Preparation and characterization of bimodal magnetofluorescent nanoprobes for biomedical application

CHIN. PHYS. LETT. Vol. 29, No. 9 (2012) 097803 Preparation and Characterization of Bimodal Magnetofluorescent Nanoprobes for Biomedical Application * LEI Jie-Mei(雷洁梅) 1 , XU Xiao-Liang(许小亮) 1** , LIU Ling(刘玲) 1 , YIN Nai-Qiang(尹乃强) 1 , ZHU Li-Xin(朱立新) 2** 1 Department of Physics, University of Science and Technology of China, Hefei 230026 2 Center Laboratory, First Affiliated Hospital, Anhui Medical University, Hefei 230022 (Received 18 June 2012) Magnetic-fluorescent bifunctional Fe 3 O 4 /SiO 2 -CdTeS nanocomposites are synthesized. Fe 3 O 4 superparamagnetic nanoparticles are firstly prepared through the thermal decomposition of Fe oleate precursors and coated with a mesoporous silica shell using the Stöber method, and the silica surface is then modified with positively charged amino groups by adding 3-aminopropyltrimethoxysilane. Finally, negatively charged CdTeS quantum dots are linked and assembled onto the positively charged surface of Fe 3 O 4 /SiO 2 through electrostatic interactions. X-ray diffraction, transmission electron microscopy, photoluminescence spectroscopy, and magnetometry are applied to characterize the nanocomposites. The results show that the bifunctional nanocomposites combine the opti- cal properties of near-infrared CdTeS quantum dots with the superparamagnetic properties of Fe 3 O 4 perfectly, expressing the potential application as a biocompatible magnetofuorescent nanoprobe for in vivo labelling. PACS: 78.30.Fs, 75.70.Cn, 87.64.kv DOI: 10.1088/0256-307X/29/9/097803 In the past decades, magnetic nanoparticles such as magnetite (Fe 3 O 4 ) have attracted considerable in- terest in various fields of biomedicine, ranging from clinical diagnosis to cancer diagnosis by magnetic res- onance imaging (MRI), drug targeting, cell separation, and hyperthermia. [1−7] One of the most important characteristics of Fe 3 O 4 at nanoscale (∼10 nm) for ap- plications is its superparamagnetic property. Mean- while, semiconductor quantum dots (QDs) have also been widely studied due to their unique optical prop- erties, such as strong size-dependent emission wave- length, continuous excitation spectrum, excellent nar- row emission spectrum, and high stability against pho- tobleaching compared with organic dyes. [8−13] The most attractive application for QDs is as fluorescent probes for bioimaging. Along with the research and development of magnetic nanoparticles and semicon- ductor QDs, respectively, magnetic-fluorescent bifunc- tional nanostructures have also drawn considerable increasing attention recently, because this nanocom- posite probe has both favorable magnetic and fluo- rescent properties in the same structure. [14−18] For example, Xu and co-workers [17] have successfully as- sembled semiconductor quantum dots around silica- coated superparmagnetic Fe 3 O 4 nanoparticles, and the nanocomposites exhibit magnetic and photolumi- nescent properties simultaneously. However, Fe 3 O 4 nanoparticles prepared by using the co-precipitation method have bad uniformity and monodispersion. In addition, the semiconductor QDs were firstly syn- thesized in organic phase and then transferred to the water by another modification step before link- ing Fe 3 O 4 nanoparticles. The method is relatively complicated and will lead to fluorescence decrease of QDs as well. Therefore, how to obtain monodis- perse high-quality paramagnetic nanoparticles and water-dispersed QDs with high fluorescent intensity still makes a lot of sense for us. We have obtained highly uniform and monodispersed Fe 3 O 4 nanoparti- cles through thermal decomposition [20] of iron oleates in organic phase, and water-dispersed alloyed semi- conductor QDs (CdTeS) with high photoluminescence intensity by using a facile one-pot method. Compared with semiconductor QDs, the emission wavelengths of alloyed semiconductor QDs (CdTeS) can be easily tuned from visible to near-infrared by changing the reaction time. [12] In this Letter, we just use CdTeS alloyed QDs with an emission peak at 625nm. The typical procedure for the synthesis of the Fe 3 O 4 /SiO 2 - CdTeS magnetic/fluorescent nanocomposites is shown in Fig. 1. Hydrophobic Fe 3 O 4 nanoparticles capped with oleic acid and oleyamine were transferred to the aqueous solution using cetyltrimethylammonium bro- mide (CTAB) and then coated with silica. CTAB served not only as the secondary stabilizing surfactant for the transfer of nanocrystals to aqueous phase but also as the organic template for the formation of the mesoporous silica shell. [6,16] These mesoporous silica- coated Fe 3 O 4 nanoparticles were then modified by 3- aminopropyltrimethoxysilane (APS) to terminate the silica surface with amino groups. The negatively charged MPA-capped CdTeS QDs were finally electro- statically assembled onto the surface of APS modified silica-coated Fe 3 O 4 nanoparticles. This biocompatible * Supported by the National Natural Science Foundation of China under Grant Nos 50872129 and 81172082. ** Corresponding author. Email: xlxu@ustc.edu.cn; lx-zhu@163.com © 2012 Chinese Physical Society and IOP Publishing Ltd 097803-1 CHIN. PHYS. LETT. Vol. 29, No. 9 (2012) 097803 magnetofuorescent nanoprobe retained favorable mag- netic and fluorescent properties simultaneously, which can be applied to targeted drug delivery, hyperther- mia, bioimaging, as well as bioseparation by a single material. Oleic acid and oleymine coated Fe 3 O 4 CTAB coated Fe 3 O 4 Mesoporous silica coated Fe 3 O 4 Fe 3 O 4 /CdTeS Magnetic/Fluorescent Nanocomposites CTAB Heating Removing CTAB Silica coating Electrostatic self - assembly Fig. 1. Synthetic procedure of Fe 3 O 4 /SiO 2 -CdTeS mag- netic/fluorescent nanocomposites. Iron (III) chloride hexadydrate (FeCl 3 ·6H 2 O, 99%), cetyltrimethylammonium bromide (CTAB), sodium oleate (99%), oleic acid, ethanol (99.7%), ethyl ac- etate (99.5%), trichloromethane (99%), ammonia so- lution (25%–28%), tetraethyl orthosilicate (TEOS), n-hexane (97%), Tellurium (Te,99%), cadmium chlo- ride (CdCl 2 ), sodium borohydride (NaBH 4 ,96%) and sodium hydroxide (NaOH,96%) were obtained from the Sinopharm Chemical Reagent Co., Ltd, China. Oleylamine was purchased from Aldrich. The 3- mercaptopropionic acid (MPA, 99%) was received from Acros Organics. All chemicals were used with no further purification. Iron oxide nanoparticles (Fe 3 O 4 ) stabilized by oleic acid and oleyamine were prepared through the ther- mal decomposition method. [20] Firstly, iron-oleate was synthesized by reacting metal chlorides and sodium oleate according to the reported method. [1] Then, 3.6 g of iron-oleate, 1.91mL of oleylamine and 0.64 mL of oleic acid were mixed together and were heated at 120 ∘ C for one hour. After that, the dark solution was quickly heated up to 200 ∘ C and kept at this tempera- ture for two hours under a nitrogen atmosphere. Fi- nally, the solution was further heated up to 300 ∘ C for another two hours. Nitrogen gas was gently blown through the reaction system in the whole process to remove the trace hydrate vapor during the heating. The resulting solution containing Fe 3 O 4 nanoparticles was then cooled to room temperature and abundant ethanol was added to the solution to precipitate Fe 3 O 4 nanoparticles. The Fe 3 O 4 nanoparticles were sepa- rated by centrifugation and washed with hexane and ethanol several times. CdTeS alloyed QDs with MPA as the stabiliz- ing agent were synthesized through the hydrother- mal route [12] according to previously published litera- ture. Briefly, 127 mg of tellurium powder was reacted with 80 mg of sodium borohydride (NaBH 4 ) in 2 mL of deionized water for preparation of sodium hydro- gen telluride (NaHTe) solution. Then, the fresh pre- pared solutions of NaHTe were swiftly injected into 100 mL N 2 -saturated CdCl 2 solution with the presence of MPA at PH 9.0 under vigorous stirring. The molar ratio of [CdCl 2 ]:[MPA]:[NaHTe] was fixed at 1:1.8:0.5. Finally, a 35 mL of the mixture precursor solution was sealed in a Teflon-lined stainless steel autoclave and maintained at 180 ∘ C for 90min. After a hydrocooling process, the particles was precipitated by centrifuga- tion with ethanol and redispersed in 35 mL deionized water. Fig. 2. XRD patterns of Fe 3 O 4 nanoparticles (a), Fe 3 O 4 /SiO 2 nanoparticles (b), CdTeS QDs (c), and Fe 3 O 4 /SiO 2 -CdTeS nanocomposites (d). Firstly, 7.5 mg of oil-soluble (hydrophobic) Fe 3 O 4 nanoparticles were transferred to aqueous solu- tion using CTAB; [16] and 0.5 mL of the resulting Fe 3 O 4 /CTAB aqueous solution were diluted with 10 mL of water. Then 0.3 mL of NH 4 OH aqueous solution, 0.03 mL of tetraethylorthosilicate (TEOS), and 0.5 mL of ethyl acetate (EtOAc) were continu- ously added to the diluted magnetite nanoparticles aqueous solution under stirring. The reaction was continued under stirring for 30 s, and then aged for three hours. The Fe 3 O 4 /SiO 2 nanoparticles were col- lected by centrifugation and washed with water and ethanol several times, and then redispersed in 10 mL of ethanol. After that, 0.05 mL of APS was added to 10 mL of Fe 3 O 4 /SiO 2 ethanol solution for amino- functionalization. The solution was gently stirred at 60 ∘ C for 12 h. Finally, centrifugation was used to collect the amino-functionalized particles and remove the unbounded The amino-functionalized Fe 3 O 4 /SiO 2 nanoparticles were conjugated with the as-prepared MPA-functionalized CdTeS QDs through bonding be- tween carboxyl groups on QDs and amino groups on silica shells. In brief, 2 mL of as-synthesized MPA- stabilized CdTeS QDs were injected to 10 mL of func- tionalized Fe 3 O 4 /SiO 2 aqueous solution under me- chanical stirring. After 6-h reaction at room temper- ature, the magnetic/fluorescent nanocomposites were collected by magnetic decantation. X-ray diffraction patterns were determined using a rotating anode x-ray diffractometer (XRD, MX- 097803-2 CHIN. PHYS. LETT. Vol. 29, No. 9 (2012) 097803 PAHF, Cu K 𝛼 radiation). The morphology of the samples was investigated by transmission electronic microscopy (TEM, JEOL JEM-2010 (HT)) and high- resolving transmission electron microscopy (HRTEM, JEM-2010 FET(UHR)). Photoluminescence spectra were obtained using a Sahimadzu RF-5301PC spec- trofluorophotometer with an excitation wavelength of 470 nm. The magnetic properties of samples were measured by using a superconducting quantum inter- ference device magnetometer (SQUID, MPMS XL-7) 80 nm (a) (c) 10 nm 3.50 A (b) 80 nm (d) 20 nm 30 nm 5 6 7 8 9 10 11 12 0 10 20 30 Frequency (%) Particle size (nm) Fig. 3. TEM images of Fe 3 O 4 nanoparticles. Inset: par- ticle size distribution of Fe 3 O 4 nanoparticles (a), HRTEM image of CdTeS QDs (b), Fe 3 O 4 /SiO 2 nanoparticles(c), Fe 3 O 4 /SiO 2 -CdTeS nanocomposites (d). XRD patterns as shown in Fig. 2 were used to confirm the crystal structure of the as-synthesized nanoparticles. All of the diffraction peaks of Fe 3 O 4 in Fig. 2(a) can be perfectly indexed to the character- istic (220), (311), (400), (422), (511), and (440) peaks (marked with asterisks) of the cubic spinel magnetite structure of Fe 3 O 4 . The size of Fe 3 O 4 was calculated to be about 8 nm in diameter by using the Scher- rer equation 𝐷 = 𝑘𝜆/𝛽 cos(2𝜃), where 𝐷 (in nm) is the size of the nanocrystal, 𝜆 is the wavelength (in nm) of the x-ray (𝜆 = 0.154178 nm in our experi- ment), 2𝜃 is the angle at which the peak is observed, and 𝛽 (in radian) is the full width at the half max- imum of the peak given by the XRD pattern. In Fig. 2(b), besides the characteristic peaks of the Fe 3 O 4 as shown Fig. 2(a), a strong and broad peak around 2𝜃=23 ∘ derived from the amorphous mesoporous silica can be observed, suggesting the successfully coating of amorphous mesoporous silica on the surface of Fe 3 O 4 nanoparticles. All of the diffraction peaks of CdTeS QDs in Fig. 2(c) are between the diffraction peaks of the cubic CdTe and CdS structures, indicating that the CdTeS alloy QDs were synthesized. [12] The XRD pattern of the final Fe 3 O 4 /SiO 2 -CdTeS mag- netic/fluorescent nanocomposites shown in Fig.2(d) proves that the material is a mixture of Fe 3 O 4 , SiO 2 , and CdTeS. Furthermore, the much stronger peaks of the CdTeS phase in the pattern than those of Fe 3 O 4 phase can be attributed to the larger proportion of CdTeS in the final nanocomposites. Fig. 4. PL spectra of CdTeS QDs (solid line) and Fe 3 O 4 /SiO 2 -CdTeS nanocomposites (dashed line). The insets are photographs of Fe 3 O 4 /SiO 2 -CdTeS nanocom- posites under normal (left) and ultraviolet (right) irradia- tion. As is known, the easy aggregation of nanoparti- cles to form large clusters can be due to their high surface energy. However, Fe 3 O 4 nanoparticles more easily aggregate to bigger particles owing to the mag- netic attraction between particles. In our experiment, oleic acid and oleyamine, as the capping agents, tend to adsorb on the particular high-energy facets of the nanoparticles. Their overall specific surface energy was more or less reduced, thereby, reducing their ten- dency to aggregate. The topography of the oleic acid and oleyamine-stabilized Fe 3 O 4 MNPs is shown in Fig. 3(a). The TEM image clearly demonstrates that the Fe 3 O 4 nanoparticles are highly uniform in particle- size distribution and no aggregation can be observed. The average diameter of the observed Fe 3 O 4 nanopar- ticles was measured to be about 8nm (as shown in the inset of Fig. 3(a)), which is nearly the same as their crystallite size estimated by XRD using Scherrer’s for- mula. Figure 3(b) provides HRTEM overviews of the as-prepared CdTeS alloyed QDs emitting at 625 nm, indicating that the QDs were spherical particles with good monodispersity. The as-prepared CdTeS alloyed quantum dots show a lattice plane distance of 3.50 Å, which is intermediate between cubic CdTe (3.74 Å) and cubic CdS (3.36 Å). The existence of lattice planes on the HRTEM image confirmed the crystallinity of the as-prepared CdTeS alloyed QDs. Fe 3 O 4 nanopar- ticles were coated with a silica shell by taking a sol–gel approach through the hydrolysis and condensation of TEOS, relying on the well-known Stöber method. The TEM image of Fe 3 O 4 /SiO 2 , as shown in Fig. 3(c), re- veals that the rod-like mesoporous silica outside Fe 3 O 4 nanoparticles is synthesized. Each rod-like silica shell contains one or more monodisperse Fe 3 O 4 nanopar- ticles and the major axis rod-like mesoporous silica 097803-3 CHIN. PHYS. LETT. Vol. 29, No. 9 (2012) 097803 is around 100 nm, which is within the applicable size range for drug and gene delivery. [16,21] CTAB on the magnetic nanoparticles plays an important role in the formation of rod-like silica shell. CTAB alone in wa- ter makes rod-like micelles, and therefore it is thought that rod-like silica shell will grow due to CTAB being a soft template. [22] The mesoporous silica structures are considered as the result of removal of CTAB or- ganic templates by a small amount of ethyl acetate during sol-gel reaction. [16] These rod-like mesoporous silica shells were functionalized with APS, and the end of amino group in APS makes the silica positively charged. While QDs were functionalized by MPA, the end of carbocyl group in MPA makes the QDs nega- tively charged. Driven by the electrostatic interaction between amino-terminated Fe 3 O 4 /SiO 2 nanoparticles and carboxyl-terminated CdTeS QDs, the CdTeS QDs can be firmly linked onto the rod-like mesoporous sil- ica shells. The TEM image of Fe 3 O 4 /SiO 2 -CdTeS magnetic/fluorescent is shown in Fig.3(d), which confirms the structure of Fe 3 O 4 /SiO 2 -CdTeS mag- netic/fluorescent nanocomposite nanoparticles. That is to say, a mass of CdTeS QDs was anchored onto the surface of rod-like mesoporous silica shell with several Fe 3 O 4 nanoparticles as cores. Fig. 5. Hysteresis loop of Fe 3 O 4 nanoparticles, Fe 3 O 4 /SiO 2 nanoparticles and Fe 3 O 4 /SiO 2 -CdTeS nanocomposites. Inset: the picture of Fe 3 O 4 /SiO 2 - CdTeS nanocomposites separated by a magnet in the solution under ultraviolet light. Figure 4 shows the PL spectra of CdTeS before (solid line) and after (dashed line) deposition on the surfaces of Fe 3 O 4 /SiO 2 nanoparticles. The free CdTeS QDs in aqueous solution have a fluorescent emis- sion peak at 625 nm. However, the emission peak of Fe 3 O 4 /SiO 2 -CdTeS nanocomposites is slightly blue- shifted (around 7 nm) as compared to the free CdTeS QDs in aqueous solution, which have also been re- ported in other papers. [15,23,24] Based on the previ- ous work, [25] there are strong interactions between Fe 3 O 4 MNPs and QDs. This kind of interaction would lead to energy transfer between the Fe 3 O 4 MNPs and QDs, and hence influences the PL properties of the nanocomposites. The interaction is extremely sen- sitive to the separation distance between these two kinds of materials and the influence will be drastically diminished with the increase of distance. In our exper- iment, shown in the inset of Fig.4, it can be concluded that the Fe 3 O 4 /SiO 2 -CdTeS nanocomposites are well dispersed in the aqueous solution and show strong flu- orescent emission under UV irradiation. Figure 5 shows the hysteresis loop of the oleic acid and oleyamine stabilized Fe 3 O 4 nanoparticles, the Fe 3 O 4 /SiO 2 nanoparticles, and the Fe 3 O 4 /SiO 2 - CdTeS magnetic/fluorescent nanocomposites, where the magnetic field was cycled between −8 and 8 kOe at 300 K. It is illustrated that the saturation magne- tization (𝑀 𝑠 ) of oleic acid and oleyamine stabilized Fe 3 O 4 nanoparticles with the diameter of about 8 nm was 38.26 emu/g, and zero coercivity was found. How- ever, the value of 𝑀 𝑠 of Fe 3 O 4 /SiO 2 decreases to 13.57 emu/g, which results from the formation of a silica shell on the surface of Fe 3 O 4 . This reduction of the 𝑀 𝑠 value could be attributed to the lower mass ratio of the magnetic component in the Fe 3 O 4 /SiO 2 sample. Hence, the thicker the silica shell, the lower the 𝑀 𝑠 value. [26] As shown in Fig. 5, the 𝑀 𝑠 value of Fe 3 O 4 /SiO 2 -CdTeS magnetic/fluorescent nanocom- posites further reduces to 9.09emu/g because of the coating of CdTeS QDs on the surface of Fe 3 O 4 /SiO 2 . These phenomena demonstrate that both the thick sil- ica shell and CdTeS QDs surrounding can affect the magnetic properties of Fe 3 O 4 /SiO 2 -CdTeS nanocom- posites and lead to a lower 𝑀 𝑠 value. The thicker silica shell can screen the effect of Fe 3 O 4 MNP to the PL intensity of QDs on the outside shell but reduced the 𝑀 𝑠 values of magnetic/fluorescent nanocompos- ites simultaneously. Therefore, the SiO 2 should be neither too thick nor too thin to retain favorable mag- netic and fluorescent properties of Fe 3 O 4 /SiO 2 -CdTeS magnetic/fluorescent nanocomposites. As shown the inset of Fig. 5, when a magnetic bar is placed near the solution, the nanocomposites are attracted and ac- cumulate toward the magnet while the bulk solution becomes a clear phase, indicating that magnetic sepa- ration occurs. The aggregated magnetic/fluorescent nanocomposites under ultraviolet light show strong fluorescence intensity, illustrating the Fe 3 O 4 /SiO 2 - CdTeS magnetic/fluorescent nanocomposites retain favorable magnetic and fluorescent properties at the same time in our experiment. These results suggest that our nanocomposites can find potential applica- tions as magnetofluorescent nanoprobes in magnetic guiding and separation, targeted drug delivery, hyper- thermia, bioimaging, etc. In summary, we have synthesized superparam- agnetic Fe 3 O 4 nanoparticles with good monodisper- sity and water-soluble CdTeS quantum dots (QDs) 097803-4 CHIN. PHYS. LETT. Vol. 29, No. 9 (2012) 097803 with high fluorescent intensity separately. Then water-soluble magnetic fluorescent multifunctional nanocomposite particles of about 100 nm in size are prepared through electrostatic assembly between pos- itively charged amino-modified Fe 3 O 4 /SiO 2 nanopar- ticles and negatively charged CdTeS quantum dots (QDs). Although the magnetization intensity de- creases at a certain extent because of the appearance of SiO 2 and QD layers, the multifunctional nanocom- posites still retain favorable magnetic and fluorescent properties simultaneously, which can be further ex- plored as a magneto-fluorescent nanoprobe for appli- cations in biomedicine areas. References [1] Park J N, An K J, Hwang Y S, Park J G, Noh H J, Kim J Y Park J H, Hwang N M and Hyeon T W 2004 Nat. Mater. 3 891 [2] Ito A, Shinkai M, Honda H and Kobayashi T 2005 J. Biosci. Bioeng. 100 1 [3] Bronstein L M, Huang X L Retrum J, Schmucker A, Pink M Stein B D and Dragnea B 2007 Chem. Mater. 19 3624 [4] Jin Y D Jia C X, Huang S W, O’Donnell M and Gao X H 2010 Nat. Commun. 1 41 [5] Mohammad F, Balaji G, Weber A, Uppu R M and Kumar C S S R 2010 J. Phys. Chem. C 114 19194 [6] Kim T, Momin E, Choi J, Yuan K, Zaidi H, Kim J, Park M, Lee N, McMahon M T, Quinones-Hinojosa A, Bulte J W M, Hyeon T and Gilad A A 2011 J. Am. Chem. 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B 108 15461 [25] Hong X, Li J, Wang M J, Xu J J, Guo W, Li J H Bai Y B and Li T J 2004 Chem. Mater. 16 4022 [26] Yi D K, Lee S S, Papaefthymiou G C and Ying J Y 2006 Chem. Mater. 18 614 097803-5 . CHIN. PHYS. LETT. Vol. 29, No. 9 (2012) 097803 Preparation and Characterization of Bimodal Magnetofluorescent Nanoprobes for Biomedical Application * LEI Jie-Mei(雷洁梅) 1 , XU Xiao-Liang(许小亮) 1** ,. g of iron-oleate, 1.91mL of oleylamine and 0.64 mL of oleic acid were mixed together and were heated at 120 ∘ C for one hour. After that, the dark solution was quickly heated up to 200 ∘ C and. stirring for 30 s, and then aged for three hours. The Fe 3 O 4 /SiO 2 nanoparticles were col- lected by centrifugation and washed with water and ethanol several times, and then redispersed in 10 mL of