Highly selective fluorescent chemosensor for Zn2+ derived from inorganic-organic hybrid magnetic core/shell Fe3O4@SiO2 nanoparticles

1556-276X-7-861556-276X Nano Express Highly selective fluorescent chemosensor for Zn2+ derived from inorganic-organic hybrid magnetic core/shell Fe3O4@SiO2 nanoparticles WangYujiaowyj2009@lzu.cn PengXiaohongpengxh08@lzu.cn ShiJinminshijm07@lzu.cn TangXiaoliangtangxiaol@lzu.edu.cn JiangJiejiangj2007@lzu.cn LiuWeishengliuws@lzu.edu.cn

Key Laboratory of Nonferrous Metals Chemistry and Resources Utilization of Gansu Province and State Key Laboratory of Applied Organic Chemistry, College of Chemistry and Chemical Engineering, Lanzhou University, Lanzhou, 730000, People's Republic of China

Nanoscale Research Letters 1556-276X 2012 7 1 86 http://www.nanoscalereslett.com/content/7/1/86 10.1186/1556-276X-7-8622277075 67201125120122512012 2012Wang et al; licensee Springer.This is an Open Access article distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/2.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

Abstract

Magnetic nanoparticles with attractive optical properties have been proposed for applications in such areas as separation and magnetic resonance imaging. In this paper, a simple and novel fluorescent sensor of Zn²⁺was designed with 3,5-di-tert-butyl-2-hydroxybenzaldehyde [DTH] covalently grafted onto the surface of magnetic core/shell Fe₃O₄@SiO₂nanoparticles [NPs] (DTH-Fe₃O₄@SiO₂NPs) using the silanol hydrolysis approach. The DTH-Fe₃O₄@SiO₂inorganic-organic hybrid material was characterized by transmission electron microscopy, dynamic light scattering, X-ray power diffraction, diffuse reflectance infrared Fourier transform, UV-visible absorption and emission spectrometry. The compound DTH exhibited fluorescence response towards Zn²⁺and Mg²⁺ions, but the DTH-Fe₃O₄@SiO₂NPs only effectively recognized Zn²⁺ion by significant fluorescent enhancement in the presence of various ions, which is due to the restriction of the N-C rotation of DTH-Fe₃O₄@SiO₂NPs and the formation of the rigid plane with conjugation when the DTH-Fe₃O₄@SiO₂is coordinated with Zn²⁺. Moreover, this DTH-Fe₃O₄@SiO₂fluorescent chemosensor also displayed superparamagnetic properties, and thus, it can be recycled by magnetic attraction.

Background

Zinc is the second abundant transition metal ion in the human body, which plays a vital role in various biological processes, such as gene expression 1 , apoptosis 2 , enzyme regulation 3 , and neurotransmission 4 5 . It is also believed that the Zn²⁺homeostasis may have some bearing on the pathology of Alzheimer's disease and other neurological problems 6 7 8 . Therefore, there is an urgency to develop approaches to detect Zn²⁺ in vivo. Besides, techniques for the separation and removal of metal ions and additives in the detection process are very important to prevent poisoning in environmental and biological fields. Conventional analytical methods including atomic absorption spectrophotometry 9 , inductively coupled plasma atomic emission spectrometry 10 , and electrochemical method 11 can hardly be applied for Zn²⁺ion detection in biological systems due to their complicated pretreatment steps and expensive equipment. Hence, for convenience in future in vivo applications, various fluorescent probes based on small molecules have been designed. They were fairly efficient as reported 12 13 14 15 16 17 18 19 20 21 22 ; however, the small molecules would be toxic 23 , and it is impossible to recover or remove them from organisms 24 . The limitation of recoverability blocked the practical applications of small molecular fluorescent probes. To resolve this challenge, the inorganic supports incorporated with small molecular fluorescent probes were applied for the improvement on recoverability.

Various mesoscopic or nanoscopic materials can be acted as the inorganic supports in the design of fluorescent probes, including magnetic nanoparticles, nanotubes, mesoporous silica, metal nanoparticles, and TiO₂ 25 26 27 28 29 30 31 32 33 34 . Among all these inorganic materials, magnetic silica core/shell nanoparticles have advantages over other competitors for biological and environmental applications 35 36 37 38 39 40 41 . Firstly, they could be simply separated or recovered via external magnetic field. Besides, with magnetic silica core/shell nanoparticles as delivery, their low toxicity and biocompatibility also had advantages for the design of biological fluorescent probes. Furthermore, the silica shell around magnetic core has large surface area, and it can be grafted by fluorescent probes. Therefore, to develop nontoxic, biocompatible, and recoverable fluorimetric Zn²⁺sensors, introducing the magnetic silica nanoparticles with small molecular fluorescent probes incorporated is very necessary and highly desirable.

In this work, we designed and synthesized a magnetic recoverable fluorescence Zn²⁺sensor based on 3,5-di-tert-butyl-2-hydroxybenzaldehyde [DTH] covalently grafted onto Fe₃O₄@SiO₂nanoparticles [NPs] (DTH-Fe₃O₄@SiO₂) to provide highly selective fluorescence changes and efficient magnetic recoverability (Figure 1). This Zn²⁺-selective fluorescent switch of the immobilized chemosensors displayed excellent reversibility, combined with its superparamagnetic property, enabling the recovery of material and repeated uses for Zn²⁺sensing.

Figure 1

Syntheses of DTH-APTES and DTH-Fe₃O₄@SiO₂

Syntheses of DTH-APTES and DTH-Fe₃O₄@SiO₂.

Experimental details

Materials and methods

All reagents are purchased commercially. Besides, ethanol was used after purification by standard methods. Other chemicals were used as received without further purification.

Thermal gravimetric analysis [TGA] (P.E. Diamond TG/DTA/SPAECTRUN ONE thermal analyzer, PerkinElmer Inc., Waltham, MA, USA), dynamic light scattering (BI-200SM, Brookhaven Instruments Corporation, Holtsville, NY, USA), transmission electron microscopy [TEM] (Tecnai G²F30, 300 kV, FEI Company, OR, USA), and energy-dispersive X-ray spectrometer [EDX] were used to characterize the materials. X-ray diffraction [XRD] pattern of the synthesized products was recorded with a Rigaku D/MAX 2400 X-ray diffractometer (Tokyo, Japan) using Cu Kα radiation (λ = 0.154056 Å). The scan range (2θ) was from 10° to 80°. Solid-state infrared [IR] using diffuse-reflectance infrared Fourier transform [DRIFT] spectroscopy was performed in the 400- to 4,000-cm^-1region using a Bruker Vertex 70v (Bremen, Germany) and IR-grade KBr (Sigma-Aldrich Corporation, St. Louis, MO, USA) as the internal standard. ¹H NMR and ¹³C NMR spectra were measured on a Bruker DRX 400 spectrometer in a CDCl₃solution with TMS as the internal standard. Chemical shift multiplicities are reported as s = singlet, t = triplet, q = quartet, and m = multiplet. Mass spectra were recorded on a Bruker Daltonics esquire6000 mass spectrometer. UV absorption spectra were recorded on a Varian Cary 100 spectrophotometer (Palo Alto, CA, USA) using quartz cells of 1.0-cm path length. Fluorescence measurements were made on a Hitachi F-4500 spectrophotometer (Tokyo, Japan) and a Shimadzu RF-540 spectrofluorophotometer (Chorley, UK) equipped with quartz cuvettes of 1.0-cm path length with a xenon lamp as the excitation source. An excitation and emission slit of 10.0 nm was used for the measurements in the solution state. All spectrophotometric titrations were performed with a suspension of the sample dispersed in ethanol.

Synthesis of Fe₃O₄@SiO₂NPs

Fe₃O₄@SiO₂NPs were synthesized according to the study of Nigam et al. 42 . The process can be briefly described in the following two steps: (1) FeCl₂and FeCl₃(molar ratio, 1:2) were added to a concentrated solution of base (25% ammonium hydroxide) under N₂. The solution was mechanically stirred for 1 h at 20°C and then heated at 70°C for 1 h. The mixture was then stirred for 30 min at 90°C upon addition of citric acid (0.5 g/ml). After cooling the reaction mixture to room temperature, the magnetite NPs were obtained by permanent magnet, and then it was rinsed with deionized water to remove excess citric acid and other nonmagnetic particles thoroughly. (2) Then, the magnetite NPs were further coated with a thin silica layer via a modified Stöber method 43 to obtain stable Fe₃O₄@SiO₂. Tetraethyl orthosilicate was hydrolyzed with magnetic NPs as seeds in an ethanol/water mixture. The resulting silica-coated magnetite NPs with an average diameter of 60 to 70 nm were used.

Synthesis of DTH-Fe₃O₄@SiO₂NPs

As shown in Figure 1, the synthetic procedure for 2,4-di-tert-butyl-6-((3-(triethoxysilyl)propylimino)methyl)phenol [DTH-APTES] followed the method previously described in the literatures 44 45 . DTH (234 mg, 1 mmol) and (3-aminopropyl) triethoxysilane [APTES] (221 mg, 1 mmol) were mixed in dry ethanol (15 mL) at room temperature. Then, the solution was refluxed for 3 h under N₂. After that, the solvent was evaporated, and the crude product was further purified by flash column chromatography (silica gel, ethyl acetate/petroleum ether 1:2) to produce 371 mg (84.9%) of DTH-APTES as yellow oil. ESI-MS: m/z 438.5 (M + H⁺). ¹H NMR: (400 MHz, CDCl₃): δ (ppm) 0.69 (t, 2H, CH₂Si); 1.22 (t, 9H, CH₃); 1.30 (s, 9H, C(CH₃)₃); 1.43 (s, 9H, C(CH₃)₃); 1.82 (m, 2H, CH₂); 3.58 (t, 2H, NCH₂); 3.82 (q, 6H, SiOCH₂); 7.07, 7.36 (d, 2H, Ar); 8.34 (s, 1H, HC = N). ¹³C NMR (100 MHz, CDCl₃): 7.92 (CH₂Si); 18.30 (CH₃); 24.38, 29.40, 29.70, 31.50 (CH₃); 34.11 (C), 35.01 (C); 58.41 (CH₂); 62.08 (CH₂); 117.83, 125.69, 126.66, 136.65, 139.75, 158.27 (Ar); 165.80 (C = N). FT-IR (KBr pellet) (cm^-1): 1,637 (ν _{C = N}), 1,275-1,252 (ν _C-O), 1,596-1,342 (ν _{C = C}), 1,106-1,085 (ν _Si-O).

One hundred milligrams of dried Fe₃O₄@SiO₂NPs and 356 mg (0.81 mmol) of DTH-APTES were suspended in 10 mL of anhydrous ethanol. The mixture was refluxed for 8 h at 80°C under N₂to obtain DTH-Fe₃O₄@SiO₂. The nanoparticles were collected by centrifugation and repeatedly washed with anhydrous ethanol thoroughly. Unreacted organic molecules were removed completely and monitored by the fluorescence of the upper liquid. Then, the DTH-Fe₃O₄@SiO₂NPs were finally dried under vacuum over night. About 2.81% DTH-APTES in the precursors was finally grafted on the NPs, and the rest could be recycled if no hydrolysis occurred.

Results and discussion

Characterization of DTH-Fe₃O₄@SiO₂

The TEM image (Figure 2A) of DTH-Fe₃O₄@SiO₂reveals that iron oxide NPs have entrapped in the silica shell successfully, in which the core/shell structures are in a narrow size distribution of 60 to 70 nm 46 47 , and the diameter of the magnetic core is about 10 nm. The weight ratio of iron vs. silicon was measured to be 2.63:38.94 by EDX. Hence, according to TGA, each magnetic NP has about 6,000 DTH-APTES molecules grafted (see Additional file 1). More importantly, the right size of magnetic core/shell NPs smaller than 100 nm is an advantage for their good dispersibility. In addition, an inert silica coating on the surface of magnetite nanoparticles prevents their aggregation in liquid 48 . Hence, such a good performance on the dispersibility can improve their chemical stability and provide better protection against toxicity.

Figure 2

TEM image (A) and the particle size histogram from DLS (B) of DTH-Fe₃O₄@SiO₂

TEM image (A) and the particle size histogram from DLS (B) of DTH-Fe₃O₄@SiO₂.

Additional file 1

Characterization and properties of DTH-Fe₃O₄@SiO₂ . Figure S1, XRD patterns of Fe₃O₄core; Figure S2, TGA curves of Fe₃O₄@SiO₂(a) and DTH-Fe₃O₄@SiO₂(b); Figure S3, selectivity of DTH-Fe₃O₄@SiO₂for Zn²⁺in the presence of other metal ions in ethanol; and Table S1, the loading of DTH-APTES in the Fe₃O₄@SiO₂NPs as estimated by different methods.

Click here for file

In addition, dynamic light scattering [DLS] was performed to further reveal the colloidal stability of NPs. According to DLS results (Figure 2B), DTH-Fe₃O₄@SiO₂presents good stabilization and a narrow size distribution with peak centered at 147 nm, confirming its good stabilization in ethanol. In a common sense, the diameter achieved by DLS is mostly higher than the one observed in TEM since the size of NPs identified by DLS includes the grafted molecules' steric hindering and the hydrodynamic radius of first few solvent layers 49 50 51 . Besides, according to the calculated size of DTH-APTES which covalently grafted on the surface of Fe₃O₄@SiO₂, the grafted molecules' steric hindering could increase the diameter by about 2.72 nm.

Figure 3 shows the XRD powder diffraction patterns of two NPs for the identification of Fe₃O₄in core/shell NPs. XRD patterns of the synthesized Fe₃O₄@SiO₂(a) and DTH-Fe₃O₄@SiO₂(b) display relative diffraction peaks in the 2θ region of 10° to 80°. We could find that XRD patterns show very low intensities for the peaks attributed to the Fe₃O₄cores, due to the coating of amorphous silica shell, which deduced the efficient content of Fe₃O₄cores and then affected the peak intensities. However, the diffraction peaks of DTH-Fe₃O₄@SiO₂still maintain the same position as the magnetite core (Figure S1 in Additional file 1) 52 . The six characteristic diffraction peaks in Figure 3 can be indexed to (220), (311), (400), (422), (511), and (440), which well agree with the database of magnetite in the Joint Committee on Powder Diffraction Standards [JCPDS] (JCPDS card: 19-629) file 42 46 53 54 . Also, the broad XRD peak at a low diffraction angle of 20° to 30° corresponds to the amorphous-state SiO₂shells surrounding the Fe₃O₄NPs 53 .

Figure 3

XRD patterns of Fe₃O₄@SiO₂(a) and DTH-Fe₃O₄@SiO₂(b)

XRD patterns of Fe₃O₄@SiO₂(a) and DTH-Fe₃O₄@SiO₂(b).

The successful conjugation of DTH onto the surface of the Fe₃O₄@SiO₂NPs can be confirmed by DRIFT (Figure 4). The bands at 3,400 to 3,500 cm^-1and 1,000 to 1,250 cm^-1are due to -OH stretching on silanol 55 . It indicates that not all the silanol on Fe₃O₄@SiO₂NPs have been covalently modified. The band at 1,630 cm^-1represents the bending mode of -OH vibrations 56 . DTH-Fe₃O₄@SiO₂(see Figure 1) has additional peaks at 2,918 and 2,850 cm^-1that correspond to the -CH vibration of aliphatic and aromatic groups 28 57 58 . The bands at 1,473 and 1,463 cm^-1of DTH-Fe₃O₄@SiO₂are probably due to the bending vibrations of -CH₃, which come from the DTH part 59 . According to the spectra of Fe₃O₄@SiO₂and DTH-Fe₃O₄@SiO₂, the bands which appear as broad and strong and are centered at 1,102 (ν _as) and 800 cm^-1can be attributed to the siloxane (-Si-O-Si-) 60 . These results support the presence of the organic DTH-APTES in the magnetic material DTH-Fe₃O₄@SiO₂.

Figure 4

DRIFT spectra of Fe₃O₄@SiO₂(a) and DTH-Fe₃O₄@SiO₂(b)

DRIFT spectra of Fe₃O₄@SiO₂(a) and DTH-Fe₃O₄@SiO₂(b).

The UV-visible [UV-Vis] spectra of DTH-APTES (1.0 × 10^-5M), Fe₃O₄@SiO₂(0.3 g/L), and DTH-Fe₃O₄@SiO₂(0.3 g/L) can provide further evidence on the grafting of DTH onto the surface of the Fe₃O₄@SiO₂NPs (Figure 5). Compared to Fe₃O₄@SiO₂(b), a new absorption band centered at about 330 nm of DTH-Fe₃O₄@SiO₂can be attributed to the typical electronic transition of an aromatic ring and -C = N- conjugate system in a Schiff base molecule 29 . This result can also imply the successful immobilization of DTH-APTES onto the magnetic core/shell NPs.

Figure 5

UV-Vis spectra of DTH-APTES (a), Fe₃O₄@SiO₂(b), and DTH-Fe₃O₄@SiO₂(c)

UV-Vis spectra of DTH-APTES (a), Fe₃O₄@SiO₂(b), and DTH-Fe₃O₄@SiO₂(c).

The superparamagnetic property of the magnetic NPs plays a vital role for its biological application. Figure 6 shows the magnetization curves of the Fe₃O₄@SiO₂and DTH-Fe₃O₄@SiO₂which were investigated with a vibrating sample magnetometer tuned from -15,000 to 15,000 Oe at 300 K. The result was consistent with the conclusion that magnetic Fe₃O₄NPs smaller than 30 nm are usually superparamagnetic at room temperature 47 . The saturation magnetization value for synthesized DTH-Fe₃O₄@SiO₂is about 3.96 emu/g. The saturation magnetization value for Fe₃O₄@SiO₂support was measured to be 4.24 emu/g. Considering the grafting rate of 7.64% (according to TGA, Figure S2 and Table S1 in Additional file 1), the difference of saturation magnetization values between DTH-Fe₃O₄@SiO₂and its support could be due to the decreased weight ratio of magnetic support after grafting. More importantly, from the hysteresis loops of Fe₃O₄@SiO₂NPs and the DTH-Fe₃O₄@SiO₂NPs, it can be found that both exhibited superparamagnetic properties for no remanence was observed when the applied magnetic field was removed. These phenomena were due to the fact that the magnetite core is smaller than 30 nm in core/shell NPs (Figure 2A). As a result of this superparamagnetic property, DTH-Fe₃O₄@SiO₂had a reversal magnetic responsivity. It could be easily separated from dispersion after only 5 min using a magnet (Figure 6, inset) and then redispersed by mild agitation when the magnet was removed. The reversal magnetic responsivity of DTH-Fe₃O₄@SiO₂would be a key factor when evaluating their recoverability 61 . The magnetic separation capability of DTH-Fe₃O₄@SiO₂NPs and the reversibility of the combination between DTH-Fe₃O₄@SiO₂and Zn²⁺could also provide a simple and efficient route to separate Zn²⁺rather than through filtration approach (see Figure 6 inset).

Figure 6

Magnetization curves of the Fe₃O₄@SiO₂(a) and DTH-Fe₃O₄@SiO₂(b)

Magnetization curves of the Fe₃O₄@SiO₂(a) and DTH-Fe₃O₄@SiO₂(b). Inset shows that DTH-Fe₃O₄@SiO₂was dispersed to an external magnet in ethanol.

Fluorescence response of DTH-Fe₃O₄@SiO₂

To verify its fluorescence response towards various metal ions, we investigated fluorescence properties of DTH-Fe₃O₄@SiO₂NPs (0.3 g/L, containing 5.2 × 10^-5M DTH-APTES according to TGA in Figure S2 and Table S1 in Additional file 1) towards various metal ions Ag⁺, Ca²⁺, Cd²⁺, Co²⁺, Cr³⁺, Cu²⁺, Fe³⁺, Hg²⁺, K⁺, Mg²⁺, Mn²⁺, Na⁺, Ni²⁺, and Zn²⁺in ethanol solution (all as perchlorates, 1.0 × 10^-4M). As shown in Figure 7A, DTH-Fe₃O₄@SiO₂NPs exhibited significant 'off-on' changes in fluorescence emission only for Zn²⁺, but not for the others. It is noted that Cd²⁺with a d ¹⁰electron configuration, which often exhibited coordination properties similar to Zn²⁺ 19 , do not influence the fluorescence intensity of DTH-Fe₃O₄@SiO₂NPs significantly. As a comparison, DTH (1.0 × 10^-5M) exhibited fluorescence response towards both Zn²⁺and Mg²⁺ions (1.0 × 10^-4M) in the same solution, which is not as selective as DTH-Fe₃O₄@SiO₂for Zn²⁺detection (Figure 7B). Compared to the single aldehyde DTH, the origin of selectivity for DTH-Fe₃O₄@SiO₂may come from its Schiff base structure, which prefers to coordinate with Zn²⁺under the interference of Mg²⁺.

Figure 7

Fluorescence response of DTH-Fe₃O₄@SiO₂(A) and DTH (B) to various cations

Fluorescence response of DTH-Fe₃O₄@SiO₂(A) and DTH (B) to various cations. Excitation wavelength was 397 nm. Spectra were recorded every 25 min after adding Zn²⁺.

The remarkable increase of fluorescence intensity can be explained as follows: DTH-Fe₃O₄@SiO₂is poorly fluorescent due to the rotation of the N-C bond of DTH-APTES part. When stably chelated with Zn²⁺, the N-C rotation of DTH-APTES part is restricted and the rigid plane with conjugation is formed and the fluorescence enhanced, which consists of our previous work 62 . The emission spectra of DTH-Fe₃O₄@SiO₂, which is excited at 397 nm, exhibit the emission maximum at 452 nm with a low quantum yield (Φ = 0.0042) at room temperature in ethanol. Upon the addition of excess Zn²⁺, the fluorescence intensity of DTH-Fe₃O₄@SiO₂increased by more than 25-fold, the emission maximum shifts from 452 to 470 nm, and the quantum yield (Φ = 0.11) results in a 26-fold increase.

As illustrated in Figure 8A, the fluorescence emission of DTH-Fe₃O₄@SiO₂(0.3 g/L) increases gradually when adding various concentrations (0 to 30 μM) of Zn²⁺in ethanol, indicating that Zn²⁺is quantitatively bound to the Schiff base moiety attached to the NPs. Fluorescence titration experiment suggests that the association constant (K_d ) for Zn²⁺binding to DTH-Fe₃O₄@SiO₂is calculated to be 51.08 M^-2(log K = 1.71; Figure 8A). Job's plot suggested a 1:2 binding ratio for Zn²⁺with DTH-APTES (Figure 8B).

Figure 8

Fluorescence titrations and Job's plot

Fluorescence titrations and Job's plot. (A) Fluorescence titrations of DTH-Fe₃O₄@SiO₂with Zn²⁺. (B) Job's plot of DTH-APTES with Zn²⁺. Spectra were recorded every 25 min after adding Zn²⁺.

The competition experiments indicated that the presence of most metal ions, especially Na⁺, K⁺, Ca²⁺, and Mg²⁺, which are abundant in the biological environment, had a negligible effect on Zn²⁺sensing (Figure 9A). Since Cr³⁺, Cu²⁺, Fe³⁺, and Hg²⁺also appeared to bind DTH-Fe₃O₄@SiO₂sensors (Figure S3 in Additional file 1), they quenched the fluorescence of the Zn²⁺-DTH-Fe₃O₄@SiO₂, owing to an electron or energy transfer between the metal cation and fluorophore known as the fluorescence quenching mechanism 63 64 65 66 . The fluorescence enhancement that occurred upon exposure to Zn²⁺was fully reversible as the addition of EDTA (2.5 × 10^-4M; Figure 9B and inset) restored the emission band. Combined with its magnetic property, the results above implied that DTH-Fe₃O₄@SiO₂was considerably applicable to some field as a new inorganic-organic hybrid sensor for the Zn²⁺ion.

Figure 9

Competition of DTH-Fe₃O₄@SiO₂towards cations and reversibility of DTH-Fe₃O₄@SiO₂towards Zn²⁺

Competition of DTH-Fe₃O₄@SiO₂towards cations and reversibility of DTH-Fe₃O₄@SiO₂towards Zn²⁺. (A) Fluorescent emission changes of DTH-Fe₃O₄@SiO₂(0.3 g/L) upon addition of 1, blank; 2, Zn²⁺; 3, Na⁺; 4, Na⁺+ Zn²⁺; 5, K⁺; 6, K⁺+ Zn²⁺; 7, Ca²⁺; 8, Ca²⁺+ Zn²⁺; 9, Mg²⁺; and 10, Mg²⁺+ Zn²⁺(each metal ion is 100 μM) in ethanol at room temperature. (B) Fluorescence spectra of DTH-Fe₃O₄@SiO₂(0.3 g/L) in (a) without, (b) with Zn²⁺(1.0 × 10^-4M), and (c) after treatment with EDTA (2.5 × 10^-4M) in (b) solution. The inset picture shows the photograph of DTH-Fe₃O₄@SiO₂with Zn²⁺by treatment of EDTA (2.5 × 10^-4M) under a 365-nm UV light.

Figure 10A depicts the UV-Vis spectra of DTH-APTES (10 μM), DTH-APTES (10 μM) + Zn²⁺(100 μM), DTH-Fe₃O₄@SiO₂(0.3 g/L), and DTH-Fe₃O₄@SiO₂(0.3 g/L) + Zn²⁺(100 μM). It can be seen that the absorbance peaks at around 390 nm are formed when Zn²⁺is added in both DTH-APTES and DTH-Fe₃O₄@SiO₂systems. The absorption spectra of DTH-Fe₃O₄@SiO₂(0.3 g/L) in the presence of various concentrations of Zn²⁺(0 to 240 μM) were investigated in ethanol at room temperature, as shown in Figure 10B. When Zn²⁺was added gradually, the absorbance of DTH-Fe₃O₄@SiO₂at 390 nm gradually increases, which indicated that DTH-Fe₃O₄@SiO₂NPs coordinated with Zn²⁺gradually.

Figure 10

UV-Vis spectra

UV-Vis spectra. (A) Absorption spectra of (a) DTH-APTES (1.0 × 10^-5M), (b) DTH-APTES + Zn²⁺(1.0 × 10^-4M), (c) DTH-Fe₃O₄@SiO₂(0.3 g/L), and (d) DTH-Fe₃O₄@SiO₂(0.3 g/L) + Zn²⁺(1.0 × 10^-4M) in ethanol. (B) UV-Vis spectra of DTH-Fe₃O₄@SiO₂(0.3 g/L) in ethanol in the presence of different amounts of Zn²⁺(0 to 240 μM).

Conclusions

In summary, we have successfully designed and synthesized functionalized magnetic core/shell Fe₃O₄@SiO₂NPs (DTH-Fe₃O₄@SiO₂NPs) which could act as a new type of fluorescent chemosensor for efficient sensing and separation of Zn²⁺in ethanol. The inorganic-organic hybrid fluorescent chemosensor DTH-Fe₃O₄@SiO₂was able to recognize and adsorb Zn²⁺with a selective and sensitive fluorescence response in ethanol. The magnetic separation capability of Fe₃O₄@SiO₂NPs and the reversibility of the combination between DTH-Fe₃O₄@SiO₂and Zn²⁺would also provide a simple route to separate Zn²⁺from the environment (Figure 6, inset).

Abbreviations

APTES: (3-aminopropyl)triethoxysilane; DLS: dynamic light scattering; DRIFT: diffuse-reflectance infrared Fourier transform; DTH: 3,5-di-tert-butyl-2-hydroxybenzaldehyde; DTH-APTES: 2,4-di-tert-butyl-6-((3-(triethoxysilyl)propylimino)methyl)phenol; EDX: energy-dispersive X-ray spectrometer; NPs: nanoparticles; TEM: transmission electron microscopy; TEOS: tetraethyl orthosilicate; TGA: thermal gravimetric analysis; XRD: X-ray power diffraction.

Competing interests

The authors declare that they have no competing interests.

Authors' contributions

YW supervised and participated in all the studies and wrote this paper. XP conceived the study and participated in its design. JS participated in the synthesis of the nanoparticles and the testing of fluorescence property. XT, JJ, and WL participated in the revision of the manuscript. All authors read and approved the final manuscript.

Acknowledgements

The authors acknowledge the financial support from the NSFC (grant nos. 20931003 and 91122007) and the Specialized Research Fund for the Doctoral Program of Higher Education (grant no. 20110211130002).

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