Magnetic and Optical Properties of Ag@Fe3O4@C-CdTe@SiO2 Hybrid Nanoparticles

SHEN Mao, CHEN Su-qing, JIA Wen-ping, JIN Yan-xian, LIANG Hua-ding

(College of Pharmaceutical and Chemical Engineering, Taizhou University, Taizhou 317028, China)

Magnetic and Optical Properties of Ag@Fe3O4@C-CdTe@SiO2Hybrid Nanoparticles

SHEN Mao, CHEN Su-qing, JIA Wen-ping, JIN Yan-xian, LIANG Hua-ding*

(CollegeofPharmaceuticalandChemicalEngineering,TaizhouUniversity,Taizhou317028,China)

Novel hybrid nanoparticles Ag@Fe3O4@C-CdTe@SiO2were prepared. The synthesis method is included of a Fe3O4magnetic embedded Ag core (Ag@Fe3O4), an interlayer of carbon modified by PEI to form sufficient amounts of amine functional groups (Ag@Fe3O4@C-PEI), the grafting of CdTe quantum dots (QDs) on the surface of Ag@Fe3O4@C-PEI (Ag@Fe3O4@C-CdTe), and an ordered SiO2structured shell. The nanocomposites possess an average diameter of 150 nm, orange emission feature and the saturation magnetization of 224 A/g (22.4 emu/g), which has room temperature ferro-magnetism. The photoluminescence measurement reveals that the fluorescence of Ag@Fe3O4@C-CdTe nanostructures in this composite with the Ag cores of about 45 nm has more enhancement compared with that of Fe3O4@C-CdTe nanostructures due to the strong surface plasmonic resonance of the Ag cores.

multifunctional nanoparticles; noble metal; enhanced-luminescent; core-shell

1 Introduction

In the past few years, labeling and separation have become crucial processes in modern biomedical technology, especially if the labeling and the separation process can be performed at the same time. Nanoparticles simultaneously exhibiting both fluorescent and magnetic properties have been particularly investigated because of their potential to be used in biomedical applications such as selection, labeling, separation, and detection in one single nanoparticle. They have also shown great potential in immunoassay, targeted therapy, and cell separation[1-4].

Quantum dots are semiconductor nanoparticles composed of Ⅱ-Ⅵ and Ⅲ-Ⅴ elements in the size range of 2-10 nm. They have been actively studied for bioimaging applications due to their excellent optical properties such as narrow emission bands, continuous broad absorption band, and high resistance to photobleaching in comparison with organic dyes[5]. At the same time, a variety of Fe3O4nanoparticles has been widely used in biomedical applications, such as targeted drug delivery, rapid biological separation, biosensors, and magnetic hyperthermia therapy[6]. Therefore, combining QDs and Fe3O4nanoparticles to get fluorescent/magnetic bifunctional composite nanoparticles has attracted intense attention due to its appealing applications. However, the preparation of fluorescent/magnetic bifunctional composite nanoparticles is challenging because of their low chemical stability and quench of fluorophores. In such a context, enhancement of photoluminescence by surface plasmon resonance from noble-metals (such as gold, silver and copper nanostructures) appears to be an interesting approach not only to circumvent quenching of fluorophores but also to provide hybrid nanoparticles with high fluorescence properties and chemical stability[7-11].

In the present study, we report the fabrication and characterization of Ag@Fe3O4@C-CdTe@SiO2hybrid nanoparticles involving a Fe3O4magnetic embedded Ag core, an interlayer of carbon modified by PEI to form sufficient amounts of amine functional groups (Ag@Fe3O4@C-PEI), the grafting of CdTe quantum dots (QDs) on the surface of Ag@Fe3O4@C-PEI (Ag@Fe3O4@C-CdTe), and an ordered SiO2structured shell. The magnetic and metal-enhanced fluorescence properties appeared simultaneously when the Ag@Fe3O4@C-CdTe@SiO2hybrid nanoparticles were dispersed in a solution which can be potentially used as better alternative probes for biomolecules imaging.

2 Experiments

2.1 Materials

Ethylene glycol (EG), anhydrous sodium acetate (NaOAc), iron nitrate (Fe(NO3)3·9H2O)，silver nitrate (AgNO3), poly(vinyl-pyrrolidone) (PVPMW=30 000), and polyetherimide (PEI) were obtained from Tianjin Guangfu Fine Chemical Research Institute. 1-[3-(dimethylamino)propyl]-3-ethylcarbodii midehydrochloride (EDC), and N-hydroxysuccinimide(NHS), tellurium dioxide (TeO2, 99.99%), cadmium chloride hemi (pentahydrate) (CdCl2·2.5H2O, 99%), 3-mercaptopropionic acid (MPA, 99%), polyvinylpyrrolidone (PVP), and glucose were purchased from Aldrich Corporation (MO, USA). All chemicals were used without additional purification. All the solutions were prepared with water purified by a Milli-Q system (Millipore, Bedford, MA, USA).

2.2 Characterization

The photoluminescence (PL) spectra were measured using a CARY ECLIPSE (Agilent Technologies, Santa Clara, USA) fluorescence spectrometer. X-ray powder diffraction (XRD) analysis was performed using a Dmax-2500 (Cu Kα,λ=0.154 06 nm, Rigaku Corporation, Tokyo). The morphology of as-obtained products was characterized using JEM-2100 transmission electron microscopy (TEM， Jeol Ltd., Tokyo). Scanning electron microscopy (SEM) was carried out on a S4800 (Hitachi, Japan). The zeta potential of these particles was measured by dynamic light scattering (DLS) with a DelsaTM NanoC Particle Size Analyzer (Beckman Coulter). The magnetic properties of the samples were studied in the dried state with a vibrating sample magnetometer (PPMS-9 VSM, Quantum Design) at room temperature.

2.3 Synthesis of Ag@Fe3O4@C Nanoparticles

Functionalized Ag@Fe3O4nanoparticles were synthesizedviaa versatile solvothermal reaction as reported with a slight modification[12]. Briefly, Fe-(NO3)3·9H2O (2.0 g), NaOAc (3.5 g), PVP (1.0 g) and AgNO3(0.25 g) were dissolved in EG (70 mL) with magnetic stirring, followed by the transfer of the resulting mixture into a 100 mL Teflon-lined stainless-steel autoclave and heated at 200 ℃ for 8 h. Finally, the products were washed several times with ethanol, collected with a magnet and dried in a vacuum oven at 60 ℃ for further use.

Carbon is also an efficient barrier to avoid quenching of fluorophores by magnetic nanoparticles. The Ag@Fe3O4@C nanospheres were obtained following the method reported by Zhu[13]. 0.1 g Ag@Fe3O4nanoparticles and 1.0 g glucose were immersed in 70 mL deionized water by ultrasonication for 30 min. Then the solution was transferred to a 100 mL Teflon-sealed autoclave for treatment at 200 ℃ for 12 h. The products were washed several times with ethanol and separated by a magnet. The as-obtained products were dried at 60 ℃ for further use.

2.4 Synthesis of Ag@Fe3O4@C-CdTe Core@Shell Microspheres

MPA-CdTe QDs was prepared according to the reported methods[14]. The deposition of MPA-CdTe QDs onto Ag@Fe3O4@C was performed as follows: 0.1 g of Ag@Fe3O4@C was dispersed into 100 mL (0.5 mg/mL) PEI aqueous solution under mechanical stirring for 2 h, then EDC (0.83 mmol) and NHS (0.83 mmol) were added into the solution under stirring for 12 h. Last, 10 mL (0.5 mmol/L) MPA-CdTe QDs solution was dropped into the above solution under stirring for 12 h. The precipitate was washed several times with deionized water, and separated by a magnet.

Fe3O4@C-CdTe microspheres were prepared using a procedure similar to the above with Fe3O4@C.

2.5 Synthesis of Ag@Fe3O4@C-CdTe@SiO2 Hybrid Nanoparticles

In the first step, 0.05g Ag@Fe3O4@C-CdTe hybrid nanoparticles were redispersed in a mixture of water/ethanol (1∶4) and 0.1 mL of TEOS was added into the dispersion with mechanical stirring for 15 min. After that, 1.5 mL ammonium hydroxide was added dropwise into the solution. The reaction system was sealed and reacted for 6 h at room temperature. The products were separated by a magnet, and washed several times with ethanol and deionized water.

3 Results and Discussion

3.1 Structure Characterization

The synthesis strategy is shown in Fig.1. Ag@Fe3O4nanocomposite composed of two distinct components, which had an average size of 130 nm with a very good monodispersity (Fig.2(a) and (d)) was first synthesized. The Fe3O4shell composed of many fine primary magnetite nanocrystals was about 40 nm and the black of Ag cores was about 45 nm. This nanostructure of Ag@Fe3O4was able to overcome the aggregation of Ag nanoparticles, because the magnetic shell acted as a physical barrier to protect the Ag cores from irreversible aggregation[12]. After the hydrothermal reaction, it was evident that the carbon coated Ag@Fe3O4composite nanospheres were perfectly spherical in shape with smooth surfaces and the shell layer of carbon was about 8 nm in thickness, as shown in Fig.2 (b) and (e). The surfaces of the obtained Ag@Fe3O4@C composite microspheres were further modified by PEI to form sufficient amounts of amine functional groups, and the strong chemical bonding between carboxylic and amino groups under activation of NHS and EDC ensured the grafting of CdTe QDs on the surface of Ag@Fe3O4@C. After the CdTe QDs and SiO2shell were successively coated on the Ag@Fe3O4@C microspheres, the diameter of the resulting microspheres was increased to about 150 nm, the corresponding SEM and TEM images are shown in Fig.2 (c) and (f).

Fig.1 Preparation procedure of the Ag@Fe3O4@C-CdTe@SiO2 microspheres

Fig.2 SEM and TEM image. (a, d) Ag@Fe3O4. (b, e) Ag@Fe3O4@C. (c, f) Ag@Fe3O4@C-CdTe@SiO2.

The energy-dispersive X-ray analysis spectrum (Fig.3) of the obtained Ag@Fe3O4@C-CdTe@SiO2hybrid nanoparticles reveals the existence of Ag, C, O, Cd, Te, Si and Fe elements, confirming the incorporation of Ag@Fe3O4@C and CdTe into silica.

The crystal structures of Ag@Fe3O4@C-CdTe@SiO2microspheres were identified by means of XRD, and for a comparison, XRD patterns of the initial Ag@Fe3O4@C microspheres and CdTe QDs are also shown in Fig.4C and A. For Ag@Fe3O4@C as prepared in this work (Fig.4C), six diffraction peaks at 30.1°, 35.4°, 43.1°, 53.6°, 57.0° and 62.7° are indexed to (220), (311), (400), (422), (511) and (440) planes of the Fe3O4cubic inverse spinel phase[15]. The CdTe diffraction peaks at 2θ=24.5°, 40.6°, and 48.0° can be readily assigned to (111), (220), and (311) planes (JCPDS No. 19-0629), respectively. Ag@Fe3O4@ C-CdTe@SiO2is given in Fig. 3B. In the spectrum of the obtained product, three other diffraction peaks at 38.2°, 44.3°, and 64.4° are indexed to (111), (200), and (220) planes of Ag cubic phase (JCPDS No.04-0783).

Fig.3 EDS spectra of Ag@Fe3O4@C-CdTe@SiO2nanocomposite

Fig.4 XRD patterns of CdTe QDs (A), Ag@Fe3O4@C- CdTe@SiO2(B), and Ag@Fe3O4@C microspheres (C).

Fig.5 FTIR spectra of Ag@Fe3O4@C(A) and Ag@Fe3O4@C-CdTe@SiO2(B)

The products can be controlled by variation in their surface charges, which can be determined by measuring the zeta potential of these particles. Compared with that of Ag@Fe3O4@C nanoparticles (Fig.6(a)), the Zeta potential of Ag@Fe3O4@C-NH2possessed a higher positive charge(Fig.6(b)), which indicates that PEI has been functionalized successfully followed by EDC and NHS for activating the carboxylic acid groups of Ag@Fe3O4@C nanocomposites. In addition, as shown in Fig.6(c), the Zeta potential of CdTe QDs possessed a lower negative charge.

Fig.6 Zeta potential of the as-prepared samples. (a) Ag@Fe3O4@C. (b) Ag@Fe3O4@C-NH2. (c) CdTe QDs.

3.2 Fluorescent Spectra and Magnetic Property of The As-synthesized Nanocomposites

Fig.7 displays the fluorescence spectra of Ag@Fe3O4@C-CdTe, Fe3O4@C-CdTe and Ag@Fe3O4@C-CdTe@SiO2nanocomposites (the volume of these samples is 3 mL, the concentration is 1 mg/mL). The fluorescence and color of Fe3O4@C-CdTe, Ag@Fe3O4@C-CdTe and Ag@Fe3O4@C-CdTe@SiO2nano-composites present a little variation with each other. Compared with that of Fe3O4@C-CdTe (Fig.7(a)), the emission of Ag@Fe3O4@C-CdTe (Fig.7(b)) has a red-shift with the central emission peak moving from 561.6 to 570.2 nm, and the fluorescent color under UV irradiation changed from green-yellow to yellow. Furthermore, the fluorescence of Ag@Fe3O4@C-CdTe nanostructures in this composite with Ag cores about 45 nm has more enhancement compared with that of Fe3O4@C-CdTe nanostructures. The reason for this result may be caused by the strong surface plasmonic resonance of Ag cores. After further SiO2coating, the emission of Ag@Fe3O4@C-CdTe@SiO2nanocomposite has a red-shift with the central emission peak moving from 570.2 to 586.1 nm (Fig.7(c)), and the fluorescent color under UV irradiation changed from yellow to orange. However, the fluorescence intensity of Ag@Fe3O4@C- CdTe@SiO2nanocomposite was diminished obviously compared to the Ag@Fe3O4@C-CdTe.

Fig.7 Fluorescence emission spectra of the as-prepared samples under UV (365 nm) irradiation. (a) Fe3O4@C-CdTe. (b) Ag@Fe3O4@C-CdTe. (c) Ag@Fe3O4@C-CdTe@SiO2.

To gain a better understanding of the magnetic properties of as-synthesized nanocomposites, the magnetization curves were produced. As shown in Fig.8, the magnetic properties of the synthesized Ag@Fe3O4, Ag@Fe3O4@C, Ag@Fe3O4@C-CdTe, and Ag@Fe3O4@C-CdTe@SiO2nanocomposites were measured by vibrating sample magnetometer (VSM) at room temperature. Fig. 8A shows that the saturation magnetization (Ms) of Ag@Fe3O4nanocomposites was 763 A/g(76.3 emu/g), the values ofMsof carbon coated Ag@Fe3O4, Ag@Fe3O4@C-CdTe and Ag@Fe3O4@C-CdTe@SiO2nanocomposites were decreased to 658, 405, and 224 A/g, successively.

Fig.8 Magnetic hysteresis curves at room temperature of Ag@Fe3O4(A), Ag@Fe3O4@C(B), Ag@Fe3O4@C-CdTe(C), and Ag@Fe3O4@C-CdTe@SiO2(D).

4 Conclusion

In summary, we have designed and developed a type of multifunctional nanocomposites Ag@Fe3O4@C-CdTe@SiO2by incorporating Ag@Fe3O4@C and CdTe into silica with exhibited metal-enhanced fluorescence and magnetic properties. Their structural and fluorescence properties were investigated. The nanocomposites possessed structure with an average diameter of 150 nm, orange emission feature and the saturation magnetization of Ag@Fe3O4@C-CdTe@SiO2hybrid nanoparticles was 224 A/g (22.4 emu/g), which has room temperature ferro-magnetism. The fluorescence enhancement factor can be monitored by the strong surface plasmonic resonance of Ag cores. Because these brightly luminescent beads exhibited high stability, magnetic behavior, we will next focus on their bio-applications, such as magnetic resonance imaging, drug delivery, cell labeling, and magnetic cell separation.

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沈茂(1982-)，男，浙江臨海人，碩士，實(shí)驗(yàn)師，2009年于陜西科技大學(xué)獲得碩士學(xué)位，主要從事無機(jī)納米材料的可控制備及應(yīng)用的研究。

E-mail: shenmao19820808@163.com

梁華定(1964-)，男，浙江臨海人，教授，1990年于浙江師范大學(xué)獲得學(xué)士學(xué)位，浙江省化學(xué)會(huì)理事，《光譜實(shí)驗(yàn)室》編委，主要從事無機(jī)化學(xué)、無機(jī)及分析化學(xué)教學(xué)及納米材料的制備與應(yīng)用的研究。

E-mail: lianghuading@tzc.edu.cn

2016-09-11；

2016-10-19

浙江省應(yīng)用化學(xué)重點(diǎn)學(xué)科，臺(tái)州學(xué)院； 國家自然科學(xué)基金(21403150)； 浙江省教育廳科研基金(Y201224099)資助項(xiàng)目 Supported by Key Disciplines of Applied Chemistry of Zhejiang Province for Taizhou University; National Natural Science Foundation of China(21403150)； Scientific Research Fund of Zhejiang Provincial Education Department(Y201224099)

Ag@Fe3O4@C-CdTe@SiO2磁性熒光復(fù)合微球的制備與光學(xué)特征

沈 茂， 陳素清， 賈文平， 金燕仙， 梁華定*

(臺(tái)州學(xué)院 醫(yī)藥化工學(xué)院, 浙江 臺(tái)州 317028)

先采用一步溶劑熱法和水熱法制備了碳包覆的Ag@Fe3O4核殼型磁性納米粒子，然后通過表面氨基化改性后與巰基乙酸修飾的CdTe量子點(diǎn)反應(yīng)，將量子點(diǎn)鍵合到磁性微球上，最后在其表面包覆上一層二氧化硅殼層，制備出具有熒光增強(qiáng)的Ag@Fe3O4@C-CdTe@SiO2磁性熒光復(fù)合材料。實(shí)驗(yàn)結(jié)果表明，該納米粒子的平均粒徑大約為150 nm，磁飽和強(qiáng)度為224 A/g (22.4 emu/g)，在室溫下具有較好的磁性能。其中Ag@Fe3O4@C-CdTe磁性熒光納米粒子的熒光強(qiáng)度大于Fe3O4@C-CdTe，其主要原因是內(nèi)核為45 nm的 Ag納米粒子具有表面等離子體共振作用，能夠使其表面或附近的量子點(diǎn)熒光得到增強(qiáng)。

多功能納米粒子； 貴金屬； 熒光增強(qiáng)； 核殼結(jié)構(gòu)

1000-7032(2017)03-0274-07

O61; O482.31 Document code： A

10.3788/fgxb20173803.0274

*CorrespondingAuthor,E-mail:lianghuading@tzc.edu.cn