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A Fluorescent Sensor for Zinc Detection and Removal Based on Core-Shell Functionalized Fe3O4@SiO2 Nanoparticles
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+.
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 [1–3]. 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 [6–10]. For example, Li et al.  reported a “switching on” fluorescent chemodosimeter of selectivity to Zn2+ and its application to MCF-7 cells. Lee et al.  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.  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.  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 [15–18]. Among these nanostructured materials, core-shell Fe3O4@SiO2 NPs have been widely favored in biological and environmental applications [19–21]. 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 [22–24]. 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.
2. Experimental Section
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.
The synthesis of 8-chloroacetylaminoquinoline (CAAQ) was based on the pervious report . 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 . 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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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