Article pubs.acs.org/Langmuir
Janus Magnetic Nanoparticles with a Bicompartmental Polymer Brush Prepared Using Electrostatic Adsorption to Facilitate Toposelective Surface-Initiated ATRP Erick S. Vasquez,† I-Wei Chu,†,‡ and Keisha B. Walters*,† †
Dave C. Swalm School of Chemical Engineering and ‡Institute for Imaging and Analytical Technologies, Mississippi State University, Mississippi State, Mississippi 39762, United States S Supporting Information *
ABSTRACT: Utilizing the inherent negative charge of mica surfaces, amine-functionalized magnetic nanoparticles (Fe3O4/ NH2) were electrostatically adsorbed onto the mica such that surface-initiated ATRP could be used to grow poly(nisopropylacrylamide) (PNIPAM) from the exposed hemisphere. By reducing the solution pH, a positive charge generated on the mica was used to release the nanoparticles from the substrate. A second ATRP reaction was carried out to grow poly(methacrylic acid) (PMAA) from the initiated surfaces. As a result, the Fe3O4/NH2 core has a polymer shell with one hemisphere PMAA and the other hemisphere PNIPAM-b-PMAA resulting in the PMAA−Fe3O4−PNIPAMb-PMAA bicompartmental polymer Janus nanoparticles. Elemental and functional group compositions were confirmed using ATR-FTIR, XPS, and EDS. Imaging with AFM, SEM, and TEM showed the evolution of the Janus nanoparticle morphology. This study demonstrates a facile and innovative scheme involving a noncovalent solid protection technique combined with sequential, surface-confined controlled radical polymerizations for the production of multicomponent nanocomposites.
1. INTRODUCTION Studies of asymmetrical spherical particles, also known as Janus particles, have increased substantially over the last three decades. Janus particles are composed of at least two physically and/or chemically distinct surfaces. Properties inherently resulting from their structure make them great candidates for a wide range of applications, such as photovoltaic cells, electric paper, antireflection coatings, optical and optoelectronic displays, drug delivery, microrheology probes, and sensing devices.1−6 Janus particles (JPs) were first introduced as Janus grains7 and Janus beads in the experimental efforts of Casagrande et al.,8 where a glass core was modified to have distinct hydrophilic and hydrophobic “faces”. Synthesis processes reported for producing asymmetrical particles include surface templates and area selective modification,9 templateassisted self-assembly,10 emulsion/phase separation,11 surfacecontrolled nucleation and growth, 12,13 and microfluidic techniques.14 Regardless of the synthesis route, the major advantage of the JP structure is the duality of the resultant particle that can allow for distinct and even divergent properties within the same physical system, dynamic and adaptive responses to external conditions, and/or multifunctional composite capabilities. Different methods and synthesis routes for a variety of multicomponent JPs have been previously reviewed, introducing a vast range of asymmetrical geometries including © 2014 American Chemical Society
mushroom- and snowman-shaped structures, just to mention a couple.4,15−17 Hence, multiple properties (e.g., magnetic, catalytic, optical, electrical) can be tuned in a multicomponent JP system due to variations in particle chemistry and/or morphology. In addition to the JP physical properties and chemical composition, the synthesis route plays an important role in producing stable, useful, and long-lasting particles. Thus, new synthesis routes and characterization methods for chemically distinct surface-modified particles are all current areas of study for JP structures.17,18 Several methods have been used to synthesize multiregion, multicomponent, or multicompartment JP structures with a characteristic core/shell shape using inorganic and/or organic matter. In general, a multicomponent core/shell polymer Janus particle has a distinctive inorganic/organic core and two (biphasic) or more (multicomponent) materials (e.g., polymers) comprising the shell.15,19,20 Janus micelles, composed of organic cores and organic shells, can be formed via distinctive self-assembly processes from amphiphilic block copolymers, triblock copolymer, or linear block terpolymers in aqueous or organic solvents.15 Similarly, JP structures have been formed with inorganic cores including silica beads,21 gold,22−24 and/or Received: March 24, 2014 Revised: May 16, 2014 Published: May 20, 2014 6858
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Figure 1. Synthesis scheme for bicompartmental PMAA−Fe3O4−PNIPAM-b-PMAA Janus nanoparticles (JPs): (a) electrostatic toposelective deposition of Fe3O4/NH2 nanoparticles onto a mica substrate, (b) Br-initiation reaction with the exposed Fe3O4/NH2 nanoparticle surface, (c) polymerization of PNIPAM by surface-initiated ATRP, (d) release of modified Fe3O4/NH2−PNIPAM particles from the mica sheet by sonication in an acidic solution, (e) reinitiation of unreacted amine groups on the Fe3O4/NH2 nanoparticle surface, and (f) ATRP polymerization of PMAA.
magnetic nanoparticles.12,25 When magnetic nanoparticles (MNPs) are used as the inorganic core, the resultant JP can be manipulated via an external magnetic field which has generated tremendous interest in magnetic-core JP structures.26 In the present work, bicompartmental Janus magnetic nanoparticles are described as a core−shell structure with MNPs as the inorganic core and a shell made up of two dissimilar polymers to create a bicompartmental polymer-shell magneticcore structure. Controlled radical polymerizations (CRP) have been used to develop core−shell nanostructures and are currently being used to develop new JP materials. Using this method, almost any inorganic nanoparticle can act as the core with one or more polymers attached as the shell to the inorganic substrate using various approaches. Hatton’s group produced inorganic polymeric core−shell JPs using the surface-confined modification of nanoparticle substrates both with “grafting to” and “grafting from” approaches.12 Berger et al. worked with stimuliresponsive acrylate polymers, formed using surface-initiated atom-transfer radical polymerization (ATRP) and the graftingto approach to produce stimuli-responsive biphasic JPs using silica nanoparticle cores.13 Another study by Li et al. involved “hairy” nanoparticles produced via CRP of poly(tert-butyl acrylate) (PtBA) and then used hydrolysis to create a mixed poly(acrylic acid) (PAA) and PS polymer brush on the surface of silica (Si) nanoparticles, obtaining a PAA-Si-PS hybrid system.21 Gold (Au) nanoparticles have also been used as the substrate with grafting-to and grafting-from approaches to create biphasic JPs with different polymeric shells. For example, Au nanoparticles have been partially modified using a thiol endfunctionalized single-crystal poly(ethylene oxide) (PEO) substrate and ATRP of PAA and/or PtBA on only one side of the Au nanoparticles.23 Other single-crystal template methods have been reported for the synthesis of bicompartmental Janus nanoparticles using poly(n-isopropylacrylamide) (PNIPAM), silicon dioxide as a core, and poly(ε-caprolactone).27 These methods were found to avoid particle aggregation and result in an increased areal density.23,28 In the present work, reversible pH-modulated electrostatic interaction between amine-functionalized magnetic iron oxide
nanoparticles (Fe3O4/NH2) and a negatively charged mica substrate is explored as an alternative technique for producing bicompartmental/biphasic polymer Janus magnetic nanoparticles with the aid of surface template assistance. The negative charge found in the mica is the result of the dissociation of K+ ions in water that produce a negative (AlO2−) outer substrate layer.29 Thus, the partial charge of the amine group NH3+ and AlO2− is used for the electrostatic toposelective attachment of Fe3O4/NH2 nanoparticles onto the mica substrate. A surfaceconfined-initiator grafting from approach is used with controlled radical polymerization to produce a bicompartmental polymer shell on the Fe3O4/NH2 core, allowing for controlled molecular weight and thickness of the brush. A new synthesis route for the fabrication of three-dimensionally amphiphilic and asymmetric Janus magnetic nanoparticles is presented. The synthesis involves a combination of two methods: (1) particle retention on a solid substrate by electrostatic interactions and (2) surface-confined ATRP. The bicompartmental surface-grafted polymers are both stimuli-responsive polymers, poly(methacrylic acid) (PMAA) and PNIPAM, with each grafted onto opposite hemispheres of amine-functionalized magnetic iron oxide nanoparticles (Fe3O4/NH2). The formation of surface-grafted bicompartmental PMAA/PNIPAM-b-PMAA Janus magnetic nanoparticles (PMAA−Fe3O4−PNIPAM-b-PMAA) via a combination of noncovalent solid protection chemistry and surface-confined ATRP was successfully confirmed. The chemical composition and structure were analyzed by attenuated transmitted reflectance Fourier transform infrared spectroscopy (ATRFTIR), X-ray photoelectron spectroscopy (XPS), and energydispersive X-ray spectroscopy (EDS). The morphological structure of this polymeric−inorganic−polymeric nanohybrid system of 3D amphiphilic and asymmetric magnetic JPs was examined using atomic force microscopy (AFM) and transmission electron microscopy (TEM). In addition, the PMAA− Fe3O4−PNIPAM-b-PMAA JP nanostructures described in this work could potentially be used in different scenarios due to the dual stimuli responses of the two different polymer brushes grafted on each side of the particles and the ease of manipulation of these particles in the presence of a magnetic 6859
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field. Moreover, self-assembly, nanocarrier systems, and electrophoresis are topics of interest for further multicompartment polymer Janus magnetic nanoparticles.
becomes stronger than the electrostatic interaction between the particles and the substrate. In this manner, partially polymerized hemispherically shaped PNIPAM-Fe3O4/NH2 nanoparticles were released from the mica substrate. The solvent was then evaporated using rotoevaporation, and the particles from the first polymerization were transferred to 30 mL of ethanol, sonicated, and subjected again to rotoevaporation; this “washing” procedure was repeated in triplicate. The particles were stored in toluene at ∼4 °C prior to the next polymerization step. 2.2.4. Preparation of Br−Fe3O4−PNIPAM−Br Nanoparticles. The stored NH 2/Fe3O4−PNIPAM nanoparticles were dried using rotoevaporation prior to the reaction of the bromine initiator to the unreacted surface amines. Modified NH2/Fe3O4−PNIPAM particles were placed in a 0.01 M 2-bromoisobutyryl bromide/triethylamine/ toluene solution and allowed to react for 12 h.32 Thus, the unreacted amines on NH2/Fe3O4−PNIPAM were surface-initiated to create a Br−Fe3O4−PNIPAM−Br particle system (Figure 1e). The resultant particles were then washed using centrifugation and isopropanol addition three times and then transferred to a clean container. Particles were washed two more times with rotoevaporation and finally stored in water at 4 °C. 2.2.5. Synthesis of Janus (PMAA−Fe3O4−PNIPAM-b-PMAA) Nanoparticles. A 2 M solution of MAA/water was prepared, and NaOH (1 M) was used to neutralize the solution to pH 7.33 The neutralized MAA solution was placed under a nitrogen stream for 1 h and submitted to three freeze−pump−thaw cycles. This degassed monomer solution was added to the dried and cleaned particles (section 2.2.4) using a cannula under an inert (N2) atmosphere. Then, a degassed 0.1 mM solution of Me6TREN and Cu(I)Cl was added to the monomer/particle solution to start the ATRP polymerization from the surface of the Br−Fe3O4−PNIPAM−Br particles. This reaction was allowed to proceed for 8 h, obtaining the bicompartmental PMAA−Fe3O4−PNIPAM-b-PMAA JPs (Figure 1f and detailed further in section S1). 2.3. Characterization. ATR-FTIR spectra were collected using a Miracle-ATR accessory (PIKE Technologies) with a diamond-ZnSe crystal. For ATR-FTIR characterization, nanoparticles were deposited as a solution droplet, allowing it to dry on the crystal surface, leaving a thin nanoparticle film. A minimum of 800 scans per sample were collected on a Nicolet 6700 FTIR spectrophotometer (Thermo Electron Corporation) with a He−Ne laser, MCT-A* detector, and KBr beam splitter using Omnic software (v8.1.10, 1992−2009, Thermo Fisher Scientific Inc.). FTIR peak assignments were obtained from selected literature sources unless otherwise referenced.34,35 A PHI 1600 X-ray photoelectron spectroscopy (XPS) surface analysis system (Physical Electronics) with a Mg Kα X-ray source (300 W, 15 kV) and a 45° takeoff angle was used to collect XPS data. A PHI 10360 spherical capacitor energy analyzer and an Omni Focus II smallarea lens were utilized to focus the incident source. Survey spectra were collected using a minimum of 10 scans with a 26.95 eV pass energy across the 1100 to 0 eV range. High-resolution spectra were gathered using a minimum of 15 scans with a 23.5 eV pass energy and a 0.1 eV step size. For XPS analysis, particles were deposited from solution onto UV/O-treated, Au-coated Si wafers and allowed to dry in a ventilated hood. Measurements were collected on at least three spots per sample. Gaussian peak fitting was performed using CasaXPS software v.2.2.88, and average results are reported. Scanning electron micrographs (SEM) and energy-dispersive X-ray spectroscopy (EDS) data were collected using a JEOL JSM-6500 field emission scanning electron microscope (FE-SEM) operated at 15 keV. SEM samples were deposited on carbon tape and dried in a ventilated hood prior to placement into the FE-SEM. Transmission electron microscopy (TEM) was performed using a JEOL 100CXII at 100 kV, and highresolution images were obtained using a JEOL 2100 at 200 kV. Sample droplets were placed on a carbon Formvar Cu grid (Electron Microscopy Science), and the solvent was evaporated at ambient temperature inside a ventilated hood. AFM data was collected on a Dimension Icon operated in tapping and ScanAsyst modes. A drop of the sample solution was placed onto mica and allowed to air dry. NCHV probes from Bruker with a 320 kHz resonance frequency, 42
2. EXPERIMENTAL SECTION 2.1. Materials. Muscovite mica, K[Si3Al]O10Al2(OH)2 (Electron Microscopy Sciences, Hatfield, PA, USA) was used as the substrate in this work. Aqueous ferrofluid fluidMAG-Amine (Chemicell GmbH, Berlin, Germany) contains magnetic iron oxide (Fe3O4) nanoparticles functionalized with amine groups (50 nm hydrodynamic diameter; 50 mg/mL concentration) and were used as the core for the preparation of Janus magnetic nanoparticles. The following reagents were obtained from Sigma-Aldrich: copper(I) chloride (CuCl, 97%), copper(I) bromide (CuBr, 98%), triethylamine (99.5%), hexamethylated tris(2aminoethyl)amine (Me6TREN, 96%), 2,2′-bipyridyl (bipyridine) (Aldrich, 99%), methacrylic acid (MAA, 99%), water (HPLC grade), 2-bromoisobutyryl bromide (98%), and propanol (99.7%).20 Ethanol (99.9%), tetrahydrofuran (THF, 99.9%), and glacial acetic acid (CH3COOH, 17.4 N) were obtained from Fisher Scientific. N(Isopropylacrylamide) (NIPAM, 98%) was received from TCI, and toluene (99.8%) was received from Acros Organics. All materials were used as received. 2.2. Synthesis. 2.2.1. Preparation of Surface-Initiated Magnetic Nanoparticles (Fe3O4−Br). A small amount of fluidMAG-amine aqueous solution, 10−15 μL, was deposited onto the surface of a mica sheet, and the solution was allowed to dry overnight at room temperature under a ventilated hood. After the solvent had evaporated, the mica substrate with adsorbed nanoparticles was rinsed thoroughly with distilled water and dried with a nitrogen gun. Thus, particles remaining on the surface were attached primarily due to the electrostatic interactions between the amine-functionalized nanoparticles and the negatively charged mica substrate (Figure 1a).30 Hereafter, dried nanoparticles are referred to as Fe3O4/NH2. To covalently react the initiator for polymerization to the exposed Fe3O4/ NH2 nanoparticle surface, the mica-Fe3O4/NH2 substrate was placed in a 5 mL THF solution of 2-bromoisobutyryl bromide (0.01 M) and triethylamine (0.012 M) (Figure 1b). The reaction of 2-bromoisobutyril bromide with the available amine groups was allowed to proceed overnight with an inert nitrogen sparge and sonication. After 12 h, the reaction solution was opened to ambient conditions, and the mica substrate was removed from the solution. The mica substrate with electrostatically attached brominated Fe3O4/NH2 nanoparticles (micaFe3O4/NH2−Br, Figure 1b) was washed thoroughly with ethanol, three times, ensuring that only the magnetic particles with strong electrostatic interactions would remain on the mica surface. The remaining adsorbed nanoparticles, with bromine initiator moieties on their exposed surface, were used for PNIPAM polymerization. 2.2.2. Synthesis of Mica-Fe3O4/NH2−PNIPAM. ATRP using NIPAM as the monomer was started immediately after the reaction of the bromine initiator onto the exposed amines on the micaadsorbed Fe3O4/NH2 NPs. NIPAM ATRP was performed in propanol/water (70/30 v/v) with a 100/1/1 monomer/catalyst/ ligand molar ratio (Figure 1c).31 First, Cu(I)Br (catalyst) and bipyridine (ligand) were placed in 5 mL of propanol and subjected to freeze−pump−thaw cycles until no gas bubbles were observed during thawing. Afterward, this solution was nitrogen sparged for 1 h in a sealed test tube. Then, NIPAM monomer was added to 30 mL of water/propanol solution (70/30 v/v), and at least three freeze− pump−thaw cycles were performed. The catalyst/ligand and monomer solutions were added under inert conditions using a cannula to a separate a purged vessel containing the mica-Fe3O4/NH2−Br substrate. NIPAM polymerization proceeded for 12 h under an inert atmosphere (N2 flow) at room temperature (Figure 1c). This reaction was quenched by introducing air into the reaction. 2.2.3. Detachment of Fe3O4/NH2−PNIPAM. The electrostatic interaction holding the Fe3O4/NH2−PNIPAM nanocomposite to the mica was reversed by decreasing the pH to a value of 2 using the dropwise addition of 2 M acetic acid (Figure 1d). At this low pH value, the attraction between the carboxylate anion (COO−) and NH3+ 6860
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First, the peaks at ∼2000 cm−1 associated with uncoated/ oxidized Fe3O4 surfaces were no longer observed. Second, a broad band now observed at 3360 cm−1 can be attributed to O−H stretching in PMAA.38 Third, in the range of 1600−1720 cm−1 two overlapping peaks (1650 and 1710 cm−1) were observed that are characteristic carbonyl absorbances for PNIPAM (−C(O)N) and PMAA (−C(O)OH), respectively.40,41 Lastly, at 1540 cm−1 a sharp peak was observed from the C−O structure of the PMAA carboxylate anion, as discussed elsewhere.42 (Note that a discussion of the 1080 cm−1 peak is included in section S2.) As a result, ATR-FTIR analysis supports the formation of bicompartmental PMAA− Fe3O4−PNIPAM-b-PMAA JPs. X-ray photoelectron spectroscopy (XPS) was used to examine the surface elemental composition of the Fe3O4/ NH2 nanoparticles and the resulting bicompartmental JPs. Figure 3 shows representative XPS survey scans collected for (a) Fe3O4/NH2 nanoparticles and (b) PMAA−Fe3O4− PNIPAM-b-PMAA JPs. The presence of carbon (28.7 ± 3.6%), oxygen (49.2 ± 2.2%), iron (17.4 ± 3.0%), nitrogen (1.7 ± 1.6%), and silicon (3.0 ± 1.8%) was detected for the Fe3O4/ NH2 nanoparticles. The average atomic compositions measured for the Janus nanoparticles were 55.1 ± 8.1% C, 24.1 ± 0.7% O, 3.7 ± 1.8% N, 0.5 ± 0.1% Br, and 16.6 ± 6.6% Si. The silicon content and the presence of other background peaks (represented by dotted lines in Figure 3b) in the JP samples are attributable to mica contamination and background effects (sections S2 and S3). For comparison purposes, theoretical NIPAM atomic compositions for carbon, oxygen, and nitrogen are 75, 12.5, and 12.5%, and MAA theoretical atomic percentages for carbon and oxygen are 67% and 33%. The atomic compositions measured using XPS for the JPs without considering silicon (carbon 65.8%, oxygen 29.0%, nitrogen 4.5%, and bromine 0.7%) are within the expected range for a mixed PNIPAM/PMAA polymer brush. After the release of the half-moon NPs from the mica substrate, the surface-confined initiation and polymerization reactions were performed again, resulting in the second polymerization forming a PMAA brush from the bare NH2-funtionalized NP surface and from the hemispherical PNIPAM brush. Therefore, the PNIPAM compartment was covered with PMAA in a PNIPAM-bPMAA copolymer structure and was not probed by the shallow XPS depth of penetration (
