Article pubs.acs.org/Langmuir
Highly Cross-Linked and Biocompatible Polyphosphazene-Coated Superparamagnetic Fe3O4 Nanoparticles for Magnetic Resonance Imaging Ying Hu,† Lingjie Meng,*,‡ Lvye Niu,† and Qinghua Lu*,†,§ †
School of Chemistry and Chemical Technology, Shanghai Jiao Tong University, Shanghai, 200240, P. R. China School of Science, Xi’an Jiao Tong University, Xi’an, 710049, P.R. China § State Key Laboratory of Metal Matrix Composites, Shanghai Jiao Tong University, Shanghai 200240, P. R. China ‡
S Supporting Information *
ABSTRACT: Highly cross-linked and biocompatible poly(cyclotriphosphazene-co-4,4′-sulfonyldiphenol) (PZS) were used to directly coat hydrophilic superparamagnetic Fe3O4 nanoparticles by a facile but effective one-pot polycondensation. The obtained core−shell Fe3O4@PZS nanohybrids were characterized by transmission electron microscopy (TEM), scanning electron microscopy (SEM), Fourier-transform infrared (FTIR) and X-ray diffraction spectra. Interesting, the size and T2 relaxivity of Fe3O4@PZS increased with increasing the mass ratio of Fe3O4 to PZS. All these nanohybrids could be internalized by HeLa cells but show negligible cytotoxicity. The PZS layer slowly degraded into less dangerous forms such as 4,4′-sulfonyldiphenol, phosphate and ammonia at neutral or acid atmosphere. Considering their excellent water dispersibility, colloidal and chemical stability, magnetic manipulation, and magnetic resonance imaging (MRI) properties, Fe3O4@PZS nanohybrids have great potential in MRI diagnosis of cancer.
1. INTRODUCTION Superparamagnetic iron oxide nanoparticles (SPIONs) are being increasingly used as magnetic resonance imaging (MRI) contrast agents to improve the detection and monitoring of cancers and other organ/tissue structures.1−4 With the diameter less than a single magnetic domain (ca. 20 nm), iron oxide (magnetite, Fe3O4; maghemite, γ-Fe2O3) nanoparticles will exhibit superparamagnetic properties at room temperature, i.e., high magnetic susceptibility and loss of magnetization after removal of the magnetic field.2,3 However, these finely divided SPIONs are unstable and extremely reactive toward oxidizing agents in the presence of water or humid air.4−6 It is crucial to protect SPION by surface coating for obtaining physically and chemically stable colloidal systems. In addition, the surface coating can also improve the water-dispersibility and biocompatibility of SPION, and provide tailored surface chemistry for conjugation with other bioactive molecules or targeting ligands as well.7 Silica oxide (SiO2)8−10 and some polymeric stabilizers/surfactants including liposomes,11,12 poly(ethylene glycol) (PEG),13,14 poly(vinyl alcohol) (PVA),15 dextran,16,17 carboxydextran17 and so forth have been used to coat on SPIONs. The silica coatings are of particular interest due to their good biocompatibility and excellent chemical and mechanical stability against variation in pH or temperature of the environment.18,19 It should be remarkable that the coatings on magnetic nanoparticles may strongly affect the MRI contrast © 2013 American Chemical Society
because they can hamper the diffusion of water molecules, or increase the residence time of water molecules by forming hydrogen bonds.20 Unfortunately, the T2 relaxivity values of SPION were often dramatically reduced after being coated with SiO2.21,22 Although coating with hydrophilic polymer can enhance interactions between the protons with the magnetic core and accordingly increasing the T2 relaxivity,23 the polymeric surfactants tend to be desorbed from the nanoparticle surface due to the relatively weak adsorption.8,24 Additionally, it is commonly complicated and time-consuming to synthesize organic cross-linkable surfactants. Therefore, all these coating materials are far from ideal. Poly-(cyclotriphosphazene-co-4,4′-sulfonyldiphenol) (PZS) is a versatile highly cross-linked polymer with outstanding thermal stability and solvent resistance. It is biodegradable, water dispersible, and biocompatible.25,26 In addition, PZS is also rich in N, P, and S atoms and has plenty of phenolic hydroxyl groups, benefiting to increase the water residence time and conjugate with bioactive molecules or targeting ligands. In this Article, water-dispersed SPIONs were prepared and then directly coated with PZS by a facile one-pot synthesis. Because the aggregation/assembly of magnetic nanoparticles can significantly affect transverse (T2) relaxation,27−30 a series of Received: June 4, 2013 Published: June 25, 2013 9156
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WST-1 assay was also used to quantitatively assess the biocompatibility of Fe3O4@PZS nanohybrids.31 HeLa cells were seeded into a 96-well flat culture plate (Corning). After incubation overnight to allow cell attachment, the cells were incubated with Fe3O4@PZS-1,2,3 (10, 20, 50, 100 μg/mL, respectively) in a FBS-free culture medium at 37 °C for 24 and 48 h, respectively. Following this, the cells were washed with PBS, and 200 mL PBS was used as a substitute for the culture medium before adding 1: 10 (v: v) of the WST-1 reagent. After incubation for another 2 h, the absorbance was measured at 450 nm. Cells cultured without Fe3O4@PZSs at the same time intervals were used as controls. 2.6. The Distribution of Fe3O4@PZS in HeLa Cells. HeLa cells were seeded in a culture dish of diameter of 60 mm (Corning). The cells were cultured overnight to allow cell attachment, then incubated with Fe3O4@PZS-2 (10 μg/mL) in FBS-free culture medium for 6 h. The cells were then rinsed with sterilized PBS and fixed with 2% glutaraldehyde and 1% osmium tetroxide for 2 h at 4 °C. The cells were then dehydrated in a graded ethanol series (30%, 50%, 70% with 3% uranyl acetate, 80%, 95%, and 100%) for 10 min at each concentration followed by two changes in 100% propylene oxide. After infiltration and embedding in epoxy resins at 60 °C for 48 h, the sections were stained with lead citrate and investigated by TEM. 2.7. Degradation of Fe3O4@PZS. The Fe3O4@PZS-2 (20 mg) were dispersed in phosphate buffer (20 mL, pH = 7.4 and 5.5) at 37 °C. At different times, the Fe3O4@PZS were collected by a magnet and washed with distilled water, and then dried at 40 °C in vacuum for 12 h before being subjected to weight loss. The final morphology of the samples soaking in different pH solutions for 3 months was examined by TEM. 2.8. Characterazation. Transmission electron microscopy (TEM) was carried out on a CM120 (Philips). High resolution-transmission electron microscopy (HR-TEM) was conducted on a JEOL TEM2100 operated at 200 kV. Selected area electron diffraction (SAED) patterns were collected by HR-TEM. Scanning electron microscopy (SEM) was performed using a field-emission SEM (JSM-7401F, JEOL, Japan) at an acceleration voltage of 5 kV. The size and distribution of all as-prepared nanomaterials were determined from TEM micrographs using Image J (V1.41, NIH, USA) for image analysis. Dynamic light scattering (DLS) measurements were carried out on a Zeta sizer Nano ZS from Malvern Instruments, with a laser at 633 nm. Photographs were taken with a digital camera (IXUS 800IS, Canon, Japan). Fourier-transform infrared (FTIR) spectra were recorded on a Paragon 1000 (Perkin-Elmer) spectrometer. Samples were dried overnight at 45 °C under vacuum and thoroughly mixed and crushed with KBr to fabricate KBr pellets. XRD patterns were collected on a powder diffractometer (D/max-2200/PC, Rigaku, Japan) using Cu−K irradiation (40 kV, 20 mA). Diffraction patterns were collected from 10° to 90° at a speed of 3° min−1. The magnetization curves were measured at 300 K under a varying magnetic field with physical property measurement system (PPMS-9T, Quantum Design, USA). The Fe concentration was determined by ICP-AES (VISTAMPXICP Varian, USA). T2 relaxation time was conducted by PQ001 MRI Analyst (Shanghai Niumag Corporation, China). T2-weighted images were measured by NMI20-Analyst (Shanghai Niumag Corporation, China). The fluorescence images were obtained using an inverted fluorescence microscope (IX 71, Olympus) and a charge-coupled device (CCD, Cascade 650).
Fe3O4@PZS nanoparticles with a diverse mass ratio of SPIONs to PZS were prepared. The morphology, structure, magnetic and MRI properties, colloidal stability, biocompatibility, and biodegradability of the obtained Fe3O4@PZS particles were investigated as well.
2. EXPERIMENTAL SECTION 2.1. Chemicals. Iron(III) acetylacetonate (Fe(acac)3), triethylene glycol (TREG), acridine orange (AO), ethidium bromide (EB), hexachloro-cyclotriphosphazene (HCCP, 98%), and 4,4′-sulfonyldiphenol (BPS, 98%) were purchased from Aldrich. Fetal bovine serum (FBS) and high-glucose Dulbecco’s Modified Eagle’s mediums (DMEM) were obtained from Hyclone. WST-1 reagent was purchased from Beyondtime Bio-Tech in China. Triethylamine (TEA) and organic solvents such as tetrahydrofuran (THF) and anhydrous ethanol were purchased from Shanghai Chemical Reagent Corporation and used as received. Water was purified using a Milli-Q-system (Millipore, Bedford). 2.2. Preparation of Hydrophilic Fe3O4 Nanoparticles. Waterdispersed Fe3O4 nanoparticles were prepared as previously reported.30 In a typical experiment, Fe(acac)3 (706 mg, 2 mmol) and TREG (60 mL) were mixed and heated to reflux (∼ 278 °C) for 45 min under N2 protection, giving a black homogeneous colloidal suspension. After cooling down to room temperature, ethyl acetate (60 mL) was added to the reaction solution resulting in a dark precipitation of magnetite nanoparticles, which were then separated from the solution by centrifugation. After washed with ethyl acetate (3 × 20 mL) and ethanol (3 × 20 mL) respectively, the solid product was obtained by drying the precipitation under vacuum overnight. 2.3. Preparation of Fe3O4@PZS. Fe3O4@PZS-1 was prepared as follows: Fe3O4 nanoparticles (5 mg), TEA (2.0 mL), HCCP (40 mg, 115.0 mmol), and a mixture of THF and anhydrous alcohol (60 mL, 9:1 by volume) were added into a 100 mL round-bottom flask. After ultrasonic irradiation for 20 min (50 W, 40 kHz), BPS (90 mg, 360.0 mmol) was added. The solution was then maintained at room temperature for 6 h under ultrasonic irradiation (50 W, 40 kHz). As soon as the reaction was complete, the resulting solids were collected by a magnet, washed with THF and anhydrous alcohol, and dried at 40 °C under vacuum overnight. Fe3O4@PZS-2 and -3 were obtained according to a same procedure by increasing Fe3O4 nanoparticles to 10 and 20 mg respectively. 2.4. Measurement of Magnetization Curves, T2 Relaxation Time, and T2-Weighted Images. Hysteresis cycles at room temperature were performed with a physical property measurement system (PPMS-9T, Quantum Design, USA) from −15000 to 15000 Oe. For MRI experiments, samples at given concentrations were suspended in phosphate buffered saline (PBS) and placed in 5 mL tubes. The transverse relaxation time T2 was measured with varying Fe concentrations using a MRI scanner (PQ001 Analyst, Shanghai Niumag Corporation, China) at a magnetic field strength of 0.5 T. Spin−echo pulse sequences with multiple spin echoes of various echo times were utilized to obtain pixel-by-pixel T2 maps of each sample (TR/TE, 3000/60 ms; matrix, 15.0 × 15.0 mm; section thickness, 0.6 mm). The relaxivities r2 can be obtained from the slopes of the plot from the concentration vs relaxation rate. MR imaging capabilities of the Fe3O4@PZS nanoparticles were examined at 0.5 T using an NMI20-Analyst (Shanghai Niumag Corporation, China) with the following parameters: TR = 2000 ms, TE = 100 ms, Slices = 1, Slice thickness = 5 mm. 2.5. Biocompatibility of Fe3O4@PZS. Human cervical cancer HeLa cells were cultured in DMEM (high glucose) supplemented with 10% FBS in a humidified incubator kept at 37 °C (95% room air, 5% CO2). Cells were cultured overnight to allow cell attachment and subsequently washed with FBS-free DMEM. Fe3O4@PZS-1,2,3 suspensions were then added respectively, and the resulting mixture was incubated at 37 °C for 24 h. The Fe3O4@PZS concentration in the culture was typically 0.1 mg/mL. The cell viability were assessed qualitatively by AO/EB double staining according to literature.27
3. RESULTS AND DISCUSSION The synthesis of Fe3O4@PZS core−shell particles is a relatively straightforward process (Scheme 1). First, the hydrophilic and monodispersed Fe3O4 nanoparticles were prepared according to a literature method.32 Intrinsically, a layer of hydrophilic polyol molecules was adsorbed on the surface of Fe3O4 nanoparticles during the synthesis process, and therefore the nanoparticles can be well redispersed in a mixture of THF and ethanol (9: 1, v/v) as a ferrofluid. After addition of a base, TEA, into the solution, the hydroxyl groups on the surface of Fe3O4 9157
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Scheme 1. Preparation Procedure of Superparamagnetic Fe3O4@PZS
Figure 2. (a) FTIR spectra and (b) XRD patterns of Fe3O4, PZS, and Fe3O4@PZS-2.
nanoparticles were activated under ultrasonic condition (Scheme 1) because the signal of 1H NMR of the hydroxyl groups on Fe3O4 completely disappeared (see the Supporting
Figure 1. TEM images of (a,b) Fe3O4@PZS-1, (d,e) Fe3O4@PZS-2, and (g,h) Fe3O4@PZS-3. FE-SEM images of (e) Fe3O4@PZS-1, (f) Fe3O4@ PZS-2, and (i) Fe3O4@PZS-3. 9158
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Figure 4. (a) T2-weighted MR images and (b) T2 relaxivity plot of aqueous suspension of Fe3O4@PZS-1, Fe3O4@PZS-2 and Fe3O4@ PZS-3.
Figure 3. (a) The field-dependent magnetization curve of the assynthesized materials measured at 300 K. (b) Magnified curve of the surrounding origin in panel a. (c) Separation of Fe3O4@PZS-2 from the solution under an external magnetic field.
Information, Figure S1). The activated hydroxyl groups tend to attack the phosphorus atoms’ nucleus on HCCP, and then the polycondensation of HCCP with BPS generated highly crosslinked polymer on the surface of Fe3O4 in an ultrasonic bath. Excess TEA also acted as an acid acceptor to absorb the resulting HCl and accelerated the polymerization. The Fe3O4 nanoparticles tend to aggregate and be embedded into the resulting PZS particles, depending on the mass ratio of Fe3O4 nanoparticles to PZS. The obtained Fe3O4@PZS particles were collected by centrifugation. Interestingly, Fe3O4@PZS can be well redispersed in common solvents such as water, N,Ndimethylformamide, THF, and PBS solutions to form a stable colloid solution against variation in pH of the environment due to the hydrophilic and highly cross-linking nature of PZS, benefiting further surface modification and biological applications. Figure S2a shows representative TEM images of the hydrophilic magnetite nanoparticles. The as-prepared Fe3O4 nanoparticles are monodispersed with a diameter of 8.2 ± 1.1
Figure 5. Fluorescence images of HeLa cells incubated with (a) control, (b) Fe3O4@PZS-1, (c) Fe3O4@PZS-2, and (d) Fe3O4@PZS-3 for 24 h. The cells were stained with acridine orange/ethidium bromide.
nm. To investigate its crystal structure, an HR-TEM image of a Fe3O4 nanoparticle is shown in Figure S2b. The lattice plane distance is 0.25 nm and corresponds to the [311] lattice plane of magnetite. SAED and X-ray diffraction (XRD) analysis further prove that the nanoparticles are highly crystalline magnetite particles with a face-centered cubic phase (see the Supporting Information, Figure S2c). The morphology and structure of the as-synthesized Fe3O4@ PZS materials were investigated by TEM and SEM. When we fixed the HCCP mass at 40.0 mg (43.0 mmol) and adjusted the 9159
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black cores were obtained (Figure 1g,h). As a consequence, the size of Fe3O4@PZS-3 increased to about 293.5 ± 8 nm in diameter (Figure 1i). The size distributions of Fe3O4@PZS1,2,3 were also analyzed from the SEM images (see the Supporting Information, Figure S3). The DLS measurements revealed that the hydrodynamic diameters of the as-prepared Fe3O4@PZS-1,2,3 were 239.7 nm, 263.1 and 335.2 nm, respectively (see the Supporting Information, Figure S4). These values are a bit bigger than that observed from SEM due to the hydrophilic nature of PZS. On the basis of these results, the size of magnetite cores and core−shell Fe3O4@PZS particles can be nicely regulated by adjusting the mass ratio of Fe3O4 to PZS. FTIR spectroscopy was used to confirm the successful formation of PZS (Figure 2a). The absorption at 589 cm−1 of Fe3O4 was ascribed to n(Fe−O) in Fe3O4 nanoparticles,33 and the other absorptions at 3391, 2863, 1069, and 1638 cm−1 belong to the stabilizer, TREG.32 For the IR spectrum of PZS, the absorption at 943 cm−1 is assigned to the P−O−Ar band, which is a direct evidence of the polymerization of comonomers HCCP and BPS.13 Other characteristic peaks of PZS can also be observed, including 1184 cm−1 (P = N), 882 cm−1 (P−N) in the cyclo-triphosphazene structure, and 1284 and 1153 cm−1 (OSO), 1588 and 1490 cm−1 (CC) in the sulfonylphenol units. The IR spectrum of the Fe3O4@PZS2 is very similar to that of PZS, and the characteristic peaks of the Fe3O4 almost disappeared because they were pretty weak and were overlapped by the peaks of PZS. It is noted that the peaks at 3093 cm−1 of Fe3O4@PZS and PZS are assigned to the stretching vibration of the phenolic hydroxyl groups, offering high surface activity to bind other biomolecules or drugs. The XRD analysis further proved that Fe3O4 nanoparticles were coated with PZS (Figure 2b). All the diffraction peaks of Fe3O4 nanoparticles match well with those from the JCPDS card (19-0629) for magnetite. The average particle size calculated using Scherrer’s formula is about 8.6 nm, which rightly agrees with the TEM result (see the Supporting Information, Figure S2). The Fe3O4@PZS sample possessed one broad diffraction peak and six sharp diffraction peaks, which corresponds to the reflection of PZS and magnetite respectively. These results demonstrate that the present method makes cross-linking polymer PZS successfully coated on the Fe3O4 nanoparticles. The XRD patterns of Fe3O4@PZS2 did not show an obvious change after storage in the air for 4 weeks, indicating that the Fe3O4 nanoparticles have a good chemical stability after coating by PZS. Since the superparamagnetism is very useful in biotechnology,18 the room-temperature magnetization curves of Fe3O4@ PZS were measured (Figure 3a,b). The saturation magnetization (Ms) values of the Fe3O4@PZS-1,2,3 and Fe3O4 are 10.9, 30.8, 54.0, and 79.7 emu/g, respectively. The Ms decrease of Fe3O4@PZS could be attributed to the lower magnetic component in the composite. All these samples are essentially superparamagnetic with negligible hysteresis (Figure 3b), suggesting that Fe3O4 nanoparticles encapsulated in the crosslinked PZS shell can preserve their superparamagnetic properties. The superparamagnetism can not only provide enhancement of the proton relaxation times T1 and T2,34,35 but also endow Fe3O4@PZS with good magnetic manipulation for many other biomedical applications such as MRI and drug delivery.35,36 The magnetic manipulation of Fe3O4@PZS-2 was tested in water by placing a magnet near the glass bottle. The gray particles can be totally attracted toward the magnet within
Figure 6. Viability of HeLa cells incubated with (a) Fe3O4@PZS-1, (b) Fe3O4@PZS-2, and (c) Fe3O4@PZS-3 at diverse concentrations for 24 and 48 h determined by the WST-1 assay.
mass of Fe3O4 from 5.0 to 20 mg, the core−shell structured Fe3O4@PZS spheres were prepared. At relatively low Fe3O4 mass amount (5.0 mg, 21.5 mmol), diverse sizes of particles embedded with few Fe3O4 nanoparticles (termed as Fe3O4@ PZS-1) were obtained (Figure 1a,b). Some of the particles do not contain any magnetite cores due to the lack of Fe3O4 nanoparticles. When the mass of Fe3O4 increased to 10 mg (43 mmol), monodispersed particles (termed as Fe3O4@PZS-2) were prepared. Each Fe3O4@PZS-2 particle has an obvious black core composed of many Fe3O4 nanoparticles (Figure1d,e). The Fe3O4@PZS-2 is about 228.5 ± 15 nm with relatively smooth outer surfaces (Figure 1f). If the mass of Fe 3 O 4 was further increased to 20 mg (86 mmol), monodispersed particles (termed as Fe3O4@PZS-3) with bigger 9160
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Figure 7. TEM images of HeLa cells incubated with Fe3O4@PZS-2 (10 μg/mL) for 6 h.
PZS can increase the water residence time and therefore increase the r2.20,23 Any biomaterials are expected to have an intrinsically low toxicity, and thus, the biocompatibility of Fe3O4@PZS was first qualitatively studied by AO/EB double staining. Generally, AO can cross intact cytomembrane and insert into DNA with a bright green color, while EB can only enter the damaged cytomembrane and insert into DNA with red color. Therefore, healthy cells have green nuclei and uniform chromatin with an intact cell membrane, whereas the cells in necrosis or at a late stage of apoptosis have red nuclei with a damaged cell membrane. Almost all the HeLa cells remain healthy, with nuclei being green even after incubating with Fe3O4@PZS-1,2,3 for 24 h at the concentration of 0.5 ug/mL (Figure 5b-d), suggesting that the PZS shells can endow the Fe3O4 with an excellent biocompatibility. WST-1 assay was also used to quantitatively assess the biocompatibility of Fe3O4@PZS nanohybrids. The HeLa cells were incubated in PBS buffer containing Fe3O4@PZS-1,2,3 with different concentrations, respectively. All the Fe3O4@PZS nanohybrids are of biologically low cytotoxicity in WST-1 analysis in a wide concentration range from 10 to 100 ug/mL for 24 and 48 h due to the excellent biocompatibility of PZS coatings (Figure 6). These results fairly agreed with those of AO/EB double staining experiments. TEM analysis was further used to track the distribution of Fe3O4@PZS inside the cells. TEM observations showed that Fe3O4@PZS-2 were internalized by HeLa cells and localized mostly in the cytoplasm (Figure 7). The black dots on the surface of Fe3O4@PZS and in the cytoplasm are Fe3O4 nanoparticles leaked during the process of making the ultrathin section. Herefore, the Fe3O4@PZS tends to enter HeLa cells but shows negligible cytotoxicity, suggesting that PZS is an ideal coating materials for biological applications. The degradation by hydrolysis of Fe3O4@PZS-2 in PBS solution was evaluated at 37 °C and different pH, 7.4 and 5.5 (corresponding to blood serum and lysosomal pH, respectively). The Fe3O4@PZS-2 has a very similar degradation curve at pH 7.4 and 5.5, and loss about 37% and 44% weight at the beginning 100 days, respectively (Figure 8). The PZS gradually degrades into BPS (see the Supporting Information, Figure S5) and other biologically benign products including phosphate and ammonia.36−38 The degradation rate of PZS at low pH environment decreases because the degradation product, BPS, is slightly acidic. The morphology and size distribution of Fe3O4@PZS-2 after degradation in pH 7.4 PBS solution for 160 days were also investigated. The degradation of PZS coating
Figure 8. The degradation curves of Fe3O4@PZS-2 in pH 7.4 and 5.5 PBS solutions at 37 °C.
several minutes, leaving the gray suspension a clear solution. After removal of the magnet, the Fe3O4@PZS-2 can be easily redispersed with slight shaking (Figure 3c). SPIONs have been proven to be an effective contrast agent in MRI because of their negative-enhancement effect on T2weighted sequences.1,3,33 The T2-weighted MR images of Fe3O4@PZS-1,2,3 aqueous dispersions at different Fe concentrations were investigated (Figure 4a). With increasing the Fe concentration for the all three Fe3O4@PZS samples, the signal intensity of the MR image decreased. More importantly, by comparing the T2-weighted images of Fe3O4@PZS-1,2,3, the T2 relaxivity was increased dramatically by increasing the magnetite core under the same Fe concentration. We believe that the aggregation of Fe3O4 nanoparticles in Fe3O4@PZS-2,3 provide higher contrast. It has been reported that the surface coating and aggregation/assembly of magnetic nanoparticles significantly affected transverse (R2) relaxation.27−29 The specific relaxivity (r2) of Fe3O4@PZS-1,2,3 was measured and calculated as well (Figure 4b). Fe3O4@PZS-3 was calculated to be 292.8 mM−1 s−1, which was significantly higher than that of Fe3O4@PZS-2 (177.4 mM−1 s−1) and Fe3O4 @PZS-1(108.1 mM−1 s−1). These results can well explain the effect of aggregation on MR contrast. Therefore, we can regulate not only the size and structure but also the MR nature of Fe3O4@ PZS by adjusting the mass ratio of Fe3O4 to PZS. It should also be noted that all the Fe3O4@PZS samples have a relatively higher r 2 than that SiO2 or copolymers coated Fe 3 O 4 samples,21−23,27 probably because the hydrophilic nature of 9161
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leads to the thinning of PZS layer, and the average diameters of the Fe3O4@PZS-2 after degradation fell to 181.5 ± 15 nm (see the Supporting Information, Figures S6 and S7).
4. CONCLUSIONS The core−shell Fe3O4@PZS nanohybrids were successfully prepared by a facile one-pot polycondensation of highly crosslinked PZS on the superparamagnetic Fe3O4 nanoparticles. All these nanohybrids have excellent water dispersion, colloidal and chemical stability, and tailored surface chemistry due to the nature of PZS. The magnetic core content can be easily tuned by varying the mass ratio of Fe3O4 to PZS. The T2 relaxation and MRI properties of Fe3O4@PZS was enhanced with the increasing of Fe3O4 cores. The Fe3O4@PZS tends to enter HeLa cells and mainly localize in the cytoplasm but show negligible cytotoxicity. The PZS layer slowly degraded into less dangerous forms such as 4,4′-sulfonyldiphenol, phosphate, and ammonia both at neutral and acid atmosphere. Therefore, the Fe3O4@PZS nanohybrids may be used as effective MR contrast agents for cancer diagnosis.
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ASSOCIATED CONTENT
S Supporting Information *
1
H NMR spectra of TREG, TEA and TREG+TEA; TEM images and SAED pattern of Fe3O4; the size distribution of Fe3O4@PZS-1,2,3 analyzed from SEM and DLS; the liquid chromatography−mass spectroscopy of BPS and the degradation product of Fe3O4@PZS- 2; TEM image and the size distribution of Fe3O4@PZS- 2 after 160 days degradation in pH 7.4 PBS solution. This material is available free of charge via the Internet at http://pubs.acs.org.
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected] (L.M.);
[email protected] (Q.L.). Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This work was supported by National Science Foundation for Distinguished Young Scholars (50925310), the National Science Foundation of China (20874059, 21174087), the Shanghai Municipal Natural Science Foundation (11ZR1448200, 114119a0600), and Evonik Degussa Specialty Chemicals (Shanghai) Co., Ltd. We also thank Prof. Sheng Dong and Mr. Da Xi for their help with magnetic measurement.
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