald ripening, most of them focus on the synthesis of inorganic nanocontainers.20−22 For example, Hyeon et al. prepared spindlelike iron oxide nanocapsules by using silica coated β-FeOOH as starting material and template.23 Moreover, Xia and co-workers synthesized Au−Ag nanoboxes via galvanic replacement. The obtained nanoboxes exhibited controllable pores at corners and tunable SPR peaks at the near-infrared region.24 Metal−Organic Frameworks (MOFs), as one of the emerging materials, have shown great potential in gas storage/separation, catalysis, sensors, and drug delivery due to their superhigh porosity, chemical stability, and tunable cavity sizes.25−27 It is worth noting that MOFs have also been utilized in hollow structure preparation. Chen et al. prepared ZnS nanocages by using a zeolitic imidazolate framework (ZIF-8) as template, perfectly holding the truncated rhombic dodecahedral morphology.28 In addition, hollow MOFs are prepared for wide

INTRODUCTION Nanocontainers, regarded as an important family member of functional hollow nanomaterials, have offered great promise in various application fields owing to their integration of functional groups, inner voids, and large specific surface area.1−4 Among them, polymer based nanocontainers have attracted a tremendous amount of interests for applications in biomedicine, electrochemistry, and sensing, due to their fascinating properties (physical/chemical stability, biocompatibility, and adjustability).5−11 With the deepening of the nanoscience, preparing nanocontainers with specific nonspherical geometric morphologies and compositions becomes a tendency, since these factors may endow novel functionalities to the nanocontainers and provide them new opportunities in biomedical and energy related fields.12−16 So far, the most mature strategy for nanocontainers synthesis is the templating method. The traditional templating method is always conducted by coating the desired shell onto the “cores” (usually solid/hard templates), followed by a postprocessing for templates removal.17 Although many hollow structures are obtained, limitations also exist in the method, such as lack of nonspherical template materials, complex and tedious postprocessing, difficulties in precisely controlling the manipulation, © 2017 American Chemical Society

Received: June 2, 2017 Revised: June 30, 2017 Published: July 18, 2017 6536

DOI: 10.1021/acs.chemmater.7b02274 Chem. Mater. 2017, 29, 6536−6543

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Chemistry of Materials application fields. A series of MOF hollow nanostructures have shown highly enhanced liquid-phase selective hydrogenation performance.29 Moreover, Hu et al. synthesized hollow MOFs through a surface functionalization-assisted etching method, displaying superior hyperthermia performance under NIR irradiation.30 MOFs as template materials provide both special morphologies and functionalities due to their unique chemical/ physical properties for the hollow nanostructures. Compared to the above-mentioned inorganic hollow nanostructures, polymers with their flexibility, modifiability, and biocompatibility have been widely applied in biomedicine, catalysis, energy storage, etc.10,31−33 Constructing different polymer based nanocontainers, especially with special nonspherical morphologies and versatile functionalities, is becoming a more and more popular research topic. Recently, Caruso et al. have successfully prepared hollow faceted polymer microcapsules by using ZIF-8 as template, providing a train of thought for construction of hollow polymer structures by an LBL method.34 However, it is still imperative for us to exploit synthetic approaches with novel mechanisms for preparation of these unique nanostructures. In previous work, we exploited a strategy, chelation competition induced polymerization (CCIP), to prepare nonspherical polydopamine nanocontainers loaded with gold nanorods (Au NRs@PDA NCTs) via a ZIF-8 template, which have displayed excellent performance in synergistic thermochemo cancer eradication.35 However, the mechanism of this strategy remains indistinct and needs to be further studied. Here, the hollowing process is understood carefully from both experimental and theoretical perspectives. In this method, the hollowing process derives from the stronger chelation ability between metal cations and monomer than that between metal cations and organic linker in templates, which can be further ascribed to the different binding energy between metal cations and ligands. Theoretical calculation results have been provided to prove the mechanism. In addition, the generality of the CCIP method has also been studied. On the basis of the corresponding binding energy, polymer nanocontainers with different morphologies, chelated metal cations, and monomer can be successfully prepared. The disassembly of the templates and polymerization of the monomer occur simultaneously, forming the nanocontainers without further templating removal. The obtained polymer nanocontainers exhibit excellent performance and versatile functionalities, which is derived from the structures, polymers’ inherent and modification properties, showing promising candidates in various applications such as multifunctional cancer eradication and Liion batteries. This method may provide a platform and endow great significance to prepare functional nonspherical polymer based nanocontainers.

monomers. Theoretical calculation of binding energies was conducted, further proving our conclusion. The physical/ chemical properties of the obtained polymer nanocontainers were also investigated, which have shown great performance in bio- and energy-related applications. Synthesis of Zn(ZIF‑8)-PDA Nanocontainers. To synthesize the Zn(ZIF‑8)-PDA nanocontainers, the ZIF-8 nanostructure was first chosen as template. The synthesis process is mentioned in the experimental section in the Supporting Information. Zn(NO3)2·6H2O and 2-methylimidazole methanol solution were mixed together and held at room temperature.25 Then, the white ZIF-8 nanocrystals were isolated by centrifugation and redispersed in methanol. After adding DA and being refluxed at 55 °C for 7 h, the resultants, Zn(ZIF‑8)-PDA nanocontainers, were obtained by centrifugation and washing with methanol for several times. Electron microscopies were conducted to characterize the morphology, as shown in Figure 1. The TEM image shows that the obtained

Figure 1. TEM images of ZIF-8 nanostructures (A) and Zn(ZIF‑8)-PDA nanocontainers (B). SEM images of Zn(ZIF‑8)-PDA nanocontainers (C) and a magnified broken nanocontainer (D).

Zn(ZIF‑8)-PDA nanocontainers display a dodecahedral shape which is very similar to ZIF-8 templates (Figure 1A), while the brighter contrast of the inner part is attributed to the hollow structure (Figure 1B). The achieved Zn(ZIF‑8)-PDA nanocontainers were uniform with an average diameter of 155 nm. The shells possess a smooth outline and the thickness of ca. 18 nm. The SEM image shows the total appearances of Zn(ZIF‑8)PDA nanocontainers (Figure 1C), and the hollow structure can be clearly observed through a broken structure (Figure 1D). Besides, the characteristic X-ray diffraction (XRD) patterns of ZIF-8 disappeared and were replaced by a diffuse scattering peak ranging from 20° to 30°, suggesting the formation of the amorphous shell on the nanocontainers (Figure S1A). FT-IR spectra also proved that such an amorphous structure was composed of polydopamine (Figure S1B). Mechanism Study of the CCIP Method. As reported in our previous work, the formation process of Zn(ZIF‑8)-PDA

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RESULTS AND DISCUSSION A dodecahedral Zn2+ chelated polydopamine (Zn(ZIF‑8)-PDA) nanocontainer was used as an example and carefully studied to better understand such a hollowing process for the construction of nonspherical polymer nanocontainers. ZIF-8 and dopamine (DA) were chosen as the template and monomer materials, both of which played important roles in the system. The reaction conditions could be precisely controlled, which have a great effect on the structure of Zn(ZIF‑8)-PDA nanocontainers. The generality of the CCIP method was studied, which could be expanded to a series of polymer nanocontainers with different shapes and structures by changing templates and 6537

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Chemistry of Materials nanocontainers could be concluded as follows:35 DA grabbed the Zn2+ from ZIF-8, leading the disassembly of the nanostructure, and the released 2-methylimidazole (2-MIL) triggered the polymerization of DA to form PDA nanocontainers. To prove the chelation behavior, elemental distributions before and after the formation of Zn(ZIF‑8)-PDA nanocontainers are characterized in Figure S2. As the basic component of ZIF-8, element Zn (green) displays a dodecahedral shape and distributes throughout the whole nanostructure (Figure S2B). After the formation of Zn(ZIF‑8)PDA nanocontainers, the retention of Zn in the shell can still be observed clearly, exhibiting a hollow structure, due to the strong chelation between the catechol group and Zn2+.35 Similar hollow distributions could be observed in elements N and O (Figure S2G,H), which are derived from amino and catechol groups in polymer shell. The hollowing process is also monitored to help understand the mechanism. Figure 2 shows

method. As shown in Figure 3, Zn2+ displays tetradentate bonds to one of the N in each 2-MIL, and the binding energy (Eb) of

Figure 3. Theoretical calculated binding energy of Zn-2-MIL (A) and Zn-Dopamine (B).

such a unit (Zn-2-MIL) is calculated to −18.69 eV (Figure 3A). After adding DA into the system, the chelation interaction between the catechol group and Zn2+ can reach −35.97 eV (Figure 3B), which is much lower than Zn-2-MIL, actuating the occurrence of grabbing behavior. The weak alkaline environment provided by the released 2-MIL from the disassembled ZIF-8 nanostructures and the dissolved oxygen in solvent can trigger the polymerization of DA, forming Zn(ZIF‑8)-PDA nanocontainers. The control experiments have been conducted to confirm the conclusion proposed above. First, poly(vinyl alcohol) (PVA), a polymer (no need for further polymerization) with abundant and only −OH groups, is chosen to replace the DA monomer, in order to verify the stronger chelation ability between Zn and −OH groups than Zn-2-MIL. As shown in Figure S4B, after adding PVA into the ZIF-8 solution and reacting overnight, the structure of ZIF-8 was all disappeared, replaced by fusiform ZnO nanoparticles which were formed under alkaline and heat environment.37 Another control experiment was conducted to prove the role of binding energy in triggering the hollowing reaction. Aniline was chosen as monomer, due to the theoretical calculation results showing that the binding energy of both monodentate (−7.20 eV, Figure S5A) and tetradentate (−15.37 eV, Figure S5B) status of Zn-Aniline display higher binding energy than Zn-2-MIL (−18.69 eV, Figure 3A). Such results suggest that the chelation ability in Zn-Aniline is weaker than that in Zn-2-MIL, which cannot trigger the hollowing process. The experimental phenomenon also shows the consistent result. After adding aniline, the color of the solution turned from white to green, indicating the occurrence of the polymerization. However, as shown in Figure S5C, no hollow structure was observed, but only lots of small particles sticking on the surface of ZIF-8, suggesting that no hollowing occurs, but only redox polymerization of the monomer. Aniline cannot grab Zn2+ from the ZIF-8 nanostructure and form polyaniline (PANI) nanocontainers, only generating homogeneous PANI nanoparticles in Figure S5C, proving that the lower binding energy (stronger chelation ability) is the key factor for triggering the hollowing process. Therefore, the synthetic mechanism of the CCIP method can be concluded as follows: metal cations have lower binding energy with a monomer than that with organic linkers in template materials, suggesting a more stable status in metal cations-monomer. After adding monomer, the metal ions are grabbed and templates are disassembled, leading to the occurrence of the hollowing process. Simultaneously, the

Figure 2. XRD patterns of the products after adding DA into the reaction system for 0 h (red), 2 h (purple), 4 h (brown), and 7 h (blue), respectively.

the XRD patterns at different reaction times. With the reaction processing, the diffraction peaks of ZIF-8 (Figure 2, line a) gradually reduce and finally disappear, being replaced by scattering diffraction of the amorphous polymer shell (Figure 2, line d). Moreover, the morphology changes under different reaction times were also characterized by TEM, as shown in Figure S3. At the beginning, there were only some amorphous particles on the surface of the ZIF-8 nanostructure (Figure S3B), comparing to their original smooth one (Figure S3A). A “shell” was formed after 30 min (Figure S3C), with the “core” size gradually reduced and finally disappeared with the reaction processing (Figure S3D−F). Since the ZIF-8 “core” was gradually dissolved during the synthesis process, rattle-like intermediates with different core sizes could be easily achieved by simply terminating the reaction at a specific time during the formation of hollow nanocontainers. Despite that a series of experimental characterizations have been conducted, the synthetic mechanism of the CCIP method still needs in-depth investigation. The occurrence of disassembly suggests the stronger chelation ability of DA to Zn2+ than the ZIF-8 nanostructure, which can be further ascribed to the lower binding energy between Zn2+ and the catechol group of DA (Zn-Dopamine) than the N in 2-MIL (Zn-2-MIL). The structures and binding energies were calculated at the B3LYP/LanL2DZ level by using the Gaussion 09 program, and more details are provided in the Supporting Information.36 We expect that these theoretical calculations can be further utilized to guide the generality study of the CCIP 6538

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Chemistry of Materials polymerization of the monomer is triggered by metal ions or organic linkers from templates, forming polymer nanocontainers with nonspherical morphologies (Scheme 1). Scheme 1. Schematic Illustration of the CCIP Method

Figure 4. TEM images of ZIF-67 (A), Co(ZIF‑67)-PDA (B), ZIF-11 (C), and Zn(ZIF‑11)-PDA (D).

nanocontainers (Figure 4B). The as-prepared Zn(ZIF‑11)-PDA maintained the shape of the frameworks, while the interior of which were hollowed (Figure 4D). Other MOFs, such as MIL-101 (composed of Fe3+ and H2BDC, Figure 5A), could also be employed to prepare PDA nanocontainers. According to the theoretical calculation, the binding energies of Fe3+-H2BDC and Fe-Dopamine are −41.96 and −73.34 eV, respectively, showing the feasibility of Fe(MIL‑101)-PDA (Figure 5B). In addition, template materials can also jump out of MOFs to a series of metal−organic chelation compounds (such as Cu(NH2CH2CH2NH2)2). Theoretical calculations indicate the lower binding energy between Cu-Dopamine (−38.00 eV) than Cu-[(CH2NH2)2]2 (−17.67 eV) in Chart 1, which are ascribed to the strong chelation of DA and PDA to multivalent metal cations (Zn2+, Cu2+, Mn3+, Fe3+, etc.).38 TEM images also show that PDA nanocontainers with rectangular frame morphology were successfully obtained, retaining the original shape of the template materials (Figure 5C,D). Not only can PDA nanocontainers with different morphologies and chelated metal cations be exploited but also other

Generality Study of CCIP Method for Forming Nonspherical Polymer Nanocontainers. The generality of the CCIP method is studied next. The strategy is first expanded for constructing a series of PDA nanocontainers by replacing template materials, since the hollowing process is attributed to the different binding energy between metal cations and organic ligands (monomer or organic linkers) as mentioned in the Mechanism Study of the CCIP Method section. On the basis of ZIF-8, other ZIFs nanostructures with different metal cations, for example, ZIF-67 and ZIF-11, are chosen as substitutes (ZIF67 is constructed by Co2+ and 2-MIL, and ZIF-11 is composed of Zn2+ and benzimidazole (BenzMIL)). After the theoretical calculation (listed in Chart 1), metal cations-Dopamine shows lower binding energy than that in template materials (Chart 1), suggesting the feasibility of the reaction. Then, the hollowing experiments are conducted to prove the calculation. As shown in Figure 4, hollow structures could be clearly observed in Co(ZIF‑67)-PDA, indicating the successfully preparation of

Chart 1. Structure and the Corresponding Binding Energy of Metal−Organic Linkers in Template Materials and Metal CationsDopamine

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Figure 5. TEM images of MIL-101 (A), Fe(MIL‑101)-PDA (B), Cu(NH2CH2CH2NH2)2 (C), and the obtained Cu-PDA (D).

polymer nanocontainers can be prepared by similar hollow processing. For example, polypyrrole (Zn(ZIF‑8)-PPY) nanocontainers are prepared by choosing pyrrole as monomer and ZIF-8 as template (Figure 6A). Moreover, other types of PPY nanocontainers are successfully prepared by changing different templates (Figure 6B,C). The parameters of the obtained PPY nanocontainers (size, shell thickness, etc.) can be adjusted by regulating the reaction conditions. Therefore, we can further speculate that more types of polymer nanocontainers can be prepared, as long as the system meets the demands of the CCIP mechanism. Properties Characterization and Application Prospect of the Polymer Nanocontainers. In the above sections, the mechanism of the CCIP method has been clarified, and the method’s generality has also been proved successfully. It is worth noting that not only did we exploit a method which provides a platform for a series of polymer nanocontainers synthesis but also these as-prepared nanocontainers show significance in many application fields. First, the combination of nanostructure and the inherent properties of the materials can be utilized to realize the multiple functions. For example, Zn(ZIF‑8)-PDA, a typical nanostructure in our system, has displayed great photothermal behavior due to the strong nearinfrared absorption of PDA.39 As shown in Figure 7A, after being irradiated under 808 nm NIR laser for 10 min, Zn(ZIF‑8)PDA with different concentrations have exhibited a temperature increase of 8.8, 17.2, and 26.8 °C, respectively, while the temperature of pure water increased only 1 °C under the same laser exposure. In addition, the drug loading behavior of the obtained Zn(ZIF‑8)-PDA was also inspected. Doxorubicin

Figure 7. Temperature elevation of Zn(ZIF‑8)-PDA with different concentrations under exposure to an NIR laser (808 nm, 2 W cm−2) for 10 min and measured every 30 s (A). UV−vis absorbance spectra of Zn(ZIF‑8)-PDA loaded with different concentrations of DOX (B); the inset is the quantification of DOX loading at different ratios. Error bars are based on at least triplicate measurements.

(DOX), a typical anticancer drug, was mixed with Zn(ZIF‑8)PDA nanocontainers under room temperature for 24 h. After centrifugation at 7000 rpm for 10 min to remove the excess unbound drug, the obtained Zn (ZIF‑8) -PDA-DOX were redispersed in water. Figure 7B shows the UV−vis−NIR absorption spectra of Zn(ZIF‑8)-PDA-DOX with different loading concentrations. The existence of the characteristic peak of DOX at about 490 nm suggests that the drug has been successfully loaded to the Zn(ZIF‑8)-PDA nanocontainers. The corresponding loading capacity has been measured by the widely adapted approach of subtracting the absorbance of unbounded DOX. A positive correlation between loading efficiency and the addition of DOX can be observed, while the

Figure 6. TEM images of Zn(ZIF‑8)-PPY (A), Co(ZIF‑67)-PPY (B), and Fe(MIL‑101)-PPY (C). 6540

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Figure 8. Cycling performance and the corresponding CE at 100 mA g−1 (A), charge−discharge voltage profiles (B), cyclic voltammograms (C), and the rate capability (D) of the Fe3O4/C nanocages.

materials in a Li-ion battery. Cycling at 100 mA g−1, Fe3O4/C nanocages exhibit a high initial discharge capacity of 2275 mAh g−1 (Figure 8B) and deliver a reversible capacity of 1278 mAh g−1 (Figure 8A) after 90 cycles, which benefits from the incorporation of functional Fe3O4 NPs (theoretical capacity of 924 mAh g−1), improved conductivity (Figure S8) and stability of the structure.40 Cyclic voltammograms show negligible changes of redox peaks among three cycles, indicating the good reversibility of the electrochemical reaction (Figure 8C).41,42 Moreover, Fe3O4/C nanocages also show an excellent rate capacity of 603 mAh g−1 when current density reaches 3000 mA g−1 (Figure 8D), suggesting great promise in Li-ion batteries. All of these results show that the as-prepared nanocontainers have shown great potential in a variety of application fields, further proving the significance and value of the method.

saturated maximum loading capacity is ca. 56.3% (inset in Figure 7B). Such high loading efficiency is attributed to the interaction such as π−π stacking between DOX and the PDA, and the physical adsorption from the large interior of the nanocontainers. All of these excellent properties endow Zn(ZIF‑8)-PDA with a great promise as multifunctional nanomaterials in biomedical applications. Second, rather than merely limited to the biological field, the obtained polymer nanocontainers can greatly broaden their application prospect by utilizing the properties of chelated metal through further processing. After carbonizing the Co(ZIF‑67)-PDA at 550 °C under N2 flow, the Cobalt oxide/C nanocontainers were obtained (Figure S6A), which were used as anode materials in Li-ion batteries (LIBs). Figure S6B shows the cycling performance and the corresponding Coulombic efficiency (CE) of the Cobalt oxide/C nanocontainers, which is further achieved from Co(ZIF‑67)-PDA by carbonizing under N2 flow. The first discharge capacity of Cobalt oxide/C was 1301 mAh g−1 and stable at 710 mAh g−1 with the prolonging of cycling. The corresponding CE reached and maintained above 95% after 3 cycles. The good cycling performance and the stable retention of the Cobalt oxide/C nanocontainers were ascribed to the transformation from metal cation (Co2+) to metal oxide and the hollow structure obtained from our CCIP method, providing a buffer for the volume change during the charge/discharge process. Third, the functionalities of the as-prepared polymer nanocontainers can also be enhanced and tailored by incorporating functional nanoparticles. After modifying with PVP, nanoparticles could be encapsulated into the template materials.25 As shown in Figure S7A, Fe3O4 nanoparticles were successfully introduced into the ZIF-8 template materials and reserving in the nanocontainers. After carbonizing under argon at 950 °C, the obtained Fe3O4/C nanocages not only maintain the dodecahedral morphology perfectly well (Figure S7B) but also show superior electrochemical performance as anode

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CONCLUSION

In conclusion, a facile and effective strategy has been demonstrated to prepare various polymer nanocontainers in a controllable way. The synthetic mechanism has been studied and verified by theoretical calculation of binding energy between metal cations and the chelated groups. A broad range of templates and monomers are applicable for preparation of nanocontainers with multiple geometric morphologies and properties. The as-prepared polymer nanocontainers encompass the benefit from the specific structures, intrinsic properties of the polymer shell, and the tailorable functional metal cations and have shown excellent performance in biomedical and energy-related applications. We believe that these functional nanocontainers can be further applied to versatile practical fields. 6541

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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/acs.chemmater.7b02274. Experimental section, XRD patterns, FT-IR spectrum, elemental mapping images, TEM images, theoretical calculation of binding energy, cycling performance, and electrochemical impedance spectra (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Hu-Jun Qian: 0000-0001-8149-8776 Kai Zhang: 0000-0002-5507-3528 Hongchen Sun: 0000-0002-5572-508X Bai Yang: 0000-0002-3873-075X Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the Major Project of the Ministry of Science and Technology of China (2016YFC1102800), NSFC (81320108011), and the contribution to this work by H.-J.Q. was supported by the National Natural Science Foundation of China (21374003, 21522401). The authors also thank Ying Wang and the High Performance Computing Center of Jilin University for providing computing resources.

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