Article pubs.acs.org/Langmuir
Hydrophilic Magnetofluorescent Nanobowls: Rapid Magnetic Response and Efficient Photoluminescence Shun Chen,† Junjun Zhang,† Shaokun Song,† Rui Feng,† Yanyun Ju,† Chuanxi Xiong,† and Lijie Dong*,†,‡ †
School of Materials Science and Engineering, Wuhan University of Technology, Luoshi Road 122, Wuhan 430070, P.R. China Department of Materials Science and Engineering, Cornell University, Ithaca, New York 14850, United States
‡
S Supporting Information *
ABSTRACT: Multifunctional integration based on a single nanostructure is emerging as a promising paradigm to future functional materials. In this paper, novel magnetofluorescence nanobowls built with ferroferric mandrel and quantum dots exoderm is reported. Magnetic mandrels are stacked into nanobowls though hydrophobic primary Fe3O4 nanocrystals dragged into anion polyelectrolyte aqueous solution via forced solvent evaporation. Bright luminescence core/shell/shell CdSe/ CdS/ZnS quantum dots (QDs) are modified with cationic hyperbranched polyethylenimine (PEI). Through electrostatic interactions, positively charged PEI-coated QDs are anchored on the surface of magnetic mandrel. Under this method, the luminescence of QDs is not quenched by magnetic partners in the resultant magnetoflurescence nanobowls. Such magnetoflurescence nanobowls exhibit high saturation magnetization, superparamagnetic characteristics at room temperature, superior water dispersibility, and excellent photoluminescence properties. The newly developed magnetoflurescence nanobowls open a new dimension in efforts toward multimodal imaging probes combining strong magnetization and efficient fluorescence in tandem for biosensors and clinical diagnostic imaging.
1. INTRODUCTION Multimodal imaging probes integrated with multifunctional entities into single nanostructures synergistically extract accurate and reliable biomedical information, thus improving the efficacy and sensitivity of clinical imaging diagnostics.1−4 Magnetofluorescent nanomaterials which combine rapid magnetic response and efficient luminescence show a great potential in practical applications, such as precise and susceptive biosensors and targeted optical imaging.5−9 Current strategies on integrating fluorescent QDs with magnetic materials generally include four distinct approaches: coencapsulation or assembly of preformed QDs and magnetic nanoparticles,10−14 heterocrystalline growth,15−18 conjugation of magnetic chelates onto QDs,19−22 and doping of QDs with transition metal ions.23−26 As a facile and efficient approach, coencapsulation and assembly strategies have been intensely developed which mainly include discrete preformed QDs and magnetic nanocrystals into silica or polymer beads, micelles, or liposomes. As a general substrate, silica was usually employed as the intermediate layer to host QDs and magnetic particles.27−32 Ying et al. reported water-soluble hybrid materials consisting of silica-capped QDs and γ-Fe2O3 encapsulated in silica shells synthesized in reverse emulsion medium.33 Hyeon et al.34 put forward to a general method for magnetic nanocrystals (MNCs) and QDs embedded in mesoporous silica spheres for controlled uptake and release of drugs. Similarly, γ-Fe2O3 © 2015 American Chemical Society
MNCs and CdSe/Cd/ZnS QDs were incorporated into silica shells on prefabricated silica microsphere cores through a sol− gel process.35 Magnetofluorescent nanosystems were also fabricated by utilizing silica-coated magnetic nanoparticles as cores, followed by layer-by-layer assembly of polyelectrolyte and QDs onto the core surfaces.32,35 However, silica-mediated magnetic fluorescent nanocomposites usually display weak magnetic response due to the diamagnetic amorphous silica and the low mass fraction of magnetic nanocrystals.36 Recently, it is available to increase the saturation magnetization of these nanocomposites using assembled three-dimensional (3-D) nanocrystal clusters as magnetic cores owing to the cumulative magnetization effect from considerable magnetic moments.37−42 Based on this strategy, several approaches toward core/shell magneto-optical architecture have emerged, such as core/shell fluorescent polystyrene-Fe3O4 nanospheres by immobilizing QDs on the surfaces of 100 nm magnetic polystyrene nanospheres and hybrid nanoparticles composed of luminescent QDs and superparamagnetic iron oxides based on the ligand-exchange mechanism.43 Therefore, magnetic matrices possessing a compact structure are more suitable for achieving desirable robust structures; on the other hand, and in such nanostructures, it is still a huge challenge for preventing Received: October 28, 2015 Published: December 14, 2015 611
DOI: 10.1021/acs.langmuir.5b03978 Langmuir 2016, 32, 611−618
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Langmuir the luminescence quenching of QDs from aggressive magnetic partners.44 Herein, we constructed novel magnetofluorescent nanobowls with mandrel-exoderm architecture which show desirable superparamagnetic behavior, rapid magnetic response, efficient photoluminescence, outstanding water dispersibility, and colloidal stability. By an efficient evaporation-induced strategy, stacked magnetic nanobowls with enhanced saturation magnetization are finally assembled from individual magnetite nanocrystals during the oil−water phase transfer process. The bowllike magnetic superparticles with an average size of 100 nm exhibit robust superparamagnetic behavior at room temperature. Luminescent multishell CdSe/CdS/ZnS QDs are employed as the outer exoderm due to their intense photoluminescence, uniform distribution, and chemical stability. Further ligand-exchanged by positive hyperbranched polyelectrolytes, octadecylamine (ODA)-passivated CdSe/ CdS/ZnS QDs framed branched molecular interspaces are firmly anchored onto the magnetic nanobowls.
2. RESULTS AND DISCUSSION 2.1. Magnetic Nanobowls Mandrel. Uniform oleic acid capped Fe3O4 nanocrystals were initially synthesized by general thermal decomposition of iron-oleate complex.45 Owing to the strong coordination of carboxylate groups with iron cations on the magnetite surface, polyelectrolyte polyepoxysuccinic acid (PESA) was adopted to build 3-D architecture. A volume of hexane-dispersed as-prepared Fe3O4 MNCs was loaded to proportional PESA aqueous solution. By sonicating continuously and increasing temperature, the primary Fe3O4 magnetic nanocrystals were assembled into clusters. Magnetic assemblies (designated as Fe3O4−PESA) with bowllike morphology were gradually formed during the oil phase evaporation. As it has been reported, Fe3O4 nanocrystals synthesized via thermal decomposition usually exhibit a narrow size distribution with uniform spherical structure (Figure 1a). The average size of the monodisperse primary Fe3O4 MNCs is around 14 nm. The X-ray diffraction (XRD) pattern of Fe3O4 MNCs (Figure 1b(i)) reveals an inverse spinel structure. By PESAassisted and oil phase evaporation, the monodisperse Fe3O4 MNCs were further assembled into nanobowls, as shown in Figure 1c, d. It is clearly seen that Fe3O4−PESA nanobowls46,47 with an average size of about 100 nm are consisted of densely packed Fe3O4 nanocrystals. Owing to low oil/water ratio, Fe3O4 MNCs in hexane trended to form microemulsion oil droplets in PESA aqueous solution. By means of continuous ultrasonication, the microemulsion oil droplets were kept dynamically stable without or with slight coalescence. The hydrophobic Fe3O4 MNCs were confined in the micrometersized 3-D oil droplets stabilized by PESA. With temperature increasing and subsequent evaporation of low-boiling n-hexane, the oil droplets began to shrink and cave and the Fe3O4 MNCs were gathered and coalesced together spontaneously. Finally, after evaporation under vacuum to remove hexane completely, the Fe3O4 nanocrystals were densely clustered to form magnetic nanobowls (Figures S1 and S2). In the XRD pattern of Fe3O4−PESA nanobowls as shown in Figure 1b(ii), the peak and intensities are consistent with those of individual Fe3O4 MNCs, which confirms that these magnetic nanobowls are fabricated by individual Fe3O4 nanocrystals as building blocks. From the transmission electron microscopy (TEM) images, the final particles became bowllike in Figure 1c, d. The morphology of bowllike superparticles was further investigated by SEM in
Figure 1. (a) TEM image of oleic acid capped Fe3O4 nanocrystals; (b) X-ray diffraction (XRD) pattern of (i) Fe3O4 nanocrystals and (ii) Fe3O4−PESA nanobowls. Literature values (JCPDS card No. 190629) for the peak positions and intensities for bulk magnetite samples are indicated by the vertical bars in blue color. TEM images showing (c) Fe3O4−PESA nanobowls and (d) an individual Fe3O4−PESA nanobowl; SEM images of Fe3O4 magnetic nanobowls stabilized by PESA (e, f).
Figure 1e, f. Single bowllike superparticle was characterized and the high crystallinity of Fe3O4 nanocrystals is seen in Figure S3. The distance between two adjacent planes in Fe3O4 is measured to be 0.197 nm, corresponding to (331) planes in the inverse spinel structured Fe3O4. Based on the TEM image of a single superparticle (Figure 1d), it is calculated that single superparticle consisted of about 200 primary Fe3O4 naocrystals. The organic content in Fe3O4−PESA nanobowls was measured by thermogravimetric analysis (TGA) (Figure 2a). Based on the weight loss, the organic contents of oleic acidcapped Fe3O4 nanocrystals is calculated to be 16.5 wt % while that of Fe3O4−PESA nanobowls is 19.3 wt %. It is to say, about 39 vol % PESA was filled in a single Fe3O4−PESA nanobowls. Fourier transforms infared (FT-IR) spectra of Fe3O4−PESA nanobowls and pure PESA are shown in Figure 2b. Compared to oleic acid capped Fe3O4 MNCs, the magnetic nanobowls exhibit the same strong bands at 2923 and 2852 cm−1, which can be attributed to the asymmetric and symmetric stretching vibrations of −CH2. The bands at 1619, 1400, and 1046 cm−1 in the spectrum of Fe3O4−PESA nanobowls could be assigned to the antisymmetric and symmetric vibrations of the −COO− groups, stretching vibrations of C−O, respectively, which imply the presence of PESA in the magnetic nanobowls. The carboxylate groups on the PESA chains strongly coordinated 612
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of Fe3O4−PESA nanobowls, which indicates that Fe3O4−PESA nanobowls inherit the similar superparamagnetic behavior of the parent Fe3O4 MNCs. Most importantly, the as-assembled Fe3O4−PESA nanobowls show an enhanced saturation magnetization value of 38.4 emu·g−1 in contrast with that of monodisperse Fe3O4 nanocrystals. The large number density of Fe3O4 nanocrystals entrapped in a nanostructure leads to an accumulative effect of magnetic properties. In Figure 3b, the hydrophilic Fe3O4−PESA nanobowls display rapid magnetic response under an external magnetic field. Even the concentration of Fe3O4−PESA aqueous solution is low to 1 mg·mL−1; it is also easy to manipulate them within a few minutes. 2.2. Luminescence CdSe/CdS/ZnS Exoderm. According to successive ion layer adsorption and reaction method (SILAR), monodisperse octadecylamine (ODA)-passivated multishell CdSe/CdS/ZnS QDs were synthesized (Figures S4 and S5).48−50 Due to the size-dependent photoluminescence, semiconductor nanocrystals have become the most attractive fluorescent materials.51−54 Especially, covered by CdS or ZnS as a shell layer, CdSe semiconductor nanocrystals lead to high fluorescence quantum yield and robustness against chemical degradation or photo-oxidation. With a hyperbranched structure typically comprising abundant amine groups, polyethylenimine (PEI) is among the most densely charged of all organic polymers as amine groups can carry positive charges within a certain range of pH. Based on ligand-exchange method, positively water-soluble QDs (QDs-PEI) capped by PEI were obtained. TEM image of monodisperse octadecylamine (ODA)passivated core/shell/shell CdSe/CdS/ZnS QDs is shown in Figure 4a. The core/shell/shell QDs have an average diameter of 6.7 nm. Hyperbranched PEI is used to decorate QDs by ligand exchange. Since amino groups show great affinity to zinc atoms, it is easy to achieve PEI-coated QDs. The absorption spectrum of QDs-PEI in aqueous solution is very similar to that of the initial QDs in chloroform (Figure 4c). In addition, abundant uncoordinated groups on each polymer chain extend into water, rendering the nanocrystals a high degree of solubility in water. After efficient surface modification by PEI, CdSe/CdS/ZnS QDs are also kept in a monodisperse state, as shown in Figure 4b. The water-soluble QDs-PEI nanoparticles are well dispersed without any agglomeration owing to the steric hindrance and repulsive interaction between nanocrystal. Besides, the transfer of CdSe/CdS/ZnS QDs from organic phase into water can be easily detected by the fluorescent
Figure 2. (a) Thermogravimetric analysis (TGA) curves of (i) oleic acid-capped Fe3O4 nanocrystals, (ii) Fe3O4−PESA nanobowls. (b) Fourier transform infrared (FT-IR) spectra of (i) Fe3O4 nanocrystals, (ii) Fe3O4−PESA nanobowls, and (iii) pure PESA. (c) Zeta potential of the negatively charged Fe3O4−PESA nanobowls at pH = 7.
to the surface of as-fabricated magnetic nanobowls, while the uncoordinated carboxylate groups extended into the surrounding water, making the nanobowls well dispersed in aqueous solution. From zeta potentiometer measurements it can be seen that the Fe3O4−PESA nanobowls have negative surface potential (−46.65 mV) at neutral pH (Figure 2c), which also demonstrates the existence of abundant carboxylate groups on the surface of magnetic nanobowls. In this case, much more negative charges make it possible for further Fe3O4−PESA nanobowls assembling with modified QDs through electrostatic interactions. The magnetic properties of Fe3O4−PESA nanobowls were measured on a vibrating sample magnetometer (VSM) by cycling the field between −20 and 20 kOe at 300 K. Figure 3a shows the field-dependent hysteresis loops of primary MNCs and clustered Fe3O4−PESA nanobowls. It is observed that the primary Fe3O4 MNCs show superparamagnetic behavior at room temperature with an saturation magnetization value of 33.2 emu·g−1, which is near the magnetic saturation values of Fe3O4 nanocrystals reported in the literature.40−42 It is also observed that no remanence or coercivity is shown in the case
Figure 3. (a) Magnetization hysteresis loops of Fe3O4 primary nanocrystals and Fe3O4−PESA nanobowls from −20 to 20 kOe at 300 K. The inset is a magnified view of the magnetization curves at low applied fields. (b) Optical photographs of Fe3O4−PESA nanobowls dispersed in aqueous media (left) and with magnet (right). 613
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2.3. Magnetofluorescent Mandrel-Exoderm Superparticles (MMESs). Through intense electrostatic interactions, magnetofluorescent nanobowls were self-assembled by anchoring luminescent QDs-PEI onto as-derived Fe3O4−PESA nanobowls surface. Typically, moderate solution of PEImodified QDs was slowly injected into the dispersion of Fe3O4−PESA nanobowls by vigorous stirring in neutral aqueous solution. After 4 h of electrostatic capture, the resultant magnetofluorescent mandrel-exoderm superparticles were collected by NdFeB permanent magnet and further purified to remove excess QDs-PEI. TEM images depict that the obtained magnetofluorescent mandrel-exoderm superparticles are perfectly dispersed without any aggregation (Figure 6a), due to the electrostatic repulsions and steric stabilization between the PEI chains surrounding the nanoparticles. When positively charged PEI-modified QDs were added into the aqueous solution, the negative Fe3O4−PESA mandrel with huge surface area began to harvest the surrounding QDs by electrostatic force, and a layer of QDs smoothly deposited on the surface of Fe3O4−PESA mandrels. Owing to the structural obstruct from PEI and PESA, the fluorescence of QDs-PEI nanoparticles were preserved well on the surface of Fe3O4−PESA nanobowls. In contrast with Fe3O4−PESA mandrels, most of the magnetofluorescent nanobowls58,59 exhibit a larger size of around 150 nm, as confirmed by the magnification TEM image shown in Figure 4b. More comprehensive structural information came from TEM and detailed high resolution TEM (HRTEM) analysis of individual magnetofluorescent nanobowls. Compared with the PEI-modified QDs, Fe3O4 nanocrystals are darker from the HRTEM image took from the edge of each individual magnetofluorescent nanobowls. The negligible interspace between the two assembling components indicates that QDs attach tightly to the surface of the Fe3O4−PESA mandrels. Energy-dispersive X-ray spectrum of the MMESs in Figure 6c demonstrates the existence of the elements including Fe, Cd, Se, S, Zn, and Cu. Apart from Cu and C peaks according to the carbon-coated copper grid, the Cd, Se, S, and Zn peaks indicate the presence of CdSe/CdS/ZnS QDs on the surface of the Fe3O4−PESA nanobowls, which is in accordance with TEM observations. The intense Fe and O peaks is relative to the densely packed magnetite nanocrystals within the MNCs. The excellent monodispersibility of such MMESs can also be verified from the SEM image in Figure 6d. The photoluminescence emission peak of MMESs is localized at 619 nm in Figure 7a, with a tiny blue shift in contrast with that of PEI-modified QDs at 621 nm. The blue shift phenomenon is attributed to the change in surface states of the QDs due to the immobilization to the Fe3O4−PESA mandrels. Remarkably, the PL intensity of MMESs is nearly unchanged during storage under ambient conditions, in darkness and in ambient air for more than 6 months, indicating that the luminescence from QDs are hardly disturbed by the Fe3O4−PESA mandrels. The magnetofluorescent nanobowls maintain their efficient luminescence at a prolonged time. CLSM image of the magnetofluorescent nanobowls is shown in Figure 7b. A drop of MMESs suspension was smeared on a glass slide, and was subsequently evaluated after drying. The MMESs exhibit bright red-emitting luminescence against the dark background and no aggregates are observed, indicating the immobilization of abundant QDs on the surface of Fe3O4− PESA nanobowls. Meanwhile, there exist no fluorescent images of individual PEI-modified QDs in the visual field, that is, the
Figure 4. TEM images of (a) hydrophobic CdSe/CdS/ZnS core/ shell/shell QDs and (b) hydrophilic QDs-PEI. Inset: As-synthesized CdSe/CdS/ZnS core/shel/shell QDs in chloroform in daylight (i) and under UV excitation (ii); QDs-PEI after ligand exchange in deionized water in daylight (iii) and under UV excitation (iv). (c) UV−vis spectra and (d) PL spectra of original CdSe/CdS/ZnS QDs (i) and water-soluble QDs-PEI(ii).
maintain in different solvent in daylight and under UV light (inset in Figure 4a, b). Optical properties of original CdSe/CdS/ZnS QDs and water-soluble QDs-PEI have been investigated by UV−vis spectrometer and PL spectroscopy shown in Figure 4c and d. The PL spectra of QDs-PEI are similar to those of CdSe/CdS/ ZnS QDs.55−57 A slight blue shift of the PL peak wavelength of the original QDs can be observed from 623 to 621 nm after ligand exchange with hyperbrached PEI, which implies that more surface defects are generated. The QY of the original CdSe/CdS/ZnS QDs was measured to be as high as 69%. And the relative QY of the QDs-PEI declined to about 50%. Since the PL properties are closely related to the surface state of the QDs, this result is attributed to the imperfect protective environment of PEI coated on the surface of QDs. Zeta potentiometer characterization showed that the PEIstabilized QDs had positive potential of +19.17 mV due to the -NH3+ groups in aqueous environment (Figure 5). Thus, the two functional nanounits with opposite surface charges, that is, Fe3O4−PESA nanobowls and QDs-PEI, have provided the possibility of building multifunctional nanosystems combined with superparamagnetism, high magnetization and fluorescent properties.
Figure 5. Zeta potential of the positively charged QDs-PEI at pH = 7 in aqueous phase. 614
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Figure 6. (a) TEM image of MMESs dispersed in aqueous medium, the scale bar is 500 nm. (b) TEM image of individual MMES; scale bar is 50 nm. In contrast to the PEI-modified QDs picture, the Fe3O4 nanocrystals are darker from the HRTEM images. The close interspace between the two subunits indicates that QDs attach tightly to the surface of the MNCs. (c) Energy dispersive X-ray spectrum of MMESs. (d) SEM image of asprepared MMESs; scale bar is 5 μm.
Figure 7. (a) PL spectra of MMESs dispersed in deionized water. (b) Dark-field confocal laser scanning microscopy (CLSM) image of as-assembled MMESs. (c) Magnetic and optical properties of MMESs: photographs of the MMESs dispersed in deionized water without (i) and with (ii) UV irradiation and those applied with an external magnetic field without (iii) and with (iv) UV irradiation under 365 nm UV light.
unconjugated QDs-PEI is completely removed. In particular, each multifunctional nanobowl exhibits fluorescence of similar intensities, implying that the QDs are capped and distributed uniformly. The combination of magnetic and optical properties should enable the magnetofluorescent nanobowls to be utilized simultaneously for MRI and biological labeling. Figure 7c illustrates the separation and redispersion process of such magnetofluorescent nanobowls in aqueous solution. The dispersion is yellowish under sunlight and exhibited luminous red color under UV light irradiation. Under an external magnetic field, the as-assembled magnetofluorescent nanobowls subsequently are totally localized on the wall of the sample vial
in a few minutes, leaving the solution transparent and colorless. It can be inferred that QDs-PEI are closely attached on the surface of Fe3O4−PESA mandrel. The yellowish-brown aggregation glows red color under UV light, and can redisperse in the deionized water again by gentle shaking without magnetic field. After 6 months, the luminescence can still be illuminated by 365 nm lamp with a bright color. Benefiting from the mandrel-exoderm structure and excellent water dispersibility, such magnetofluorescent nanobowls exhibit desirable colloidal stability for at least 6 months. 615
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reaction (SILAR) technique.54,60 The as-synthesized CdSe/CdS/ZnS core/shell/shell QDs were stored in chloroform in a dark place as stock solution. The as-synthesized QDs exhibit a high photoluminescence (PL) quantum yield (QY) as high as 70% at room temperature. In order to prepare hydrophilic QDs coated by hyperbrached polyethleneimine (PEI), 100 mg of PEI was dissolved in 10 mL of QDs stock solution (20 mM) and treated by ultrasonication for 2 h at 40 °C. The mixture was precipitated by nhexane and collected by centrifugation at 6000 rpm for 6 min. Deionized water was then added to dissolve the PEI-QDs. The yellow solution was dialyzed for 3 days to remove the excess PEI in a dialysis tube (MD34 (7000)) against deionized water. The final product was stored in a dark place at 4 °C. Synthesis of Core/Shell Magnetofluorescent Nanobowls. In a typical procedure, a volume of 125 μL 4 mg·mL−1 particles was distilled to 5 mL, followed by adding 5 mL of QDs-PEI aqueous solution (5 mM) under ultrasonication and mechanical stirring at 200 rpm. After ultrasonic treatment for 20 min, the mixture was kept on stirring for another 4 h. Finally, the raw products were separated by external magnetic field and dispersed in deionized water. This cycle was repeated until the supernatant after magnetic separation could not show the photoluminescence spectrum measured by a HITACHIF2500 fluorescence spectrophotometer at an excitation wavelength of 365 nm. Characterization. Transmission electron microscopy (TEM) images, high-resolution TEM (HRTEM) images, and energydispersive X-ray spectroscopy (EDX) of the samples were recorded on a field-emission transmission electron microscope (JoelJEM-2001F, coupled with an EDX analysis system, accelerating voltage 200 kV) to measure the size, size distribution, element types, and composition percentage of the Fe3O4 nanocrystals, Fe3O4−PESA nanobowls, and magnetofluorescent superparticles by depositing them on carboncoated copper grids (holey carbon, 200 mesh Cu) and left to dry at room temperature. Scanning electron microscopy (SEM) images were obtained using a field-emission gun environmental scanning electron microscope (S-4800, HITACHI) at an acceleration voltage of 30 kV. Thermogravimetric analysis (TGA) was performed using a simultaneous thermal analyzer (STA 499C, Netzsch) at a heating rate of 10 °C·min−1 in a flowing nitrogen atmosphere from room temperature to 800 °C. Fourier transform infrared (FT-IR) spectra were obtained on an FT-IR spectrometer (Thermo Nicolet Nexus) using KBr pellets. Powder X-ray diffraction (XRD) patterns were acquired on a D/MaxIIIA (Rigaku) diffractometer using Cu Kα radiation (λ = 1.54 Å) in a 2θ range from 5° to 70° at a scan rate of 0.02° per step. UV−vis absorption spectra were measured on a diode array UV−vis spectrometer (UV-2550, SHIMADZU). Photoluminescence (PL) emission spectra were collected via a RF-5301 PC spectrophotometer. PL quantum yields (QY) of the samples were estimated by comparing the fluorescent intensity to that of rhodamine 6G with known quantum yield (95% in ethanol) with the same optical density at the excitation wavelength at 480 nm. Fluorescent images of the asobtained magnetofluorescent superparticles were acquired by confocal laser scanning microscopy (Leica TCS NT, Germany). Magnetic measurements of the products were conducted on vibrating sample magnetometer (VSM, Lake Shore 7410) with external magnetic field ranging from −20 to 20 kOe at 300 K. Zeta potential measurements were performed on a Brookhaven Zeta-PALS with Bi-Mas particle sizing option. Zeta potentials were recorded for a solution of Fe3O4− PESA nanobowls and QDs-PEI in neutral deionized water. The pH values of these solutions were adjusted to 7.0 using 10 mM HCl and 10 mM NaOH with a digital pH meter (PHS-3C, LEICI).
3. CONCLUSIONS In summary, we have reported a simple and versatile method for preparing multifunctional magnetofluorescent mandrelexoderm nanobowls by assembling QDs-PEI on the surface of the negatively charged magnetite mandrel in aqueous phase via electrostatic attractions. This strategy is advantageous for incorporating diverse functionalities together. The combination of superparamagnetism at room temperature, high magnetization, efficient luminescence, excellent water-dispersity, and stability can impart magnetofluorescent mandrel-exoderm nanobowls great potential to meet the requirements in biosensors and clinical diagnostic imaging. 4. EXPERIMENTAL SECTION Materials. FeCl3·6H2O (98%) and sodium oleate (95%) were purchased from Alfa Aesar, cadmium oxide (CdO, 99.99%), selenium (Se, 99.5%), zinc oxide (ZnO, 99.99%, powder), sulfur (S, 99.98%, powder), tributylphosphine oxide (TOPO, 90%), tributylphosphine (TBP, 97%), octadecylamine (ODA, 97%), 1-octadecene (ODE, 90%), oleic acid (OA, 90%), and stearic acid (SA, 99%) were all from Sigma-Aldrich. Polyethylenimine (PEI, branched, Mw 1800, 99%) was received from AlfaAesar. Polyepoxysuccinic acid sodium salt (PESA, Mw 400−1500, 40 w/v %) was offered by Shandong Taihe Water Treatment Co., Ltd., China. n-Hexane (AR) and anhydrous ethanol (AR) were provided by Sinopharm Chemical Reagent Co., Ltd., China. The original PESA aqueous solution (40 wt %) was subsequently diluted to a concentration of 1 wt %, and then the pH value of the diluted solution was adjusted to 8.0 as a stock solution by adding 5 M hydrochloric acid. All of the other materials were used as received without further treatments. Deionized water was distilled with a Milli-Q water purification system. Synthesis of Magnetite (Fe3O4) Nanoparticles and Fe3O4− PESA Nanobowls. Monodisperse Fe3O4 nanocrystals in 14 nm capped by oleic acid was synthesized according to the procedures reported previously by the Hyoen45 group. Typically, 40 mmol of iron chloride (FeCl3·6H2O) and 120 mmol of sodium oleate were dissolved in a mixture solvent of 80 mL ethanol, 60 mL deionized water and 140 mL n-hexane. Then, the resultant solution was heated and stirred at a temperature of 70 °C for 4 h. The upper organic layer was collected and washed three times with 30 mL deionized water in a separatory funnel. After removing the hexane by evaporation, dark-brown waxy iron-oleate complex was acquired. Subsequently, 18 g of as-derived iron-oleate complex and 10 mmol oleic acid were dissolved in 100 g of 1-octadecene. The mixture was heated from room temperature to 320 °C at an elevated rate of 3.3 °C·min−1, kept for 30 min under nitrogen atmosphere, and then cooled to room temperature after reaction. The monodisperse 14 nm Fe3O4 nanocrystals were obtained by several cycles of precipitation and centrifugation and finally dispersed in nhexane at a concentration of 10 mg·mL−1. Then 1 mL of Fe3O4 dispersion was added to 20 mL of 1 wt % oligomer stock solution. This system was subsequently treated by ultrasonication in a water bath with a temperature initially set at 40 °C for 4 h continuously. During the ultrasonication, the solvent was gradually volatilized and the hydrophobic Fe3O4 nanocrystals were transferred from the oil phase into the bottom aqueous phase. After the 4 h reaction, the top nhexane layer disappeared, and the gray aqueous layer was collected and evaporated under vacuum for 30 min. After cooled to room temperature, the product was collected by external magnetic field and dissolved in deionized water by ultrasonication. This cycle of the dissolution and separation was repeated at least three times to remove free PESA. The final hydrophilic Fe3O4−PESA nanobowls were dispersed in deionized water to obtain aqueous solution with a concentration of 4 mg·mL−1. Synthesis of Core/Shell/Shell CdSe/CdS/ZnS Quantum Dots (QDs) and Ligand Exchange Reaction. Highly fluorescent ODAcapped CdSe nanocrystals were initially synthesized, and further capped by two layers of CdS, one monolayer of Cd0.5Zn0.5S, and another layer of ZnS using the successive ion layer adsorption and
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ASSOCIATED CONTENT
S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.langmuir.5b03978. TEM images of Fe3O4−PESA nanoparticles under different experimental conditions; TEM images and 616
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Applications in Immuno-Labeling and Fluorescent Imaging of Cancer Cells. Langmuir 2010, 26, 1278−1284. (14) Di Corato, R.; Bigall, N. C.; Ragusa, A.; Dorfs, D.; Genovese, A.; Marotta, R.; Manna, L.; Pellegrino, T. Multifunctional Nanobeads Based on Quantum Dots and Magnetic Nanoparticles: Synthesis and Cancer Cell Targeting and Sorting. ACS Nano 2011, 5, 1109−1121. (15) Gao, J. H.; Zhang, B.; Gao, Y.; Pan, Y.; Zhang, X. X.; Xu, B. Fluorescent Magnetic Nanocrystals by Sequential Addition of Reagents in a One-Pot Reaction: A Simple Preparation for Multifunctional Nanostructures. J. Am. Chem. Soc. 2007, 129, 11928−11935. (16) Kwon, K. W.; Shim, M. γ-Fe2O3/II-VI Sulfide Nanocrystal Heterojunctions. J. Am. Chem. Soc. 2005, 127, 10269−10275. (17) Zedan, A. F.; Abdelsayed, V.; Mohamed, M. B.; El-Shall, M. S. Rapid synthesis of magnetic/luminescent (Fe3O4/CdSe) nanocomposites by microwave irradiation. J. Nanopart. Res. 2013, 15, 1−8. (18) McDaniel, H.; Shim, M. Size and Growth Rate Dependent Structural Diversification of Fe3O4/CdS Anisotropic Nanocrystal Heterostructures. ACS Nano 2009, 3, 434−440. (19) Thiyagarajan, P.; Ahn, H. J.; Lee, J. S.; Yoon, J. C.; Jang, J. H. Nanostructures: Hierarchical Metal/Semiconductor Nanostructure for Efficient Water Splitting. Small 2013, 9, 2202. (20) Liu, J. W.; Zhang, Y.; Yan, C. Z.; Wang, C. Y.; Xu, R. Z.; Gu, N. Synthesis of Magnetic/Luminescent Alginate-Templated Composite Microparticles with Temperature-Dependent Photoluminescence under High-Frequency Magnetic Field. Langmuir 2010, 26, 19066− 19072. (21) Fan, H. M.; Olivo, M.; Shuter, B.; Yi, J. B.; Bhuvaneswari, R.; Tan, H. R.; Xing, G. C.; Ng, C. T.; Liu, L.; Lucky, S. S.; Bay, B. H.; Ding, J. Quantum Dot Capped Magnetite Nanorings as High Performance Nanoprobe for Multiphoton Fluorescence and Magnetic Resonance Imaging. J. Am. Chem. Soc. 2010, 132, 14803−14811. (22) Wang, X. B.; Wang, G.; Li, W. W.; Zhao, B. X.; Xing, B.; Leng, Y. K.; Dou, H. J.; Sun, K.; Shen, L. S.; Yuan, X. L.; Li, J. Y.; Sun, K.; Han, J. S.; Xiao, H. S.; Li, Y.; Huang, P.; Chen, X. Y. Quantum Dots: NIR-Emitting Quantum Dot-Encoded Microbeads through Membrane Emulsification for Multiplexed Immunoassays. Small 2013, 9, 3364. (23) Sitbon, G.; Bouccara, S.; Tasso, M.; Francois, A.; Bezdetnaya, L.; Marchal, F.; Beaumont, M.; Pons, T. Multimodal Mn-doped I-III-VI quantum dots for near infrared fluorescence and magnetic resonance imaging: from synthesis to in vivo application. Nanoscale 2014, 6, 9264−9272. (24) Selvan, S. T.; Patra, P. K.; Ang, C. Y.; Ying, J. Y. Synthesis of Silica-Coated Semiconductor and Magnetic Quantum Dots and Their Use in the Imaging of Live Cells. Angew. Chem. 2007, 119, 2500− 2504. (25) Kwon, K. W.; Lee, B. H.; Shim, M. Structural Evolution in Metal Oxide/Semiconductor Colloidal Nanocrystal Heterostructures. Chem. Mater. 2006, 18, 6357−6363. (26) Zhang, Y. Y.; Tang, Y. X.; Liu, X. F.; Dong, Z. L.; Hng, H. H.; Chen, Z.; Sum, T. C.; Chen, X. D. Three-Dimensional CdS-Titanate Composite Nanomaterials for Enhanced Visible-Light-Driven Hydrogen Evolution. Small 2013, 9, 996−1002. (27) Tian, Z. Q.; Zhang, Z. L.; Gao, J. H.; Huang, B. H.; Xie, H. Y.; Xie, M.; Abruna, H. D.; Pang, D. W. Color-tunable fluorescentmagnetic core/shell multifunctional nanocrystals. Chem. Commun. 2009, 4025−4027. (28) Kim, H.; Achermann, M.; Balet, L. P.; Hollingsworth, J. A.; Klimov, V. I. Synthesis and Characterization of Co/CdSe Core/Shell Nanocomposites: Bifunctional Magnetic-Optical Nanocrystals. J. Am. Chem. Soc. 2005, 127, 544−546. (29) Lee, J. E.; Lee, N.; Kim, T.; Kim, J.; Hyeon, T. Multifunctional Mesoporous Silica Nanocomposite Nanoparticles for Theranostic Applications. Acc. Chem. Res. 2011, 44, 893−902. (30) Piao, Y. Z.; Burns, A.; Kim, J.; Wiesner, U.; Hyeon, T. Designed Fabrication of Silica-Based Nanostructured Particle Systems for Nanomedicine Applications. Adv. Funct. Mater. 2008, 18, 3745−3758. (31) Guo, J.; Yang, W. L.; Wang, C. C.; He, J.; Chen, J. Y. Poly(Nisopropylacrylamide)-Coated Luminescent/Magnetic Silica Micro-
UV−vis and PL-spectra presenting the core−shell structure of CdSe/CdS/ZnS QDs (PDF)
AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS The authors are grateful to the financial support from the National Nature Science Foundation of China (Nos. 50802068 and 51273157), ESI Discipline Rank Promotion Plan of Wuhan University of Technology (No. 451-35400187), and Key Program of Natural Science Foundation of Hubei Province of China (2013CFA020).
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REFERENCES
(1) Louie, A. Multimodality imaging probes: design and challenges. Chem. Rev. 2010, 110, 3146−3195. (2) Lee, D. E.; Koo, H.; Sun, I. C.; Ryu, J. H.; Kim, K.; Kwon, I. C. Multifunctional nanoparticles for multimodal imaging and theragnosis. Chem. Soc. Rev. 2012, 41, 2656−2672. (3) Joshi, B. P.; Wang, T.D. Exogenous molecular probes for targeted imaging in cancer: focus on multi-modal imaging. Cancers 2010, 2, 1251−1287. (4) Kim, J.; Piao, Y. Z.; Hyeon, T. Multifunctional nanostructured materials for multimodal imaging, and simultaneous imaging and therapy. Chem. Soc. Rev. 2009, 38, 372−390. (5) Jing, L. H.; Ding, K.; Kershaw, S. V.; Kempson, I. M.; Rogach, A. L.; Gao, M. Y. Magnetically Engineered Semiconductor Quantum Dots as Multimodal Imaging Probes. Adv. Mater. 2014, 26, 6367− 6386. (6) Chen, O.; Riedemann, L.; Etoc, F.; Herrmann, H.; Coppey, M.; Barch, M.; Farrar, C. T.; Zhao, J.; Bruns, O. T.; Wei, H.; Guo, P.; Cui, J.; Jensen, R.; Chen, Y.; Harris, D. K.; Cordero, J. M.; Wang, Z.; Jasanoff, A.; Fukumura, D.; Reimer, R.; Dahan, M.; Jain, R. K.; Bawendi, M. G. Magneto-fluorescent core-shell supernanoparticles. Nat. Commun. 2014, 5, 5093. (7) Park, J. H.; von Maltzahn, G.; Ruoslahti, E.; Bhatia, S. N.; Sailor, M. J. Micellar Hybrid Nanoparticles for Simultaneous Magnetofluorescent Imaging and Drug Delivery. Angew. Chem., Int. Ed. 2008, 47, 7284−7288. (8) Hao, R.; Xing, R. J.; Xu, Z. C.; Hou, Y. L.; Gao, S.; Sun, S. H. Synthesis, Functionalization, and Biomedical Applications of Multifunctional Magnetic Nanoparticles. Adv. Mater. 2010, 22, 2729−2742. (9) Vollath, D. Bifunctional Nanocomposites with Magnetic and Luminescence Properties. Adv. Mater. 2010, 22, 4410−4415. (10) Kim, J.; Lee, J. E.; Lee, J.; Jang, Y. J.; Kim, S. W.; An, K.; Yu, J. H.; Hyeon, T. Generalized Fabrication of Multifunctional Nanoparticle Assemblies on Silica Spheres. Angew. Chem., Int. Ed. 2006, 45, 4789−4793. (11) Wang, D. S.; He, J. B.; Rosenzweig, N.; Rosenzweig, Z. Superparamagnetic Fe2O3 Beads-CdSe/ZnS Quantum Dots CoreShell Nanocomposite Particles for Cell Separation. Nano Lett. 2004, 4, 409−413. (12) Xu, Y.; Karmakar, A.; Wang, D. Y.; Mahmood, M. W.; Watanabe, F.; Zhang, Y. B.; Fejleh, A.; Fejleh, P.; Li, Z. R.; Kannarpady, G.; Ali, S.; Biris, A. R.; Biris, A. S. Multifunctional Fe3O4 Cored Magnetic-Quantum Dot Fluorescent Nanocomposites for RF Nanohyperthermia of Cancer Cells. J. Phys. Chem. C 2010, 114, 5020−5026. (13) Sun, P.; Zhang, H. Y.; Liu, C.; Fang, J.; Wang, M.; Chen, J.; Zhang, J. P.; Mao, C. B.; Xu, S. K. Preparation and Characterization of Fe3O4/CdTe Magnetic/Fluorescent Nanocomposites and Their 617
DOI: 10.1021/acs.langmuir.5b03978 Langmuir 2016, 32, 611−618
Article
Langmuir spheres: Preparation, Characterization, and Biomedical Applications. Chem. Mater. 2006, 18, 5554−5562. (32) Chen, Y.; Chen, H. R.; Zhang, S. J.; Chen, F.; Zhang, L. X.; Zhang, J. M.; Zhu, M.; Wu, H. X.; Guo, L. M.; Feng, J. W.; Shi, J. L. Multifunctional Mesoporous Nanoellipsoids for Biological Bimodal Imaging and Magnetically Targeted Delivery of Anticancer Drugs. Adv. Funct. Mater. 2011, 21, 270−278. (33) Yi, D. K.; Selvan, S. T.; Lee, S. S.; Papaefthymiou, G. C.; Kundaliya, D.; Ying, J. Y. Silica-Coated Nanocomposites of Magnetic Nanoparticles and Quantum Dots. J. Am. Chem. Soc. 2005, 127, 4990− 4991. (34) Kim, J.; Lee, J. E.; Lee, J.; Yu, J. H.; Kim, B. C.; An, K.; Hwang, Y.; Shin, C. H.; Park, J. G.; Kim, J.; Hyeon, T. Magnetic Fluorescent Delivery Vehicle Using Uniform Mesoporous Silica Spheres Embedded with Monodisperse Magnetic and Semiconductor Nanocrystals. J. Am. Chem. Soc. 2006, 128, 688−689. (35) Insin, N.; Tracy, J. B.; Lee, H.; Zimmer, J. P.; Westervelt, R. M.; Bawendi, M. G. Incorporation of Iron Oxide Nanoparticles and Quantum Dots into Silica Microspheres. ACS Nano 2008, 2, 197−202. (36) Salgueiriño-Maceira, V.; Correa-Duarte, M. A.; Spasova, M.; LizMarzán, L. M.; Farle, M. Composite Silica Spheres with Magnetic and Luminescent Functionalities. Adv. Funct. Mater. 2006, 16, 509−514. (37) Dong, L. J.; Liu, S. H.; Gao, H. T.; Ding, N.; Tremel, W.; Xiong, C. X.; Zhu, Q. M.; Knoll, W. Self-Assembled FeCo/Gelatin Nanospheres with Rapid Magnetic Response and High BiomoleculeLoading Capacity. Small 2009, 10, 1153−1157. (38) Tong, L. Z.; Shi, J. H.; Liu, D. M.; Li, Q. H.; Ren, X. Z.; Yang, H. Luminescent and Magnetic Properties of Fe3O4@SiO2@Y2O3:Eu3+ Composites with Core-Shell Structure. J. Phys. Chem. C 2012, 116, 7153−7157. (39) Liu, J.; Qiao, S. Z.; Chen, J. S.; Lou, X. W.; Xing, X. R.; Lu, G. Q. Yolk/shell nanoparticles: new platforms for nanoreactors, drug delivery and lithium-ion batteries. Chem. Commun. 2011, 47, 12578−12591. (40) Zhu, Y. F.; Zhao, W. R.; Chen, H. R.; Shi, J. L. A Simple OnePot Self-Assembly Route to Nanoporous and Monodispersed Fe3O4 Particles with Oriented Attachment Structure and Magnetic Property. J. Phys. Chem. C 2007, 111, 5281−5285. (41) Ge, J. P.; Hu, Y. X.; Biasini, M.; Beyermann, W. P.; Yin, Y. D. Superparamagnetic Magnetite Colloidal Nanocrystal Clusters. Angew. Chem., Int. Ed. 2007, 46, 4342−4345. (42) Zhang, L.; Qiao, S. Z.; Jin, Y. G.; Chen, Z. G.; Gu, H. C.; Lu, G. Q. Magnetic Hollow Spheres of Periodic Mesoporous Organosilica and Fe3O4 Nanocrystals: Fabrication and Structure Control. Adv. Mater. 2008, 20, 805−809. (43) Shi, D. L.; Cho, H. S.; Chen, Y.; Xu, H.; Gu, H. C.; Lian, J.; Wang, W.; Liu, G. K.; Huth, C.; Wang, L. M.; Ewing, R. C.; Budko, S.; Pauletti, G. M.; Dong, Z. Y. Fluorescent Polystyrene-Fe3O4 Composite Nanospheres for In Vivo Imaging and Hyperthermia. Adv. Mater. 2009, 21, 2170−2173. (44) Kas, R.; Sevinc, E.; Topal, U.; Acar, H. Y. A Universal Method for the Preparation of Magnetic and Luminescent Hybrid Nanoparticles. J. Phys. Chem. C 2010, 114, 7758−7766. (45) Park, J.; An, K.; Hwang, Y.; Park, J. G.; Noh, H. J.; Kim, J. Y.; Park, J. H.; Hwang, N. M.; Hyeon, T. Ultra-large-scale syntheses of monodisperse nanocrystal. Nat. Mater. 2004, 3, 891−895. (46) Deng, Z.; Dai, Y.; Chen, W.; Pei, X. M.; Liao, J. H. Synthesis and Characterization of Bowl-Like Single-Crystalline BaTiO3 Nanoparticles. Nanoscale Res. Lett. 2010, 5, 1217−1221. (47) Xia, X. H.; Tu, J. P.; Zhang, J.; Xiang, J. Y.; Wang, X. L.; Zhao, X. B. Cobalt Oxide Ordered Bowl-Like Array Films Prepared by Electrodeposition through Monolayer Polystyrene Sphere Template and Electrochromic Properties. ACS Appl. Mater. Interfaces 2010, 2, 186−192. (48) Qu, L. H.; Peng, X. G. Control of Photoluminescence Properties of CdSe Nanocrystals in Growth. J. Am. Chem. Soc. 2002, 124, 2049− 2055. (49) Li, J. J.; Wang, Y. A.; Guo, W. Z.; Keay, J. C.; Mishima, T. D.; Johnson, M. B.; Peng, X. G. Large-Scale Synthesis of Nearly
Monodisperse CdSe/CdS Core/Shell Nanocrystals Using Air-Stable Reagents via Successive Ion Layer Adsorption and Reaction. J. Am. Chem. Soc. 2003, 125, 12567−12575. (50) Xie, R. G.; Kolb, U.; Li, J. X.; Basche, T.; Mews, A. Synthesis and Characterization of Highly Luminescent CdSe-Core CdS/Zn0.5Cd0.5S/ ZnS Multishell Nanocrystals. J. Am. Chem. Soc. 2005, 127, 7480−7488. (51) Bruchez, M.; Moronne, M.; Gin, P.; Weiss, S.; Alivisatos, A. P. Semiconductor Nanocrystals as Fluorescent Biological Labels. Science 1998, 281, 2013−2016. (52) Chan, W. C. W.; Nie, S. M. Quantum Dot Bioconjugates for Ultrasensitive Nonisotopic Detection. Science 1998, 281, 2016−2018. (53) Han, M. Y.; Gao, X. H.; Su, J. Z.; Nie, S. M. Quantum-dottagged microbeads for multiplexed optical coding of biomolecules. Nat. Biotechnol. 2001, 19, 631. (54) Feng, Q. S.; Dong, L. J.; Huang, J.; Li, Q.; Fan, Y. H.; Xiong, J.; Xiong, C. X. Fluxible Monodisperse Quantum Dots with Efficient Luminescence. Angew. Chem., Int. Ed. 2010, 49, 9943−9946. (55) Zhang, B. B.; Wang, X. H.; Liu, F. J.; Cheng, Y. H.; Shi, D. L. Effective Reduction of Nonspecific Binding by Surface Engineering of Quantum Dots with Bovine Serum Albumin for Cell-Targeted Imaging. Langmuir 2012, 28, 16605−16613. (56) Deng, D. W.; Qu, L. Z.; Li, Y.; Gu, Y. Q. Versatile Self-Assembly of Water-Soluble Thiol-Capped CdTe Quantum Dots: External Destabilization and Internal Stability of Colloidal QDs. Langmuir 2013, 29, 10907−10914. (57) Speranskaya, E. S.; Beloglazova, N. V.; Abé, S.; Aubert, T.; Smet, P. F.; Poelman, D.; Goryacheva, I. Y.; De Saeger, S.; Hens, Z. Hydrophilic Bright CuInS2 Quantum Dots as Cd-Free Fluorescent Labels in Quantitative Immunoassay. Langmuir 2014, 30, 7567−7575. (58) Wang, X. D.; Lao, C. S.; Graugnard, E.; Summers, C. J.; Wang, Z. L. Large-Size Liftable Inverted-Nanobowl Sheets as Reusable Masks for Nanolithiography. Nano Lett. 2005, 5, 1784−1788. (59) Wang, X. D.; Graugnard, E.; King, J. S.; Wang, Z. L.; Summers, C. J. Large-Scale Fabrication of Ordered Nanobowl Arrays. Nano Lett. 2004, 4, 2223−2226. (60) Li, Q.; Dong, L. J.; Wang, X.; Huang, J.; Xie, H. A.; Xiong, C. X. Self-assembled quantum dots-polyhedral oligomeric silsesquioxane nanohybrids with enhanced photoluminescence. Scr. Mater. 2012, 66, 646−649.
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