Metal-Free Photoinduced Electron TransferâAtom Transfer Radical

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Metal-Free Photoinduced Electron Transfer-Atom Transfer Radical Polymerization Integrated with Bio-inspired Polydopamine Chemistry as a Green Strategy for Surface Engineering of Magnetic Nanoparticles Yang Yang, Xuegang Liu, Gang Ye, Shan Zhu, Zhe Wang, Xiaomei Huo, Krzysztof Matyjaszewski, Yuexiang Lu, and Jing Chen ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b01863 • Publication Date (Web): 27 Mar 2017 Downloaded from http://pubs.acs.org on March 27, 2017

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ACS Applied Materials & Interfaces

Metal-Free Photoinduced Electron Transfer-Atom Transfer Radical Polymerization Integrated with Bio-inspired Polydopamine Chemistry as a Green Strategy for Surface Engineering of Magnetic Nanoparticles Yang Yanga, Xuegang Liua,b, Gang Yea,b,*, Shan Zhuc, Zhe Wanga, Xiaomei Huoa, Krzysztof Matyjaszewskid,*, Yuexiang Lua,b, Jing Chen a,b,* a

Collaborative Innovation Center of Advanced Nuclear Energy Technology, Institute of Nuclear

and New Energy Technology, Tsinghua University, Beijing, 100084, China b

Beijing Key Lab of Radioactive Waste Treatment, Tsinghua University, Beijing, 100084, China

c

State Key Laboratory of Chemical Engineering, Department of Chemical Engineering,

Tsinghua University, Beijing, 100084, China d

Department of Chemistry, Carnegie Mellon University, 4400 Fifth Avenue, Pittsburgh,

Pennsylvania 15213, United States KEYWORDS. Photoinduced atom transfer radical polymerization; surface-initiated polymerization; polydopamine; core-shell structure; magnetic nanoparticles.

ACS Paragon Plus Environment

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ABSTRACT. Developing green and efficient technologies for surface modification of magnetic nanoparticles (MNPs) is of crucial importance for their biomedical and environmental applications. This study reports, for the first time, a novel strategy by integrating metal-free photoinduced electron transfer-atom transfer radical polymerization (PET-ATRP) with the bioinspired polydopamine (PDA) chemistry for controlled architecture of functional polymer brushes from MNPs. Conformal PDA encapsulation layers were initially generated on the surfaces of MNPs, which served as the protective shells while providing an ideal platform for tethering 2-bromo-2-phenylacetic acid (BPA), a highly efficient initiator. Metal-free PET-ATRP technique was then employed for controlled architecture of poly (glycidyl methacrylate) (PGMA) brushes from the core-shell MNPs by using diverse organic dyes as photoredox catalysts. Impacts of light sources including UV and visible lights, photoredox catalysts, and polymerization time on the composition and morphology of the PGMA brushes were investigated. Moreover, the versatility of the PGMA-functionalized core-shell MNPs was demonstrated by covalent attachment of ethylenediamine (EDA), a model functional molecule, which afforded the MNPs with improved hydrophilicity, dispersibility and superior binding ability to uranyl ions. The green methodology by integrating metal-free PET-ATRP with facile PDA chemistry would provide better opportunities for surface modification of MNPs and miscellaneous nanomaterials for biomedical and electronic applications.

1. INTRODUCTION

Magnetic nanoparticles (MNPs) have received significant attention in recent years due to the favorable magnetic properties and accessibility for surface modification, which are potentially exploited for biomedical and environmental applications.1,

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Since bare MNPs are prone to


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agglomeration, and are chemically unstable by oxidation and corrosion when directly exposed to surrounding environment, substantial efforts have been made for surface engineering of MNPs, aiming to improve their dispersibility, stability and bio-compatibility, as well as to functionalize them for specific applications.3 Among the diverse established methodologies, surface-initiated polymerization (SIP) via polymer brushes architecture from the surfaces of MNPs has been recognized as an efficient and robust strategy.4 Particularly, by employing the living radical polymerization methods, such as atom transfer radical polymerization (ATRP),5,

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reversible

addition-fragmentation chain transfer (RAFT) polymerization,7-9 and nitroxide-mediated polymerization (NMP),10, 11 polymer brushes with controlled molecular weight (MW), narrow molecular weight distribution (MWD), high end-group functionality and complex architecture can be generated.12 Traditional ATRP relies on the use of transition-metal catalysts to regulate the equilibrium between an active propagating radical and a dormant alkyl halide, which maintains a low concentration of radicals in the system and minimizes bimolecular termination to achieve controlled growth of polymer brushes.13-15 But the presence of transition-metal residue would affect the physical and chemical properties of the polymer products, and especially impede their electronic and biomedical applications.16, 17 In addition, the rigorous deoxygenation procedure (multiple freeze−pump−thaw steps) and purification challenge limit the development and application of this technique on a large scale.18 Although, recently, some kinds of low-ppm catalyst ATRP techniques with enhanced oxygen tolerance and improved post-processes for purifying the trace metals have been developed,19 people are still seeking for more eco-friendly and sustainable techniques.20, 21


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Lately, inspired by the seminal work of Macmillan,22, 23 Yoon,24, 25 and Stephenson26-28 on photoredox catalysts (PCs) for organic transformation, Hawker’s group reported the living radical polymerization of methacrylates regulated by visible light using an Ir-based photoredox catalyst.29, 30 The photoinduced living radical polymerization technique immediately attracted worldwide interest, due to the advantages such as extremely mild conditions, tunable and accessible light sources, minimal side reactions, and precise spatial and temporal control over the polymerization process.31-34 While this technique provides an impressive advancement for surface modification of nanomaterials, the employment of metal catalysts remains problematic for a variety of applications like microelectronics and bio-compatible devices. To break this bottleneck, in 2014, Hawker’s group developed a metal-free visible light mediated ATRP technique using an organic photoredox catalyst (10-phenylphenothiazine, PhPTZ).35 Following this pioneering work, a series of metal-free photocatalytic ATRP systems have been reported by taking advantage of photosensitive compounds including fluorescein (FL),36 1’-diethyl-2,2’-cyanine iodide,37 methylene blue,38 eosin Y (EY),39 rhodamine 6G,40 and so on.41, 42 Moreover, based on this technique, controlled growth of polymer brushes from silicon substrates and SiO2 nanoparticles has been achieved.43, 44 It is envisioned that this metal-free photoinduced ATRP technique, due to the spatial and temporal control while eliminating the metal catalyst impurities, would provide a better opportunity for surface modification of MNPs, especially for biomedical and biological applications.21 However, to the best of our knowledge, so far, employing the metal-free visible light mediated ATRP for controlled architecture of polymer brushes from the surfaces of MNPs has never been reported. Another challenge for using surface-initiated polymerization lies in the effective generation of adequate initiating species with uniform distribution on the substrates.45 Unlike the silicon or


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silica substrates, to which a family of silane coupling agents can be used for anchoring the initiator molecules, MNPs require more efficient but mild approaches for initiator tethering, due to the less active sites on the surfaces and chemical instability. Recently developed polydopamine (PDA) chemistry inspired by the adhesive properties of mussel foot proteins paves a way to reach this end.46 The significance of PDA chemistry to materials science has been realized by more and more researchers since the innovative work in Messersmith’s group in 2007.47 By manipulating the self-polymerization of dopamine under basic media in the presence of oxygen, well-defined adhesive PDA coatings can be formed on virtually all surfaces.48, 49 Though the self-polymerization process of dopamine and the structure of PDA have not been fully elucidated,50 PDA chemistry has been accepted as a facile and talented method for surface modification.51 Moreover, the PDA coatings are biocompatible due to the structural similarity to melanin/eumelanin.52 And, with high density of catechol and imine/amine groups, PDA can serve as a functional platform for secondary reactions, i.e., anchoring of initiator molecules,53, 54 biomolecules,55 or polymer chains.56-58

Scheme 1 Illustrative synthesis of core-shell MNPs by the combination of surface-initiated PETATRP and PDA chemistry.


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Herein, we report, for the first time, a robust and eco-friendly strategy employing metal-free photoinduced electron transfer (PET) ATRP, integrated with the bio-inspired PDA chemistry, for surface modification of MNPs. The synthetic pathway is illustrated in Scheme 1. Pristine MNPs with uniform size were firstly obtained. A uniform PDA encapsulation layer was then built via the self-polymerization of dopamine (DA) in mild Tris buffer solution mediated by the addition of alcohol. The well-defined PDA shells can protect the magnetic cores from agglomeration and oxidation due to the reducing properties of the catechol units therein, and, facilitate the tethering of 2-bromo-2-phenylacetic acid (BPA), a highly efficient initiator.19 Then, metal-free PET ATRP was performed for controlled architecture of poly (glycidyl methacrylate) (PGMA) brushes from the surfaces of MNPs using diverse organic dyes as photoredox catalysts (Scheme 2). Impacts of the light sources, photoredox catalysts, and polymerization time on the composition and morphology of the PGMA brushes were investigated. Due to the rich epoxy functionalities, the PGMA functionalized core-shell MNPs could be ideal candidates for biomedical and environmental applications. We further demonstrated that the epoxy platform could be used for facile immobilization of reactive molecules such as ethylenediamine (EDA), affording the MNPs with enhanced hydrophilicity, dispersibility in aqueous media, and excellent binding ability to radioactive uranium. The strategy of metal-free PET ATRP mediated by the bio-inspired PDA chemistry, without complicated operation, harsh reaction conditions and metal catalyst contamination, can be a green synthetic method for surface modification of various nanomaterials as well as bulk surfaces.


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Scheme 2 Chemical structures of the organic photoredox catalysts, initiator and monomer employed in this study.

2. EXPERIMENTAL SECTION

2.1 Synthesis of Fe3O4@PDA. Monodisperse Fe3O4 MNPs were synthesized via a solvothermal reaction.59 The products were separated using a magnet and dried under vacuum. Then, dopamine was deposited on the surfaces of Fe3O4 to build up core-shell structure. 100 mL Tris buffer solution (20 mM, pH=8.5) was prepared in advance by dissolving tris (hydroxymethyl) aminomethane in ethanol/water mixture with a volume ratio of 3:7. Fe3O4 MNPs (0.10 g) were dispersed in the Tris buffer solution under ultrasonication. Dopamine hydrochloride (0.30 g, 1.6 mmol) was dissolved in 15 mL N,N-dimethylformamide (DMF). The obtained solution was dropwise added to the reaction mixture. The reaction lasted for 5 h under stirring (150 rpm) at room temperature. The products were collected by using a magnet, and were alternately washed with ethanol and deionized water. Finally, the Fe3O4@PDA products were placed in a vacuum oven (40 °C) for 12 h drying.


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2.2 Synthesis of Fe3O4@PDA-Br. 2-bromo-2-phenylacetic acid (BPA) was anchored to the surface of PDA shell to initiate the metal-free surface-initiated ATRP. 0.10 g dry Fe3O4@PDA was dispersed in 20 mL anhydrous DCM, followed by the addition of 0.02 g DMAP (0.16 mmol) and 0.34 g BPA (1.6 mmol). Under stirring, 0.33 g DCC (1.6 mmol) dissolved in 10 mL DCM was added dropwise into the flask in an ice bath. The temperature of the reaction system was gradually raised to room temperature. After 24 h, the BPA-anchored Fe3O4@PDA were collected by a magnet. Deionized water and ethanol were used for washing the products for three times. Finally, the products were dried at room temperature under vacuum for 24 h. 2.3 PGMA brushes growth via PET-ATRP. 50 mg Fe3O4@PDA-Br was dispersed in mixture of 6 mL (45.6 mmol) of GMA and 20 mL of DMSO in a three-necked flask equipped with a sealed casing pipe. Organic dyes as photoredox catalysts were added in specific ratio according to experiment design. By using a gas tight syringe, TEA dissolved 1 mL DMSO was injected to the reaction mixture. The mixture was covered with aluminum foil while degassed by nitrogen purging for 15 min. Then, the reaction mixture was irradiated with a Blue LED strip at 460 nm. Samples were taken from the reaction at predetermined time intervals. Reactions were terminated by exposure to air in the dark and the particles were collected by a magnet. After washing with THF and ethanol in turn, the Fe3O4@PDA@PGMA particles were dried at room temperature under vacuum overnight. The DMSO eluent was separated and the final polymer product was precipitated in methanol. After collected by filtration and dried at 50 °C under vacuum, the polymer was applied for GPC to determine the molecular weight. 2.4 Synthesis of Fe3O4@PDA@PGMA-EDA. 0.1 g Fe3O4@PDA@PGMA synthesized in 3 h were dispersed in 40 mL DMA containing 5 mL ethylenediamine (EDA). After stirring for 24 h at 50 °C, the final products Fe3O4@PDA@PGMA-EDA were separated by a magnet.


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Deionized water and ethanol were used to wash the particles. Then, the products were dried under vacuum for at least 12 h before use. 2.5 Uranium binding. U(VI) solutions (100 mg/L) were prepared by diluting concentrated U(VI) stock solution (196 g/L). 0.01 mol/L NaClO4 was used to dilute the U(VI) solutions for the control of ionic strength. NaOH or HNO3 solution was then added to adjust the pH. In general, the adsorption experiments were performed by batch operation in an air bath oscillator with temperature controlled at 25 °C. After shaking for a pre-determined time, the Fe3O4@PDA@PGMA-EDA MNPs were separated using a magnet. A 0.45 µm micro-pore membrane filter was used for separating the aqueous phase. The concentration of residual uranium was measured by the arsenazo III method with a 721-type spectrophotometer at 652 nm. Adsorption capacity (q) is defined as: (C 0 − C e ) × V M (1) where C0 and Ce are the initial and equilibrium U(VI) concentration in aqueous phase, q=

respectively. V represents the volume of the solution added to the plastic tubes, and M is the weight of the Fe3O4@PDA@PGMA-EDA (g, dry-basis). 3. RESULTS AND DISCUSSION Monodisperse Fe3O4 particles were synthesized according to a previously reported method.59 TEM image of the MNPs with a diameter of ~ 200 nm is shown in Figure 1(a). The particles could be easily dispersed in water and polar organic solvents. PDA protective layers were deposited on the MNPs in Tris buffer solution through the auto-oxidative polymerization of DA. The polymerization rate was controlled by the addition of ethanol,60 resulting in uniform PDA layers (thickness ~ 30 nm) on the surfaces of the MNPs (Figure 1(b)). The presence of a large


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number of imine/amine groups in PDA allowed the facile connection of initiator molecules for surface-initiated polymerization.50, 61 An efficient initiator 2-bromo-2-phenylacetic acid (BPA)43 was covalently immobilized through a mild reaction at room temperature using DCC as catalyst. The core-shell structure of the Fe3O4@PDA, after the initiator anchoring, was well-maintained (Figure 1(c)).

Figure 1 TEM images of (a) Fe3O4, (b) Fe3O4@PDA, (c) Fe3O4@PDA-Br and (d) Fe3O4@PDA@PGMA (FL-TEA, blue LED, 3 h). Scale bar=100 nm. Then, PGMA brushes were grown from the surface of Fe3O4@PDA via PET-ATRP by using fluorescein (FL) as a photoredox catalyst. In the presence of triethylamine (TEA) as electron donors, FL could activate alkyl bromide by a reductive quenching pathway and control the activation and deactivation equilibrium of living radical polymerization.36, 40 The TEM image (Figure 1(d)) shows that an evident polymer layer with average thickness ~ 13 nm was generated from the BPA-anchored PDA shell after 3 h polymerization under a blue LED light (λmax=460


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nm, 4.8 W). The boundary of the PDA shell and the polymer layer was clearly observed due to the difference in density. Number-average molecular weight (Mn) of the polymer brushes grafted on Fe3O4@PDA was determined to be 6.68×104.60 And, the graft density was estimated to be ~ 0.26 chain/nm2 using an established method.43,

62

The 1H NMR spectrum (Figure S1) of the

polymer obtained from the polymerization system confirmed the structure of PGMA. To demonstrate the ability of the PGMA platform for further modification, EDA molecules were introduced via the ring-opening reaction of the epoxy groups. The robust core-shell architecture of Fe3O4@PDA@PGMA after 24 h reaction was still preserved (Figure S2). Table 1 Surface initiated PET-ATRP of PGMA from Fe3O4@PDA MNPs under different experimental conditions. Photoredox catalysts

[GMA]/[dye]/TEA Light Sources t (h) Thickness (nm)

Mn

PDI

Fluorescein (FL)

200:0.05:0.4

UV LED

3

random

1.37×105

2.34

Fluorescein (FL)

200:0.05:0.4

Blue LED

3

13

6.46×104

2.74

Eosin Y (EY)

200:0.02:0.1

Blue LED

3

10

3.27×104

2.52

Rhodamine 6G (R6G)

200:0.05:0.5

Blue LED

3

10

3.12×104

2.10

Influences of photoredox catalysts and light sources on the composition and morphology of the PGMA brushes grafted from the surfaces of MNPs via PET-ATRP were investigated. The experimental conditions, number-average molecular weight (Mn) and polydispersity index (PDI) (Mw/Mn) of the polymer products were listed in Table 1. When a UV LED (4.8 W) was used as light source (Figure S3(a)), polymer agglomerates were generated with nonuniform coverage on the PDA coated MNPs (Figure 2(a)). The obtained polymer in the reaction mixture showed a high Mn of 1.37×105. This might be explained by the high energy of the UV light causing ultra-


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fast initiation and uneven chain propagation at local areas in the heterogeneous polymerization system. Visible light with relatively low energy is a more favorable and environment-friendly stimulus for regulating the polymerization. Under irradiation of a blue LED ((Figure S3(b))), conformal and uniform PGMA layers were generated on the surfaces of Fe3O4@PDA (Figure 2(b-d)). It should be pointed out that the PDA coating, serving as protective shell and initiatoranchoring platform, shows no strong absorption of visible light (Figure S4). This guaranteed the effective surface-initiated PET-ATRP growth of polymer brushes.

Figure 2 TEM images of Fe3O4@PDA@PGMA under different experimental conditions: (a) FLTEA, UV LED; (b) FL-TEA, blue LED; (c) EY-TEA, blue LED; (d) R6G-TEA, blue LED. Reaction time=3 h; scale bar=100 nm. Photoredox catalyst plays a crucial role in a photo-induced living radical polymerization system. Here, using different organic dyes including FL, EY and R6G as photoredox catalysts resulted in different thickness, Mn and PDI of the PGMA brushes. This may be associated with


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the difference in optical properties of the organic dyes, i.e., absorption wavelength, reduction potential, fluorescence quantum yield, etc.36, 40 Since the absorption wavelength of FL (λmax=450 nm) was closed to the emission of the blue LED light,36 and, relatively thicker and uniform PGMA layer was obtained when using FL as photoredox catalyst in the presence of TEA, in the following section, if not particularly mentioned, Fe3O4@PDA@PGMA refers to the functionalized MNPs synthesized in FL-TEA system, with the experimental condition [GMA]/[FL]/TEA = 200:0.05:0.4.

Figure 3 (a) number-average molecular weight (Mn) and polydispersity index (PDI) of PGMA varying with exposure time; (b) molecular weight distribution of PGMA at different polymerization time. [GMA]/[FL]/TEA = 200:0.05:0.4 in DMSO.


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The growth process of PGMA brushes from the surface of Fe3O4@PDA was studied. Figure 3(a) shows a linear plot of Mn versus exposure time, suggesting that constant concentrations of propagating free radicals were maintained during the polymerization process, with an induction period less than 3 h at the beginning. Monomodal distributions of molecular weight shifted while Mw/Mn reduced with exposure time (Figure 3(b)), indicative a good control of the polymerization. But there is still room to optimize the polymerization conditions before a narrower polydispersity of the PGMA brushes can be obtained. Basically, surface-initiated polymerization from MNPs represents a more complex polymerization system due to the absorption, refraction and scattering of visible light by the MNPs, which makes it more difficult to achieve a well-controlled polymerization than that performed in clear solutions with uniform light exposure. The thickness of the PGMA layers generated on the surface of Fe3O4@PDA was examined by TEM, showing a continuous increase with prolonging the exposure time (Figure 4). A 40 nm thick PGMA layer could be obtained after 9 h. The thickness increase of the PGMA brushes was also estimated by thermogravimetric analysis (TGA). Figure S5 shows the TGA curves of Fe3O4@PDA@PGMA obtained at different polymerization time. By comparing the mass loss corresponding to the decomposition of PGMA till 400 oC, the fraction of PGMA increased with polymerization time, which was in agreement with the thickness increase of PGMA. Overall, under the experimental conditions mentioned above, the thickness of the PGMA brushes grafted from Fe3O4@PDA was tunable by controlling the exposure time.


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Figure 4 TEM images of Fe3O4@PDA@PGMA with different thickness of the PGMA shells at different polymerization time: (a) 3 h, ~ 14 nm; (b) 5 h, ~ 22 nm; (c) 7 h, ~ 32 nm; (d) 9 h, ~ 40 nm. [GMA]/[FL]/TEA = 200:0.05:0.4, blue LED light, scale bar=100 nm. Composition and structure of the functionalized MNPs prepared by metal-free PET-ATRP combined with PDA chemistry were characterized. Firstly, the FT-IR spectra of the surfacemodified MNPs were recorded. The PDA coatings exhibit broad absorption ranging from 3500 cm−1 to 3200 cm−1 (Figure 5(a)), corresponding to the -NH/-OH groups. The signals in the range of 1700-1200 cm−1 can be ascribed to the aromatic rings (1610 cm-1), amide I, amide II and C-N stretching bands (1640 cm-1, 1530 cm-1 and 1230 cm-1, respectively).63 With anchoring of the BPA initiators to the PDA shells, the Fe3O4@PDA-Br showed C=O stretching vibration at 1730 cm−1 (Figure 5(b)), indicative of the successful introduction of the BPA molecules. After the surface-initiated growth of PGMA brushes, enhanced C=O signal was observed, with the appearance of the symmetric stretching vibration of the epoxy groups (906 cm-1 and 845 cm-1) (Figure 5(c)).64 The treatment with EDA resulted in the ring-opening of the epoxy groups. The


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primary amine introduced to the particles showed absorption at 1575 cm−1. The broad absorption peak around 3500–3200 cm−1 was intensified due to the increased -NH2 and -OH. The typical epoxide bands disappeared, suggesting a complete conversion of the epoxy groups (Figure 5(d)).

Figure 5 FT-IR spectra of the functionalized MNPs during surface modification: (a) Fe3O4@PDA,

(b)

Fe3O4@PDA-Br,

(c)

Fe3O4@PDA@PGMA

(3

h)

and

(d)

Fe3O4@PDA@PGMA-EDA. X-ray photoelectron spectroscopy (XPS) was used to further characterize the surface-modified MNPs. Figure 6 shows the XPS spectra (wide scan) of the Fe3O4@PDA@PGMA-EDA and the intermediate products. Fe3O4@PDA (a) with a thick PDA shell (~ 30 nm) only exhibits C 1s, O 1s and N 1s signals. A Br 3d signal shows up at ~ 70 eV with the immobilization of BPA initiator to Fe3O4@PDA, confirming the successful preparation of Fe3O4@PDA-Br (b). It is noteworthy that, after the surface-initiated PET-ATRP growth of polymer, the N 1s signal around 400 eV disappears (c). This implies the effective generation of a PGMA layer (absence of N element) on the surface of Fe3O4@PDA, the thickness of which is higher than the analysis depth of XPS (less than 10 nm). The ring-opening reaction of the epoxy groups introduced EDA


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molecules to the polymer layer. Evident N 1s peak showed up again for the functionalized product Fe3O4@PDA@PGMA-EDA (d).

Figure 6 Wide scan XPS spectra of (a) Fe3O4@PDA, (b) Fe3O4@PDA-Br, (c) Fe3O4@PDA@PGMA (3 h) and (d) Fe3O4@PDA@PGMA-EDA. Based on XPS analysis, the molar percentage of the corresponding elements (C, N, O and Br) was obtained (Table 2). Fe3O4@PDA shows the percentages of C, N and O with 68.8 mol.%, 9.8 mol.% and 21.4 mol.%, respectively. When BPA was introduced to the PDA shells, 4.2 mol.% Br element was detected in Fe3O4@PDA-Br. Almost no N elements was detected for Fe3O4@PDA@PGMA with the surface-initiated growth of PGMA brushes due to the abovementioned reason. While the treatment with EDA resulted in the substantial increase of N elements (12.1 mol%) in the final product, indicating the high efficiency of the ring-opening reaction of the epoxy groups. Table

2 Element content analysis of (a) Fe3O4@PDA, (b) Fe3O4@PDA-Br, (c)

Fe3O4@PDA@PGMA (3 h) and (d) Fe3O4@PDA@PGMA-EDA by XPS survey.


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Percentage (mol.%) Samples C 1s

N 1s

O 1s

Br 3d

Fe3O4@PDA

68.8

9.8

21.4

/

Fe3O4@PDA-Br

67.9

7.0

20.9

4.2

Fe3O4@PDA@PGMA

70.6

0.2

29.1

0.1

Fe3O4@PDA@PGMA-EDA

64.7

12.1

22.9

0.3

Further evidences could be obtained from the narrow scan XPS spectra of C 1S and N 1S regions (Figure 7), to prove the successful modification of the MNPs by the proposed strategy. The C 1s signal of Fe3O4@PDA located between 282 eV and 290 eV corresponds to the C–H, C–N and C–O species in the PDA layers.65 With introduction of the BPA initiators, a new peak at ~ 289 eV indicative of the C 1s signal of C=O groups is observed. After the controlled growth of PGMA brushes by PET-ATRP, a third peak located at 286.3 eV shows up which can be ascribed to the C-O groups in the abundant epoxy groups. Other main peaks located at 284.8 eV and 288.5 eV still exist, which belong to the C−C and C=O species, respectively. And, the signal of C=O groups was significantly intensified. The nitrogen signal, which is observed for both Fe3O4@PDA and Fe3O4@PDA-Br, disappears from the narrow scan N 1s spectrum of Fe3O4@PDA@PGMA, indicating that the thickness of the polymer layer is higher than the analysis depth of XPS. When the EDA was introduced to Fe3O4@PDA@PGMA, the signal at 286.3 eV corresponding to the C−O signal of the epoxy groups disappears, implying a complete conversion in the ring-opening reaction. Meanwhile, strong N 1s signal is found for the EDA modified products.


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Figure 7 Narrow scan XPS spectra of C 1s and N 1s (a) Fe3O4@PDA, (b) Fe3O4@PDA-Br, (c) Fe3O4@PDA@PGMA (3 h) and (d) Fe3O4@PDA@PGMA-EDA.


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Figure 8 TGA curves (A) and DTG plots (B) of (a) Fe3O4, (b) Fe3O4@PDA, (c) Fe3O4@PDA-Br, (d) Fe3O4@PDA@PGMA (9 h) and (e) Fe3O4@PDA@PGMA-EDA. Thermogravimetric analysis (TGA) was used to study the degradation behavior for examination of the thermal stability and content of the organic components introduced to the surfaces of the MNPs. The grafting percentage of each component was obtained by comparing the mass losses in the TGA curves (Figure 8A). According to the mass loss of Fe3O4@PDA (b) compared to that of the pristine Fe3O4 (a) when temperature was raised to 900 °C, it is estimated that approximately 43.7 wt. % of PDA was deposited on the surfaces of the MNPs. Further 2.4 wt.% mass loss was found for Fe3O4@PDA-Br (c) due to the anchoring of BPA initiators. After surface-initiated PET-ATRP for 9 h, the content of the PGMA brushes grafted from


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Fe3O4@PDA was about 27.9 wt.% (d). Finally, the EDA treatment resulted in a further 3.1 wt.% mass loss of modified MNPs. The derivative thermogravimetric analysis (DTG) plots were obtained (Figure 8B) to distinguish overlapping mass loss events, and to help identify major mass loss steps. Each peak in the DTG curves represents a separate event. The pristine MNPs showed no peak till 900 °C (a). For Fe3O4@PDA, a sharp peak was observed after 600 °C (b), according to the carbonization of PDA. The anchoring of BPA caused no obvious change of the DTG profile (c). When the PGMA brushes were grown from the surfaces, two-stage mass loss event was identified at 300 oC and 600 °C (d), which was attributed to the decomposition of PGMA and carbonization of PDA, respectively. After the ring-opening reaction with EDA, the polymer layer showed enhanced stability with decomposition temperature shifted to higher range (e). This might be explained by the intensified hydrogen bonding due to the introduced large number of NH2-, as well as OHgenerated by the ring-opening of the epoxy groups. The magnetic properties and dispersibility of MNPs were of great importance for their biomedical and environmental applications. In this respect, the prepared Fe3O4@PDA@PGMA MNPs showed favorable magnetic properties and quick response in applied magnetic field. Digital camera was used to record the magnet-assisted separation processes. It shows that in Figure S6, when dispersed in ethanol, either Fe3O4@PDA@PGMA or the EDA-modified counterpart could be conveniently separated using a magnet within a few seconds. Besides, the dispersibility of the functionalized MNPs in different solvents were examined. In aqueous environment, both Fe3O4@PDA and Fe3O4@PDA@PGMA exhibited poor dispersibility, and evident sedimentation was observed within 30 min (Figure 9). Improved dispersibility was found for Fe3O4@PDA@PGMA-EDA which contained a large number of hydrophilic –NH2 and –OH


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groups. On the other hand, in organic solvents, Fe3O4@PDA@PGMA showed much better dispersibility than its EDA-modified counterpart, the sedimentation of which was observed in THF and DMF within 15 min. Therefore, the Fe3O4@PDA@PGMA MNPs could be more appropriately

utilized

in

organic

media,

while

the

EDA-modified

products

Fe3O4@PDA@PGMA-EDA in aqueous environments.

Figure 9 Digital photos of functionalized MNPs dispersed in different solvents: Fe3O4@PDA in water (a), Fe3O4@PDA@PGMA (3 h) in water (b) and THF (c), Fe3O4@PDA@PGMA-EDA in water (d), THF (e) and DMF (f) at different times: 15 min, 30 min, 1 h, and 4 h.


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Figure 10 Influence of pH values on the adsorption ability of Fe3O4@PDA@PGMA-EDA. CU(VI)=100 mg/L, T=298 K, [NaClO4]=0.01 M, contact time=24 h. Due to the presence of rich epoxy functionalities or amine ligands in the polymer layers, it is envisioned that these surface-modified MNPs are of potential for biological and environmental applications, such as biomolecules immobilization, targeted drug delivery, heavy metal decontamination, etc. Here, we demonstrated that the abundant amine ligands of Fe3O4@PDA@PGMA-EDA, together with the hydroxyl groups generated via the ring-opening of epoxy groups, could be exploited for uranium binding in aqueous solutions. As is known, uranium can be both strategic resource and environmental contaminant. Substantial efforts have been made in recent years to locate techniques for enrichment or removal of uranium from diverse environments.66, 67 A series of adsorption experiments were conducted in this work to assess the binding ability of the EDA modified Fe3O4@PDA@PGMA toward U(VI). Figure 10 shows that pH values of the solution evidently affected the adsorption capacity of the modified MNPs. This is related to the proton dissociation equilibrium of the amine groups in the polymer layer, which affects the binding ability to uranium.68 The highest adsorption capacity of U(VI) (259.4 mg/g) was obtained at pH 6.0. Besides, the adsorption to U(VI) exhibited a fast kinetics,


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with the saturation adsorption reached in 3 hours (Figure S7). After the contact with uranium solution, the Fe3O4@PDA@PGMA-EDA MNPs still maintained the core-shell architecture (Figure S8). Due to the high adsorption ability, fast kinetics and facile magnetic separation, it can be concluded that this kind of functionalized MNPs may find applications for the clean-up of uranium as well as other environmental hazards. 4. CONCLUSION In this study, we demonstrated for the first time that metal-free surface-initiated PET-ATRP, mediated by the mussel-inspired PDA chemistry, could be employed as an efficient strategy for the surface modification of MNPs. Initially, by manipulating the auto-oxidative polymerization of dopamine, a uniform PDA encapsulation layer with controlled thickness was generated on the MNPs. The PDA layer served as a protective shell as well as an ideal platform to accommodate initiators required for surface-initiated polymerization. Controlled growth of PGMA brushes were then performed via metal-free PET-ATRP using low-concentration of organic dyes as photoredox catalysts. Influences of light sources, photoredox catalysts, and polymerization time on the composition and morphology of the polymer brushes were investigated. The obtained Fe3O4@PDA@PGMA MNPs could be further modified with EDA to improve the hydrophilicity and dispersibility. With rich amine ligands and hydroxyl groups in the polymer layer, the functionalized MNPs showed superior binding ability to uranyl ions in aqueous solutions with fast kinetics. The functionalized MNPs might be useful in biomedical and environmental areas. Most importantly, the methodology by integrating the metal-free PET-ATRP with the ecofriendly PDA chemistry would provide better opportunities for surface engineering of MNPs and other advanced nanomaterials for broadened applications.


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ASSOCIATED CONTENT Supporting Information. Chemicals, characterizations, experimental setups, 1H NMR of PGMA, UV-vis spectrum of PDA, TEM images, TGA curves, adsorption kinetics, and digital photos of the magnetic separation process. This material is available free of charge via the Internet at http://pubs.acs.org. AUTHOR INFORMATION Corresponding Author E-mail: [email protected]; [email protected]; [email protected]. ACKNOWLEDGMENT The study was supported by the Changjiang Scholars and Innovative Research Team in University (IRT13026), the National Science Fund for Distinguished Young Scholars (51425403), National Natural Science Foundation of China under Project 51673109, 51473087 and U1430234. K.M. acknowledges support from the National Science Foundation (DMR 1501324). The authors appreciate the help of Ms. Yi Du at Department of Chemical Engineering, Tsinghua University for GPC measurements.

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