Synthesis, Characterization, and Application of Partially Blocked

Article Cite This: Langmuir 2017, 33, 14728−14737

pubs.acs.org/Langmuir

Synthesis, Characterization, and Application of Partially Blocked Amine-Functionalized Magnetic Nanoparticles A. Hosseinifar,†,‡ M. Shariaty-Niassar,†,‡ S. A. Seyyed Ebrahimi,*,‡ and M. Moshref-Javadi§ †

Transport Phenomena & Nanotechnology (TPNT) Lab., School of Chemical Engineering, College of Engineering, University of Tehran, Tehran 111554563, Iran ‡ Advanced Magnetic Materials Research Center, School of Metallurgy and Materials, College of Engineering, University of Tehran, Tehran 111554563, Iran § Department of Materials Science and Engineering, Monash University, Clayton, VIC 3800, Australia S Supporting Information *

ABSTRACT: In this study, a novel technique was introduced for selective surface modification of amine-functionalized magnetic nanoparticles. The method was based on alignment of magnetic nanoparticles in an external magnetic field, which resulted in formation of chain-like assemblies in diluted suspensions. The aligned chains were then modified on the surface via reaction of isocyanate species with the particle functionalities. Finally, after removal from the reactor medium, particles with segmented distribution of surface functionalities were achieved. We named these partially blocked amine-functionalized magnetic nanoparticles as “Saturn” nanoparticles. Application of the particles in fabrication of magnetic assemblies was successfully demonstrated. Using methylene diphenyl diisocyanate (MDI) as the bridging agent, structures in different forms such as chains and filaments were produced by the Saturn particles and compared with cross-linked structures of the unmodified amine-functionalized particles. It is expected that this novel nanoparticle with its unique structure will have great potential in assembly fabrication with a variety of applications in biomedical fields.

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INTRODUCTION Surface engineering and fabrication of ordered assemblies of nanoparticles are of growing interest in many fields such as micro-electromechanical systems,1,2 microfluidics,3,4 medicine,5,6 energy storage,7 and optoelectronics.8,9 These nanostructures have been fabricated through a wide variety of methods including self-assembly,10 magnetic field directed assembly,11 electric field directed assembly,12 polymer mediated assembly,13 and patterned templating.14 Among all assemblies, there has been a great interest in those that find order based on their magnetic properties. These magnetically ordered assemblies have been reported in the forms of hollow sphere,15 rod,16 wire,17 chain,18 filament,19 and cilia20 with diverse applications in sensors,21 catalysts,20 contrasting agents in magnetic resonance imaging (MRI),22 and drug delivery.23 Such assemblies are produced in either the presence or absence of external magnetic fields. In the absence of magnetic fields, dipolar interactions of magnetic nanoparticles are known to be the primary driving force for formation of the assemblies.24 In the presence of a uniform magnetic field, however, morphology and alignment are governed not only by the reorientation of the crystal magnetic easy axis toward the external magnetic field but also by the magnetophoretic attraction between the particles.25−27 In the field of magnetic assemblies, iron oxide nanoparticles are of immense popularity, thanks to their facile synthesis, © 2017 American Chemical Society

stability, nontoxicity, high magnetic susceptibility, and easy control over size and shape.28−30 Although the surface of the iron oxide particles is intrinsically decorated with hydroxyl groups, they cannot be considered as appropriate functionalization agents in assembly processes. The reason is that their density strictly depends on pH of the surrounding media, synthesis procedure, and temperature, rendering them not readily accessible for the relevant reactions. Hence, functionalization of Fe3O4 nanoparticles is deemed essential in fabrication of magnetic assemblies. Moreover, in functionalized magnetic particles, it is desired to have the minimum magnetic interference caused by the functional groups or moieties. Therefore, compared with other coatings, silica and functionalized silica coatings are predominantly utilized for this purpose. In particular, amino alkoxysilanes are frequently used as the functionalization reagent, owing mainly to their high reactivity and accessibility.31 Suitability of functionalized silica shells for the subsequent attachment and assembly of various macromolecules has been frequently reported in the literature. For instance, assembly of amine-functionalized particles can be performed using diisocyanates as the bridging agent, where amine groups react Received: June 19, 2017 Revised: October 26, 2017 Published: November 28, 2017 14728

DOI: 10.1021/acs.langmuir.7b02093 Langmuir 2017, 33, 14728−14737

Article

Langmuir Scheme 1. Possible Routes for Synthesis of Diureasil Compoundsa

a

Pathway 1, reaction of two equivalents of amino siloxane and a diisocyanate chain. Pathway 2, reaction of two equivalents of isocyanato siloxane and a diamine chain.

iron oxide nanoparticles to a selective modification process, that is, urea formation via isocyanate chemistry on the surface. The Saturn nanoparticles exhibited selectively blocked functionalities: the same reactive amine groups at the poles and nonreactive urea groups in the equatorial regions. Hence, contrary to Janus nanoparticles, the surface of the Saturn particle was divided into three faces. The particles were also innovatively applied in fabrication of magnetic assemblies such as chains and filaments. The method devised here presents an efficient technique for modifying particles surfaces that can be applied as an alternative approach in fabricating ordered assemblies.

with isocyanates. Many primary and secondary amines form ureas by a nucleophilic addition reaction with an isocyanate.32 For example, van Tilburg et al.33 reported high overall yield (70−90%) reactions of phenylisocyanate with primary amines at room temperature over a duration of 0.5−3 h. Urea, with its long-range intramolecular hydrogen bonds and the peripheral polar groups, acts as a strong bridging agent, which provides a convenient opportunity for 1D directionality. This particular characteristic allows for synthesis of stable columnar aggregates, chain architecture, and incorporation of organic chains into inorganic backbones.34 Bisurea-based compounds, as well as their corresponding chains, can be formed from either a reaction of a diamine chain with isocyanates or a diisocyanate chain with amines.35 Scheme 1 illustrates possible routes for synthesis of diureasil compounds. As shown in the scheme, a parent silane group can bear either amine or isocyanate moieties. As an example of the first pathway of bridging between Si atoms, Roy et al. reported cross-linking of amine-functionalized silica nanoparticles with hexamethylene diisocyanate oligomer (Desmodur N3200).36,37 An example of the second pathway of diureasil formation is synthesis of a bridged siloxane performed by the reaction of γ-isocyanatopropyltriethoxysilane with 1,4-diaminobenzene.38 In addition, spiral and lamellar structures of diureasil bridges were reported from the reaction of a chiral diamine and diamino dodecane with the same isocyanatosiloxane, respectively.39 Methylene diphenyl diisocyanate (MDI) was also used to form bonds between amine groups in aerogels.40 In these examples, where aromatic bridging is possible, cooperative effects of π−π stacking of aromatic cores played a role in assembling the particles in a peripheral orientation. All in all, bisurea-based compounds are well-known among the best candidates for bridging and architecting structures and assemblies. While surface modification of the particles can be carried out fully and uniformly over the entire surface, it can also be performed segmentally in a selective manner. Here, the best example is Janus nanoparticles,41 whose surface is divided into two equal distinct faces as a result of the modification. An exemplary selective modification of “pearl-necklace” magnetic assemblies has also been conducted previously by acid etching of PS−AuCoNP chains on solid supported substrates.42 Such specific surface modifications present interesting applications in catalysts, magnetic assemblies, and drug delivery.41,43,44 Nevertheless, more variations of selectively surface modified particles can be created that assist in facile development of new or improved structures and properties. In this study, we report a new generation of modified nanoparticles, which we name “Saturn” nanoparticles. The particles were synthesized by subjecting amine-functionalized

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EXPERIMENTAL SECTION

Materials. All chemicals including FeSO4·7H2O, FeCl3, absolute ethanol, NaOH, (3-aminopropyl)triethoxysilane (APTS), phenylisocyanate, and methylene diphenyldiisocyanate (MDI) (mixture of di and tri) were purchased from Merck and were used as received. Water was distilled twice before use. Magnetic Setup. A setup was designed and used to achieve uniform magnetic fields (Figure 1). The setup consisted of a current

Figure 1. (a) 3D representation of the setup developed to apply uniform magnetic fields and the reactor used for modification of the nanoparticles at the uniform level of the applied field. (b) Magnified image of the reactor, showing magnetic particles inside the solution aligned toward the field direction and a syringe used for injecting the reagents.

controllable electromagnet with an iron core and a 10 cm gap, capable of creating magnetic fields up to 600 mT. In a magnetic field of 260 mT, the field gradient was measured to be minimum at a 4 cm distance from the poles. This level was thus selected and employed as the uniform field. A 30 mL capacity reactor was made from glassware and polytetrafluoroethylene (PTFE) and held exactly in the uniform field area within the gap. Alignment of amine-functionalized magnetic nanoparticles (AFMNPs) was conducted inside the reactor so as to form the maximum number of aligned aggregates (Figure 1b). The 14729


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Langmuir Scheme 2. Synthesis Reaction of the Fe3O4 Nanoparticles and the AFMNPs

Scheme 3. (a) Reaction of AFMNPs with MDI as the Cross-linker in the Presence and Absence of the External Magnetic Field and (b) Synthesis Process of PBAFMNPsa

Steps and schematics: (I) Schematic of a single AFMNP; (II) formation of aligned AFMNPs in an external magnetic field; (III) injection of phenylisocyanate into the reactor medium and its reaction with amine functionalities on the surface of the AFMNPs; (IV) formation of aligned PBAFMNPs in an external magnetic field; (V) schematic of a single PBAFMNP.

a

nitrogen gas flow. Upon addition, black precipitates appeared in the solution. Next, the mixture was vigorously stirred for 30 min and then transferred to a Teflon sealed autoclave reactor for processing at 180 °C for 6 h. After cooling down at room temperature, the particles were removed from the reactor. They were then washed several times with doubly distilled water and were collected magnetically. Finally, a step by step separation process by a high speed centrifuge was followed to remove agglomerates and large aggregates in the powder. For this

reactor was equipped with two silicon rubber septa to inject reagents into the reactor. Synthesis of Fe3O4 Nanoparticles. Fe3O4 nanoparticles were prepared by coprecipitation method, followed by applying hydrothermal treatment (Scheme 2). Typically, 0.695 g of FeSO4·7H2O and 0.8110 g of dehydrated FeCl3 were dissolved in 50 mL of acidic solution (0.01 N HCl), and the resulting solution was added dropwise to 50 mL of alkaline solution containing 0.8 g of NaOH under 14730


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Langmuir purpose, the achieved black precipitated powder was initially dispersed in 100 mL water using sonication method. Afterward, separation was conducted at different centrifugation speeds from 1000 to 6000 rpm. At each separation step, the remaining solution (supernatant) was taken and centrifugation was applied thereon at a higher speed. Finally, after 6000 rpm, the particles were collected from the supernatant using a magnet. Synthesis of Amine-Functionalized Magnetic Nanoparticles (AFMNPs). The synthesized Fe3O4 nanoparticles were functionalized with amine groups using aminopropyl triethoxysilane (APTS) to form amine-functionalized magnetic nanoparticles (AFMNPs). For this purpose, according to the reaction shown in Scheme 2, 0.4 g of the synthesized Fe3O4 nanoparticles were dispersed in 200 mL of absolute ethanol via sonication for 1 h. Then, 100 μL of APTS was added to the suspension of nanoparticles and the sonication process continued for 10 min. Next, a mixture of 4 mL water and 1 mL of 25% ammonia solution was added dropwise to the solution. The sonication process was again performed for 30 min, followed by stirring the suspension overnight. Finally, AFMNPs were collected magnetically and washed several times with deionized water and ethanol. Cross-linking of AFMNPS in Presence and Absence of the Magnetic Field. To test the cross-linking ability of the AFMNPs with MDI, 4 mg of AFMNPs were dispersed in 15 mL of dry acetonitrile, followed by adding 10 mL of 10 mM solution of MDI in dry acetonitrile to it. After a few minutes, black gel pieces appeared in the solution, indicating the cross-linking of AFMNPs. Over time, the particles converted to condensed forms and finally precipitated at the bottom of the container. The precipitates were gently washed several times with acetonitrile to remove the unreacted reagents. This process was also performed in the presence of a 50 mT magnetic field. Scheme 3a illustrates the reactions of the AFMNPs with MDI in presence and absence of the magnetic field. Partial Blocking of Amine Functionalities on the Surface of the AFMNPs. In the next step of the surface modification, a novel strategy was devised to partially block the amine functionalities on the surface of the AFMNPs. The modification was, fundamentally, made under a uniform magnetic field. Initially, 5 mg of AFMNPs were dispersed in 30 mL of dry acetonitrile and sonicated for 30 min. This dispersion was injected into the reactor, and the magnetic field of 260 mT was switched on, upon which the particles formed chain like assemblies of magnetic particles. After 15 min, 2 mL of a 1 mM solution of phenylisocyanate in dry acetonitrile was injected slowly at a rate of 1 mL/min. During all the injections, the trapped air in the gap was drained out using another syringe needle. The reaction between phenylisocyanate and amine heads (Scheme 3b) was allowed to occur overnight, and the product of this step was termed “partially blocked amine functionalized magnetic nanoparticles (PBAFMNPs)”. At the next stage, the produced particles had to be removed from the reactor. Due to the excess amount of phenylisocyanate in the reaction medium, removing particles led to free movement of the particles in the solution, and consequently full coverage of the AFMNPs with phenylisocyanate took place. To resolve this problem, before removing magnetic nanoparticles from the reactor and while the magnetic field was still switched on, 2 mL of water was injected gently. Although the reaction of the excess amount of isocyanate with water is fast, about 4 h was considered for the reaction, due to lack of any mixing regime in the reactor. The magnetic field was then switched off, the reactor was evacuated, and magnetic nanoparticles were collected magnetically. Nanoparticles were again washed several times with acetonitrile to remove unreacted species and impurities. The samples were dried in a reduced pressure overnight at 80 °C. Bridging of the PBAFMNPs. In order to fabricate magnetic chains, MDI was used as the bridging agent between PBAFMNPs. Active functional groups at the poles of PBAFMNPs as well as the dipole−dipole interactions of the particles acted as the driving forces to pose the particles next to each other. According to the reaction shown in Scheme 4, surface amine groups and isocyanate moieties react and form diureasil bridges. For this purpose, 10 mL of a 10 mM solution of MDI in dry acetonitrile was added to a suspension of 0.5 mg of PBAFMNPs in 10 mL of dry acetonitrile inside the beaker. The

Scheme 4. Formation of Chains Based on the Reaction between PBAFMNPs and MDI as the Bridging Agent

suspension was then shaken well and was left for 4 h to allow the reaction to complete. The mixture was washed gently several times with acetonitrile to remove the extra MDI. After solvent evaporation, a dark brown precipitate remained, which was subsequently prepared for characterization. Surface Functionalization of the Glass Substrate. The synthesized nanoparticles and chains were transferred onto a substrate for subsequent examinations. A glass substrate was selected, according to the main applications of the produced chains in sensors and actuators. Functionalization of the glass substrate was performed based on self-assembly of the silane molecules to form an amine monolayer, as described by van Veggel et al.45 In brief, the method is as follows. A soda lime glass substrate was cleaned in boiling piranha and then washed several times with doubly distilled water. Next, the substrate was dried with nitrogen gas and then immersed in a 10 mM solution of APTS in dry toluene overnight. The substrate was then removed from the solution and washed with toluene (twice), dichloromethane (twice), and ethanol (twice) to remove the physisorbed species. Fabrication of Aligned Magnetic Chains on the AmineFunctionalized Glass Substrate. The PBAFMNPs were used as the building blocks for fabrication of the magnetic chains on the glass substrate. Similar to the previous section, MDI was used as the bridging agent. MDI reacts not only with amine functionalities of the PBAFMNPs but also with amine functionalities of the as-prepared glass substrate. Since a vast portion of the amine groups on PBAFMNPs were blocked by phenylisocyanate, the reaction between them and MDI was very slow compared to that between AFMNPs and MDI. To solve this, 4 mg of PBAFMNPs was dispersed in 15 mL of acetonitrile and sonicated for 30 min. In another container, 100 μL of MDI was dissolved in 5 mL of dry acetonitrile. Amine-functionalized glass substrate was placed at the bottom of a 25 mL beaker, and the beaker was placed over a weak permanent magnet to align the particles perpendicular to the glass substrate (50 mT). The suspension of PBAFMNPs and MDI solution were instantly transferred to the beaker. Bridging of the magnetic nanoparticles with MDI took place in a weak magnetic field, that is, 50 mT, in a safe place without any vibration. Laser light exposure onto the surface of the glass substrate revealed the evidence of chain formation according to the light scattering observed. After 3 h, when all the nanoparticles participated in chain formation, the solvent was removed by a syringe, and the glass substrate was slowly washed several times with fresh solvent. The solvent was then allowed to evaporate at room temperature overnight. Characterizations. Structural characterization of the materials was conducted using a PerkinElmerFourier transform infrared (FT-IR) spectroscopy instrument in the range of 4000−400 cm−1 in KBr pellets. After gold coating of the samples using a HITACHI S-4160 model, their morphologies were examined by a field emission scanning electron microscope (FESEM). The accelerating voltage of the FESEM was 30 kV. Transmission electron microscopy (TEM) was carried out using a Philips CM30 electron microscope operated at an accelerating voltage of 300 kV. The X-ray diffraction (XRD) patterns and pole figures of the nanoparticles and fabricated structures were 14731


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Langmuir recorded using a Philips Xpert Pro diffractometer with Cu Kα radiation (λ = 0.15406 nm). The patterns were measured between 10° and 100° with a step size of 0.02°, while the pole figures were recorded at 63.01°. Characterization of the magnetic properties was performed on the powder samples and thin films using a vibrating sample magnetometer (VSM) parallel to the magnetic field.

new bands clearly demonstrate successful reaction of phenylisocyanate and amine groups on the surface of the functionalized particles. The remaining peaks are similar to those of the AFMNPs. Figure 2d shows the FT-IR spectrum of chains of PBAFMNPs bridged together by MDI. The peak at 1705 cm−1, which corresponds to carbonyl groups, shows an increased intensity, attributed to the new bonds formed at the poles of the particles. Augmented intensity of the sharp peak at 1509 cm−1 also confirms the same. The appearance of a peak at 2260 cm−1 suggests some intact isocyanate groups in the chains, as they refer to the stretching vibrations of the -N CO moieties. The intact species may be due to unreacted functionalities of the trimer impurities of isocyanate (methylene triphenylisocyanate) or isocyanates far from amine groups, as depicted in Scheme S1. Microscopic Studies. Figure 3a shows FESEM micrograph of the cross-linked aggregates of AFMNPs formed using MDI

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RESULTS AND DISCUSSION FT-IR Results. Figure 2 demonstrates FT-IR spectra of bare nanoparticles, functionalized nanoparticles, and the resulting

Figure 2. FT-IR spectrum of (a) Fe3O4 nanoparticles, (b) aminefunctionalized Fe3O4 nanoparticles (AFMNPs), (c) partially blocked amine-functionalized nanoparticles (PBAFMNPs), and (d) bridged chains of PBAFMNPs.

chains. For Fe3O4 nanoparticles, the characteristic peaks at 590 and 462 cm−1 are related to bending vibrations of the Fe−O bond (Figure 2a). The broad envelope at the high energy level is attributed to the adsorbed moisture. The peak around 1620 cm−1 also signifies the adsorbed water, as it belongs to the related bending vibrations. These results are in excellent agreement with other reports on synthesized Fe3O4.46,47 Figure 2b represents FT-IR spectrum of the AFMNPs. The peak at 470 cm−1 is ascribed to the bending stretching vibrations in siloxane groups (Si−O−Si). In addition, the slight shifts from 590 to 573 cm−1 and 628 to 604 cm−1, compared to Figure 2a, indicate Fe−O−Si bond formation, while the broad peak at 1100−1200 cm−1 is attributed to its asymmetric stretching. Peaks related to silanol groups (Si−OH), amine groups on the silica surface, and C−H bonds proved to overlap to show a broad peak at ∼3400 cm−1. The weak shoulder in 2850−2950 cm−1 is related to symmetric vibrations of the C− H group. The peaks at 1637 and 800 cm−1 correspond to stretching vibrations of the water molecules. Overall, the spectrum clearly confirms grafting of the magnetite surfaces to the amino silicate shell through the silylation reaction. Figure 2c illustrates FT-IR spectrum for the PBAFMNPs. The carbonyl group of the urea bond, resulting from the reaction between the amine group of AFMNPs and the isocyanate moiety of phenylisocyanate, exhibited a characteristic peak at 1705 cm−1, attributed to the stretching vibrations of CO in amides. The sharp peaks at 1509 cm−1 as well as 1640 cm−1 represent C−C stretching in the aromatic ring. The

Figure 3. FESEM micrograph of (a) the cross-linked AFMNPs in zero magnetic field, (b) aligned aggregates of cross-linked AFMNPs formed in a 50 mT magnetic field, and (c) a single chain produced by bridging of PBAFMNPs and MDI in a highly diluted suspension of nanoparticles. (d) TEM micrograph of the chain in panel c. (e) FESEM micrograph of a filament of bridged PAFMNPs formed in a concentrated solution in a 50 mT magnetic field. (f) Optical micrograph of the aligned filaments shown in panel e after drying.

in a zero magnetic field. The morphology of the aggregated particles revealed a thoroughly randomized, shapeless, and porous structure. AFMNPs, as building blocks of the overall structure, are clearly seen on the bulk of the aggregates. TEM examinations showed AFMNPs to have core and shell sizes of 74.82 ± 8.34 nm and 11.93 ± 2.53 nm, respectively (Figure S1). Due to the high reactivity of amine surface functionalities with the cross-linking agents (Scheme 1), the reaction product, that is, bisurea bridge, filled the spaces between the nanoparticles, binding them together in all directions without any preferred alignment. In contrast, Figure 3b shows the structure of the aggregates formed in an applied magnetic field of 50 mT. 14732


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Langmuir It displays preferential alignment of the magnetic particles toward the applied field. It is obvious that MDI cross-linked the parallel assemblies of chains, leaving almost no chain free in the structure. Due to ordering of the nanoparticles as well as occupying the free spaces as a result of the applied field, the structure was also characterized by a denser packing. A chain resulting from bridging of PBAFMNPs using MDI as the bridging agent is shown in Figure 3c. The figure clearly demonstrates the preferential bridging along a single direction, implying unidirectional linking. The chains were formed using a low concentration of PBAFMNPs. Average diameter of the chain was ∼92 nm, and the chain proved to form from a single array of particles. The chain was generally straight with respect to its morphology. However, minor deviations from a straight chain could be noticed in some parts, which might originate from incomplete formation of PBAFMNPs and the resulting branching. TEM micrograph of this structure also confirmed PBAFMNPs chains to have a diameter about that of a single particle, with average core diameter and shell thickness of 75.34 ± 8.65 nm and 12.24 ± 2.99 nm, respectively (Figure 3d). This revealed that the particles were brought into intimate contact and formed a single-diameter chain with 1D directionality. Figure 3e shows a structure formed as a result of bridging a concentrated dispersion of PBAFMNPs in a 50 mT magnetic field. This assembly of chains, which can be called a filament, exhibited a thickness of 10−15 times greater than a single chain. Figure 3f shows the optical micrograph of the aligned filaments representing their long lengths ranging from 100 to 300 μm. The longer filaments up to millimeters can be fabricated by using higher concentration solutions of PBAFMNPs. The figure clearly demonstrates not only the high efficiency of MDI in bridging PBAFMNPs but also preferential alignment of the magnetic chains with minimum cross-linking between the parallel chains. X-ray Diffractions and Pole Figure Analysis. XRD technique was employed to study the crystalline structure of the as-prepared particles and AFMNPs (Figure 4). In Figure 4a, characteristic peaks at 2θ values of 30.27°, 35.71°, 43.30°, 53.81°, 57.28°, 63.01°, 71.40°, and 73.32° are assigned to (220), (331), (400), (422), (511), (440), (620), and (622) planes of the crystalline Fe3O4, respectively. These results are in a good agreement with the diffraction pattern of the cubic inverse spinel of Fe3O4 (PDF No. 01-075-0449). Diffraction pattern of the AFMNPs also revealed similar characteristics compared with that of the Fe3O4 nanoparticles, while the peaks exhibited more broadening, owing to the amorphous structure of the silica shell (Figure 4b). In order to examine the presence of preferred crystallographic orientations within the synthesized structures, the pole figure (440) was obtained for three specimens: (1) cross-linked AFMNPs in the presence of the external magnetic field, (2) cross-linked AFMNPs in absence of the external magnetic field, and (3) filaments of the bridged chains of PBAFMNPs. For pole figure analysis, selection of the appropriate crystallographic plane is generally based on the peak intensities (peak with the highest intensity) or the desired crystallographic orientation. It is well reported that one of the easy magnetization axes of Fe3O4 crystal is along the ⟨110⟩ direction,27 which is the normal of the (110) plane. However, the peak corresponding to this plane exhibited a weak intensity in the XRD pattern (at 2θ = 18°). To this end, (440) plane was selected as the appropriate plane for pole figure analysis, in that (1) it is in the same family of planes as (110) and thus their normal directions are parallel

Figure 4. (a) X-ray diffraction patterns: standard peaks of Fe3O4 (PDF No. 01-075-0449) (bottom), pattern of the Fe3O4 nanoparticles (middle), and pattern of the AFMNPs (top). Pole figures of (b) the random distribution of cross-linked AFMNPs, (c) aligned cross-linked AFMNP, and (d) aligned filaments of bridged PBAFMNPs. The pole figures were obtained from diffractions in (440) plane. The highest counter in panels b, c, and d is 5.8, 6.1, and 22.2, respectively.

and (2) it shows a high peak intensity (at 2θ = 62.5°). Figure 4b displays the (440) pole figure corresponding to the crosslinked AFMNPs formed in the absence of the magnetic field. A ring with a small pole intensity is seen at 35.2° inclination angle χ, which suggests random distribution of the particles around the easy axis. Similarly, a diffraction ring concentrated near the center implies partial alignment for the grains, which can be attributed to the dipole−dipole interaction of the particles and the consequent formation of small chains. Pole figure of the cross-linked AFMNPs formed under an external magnetic field is depicted in Figure 4c. The strong peak at the center denotes the orientation of grains along the (440) plane. The figure also demonstrates a ring pattern, which can be associated with the high proportion of nonoriented grains. This issue can be explained by the rapid formation of bonds between amine functionalities and MDI, reducing the rotation ability of magnetic particles and the chance of perfect alignment along the magnetic field. Figure 4d shows the pole figure of filaments of bridged PBAFMNPs. The figure shows that the highest counter at the center is four times greater than the previous specimen. This indicates a maximum degree of texture and almost perfect alignment of the constituent grains along the crystal easy axis. Magnetic Properties. Figure 5 shows the magnetization curves of the Fe3O4 nanoparticles before and after hydrothermal treatment (HT), amine-functionalized nanoparticles, and cross-linked nanoparticles, all measured at 300 K in an applied magnetic field of up to 12000 Oe. An expanded plot is depicted in the inset for field strengths between −500 and 500 Oe. According to this figure, a saturation magnetization (Ms) of ∼32.4 emu/g, a coercivity (Hc) of ∼35 Oe, and a remnant magnetization (Mr) of ∼3.0 emu/g was obtained for Fe3O4 14733


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emu/g and 145 Oe, respectively. Comparison of Ms value of the HT-treated magnetic nanoparticles with the results of other studies (Table 1) shows that the value is closer to those of the larger particles. This result also was confirmed by TEM. Larger particles with the higher degree of crystallinity exhibit greater Ms and Hc due to multiple domains produced by prolonging the treatment. Amine-functionalized silica core−shell magnetite nanoparticles showed a lower Ms compared to the bare particles. This can be explained by the decrease in magnetite fraction of each nanoparticle as well as the existence of diamagnetic APTS coating contributed as a nonmagnetic mass to the total sample volume.54 Accordingly, by formation of a shell, Hc and Mr decreased to 127 Oe and 11.7 emu/g, respectively. Such reduction behaviors are in good agreement with the results of other studies.50,55 We also calculated the ratio of diameter of AFMNPs (DAFMNPs) to the diameter of Fe3O4 (DFe3O4) particles based on the magnetization data and compared it with the results achieved using TEM micrographs (Figure S2 and eqs S1−7). The results obtained by calculations proved to be in excellent agreement with the microscopy results. Magnetization curve obtained for the cross-linked particles formed in a zero magnetic field showed a declined Ms to 52.5 emu/g, which can be due to the same reason as for the core− shell nanoparticles. In fact, cross-linking of the core−shell nanoparticles by the nonmagnetic MDI cross-linker resulted in reduced fraction of the magnetic particles to the total volume of the sample, leading to a decreased Ms. In addition, cross-linking of the nanoparticles restricted the rotation ability of the magnetic particles in the applied magnetic field, which resulted in thorough loss of the nanoparticles magnetization. Magnetization curves of the Fe3O4 nanoparticles (after HT), AFMNPs, cross-linked AFMNPs, and filaments of PAFMNPs were also obtained from the corresponding thin films (Figure 6). For this purpose, samples of laser-cut 7 × 7 mm2 thin films on the glass substrates were examined using VSM test. The curves of the thin films show a similar trend compared with the powder samples. The figure also indicates enhanced Ms and Hc for the filaments of bridged PBAFMNPs compared to the cross-linked aggregates. However, Ms and Mr of these filaments were still below the values related to AFMNPs, explained by the lower mass fraction of the magnetic particles in the sample. The

Figure 5. Room temperature magnetization curves of the powders of untreated and hydrothermally treated Fe3O4, AFMNPs nanoparticles, and cross-linked structures of AFMNPs obtained in zero field. The inset shows the respective expanded plots for fields between −500 and 500 Oe.

nanoparticles before HT, suggesting ferromagnetic behavior of the particles.48 Ms for Fe3O4 nanoparticles before HT was lower than that for the corresponding bulk material (Table 1).49,50 The decrease in Ms value mainly pertains to the spin disorder resulting from crystallographic defects and the incomplete bond formation on the surface of the nanoparticle.51 Furthermore, attained Ms values were lower compared to values reported in other works (Table 1). This contrast can be attributed to differences in experimental conditions such as reaction time and temperature as well as incomplete crystal formation. While in the majority of the precipitation methods, high temperatures are utilized, here this process was conducted at room temperature, in that crystal formation was intended to occur in the next step, that is, in the hydrothermal process. HT of the nanoparticles brought about improved crystallinity in the nanoparticles by eliminating α-FeOOH precursor. This improvement is the main contributing factor to the observed rise in Ms of the magnetic nanoparticles.52,53 HT also gave rise to increased Hc for the nanoparticles, originating from the shape anisotropy associated with the faceted growth of the particles.50 Overall, in the present study, magnetic parameters Ms and Hc of the Fe3O4 nanoparticles after HT reached 76.3

Table 1. Magnetic Properties of the Powder and Bulk Samples Obtained from VSM Tests sample

Ms (emu/g)

Hc (Oe)

Mr (emu/ g)

ref

Fe3O4 (NPs before HT)

32.4 68.0 76.9 60.7 76.3 52.0−84.0 57.4 82.2 65.8 34.0 62.0 68.8 52.5 92

35 0 0 0 145 0−150 b b 127 0 0 0 127 115−150

3.0 0 0 0 14.4 b b b 11.7 0 0 0 8.8 b

a 57 58 59 a 60 61 62 a 57 49 63 a 49,50,54,64

Fe3O4 (NPs after HT)

AFMNPs

cross-linked AFMNPs Fe3O4 (bulk) a

particle size (nm)

synthesis conditions

∼12 ∼9 ∼10

Fe2+/Fe3+/NaOH/80 °C Fe2+/Fe3+/NaOH/80 °C Fe2+/Fe3+/NH3·H2O/80 °C

10−40 17.1 ∼40

Fe2+/Fe3+/base/250 °C/24 h HT Fe2+/Fe3+/NaOH/N2/200 °C/1 h HT Fe2+/Fe3+/N(CH3)4OH/250 °C/24 h HT

∼13 ∼15 ∼10

Fe3O4@SiO2/toluene/APTMSc/reflux 110 °C/12 h/N2 Fe3O4/(glycerol/H2O)/APTS/acid/3 h Fe3O4/APTS/reflux 125 °C/2 h

Present work. bNot reported. c(3-aminopropyl) trimethoxysilane. 14734


Article

Langmuir

We also successfully applied the synthesized magnetic particles in fabricating assemblies of different forms, namely aggregates, chains, and filaments. MDI was used as the crosslinker between AFMNPs and demonstrated to be an efficient bridging agent for this purpose. Application of magnetic field brought about preferred alignment in the cross-linked structures. Additionally, comparison of the cross-linking of AFMNPs and of PBAFMNPs revealed that targeted partial blocking of the surface functionalities on the nanoparticles led to fabrication of unidirectional assemblies of bridged nanoparticles. Depending on the concentration of PBAFMNPs, bridging resulted in different structures: chains at low concentrations and filaments at high concentrations. PBAFMNPs were also successfully grafted on the surface of an amine-functionalized glass substrate in a weak external magnetic field. The resulting thin film of the bridged filaments exhibited a highly uniform distribution as well as excellent behavior in response to external magnetic fields. Our findings open up new opportunities to develop novel assemblies in biomedical applications such as imaging and sensing.

Figure 6. Comparative room temperature magnetization curves of thin films of Fe3O4, AFMNPs, cross-linked structures of bridged PBAFMNPs, and filaments of bridged PBAFMNPs formed in a 50 mT field. The magnetization curves were measured parallel to the magnetic field. The inset shows the respective expanded plots for fields between −500 and 500 Oe.

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ASSOCIATED CONTENT

* Supporting Information S

observed increase in Ms of the filaments compared to that of the cross-linked aggregates can be attributed to the higher level of alignment of the magnetic moments of the particles along the easy axis direction. It has also been reported that chain-like assemblies of the magnetic particles can be regarded as nanorods, for which the corresponding demagnetization factor is much smaller compared with the cross-linked aggregates. Such semirod structure clearly contributes to the enhanced magnetization.52 The alignment and the resulting ordering of the chains can be further confirmed by the increased Hc. The larger Hc of the ordered chain-like and ordered structures compared to the free particles is also consistent with the literature and indicates that there existed a residual magnetic moment in the structures.18,56

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.langmuir.7b02093. PBAFMNPs bridged together by the isocynate reagent, TEM micrographs of the AFMNPs, and calculation of the diameters ratio using VSM data (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

A. Hosseinifar: 0000-0003-0776-396X M. Shariaty-Niassar: 0000-0002-6080-3011 M. Moshref-Javadi: 0000-0001-5870-7381

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CONCLUSIONS In this study, a novel technique was devised to synthesize AFMNPs through selective surface modification. In essence, the process was based on partial blocking of amine-functionalities on the surface of the functionalized Fe3O4 nanoparticles. The modification consisted of two successive processes; alignment of the functionalized magnetic nanoparticles in an external magnetic field at the first stage, and selective surface modification at the next stage. In a low concentration solution of magnetic nanoparticles and under a 260 mT magnetic field, the particles formed chain-like alignments in the bulk solution via head to tail interactions of the magnetic dipoles. This alignment resulted in an intimate contact between magnetic particles at their poles, rendering amine groups at the contact points predominantly inaccessible and in the equatorial regions readily available to the bulk solution. Subsequently, phenylisocyanate, injected into the reactor medium, reacted with equatorial amines, giving rise to blocking of the equatorial regions. According to their appearance, we called these selectively surface modified particles “Saturn” particles, uniquely characterized by higher density of reactive amine functional groups at the poles, and maximum nonreactive urea content in equatorial regions. These nanoparticles with their unique structures can be used in various fields such as catalysts, magnetic resonance imaging (MRI), and drug delivery.

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors would like to acknowledge the financial support of University of Tehran Science and Technology Park for this research under the grant number 94009. The authors also thank Dr. Yasser Abdi in the Department of Physics at University of Tehran for the helpful discussions.

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REFERENCES

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Synthesis, Characterization, and Application of Partially Blocked

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