ARTICLE pubs.acs.org/Langmuir
Bifunctional Nanoparticles with Fluorescence and Magnetism via Surface-Initiated AGET ATRP Mediated by an Iron Catalyst Jiliang Liu, Weiwei He, Lifen Zhang, Zhengbiao Zhang, Jian Zhu, Lin Yuan, Hong Chen, Zhenping Cheng,* and Xiulin Zhu* Jiangsu Key Laboratory of Advanced Functional Polymer Design and Application, Department of Polymer Science and Engineering, College of Chemistry, Chemical Engineering and Materials Science, Soochow University, Suzhou 215123, China ABSTRACT: Fluorescent/magnetic nanoparticles are of interest in many applications in biotechnology and nanomedicine for its living detection. In this study, a novel method of surface modification of nanoparticles was first used to modify a fluorescent monomer on the surfaces of magnetic nanoparticles directly. This was achieved via iron(III)-mediated atom-transfer radical polymerization with activators generated by electron transfer (AGET ATRP). Fluorescent monomer 9-(4-vinylbenzyl)-9H-carbazole (VBK) was synthesized and was grafted from magnetic nanoparticles (ferroferric oxide) via AGET ATRP using FeCl3 3 6 H2O as the catalyst, tris(3,6-dioxaheptyl)amine (TDA-1) as the ligand, and ascorbic acid (AsAc) as the reducing agent. The initiator for ATRP was modified on magnetic nanoparticles with the reported method: ligand exchange with 3-aminopropyltriethoxysilane (APTES) and then esterification with 2-bromoisobutyryl bromide. After polymerization, a well-defined nanocomposite (Fe3O4@PVBK) was yielded with a magnetic core and a fluorescent shell (PVBK). Subsequently, well-dispersed bifunctional nanoparticles (Fe3O4@PVBK-b-P(PEGMA)) in water were obtained via consecutive AGET ATRP of hydrophilic monomer poly(ethylene glycol) methyl ether methacrylate (PEGMA). The chemical composition of the magnetic nanoparticles’ surface at different surface modification stages was investigated with Fourier transform infrared (FT-IR) spectra. The magnetic and fluorescent properties were validated with a vibrating sample magnetometer (VSM) and a fluorophotometer. The Fe3O4@PVBK-b-P(PEGMA) nanoparticles showed an effective imaging ability in enhancing the negative contrast in magnetic resonance imaging (MRI).
’ INTRODUCTION Magnetic iron oxide nanoparticles (MIONPs) have been utilized extensively for drug delivery, magnetic resonance imaging (MRI), hyperthermia techniques, cell separation, and tissue repair.1 Bifunctional nanoparticles with magnetism and fluorescence are of particular importance because of their broad range of potential applications, especially in vitro and in vivo bioimaging, biological labeling, and biomedicine.2 Most of these magnetic/ fluorescent nanoparticles are coreshell nanostructures. In general, eight main types of magnetic/fluorescent nanocomposites can be identified:3 (i) fluorescently modified magnetic nanoparticles coated with silica and a magnetic core coated with fluorescence-doped silica shells,2e,4 (ii) polymer-coated magnetic nanoparticles functionalized with fluorescence,2d,5 (iii) a magnetic core conjoining a fluorescent shell with the interaction between oppositely electric charges,6 (iv) fluorescence-labeled bilipid-coated magnetic nanoparticles,7 (v) a magnetic core directly linked to a fluorescent entity via a chain of small molecules,8 (vi) a magnetic core directly modified by a fluorescent semiconducting shell,9 (vii) magnetically mixed quantum dots (QDs) or a magnetically doped rare earth ions complex,10 and (viii) a mixture of magnetic nanoparticles and QDs encapsulated in polymer or embedded into mesoporous silicon.11 r 2011 American Chemical Society
Among these main types, polymer-coated magnetic nanoparticles have attracted a great deal of attention because of their unique advantages. These advantages include the ease with which new functional groups with bioactivity and chemical activity can be endowed to the polymer. Several methods have been used to prepare polymer-coated magnetic nanoparticles: (i) using a layer by layer technique to coat magnetic nanoparticles with polyelectrolytes (PE) and QDs;5,12 (ii) magnetic nanoparticles and QDs were coated with silicon dioxide and then modified with polymer as a shell,4,13 (iii) magnetic nanoparticles were coated with a fluorescent polymer;14 and (iv) fluorescently labeled bilipid-, dextran-, or lipidosome-coated magnetic nanoparticles (MNPs).7a,b,15 Several methods have been used to modify MNPs, especially surface-initiated living radical polymerization (LRP), such as nitroxide-mediated radical polymerization,16 atom-transfer radical polymerization (ATRP),17 and reversible additionfragmentation chain-transfer polymerization (RAFT).18 Surface-initiated ATRP is more popular than other LRP techniques because the grafting density19 and chain length of the grafted polymer20 can be Received: July 18, 2011 Revised: August 28, 2011 Published: September 01, 2011 12684
dx.doi.org/10.1021/la202749v | Langmuir 2011, 27, 12684–12692
Langmuir easily controlled, where highly functionalized chain ends and postpolymerization modification can also be achieved easily.21 However, normal ATRP has some drawbacks because the catalyst complex used in the ATRP system contains a transition-metal compound in a lower oxidation state. The transition metal is sensitive to oxygen and can be easily oxidized to a higher oxidation state. Thus, polymerization can be easily inhibited by oxygen. To overcome such drawbacks of normal ATRP, the initiation system needs to be improved.22 In 2005, a great advance in ATRP was the development of a new method of initiating polymerization with activators generated by electron transfer (AGET) ATRP.23 This development is significant because AGET ATRP can be conducted in the presence of air as a result of the presence of a reducing agent.24 To date, a number of excellent studies involving AGET ATRP mediated by a copper catalyst have been reported.25 AGET ATRP mediated by copper catalyst can also be used for surface modification.26 For example, Zhao et al.26c reported the first modification of silica nanoparticles (SiO2) with comb coil polymer brushes via copper-mediated AGET ATRP, where the Cu2+/bipyridine complex was used as the catalyst and tin(II) 2-ethylhexanoate (Sn(EH)2) was used as the reducing agent. In AGET ATRP, the active Cu(I) catalyst is generated in situ by the reduction of an air-stable copper(II) complex with a reducing agent such as ascorbic acid (AsAc), Sn(EH)2, phenols, and so forth.27 Therefore, AGET ATRP has all the advantages of normal ATRP as well as additional benefits of facile preparation and storage of catalyst. Considering the toxicity of copper to humans as in copper-mediated normal ATRP and AGET ATRP, there has been an urgent need to develop a new nontoxic, highly active catalyst system for AGET ATRP, thus allowing the synthesis of materials for biomedical applications. Fortunately, Zhu’s and Sen’s groups have recently developed new iron(III)-mediated AGET ATRP catalyst systems, which have good biocompatibility and low toxicity due to the nontoxic iron catalyst used.28 In 2009, we first introduced iron-mediated AGET ATRP to the surface modification of chitosan nanospheres in the presence of air.29 In this work, MNPs with a diameter of about 10 nm were synthesized, on which ATRP initiators were immobilized in an effort to create bifunctional nanoparticles with magnetism and fluorescence. This was accomplished using a chemical reagent synthesized in our laboratory, 9-(4-vinylbenzyl)-9H-carbazole (VBK), as a fluorescent monomer. The aim of this work is to develop a novel method for the fabrication of magnetic/fluorescent nanocomposites via iron-mediated AGET ATRP.
’ EXPERIMENTAL SECTION Materials. Aminopropyltriethoxysilane (APTES, analytical reagent) was purchased from Alfa Aesar Chemical Company. 2-Bromoisobutyryl bromide (98%) and 4-vinylbenzyl chloride (90%) were supplied by Aldrich Chemical Company. Oleic acid (OA, analytical reagent), carbazole (chemically pure), iron(III) chloride hexahydrate (FeCl3 3 6H2O, 99+%), ascorbic acid (AsAc, 99.7%), sodium hydroxide (NaOH) (analytical reagent), and tetrahydrofuran (THF, analytical reagent) were purchased from Shanghai Chemical Reagents Company. Tris(3,6-dioxaheptyl) amine (TDA-1, 97%) was purchased from Linhai Xinghua Chemical Factory (Zhejiang, China). Poly(ethylene glycol) methyl ether methacrylate (PEGMA, 97%, Mn = 300 g/mol) was obtained from Aldrich Chemical Company. It was passed through an inhibitor removing column before use. Methylbenzene was purchased from Shanghai Chemical Reagents Company (Shanghai, China) and dried by refluxing with sodium. N,N-Dimethylformamide (DMF,
ARTICLE
Scheme 1. Schematic Diagram of the Synthesis Route to Fluorescent Monomer VBK
analytical reagent) and triethylamine (Et3N, analytical reagent) were obtained from Shanghai Chemical Reagents Company and dried with activated molecular sieve (4 Å). Dichloromethane (analytical reagent) was obtained from Shanghai Chemical Reagents Company and dried with CaH2 after being refluxed. All other reagents (analytical reagent) were obtained from Shanghai Chemical Reagents Company and used as received unless otherwise indicated. Synthesis of 9-(4-Vinylbenzyl)-9H-Carbazole (VBK). Monomer VBK was synthesized according to the reported documents,30 and the synthesis route is shown in Scheme 1. In brief, carbazole (8.4 g, 50.0 mmol) was mixed with NaOH (2.0 g, 50.0 mmol) and a little KI as a catalyst in DMF (100.0 mL) under vigorous magnetic stirring for 2 h. 4-Vinylbenzyl chloride (9.2 g, 60.0 mmol) was dripped slowly into the mixture mentioned above at room temperature. After stirring at room temperature for 24 h, the reaction mixture was poured into a large amount of deionized water. The crude product was precipitated and collected by filtration. It was purified by recrystallization from acetone to give white crystals (85.0%). 1H NMR (DMSO-d6, tetramethylsilane): δ 5.71 (1H, CH2dCH), 5.17 (1H, CH2dCH), 5.47 (2H, CH2), 6.64 (1H, CH2dCH), 7.34, 7.12, 8.17, 7.42, 7.19, 7.62 (12H, ArH). Anal. Calcd for C21H17N: C, 89.01; H, 6.05; N, 4.94. Found: C, 88.62; H, 6.05; N, 4.96. HPLC (Waters 515) indicated that the purity of VBK was greater than 98.0% Synthesis of Magnetic Nanoparticles (MNPs). MNPs were synthesized by the coprecipitation of ferric and ferrous ions in sodium hydroxide solution. The procedure is similar to that in ref 31, but FeSO4 3 6H2O was used to replace FeCl2 3 4H2O. Briefly, a mixture of 1.28 M FeCl3 3 6H2O, 0.64 M FeSO4 3 6H2O, and 0.4 M HCl was obtained as a source of iron that was dissolved in Milli-Q water and then stirred to produce a homogeneous solution. The molar ratio of Fe2+/Fe3+ was 1:2. In the same way, 1.0 M NaOH was prepared as the alkali source. A solution of 0.01 M HCl was prepared for surface neutralization. The MNPs were prepared by mixing the solution of iron and alkali sources at room temperature under vigorous stirring. Ar gas was bubbled into the system throughout all of the procedures to provide an oxygen-free environment. The detailed procedure is given in the literature.31 To a 500 mL three-necked bottle, 25 mL of the iron source above was added dropwise to 250 mL of the alkali source under vigorous mechanical stirring for 30 min at room temperature (25 °C). Then a black solid product was obtained. The precipitated nanoparticles were separated through an external permanent magnet. The nanoparticles were washed with 0.01 M HCl and then deoxygenated deionized water five times. Finally, the MNPs were coated with oleic acid (OA) using a surfactant solution of OA under vigorous mechanical stirring for 30 min at 90 °C. The solution obtained above (80 mL) was mixed with 80 mL of toluene, and then 0.5 g of NaCl was added to the mixture. By shaking the mixture, single-layer OA-coated nanoparticles were transferred into the toluene phase. Then the toluene-based OA-coated MNPs solution was centrifuged at 18 000 rpm for 20 min to collect the OA-coated MNPs.32 Immobilization of Initiator onto MNPs (MNPs@Br). APTESmodified MNPs were obtained according to a previous report.32 Briefly, 1.91 g of MNPs was mixed with dried toluene (50 mL) under ultrasonic mixing to produce a homogeneous nanoparticle suspension, to which 6 mL 12685
dx.doi.org/10.1021/la202749v |Langmuir 2011, 27, 12684–12692
Langmuir
ARTICLE
Scheme 2. Schematic Diagram of the Synthesis Route for the Polymer-Grafted MNPs (Fe3O4@PVBK and Fe3O4@PVBK-bP(PEGMA)) with Core/Shell Structure
of APTES was added. After air was evacuated with Ar for 30 min, the reaction mixture was kept at room temperature for 6 h under vigorous stirring. The obtained APTES-modified MNPs were washed with ethanol (2 30 mL) and dichloromethane (2 30 mL) in turn to remove the unreacted APTES. After magnetic separation, the obtained amino-functionalized MNPs (MNPs@NH2) were used to immobilize the initiator. The procedure is as follows: 2.1 g of MNPs@NH2, 3.0 mL (0.021 mmol) of triethylamine, and 50.0 mL of dried dichloromethane were placed in a dried 100 mL flask immersed in an icewater bath. The mixture was stirred, and then 2.3 mL (0.018 mmol) of 2-bromoisobutyryl bromide was added dropwise to the mixture. After the mixture was stirred for 12 h under an argon atmosphere, the product (MNPs@Br) was separated by centrifuging and then washed with a wateralcohol mixture at pH 4.0 (1:1 v/v, pH adjusted with acetic acid), a wateralcohol (1:1 v/v), and finally with ethyl ether and was then dried under vacuum at room temperature overnight. Surface-Initiated AGET ATRP of VBK from MNPs. A typical polymerization procedure was as follows: To a 5 mL dried ampule, VBK (300.0 mg, 1.06 mmol), FeCl3 3 H2O (10.0 mg, 0.037 mmol), MNPs@Br (15.0 mg), TDA-1 (23.4 μL, 0.074 mmol), AsAc (13.1 mg, 0.074 mmol), and DMF (2.0 mL) were added. The mixture was sonicated to be dispersed and stirred until a homogeneous solution suspension was formed. The ampule containing the above mixture was flame sealed directly in the presence of air and transferred into an oil bath with a constant temperature at 110 °C under a magnetic stirrer for the desired time. [O2] = 3.0 102 mol/L based on the reaction solution (2 mL) was calculated from the residual volume (6.3 mL of air) of the ampule after adding the reaction mixture and ignoring the amount of oxygen dissolved in the liquids. Polymerization was stopped by immersing the ampule in ice water, and then the product was diluted with THF and centrifuged (18 000 rpm for 30 min) to collect PVBK-grafted MNPs. This cycle of centrifugation and redispersion in THF was repeated several times to obtain “pure” polymer-grafted nanoparticles (MNPs@PVBK). To obtain the molecular weight of the grafted PVBK, the obtained MNPs@PVBK was cleaved with HF and then subjected to GPC measurement. The graft percentage (G) was calculated according to the following equation G% ¼
Wg W0 100% W0
where Wg and W0 are the weights of MNPs@PVBK and original MNPs@Br, respectively.
Block Copolymerization of PEGMA Using MNPs@PVBK as the AGET ATRP Initiator. MNPs@PVBK (30 mg), PEGMA (1 mL, 3.5 mmol), FeCl3 3 H2O (10 mg, 0.037 mmol), PPh3 (18.5 mg, 0.07 mmol), AsAc (18 mg, 0.102 mmol), and DMF (3.0 mL) were added to a dry ampule and mixed ultrasonically for 10 min to give a well-dispersed solution, and then the ampule was flame sealed. Polymerization proceeded in an oil bath at the desired temperature (90 °C). After the desired polymerization time, the polymerization was stopped by quenching the ampule in ice water. The reaction mixture was diluted with THF and centrifugated (at 18 000 rpm for 30 min) to separate free P(PEGMA) from P(PEGMA)-grafted nanoparticles. This cycle of redispersion in THF and centrifugation was repeated five times to make sure that the free polymers were removed completely. The final product, Fe3O4@PVBK-b-P(PEGMA), was dried in a vacuum oven at room temperature. Characterization. The number-average molecular weight (Mn,GPC) values and molecular weight distribution (Mw/Mn) values of the polymers were determined using HR 1 (pore size 100 Å, 1005000 Da), HR 2 (pore size 500 Å, 50020 000 Da), and HR 4 (pore size 10 000 Å, 50100 000 Da) columns (7.8 300 mm, 5 μm bead size) with measurable molecular weights ranging from 102 to 5 105 g/mol. THF was used as an eluent at a flow rate of 1.0 mL/min and 30 °C. The GPC samples were injected using a Waters 717 plus autosampler and calibrated with polystyrene standards from Waters. Transmission electron microscopy (TEM) images were recorded on an FEI Tecnai G220 transmission electron microscope at 200 KV. The sample was prepared by mounting a drop of the nanoparticle dispersion on a carbon-coated Cu grid and allowing the sample to dry in air. Fourier transform infrared (FT-IR) spectra were recorded on a Nicolet 380 FT-IR spectrometer. Thermogravimetric analysis (TGA) was performed in air at a heating rate of 10 °C/min with a TA Instruments SDT-2960 TG/ DTA from room temperature to 600 °C. Power X-ray diffraction (XRD) measurements were performed on an Xpert-PRO MPO. The XRD patterns were recorded using Cu Kα irradiation (λ = 1.54178 Å). Magnetic hysteresis loops of the nanoparticles were recorded at 300 K on a VSM7407 (LakeShore). The fluorescence emission spectra were obtained on a PerkinElmer LS-50B fluorescence spectrophotometer with THF as the solvent at room temperature.
’ RESULTS AND DISCUSSION Synthesis Pathway of Magnetic/Fluorescent Nanoparticles via Surface-Initiated AGET ATRP. Scheme 2 shows the 12686
dx.doi.org/10.1021/la202749v |Langmuir 2011, 27, 12684–12692
Langmuir
ARTICLE
Figure 1. TEM images and corresponding histograms of (a) Fe3O4, (b) Fe3O4@Br, and (c) Fe3O4@PVBK (Mn,GPC = 19 400 g/mol, Mw/Mn = 1.85) obtained by the polymerization of VBK via AGET ATRP for 10 h.
synthesis pathway to preparing magnetic/fluorescent bifunctional nanoparticles. First, the magnetic core was prepared using the chemical coprecipitation method, which has been well known to produce magnetic nanoparticles simply. In this article, magnetic nanoparticles with an average diameter of 10 nm, as determined by TEM (Figure 1a), were used as the magnetic core. Second, amino groups were first immobilized on the surfaces of the magnetic core to obtain Fe3O4@NH2 nanoparticles by the self-assembly of APTES. Third, Fe3O4@NH2 nanoparticles were further reacted with 2-bromoisobutyryl bromide, leading to ATRP initiator Fe3O4@Br. Finally, the surfaceinitiated AGET ATRP of VBK was conducted with Fe3O4@Br nanoparticles using FeCl3/TDA-1 as the catalyst and AsAc as the reducing agent. The whole process is schematically shown in Scheme 2. Morphology and XRD Spectra of Magnetic/Fluorescent Nanoparticles. Coreshell nanoparticles with a PVBK shell provide a facile way to synthesize different hybrid nanomaterials. In this work, the Fe3O4@PVBK nanoparticles were prepared using surface-initiated AGET ATRP mediated by an iron catalyst from magnetic nanoparticles. Figure 1 shows the morphologies of Fe3O4, Fe3O4@Br, and Fe3O4@PVBK nanoparticles. It can be seen that well-dispersed nanoparticles with an average diameter of about 10 nm were obtained via the coprecipitation of ferric and ferrous ions in sodium hydroxide solution after being coated with OA (Figure 1a). After immobilizing ATRP initiators and grafting PVBK on the surfaces of nanoparticles, similar morphologies were observed for the corresponding nanoparticles and the diameters of the nanoparticles increased to 11 and 13 nm, respectively. As shown in Figure 2, the pure magnetic nanoparticles (Figure 2a) and magnetic nanoparticles grafted with fluorescent polymer PVBK (Figures 2b) were examined by XRD. The peaks in the XRD pattern match the magnetite (Fe3O4) crystal structure data well (Joint Committee for Powder Diffraction file,
Figure 2. X-ray powder diffraction patterns of the (a) Fe3O4 and (b) Fe3O4@PVBK (Mn,GPC = 19 400 g/mol, Mw/Mn = 1.85) nanoparticles obtained by the polymerization of VBK via AGET ATRP for 10 h.
JCPDS card no. 19-0629), indicating that the obtained nanoparticles are magnetite. After the grafting of PVBK, all of the peaks (Figures 2b) related to magnetic Fe3O4 were also observed, although they are weaker than that of pure Fe3O4, indicating that the magnetic Fe3O4 core remained intact during the synthesis process. The reason for the decrease in the peaks after PVBK grafting was due to the fact that the content of magnetic Fe3O4 in nanoparticles after PVBK grafting is lower than that of pure Fe3O4. Immobilization of Initiator onto MNPs. ATRP initiator was immobilized on the nanosphere surfaces to obtain the surface initiators (Fe3O4@Br) by the interaction of the NH2 groups on the Fe3O4@APTES surfaces with 2-bromoisbutyl bromide. FTIR was used to monitor the immobilization process, as shown in Figure 3. As compared to bare Fe3O4 (Figure 3a), besides the characteristic peaks of Fe3O4 at 623 and 590 cm1, these characteristic peaks at 1521 and 1459 cm1 and at 2922 and 2850 cm1 from the COO and CH2 groups, respectively, 12687
dx.doi.org/10.1021/la202749v |Langmuir 2011, 27, 12684–12692
Langmuir
ARTICLE
Figure 3. FT-IR spectra of (a) uncoated Fe3O4, (b) OA-coated Fe3O4, (c) Fe3O4@Br, (d) Fe3O4@PVBK (Mn,GPC = 19 400 g/mol, Mw/Mn = 1.85), and (e) Fe3O4@PVBK-b-P(PEGMA).
in the chain of OA can be observed (Figure 3b) after OA coating.17a It can be seen from Figure 3c that two strong peaks at 1108 cm1 belong to SiO vibrations from APTES moieties and a peak 1647 cm1 contributes to the presence of the OdCN group from ATRP moieties. All of these results indicate that the ATRP initiators have been successfully immobilized on the surfaces of the MNPs to form Fe3O4@Br. Surface-Initiated AGET ATRP of VBK Mediated by an Iron Catalyst on the Surface of Fe3O4@Br. Compared to coppermediated ATRP and AGET ATRP, an iron catalyst has more advantages such as low toxicity to humans, which makes it more applicable in the biomedical and biotechnology fields. In this work, a VBK fluorescent monomer was grafted from MNPs directly by iron-mediated AGET ATRP without free initiator to form bifunctional fluorescent/magnetic nanoparticles. The successful grafting of PVBK can be confirmed by the FT-IR spectrum, as shown in Figure 3d, where the characteristic peaks at 1600, 1500, and 1452 cm1 for aromatic groups in the chain of PVBK were observed. In addition, after grafting P(PEGMA) using MNPs@PVBK as the macroinitiators, the characteristic absorption bands at 1731 cm1 (CdO stretching vibration) and 1100 cm1 (COC stretching vibration) in Figure 3e are consistent with the presence of the P(PEGMA) block on the surfaces of the MNPs@PVBK-b-P(PEGMA) nanoparticles.33 To investigate the polymerization behavior further, the polymerization kinetics was studied in detail. As shown in Figure 4a, a first-order kinetic plot of ln([M0]/[M]) versus time for the AGET ATRP of VBK was observed, which indicates that the concentration of the propagating free radical was constant during the polymerization process. Figure 4b shows the evolution of Mn,GPC and Mw/Mn of PVBK cleaved from Fe3O4@PVBK versus monomer conversion. It can be seen that Mn,GPC values of the grafting polymer increased linearly with the increase in monomer conversion. However, Mw/Mn values remained broad (∼1.8). This may be attributed to the drawbacks of Fe3O4@Br and the side reactions during polymerization and etching processes. To compare Mn,GPC and Mw/Mn of PVBK obtained via a conventional radical polymerization, a reference experiment using AIBN as an initiator (VBK, 600 mg, 2.12 mmol; AIBN, 1.43 mg, 0.0087 mmol; DMF, 4 mL; polymerization time, 2 h) at 110 °C was performed and polymer with Mn,GPC = 8690 g/mol and Mw/Mn = 3.13 was obtained. Obviously, the value of Mw/Mn
Figure 4. (a) Plot of ln([M]0/[M]) versus time and (b) evolution of the molecular weight (Mn,GPC) and molecular weight distribution (Mw/Mn) of PVBK grafted from MNPs with monomer conversion for the surface-initiated AGET ATRP polymerization of VBK using MNPsBr as the initiator in DMF at 110 °C. Polymerization conditions: [VBK]0/ [Fe3O4@Br]0/[FeCl3 3 6H2O]0/[TDA-1]0/[AsAc]0 = 126:1:4:8:8. The weight of initiator Fe3O4@Br is 20 mg.
from conventional radical polymerization was much higher than that obtained via the AGET ATRP technique. In addition, the grafting percentage was also investigated at the same time, as shown in Figure 5. There was a linear increase in the graft percentage with the polymerization time. It is noted that there might be a small amount of weight loss during the whole reaction and purification, which resulted in some errors in the grafting percentage. All of these results indicated that surface-initiated AGET ATRP demonstrated the characteristics of a controlled/ “living” polymerization process. TGA Analysis. Figure 6 shows the TGA analysis of the MNPs modified with OA, initiator, and PVBK. The TGA curve of OAcoated MNPs (Figure 6a) shows a weight loss of OA on MNPs. The total weight loss is about 21.4%. If Fe3O4 is assumed to be oxidized to Fe2O3, then we can roughly estimate that the weight of OA in OA-coated MNPs is about 23.6%. As shown in Figure 6b, the TGA curve of MNPs@Br shows that the total loss is close to that of OA-coated MNPs but the initiated weight loss temperature (121 °C) is lower than that (206 °C) of OAcoated MNPs and the weight loss is quicker than that of OAcoated MNPs from the graphs (Figure 6a,b), which shows that the disintegration of initiator-modified MNPs is easier than 12688
dx.doi.org/10.1021/la202749v |Langmuir 2011, 27, 12684–12692
Langmuir
ARTICLE
Figure 7. Photograph of Fe3O4@PVBK dispersed in DMF (a) before and (b) after an external magnetic field.
Figure 5. Grafting percentage as a function of polymerization time for the surface-initiated AGET ATRP of VBK using Fe3O4@Br as the initiator in DMF at 110 °C. Polymerization conditions: [VBK]0/[Fe3O4@Br]0/ [FeCl3 3 6H2O]0/[TDA-1]0/[AsAc]0 = 126:1:4:8:8. The weight of initiator Fe3O4@Br is 20 mg.
Figure 8. Magnetic hysteresis loops at 300 K for (insert) the obtained Fe3O4 NPs, (a) Fe3O4@Br, and (b) Fe3O4@PVBK NPs by AGET ATRP for 10 h.
Figure 6. Thermogravimetric analysis (TGA) of (a) OA-coated magnetic nanoparticles (Fe3O4), (b) Fe3O4@Br, and (c) Fe3O4@PVBK obtained by the polymerization of VBK via AGET ATRP for 10 h.
coating MNPs with OA. This also indicates that the initiator has successfully been immobilized on the surfaces of the MNPs to form MNPs@Br, as confirmed by the results of FT-IR spectra (Figure 3). In addition, from the weight loss of the TGA curve, it is easy to estimate that the initiator accounts for about 12.1% of the total weight of MNPs@Br. Considering that the silicon dioxide ash remained after the oxidation of initiator, it is estimated that the content of initiators in MNPs@Br is 0.58 mmol/g. Therefore, we can estimate the loading density (LD) of ATRP initiators on the surfaces of MNPs to be about 3.0 initiators/nm2. The LD (1/nm2) of the ATRP initiator was estimated using the following equation as reported in our previous publication.34 LD ¼
N Br N A Sparticle FMNPs V particle
where NBr is the number of moles of 1 g of MNPs@Br, NA is Avogadro’s number, Sparticle and Vparticle are the surface area and volume of one particle, respectively, and FMNPs is the density of the Fe3O4 nanoparticles. In the same way, the graft density of PVBK on the surfaces of MNPs can also be estimated using the above equation. If we use
the mass retention of MNPs@Br as the reference and the weight retention at 600 °C obtained for PVBK-grafted MNPs at 8.5% total monomer conversion (37.3%), then the PVBK weight relative to Fe3O4@PVBK was roughly calculated to be 58.2%. The Mn of the grafted PVBK chains was determined to be 19 400 g/mol by GPC, and the content of PVBK in the MNPs cores (MNPs@Br) was estimated to be 0.07 mmol/g. The initiator efficiency and the graft density for the surface-initiated AGET ATRP of VBK can be estimated to be 14.3% and 0.43 chains/ nm2, respectively. Properties of As-Prepared Nanoparticles. To determine the superparamagnetic behavior of the obtained bifunctional nanoparticles, an external magnetic field was used, as shown in Figure 7. Because of the superparamagnetic property and the screening effect of the grafted PVBK layer, fluorescent magnetic Fe3O4@PVBK nanoparticles could be easily and stably dispersed in DMF in the absence of an external magnetic field (Figure 7a). Interestingly, these nanoparticles could also be completely separated from the solution within several minutes when subjected to a strong magnetic field (Figure 7b). They could redisperse very well with minimal agitation when the magnetic field was removed. This suggests that these Fe3O4@PVBK nanoparticles have a high dispersibility and a high sensitivity to the magnetic field, which are two important factors for bioapplication. To study the nanoparticle magnetization further, the magnetization curves of as-synthesized nanoparticles were measured by VSM. Figure 8 shows the magnetization curves of Fe3O4, Fe3O4@Br, and Fe3O4@PVBK nanoparticles at 300 K. As shown in Figure 8, the saturation magnetization value of the unmodified 12689
dx.doi.org/10.1021/la202749v |Langmuir 2011, 27, 12684–12692
Langmuir
ARTICLE
Figure 9. T2-weighted MRI images (1.5 T; TR, 4240.00 ms; TE, 108.47 ms) of the Fe3O4@PVBK-b-P(PEGMA) nanoparticles. The iron concentration from left to right is 0, 18.75, 37.5, 150, 300, 450, and 600 μmol/L, respectively.
Figure 10. T2 relaxation rates (1/T2, s1) as a function of the iron concentration of Fe3O4@PVBK-b-P(PEGMA) nanoparticles in aqueous solution (1.5 T; 25 °C; TR, 4240.00 ms; TE, 108.47 ms).
Fe3O4 nanoparticles (insert graph) is about 61.2 emu/g at 300 K. It can also be seen that the saturation magnetization of Fe3O4@Br nanoparticles (Figure 8a) and Fe3O4@PVBK nanoparticles (Figure 8b), 34.4 and 21.8 emu/g, respectively, decreased successively, indicating its suitability for magnetic guiding and imaging. In addition, neither coercivity nor remanence was observed among the three magnetization curves in Figure 8, indicating the superparamagnetic behavior of the asprepared nanoparticles at 300 K. Because Fe3O4 nanoparticles are good T2-type contrast agents in MRI, it is interesting to know whether the as-prepared nanoparticles are also good at contrasting MRI images. For this reason, we first modified the PVBK-grafted nanoparticles with hydrophilic polymer chain P(PEGMA) via the block copolymerization of PEGMA using Fe3O4@PVBK as the macroinitiator, which could allow the nanoparticles to disperse in water well, and then the MRI test was carried out at 1.5 T. Figures 9 and 10 show T2-weighted MR images and T2 relaxation rates under various Fe concentrations in Fe3O4@PVBK-b-P(PEGMA) nanoparticles (0, 18.75, 37.5, 150, 300, 450, and 600 μmol/L), respectively. It can be easily seen that the smallest Fe concentration in Fe3O4@PVBK-b-P(PEGMA) gives much brighter T2*-weighted images and that the R2 value per millimole of Fe in the Fe 3 O 4 @PVBK-g-P(PEGMA) nanoparticles is 110.3 ( 5.2 mM 1 s 1 . We know that the larger the Fe concentration, the more enhanced the MR signals. All of these results confirmed that the as-prepared Fe3O4@PVBK-b-P(PEGMA) nanoparticles have an effective imaging ability in enhancing the negative contrast in MRI. The fluorescence properties of Fe3O4@PVBK-b-P(PEGMA) nanoparticles were investigated in THF with an excitation peak at 294 nm. The results are shown in Figure 11. It can be seen that there is no fluorescence for Fe3O4 and Fe3O4@Br nanoparticles; however, two intense peaks at 350 and 365 nm were observed for
Figure 11. Fluorescence spectra of Fe3O4@PVBK-b-P(PEGMA) in THF at different concentrations at room temperature with an excitation wavelength of 294 nm. The concentration of Fe3O4@PVBK-b-P(PEGMA) changed from high to low, as shown by the arrow: (A) 500, 400, 300, 200, 100, 90, and 80 μg/mL. (B) 80, 60, 40, 20, 10, 8, 6, and 4 μg/mL. The fluorescence spectra of THF and Fe3O4@Br are at the bottom with no fluorescence.
the Fe3O4@PVBK-b-P(PEGMA) nanoparticles. The fluorescence intensity increased first (Figure 11 A) and then decreased (Figure 11 B) with the decrease in the concentration of Fe3O4@PVBK-b-P(PEGMA) nanoparticles (from 500 to 4 μg/mL) as a result of the fluorescence self-quenching and magnetism of the MNPs. Therefore, a suitable concentration of the nanoparticles is 80 μg/mL.
’ CONCLUSIONS A novel strategy to synthesize magnetic/fluorescent nanoparticles with well-defined coreshell structure was demonstrated via surface-initiated AGET ATRP mediated by an iron(III) catalyst using a fluorescent monomer (VBK) and a magnetic Fe3O4 core. This procedure has all the advantages of normal ATRP as well as additional benefits of the facile preparation, storage, and handing of ATRP catalysts, good biocompatibility, and low toxicity due to the iron catalyst used. The obtained 12690
dx.doi.org/10.1021/la202749v |Langmuir 2011, 27, 12684–12692
Langmuir bifunctional nanoparticles exhibit good fluorescence and magnetic properties.
’ AUTHOR INFORMATION Corresponding Author
*(Z.C.) E-mail:
[email protected]. Fax: 86-512-65882787. (X.Z.) E-mail:
[email protected]. Fax: 86-512-65112796.
’ ACKNOWLEDGMENT Financial support from the National Natural Science Foundation of China (nos. 20974071, 20904036, and 21174096), the Specialized Research Fund for the Doctoral Program of Higher Education (no. 20103201110005), the Project of Science and Technology Development Planning of Suzhou (no. SYG201026), the Project of International Cooperation of the Ministry of Science and Technology of China (no. 2011DFA50530), the Qing Lan Project, the Program of Innovative Research Team of Soochow University, and the project funded by the Priority Academic Program Development of Jiangsu Higher Education Institutions (PAPD) is gratefully acknowledged. ’ REFERENCES (1) (a) Won, J.; Kim, M.; Yi, Y.; Kim, Y. H.; Jung, N.; Kim, T. K. Science 2005, 309, 121–125. (b) Weissleder, R. Science 2006, 312, 1168–1171. (c) Huh, Y.; Jun, Y.; Song, H.; Kim, S.; Choi, J.; Lee, J.; Yoon, S.; Kim, K.; Shin, J.; Suh, J.; Cheon, J. J. Am. Chem. Soc. 2005, 127, 12387–12391. (d) Corot, C.; Robert, P.; Port, M. Adv. Drug Delivery Rev 2006, 58, 1471–1504. (e) Ma, M.; Wu, Y.; Zhou, J.; Sun, Y.; Zhang, Y.; Gu, N. J. Magn. Magn. Mater. 2004, 268, 33–39. (f) Shi, X.; Wang, S. H.; Swanson, S. D.; Ge, S.; Cao, Z.; Van Antwerp, M. E.; Landmark, K. J.; Baker, J. R., Jr. Adv. Mater. 2008, 20, 1671–1678. (g) Lee, E. S. M.; Shuter, B.; Chan, J.; Chong, M. S. K.; Ding, J.; Teoh, S. H.; Beuf, O.; Briguet, A.; Tam, K. C.; Choolani, A.; Wang, S. C. Biomaterials 2010, 31, 3296–3306. (h) Marina, T.; Cristianne, J. F. R.; Twan, L.; Peter, R. S.; Gert, S.; Cornelus, F. N.; Wim, E. H. Langmuir 2009, 25, 2060–2067. (2) (a) Bertorelle, F.; Wilhelm, C.; Roger, J.; Gazeau, F.; Menager, C.; Cabuil, V. Langmuir 2006, 22, 5385–5391. (b) Corr, S. A.; O’Byrne, A.; Gun’ko, Y. K.; Ghosh, S.; Brougham, D. F.; Mitchell, S.; Volkov, Y.; Prina-Mello, A. Chem. Commun. 2006, 43, 4474–4476. (c) Gu, H. W.; Xu, K. M.; Yang, Z. M.; Chang, C. K.; Xu, B. Chem. Commun. 2005, 34, 4270–4272. (d) Wang, L.; Neoh, K. G.; Kang, E. T.; Shuter, B.; Wang, S. C. Biomaterials 2010, 31, 3502–3511. (e) Sun, P.; Zhang, H. Y.; Liu, C.; Fang, J.; Wang, M.; Chen, J.; Zhang, J. P.; Mao, C. B.; Xu, S. K. Langmuir 2010, 26, 1278–1284. (f) Lou, L.; Yu, K.; Zhang, Z. L.; Li, B.; Zhu, J. Z.; Wang, Y. T.; Huang, R.; Zhu, Z. Q. Nanoscale 2011, 3, 2315–2323. (g) Wang, G. N.; Su, X. G. Analyst 2011, 136, 1783–1798. (3) Corr, S. A.; Rakovich, Y. P.; Gun’ko, Y. K. Nanoscale Res. Lett. 2008, 3, 87–104. (4) (a) Lu, Y.; Yin, Y. D.; Mayers, B. T.; Xia, Y. N. Nano Lett. 2002, 2, 183–186. (b) Chen, D. Y.; Jiang, M. J.; Li, N. J.; Gu, H. W.; Xu, Q. F.; Ge, J. F.; Xia, X. W.; Lu, J. M. J. Mater. Chem. 2010, 20, 6422–6429. (c) Li, G. L.; Zeng, D. L.; Wang, L.; Zong, B. Y.; Neoh, K. G.; Kang., E. T. Macromolecules 2009, 42, 8561–8565. (d) Heitsch, A. T.; Smith, D. K.; Patel, R. N.; Ress, D.; Korgel, B. A. J. Solid State Chem. 2008, 181, 1590–1599. (5) (a) Hong, X.; Li, J.; Wang, M. J.; Xu, J. J.; Guo, W.; Li, J. H.; Bai, Y. B.; Li, T. J. Chem. Mater. 2004, 16, 4022–4027. (b) Yang, J.; Lim, E. -K.; Lee, H. J.; Park, J.; Lee, S. C.; Lee, K.; Yoon, H. -G.; Suh, J. -S.; Huh, Y. -M.; Haam, S. Biomaterials 2008, 29, 2548–2555. (6) Corr, S. A.; O’Byrne, A.; Gun’ko, Y. K.; Ghosh, S.; Brougham, D. F.; Mitchell, S.; Volkov, Y.; Prina-Mello, A. Chem. Commun. 2006, 43, 4474–4476. (7) (a) Becker, C.; Hodenius, M.; Blendinger, G.; Sechi, A.; Hieronymus, T.; Muller-Schulte, D.; Schmitz-Rode, T.; Zenke, M.
ARTICLE
J. Magn. Magn. Mater. 2007, 311, 234–237. (b) Li, X. Z.; Wang, L.; Zhou, C.; Guan, T. T.; Li, J.; Zhang, Y. H. Clin. Chim. Acta 2007, 378, 168–174. (c) Vuu, K.; Xie, J. W.; McDonald, M. A.; Bernardo, M.; Hunter, F.; Zhang, Y. T.; Li, K.; Bednarski, M.; Guccione, S. Bioconjug. Chem. 2005, 16, 995–999. (8) (a) Gu, H. W.; Xu, K. M.; Yang, Z. M.; Chang, C. K.; Xu, B. Chem. Commun. 2005, 34, 4270–4272. (b) Tanase, M.; Bauer, L. A.; Hultgren, A.; Silevitch, D. M.; Sun, L.; Reich, D. H.; Searson, P. C.; Meyer, G. J. Nano Lett. 2001, 1, 155–158. (c) Huh, Y. M.; Jun, Y. W.; Song, H. T.; Kim, S.; Choi, J. S.; Lee, J. H.; Yoon, S.; Kim, K. S.; Shin, J. S.; Suh, J. S.; Cheon, J. J. Am. Chem. Soc. 2005, 127, 12387–12391. (d) Banerjee, S. S.; Chen, D. H. Nanotechnology 2009, 20, 185103–185112. (9) (a) Gu, H. W.; Zheng, R. K.; Zhang, X. X.; Xu, B. J. Am. Chem. Soc. 2004, 126, 5664–5665. (b) Kim, H.; Achermann, M.; Balet, L. P.; Hollingsworth, J. A.; Klimov, V. I. J. Am. Chem. Soc. 2005, 127, 544–546. (c) Yang, J.; Lim, E. -K.; Lee, H. J.; Park, J.; Lee, S. C.; Lee, K.; Yoon, H. -G.; Suh, J. -S.; Huh, Y. -M.; Haam, S. Biomaterials 2008, 29, 2548–2555. (10) (a) Santra, S.; Yang, H. S.; Holloway, P. H.; Stanley, J. T.; Mericle, R. A. J. Am. Chem. Soc. 2005, 127, 1656–1657. (b) Zhang, Y. X.; Das, G. K.; Xu, R.; Tan, T. T. Y. J. Mater. Chem. 2009, 19, 3696–3703. (11) (a) Philipse, A. P.; Vanbruggen, M. P. B.; Pathmamanoharan, C. Langmuir 1994, 10, 92–99. (b) Gaponik, N.; Radtchenko, I. L.; Sukhorukov, G. B.; Rogach, A. L. Langmuir 2004, 20, 1449–1452. (c) Kim, J.; Lee, J. E.; Lee, J.; Yu, J. H.; Kim, B. C.; An, K.; Hwang, Y.; Shin, C. H.; Park, J. G.; Kim, J.; Hyeon, T. J. Am. Chem. Soc. 2005, 128, 688–689. (d) Hu, J.; Xie, M.; Wen, C. Y.; Zhang, Z. L.; Xie, H. Y.; Liu, A. A.; Chen, Y. Y.; Zhou, S. M.; Pang, D. W. Biomaterials 2011, 32, 1177–1184. (12) (a) Okamoto, Y.; Kitagawa, F.; Otsuka, K. Anal. Chem. 2007, 79, 3041–3047. (b) Fang, M.; Grant, P. S.; McShane, M. J.; Sukhorukov, G. B.; Golub, V. O.; Lvov, Y. M. Langmuir 2002, 18, 6338–6344. (13) Levy, L.; Sahoo, Y.; Kim, K. S.; Bergey, E. J.; Prasad, P. N. Chem. Mater. 2002, 14, 3715–3721. (14) Bertorelle, F.; Wilhelm, C.; Roger, J.; Gazeau, F.; Menager, C.; Cabuil, V. Langmuir 2006, 22, 5385–5391. (15) (a) Herdt, A. R.; Kim, B. S.; Taton, T. A. Bioconjugate Chem. 2007, 18, 183–189. (b) Nitin, N.; Laconte, L. E. W.; Zurkiya, O.; Hu, X.; Bao, G. J. Biol. Inorg. Chem. 2004, 9, 706–712. (16) (a) Matsuno, R.; Yamamoto, K.; Otsuka, H.; Takahara, A. Chem. Mater. 2003, 15, 3–5. (b) Matsuno, R.; Yamamoto, K.; Otsuka, H.; Takahara, A. Macromolecules 2004, 37, 2203–2209. (17) (a) Sun, Y. B.; Ding, X. B.; Zheng, Z. H.; Cheng, X.; Hu, X. H.; Peng, Y. X. Eur. Polym. J. 2007, 43, 762–772. (b) Wuang, S. C.; Neoh, K. G.; Kang, E. T.; Pack, D. W.; Leckband, D. E. Adv. Funct. Mater. 2006, 16, 1723–1730. (c) Vestal, C. R.; Zhang, Z. J. J. Am. Chem. Soc. 2002, 124, 14312–14313. (d) Lattuada, M.; Hatton, T. A. Langmuir 2007, 23, 2158–2168. (e) Marutani, E.; Yamamoto, S.; Ninjbadga, T.; Tsujii, Y.; Fukuda, T.; Takano, M. Polymer 2004, 45, 2231–2235. (f) Ding, S. J.; Radosz, M.; Shen, Y. Q. ACS Symp. Ser. 2006, 944, 71–84. (18) (a) Wang, W. C.; Neoh, K. G.; Kang, E. T. Macromol. Rapid Commun. 2006, 27, 1665–1669. (b) Saoud, F. M.; Tonge, M. P.; Weber, W. G.; Sanderson, R. D. Macromolecules 2008, 41, 1598–1600. (c) Li, Q.; Zhang, L. F.; Bai, L. J.; Zhang, Z. B.; Zhu, J.; Zhou, N. C.; Cheng, Z. P.; Zhu, X. L. Soft Matter 2011, 7, 6958–6966. (19) (a) Jones, D. M.; Brown, A. A.; Huck, W. T. S. Langmuir 2002, 18, 1265–1269. (b) Feng, W.; Zhu, S.; Ishihara, K.; Brash, J. L. Biomaterials 2002, 27, 847–855. (20) (a) Prucker, O.; Ruhe, J. Langmuir 1998, 14, 6393–6398. (b) Prucker, O.; Ruhe, J. Macromolecules 1998, 31, 602–613. (c) Radhakrishnan, B.; Ranjan, R.; Brittain, W. J Soft Matter 2006, 2, 386–396. (d) Peng, B.; Johannsmann, D.; Ruhe, J. Macromolecules 1999, 32, 6759–6766. (e) Xiao, D.; Wirth, M. J. Macromolecules 2002, 35, 2919–2925. (21) (a) Coessens, V.; Pintauer, T.; Matyjaszewski, K. Prog. Polym. Sci. 2001, 26, 337–377. (b) Xu, F. J.; Neoh, K. G.; Kang, E. T. Prog. Polym. Sci. 2009, 34, 719–761. 12691
dx.doi.org/10.1021/la202749v |Langmuir 2011, 27, 12684–12692
Langmuir
ARTICLE
(22) (a) Hong, S. C.; Matyjaszewski, K. Macromolecules 2002, 35, 7592–7605. (b) Shen, Y. Q.; Tang, H. D.; Ding, S. J. Prog. Polym. Sci. 2004, 29, 1053–1078. (23) Jakubowski, W.; Matyjaszewski, K. Macromolecules 2005, 38, 4139–4146. (24) (a) Tang, H. D.; Radosz, M.; Shen, Y. Q. Macromol. Rapid Commun. 2006, 27, 1127–1131. (b) Min, K.; Jakubowski, W.; Matyjaszewski, K. Macromol. Rapid Commun. 2006, 27, 594–598. (c) Matyjaszewski, K.; Dong, H.; Jakubowski, W.; Pietrasik, J.; Kusumo, A. Langmuir 2007, 23, 4528–4531. (25) (a) Hizal, G.; Tunca, U.; Aras, S.; Mert, H. J. Polym. Sci., Part A: Polym. Chem. 2006, 44, 77–87. (b) Jakubowski, W.; Min, K.; Gao, H.; Matyjaszewski, K. Macromolecules 2006, 39, 39–45. (c) Chan, N.; Cunningham, M. F.; Hutchinson, R. A. Macromol. React. Eng. 2010, 4, 369–380. (d) Kwon OH, J.; Perineau, F.; Charleux, B.; Matyjaszewski, K. J. Polym. Sci., Part A: Polym. Chem. 2009, 47, 1771–1781. (e) Stoffelbach, F.; Griffete, N.; Buiab, C.; Charleux, B. Chem. Commun. 2008, 4807–4809. (26) (a) Estevea, A. C. C.; Bombalski, L.; Trindade, T.; Matyjaszewski, K.; Timmons, A. B. Small 2007, 3, 1230–1236. (b) Bombalski, L.; Min, K.; Dong, H.; Tang, C.; Matyjaszewski, K. Macromolecules 2007, 40, 7429–7432. (c) Zhao, H. Y.; Kang, X. L.; Liu, L. Macromolecules 2005, 38, 10619–10622. (d) Qian, H.; He, L. Anal. Chem. 2009, 81, 4536–4542. (e) Qian, H.; He, L. Anal. Chem. 2009, 81, 9824–9827. (f) Kitayama, Y.; Kagawa, Y.; Minami, H.; Okubo, M. Langmuir 2010, 26, 7029–7034. (27) (a) Pietrasik, J.; Dong, H. C.; Matyjaszewski, K. Macromolecules 2006, 39, 6384–6390. (b) Yamamura, Y.; Matyjaszewski, K. J. Macromol. Sci., Part A: Pure Appl. Chem. 2007, 44, 1035–1039. (28) (a) Zhang, L. F.; Cheng, Z. P.; Tang, F.; Li, Q.; Zhu, X. L. Macromol. Chem. Phys. 2008, 209, 1705–1713. (b) Zhang, L. F.; Cheng, Z. P.; Shi, S. P.; Li, Q. H.; Zhu, X. L. Polymer 2008, 49, 3054–3059. (c) Luo, R.; Sen, A. Macromolecules 2008, 41, 4514–4518. (29) Tang, F.; Zhang, L. F.; Zhu, J.; Cheng, Z. P.; Zhu, X. L. Ind. Eng. Chem. Res. 2009, 48, 6216–6223. (30) (a) Zhang, W.; Yan, Y. F.; Zhou, N. C.; Cheng, Z. P.; Zhu, J.; Xia, C. M.; Zhu, X. L. Eur. Polym. J. 2008, 44, 3300–3305. (b) Cho, Y. S.; Kim, S. W.; Ihn, C. S.; Lee, J. S. Polymer 2001, 42, 7611–7616. (31) Kim, D. K.; Zhang, Y.; Voit, W.; Rao, K. V.; Muhammed, M. J. Magn. Magn. Mater. 2001, 225, 30–36. (32) (a) Sun, Y. B.; Ding, X. B.; Zheng, Z. H.; Cheng, X.; Hu, X. H.; Peng, Y. X. Chem. Commun. 2006, 2765–2767. (b) Lattuada, M.; Hatton, T. A. Langmuir 2007, 23, 2158–2168. (c) Hu, F. X.; Neoh, K. G.; Cen, L.; Kang, E. T. Biomacromolecules 2006, 7, 809–816. (33) Cheng, Z. P.; Zhu, X. L.; Fu, G. D.; Kang, E. T.; Neoh, K. G. Macromolecules 2005, 38, 7187–7192. (34) (a) Li, Q.; Zhang, L. F.; Zhang, Z. B.; Zhou, N. C.; Cheng, Z. P.; Zhu, X. L. J. Polym. Sci., Part A: Polym. Chem. 2010, 48, 2006–2015. (b) Liu, J. L.; Zhang, L. F.; Shi, S. P.; Chen, S.; Zhou, N. C.; Zhang, Z. B.; Cheng, Z. P.; Zhu, X. L. Langmuir 2010, 26, 14806–14813.
12692
dx.doi.org/10.1021/la202749v |Langmuir 2011, 27, 12684–12692
