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Journal of Nanomaterials
Volume 2013 (2013), Article ID 178138, 7 pages
http://dx.doi.org/10.1155/2013/178138
Research Article

A Fluorescent Sensor for Zinc Detection and Removal Based on Core-Shell Functionalized Fe3O4@SiO2 Nanoparticles

1Faculty of Metallurgical and Energy Engineering, Kunming University of Science and Technology, Kunming 650093, China
2National Engineering Laboratory for Vacuum Metallurgy, Kunming University of Science and Technology, Kunming 650093, China

Received 11 December 2012; Accepted 5 January 2013

Academic Editor: Tao Chen

Copyright © 2013 Yaohui Xu et al. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

Abstract

The magnetic Fe3O4@SiO2 nanoparticles (NPs) functionalized with 8-chloroacetylaminoquinoline as a fluorescent sensor for detection and removal of Zn2+ have been synthesized. The core-shell structures of the nanoparticles and chemical composition have been confirmed by TEM, XRD, FTIR, and XPS techniques. The addition of functionalized Fe3O4@SiO2 NPs into the acetonitrile solution of Zn2+ had an effect of visual color change as well as significant fluorescent enhancement. High-saturated magnetizations (24.7 emu/g) of functionalized Fe3O4@SiO2 NPs could help to separate the metal ions from the aqueous solution. The magnetic sensor exhibited high removal efficiency towards Zn2+ (92.37%). In this work, we provided an easy and efficient route to detect Zn2+ and simultaneously remove Zn2+.

1. Introduction

Zinc ion, as biologically important metal ions, plays a vital biological role in various pathophysiologic processes, such as gene expression, cell apoptosis, enzymatic adjustment, and neurotransmission, because of its structural properties [13]. Toxic in excess or absence, the break-up of zinc ion homeostasis may cause pathology of intellectual development and other neurological problems such as Alzheimer’s and Parkinson’s diseases [4, 5]. Therefore, the analytical approaches to detect Zn2+ have been studied intensively. So far, many selective smart-molecular fluorescent probes targeting Zn2+ have been reported, that are mostly based on quinoline, fluorescein, benzazole, or fluorophores [610]. For example, Li et al. [11] reported a “switching on” fluorescent chemodosimeter of selectivity to Zn2+ and its application to MCF-7 cells. Lee et al. [12] also synthesized and evaluated two new ratiometric chemosensors for the quantification of potentially toxic free Zn2+ ions in aqueous solutions. Recently, Chen and Li et al. [13] synthesized a new fluorescent chemosensor based on a helical imide as fluorophore and a cyclen moiety as ionophore, which not only showed enhanced fluorescent responses in the presence of Zn2+, Cd2+, and Hg2+, but also could simultaneously and selectively distinguish the three cations in a simulated physiological condition with the help of cysteine as an auxiliary reagent. Moreover, Ding et al. [14] investigated selective and sensitive “turn-on” fluorescent Zn2+ sensors based on di- and tripyrrins with readily modulated emission wavelengths. However, their chemosensors were unsuitable or inconvenient for the separation, removal, and enrichment of target species or in rapid screening applications.

Over the past decade, the modification of small molecular fluorescent probes on the surface of nanostructured materials has been attracting significant attention [1518]. Among these nanostructured materials, core-shell Fe3O4@SiO2 NPs have been widely favored in biological and environmental applications [1921]. First of all, the magnetic Fe3O4 NPs as core in the Fe3O4@SiO2 NPs would facilitate the magnetic separation and recuperation from the detection system within the magnetic field. Moreover, the coating of silica would not only stabilize the magnetic nanocore completely, but also provide sites greatly for surface modification [2224]. In addition, due to more advantages of silica, such as nontoxicity, biocompatibility chemical inertness, no swelling, and transparency, it is a possible option to develop the nontoxic, biocompatible, and recoverable Zn2+-selective fluorescent sensors on Fe3O4@SiO2 NPs surface.

In this work, 8-chloroacetylaminoquinoline (CAAQ) was synthesized and covalently linked to Fe3O4@SiO2 NPs surface to establish an efficient system for the detection and removal of Zn2+ (labeled as Fe3O4@SiO2-CAAQ). The general synthetic strategies and the functionalization of Fe3O4@SiO2 NPs are shown in Scheme 1.

178138.sch.001
Scheme 1: The synthesis of CAAQ and the functionalization of Fe3O4@SiO2-CAAQ.

2. Experimental Section

2.1. Materials

Ferric chloride hexahydrate (FeCl3·6H2O, >98%), ferrous chloride tetrahydrate (FeCl2·4H2O, >99%), dichloromethane (DCM), ammonium hydroxide (28 wt%), tetraethyl orthosilicate (TEOS), and potassium carbonate were obtained from Sinopharm Chemical Reagent Tianjin Co. Ltd. 8-Aminoquinoline, chloroacetyl chloride, triethylamine (TEA), and 3-aminopropyltriethoxysilane (APTES) were purchased from Alfa Aesar and Aladdin Co. Ltd.

2.2. Synthesis

The synthesis of 8-chloroacetylaminoquinoline (CAAQ) was based on the pervious report [25]. 8-Aminoquinoline (0.95 g, 6.6 mmol) and 1 mL TEA were mixed in 50 mL DCM in an ice bath for 30 min. Then, chloroacetyl chloride (0.6 mL, 7.5 mmol) was added dropwise. The mixture was kept in darkness with continuous magnetic stirring for 48 h at room temperature. After that, the solvent was evaporated and the crude products were purified by silica gel column chromatography (petroleum ether/ethylacetate = 3 : 1). 1HNMR (CDCl3, 400 MHz): δ10.7 (s, 1H,), 8.4 (m, 1H), 8.6 (m, 1H), 8.9 (m, 1H), 7.7 (m, 1H), 4.5 (s, 2H).

2.3. Preparation and Surface Modification of Fe3O4@SiO2  NPs

Fe3O4 NPs were prepared according to our previous report [26]. FeCl2·4H2O (2 g, 0.01 mol) and FeCl3·6H2O (5.4 g, 0.02 mol) were dissolved in 120 mL of deionized water. NH3·H2O (60 mL, 28 wt%) was added under vigorous mechanical stirring. The color of the suspension turned black immediately. Afterward, the mixture was kept at 70°C for 30 min. The mechanical stirring and nitrogen atmosphere were carried out throughout the reaction. After cooling down, the precipitated powders were collected by magnetic separation and washed with deionized water. Subsequently, trisodium citrate (150 mL, 20 mmol/L) was acceded to Fe3O4 NPs under vigorous mechanical stirring for 12 h under nitrogen atmosphere at room temperature. Finally, the citrate-functionalized Fe3O4 NPs were washed with deionized water.

The core-shell Fe3O4@SiO2 NPs were synthesized by a sol-gel process though the hydrolysis and condensation of TEOS in ethanol and ammonia mixture. Briefly, 0.56 g citrate-functionalized Fe3O4 NPs and NH3·H2O (5.0 mL, 28 wt%) were stirred in a flask charged with 120 mL ethanol. Afterward, 4.0 mL TEOS was added dropwise. The mixture was kept at room temperature for 8 h under N2 with violent mechanical stirring. The products were collected with a magnet and washed with ethanol and toluene.

The amino-functionalized Fe3O4@SiO2 NPs were synthesized by a silanization reaction. Namely, 0.8 g Fe3O4@SiO2 NPs and 80 mL toluene were stirred to form a homogeneous suspension, to which APTES (3.46 mL, 15 mmol) was added using a syringe. The reaction mixture was kept at 90°C for 24 h under N2 with vigorous mechanical stirring. The obtained amino-group immobilized Fe3O4@SiO2 NPs (labeled as Fe3O4@SiO2-APTES) were collected by magnetic separation and washed with ethanol and CH3CN (  mL).

The as-prepared Fe3O4@SiO2-APTES NPs, 0.6 g CAAQ and 0.08 g K2CO3 as catalyst were mixed into 80 mL CH3CN. The mixture was stirred at 60°C for 24 h under nitrogen atmosphere. After reaction and cooling down to room temperature, the obtained 8-chloroacetylaminoquinoline functionalized Fe3O4@SiO2 NPs (labeled as Fe3O4@SiO2-CAAQ) were magnetically separated and washed with CH3CN and ethanol (  mL) in turn.

2.4. Characterization

The morphology and size of Fe3O4@SiO2 NPs were studied by transmission electron microscopy (TEM, JEM-2100). The structural properties of Fe3O4@SiO2 NPs were studied by X-ray diffraction (XRD, RIGAKU D/MAX-2400). The variation of function groups on Fe3O4@SiO2 NPs was obtained with Fourier transform infrared spectra (FT-IR, EQUINOX55). Proton nuclear magnetic resonance (1H-NMR) spectra were recorded using a Bruker AV 400 spectrometer. The C1s and N1s curve fitting of Fe3O4@SiO2-CAAQ NPs were performed by X-ray photoelectron spectroscopy (XPS, ESCALab220i-XL). The magnetic properties of Fe3O4@SiO2-CAAQ were obtained by using a vibrating sample magnetometer (VSM, Lakeshore 7307). Atomic absorption spectrum (AAS) was implemented by Shimadzu AA-6800 atomic absorption spectrophotometer. UV-vis spectra were recorded on a TU-1901 spectrophotometer.

3. Results and Discussion

The TEM images and XRD pattern of Fe3O4@SiO2 NPs were shown in Figure 1. The average diameter of magnetic silica NPs was about 22 nm (Figure 1(a)). And from Figure 1(a), we can see clearly that the Fe3O4@SiO2 NPs have uniform spherical morphology. The inset in Figure 1(a) was the HRTEM image of Fe3O4@SiO2 NPs, which demonstrated the obvious core-shell structure of magnetic silica NPs. In Figure 1(b), the sharp peaks at can be assigned to the (220), (311), (400), (422), (511), (440), and (533), which agreed well with the crystallographic planes of Fe3O4. Meanwhile, the broad featureless peak at was consistent with the amorphous state of the SiO2 shells, which proved the coating of silica on magnetite nanoparticles.

fig1
Figure 1: TEM images (a) and XRD pattern (b) of Fe3O4@SiO2 NPs.

The variation of functional groups on Fe3O4@SiO2 NPs was characterized by FT-IR (Figure 2). The peaks for all samples at 3439, 1083, and 586 cm−1 were corresponded to the O–H, Si–O, and Fe–O. For Fe3O4@SiO2-APTES, the –(CH2)n– group was confirmed by C–H stretching at 2933 cm−1 and the C–H scissoring vibration at 1393 cm−1 (Figure 2(b)). For Fe3O4@SiO2-CAAQ (Figure 2(c)), new absorption peaks appeared at 1682 cm−1 (C=O bond), 1618 cm−1 (C=N bond), and 1487 cm−1 (N–H bond), which implied that the CAAQ was successfully bonded on the surface of Fe3O4@SiO2-APTES NPs through amino group.

178138.fig.002
Figure 2: FT-IR spectra of (a) Fe3O4@SiO2, (b) Fe3O4@SiO2-APTES, and (c) Fe3O4@SiO2-CAAQ NPs.

XPS spectra of Fe3O4@SiO2 and Fe3O4@SiO2-CAAQ were shown in Figure 3. The wide-scan spectrum of Fe3O4@SiO2 NPs (Figure 3(a)) was dominated by the signals of Fe, O, C, and Si element. Compared with the wide-scan spectrum of Fe3O4@SiO2 NPs, Fe3O4@SiO2-CAAQ NPs (Figure 3(b)) revealed the new obvious peak of N element. In Figure 3(c), the C1s core-level spectrum of Fe3O4@SiO2-CAAQ can be curve-fitted into three peak components with binding energies at about 288.2, 285.5, and 284.4 eV, attributable to the N–C=O, C–N, and C–H species, respectively. In Figure 3(d), the N1s core-level spectrum can be curve-fitted into two peak components with binding energies at about 399.8 and 399.1 eV, which were ascribed to the N–C=O and C–N species, respectively.

fig3
Figure 3: Wide scan of Fe3O4@SiO2 (a) and Fe3O4@SiO2-CAAQ NPs (b), C1s core-level (c) and N1s core-level (d) spectra of Fe3O4@SiO2-CAAQ NPs.

Figure 4 showed the magnetic properties of Fe3O4@SiO2 and Fe3O4@SiO2-CAAQ NPs. From Figure 4, we can see that the saturated magnetizations of Fe3O4@SiO2 and Fe3O4@SiO2-CAAQ NPs were 38.1 and 24.7 emu/g at 25°C, and neither remanence nor coercivity was observed, which indicated that the Fe3O4@SiO2 and Fe3O4@SiO2-CAAQ were superparamagnetic nanoparticles. Thus, it provided an easy and efficient route to separate particles from a suspension system under an external magnetic field. The decrease of the saturated magnetization for functionalized magnetic silica nanoparticles was mainly attributed to the contribution of the volume of the nonmagnetic coating layer to the total sample volume. The strong magnetic sensitivity of the magnetic nanohybrid was confirmed by the inset in Figure 4, which showed the complete magnetic separation by supplying an external magnetic field near the Fe3O4@SiO2-CAAQ NPs suspension system. The Fe3O4@SiO2-CAAQ NPs are magnetic, so they can easily be removed by applying an external magnetic field after binding with Zn ions in aqueous solution, which makes them recyclable. Actually this is an important property for decontaminating agent.

178138.fig.004
Figure 4: Room-temperature magnetization hysteresis loops of Fe3O4@SiO2-CAAQ NPs (inset is the photograph of Fe3O4@SiO2-CAAQ NPs suspension with a magnet).

The atomic absorption spectrum (AAS) was employed to monitor the variation of zinc ions before and after Fe3O4@SiO2-CAAQ treatment. Figure 5 showed the effective removal of Fe3O4@SiO2-CAAQ NPs for Zn2+ in aqueous sample solution. The accurate removal response was observed from 100 ppm to as low as 7.63 ppm at room temperature and the removal efficiency could reach 92.37%. The effective adsorption capacity implied that the proposed method for Zn2+ removal at room temperature was a successful attempt.

178138.fig.005
Figure 5: Removal-efficiency histograms of Zn2+ on Fe3O4@SiO2-CAAQ surface: concentration of Zn2+ (1) before and (2) after Fe3O4@SiO2-CAAQ treatment. The initial concentration of Zn2+ is 100 ppm, the volume of Zn2+ is 10 mL, the weight of nanoparticles is 20 mg, the adsorption time is 12 h, and the temperature is 25°C.

The sensitivity of the Fe3O4@SiO2-CAAQ NPs to Zn2+ in C2H3N was investigated by UV-vis spectra (Figure 6). With Zn2+ attachment, the Fe3O4@SiO2-CAAQ NPs exhibited two evident absorption peaks at about 374 and 301 nm, and the intensity of absorption gradually enhanced upon binding with increasing the concentration of Zn2+, which was attributed to the formation of a Fe3O4@SiO2-CAAQ + Zn2+ complex.

178138.fig.006
Figure 6: UV-vis curves of the Fe3O4@SiO2-CAAQ NPs (0.3 g/L) in C2H3N with various amounts of Zn2+ (0–10 μM).

Figure 7 depicted the Zn2+-selective sensing mechanism and fluorescent change of Fe3O4@SiO2-CAAQ with Zn2+. The considerable blue-yellow emission of the acetonitrile solution can easily be observed by naked eye for Fe3O4@SiO2-CAAQ upon binding Zn2+ by comparison with that of only Fe3O4@SiO2-CAAQ. The fluorescence enhancement of Fe3O4@SiO2-CAAQ + Zn2+ could be attributed to the induction from Zn2+ attachment. Namely, when the Fe3O4@SiO2-CAAQ was selectively coordinated with metal ions, the fluorescence from Fe3O4@SiO2-CAAQ was modified appropriately by the metal ions. Our previous efforts in our group were made to demonstrate that Cu2+ with Fe3O4@SiO2-CAAQ treatment could result in fluorescence quenching when other metal ions (Ag+, Hg2+, Ni2+, Co2+, Mn2+, Pb2+, Cr3+, and Fe3+) caused no observable fluorescent changes.

178138.fig.007
Figure 7: The complexation model of Fe3O4@SiO2-CAAQ and Zn2+ (inset is the fluorescent change of Fe3O4@SiO2-CAAQ (0.3 g/L) with Zn2+ (100 μM) in CH3CN media).

4. Conclusions

The Fe3O4 NPs coated with silica nanoparticles were prepared, and an attempt had been made that the Fe3O4@SiO2 NPs were modified by 8-chloroacetylaminoquinoline. The functionalized Fe3O4@SiO2 NPs as a fluorescent sensor for detection of Zn2+ were available. Meanwhile, the hybrid fluorescent material showed an efficient removal of Zn2+ (92.37%) and exhibited excellent magnetic properties for further biological and environmental applications. Aside from the applications for the detection of Zn2+, owing to the require of the diversity and controllable ability of functional molecules on surface of nanoparticles, our efforts are concentrated to extend current work to applications in selective detection and removal of heavy-metal ions such as Hg (II), Pb (II), Cu (II), and Cd (II).

Conflict of Interests

The authors declare that they have no conflict of interests.

Acknowledgments

Financial support of this work from NSFC (50903041), Natural Science Foundation of Yunnan Province (2009CD026 and 2010CA019), and Inspection and Quarantine of the People’s Republic of China (2009QK406) was gratefully acknowledged.

References

  1. K. H. Falchuk, “The molecular basis for the role of zinc in developmental biology,” Molecular and Cellular Biochemistry, vol. 188, no. 1-2, pp. 41–48, 1998. View at Publisher · View at Google Scholar · View at Scopus
  2. W. Maret, C. Jacob, B. L. Vallee, and E. H. Fischer, “Inhibitory sites in enzymes: zinc removal and reactivation by thionein,” Proceedings of the National Academy of Sciences of the United States of America, vol. 96, no. 5, pp. 1936–1940, 1999. View at Publisher · View at Google Scholar · View at Scopus
  3. H. H. Sandstead, “Zinc is essential for brain development and function,” The Journal of Trace Elements in Experimental Medicine, vol. 16, no. 4, pp. 165–173, 2003. View at Publisher · View at Google Scholar · View at Scopus
  4. J. A. Duce, A. Tsatsanis, M. A. Cater et al., “Iron-export ferroxidase activity of β-amyloid precursor protein is inhibited by Zinc in Alzheimer's disease,” Cell, vol. 142, no. 6, pp. 857–867, 2010. View at Publisher · View at Google Scholar · View at Scopus
  5. E. M. Alvarez, R. S. Otero, A. H. Ameijeiras, A. M. L. Real, and J. L. L. Garcia, “Effects of aluminum and zinc on the oxidative stress caused by 6-hydroxydopamine autoxidation: relevance for the pathogenesis of Parkinson's disease,” Biochimica et Biophysica Acta, vol. 1586, no. 2, pp. 155–168, 2002. View at Publisher · View at Google Scholar · View at Scopus
  6. T. Liu and S. Liu, “Responsive polymers-based dual fluorescent chemosensors for Zn2+ ions and temperatures working in purely aqueous media,” Analytical Chemistry, vol. 83, no. 7, pp. 2775–2785, 2011. View at Publisher · View at Google Scholar · View at Scopus
  7. D. Dong, X. Jing, X. Zhang, X. Hu, Y. Wu, and C. Duan, “Gadolinium (III)-fluorescein complex as a dual modal probe for MRI and fluorescence zinc sensing,” Tetrahedron, vol. 68, pp. 306–310, 2012.
  8. A. Helal, M. Harun, M. Rashid, C. H. Choi, and H. S. Kim, “New regioisomeric naphthol-substituted thiazole based ratiometric fluorescence sensor for Zn2+ with a remarkable red shift in emission spectra,” Tetrahedron, vol. 68, pp. 647–653, 2012.
  9. P. Ashokkumar, V. T. Ramakrishnan, and P. Ramamurthy, “Photoinduced electron transfer (PET) based Zn2+ fluorescent probe: transformation of turn-on sensors into ratiometric ones with dual emission in acetonitrile,” The Journal of Physical Chemistry A, vol. 115, pp. 14292–14299, 2011.
  10. Z. Xu, J. Yoon, and D. R. Spring, “Fluorescent chemosensors for Zn2+,” Chemical Society Reviews, vol. 39, no. 6, pp. 1996–2006, 2010. View at Publisher · View at Google Scholar · View at Scopus
  11. Z. Li, M. Yu, L. Zhang et al., “A “switching on” fluorescent chemodosimeter of selectivity to Zn2+ and its application to MCF-7 cells,” Chemical Communications, vol. 46, no. 38, pp. 7169–7171, 2010. View at Publisher · View at Google Scholar · View at Scopus
  12. A. E. Lee, M. R. Grace, A. G. Meyer, and K. L. Tuck, “Fluorescent Zn2+ chemosensors, functional in aqueous solution under environmentally relevant conditions,” Tetrahedron Letters, vol. 51, no. 8, pp. 1161–1165, 2010. View at Publisher · View at Google Scholar · View at Scopus
  13. M. Li, H. Lu, R. Liu, J. Chen, and C. Chen, “Turn-on fluorescent sensor for selective detection of Zn2+, Cd2+, and Hg2+ in water,” The Journal of Organic Chemistry, vol. 77, pp. 3670–3673, 2012.
  14. Y. Ding, Y. Xie, X. Li, J. P. Hill, W. Zhang, and W. Zhu, “Selective and sensitive “turn-on” fluorescent Zn2+ sensors based on di- and tripyrrins with readily modulated emission wavelengths,” Chemical Communications, vol. 47, no. 19, pp. 5431–5433, 2011. View at Publisher · View at Google Scholar · View at Scopus
  15. J. Kim, Y. Piao, and T. Hyeon, “Multifunctional nanostructured materials for multimodal imaging, and simultaneous imaging and therapy,” Chemical Society Reviews, vol. 38, no. 2, pp. 372–390, 2009. View at Publisher · View at Google Scholar · View at Scopus
  16. A. Quarta, R. D. Corato, L. Manna et al., “Multifunctional nanostructures based on inorganic nanoparticles and oligothiophenes and their exploitation for cellular studies,” Journal of the American Chemical Society, vol. 130, no. 32, pp. 10545–10555, 2008. View at Publisher · View at Google Scholar · View at Scopus
  17. R. Bardhan, N. K. Grady, J. R. Cole, A. Joshi, and N. J. Halas, “Fluorescence enhancement by au nanostructures: nanoshells and nanorods,” ACS Nano, vol. 3, no. 3, pp. 744–752, 2009. View at Publisher · View at Google Scholar · View at Scopus
  18. J. Gao, H. Gu, and B. Xu, “Multifunctional magnetic nanoparticles: design, synthesis, and biomedical applications,” Accounts of Chemical Research, vol. 42, no. 8, pp. 1097–1107, 2009. View at Publisher · View at Google Scholar · View at Scopus
  19. X. Zhang, H. Niu, Y. Pan, Y. Shi, and Y. Cai, “Modifying the surface of Fe3O4/SiO2 magnetic nanoparticles with C18/NH2 mixed group to get an efficient sorbent for anionic organic pollutants,” Journal of Colloid and Interface Science, vol. 362, no. 1, pp. 107–112, 2011. View at Publisher · View at Google Scholar · View at Scopus
  20. M. Shi, Y. Liu, M. Xu, H. Yang, and C. Wu, “Core/shell Fe3O4@SiO2 nanoparticles modified with PAH as a vector for EGFP plasmid DNA delivery into HeLa cells, macromol,” Bioscience, vol. 11, pp. 1563–1569, 2011.
  21. Y. Deng, D. Qi, C. Deng, X. Zhang, and D. Zhao, “Superparamagnetic high-magnetization microspheres with an Fe3O4@SiO2 core and perpendicularly aligned mesoporous SiO2 shell for removal of microcystins,” Journal of the American Chemical Society, vol. 130, no. 1, pp. 28–29, 2008. View at Publisher · View at Google Scholar · View at Scopus
  22. P. Kluth, C. S. Schnohr, O. H. Pakarinen et al., “Fine structure in swift heavy ion tracks in amorphous SiO2,” Physical Review Letters, vol. 101, no. 17, Article ID 175503, 2008. View at Publisher · View at Google Scholar · View at Scopus
  23. C. R. Li, A. L. Ji, L. Gao, and Z. X. Cao, “Stressed triangular tessellations and fibonacci parastichous spirals on Ag core/SiO2 shell microstructures,” Advanced Materials, vol. 21, no. 45, pp. 4652–4657, 2009. View at Publisher · View at Google Scholar · View at Scopus
  24. H. Chen, C. Deng, and X. Zhang, “Synthesis of Fe304@SiO2@PMMA core-shell-shell magnetic microspheres for highly efficient enrichment of peptides and proteins for MALDI-ToF MS analysis,” Angewandte Chemie - International Edition, vol. 49, no. 3, pp. 607–611, 2010. View at Publisher · View at Google Scholar · View at Scopus
  25. X. Kong, Y. Chen, Z. Yang, S. Wang, and Y. Zhou, “A highly sensitive and efficient functionalized magnetic chemosensor for Cu2+ removal,” Physics Procedia, vol. 25, pp. 2125–2130, 2012.
  26. Y. Zhou, S. Wang, Y. Xie et al., “1, 3-Dipolar cycloaddition as a general route for functionalization of Fe3O4,” Nanotechnology, vol. 19, no. 17, Article ID 175601, 2008. View at Scopus