Double-Shelled Polymer Nanocontainers Decorated with Poly

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Double-Shelled Polymer Nanocontainers Decorated with Poly(ethylene glycol) Brushes by Combined Distillation Precipitation Polymerization and Thiol−Yne Surface Chemistry Guo Liang Li,*,†,‡ Ran Yu,§ Tao Qi,*,† Helmuth Möhwald,‡ and Dmitry G. Shchukin∥ †

National Engineering Laboratory for Hydrometallurgical Cleaner Production Technology, Institute of Process Engineering, Chinese Academy of Sciences, Beijing 100190, P. R. China ‡ Max Planck Institute of Colloids and Interfaces, Wissenschaftspark Golm, 14476 Potsdam, Germany § Institute of Chemistry, Chinese Academy of Sciences, Beijing 100190, P. R. China ∥ Stephenson Institute for Renewable Energy, Department of Chemistry, University of Liverpool, Crown Street, Liverpool L69 7ZD, United Kingdom S Supporting Information *

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nanospheres.36−38 Herein we report a synthetic strategy to construct a multifunctional container with many beneficial factors such as uniform size, well-defined morphology, and enhanced surface function via combined distillation precipitation polymerization and thiol−yne “click” chemistry. Distillation precipitation copolymerization was explored to provide a cross-linked shell with “clickable” alkyne groups on silica template surfaces. The thiol−yne coupling reaction was carried out through grafting of thiol-terminated PEG chains to the container surfaces, leading to double-shelled nanocontainers with enhanced PEG brushes on surfaces upon removal of the silica templates from the core−double shell hybrid nanospheres.

INTRODUCTION Nanocontainers with hollow nanostructures have attracted tremendous attention because of diverse applications in drug delivery, confined catalysis, and energy storage and as selfhealing composite materials.1−8 Nanocontainers with beneficial factors of precisely controlled size, narrow size distribution, sophisticated structure, and surface function can help for a better understanding of the structure−property relationship and for further improving practical applications.9−15 However, it is very challenging to have all these beneficial factors in one nanocontainer system. Moreover, the fate of nanocontainers in biological systems depends on their surface functions such as hydrophobicity/hydrophilicity, roughness, adhesion, or biocompatibility.16 For instance, surface decoration with poly(ethylene glycol) (PEG) chains helps nanoparticles to evade the reticuloendothelial system (RES) uptake and thus to circulate in the blood for a longer time.17−19 It is highly demanded to explore efficient approaches to functionalize the surfaces of nanocontainers with polymer brushes,20 and decoration of PEG brushes on substrate surfaces via “living” radical polymerization such as ATRP and RAFT has been reported.21,22 However, the grafting density of polymer brushes on particle surfaces is still limited, which is insufficient for defined surface properties in specific applications. This is probably due to the low reactive anchor sites or surface reaction efficiency, and it will be improved in this work. On the one hand, “click” chemistry including Cu(I)-catalyzed Huisgen 1,3-dipolar cycloaddition and thiol−ene coupling reactions with advantages of mild condition and high efficiency provides a robust synthetic strategy for complex molecular structures and functional materials.23−32 In contrast to the Cu(I)-catalyzed Huisgen 1,3-dipolar cycloaddition and thiol− ene coupling reactions, the thiol−yne reaction allows for addition of two thiol groups to one alkyne site.33−35 If this is the case, the double hydrothiolation of alkyne groups on container surfaces should allow for more efficient grafting of thiol-PEG brushes on the same reactive site anchored on container surfaces. On the other hand, distillation precipitation polymerization provides a robust approach for the efficient fabrication of surfactant-free, functional, and uniform micro/ © XXXX American Chemical Society

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EXPERIMENTAL SECTION

Materials. Propargyl methacrylate (PMA, Alfa Aesar, 98%), methacrylic acid (MAA, Sigma-Aldrich, 99%), and ethylene glycol dimethacrylate (EGDMA, Sigma-Aldrich, 98%) were purified by passing through an inhibitor removing column prior to being stored under an argon atmosphere at −10 °C. 2,2′-Azobis(isobutyronitrile) (AIBN,WAKO, 97%) was recrystallized from methanol. Tetraethyl orthosilicate (TEOS, Sigma-Aldrich, 98%), 3-(trimethoxysilyl)propyl methacrylate (MPS, Sigma-Aldrich, 98%), 2,2-dimethoxy-2-phenylacetophenone (DMPA, Sigma-Aldrich, 99%), O-[2-(3-mercaptopropionylamino)ethyl]-O′-methylpoly(ethylene glycol) 5000 (PEGSH, Sigma-Aldrich, Mn = 5000 g/mol), hydrofluoric acid (HF, Riedelde Häen, 48%), ammonia solution (NH3·H2O, Merck, 25 wt %), acetonitrile (Merck, HLPC grade), and dimethylformamide (DMF, Merck, HPLC grade) were used as received without further purification. Synthesis of SiO2@Poly(propargyl methacrylate-co-methacrylic acid-co-ethylene glycol dimethacrylate) Core−Shell Hybrid Nanospheres by Distillation Precipitation Copolymerization. Initially, carbon−carbon double bond modified silica templates were prepared via sol−gel reaction according to the Stöber method.39,40 Then, well-defined silica@poly(propargyl methacrylateco-methacrylic acid-co-ethylene glycol dimethacrylate) (SiO2@poly(PMA-co-MAA-co-EGDMA)) core−shell nanospheres were prepared by precipitation copolymerization of propargyl methacrylate (PMA) Received: November 5, 2015 Revised: January 11, 2016

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DOI: 10.1021/acs.macromol.5b02406 Macromolecules XXXX, XXX, XXX−XXX

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Scheme 1. Schematic Illustration of the SiO2@Poly(PMA-co-MAA-co-EGDMA)-click-PEG Core−Double Shell Nanospheres via Combined Distillation Precipitation Polymerization and Thiol−Yne Surface “Click” Reaction

Figure 1. FESEM and TEM images of the silica templates (A, B) and SiO2@poly(PMA-co-MAA-co-EGDMA) core−shell nanospheres (C, D). was introduced into the flask. Then, the polymerization temperature was increased to the reflux temperature of the reaction mixture in 30 min. The polymerization reaction continued for 6 h under reflux conditions, and finally half of the solvent was distilled out of the polymerization system. The resulting core−shell hybrid nanospheres were collected by centrifugation and cleaned by three cycles of

with methacrylic acid (MAA) and ethylene glycol dimethacrylate (EGDMA) in acetonitrile. Briefly, 0.30 g of the silica templates were dispersed in 60 mL of acetonitrile with the aid of sonication for 0.5 h in a 100 mL two-neck round-bottom flask, equipped with a reflux condenser. A mixture of PMA (0.50 mL, 4 mmol), MAA (0.34 mL, 4 mmol), EGDMA (0.38 mL, 2 mmol), and AIBN (25 mg, 0.15 mmol) B


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Macromolecules washing using 40 mL of an acetone/ethanol mixture to remove the unreacted monomers and oligomers. The as-prepared SiO2@poly(PMA-co-MAA-co-EGDMA) core−shell nanospheres were finally dried overnight in a vacuum oven at 50 °C. Surface Grafting of PEG Brushes on the SiO2@poly(PMA-coMAA-co-EGDMA) Core−Shell Hybrid Nanospheres via Thiol− Yne Surface Reaction. About 0.15 g of the SiO2@poly(PMA-coMAA-co-EGDMA) core−shell hybrid nanospheres and 0.40 g of thiolterminated PEG chains (PEG-SH, Mn = 5000 g/mol) were introduced under sonification into 10 mL of DMF in a Pyrex tube. The reaction mixture was degassed with argon for 15 min. Then, 0.08 g of the photoinitiator DMPA (20 wt % relative to the PEG-SH) was added into the reaction mixture. The Pyrex tube was sealed under an argon atmosphere. The reaction was performed by irradiation with a highpressure mercury lamp (Riko rotary photochemical reactor, RH 40010W, Japan) at room temperature for 3−6 h. The resulting SiO2@ poly(PMA-co-MAA-co-EGDMA)-click-PEG core−double shell nanospheres were washed three times with a mixture of DMF, ethanol, and acetone to remove the unreacted PEG-SH chains. The dispersion of SiO2@poly(PMA-co-MAA-co-EGDMA)-clickPEG core−double shell nanospheres in 10 mL of 20% HF was stirred at room temperature for 24 h to remove the silica template core. The excess of HF and SiF4 was removed from the samples by dialysis in deionized water for 1 week. Finally, the double-shelled nanocontainers were collected by freeze-drying. Characterization. Field-emission scanning electron microscopy (FESEM) was performed with a JEOL JSM-6700 SEM. Transmission electron microscopy (TEM) images were obtained with a JEOL JEM2010 TEM. X-ray photoelectron spectroscopy (XPS) measurements were carried out with a Kratos AXIS Ultra HSA spectrometer equipped with a monochromatized Al Kα X-ray source (1468.6 eV photons). Fourier-transform infrared (FT-IR) spectroscopy analysis was carried out with a Shimadzu 8400 FT-IR spectrophotometer over a range of 4000−400 cm−1. Dynamic laser scattering (DLS) measurements were performed with a Brookhaven 90 plus laser light scattering spectrometer at a scattering angle of θ = 90°. The hydrodynamic diameter of the nanospheres was obtained by averaging over five measurements. The polydispersity index (PDI), or the size distribution, of the nanospheres was determined with the following statistical formulas: k

PDI = Dw /Dn

Dn =

k

∑ niDi /∑ ni i=1

i=1

k

Dw =

Table 1. Size, Size Distribution, and Shell Thickness of the As-Synthesized Nanospheres sample

SiO2 templates SiO2@poly(PMA-co-MAAco-EGDMA) SiO2@poly(PMA-co-MAAco-EGDMA)-click-PEG

shell thicknessc (nm)

Dnb (nm)

Dhb (nm)

PDI

223 271

227 315

1.11 1.24

24

6 7

287

352

1.19

24/8

9

b

CVd (%)

a

PMA = propargyl methacrylate, PEG = poly(ethylene glycol). bDn is the number-average diameter from FESEM images, Dh is the hydrodynamic diameter in ethanol from dynamic light scattering (DLS), and PDI is the polydispersity index. cThe shell thickness of silica@polymer nanospheres was measured from the FESEM images. d CV is the coefficient of variation or ratio of the standard deviation to the mean of nanoparticle size (CV = δ/Dn).

Figure 2. FT-IR spectra of the SiO2@poly(PMA-co-MAA-coEGDMA) (A), SiO2@poly(PMA-co-MAA-co-EGDMA)-click-PEG (B), and air@ poly(PMA-co-MAA-co-EGDMA)-click-PEG nanocontainers (C).

k

∑ niDi 4 /∑ niDi 3 i=1

a

i=1

with poly(ethylene glycol) (PEG) brushes on surfaces is illustrated in Scheme 1. Initially, uniform SiO2@poly(PMA-coMAA-co-EGDMA) core−shell hybrid nanospheres were synthesized using the silica templates by sol−gel reaction of tetraethyl orthosilicate and 3-(trimethoxysilyl)propyl methacrylate, followed by distillation precipitation copolymerization of propargyl methacrylate with methacrylic acid and ethylene glycol dimethacrylate in acetonitrile. The SiO2@poly(PMA-coMAA-co-EGDMA)-click-PEG core−double shell hybrid nanospheres were further synthesized by thiol−yne surface reaction between PEG-SH chains and alkyne groups on the core−shell surfaces. Finally, well-defined double-shelled nanocontainers with PEG brushes on surfaces were obtained after removal of the silica core templates. The FESEM and TEM images of the silica templates and SiO 2 @poly(PMA-co-MAA-co-EGDMA) core−shell nanospheres are shown in Figure 1. The size of the core−shell hybrid nanospheres after precipitation copolymerization of PMA on silica templates increases to 271 nm from 223 nm with a coefficient of variation (CV) of 6−7%. The size and size distribution of the nanospheres are summarized in Table 1. The spherical shape and uniform morphology are well retained after the polymerization reaction. The TEM image in Figure 1d reveals a polymer shell encapsulating a dense silica core, giving

Dn is the number-average diameter, Dw is the weight-average diameter, and Di is the particle diameter. The calculation is based on the cumulative diameters of 50−100 nanoparticles in the SEM images. The coefficient of variation (CV = δ/Dn), defined as the ratio of the standard deviation to the mean, was used to estimate the error in nanoparticle size of Dn. Thermogravimetric analysis (TGA) was carried out with a thermogravimetric analyzer (TA Instruments, Model 2050) at a heating rate of 10 °C/min in nitrogen. The surface grafting density in terms of the number of chains per unit surface area (Ds, chains/nm2) was calculated based on a unit weight of the nanospheres, according to the equation of Ds = number of polymer chains/unit area = WNa/MnS (W, Na, Mn, and S are the graft amount, Avogadro’s number, molecular weight, and surface area, respectively). The graft amount, W, was obtained from weight loss variations between the SiO2, SiO2@poly(PMA-co-MAA-co-EGDMA), and SiO2@poly(PMAco-MAA-co-EGDMA)-click-PEG nanospheres in TGA measurements. The surface area, S, was calculated from the average size and number of nanospheres per unit weight. The latter in turn was estimated from the density of the core−shell nanospheres (1.5 g/cm3, from the average density of the silica core and the polymer shell) and the average size of the core−shell nanospheres.

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RESULTS AND DISCUSSION The procedure for fabrication of the SiO2@poly(PMA-coMAA-co-EGDMA)-click-PEG core−double shell nanospheres C


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Figure 3. XPS spectra with wide scan (A) and C 1s core level (B) of the SiO2@poly(PMA-co-MAA-co-EGDMA) core−shell nanospheres.

rise to a distinctive core−shell nanostructure because of different contrast between polymer shell and inorganic silica core. The thickness of the polymer shell derived from the FESEM images is around 24 nm. To ascertain that the asfabricated SiO2@poly(PMA-co-MAA-co-EGDMA) core−shell nanospheres possess functional or “clickable” alkyne groups on the copolymer shell, the core−shell nanospheres were further characterized by FT-IR and XPS spectroscopy. The FT-IR spectra of the silica template before and after polymer shell growth are shown in Figure 2. The absorption bands at 1103, 803, and 471 cm−1 correspond to the asymmetric stretching, the symmetric stretching/bending, and the rocking modes of the Si−O−Si bridges. The absorption peak at 2130 cm−1 is associated with alkyne groups on the exterior surface of the core−shell hybrid nanospheres.41 Furthermore, Figures 3a and 3b show the XPS wide-scan and C 1s core-level spectra of the SiO 2 @poly(PMA-co-MAA-co-EGDMA) core−shell nanospheres, respectively. The photoelectron lines at binding energy (BE) of about 102, 153, 284, and 530 eV are associated with the Si 2p, Si 2s, C 1s, and O 1s species. The C 1s core-level spectrum of the SiO2@poly(PMA-co-MAA-co-EGDMA) core− shell nanospheres in Figure 3b can be curve-fitted with four peak components having BE at about 284.6, 285.2, 286.2, and 288.6 eV, attributable to the C−C/C−H, CC, C−O, and OC−O species, respectively.42 The peak signal of the −C C group confirms that there are “clickable” sites on the core− shell hybrid particle surfaces. The thiol-terminated poly(ethylene glycol) chains (PEG-SH) are selected for the thiol−yne surface reaction because of the high demand of surface PEGylation of nanospheres. The thiol− yne coupling reaction can be regarded as a “grafting to” process. One advantage of the “grafting to” approach is that the

Figure 4. 1H NMR spectra of the SiO2@poly(PMA-co-MAA-coEGDMA) core−shell hybrid nanospheres (a) before and (b) after grafting of the PEG brushes on surface via thiol−yne reaction.

molecular information on grafted polymer chains can be known before surface coupling to the substrate. The PEG-SH telechelic chains with reactive thiol end groups and molecular weight Mn of 5000 g/mol are characterized by X-ray photoelectron spectroscopy (XPS) and FT-IR spectroscopy (Supporting Information, Figures S1 and S2, respectively). The SiO 2 @poly(PMA-co-MAA-co-EGDMA)-click-PEG doubleshelled nanoparticles were synthesized via grafting of PEG chains onto the SiO2@poly(PMA-co-MAA-co-EGDMA) core− shell nanospheres by the thiol−yne coupling reaction. The FTIR absorption spectrum of the SiO2@poly(PMA-co-MAA-coEGDMA)-click-PEG core−double shell nanospheres is shown in Figure 2b. Upon surface grafting of PEG-SH brushes on the nanocontainer surfaces, the stretching vibration peak of alkyne groups at 2130 cm−1 disappears, and a new absorption band at 2986 and 2921 cm−1associated with −CH3 and −CH2−CH2− groups from PEG chains appears simultaneously. It is hard to discern the characteristic absorption of the CH2−O−CH2 groups, which are covered by the absorption of the Si−O−Si bonds in the range of 1100−1300 cm−1. The successful surface double hydrothiolation of alkyne groups on surfaces is further confirmed by 1H NMR spectroscopy of the SiO2@poly(PMAco-MAA-co-EGDMA) and SiO 2 @poly(PMA-co-MAA-coEGDMA)-click-PEG in Figure 4. Before the thiol−yne coupling reaction, the chemical shifts for the proton at 4.0 and 3.6 ppm D


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Figure 5. FESEM and TEM images of the SiO2@poly(PMA-co-MAA-co-EGDMA)-click-PEG core−double shell nanospheres.

co-EGDMA)-click-PEG core−double shell nanospheres in Figure 5 suggest that the SiO2@poly(PMA-co-MAA-coEGDMA) core−shell nanospheres are covered by an outer shell of core−shell hybrid surfaces, leading to well-defined SiO2@poly(PMA-co-MAA-co-EGDMA)-click-PEG core−double shell nanospheres. To show that the thiol−yne “click” reaction can be regarded as a general approach for surface modification, the small molecule cysteamine (HSCH2 CH2NH2) was also hydrothiolated onto the alkyne groups of the SiO2@poly(PMA-co-MAA-co-EGDMA) nanospheres (Figure S3 and Table S1). The grafting density of PEG brushes on the SiO2@ poly(PMA-co-MAA-co-EGDMA)-click-PEG nanospheres was determined by the TGA difference before and after grafting of PEG telechelic chains onto SiO2@poly(PMA-co-MAA-coEGDMA) core−shell particle surfaces in Figure 6. It is calculated to be 0.95 chains/nm2, which is significantly higher than previous report by alkyne−azide and thiol−ene “click” reaction (0.017−0.7 chains/nm2, Table 2).43−46 The grafting density is also comparable to that by alkyne−azide reaction between the same particles and telechelic PEG chains with azide groups (Figure S5). The particle materials, size, and surface chemistry are specified and summarized in Table 2. Although the grafted polymer brushes and surface reactive sites before “click” reaction in these functionalization approaches are different, it shows that the thiol−yne “click” reaction in the

Figure 6. TGA curves of the SiO2@poly(PMA-co-MAA-co-EGDMA) core−shell (A) and SiO2@poly(PMA-co-MAA-co-EGDMA)-click-PEG core−double shell nanospheres (B).

(e and d signals) due to the respective ester linkage near the alkyne and end proton of the alkyne groups from poly(propargyl methacrylate) segments are observed. After “click” reaction of the PEG-SH chains to core−shell nanospheres, the chemical shift at 4.0 ppm disappears. The new chemical shifts at 3.8 and 3.5 ppm (f′ and e′) correspond to the proton from −CH2CH2O− and −OCH3 groups of PEG brushes. Moreover, the FESEM and TEM images of the SiO2@poly(PMA-co-MAA-

Table 2. Grafting Density of Polymer Brushes on Particle Surfaces by “Click” Reactions

E


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Figure 7. FESEM and TEM images of the air@poly(PMA-co-MAA-co-EGDMA)-click-PEG nanocontainers (A−D) and air@poly(PMA-co-MAA-coEGDMA) nanocontainers (E, F).

shape47,48 is discernible in Figure 7a. The well-defined hollow structures of the synthesized nanospheres in Figures 7b and 7c confirm the fact that the silica core has been successfully removed from the hairy SiO 2 @poly(PMA-co-MAA-coEGDMA)-click-PEG core−double shell hybrid nanospheres. In contrast to the air@poly(PMA-co-MAA-co-EGDMA) core− shell nanospheres in Figures 7e and 7f, the air@poly(PMA-coMAA-co-EGDMA)-click-PEG nanocontainers after surface grafting of PEG chains (Figure 7c) show a distinctive doubleshelled hollow morphology. Energy-dispersive X-ray (EDX) measurements (Figure S4) were further utilized to investigate the Si signals before and after HF etching. In contrast to the EDX spectrum of the SiO2@poly(PMA-co-MAA-co-EGDMA)click-PEG nanospheres, the Si signal of the hairy air@ poly(PMA-co-MAA-co-EGDMA)-click-PEG hollow nanocon-

present work is an alternative efficient modification of particle surfaces. The high grafting density of PEG-SH chains on surfaces is probably due to the double hydrothiolation of alkyne groups with PEG-SH telechelic chains via the thiol−yne coupling reaction. Thus, it has offset the drawback of steric hindrance of surface grafting of telechelic polymers onto the particle surfaces during the “grafting to” process. The air@poly(PMA-co-MAA-co-EGDMA)-click-PEG nanocontainers decorated with PEG brushes on surfaces were fabricated via HF etching of the hairy SiO2@poly(PMA-coMAA-co-EGDMA)-click-PEG core−shell hybrid nanospheres to remove the silica template core. The morphology of hairy air@ poly(PMA-co-MAA-co-EGDMA)-click-PEG hollow nanocontainers is observed in FESEM and TEM images in Figures 7a−c. The red blood cell-like nanostructure with a biconcave F


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tainers disappeared. The mass content of Si decreases dramatically from 16.65% to 0.74% as the silica nanocore is removed. Thus, air@poly(PMA-co-MAA-co-EGDMA)-clickPEG nanocontainers with well-defined hollow nanostructure, uniform size, and hairy PEG brushes on the surface were obtained. Because of the efficient thiol−yne coupling reaction, the highly grafted PEG brushes on the cross-linked poly(PMAco-MAA-co-EGDMA) shell form a dense hairy outer shell.

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CONCLUSIONS A facile synthesis of double-shelled nanocontainers with highly functionalized PEG brushes on the surface has been demonstrated by combining distillation precipitation polymerization and thiol−yne coupling reaction. Many beneficial factors such as uniform size, complex hollow structures, and hairy functional surface are unified in one multifunctional container system. The functional PEG brushes can be grafted on the container surfaces with a grafting density of 0.95 chains/nm2. This high grafting density is probably due to the efficient hydrothiolation reaction of alkyne groups on nanoparticle surfaces. The combined precipitation polymerization and thiol−yne surface reaction provide a new and efficient synthetic strategy to construct double-shelled polymer nanocontainers with highly functional surfaces.

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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.macromol.5b02406. X-ray photoelectron spectroscopy (XPS), Fourier-transform infrared (FT-IR) spectroscopy of thiol-terminated poly(ethylene glycol) (PEG-SH) chains, energy dispersive X-ray (EDX) spectroscopy of the SiO2@poly(PMA-co-MAA-co-EGDMA)-click-PEG core−double shell nanospheres and the air@poly(PMA-co-MAA-coEGDMA)-click-PEG nanocontainers (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*E-mail [email protected] (G.L.L.). *E-mail [email protected] (T.Q.). Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS G.L. acknowledges financial support by the One Hundred Talent Program of the Institute of Process Engineering, Chinese Academy of Sciences, as well as by an Alexander von Humboldt Fellowship. We thank Professor Zebing Zeng for 1H NMR measurement. D.S. acknowledges the support from the ERC ENERCAPSULE project.

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REFERENCES

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