Highly water-dispersed superparamagnetic magnetite colloidal nanocrystal clusters from multifunctional polymeric nanoreactors: synthesis and properties

Abstract

An unconventional but robust strategy to fabricate uniform hybrid inorganic–organic core–shell superparamagnetic magnetite (Fe₃O₄) colloidal nanoclusters in situ was introduced based on water-soluble multi-arm star-shaped brush-like block copolymers as multifunctional polymeric nanoreactors, composed of poly(ethylene oxide) (PEO) as the main chain, poly(acrylic acid) (PAA) as functional graft chains, and the second PEO block as a shell (i.e., multi-arm star-shaped brush-like block copolymer [(PEO-g-PAA)-b-PEO]₁₈) with different molecular weights and grafting densities. FeCl₃ and FeCl₂ as precursors of Fe₃O₄ were loaded into the graft chain PAA template domain of a multi-arm star-shaped brush-like block copolymer [(PEO-g-PAA)-b-PEO]₁₈ polymeric nanoreactors, followed by an in situ reaction to form Fe₃O₄ nanoclusters. The dimensions of the clusters can be tuned precisely by changing the chain lengths of the PEO backbones of the PAA grafting region. In addition, the density of the subunits can also be tailored by adjusting grafting density of the PAA side chains, determined by the molar ratio of ethoxyethyl glycidyl ether (EEGE) to EO during the anionic copolymerization. The Fe₃O₄ colloidal nanocrystal clusters with superparamagnetic behavior at room temperature are highly water-dispersed because of the hydrophilic ligands of the surface-tethered PEO polymer shell.

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Introduction

Owing to their unique optical, electrical, catalytic and magnetic properties, the fabrication of high-quality colloidal inorganic nanocrystals with tunable dimensions and shape is of key importance for applications in optics, catalysis, electronics, and magnetic resonance imaging for medical diagnosis, to name but a few.^1–3 As one of the most important manipulations for these applications, the assembling of inorganic nanocrystals into secondary nanostructures is highly desirable in order to combine the properties of individual nanocrystals and their secondary structures due to interactions of the subunits.^4–6 Fabrication approaches of these secondary nanostructures, nanocrystal clusters, by self-assembly or solution growth in situ have become a focus of synthetic efforts in recent years.^7–10 Among various strategies for the fabrication of secondary nanostructures of nanocrystals, the template-assisted method is an important process for the synthesis of nanocrystal clusters. For example, amphiphilic block copolymers, polystyrene-block-poly(4-vinylpyridine) (PS-b-P4VP), have been utilized as soft templates to fabricate a series of nanoclusters with highly ordered secondary patterns (e.g., Au, Ag and Pt, etc.).^11,12 However, these macromolecular micelles as templates by conventional linear amphiphilic block copolymers were formed by equilibrium multi-molecular aggregates.^13–15 The shapes and characteristics of these multi-molecular micelles with equilibrium structures can change with varying ambient conditions, such as concentration, solvent, temperature, and pH.¹⁶ In addition to block copolymers, dendritic polymers as soft templates have been shown to be useful for fabricating different nanoclusters.¹⁷ However, these approaches often require certain stringent experimental conditions, and it is difficult to tune the dimensions of the secondary structures and the density of the subunits.

Magnetite (Fe₃O₄), a common magnetic iron oxide with a cubic inverse spinel structure, has been broadly applied in ferrofluids, colloidal liquids made of nanoscale ferromagnetic, or ferrimagnetic, particles suspended in a carrier fluid (usually an organic solvent or water).^18,19 Especially, colloidal Fe₃O₄ nanocrystals with superparamagnetic behavior and water dispersibility have various medical applications; examples include magnetic resonance imaging for medical diagnosis, AC magnetic field-assisted cancer therapy, drug delivery, and bioseperation.^18,20 In addition, a narrow size distribution of nanocrystals is also important, so that the nanostructures have uniform physical and chemical properties. Owing to interactions between the primary nanocrystals, assembling these small Fe₃O₄ nanocrystals into large clusters with controllable size and shape is still desirable for future applications. In recent years, copolymer brushes have received considerable attention due to their unique chemical and physical properties as well as their applications in biomaterials, nanotechnology, supramolecular science, etc.^21,22 Even though various well-defined copolymers with complicated structures have been synthesized by different controlled/living polymerization techniques (e.g., anionic polymerization and ATRP),^23,24 limited work has been reported about the assembling of copolymer brushes with two functional hydrophilic segments into star-shaped architectures.²⁵

Here, we report an unconventional but robust method for the in situ fabrication of a series of water-dispersed Fe₃O₄ colloidal secondary nanostructures, nanocrystal clusters, composed of small primary Fe₃O₄ nanocrystals as subunits, with precisely tunable dimensions and subunit densities, by exploiting rationally designed, novel water-soluble star-shaped brush-like block copolymers as multifunctional polymeric nanoreactors; these are composed of PEO as the main chain, PAA as functional graft chains and a second PEO block as the hydrophilic ligand (i.e., multi-arm star-shaped brush-like block copolymer [(PEO-g-PAA)-b-PEO]₁₈) with different molecular weights and grafting densities. A series of water-soluble multi-arm star-shaped graft block copolymers based on α-cyclodextrin ​ (α-CD) as the core, star-shaped brush-like block copolymer [(PEO-g-PAA)-b-PEO]₁₈, were firstly prepared by a combination of anionic copolymerization and atom transfer radical polymerization (ATRP).²⁵ Subsequently, FeCl₃ and FeCl₂, as precursors of Fe₃O₄, were encapsulated into the graft chain PAA template domain of star-shaped brush-like block copolymer [(PEO-g-PAA)-b-PEO]₁₈ polymeric nanoreactors by coordination interactions between the precursors and carboxyl groups of the PAA domain, followed by an in situ reaction to form Fe₃O₄ nanocrystal clusters. The dimensions of the clusters can be adjusted precisely by changing the length of the PEO backbones in the grafting domains. In addition, the density of the subunits can be also tailored by adjusting the grafting density of the PAA graft side chains, determined by the feed molar ratio of EEGE to EO, during the anionic copolymerization. These Fe₃O₄ colloidal nanocrystal clusters with superparamagnetic behavior at room temperature are highly water-dispersed because of the surface-tethered hydrophilic PEO polymer shell.

Experimental section

Materials

α-Cyclodextrin (α-CD, ≥99%), ethylene oxide (EO, ≥99.5%), N,N,N′,N′′,N′′-pentamethyldiethylene triamine (PMDETA, 99%), benzenecarbonyl chloride (≥99.0%), 1-methyl-2-pyrrolidinone (NMP, anhydrous, 99.5%), and trifluoroacetic acid (TFA, 99%) were purchased from Sigma-Aldrich and used without further purification. CuBr (98%, Sigma-Aldrich) was purified by stirring in acetic acid and was subsequently filtered, washed with ethanol and diethyl ether successively, and dried under vacuum. N,N-Dimethylformamide (DMF, Fisher Scientific, 99.9%), tert-butyl acrylate (tBA, Sigma-Aldrich 98%) and methyl ethyl ketone (99.9%, Fisher Scientific) were dried with CaH₂ and distilled under reduced pressure prior to use. Tetrahydrofuran (THF, 99%, BDH) was dried by refluxing over sodium, and then distilled from sodium naphthalenide solution. A THF solution of diphenylmethyl sodium (DPMNa) (c = 0.51 M) was synthesized according to a literature procedure.²⁶ Ethoxyethyl glycidyl ether (EEGE) was synthesized by protecting the hydroxyl group of glycidol (Gly) with ethyl vinyl ether based on a previously reported method.²⁶ Iron(II) chloride tetrahydrate (FeCl₂·4H₂O, ≥99%), iron(III) chloride hexahydrate (FeCl₃·6H₂O, ≥98%) and ammonium hydroxide solution (NH₃·H₂O, ACS reagent, 28.0–30.0% NH₃ basis) were purchased from Sigma-Aldrich and used as starting materials without further purification. All other reagents were purified by common purification methods.

Characterizations

The molecular weights of the copolymers were measured by gel permeation chromatography (GPC, Agilent 1100) equipped with a G1310A pump, a G1362A refractive detector and a G1314A variable wavelength detector. THF was used as the mobile phase at a flow rate of 1.0 mL min⁻¹ at 35 °C. One LP 5 μm gel column and two 5 μm gel LP mixed bed columns were calibrated with 10 polystyrene (PS) standard samples with molecular weights ranging from 200 to 3 × 10⁶ g mol^{−1 1}H nuclear magnetic resonance (¹H-NMR) spectra were obtained using a Bruker 400 MHz spectrometer with tetramethylsilane (TMS) as the internal standard. CDCl₃ and methanol-d₄ (CD₃OD) were used as the solvents. FTIR spectra were obtained on a Magna-550 Fourier transform infrared spectrometer. The morphologies of the star-shaped brush-like (PEO-g-PAA)-b-PEO architectures and resulting colloidal Fe₃O₄ nanocrystal clusters were characterized by transmission electron microscopy (JEOL 1200EX TEM; operated at 80 kV). TEM samples of the template copolymers were prepared by applying a drop of star-shaped brush-like block copolymer (PEO-g-PAA)-b-PEO DMF solution (∼5 μL at c = 1 mg mL⁻¹) onto a carbon coated copper TEM grid (300 mesh) and allowing DMF to evaporate in a vacuum oven at room temperature. Then the copolymer samples were stained with uranyl acetate. A droplet of freshly prepared saturated uranyl acetate aqueous solution (∼20 μL) was deposited onto the dried TEM samples.^27,28 The excess solution was removed using a filter paper after 5 min. The TEM images were analyzed using standard TEM image analysis software (Image Pro). The morphologies of the structures of the multi-arm star-shaped brush-like block copolymer [(PEO-g-PAA)-b-PEO]₁₈ were measured by atomic force microscopy (AFM, Bruker Dimension 3000) in the tapping mode. AFM characterization samples were prepared by spin-coating the DMF solution (c = 1 mg mL⁻¹) on a silicon substrate at 2000 rpm for 1 min. The crystalline structures of the samples were measured by X-ray diffraction (XRD; SCINTAG XDS-2000, Cu Kα radiation). Energy dispersive spectroscopy (EDS) microanalysis of the samples was conducted by field emission scanning electron microscopy (FE-SEM; FEI Quanta 250). A superconducting quantum interference device (SQUID) magnetometer (Quantum Design MPMS-5S) was used to determine the magnetic properties of Fe₃O₄ nanocrystal clusters at room temperature.

Synthesis of water-soluble 18-arm star-shaped brush-like block copolymer [(PEO-g-PAA)-b-PEO]₁₈

Firstly, multi-arm star-shaped brush-like block copolymer [(PEO-g-PtBA)-b-PEO]₁₈ was synthesized by the ATRP technique. ATRP of tert-butyl acrylate (tBA) was carried out using multi-arm star-shaped block copolymer [poly(EO-co-BiBGE)-b-PEO]₁₈ as a star-shaped macroinitiator possessing side initiation sites on each arm, according to our earlier work (ESI†).²⁵ In a typical process, a dry ampoule charged with CuBr (0.0707 g), PMDETA (0.1707 g), star-shaped macroinitiator (0.1 g), tBA (23.4 mL), and 23 mL methyl ethyl ketone as solvent was degassed by three freeze–pump–thaw cycles in liquid nitrogen, then sealed and placed in an oil bath at 60 °C. The polymerization process was terminated by dipping the ampoule into an ice/water bath at the desired reaction time. The crude solution was then diluted with acetone, passed through a neutral alumina column to remove the catalyst, and precipitated in a mixture of methanol/water (v/v = 1/1). The final product, multi-arm star-shaped brush-like block copolymer [(PEO-g-PtBA)-b-PEO]₁₈, was purified by dissolution/precipitation twice with acetone and methanol/water, and dried at 50 °C in a vacuum oven for 12 h.

After multi-arm star-shaped brush-like block copolymer [(PEO-g-PtBA)-b-PEO]₁₈ was obtained, water-soluble star-shaped brush-Like block copolymer [(PEO-g-PAA)-b-PEO]₁₈ was prepared by hydrolysis of the tert-butyl ester groups of the PtBA graft chains. In a typical hydrolysis process, purified multi-arm star-shaped brush-like block copolymer [(PEO-g-PtBA)-b-PEO]₁₈ (1.0 g) was dissolved in 50 mL CH₂Cl₂, and then 10 mL trifluoroacetic acid (TFA) was introduced. The reaction system was stirred at room temperature for 24 h. Notably, the resulting water-soluble star-shaped brush-like copolymer [(PEO-g-PAA)-b-PEO]₁₈ was gradually precipitated from CH₂Cl₂ during the hydrolysis process. The final product was washed with CH₂Cl₂ and then dried under vacuum at 50 °C for 10 h.

Fabrication of highly water-dispersed hybrid inorganic–organic core–shell superparamagnetic Fe₃O₄ colloidal nanocrystal clusters capped with hydrophilic PEO as a shell

The fabrication of superparamagnetic Fe₃O₄ colloidal nanocrystal clusters capped with hydrophilic PEO chains as a shell, using water-soluble multi-arm star-shaped brush-like block copolymer [(PEO-g-PAA)-b-PEO]₁₈ as templates, can be described as follows: 10 mg star-shaped brush-like block copolymer [(PEO-g-PAA)-b-PEO]₁₈ template was dissolved in 10 mL DMF at room temperature, followed by the addition of appropriate amounts of precursors (FeCl₂·4H₂O, FeCl₃·6H₂O, and NH₃·H₂O as the precursor system) that were selectively loaded into the inner grafting PAA polymer chain template domain. The fabrication reaction was carried out at 100 °C for 1 h under argon. The final products were purified by magnet, sequentially washed with ethanol three times, and then dispersed in DI water.

Results and discussion

As illustrated in Scheme 1, according to our earlier work,²⁵ a series of novel water-soluble multi-arm star-shaped brush-like block copolymers with narrow molecular distribution of each segment based on α-cyclodextrin as the core were firstly rationally designed and synthesized by a combination of anionic copolymerization and atom transfer radical polymerization (ATRP); they are composed of poly(ethylene oxide) (PEO) as the main chain, poly(acrylic acid) (PAA) as functional graft chains, and the second PEO block as the shell (i.e., multi-arm star-shaped brush-like block copolymer [(PEO-g-PAA)-b-PEO]₁₈) (see the ESI†).²⁵ Four samples were synthesized, as shown in Table 1. Subsequently, these water-soluble multi-arm star-shaped brush-like block copolymer [(PEO-g-PAA)-b-PEO]₁₈ were utilized as multifunctional polymeric nanoreactors to deposit in situ a series of water-dispersed Fe₃O₄ colloidal secondary nanostructures, nanocrystal clusters, composed of small primary Fe₃O₄ nanocrystals as subunits, with precisely tunable dimensions and subunit densities.

Scheme 1 Schematic of the fabrication of superparamagnetic Fe₃O₄ colloidal nanocrystal clusters capped with PEO as ligands using a water-soluble multi-arm star-shaped brush-like block copolymer [(PEO-g-PAA)-b-PEO]₁₈ as multifunctional polymeric nanoreactors.

Table 1 Summary of multi-arm star-shaped brush-like copolymers of [(PEO-g-PtBA)-b-PEO]₁₈ and their corresponding water-soluble multi-arm star-shaped brush-like block copolymers of [(PEO-g-PAA)-b-PEO]₁₈

Sample	Mn,GPC^a (kg mol^−1)	PDI^b	Mn,PtBA^c (kg mol^−1)	Mn,PAA^d (kg mol^−1)
a Number-average molecular weights of multi-arm star-shaped brush-like copolymers of [(PEO-g-PtBA)-b-PEO]18 determined by GPC.b The polydispersity (PDI) of multi-arm star-shaped brush-like copolymers of [(PEO-g-PtBA)-b-PEO]18 determined by GPC.c Mn of each PtBA graft chain calculated from ^1H-NMR data.d Number average molecular weight, Mn, of each PAA block calculated from the molecular weight difference between the PtBA block (before hydrolysis) and the PAA block (after hydrolysis).
A-1b	218.5	1.12	4.2	2.4
A-2b	321.3	1.15	4.9	2.8
B-1b	179.5	1.14	4.6	2.6
B-2b	268.9	1.10	5.1	2.9

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Owing to their ability to direct the synthesis of functional inorganic nanomaterials with well-defined structures, multimolecular micelles by aggregation of linear amphiphilic block copolymers were used as an attractive preparation strategy to fabricate functional colloidal nanocrystals by combination with sol–gel processes.¹³ Multimolecular micelles based on equilibrium aggregates of linear amphiphilic block copolymers are formed when the concentration is above the critical micelle concentration, and their morphology and structures in a given system depend heavily on the temperature and solvent properties.^13,29 With changes in concentration, solvent properties, temperature, and pH values, the shapes and structures of multimolecular micelles may vary.¹⁵ Compared with multimolecular templates, the multifunctional polymeric nanoreactors from multi-arm star-shaped brush-like block copolymer [(PEO-g-PAA)-b-PEO]₁₈ with uniform and structurally stable spherical structures are static rather than dynamic.^16,30

Specific interactions between PAA and PEO segments in aqueous solutions resulting in the formation of interpolymer complexes have been extensively studied for more than four decades.^31,32 In aqueous solution, the pH value of the solution governs the behavior of PAA/PEO mixtures. When the pH value of the aqueous solution was below a certain critical pH of complexation between the PAA and PEO segments (usually in acidic conditions), the polymers can form interpolymer associations. Above the critical pH of complexation, copolymers consisting of PAA and PEO segments do not form association products or polycomplexes. In the polymeric template system of multi-arm star-shaped brush-like block copolymers of [(PEO-g-PAA)-b-PEO]₁₈, in order to exploit them as polymeric nanoreactors, interpolymer association from complexation between the PAA and PEO segments should be avoided. In our system, pure DMF was chosen as a polar aprotic solvent; it is a good solvent for both PAA and PEO segments. After the water-soluble multi-arm star-shaped brush-like block copolymer (PEO-g-PAA)-b-PEO was completely dissolved in the polar solvent DMF, polymeric template systems were formed. AFM characterizations were performed to measure the characteristics of the polymeric template structures. Clearly, the water-soluble multi-arm star-shaped brush-like block copolymer [(PEO-g-PAA)-b-PEO]₁₈ formed spherical structures with an average diameter of 42 ± 4.6 nm (Fig. S12†). In order to further investigate the architectures of the star-shaped brush-like block copolymer [(PEO-g-PAA)-b-PEO]₁₈ structures, TEM characterization was carried out. The dark spherical structures in the TEM micrographs corresponded to the hydrophilic PAA brush cores, as uranyl acetate preferentially stained the PAA segments of the polymeric architectures (Fig. 1). The average diameter of the cores was 33 ± 3.6 nm, and the size was smaller than that found from the AFM results by about 9 nm, owing to the presence of the second PEO as the shell.

Fig. 1 TEM images of polymeric nanoreactors from the water-soluble multi-arm star-shaped brush-like block copolymer [(PEO-g-PAA)-b-PEO]₁₈ (sample A-1b as precursor in Table 1) with different scale bars. The samples were treated with uranyl acetate before imaging to selectively stain the hydrophilic core PAA segments.

Subsequently, the water-soluble multi-arm star-shaped brush-like block copolymer [(PEO-g-PAA)-b-PEO]₁₈ was utilized as a multifunctional polymeric nanoreactor to structure-direct the precursors of Fe₃O₄ into superparamagnetic magnetite colloidal nanocrystal clusters. Highly water-dispersed Fe₃O₄ colloidal nanocrystal clusters were fabricated in situ by exploiting the water-soluble multi-arm star-shaped brush-like block copolymer [(PEO-g-PAA)-b-PEO]₁₈ as a template. FeCl₂·4H₂O, FeCl₃·6H₂O, and NH₃·H₂O were used as the precursor system (Fe³⁺ : Fe²⁺ = 3 : 2; the molar ratios of Fe₃O₄ precursors to AA units in PAA domain = 2 : 1), and DMF was used as a polar solvent. Precursors were loaded into the PAA region of the templates, owing to the strong coordination of carboxyl groups with metal ions (e.g., iron cations).^3,33,34 Introduction of NH₃·H₂O into the reaction system increased the alkalinity of the reaction solution, and is propitious to the hydrolysis of FeCl₂ and FeCl₃ for the formation of Fe₃O₄ colloidal nanocrystal clusters. Finally, Fe₃O₄ nanoclusters are formed through dehydration of the resulting Fe(OH)₃ and Fe(OH)₂ species, within which spherical Fe₃O₄ colloidal nanocrystal clusters were intimately and permanently capped with hydrophilic PEO chains on the surface. The morphology of the three-dimensional Fe₃O₄ nanocrystal clusters with an average diameter of 30 ± 3.2 nm was characterized by TEM, as demonstrated in the representative TEM images at different magnifications in Fig. 2. According to the close observation of these images, these uniform spherical colloids comprise small primary Fe₃O₄ nanocrystals. The subunits of the clusters can be investigated more clearly, and the crystal lattices are clearly shown in the high resolution TEM (HR-TEM) image shown in Fig. 2(d). Clearly, the spherical clusters consist of small primary nanocrystals as subunits with an average diameter of ∼3.6 nm. X-ray diffraction (XRD) measurements also confirmed the subunit crystal structure of Fe₃O₄ colloidal nanocrystal clusters, and the diffraction pattern of cubic magnetite was shown in Fig. S13.† In addition, energy dispersive spectroscopy (EDS) microanalysis was also used to confirm the composition of iron oxide as magnetite by combining the XRD characterization, as shown in Fig. S14.†

Fig. 2 Representative TEM and HR-TEM images of Fe₃O₄ colloidal nanocrystal clusters with different scale bars; the water-soluble multi-arm star-shaped brush-like block copolymer [(PEO-g-PAA)-b-PEO]₁₈ (sample A-1b as the precursor in Table 1) was used as a multifunctional polymeric nanoreactor. (a–c) Representative TEM images. (d) Typical HR-TEM image.

The dimension of Fe₃O₄ colloidal nanocrystal clusters can be precisely tuned by adjusting the molecular weight of the PEO backbones in the PAA grafting chain region of the multi-arm star-shaped brush-like block copolymer [(PEO-g-PAA)-b-PEO]₁₈. When multi-arm star-shaped copolymer [poly(EO-co-EEGE)]₁₈ was synthesized, the molecular weight – in other words, the length of the grafting backbone of the final template copolymers – can be tailored by changing the monomer conversion and the concentration of initiators during the anionic copolymerization. As the length of the grafting backbones increases, the diameter of the spherical domain of the PAA grafting chains may also increase; therefore, the size of the resulting Fe₃O₄ colloidal nanocrystal clusters based on copolymer templates increases. As shown in Fig. 3, when the molecular weight of each arm poly(EO-co-EEGE) in the multi-arm star-shaped copolymer [poly(EO-co-EEGE)]₁₈ was increased from 17.2 kg mol⁻¹ to 34.9 kg mol⁻¹ (sample A-1 and sample A-2 in Table S1,† respectively), the average diameter of the Fe₃O₄ colloidal nanocrystal clusters increased from 30 ± 3.2 nm to 42 ± 4.5 nm. Clearly, the density of the Fe₃O₄ primary nanocrystals in the spherical clusters can also be changed by tuning the grafting density of the PAA side chains (i.e., the number of EEGE units in multi-arm star-shaped copolymers of [poly(EO-co-EEGE)]₁₈) while keeping other parameters fixed (Fig. 4). Owing to the change of the number of EEGE units in the multi-arm star-shaped copolymers of [poly(EO-co-EEGE)]₁₈, the average number of grafting PAA chains on each arm of the final template copolymer can be accordingly adjusted. Sequentially, the PAA region as the template phase with a lower density of PAA side chains can load smaller precursors, even if the molar ratio of the precursors to the AA unit is fixed. As demonstrated in Fig. 4, according to the approximate calculation based on the volume of spherical clusters and the average number of subunits in one cluster, the density of Fe₃O₄ primary nanocrystals can be adjusted from about 29 nanocrystals per 1 × 10³ nm³ to about 18 nanocrystals per 1 × 10³ nm³, while the number of EEGE units in each arm of the multi-arm star-shaped copolymers of [poly(EO-co-EEGE)]₁₈ was changed from 41 to 20 by tuning the feed molar ratio of the two monomers at the stage of preparing the multi-arm star-shaped copolymers of [poly(EO-co-EEGE)]₁₈.

Fig. 3 Dependence of the diameter of Fe₃O₄ colloidal nanocrystal clusters (i.e., 30 ± 3.2 nm and 42 ± 4.5 nm) on the molecular weights of each poly(EO-co-EEGE) arm in multi-arm star-shaped copolymer [poly(EO-co-EEGE)]₁₈ (sample A-1 and sample A-2 in Table S1,† respectively).

Fig. 4 Dependence of the density of Fe₃O₄ primary nanocrystals on the grafting density of the PAA side chains (i.e., the number of EEGE units in each arm of the multi-arm star-shaped copolymers of [poly(EO-co-EEGE)]₁₈, sample A-1 and sample B-1 in Table S1,† respectively).

Fe₃O₄ colloidal nanocrystal clusters with unique architectures (i.e., the aggregates of small primary Fe₃O₄ nanocrystals) show superparamagnetic behavior at room temperature. The hysteresis loops of ∼30 nm Fe₃O₄ colloidal nanocrystal clusters composed of primary nanocrystals prepared by using different molar ratios of Fe₃O₄ precursors to AA units in the PAA domain were measured at 300 K, as shown in Fig. 5(a). Both the clusters (i.e., the aggregates of small primary Fe₃O₄ nanocrystals, not the individual primary magnetite nanoparticles) show no remanence or coercivity at 300 K, that is, superparamagnetic behavior.³⁵ In order to quantitatively evaluate the magnetic response of the Fe₃O₄ colloidal nanocrystal clusters to an applied magnetic field, the mass magnetization was characterized at 300 K. The saturation magnetizations of the Fe₃O₄ colloidal nanocrystal clusters capped with hydrophilic PEO on the surface were determined to be 21.9 and 15.5 emu g⁻¹ for Fe₃O₄ colloidal nanocrystal clusters (D: ∼30 nm) consisting of Fe₃O₄ primary nanocrystals when different molar ratios of Fe₃O₄ precursors to AA units in the PAA domain were used. Even if the sizes of two Fe₃O₄ colloidal nanocrystal clusters are the same (D: ∼30 nm), their saturation magnetization values are different. The increasing saturation magnetization should be due to the increasing size of the subunit.³⁵ This size increase of the primary nanocrystals may be the result of a higher concentration of precursors, and caused more precursors to be loaded into the PAA compartment for the formation of larger nanocrystals.

Fig. 5 Superparamagnetic behavior of Fe₃O₄ colloidal nanocrystal clusters (D: ∼30 nm) by using multi-arm star-shaped brush-like block copolymer [(PEO-g-PAA)-b-PEO]₁₈ as a multifunctional polymeric nanoreactor (sample A-1b as precursor in Table 1). (a) Mass magnetization M as a function of applied external field H at 300 K when different molar ratios of Fe₃O₄ precursors to PAA grafting chain repeating units were used (A = 2 : 1; B = 6 : 1). (b) Digital images of the aqueous Fe₃O₄ colloidal nanocrystal clusters shown in Fig. 2. Without a magnetic field, with a magnetic field for 0 and 10 min, and after the magnetic field was removed (from left to right).

Owing to their surface-tethered hydrophilic PEO polymer chain shells, the resulting hybrid inorganic–organic core–shell Fe₃O₄ colloidal nanocrystal clusters synthesized by multifunctional polymeric nanoreactor are highly water-dispersed. The external magnetic responses were observed by dispersing Fe₃O₄ colloidal nanocrystal clusters into DI water. As shown in Fig. 5(b), Fe₃O₄ colloidal nanocrystal clusters deposited on the wall of the vial under the influence of an external magnetic field within several minutes. When the external magnet was removed, the aggregated Fe₃O₄ colloidal nanocrystal clusters returned to the original aqueous solution. Even if six cycles of magnetic sequestration/redispersion were performed, when compared with the TEM images in Fig. 2, TEM measurements show that the morphology of the Fe₃O₄ colloidal nanocrystal clusters remained almost unchanged (Fig. S15†).

Conclusion

In summary, we have developed an unconventional but robust strategy for the in situ creation of water-dispersed hybrid inorganic–organic core–shell Fe₃O₄ colloidal nanocrystal clusters composed of small primary nanocrystals as subunits, with precisely tunable dimensions and subunit densities, by exploiting rationally designed, novel water-soluble star-shaped brush-like block copolymers as multifunctional polymeric nanoreactors; these consisted of PEO as the backbone, PAA as functional graft chains, and a second PEO block as the hydrophilic ligand (i.e., multi-arm star-shaped brush-like block copolymer [(PEO-g-PAA)-b-PEO]₁₈) with different molecular weights and grafting densities. The dimensions of the clusters can be tuned precisely by adjusting the length of the PEO backbones of the grafting domains. In addition, the density of the subunits can be also tailored by adjusting the grafting density of the PAA graft side chains, as determined by the molar ratio of EEGE to EO during the anionic copolymerization. These Fe₃O₄ colloidal nanocrystal clusters with superparamagnetic behavior at room temperature are highly water-dispersed because of the presence of surface-tethered PEO polymer chains as a hydrophilic shell. Hence, this multifunctional polymeric nanoreactor strategy may stand out as an emerging methodology to fabricate a myriad of other functional inorganic colloidal nanocrystal clusters with novel and tailorable physical properties for a range of intriguing applications, such as nanoelectrics, sensors, magnetic devices, and catalysis.

Acknowledgements

We gratefully acknowledge funding support from the National High Technology Research and Development Program of China (863 Program, No. 2011AA02A204-03), the 1000 Young Talent (to Xinchang Pang), the Postdoctoral Research Sponsorship in Henan Province (2014004, to Zhe Cui) and the Outstanding Young Teacher Development Fund of Zhengzhou University (1421320045, to Peng Fu).

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Footnotes

† Electronic supplementary information (ESI) available: Synthesis procedures and summary of characterization data for the final multi-arm star-shaped brush-like block copolymer [(PEO-g-PAA)-b-PEO]₁₈ as template and intermediates: GPC, ¹H-NMR and FTIR; XRD and EDS of Fe₃O₄ colloidal secondary nanostructures. See DOI: 10.1039/c5ra17869j

‡ These authors contributed equally to this work.

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