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Facile and efficient synthesis of magnetic fluorescent nanocomposites based on carbon nanotubes
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Ceramics International journal homepage: www.elsevier.com/locate/ceramint
Facile and efficient synthesis of magnetic fluorescent nanocomposites based on carbon nanotubes Huaqiao Wan, Chen Li, Zhaodongfang Gao, Zhikang Liu, Lijie Dong, Quanling Yang∗∗, Chuanxi Xiong∗ State Key Laboratory of Silicate Materials for Architectures, And School of Materials Science and Engineering, Wuhan University of Technology, Wuhan, 430070, China
A R T I C LE I N FO
A B S T R A C T
Keywords: Nanocomposites Fluorescence Magnetic Carbon nanotubes
Multifunctional nanomaterials composed of magnetic and fluorescent nanoparticles have been one of the most extensive pursuits because of the potential application in bio-research. In this paper, we demonstrated an efficient method by coupling CdSe/CdS/ZnS quantum dots (QDs) with Fe3O4 magnetic nanoparticles(MNPs) while functionalized multiwall carbon nanotubes (f-MWCNTs) were used as matrix to synthesize a kind of magnetic fluorescent nanocomposite. Compared with other matrix materials, carbon nanotubes have the advantages of high surface areas and good biocompatibility. The incorporation of f-MWCNTs supplies plenty of nucleation sites for the preferential growth of Fe3O4 nanoparticles, avoiding the agglomeration phenomenon of Fe3O4 MNPs in traditional co-precipitation method. Moreover, the un-reacted functional groups of f-MCNTs can further adsorb biological species and drugs, averting the decline of fluorescent intensity caused by the modification of biological species and drugs. The synthetic product maintains the unique properties of rapid magnetic response and efficient fluorescence, which shows a broad application prospect in fluorescent labeling, biological imaging, cell tracking and drug delivery.
1. Introduction Semiconductor quantum dots (QDs) have been actively studied in a series of biological applicationsas fluorescent labels [7,18,37] because of their fascinating properties such as broad excitation band, narrow emission spectra, size-tunable optical properties and high photochemical stability [2,6,24]. However, single-modal QDs cannot satisfy all the demanded steps in bio-technology research including imaging, tracking, and separating biological molecules and/or cells [29,31]. Developing multi-functional nanoparticles can be an efficient and widely used method to overcome such limitation [32], for instance, cooperating magnetic nanoparticles with QDs to obtain multifunctional nanocomposites featuring optical and properties [35]. A series of methods have been reported to fabricate magnetic luminescent nanocomposite in recent years, and these methods can be divided into three categories: coupling modified component particles by covalent linking or electrostatic absorption [4,8], depositing QDs onto magnetic core based on inorganic synthesis [28], and encapsulating magnetic nanoparticles and quantum dots in a matrix (polymer microbeads, silica matrix or alternate polymer layers) [17]. By
∗
comparison, coupling modified component particles can simplify the synthetic process, and obtain multifunctional nanoparticles with overall smaller size, which has already become the most commonly used approach to prepare magnetic luminescent nanomaterials [34]. However, one of the major challenges still prevents the application of these coupled magnetic fluorescent nanoparticles, that the conventional ligands on the surface of nanoparticles do not contain additional active functional groups for further modification with biological species and drugs [23]. Introducing a biocompatible nanomaterial as substrate and nanocarrier could be a good solution to solve this problem. Carbon nanotubes(CNTs) hold great promise in a wide range of bio-applications because of their atomic structure, high aspect ratio and excellent electrical and mechanical properties [1]. Furthermore, due to the maturation of functionalized methods and the ability of CNTs to penetrate into cells, CNTs can be applied in more biomedical issues, especially as carriers for drugs, DNA, RNA and other biomolecules [15,22]. Since papers have been reported on the preparation of magnetic luminescent nanocomposites almost are binary system, a refined but efficient synthetic strategy of fabricating a ternary complex based on carbon nanotubes is highly desired.
Corresponding author. Corresponding author. E-mail addresses: [email protected] (Q. Yang), [email protected] (C. Xiong).
∗∗
https://doi.org/10.1016/j.ceramint.2019.12.139 Received 24 October 2019; Received in revised form 1 November 2019; Accepted 14 December 2019 0272-8842/ © 2019 Elsevier Ltd and Techna Group S.r.l. All rights reserved.
Please cite this article as: Huaqiao Wan, et al., Ceramics International, https://doi.org/10.1016/j.ceramint.2019.12.139
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99.5%) were bought from Sinopharm Chemical Reagent Co., Ltd, China. γ-aminopropyltriethoxysilane (H2N(CH2)3Si(OC2H5)3) (WD-50, 95%) was obtained from Wuhan University Silicone New Material Co., Ltd. Cadmium oxide (CdO, 99.99%), stearic acid (SA, 99%), zinc oxide (ZnO, 99.99%, powder), selenium (Se, 99.5%), sulfur (S, 99.98%, powder), tributylphosphine (TBP, 97%), tributylphosphine oxide (TOPO, 90%), 1-octadecene (1-ODE, 90%), oleic acid (OA, 90%), octadecylamine (ODA, 97%), 1-ethyl-(2-dimethylaminopropyl) carbodiimide hydrochloride (EDC), N-Hydroxysuccinimide (NHS, BR) and mercaptopropionic acid (MPA, 98%), were all purchased from SigmaAldrich. All chemicals were used directly without further purification.
In this paper, we explored a novel and facile method by coupling Fe3O4 MNPs with CdSe/CdS/ZnS QDs based on a ligand-exchange mechanism while functionalized multiwall carbon nanotubes (fMWCNTs) were used as matrix to prepare a magnetic fluorescent nanomaterial. CdSe/CdS/ZnS QDs exhibit efficient luminescence and much better stability compared with conventional core/shell QDs, the double shell (or core-shell-shell) structure of CdSe/CdS/ZnS QDs allows a reduction of strain at the interface between the core and outer shell [30]. And compared with other heavy metal-based magnetic nanoparticles in biomedical application, iron oxide nanoparticles have the advantages of non-toxicity and have the ability of natural integration into tissue physiology [5,13,33]. Different from the traditional coupling method, we introduce multiwall carbon nanotubes (MWCNTs) as an ideal matrix, which have attracted extensive attention for the unique physical and chemical properties during the past decades. Functionalized MWCNTs process plenty of nucleation sites for the preferential growth of Fe3O4 nanoparticles, making contribution to the decrease of agglomeration phenomenon in the traditional co-precipitation method. In addition, the un-reacted function groups of f-MCNTs can further adsorb biological species and drugs, averting the decline of fluorescent intensity caused by the modification of biological species and drugs [19,21]. The whole reaction process is shown in Fig. 1. As expected, these synthesized multifunctional nanomaterials have the abilities of rapid magnetic response, efficient fluorescence and good water dispersibility, which hold great potential in many prospective bio-applications, such as bio-imaging and sorting, drug delivery and therapy, the detection and separation of protein and DNA.
2.2. Synthesis of magnetic Fe3O4/f-MWCNTs nanocomposites
2. Experimental section
Functionalized multiwall carbon nanotubes (f-MWCNTs) were prepared according to our previous reported method [20]. Fe3O4 magnetic nanoparticles were synthesized on f-MWCNTs via co-precipitate. Typically, 200 mg f-MWCNTs was suspended in 100 ml deionized water and sonicated for 1 h to get stable f-MWCNTs dispersions. Then 300 mg FeCl2·4H2O and 123 mg FeCl3·6H2O were successively added to the above f-MWCNTs dispersion under mechanical stirring and nitrogen protection. After stirred for 1 h, the resultant mixture was dialyzed in deionized water for 4 h to remove the un-adsorbed Fe3+ and Fe2+. The suspension was then transferred to a three-necked flask with nitrogen protection and then heated to 80 °C followed by dropwise adding 15 ml NH3·H2O to adjust PH between 10~11, the reaction continued for an hour under constant mechanical stirring and N2 gas flow. The final precipitate was separated under external magnet field (1 T) and washed with deionized water, followed by acuum drying at 70 °C for 24 h.
2.1. Materials
2.3. Aminoalkylation of magnetic Fe3O4/f-MWCNTs nanocomposites
Multiwall carbon nanotubes (MWCNTs; diameter, 8–15 nm; length, 10–30 μm; purity, 95%) were got from ChenDu Organic Chemistry Co., Ltd, Chinese Academy of Science. Ferrous chloride tetrahydrate (FeCl2·4H2O, 99.99%) and ferric chloride hexahydrate (FeCl3·6H2O,
Purified Fe3O4/f-MWCNTs nanocomposites were sonicated with 150 ml alcohol for 30 min, then the suspension was heated up to 65 °C under moderate stirring. Then 10 ml WD-50 was added to the suspension and shaked for 6 h at 50 °C. The precipitation was separated from
Fig. 1. Schematic diagram for the formation of magnetic (a) Fe3O4/f-MWCNTs,(b) Fe3O4–NH2/f-MWCNTs and (c) magnetic and luminescent nanocompositesMPAQDs/Fe3O4–NH2/f-MWCNTs. 2
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Fig. 2. (a) FT-IR spectra of MWCNTs, f-MWCNTs, Fe3O4/f-MWCNTs and Fe3O4–NH2/f-MWCNTs. (b)XRD patterns of MWCNTs, Fe3O4, Fe3O4/f-MWCNTs and Fe3O4–NH2/f-MWCNTs.
the solution after filtration and then washed three times with ethanol to remove excess WD-50. The final product was obtained and defined as Fe3O4–NH2/f-MWCNTs.
deionized water.
2.4. Synthesis of CdSe/CdS/ZnS QDs
Infrared spectra (IR) were measured on a Thermo Nicolet NEXUS IR spectrometer using KBr pellets. A JEOL-JEM 2001F transmission electron microscope (TEM) operating at an acceleration voltage of 200 kV was used to analyze morphologies and sizes of resulted samples. X-ray diffraction (XRD) was taken by a Rigaku D/Max-IIIA powder diffractometer, using Cu Kα (λ = 1.54 Å) as the incident radiation. Magnetic measurements of the products were conducted on Vibrating Sample Magnetometer (VSM-220) with external magnetic field ranging from −20 kOe to 20 kOe at 300 K. UV–vis absorption spectra were acquired on a diode array UV–vis spectrometer (UV-2550, SHIMADZU). Photoluminescence (PL) emission spectra were obtained from a RF5301 PC fluorometer. Fluorescent images were carried out using a XYS13 microscope, samples were prepared by spin coating the suspension onto a glass slide for microscope imaging.
2.7. Characterization
Highly fluorescent ODA-capped CdSe nanocrystals were prepared by modifying the previous method in literatures [11,26,36]. The particle concentration of the purified CdSe solution in hexanes/ODE, as stock solution for core/shell growth, was measured using Beer's law with the reported extinction coefficients of CdSe nanocrystals [38], CdSe (3.69 × 10−2 mmol/L, 3.3 nm in diameter) solution in hexanes/ ODE with the first adsorption peak around 560 nm. To achieve completed growth of shells around the above CdSe-core particle, we used Successive Ion Layer Adsorption and Reaction (SILAR) technique [25,27]. There were four monolayers around the CdSe-core nanocrystals: two monolayers of CdS, one monolayer of Cd0.5Zn0.5S and one monolayer of ZnS. For each layer growth, the amount of given precursor was calculated accurately. Synthesized CdSe/CdS/ZnS nanocrystals were extracted three times with acetoneand stored in dark place.
3. Results and discussion 3.1. Characterization of the magnetic Fe3O4/f-MWCNTs nanocomposites
2.5. Synthesis of water dispersible CdSe/CdS/ZnS QDs
Combining the Fe3O4–NH2/MWCNTs nanocomposites and CdSe/ CdS/ZnS QDs modified with MPA, magnetic and luminescent nanocomposite particles based on CNTs were prepared via electrostatic interaction between carboxyl group and amino group. Firstly, the surface groups of MWCNTs, f-MWCNTs, Fe3O4/f-MWCNTs and Fe3O4–NH2/fMWCNTs nanocomposites were investigated by FT-IR spectra. As shown in Fig. 2a, the peaks at 1712, 1093 and 1585 cm−1 in the spectrum of f-MWCNTs correspond to C]O, C–O stretching vibrations of –COOH and in-plane bending vibration of –COO–, respectively, indicating the successful incorporation of carboxylic function groups by the chemical oxidation. The FTIR spectra of Fe3O4/f-MWCNTs and Fe3O4–NH2/f-MWCNTs show a broad band at 582 cm−1 and 585 cm−1, respectively, belonged to the stretching vibration of Fe–O–Fe, indicating that Fe3O4was successfully co-precipitated on functionalized multiwall carbon nanotubes. Moreover, a shift of C]O stretching vibration peak from 1712 to 1722 cm−1 was also observed between the two FTIR spectra and the FTIR spectra of f-MWCNTs, further proving that Fe3O4 nanoparticles were anchored to the carboxyl groups of fMWCNTs [9]. In the spectrum of Fe3O4–NH2/MWCNTs, the successful modification of WD-50 on the surface of magnetic Fe3O4/MWCNTs is confirmed by Si–O stretching vibration peak at 1034 cm−1, N–H stretching vibrations of –NH2 at 1104 cm−1, and symmetric and unsymmetric stretching vibrations of –CH2 at 2922 cm−1 and 2854 cm−1, respectively. X-ray diffraction (XRD) was used to identify the crystallographic properties of MWCNTs, Fe3O4/f-MWCNTs and Fe3O4–NH2/f-MWCNTs,
MPA-coated QDs were prepared by phase transfer method. Typically, CdSe/CdS/ZnS nanocrystals were dissolved in chloroform followed by adding a certain amount of MPA (CdSe/CdS/ZnS: MPA = 1: 2.5). The mixture was sonicated for 1–2 h at room temperature till yellow flocculent precipitate appeared abundantly. The precipitate was isolated from suspension by centrifugation at 8000 rpm for 10 min and washed twice with chloroform to remove excess MPA and ODA. Then the QDs samples were redissolved in 0.1 M PBS buffer (PH 8.5) and incubated at room temperature for 12 h to accomplish ligand exchange. The product was centrifuged at 8000 rpm for 10 min to remove the excess organic ligands. Finally, the final product was dissolved in PBS butter. 2.6. Synthesis of magnetic-fluorescent nanocomposites Self-assembly Technology was used to prepared magnetic-fluorescent nanocomposites composed of Fe3O4–NH2/MWCNTs and QDsMPA. 60 mg carbodiimide hydrochloride (EDC) and 80 mg NHydroxysuccinimide (NHS) were added to 15 g deionized water to form an EDC-NHS solution. Then 2 ml of EDC-NHS solution and 2 ml of MPAcoated QDs solution were added to 1 ml well-dispersed Fe3O4–NH2/fMWCNTs aqueous solution (0.02 M). The above mixture was ultrasonicated for 30 min at room temperature, then shaked for 6 h. Subsequently, the QDs/Fe3O4/MWCNTs nanocomposites were separated under external magnet field (1 T) and washed three times with 3
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Fig. 3. TEM images of (a) Fe3O4/MWCNTs, (b) Fe3O4–NH2/MWCNTs.
the results can further indicate the successful synthesis of Fe3O4 magnetic nanoparticles on f-MWCNTs. In Fig. 2b, the diffraction peak at 25.8° is estimated to be the (0 0 2) plane of CNTs [39]. Moreover, comparing the XRD patterns of Fe3O4/MWCNTs and Fe3O4–NH2/ MWCNTs with pure MWCNTs, the characteristic peak of CNTs still exist in the patterns of two nanocomposites, distinctive only difference is that the peak intensity decreases. The diffraction peaks at 30.1°, 35.6°, 43.2°, 53.5°, 57.0° and 62.9° are indexed to (2 2 0), (3 1 1), (4 0 0), (4 2 2), (5 1 1), (4 4 0) crystal planes of cubic Fe3O4 [3], respectively. It is observed that both Fe3O4/MWCNTs and Fe3O4–NH2/MWCNTs patterns all comprise two phases of cubic Fe3O4 and CNTs, confirming that aminoalkylation operation make no difference in crystallographic properties of the nanocomposites. Meanwhile, the diffraction peaks of Fe3O4 in these patterns are broadened, indicating the crystalline sizes of the Fe3O4 nanoparticles are small [14]. In order to examine microstructure of Fe3O4/MWCNTs and Fe3O4–NH2/MWCNTs, transmission electron microscopy (TEM) images were carried out, respectively. Shown from Fig. 3a, the Fe3O4 nanoparticles with an average size of 20 nm were uniformly dispersed on functionalized multi-walled carbon nanotubes by co-precipitation technique. Significantly, depositing Fe3O4 nanoparticles on CNTs could prevent the agglomeration phenomenon in the traditional co-precipitation method, comfirming the preferential growth of Fe3O4 nanoparticles on functionalized multi-walled carbon nanotubes. These could be attributed to the sufficient combination of Fe2+ and Fe3+ with nucleation sites(–COOH) provided by functionalized multi-walled carbon nanotubes. Only few free Fe3O4 nanoparticles can be observed due to the removal of excess Fe2+ and Fe3+ by dialyzing process. Comparing Fig. 3a and b, the nanocomposites all maintain good dispersibility and little differences exist between them, which could be deduced that the present modification strategy has very minor impact on the microstructure of nanocomposites. The magnetic properties of Fe3O4/f-MWCNTs and Fe3O4–NH2/fMWCNTs are measured on VSM at room temperature in the applied field sweeping from −20 k to 20 kOe. As illustrated in Fig. 4a, no hysteresis exists in the magnetization curves, this result ascribes to the fact that both nanocomposites are superparamagnetic. Saturation magnetization of Fe3O4/f-MWCNTs and Fe3O4–NH2/f-MWCNTs Fe3O4 are ~54.8 and ~27.4 emu/g, respectively. The decline in saturation magnetization can be explained by the hydrophilic group surrounding the Fe3O4 NPs. It is observed that Fe3O4–NH2/f-MWCNTs are welldispersed in water, while the Fe3O4/f-MWCNTs nanocomposites aggregate on the bottom of the mixture solution. When an external magnetic field applied, magnetic nanocomposites were attracted and gathered on the side of the bottle, which was close to the magnet. The whole process is accomplished in 5 s and 20 s for Fe3O4/f-MWCNTs, and Fe3O4–NH2/f-MWCNTs, respectively, suggesting that the rapid magnetic response of nanocomposites derived from the Fe3O4 nanoparticles. Meanwhile, the dispersion became clear and transparent after external magnetic field applied, simultaneously proving that Fe3O4 magnetic nanoparticles were embedded into the nanocomposites.
Fig. 4. Optical photographs of (a) Magnetization curves of Fe3O4/f-MWCNTs and Fe3O4–NH2/f-MWCNTs, the inset is a magnified view of the magnetization curves at low applied fields. (b)Fe3O4/f-MWCNTs, (c)Fe3O4–NH2/f-MWCNTs nanomaterials dispersed in deionized water, without (left) and with (right) an external magnetic field.
3.2. Characterization of mercaptopropionic acid (MPA)-coated CdSe/CdS/ ZnS QDs Highly fluorescent CdSe/CdS/ZnS nanocrystals were prepared according to the reported literatures with little modification [11,26,36] and modified by MPA via ligands change for further self-assembling procedure. It can be seen from TEM images that the morphology of CdSe/CdS/ZnS nanocrystals is nearly spherical and monodispersed with approximate diameter between 6 and 10 nm. After modification, the obtained water dispersible QDs are still monodispersed without noticeable aggregation, though they become easily aggregated than initial QDs owing to the lack of steric hindrance between nanocrystals. In the inset of Fig. 5a and b, the dispersion of QDs in hexane appears red color under sunlight and salmon pink under ultraviolet light. MPA-QDs were finely dispersed in water due to the hydrophilic carboxylic groups provided by MPA, forming a homogeneous orange solution under daylight. In addition, the solution turns to a similar color when UV light 4
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Fig. 5. (a) TEM image of CdSe/CdS/ZnS QDs, the inset is QDs dispersed in hexane under daylight and UV light; (b) TEM image of MPA-QDs, the inset is MPA-QDs dispersed in water under daylight and UV light.
applied as the dispersion of QDs under UV light. The optical properties can be demonstrated by the UV–vis and PL spectra of synthetic CdSe/ CdS/ZnS QDs in hexane and MPA-QDs in water, as shown in Fig. 6. There's almost no difference in absorption spectra between QDs and MPA-QDs. This result provides evidence that no harm was done on the structure of QDs in ligands change process. In the PL spectra, the initial QDs show higher PL efficiency with an emission peak at 628 nm. With a tiny blue shift, the emission peak of MAP-QDs is at 625 nm. The quantum yield (QY) of resulting water-soluble QDs is decreased compared to the original CdSe/CdS/ZnS QDs, caused by the increase number of dangling bonds on the surface of QDs during ligands change [12]. Fig. 7. (a) Magnetization curves of Fe3O4/f-MWCNTs, Fe3O4–NH2/f-MWCNTs and QDs/Fe3O4–NH2/f-MWCNTs, the inset is a magnified view of the magnetization curves at low applied fields. (b) Photographs of QDs/Fe3O4–NH2/fMWCNTs nanocomposites dispersed in deionized water without (left) and with (right) an external magnetic field.
3.3. Characterization of the self-assembled magnetic luminescent nanocomposites based on carbon nanotubes The self-assembled magnetic fluorescent nanocomposites were synthesized via electrostatic interaction between carboxyl group and amino group, which are from MPA-QDs and Fe3O4–NH2/MWCNTs nanocomposites respectively. The nanocomposites we prepared exhibit superparamagnetic properties at room temperature, which can be deduced from Fig. 7b. After conjugation, the synthesized magnetic fluorescent nanocomposites display a little lower saturation magnetization of 24.7 emu/g. However, it is sufficiently high to be applied in biological application [16]. The good magnetic properties could be further proved by Fig. 7a. A homogeneous black aqueous solution is demonstrated and indicates that the dispersion of nanocomposites in water is still desirable after self-assembling process. The nanocomposites show a quick response to the magnetic field manifest as effective separation within 20 s, which could be inferred that MPA-QDs are tightly bound to the Fe3O4–NH2/f-MWCNTs and has little effect on the magnetic properties of nanocomposites. This process is reversible. After the magnetic field was removed, just gentle shaking could lead to a uniform redispersion. From Fig. 8a, it can be clearly observed that the
MPA-QDs were assembled with the Fe3O4–NH2/MWCNTs nanocomposites and few aggregations exist even in a wide field view. Considering the interaction between carboxyl groups and amino groups, and Fe3O4 nanoparticles is more than twice the diameter of MPA-QDs, we could deduce that MPA-QDs are mainly attached to the Fe3O4 nanoparticles. Fig. 8b shows the fluorescence photograph of MPA-QDs/Fe3O4–NH2/fMWCNTs nanocomposites, bright red luminescence against the dark background is clearly seen. The major luminescent region is similar to the branching shape of multiwalled carbon nanotubes, indicating that QDs are uniformly dispersed on carbon nanotubes. In Fig. 8c, the synthesized magnetic fluorescent nanocomposites show a slight loss in PL intensity and a tiny blue shift in the emission peak (628 nm) compared to MPA-QDs. The above two phenomena are caused by the change in surface states of the QDs in coupling process.
Fig. 6. (a) UV–vis and (b) photoluminescence spectra of synthetic CdSe/CdS/ZnS QDs in hexane and MPA-QDs in water. 5
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Fig. 8. TEM image (a) and fluorescent photograph (b) of Fe3O4–NH2/f-MWCNTs
assembled with MPA-modified CdSe/CdS/ZnS QDs. (c)Photoluminescence spectra of synthetic magnetic fluorescent nanocomposites in water.
4. Conclusions In summary, we developed a facile pathway for fabricatinga new type of magnetic luminescent nanomaterial by conjugating Fe3O4–NH2/ f-MWCNTs nanocomposites and MPA-modified CdSe/CdS/ZnS QDs via ligand-exchange process. Different from traditional coupling method, functionalized multiwall carbon nanotubes were introduced as nanocarriers to provide active functional groups for further modifying with biological species and drugs. This procedure verified the feasibility to synthesize multifunctional nanomaterials with admirable magnetic performance, efficient luminescence and stable water dispersity. Meanwhile, because of the development toward strategies for the chemical modification and functionalization of carbon nanotubes, more multifunctional nanomaterials would be exploited for bio-applications based on this approach. The synthesized magnetic luminescent nanocomposites are promisingcandidates for various applications, such as fluorescence labeling and imaging, biosensors, cancer treatment and drug delivery system.
[11]
[12]
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Declaration of competing interest The authors declare that they have no known competing financialinterestsor personal relationships that could have appeared to influence the work reported in this paper.
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Acknowledgements
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This work was supported by the National Natural Science Foundation of China (No. 51673154, 51703177, 21704079), and the Fundamental Research Funds for the Central Universities (WUT: 2018III009, 2018IVB022, 2018IVB041).
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Ceramics International journal homepage: www.elsevier.com/locate/ceramint
Facile and efficient synthesis of magnetic fluorescent nanocomposites based on carbon nanotubes Huaqiao Wan, Chen Li, Zhaodongfang Gao, Zhikang Liu, Lijie Dong, Quanling Yang∗∗, Chuanxi Xiong∗ State Key Laboratory of Silicate Materials for Architectures, And School of Materials Science and Engineering, Wuhan University of Technology, Wuhan, 430070, China
A R T I C LE I N FO
A B S T R A C T
Keywords: Nanocomposites Fluorescence Magnetic Carbon nanotubes
Multifunctional nanomaterials composed of magnetic and fluorescent nanoparticles have been one of the most extensive pursuits because of the potential application in bio-research. In this paper, we demonstrated an efficient method by coupling CdSe/CdS/ZnS quantum dots (QDs) with Fe3O4 magnetic nanoparticles(MNPs) while functionalized multiwall carbon nanotubes (f-MWCNTs) were used as matrix to synthesize a kind of magnetic fluorescent nanocomposite. Compared with other matrix materials, carbon nanotubes have the advantages of high surface areas and good biocompatibility. The incorporation of f-MWCNTs supplies plenty of nucleation sites for the preferential growth of Fe3O4 nanoparticles, avoiding the agglomeration phenomenon of Fe3O4 MNPs in traditional co-precipitation method. Moreover, the un-reacted functional groups of f-MCNTs can further adsorb biological species and drugs, averting the decline of fluorescent intensity caused by the modification of biological species and drugs. The synthetic product maintains the unique properties of rapid magnetic response and efficient fluorescence, which shows a broad application prospect in fluorescent labeling, biological imaging, cell tracking and drug delivery.
1. Introduction Semiconductor quantum dots (QDs) have been actively studied in a series of biological applicationsas fluorescent labels [7,18,37] because of their fascinating properties such as broad excitation band, narrow emission spectra, size-tunable optical properties and high photochemical stability [2,6,24]. However, single-modal QDs cannot satisfy all the demanded steps in bio-technology research including imaging, tracking, and separating biological molecules and/or cells [29,31]. Developing multi-functional nanoparticles can be an efficient and widely used method to overcome such limitation [32], for instance, cooperating magnetic nanoparticles with QDs to obtain multifunctional nanocomposites featuring optical and properties [35]. A series of methods have been reported to fabricate magnetic luminescent nanocomposite in recent years, and these methods can be divided into three categories: coupling modified component particles by covalent linking or electrostatic absorption [4,8], depositing QDs onto magnetic core based on inorganic synthesis [28], and encapsulating magnetic nanoparticles and quantum dots in a matrix (polymer microbeads, silica matrix or alternate polymer layers) [17]. By
∗
comparison, coupling modified component particles can simplify the synthetic process, and obtain multifunctional nanoparticles with overall smaller size, which has already become the most commonly used approach to prepare magnetic luminescent nanomaterials [34]. However, one of the major challenges still prevents the application of these coupled magnetic fluorescent nanoparticles, that the conventional ligands on the surface of nanoparticles do not contain additional active functional groups for further modification with biological species and drugs [23]. Introducing a biocompatible nanomaterial as substrate and nanocarrier could be a good solution to solve this problem. Carbon nanotubes(CNTs) hold great promise in a wide range of bio-applications because of their atomic structure, high aspect ratio and excellent electrical and mechanical properties [1]. Furthermore, due to the maturation of functionalized methods and the ability of CNTs to penetrate into cells, CNTs can be applied in more biomedical issues, especially as carriers for drugs, DNA, RNA and other biomolecules [15,22]. Since papers have been reported on the preparation of magnetic luminescent nanocomposites almost are binary system, a refined but efficient synthetic strategy of fabricating a ternary complex based on carbon nanotubes is highly desired.
Corresponding author. Corresponding author. E-mail addresses: [email protected] (Q. Yang), [email protected] (C. Xiong).
∗∗
https://doi.org/10.1016/j.ceramint.2019.12.139 Received 24 October 2019; Received in revised form 1 November 2019; Accepted 14 December 2019 0272-8842/ © 2019 Elsevier Ltd and Techna Group S.r.l. All rights reserved.
Please cite this article as: Huaqiao Wan, et al., Ceramics International, https://doi.org/10.1016/j.ceramint.2019.12.139
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99.5%) were bought from Sinopharm Chemical Reagent Co., Ltd, China. γ-aminopropyltriethoxysilane (H2N(CH2)3Si(OC2H5)3) (WD-50, 95%) was obtained from Wuhan University Silicone New Material Co., Ltd. Cadmium oxide (CdO, 99.99%), stearic acid (SA, 99%), zinc oxide (ZnO, 99.99%, powder), selenium (Se, 99.5%), sulfur (S, 99.98%, powder), tributylphosphine (TBP, 97%), tributylphosphine oxide (TOPO, 90%), 1-octadecene (1-ODE, 90%), oleic acid (OA, 90%), octadecylamine (ODA, 97%), 1-ethyl-(2-dimethylaminopropyl) carbodiimide hydrochloride (EDC), N-Hydroxysuccinimide (NHS, BR) and mercaptopropionic acid (MPA, 98%), were all purchased from SigmaAldrich. All chemicals were used directly without further purification.
In this paper, we explored a novel and facile method by coupling Fe3O4 MNPs with CdSe/CdS/ZnS QDs based on a ligand-exchange mechanism while functionalized multiwall carbon nanotubes (fMWCNTs) were used as matrix to prepare a magnetic fluorescent nanomaterial. CdSe/CdS/ZnS QDs exhibit efficient luminescence and much better stability compared with conventional core/shell QDs, the double shell (or core-shell-shell) structure of CdSe/CdS/ZnS QDs allows a reduction of strain at the interface between the core and outer shell [30]. And compared with other heavy metal-based magnetic nanoparticles in biomedical application, iron oxide nanoparticles have the advantages of non-toxicity and have the ability of natural integration into tissue physiology [5,13,33]. Different from the traditional coupling method, we introduce multiwall carbon nanotubes (MWCNTs) as an ideal matrix, which have attracted extensive attention for the unique physical and chemical properties during the past decades. Functionalized MWCNTs process plenty of nucleation sites for the preferential growth of Fe3O4 nanoparticles, making contribution to the decrease of agglomeration phenomenon in the traditional co-precipitation method. In addition, the un-reacted function groups of f-MCNTs can further adsorb biological species and drugs, averting the decline of fluorescent intensity caused by the modification of biological species and drugs [19,21]. The whole reaction process is shown in Fig. 1. As expected, these synthesized multifunctional nanomaterials have the abilities of rapid magnetic response, efficient fluorescence and good water dispersibility, which hold great potential in many prospective bio-applications, such as bio-imaging and sorting, drug delivery and therapy, the detection and separation of protein and DNA.
2.2. Synthesis of magnetic Fe3O4/f-MWCNTs nanocomposites
2. Experimental section
Functionalized multiwall carbon nanotubes (f-MWCNTs) were prepared according to our previous reported method [20]. Fe3O4 magnetic nanoparticles were synthesized on f-MWCNTs via co-precipitate. Typically, 200 mg f-MWCNTs was suspended in 100 ml deionized water and sonicated for 1 h to get stable f-MWCNTs dispersions. Then 300 mg FeCl2·4H2O and 123 mg FeCl3·6H2O were successively added to the above f-MWCNTs dispersion under mechanical stirring and nitrogen protection. After stirred for 1 h, the resultant mixture was dialyzed in deionized water for 4 h to remove the un-adsorbed Fe3+ and Fe2+. The suspension was then transferred to a three-necked flask with nitrogen protection and then heated to 80 °C followed by dropwise adding 15 ml NH3·H2O to adjust PH between 10~11, the reaction continued for an hour under constant mechanical stirring and N2 gas flow. The final precipitate was separated under external magnet field (1 T) and washed with deionized water, followed by acuum drying at 70 °C for 24 h.
2.1. Materials
2.3. Aminoalkylation of magnetic Fe3O4/f-MWCNTs nanocomposites
Multiwall carbon nanotubes (MWCNTs; diameter, 8–15 nm; length, 10–30 μm; purity, 95%) were got from ChenDu Organic Chemistry Co., Ltd, Chinese Academy of Science. Ferrous chloride tetrahydrate (FeCl2·4H2O, 99.99%) and ferric chloride hexahydrate (FeCl3·6H2O,
Purified Fe3O4/f-MWCNTs nanocomposites were sonicated with 150 ml alcohol for 30 min, then the suspension was heated up to 65 °C under moderate stirring. Then 10 ml WD-50 was added to the suspension and shaked for 6 h at 50 °C. The precipitation was separated from
Fig. 1. Schematic diagram for the formation of magnetic (a) Fe3O4/f-MWCNTs,(b) Fe3O4–NH2/f-MWCNTs and (c) magnetic and luminescent nanocompositesMPAQDs/Fe3O4–NH2/f-MWCNTs. 2
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Fig. 2. (a) FT-IR spectra of MWCNTs, f-MWCNTs, Fe3O4/f-MWCNTs and Fe3O4–NH2/f-MWCNTs. (b)XRD patterns of MWCNTs, Fe3O4, Fe3O4/f-MWCNTs and Fe3O4–NH2/f-MWCNTs.
the solution after filtration and then washed three times with ethanol to remove excess WD-50. The final product was obtained and defined as Fe3O4–NH2/f-MWCNTs.
deionized water.
2.4. Synthesis of CdSe/CdS/ZnS QDs
Infrared spectra (IR) were measured on a Thermo Nicolet NEXUS IR spectrometer using KBr pellets. A JEOL-JEM 2001F transmission electron microscope (TEM) operating at an acceleration voltage of 200 kV was used to analyze morphologies and sizes of resulted samples. X-ray diffraction (XRD) was taken by a Rigaku D/Max-IIIA powder diffractometer, using Cu Kα (λ = 1.54 Å) as the incident radiation. Magnetic measurements of the products were conducted on Vibrating Sample Magnetometer (VSM-220) with external magnetic field ranging from −20 kOe to 20 kOe at 300 K. UV–vis absorption spectra were acquired on a diode array UV–vis spectrometer (UV-2550, SHIMADZU). Photoluminescence (PL) emission spectra were obtained from a RF5301 PC fluorometer. Fluorescent images were carried out using a XYS13 microscope, samples were prepared by spin coating the suspension onto a glass slide for microscope imaging.
2.7. Characterization
Highly fluorescent ODA-capped CdSe nanocrystals were prepared by modifying the previous method in literatures [11,26,36]. The particle concentration of the purified CdSe solution in hexanes/ODE, as stock solution for core/shell growth, was measured using Beer's law with the reported extinction coefficients of CdSe nanocrystals [38], CdSe (3.69 × 10−2 mmol/L, 3.3 nm in diameter) solution in hexanes/ ODE with the first adsorption peak around 560 nm. To achieve completed growth of shells around the above CdSe-core particle, we used Successive Ion Layer Adsorption and Reaction (SILAR) technique [25,27]. There were four monolayers around the CdSe-core nanocrystals: two monolayers of CdS, one monolayer of Cd0.5Zn0.5S and one monolayer of ZnS. For each layer growth, the amount of given precursor was calculated accurately. Synthesized CdSe/CdS/ZnS nanocrystals were extracted three times with acetoneand stored in dark place.
3. Results and discussion 3.1. Characterization of the magnetic Fe3O4/f-MWCNTs nanocomposites
2.5. Synthesis of water dispersible CdSe/CdS/ZnS QDs
Combining the Fe3O4–NH2/MWCNTs nanocomposites and CdSe/ CdS/ZnS QDs modified with MPA, magnetic and luminescent nanocomposite particles based on CNTs were prepared via electrostatic interaction between carboxyl group and amino group. Firstly, the surface groups of MWCNTs, f-MWCNTs, Fe3O4/f-MWCNTs and Fe3O4–NH2/fMWCNTs nanocomposites were investigated by FT-IR spectra. As shown in Fig. 2a, the peaks at 1712, 1093 and 1585 cm−1 in the spectrum of f-MWCNTs correspond to C]O, C–O stretching vibrations of –COOH and in-plane bending vibration of –COO–, respectively, indicating the successful incorporation of carboxylic function groups by the chemical oxidation. The FTIR spectra of Fe3O4/f-MWCNTs and Fe3O4–NH2/f-MWCNTs show a broad band at 582 cm−1 and 585 cm−1, respectively, belonged to the stretching vibration of Fe–O–Fe, indicating that Fe3O4was successfully co-precipitated on functionalized multiwall carbon nanotubes. Moreover, a shift of C]O stretching vibration peak from 1712 to 1722 cm−1 was also observed between the two FTIR spectra and the FTIR spectra of f-MWCNTs, further proving that Fe3O4 nanoparticles were anchored to the carboxyl groups of fMWCNTs [9]. In the spectrum of Fe3O4–NH2/MWCNTs, the successful modification of WD-50 on the surface of magnetic Fe3O4/MWCNTs is confirmed by Si–O stretching vibration peak at 1034 cm−1, N–H stretching vibrations of –NH2 at 1104 cm−1, and symmetric and unsymmetric stretching vibrations of –CH2 at 2922 cm−1 and 2854 cm−1, respectively. X-ray diffraction (XRD) was used to identify the crystallographic properties of MWCNTs, Fe3O4/f-MWCNTs and Fe3O4–NH2/f-MWCNTs,
MPA-coated QDs were prepared by phase transfer method. Typically, CdSe/CdS/ZnS nanocrystals were dissolved in chloroform followed by adding a certain amount of MPA (CdSe/CdS/ZnS: MPA = 1: 2.5). The mixture was sonicated for 1–2 h at room temperature till yellow flocculent precipitate appeared abundantly. The precipitate was isolated from suspension by centrifugation at 8000 rpm for 10 min and washed twice with chloroform to remove excess MPA and ODA. Then the QDs samples were redissolved in 0.1 M PBS buffer (PH 8.5) and incubated at room temperature for 12 h to accomplish ligand exchange. The product was centrifuged at 8000 rpm for 10 min to remove the excess organic ligands. Finally, the final product was dissolved in PBS butter. 2.6. Synthesis of magnetic-fluorescent nanocomposites Self-assembly Technology was used to prepared magnetic-fluorescent nanocomposites composed of Fe3O4–NH2/MWCNTs and QDsMPA. 60 mg carbodiimide hydrochloride (EDC) and 80 mg NHydroxysuccinimide (NHS) were added to 15 g deionized water to form an EDC-NHS solution. Then 2 ml of EDC-NHS solution and 2 ml of MPAcoated QDs solution were added to 1 ml well-dispersed Fe3O4–NH2/fMWCNTs aqueous solution (0.02 M). The above mixture was ultrasonicated for 30 min at room temperature, then shaked for 6 h. Subsequently, the QDs/Fe3O4/MWCNTs nanocomposites were separated under external magnet field (1 T) and washed three times with 3
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Fig. 3. TEM images of (a) Fe3O4/MWCNTs, (b) Fe3O4–NH2/MWCNTs.
the results can further indicate the successful synthesis of Fe3O4 magnetic nanoparticles on f-MWCNTs. In Fig. 2b, the diffraction peak at 25.8° is estimated to be the (0 0 2) plane of CNTs [39]. Moreover, comparing the XRD patterns of Fe3O4/MWCNTs and Fe3O4–NH2/ MWCNTs with pure MWCNTs, the characteristic peak of CNTs still exist in the patterns of two nanocomposites, distinctive only difference is that the peak intensity decreases. The diffraction peaks at 30.1°, 35.6°, 43.2°, 53.5°, 57.0° and 62.9° are indexed to (2 2 0), (3 1 1), (4 0 0), (4 2 2), (5 1 1), (4 4 0) crystal planes of cubic Fe3O4 [3], respectively. It is observed that both Fe3O4/MWCNTs and Fe3O4–NH2/MWCNTs patterns all comprise two phases of cubic Fe3O4 and CNTs, confirming that aminoalkylation operation make no difference in crystallographic properties of the nanocomposites. Meanwhile, the diffraction peaks of Fe3O4 in these patterns are broadened, indicating the crystalline sizes of the Fe3O4 nanoparticles are small [14]. In order to examine microstructure of Fe3O4/MWCNTs and Fe3O4–NH2/MWCNTs, transmission electron microscopy (TEM) images were carried out, respectively. Shown from Fig. 3a, the Fe3O4 nanoparticles with an average size of 20 nm were uniformly dispersed on functionalized multi-walled carbon nanotubes by co-precipitation technique. Significantly, depositing Fe3O4 nanoparticles on CNTs could prevent the agglomeration phenomenon in the traditional co-precipitation method, comfirming the preferential growth of Fe3O4 nanoparticles on functionalized multi-walled carbon nanotubes. These could be attributed to the sufficient combination of Fe2+ and Fe3+ with nucleation sites(–COOH) provided by functionalized multi-walled carbon nanotubes. Only few free Fe3O4 nanoparticles can be observed due to the removal of excess Fe2+ and Fe3+ by dialyzing process. Comparing Fig. 3a and b, the nanocomposites all maintain good dispersibility and little differences exist between them, which could be deduced that the present modification strategy has very minor impact on the microstructure of nanocomposites. The magnetic properties of Fe3O4/f-MWCNTs and Fe3O4–NH2/fMWCNTs are measured on VSM at room temperature in the applied field sweeping from −20 k to 20 kOe. As illustrated in Fig. 4a, no hysteresis exists in the magnetization curves, this result ascribes to the fact that both nanocomposites are superparamagnetic. Saturation magnetization of Fe3O4/f-MWCNTs and Fe3O4–NH2/f-MWCNTs Fe3O4 are ~54.8 and ~27.4 emu/g, respectively. The decline in saturation magnetization can be explained by the hydrophilic group surrounding the Fe3O4 NPs. It is observed that Fe3O4–NH2/f-MWCNTs are welldispersed in water, while the Fe3O4/f-MWCNTs nanocomposites aggregate on the bottom of the mixture solution. When an external magnetic field applied, magnetic nanocomposites were attracted and gathered on the side of the bottle, which was close to the magnet. The whole process is accomplished in 5 s and 20 s for Fe3O4/f-MWCNTs, and Fe3O4–NH2/f-MWCNTs, respectively, suggesting that the rapid magnetic response of nanocomposites derived from the Fe3O4 nanoparticles. Meanwhile, the dispersion became clear and transparent after external magnetic field applied, simultaneously proving that Fe3O4 magnetic nanoparticles were embedded into the nanocomposites.
Fig. 4. Optical photographs of (a) Magnetization curves of Fe3O4/f-MWCNTs and Fe3O4–NH2/f-MWCNTs, the inset is a magnified view of the magnetization curves at low applied fields. (b)Fe3O4/f-MWCNTs, (c)Fe3O4–NH2/f-MWCNTs nanomaterials dispersed in deionized water, without (left) and with (right) an external magnetic field.
3.2. Characterization of mercaptopropionic acid (MPA)-coated CdSe/CdS/ ZnS QDs Highly fluorescent CdSe/CdS/ZnS nanocrystals were prepared according to the reported literatures with little modification [11,26,36] and modified by MPA via ligands change for further self-assembling procedure. It can be seen from TEM images that the morphology of CdSe/CdS/ZnS nanocrystals is nearly spherical and monodispersed with approximate diameter between 6 and 10 nm. After modification, the obtained water dispersible QDs are still monodispersed without noticeable aggregation, though they become easily aggregated than initial QDs owing to the lack of steric hindrance between nanocrystals. In the inset of Fig. 5a and b, the dispersion of QDs in hexane appears red color under sunlight and salmon pink under ultraviolet light. MPA-QDs were finely dispersed in water due to the hydrophilic carboxylic groups provided by MPA, forming a homogeneous orange solution under daylight. In addition, the solution turns to a similar color when UV light 4
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Fig. 5. (a) TEM image of CdSe/CdS/ZnS QDs, the inset is QDs dispersed in hexane under daylight and UV light; (b) TEM image of MPA-QDs, the inset is MPA-QDs dispersed in water under daylight and UV light.
applied as the dispersion of QDs under UV light. The optical properties can be demonstrated by the UV–vis and PL spectra of synthetic CdSe/ CdS/ZnS QDs in hexane and MPA-QDs in water, as shown in Fig. 6. There's almost no difference in absorption spectra between QDs and MPA-QDs. This result provides evidence that no harm was done on the structure of QDs in ligands change process. In the PL spectra, the initial QDs show higher PL efficiency with an emission peak at 628 nm. With a tiny blue shift, the emission peak of MAP-QDs is at 625 nm. The quantum yield (QY) of resulting water-soluble QDs is decreased compared to the original CdSe/CdS/ZnS QDs, caused by the increase number of dangling bonds on the surface of QDs during ligands change [12]. Fig. 7. (a) Magnetization curves of Fe3O4/f-MWCNTs, Fe3O4–NH2/f-MWCNTs and QDs/Fe3O4–NH2/f-MWCNTs, the inset is a magnified view of the magnetization curves at low applied fields. (b) Photographs of QDs/Fe3O4–NH2/fMWCNTs nanocomposites dispersed in deionized water without (left) and with (right) an external magnetic field.
3.3. Characterization of the self-assembled magnetic luminescent nanocomposites based on carbon nanotubes The self-assembled magnetic fluorescent nanocomposites were synthesized via electrostatic interaction between carboxyl group and amino group, which are from MPA-QDs and Fe3O4–NH2/MWCNTs nanocomposites respectively. The nanocomposites we prepared exhibit superparamagnetic properties at room temperature, which can be deduced from Fig. 7b. After conjugation, the synthesized magnetic fluorescent nanocomposites display a little lower saturation magnetization of 24.7 emu/g. However, it is sufficiently high to be applied in biological application [16]. The good magnetic properties could be further proved by Fig. 7a. A homogeneous black aqueous solution is demonstrated and indicates that the dispersion of nanocomposites in water is still desirable after self-assembling process. The nanocomposites show a quick response to the magnetic field manifest as effective separation within 20 s, which could be inferred that MPA-QDs are tightly bound to the Fe3O4–NH2/f-MWCNTs and has little effect on the magnetic properties of nanocomposites. This process is reversible. After the magnetic field was removed, just gentle shaking could lead to a uniform redispersion. From Fig. 8a, it can be clearly observed that the
MPA-QDs were assembled with the Fe3O4–NH2/MWCNTs nanocomposites and few aggregations exist even in a wide field view. Considering the interaction between carboxyl groups and amino groups, and Fe3O4 nanoparticles is more than twice the diameter of MPA-QDs, we could deduce that MPA-QDs are mainly attached to the Fe3O4 nanoparticles. Fig. 8b shows the fluorescence photograph of MPA-QDs/Fe3O4–NH2/fMWCNTs nanocomposites, bright red luminescence against the dark background is clearly seen. The major luminescent region is similar to the branching shape of multiwalled carbon nanotubes, indicating that QDs are uniformly dispersed on carbon nanotubes. In Fig. 8c, the synthesized magnetic fluorescent nanocomposites show a slight loss in PL intensity and a tiny blue shift in the emission peak (628 nm) compared to MPA-QDs. The above two phenomena are caused by the change in surface states of the QDs in coupling process.
Fig. 6. (a) UV–vis and (b) photoluminescence spectra of synthetic CdSe/CdS/ZnS QDs in hexane and MPA-QDs in water. 5
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Fig. 8. TEM image (a) and fluorescent photograph (b) of Fe3O4–NH2/f-MWCNTs
assembled with MPA-modified CdSe/CdS/ZnS QDs. (c)Photoluminescence spectra of synthetic magnetic fluorescent nanocomposites in water.
4. Conclusions In summary, we developed a facile pathway for fabricatinga new type of magnetic luminescent nanomaterial by conjugating Fe3O4–NH2/ f-MWCNTs nanocomposites and MPA-modified CdSe/CdS/ZnS QDs via ligand-exchange process. Different from traditional coupling method, functionalized multiwall carbon nanotubes were introduced as nanocarriers to provide active functional groups for further modifying with biological species and drugs. This procedure verified the feasibility to synthesize multifunctional nanomaterials with admirable magnetic performance, efficient luminescence and stable water dispersity. Meanwhile, because of the development toward strategies for the chemical modification and functionalization of carbon nanotubes, more multifunctional nanomaterials would be exploited for bio-applications based on this approach. The synthesized magnetic luminescent nanocomposites are promisingcandidates for various applications, such as fluorescence labeling and imaging, biosensors, cancer treatment and drug delivery system.
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[17]
[18]
Declaration of competing interest The authors declare that they have no known competing financialinterestsor personal relationships that could have appeared to influence the work reported in this paper.
[19]
Acknowledgements
[22]
This work was supported by the National Natural Science Foundation of China (No. 51673154, 51703177, 21704079), and the Fundamental Research Funds for the Central Universities (WUT: 2018III009, 2018IVB022, 2018IVB041).
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