Fluorescent magnetic nanosensors for Zn²⁺ and CN⁻ in aqueous solution prepared from adamantane-modified fluorescein and β-cyclodextrin-modified Fe₃O₄@SiO₂ via host–guest interactions

Abstract

A novel multifunctional fluorescent chemosensor has been constructed with a highly selective “off–on” behavior, recoverability and recyclability based on β-CD/AD (adamantane) host–guest self-assembly interactions. Adamantane-modified fluorescein/cyclodextrin-modified Fe₃O₄@SiO₂ inclusion complex magnetic nanoparticles (FFIC MNPs), which can provide a specific green fluorescence enhancement in response to Zn²⁺, have a detection limit of 4.5 × 10⁻⁷ mol L⁻¹ in CH₃CN : H₂O (1 : 4, v/v). The spirolactam ring in the fluorophore moiety would be open with the introduction of Zn²⁺, while it could recover as long as the zinc in the complex is removed. Meanwhile, one gram of FFIC MNPs can adsorb 6.1 mg zinc. Therefore, the derivative chemosensors, FFIC MNPs–Zn, are available to respond to CN⁻ due to the fluorescence quenching under UV (ultraviolet) radiation with a detection limit of 7.7 × 10⁻⁷ mol L⁻¹. Furthermore, the FFIC MNPs exhibit great reusability and recyclability in aqueous solution on account of their magnetism and reproducibility. We recycle the residual MNPs to detect Zn²⁺ repeatedly for at least 4 times, after readily adsorbing the complex zinc with excess CN⁻, and the same principle also works in reverse. If the fluorophore moiety is inactive, we could also wash out the useless fluorescent molecules from the MNPs (hosts) and then reassemble new fluorescent small-molecules (guests) to maintain the efficient properties of the probe in responding to Zn²⁺ and CN⁻ for at least 7 times.

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Introduction

Zinc, which plays an important role in life, is an abundant heavy metal just like iron and copper and essential as a catalytic cofactor in hundreds of metalloenzymes like carbonic anhydrase, DNA polymerase and pancreatic carboxypeptidase. Excessive zinc can result in gastrointestinal and immunological toxicity. Traces of cyanide are highly toxic to living creatures and should not be ignored either. So it is important to rapidly and quantificationally monitor trace amounts of zinc ions in vitro and in vivo or CN⁻ in aqueous environments. Thus in recent decades, the efficient detection, removal and recovery of zinc or cyanide in chemosensor applications have gained considerable attention, which could solve this technologically challenging and ecologically urgent problem. Up to now, numerous superior fluorescent molecule chemosensors for the detection and recognition of Zn²⁺ (ref. 2, 7–9 and 14) or CN⁻ (ref. 11 and 18) have been developed with high selectivity and sensitivity, mild examination conditions, and short response times by many research groups. But these small-molecule sensors have many disadvantages, such as that they cannot be reused repeatedly or be separated after detection. Therefore we have utilized the ‘host–guest’ β-CD/AD self-assembly interactions to construct a multifunctional fluorescent chemosensor, the FFIC MNPs,¹ which can respond to Zn²⁺, be separated, and be reused repeatedly with high selectivity and sensitivity. In this β-CD/AD host–guest supramolecular inclusion complexation, adamantane can be specifically identified and well-included by β-CD, because it has a number of hydrophilic hydroxyl groups on the external and hydrophobic internal cavity, in which it can accommodate suitable micromolecules. On the contrary, the exclusion is easy to control in acetonitrile and ethanol.

It is shown from the measurement results that the FFIC MNPs can adsorb zinc up to 6.1 mg g⁻¹ and when CN⁻ removes the Zn²⁺ combined with FFIC MNPs, the spirolactam ring in the fluorophore moiety will close again accompanied with fluorescence quenching and that is why the FFIC MNPs–Zn can effectively respond to CN⁻. Another reason is the higher association constant between zinc and CN⁻ than zinc and other anions,¹¹ which have nearly no influence on the probing, such as Cl⁻, I⁻, SCN⁻, Br⁻, HPO₄²⁻, F⁻, HSO₃⁻, and S₂O₃²⁻, certainly including FFIC MNPs as well. This method solves the complicated modes of action of the direct cyanide chemosensors which are based on hydrogen-bonding and bond-forming reactions. We know a great deal about the rules of coordination chemistry between the ligands of sensors and cations but have poor knowledge of the interactions between compounds and anions according to the literature.¹⁸ In brief, the probe can be reused many times either as FFIC MNPs or as FFIC MNPs–Zn with high selectivity and sensitivity (Scheme 1).

Scheme 1 A chemical and schematic illustration of the preparation of the FFIC MNPs and FFIC MNPs–Zn fluorescent sensors for Zn²⁺ and CN⁻ ions.

Materials and methods

Apparatus

Fluorescence measurements were performed using a Hitachi F-4500 spectrofluorimeter (Japan) and a 10 × 10 mm conventional quartz cuvette, and the slit width was 2.5 nm. ¹H NMR spectra were measured using a Bruker AV-400 spectrometer at 400 MHz in DMSO and CDCl₃ with tetramethylsilane as the internal standard (US). Mass spectra were measured using an Axima CFR MALDI-TOF mass spectrometer (US). The absorption spectra were recorded using Bruker Vector-22 infrared and Hitachi U-3010 UV-vis spectrophotometers (US). A Mettler-Toledo Instruments DELTA 320 pH-meter was used for the pH measurements. Scanning electron microscopy (SEM) was performed using an SII: SPI3800N microscope (Japan).

Materials and reagents

Fluorescein, hydrazine hydrate (80%) and other organic reagents were all used as delivered from Aladdin. The rest of the chemicals used were all of analytical grade. Distilled-deionized water was used throughout. Stock solutions (10⁻³ mol L⁻¹) of various ions (CaCl₂, CoCl₂, MgSO₄, BaCl₂, CdCl₂, Mn(NO₃)₂, SnCl₄, NiCl₂, AlCl₃, HgCl₂, KCl, FeCl₃, CuSO₄; Cl⁻, I⁻, SCN⁻, Br⁻, HPO₄²⁻, F⁻, HSO₃⁻, and S₂O₃²⁻) were prepared by dissolving their salts in a tris–HCl buffer (0.05 mol L⁻¹) to control the pH value (pH = 7.20), and to maintain the ionic strength of all solutions in the experiments.

Preparation of the FFIC MNPs and the FFIC MNPs–Zn

The Fluor-Ad/Fe₃O₄@SiO₂-β-CD inclusion complex magnetic nanoparticles (FFIC MNPs) were facilely prepared from cyclodextrin-functionalized magnetic silica microspheres (denoted as Fe₃O₄@SiO₂-β-CD MNPs) (host) and Fluor-Ad (guest) via a self-assembly process. The Fe₃O₄@SiO₂-β-CD MNPs were prepared via two steps as shown in Scheme 2 (see ESI†). First, the monodisperse superparamagnetic Fe₃O₄@-silica spheres were prepared^4,6,12,17,19 and then the Fe₃O₄@SiO₂ MNPs were reacted with β-cyclodextrin that had been linked with a silane coupling agent via a sol–gel grafting reaction on the surface of the Fe₃O₄@SiO₂ nanoparticles.^1,12 The preparation of the Fluor-Ad moiety is described in Scheme S1 (see ESI†),^3,5,13,15,16 and detailed procedures can be found in the ESI too. The synthetic product was well-characterized using FTIR, SEM, TEM, XRD, superconducting quantum interference measurement device (SQUID), UV-vis spectroscopy, ¹H NMR, and ¹³C NMR (see Fig. S3 ESI†). The FFIC MNPs–Zn were prepared according to the following Langmuir isotherm that one gram of FFIC MNPs can adsorb 6.1 mg Zn²⁺, and then be separated magnetically.

Results and discussion

Morphology of the FFIC MNPs

SEM imaging shows that the Fe₃O₄@SiO₂ nanoparticles have an average diameter of about 270 nm and obviously possess smooth surfaces. It was found that Fe₃O₄ nanoparticles possess diameters of about 200 nm indicating the shell thickness is about 35 nm. The Fe₃O₄@SiO₂ nanostructures are quite uniform in size with an average diameter and no aggregation. As shown in Fig. S1b (see ESI†), only a few nanoparticles with deformed or cracked structures are observed because the silica nanocomposites possess a solid structure that is seen from the TEM in Fig. S1a (see ESI†), therefore the structure is hard to be fractured mechanically.

Fourier transform IR spectroscopy analysis

We used FTIR to further confirm the fluorescein-AD moiety is successfully loaded on the surface of Fe₃O₄@SiO₂/β-CD. Fig. 1 shows the FTIR spectra of Fe₃O₄ (A), SiO₂@Fe₃O₄ (B), β-CD/SiO₂@Fe₃O₄ (C) and the FFIC MNPs (D). The broad band at 3300–3500 cm⁻¹ (A, B, C and D) is ascribed to the O–H stretching vibration of water in the Fe₃O₄ crystal. In particular, the broad band centered at 467 cm⁻¹ (simple A, B, C and D) and the band at 1120 cm⁻¹ (B, C and D) are ascribed to the Fe–O and Si–O stretching vibrations, symmetric Si–O–Si stretching and asymmetric Si–O–Si stretching of silane.^1,10 Therefore, we confirm that SiO₂ is successfully deposited on the surface of the Fe₃O₄ MNPs. In addition, sample D shows that the characteristic bands at 1546 cm⁻¹ (secondary amide N–N bending), 1100–1296 cm⁻¹ (aromatic C–N stretching), 1608 cm⁻¹ (C=N stretching), and 1693 cm⁻¹ (C=O stretching) can also be achieved. Based on these data from FT-IR, it can be concluded that the attachment of fluorophore moieties to the surface of Fe₃O₄@SiO₂-β-CD MNPs has indeed taken place via host–guest self-assembly interactions.

Fig. 1 The FTIR spectra of Fe₃O₄ (A), SiO₂@Fe₃O₄ (B), β-CD/SiO₂@Fe₃O₄ (C), and FFIC MNPs (D).

Magnetic properties

The magnetic hysteresis loop of the FFIC MNPs measured at T = 300 K (close to room temperature) demonstrated that the samples have low coercivity and no obvious hysteresis, which indicates that the FFIC MNPs have superparamagnetism (Fig. S2a ESI†). Superparamagnetism means that when the outer magnetic field withdraws, there is no residual magnetism in the nanoparticles. If the nanoparticles have residual magnetism, it is highly possible for these nanoparticles to aggregate irreversibly. The saturation magnetization (Ms) values for the Fe₃O₄ nanoparticles and FFIC MNPs are 80.33 and 28.26 emu g⁻¹, respectively. The decrease in the Ms of magnetic nanoparticles could be attributed to the increasing amount of non-magnetic materials (organic ligands and silica shell) on the particle surface, which makes up a larger percentage of the nonmagnetic fraction. The binding of silica and the molecule in Fig. S3 (see ESI†) on the surface of the particle might have quenched the magnetic moment. In addition, organic molecules on the surface lack complete coordination and thus increase the surface spin disorientation. This disordered structure in the amorphous materials and at the interface might have caused a decrease in the effective magnetic moment. However, the FFIC MNPs inherit their strong magnetic properties from the Fe₃O₄ nanoparticles. A complete magnetic separation of the FFIC MNPs was achieved in 60 s by placing a magnet near the vessels containing the CH₃CN–H₂O dispersion of the nanoparticles. The magnetic separation capability of the FFIC MNPs in this detection method can also offer a simple and efficient route for the separation and extraction of toxic metal ions from various environments.

X-ray powder diffraction pattern analysis

The crystal structure of the as-synthesized Fe₃O₄ nanocrystals and the FFIC MNPs was investigated using X-ray diffraction (XRD) – (Fig. S4 ESI†). The XRD patterns of the synthesized Fe₃O₄ and the FFIC MNPs display several relatively strong reflection peaks in the 2θ region of 10–80°. The six discernible diffraction peaks in Fig. S4a† can be indexed to (220), (311), (400), (422), (511) and (440), which match well with those on the database in the JCPDS file for magnetite. This result shows that the embedded Fe₃O₄ MNPs retain their crystalline structure of magnetite after template extraction. Besides the peak of iron oxide, the XRD pattern of the iron-oxide–SiO₂ core–shell nanoparticles presented a broad and featureless XRD peak at a low diffraction angle, which corresponds to the amorphous state of the SiO₂ shells. This result shows that the Fe₃O₄ MNPs were successfully coated and passivated with the SiO₂ shell. To further confirm the components of the products, a TEM image is shown in Fig. S1a (see ESI†).

The appropriate responsive pH range

Fluorophores are usually disturbed by protons during the detection of metal ions. Not only is the structure of the molecule broken easily, but the hydrolysis of metal ions must be considered in various pH levels, so their low sensitivity to the operational pH value was expected and investigated. Fig. S6 (see ESI†) reveals that the FFIC MNPs could respond to Zn²⁺ ions in a pH range from 6 to 10 with little change in fluorescence intensity. At a pH value of more than 10, Zn²⁺ was hydrolyzed severely along with a decline in the fluorescence response and when the pH was below 6, the fluorescence intensity distinctly decreased. What is more, no matter how the pH changed from pH 3 to 6, the intensity had almost no large variation, because the structure of the molecule on the surface of the SiO₂@Fe₃O₄ MNPs had been destroyed by the formation of the open-ring state from the strong protonation. Considering that most of the samples for the FFIC MNPs analysis of Zn²⁺ ions were neutral, the medium for the Zn²⁺ ion quantification was buffered at pH 7.10.

Fluorescence properties and detecting Zn²⁺

In order to gain insight into the signalling properties of the FFIC MNPs, fluorescence titrations were conducted. The most appropriate proportions of CH₃CN and H₂O were found using Fig. S8,† showing 1/4 (v/v) as the most appropriate proportion (see ESI†). The fluorescence titration of Zn²⁺ was carried out using a 0.1 g L⁻¹ solution of the FFIC MNPs in buffer (tris–HCl, pH = 7.1) with CH₃CN–H₂O (1/4, v/v). The excitation wavelength was 390 nm and the fluorescence emission intensity was recorded at 485 nm. Before the dropwise addition of Zn²⁺, the fluorescence intensity of the FFIC MNPs was extremely weak and was attributed to the spirolactam form. The fluorescence spectra with different concentrations of the Zn²⁺ solution are shown in Fig. 2. Upon the addition of increasing concentrations of Zn²⁺ ions, an apparent enhancement of the characteristic fluorescence emerges at 485 nm, accompanied by an obvious green fluorescence under UV. This is because the Zn²⁺ ions could chelate with the imine N, carbonyl O, and phenol O atoms with the ring opening of the spirolactam of the fluorescein moiety taking place instantaneously.⁸ When the concentration of Zn²⁺ reached 62.5 × 10⁻⁷ mol L⁻¹, the fluorescence increased smoothly until saturation and the maximum fluorescence intensity was retained. A linear relationship existed between the fluorescence intensity of the FFIC MNPs and the concentration of Zn²⁺ over the range of 2.5 × 10⁻⁷ mol L⁻¹ to 62.5 × 10⁻⁷ mol L⁻¹. The correlation coefficient was R² = 0.9966. The detection limit was 4.5 × 10⁻⁷ mol L⁻¹.

Fig. 2 Fluorescence spectra of the FFIC MNPs (0.1 g L⁻¹) in the absence and presence of Zn²⁺ (2.5 × 10⁻⁷ to 6.25 × 10⁻⁶ mol L⁻¹). The inset exhibits the fluorescence intensity as the concentration of Zn²⁺ increases (CH₃CN–H₂O, 1 : 4, v/v, buffered at pH = 7.1 with tris–HCl (0.1 M), excitation was at 390 nm and the emission was monitored at 485 nm). Slit: 2.50 × 5.00 mm.

Metal ion competition studies

To evaluate the utility of the FFIC MNPs as ion-selective fluorescence probes for Zn²⁺ ions, the fluorescence emission response of the FFIC MNPs upon addition of various biologically and environmentally relevant metal ions, including K⁺, Al³⁺, Fe³⁺, Ba²⁺, Ca²⁺, Mg²⁺, Mn²⁺, Ni²⁺, Cd²⁺, Cu²⁺, Co²⁺, Pb²⁺ and Hg²⁺ ions, each at a concentration of 50 μmol L⁻¹ (represented by black bars in Fig. 3). As we expected, the above-mentioned metal ions show a weak effect on the fluorescence intensity of the nanosensor. However, compared with the marked enhancement provoked by the Zn²⁺ ions at a concentration of 1.0 μmol L⁻¹, the influence of the above-mentioned metal ions is negligible. Although the fluorescence was affected to some extent in the solutions of Cu²⁺ and Cd²⁺, we increased Zn²⁺ from 1 μmol L⁻¹ to 5 μmol L⁻¹ and the experimental results indicated that the proportion of other ions showed no obvious interference in the detection of Zn²⁺. Thus, the FFIC MNPs exhibit excellent selectivity toward Zn²⁺, which makes their practical application feasible.

Fig. 3 Black bars: the fluorescent emission response of the FFIC MNPs at a concentration of 0.1 g L⁻¹ in the presence of different metal ions at concentrations of 50 μmol L⁻¹ in CH₃CN–H₂O solution. Red bars: the fluorescence response of the FFIC MNPs upon the addition of 1.0 μmol L⁻¹ of Zn²⁺ ions in the presence of each interfering metal ion (CH₃CN–H₂O, 1 : 4, v/v, buffered at pH = 7.1 with tris–HCl (0.1 M), the excitation wavelength was at 390 nm, the emission wavelength was at 485 nm).

Adsorption kinetics of Zn²⁺ onto FFIC MNPs

Almost all current Zn²⁺ sensors can only detect the heavy metal ions, but not remove them from solution. In this work, we endowed the FFIC MNPs with adsorptive and separable properties to remove the Zn²⁺ ions from aqueous solution.

The FFIC MNPs were added to aqueous solutions containing different concentrations of Zn²⁺ ions and were thoroughly removed using a magnet ([FFIC] = 1 g L⁻¹, [Zn²⁺] = 0–14 mg L⁻¹). It was found that the adsorption of Zn²⁺ enhanced initially with the increasing concentration and then would level off showing that q_e = 6.1 mg g⁻¹ in Fig. S11 (see ESI†). The initial enhancement in the metal adsorption might be due to the many available chelating sites on the FFIC MNPs. The concentration of Zn²⁺ ions left in the aqueous solution was measured using inductively coupled plasma mass spectrometry (ICP-MS). The experimental adsorption equilibrium data of Zn²⁺ were analyzed according to the Langmuir adsorption equation, which is given as follows: C_e/q_e = 1/K_L q_m + C_e/q_m,¹ where q_e is the equilibrium quantity of the metal ions adsorbed onto the FFIC MNPs (mg g⁻¹), C_e is the equilibrium concentration (mg L⁻¹), and q_m (mg g⁻¹) = 7.19 and K_L (L mg⁻¹) = 0.364 are the Langmuir constants related to the saturation adsorption capacity and binding energy (affinity), respectively. Fig. S12 (see ESI†) shows the Langmuir C_e/q_e versus C_e plot and a good linear relationship (R² = 0.9503) was found. The basic assumption of the Langmuir theory is that adsorption takes place at specific homogeneous sites within the adsorbent and once a metal ion occupies a reaction site, no further adsorption occurs at that location. Thus, monolayer adsorption occurred on the FFIC MNPs. The values of q_e = 6.1 mg g⁻¹, q_m, and K_L were calculated from the slope and intercept of the C_e/q_e versus C_e plot in Fig. S11.† The linear plot indicates that the Zn²⁺ ion adsorption followed the Langmuir isotherm. The adsorption capacity was 6.1 mg of Zn²⁺ ions per gram of FFIC MNPs.

Fluorescence properties and the detection of CN⁻

A 2 g L⁻¹ FFIC MNPs–Zn solution was prepared in advance. Fluorescence titrations were conducted with CN⁻ (1 × 10⁻³ mol L⁻¹) and the experimental conditions were the same as in Fig. 2. Fig. 4 shows the fluorescence intensity of the FFIC MNPs–Zn quenched from the initial intensity of 121 to the ultimate intensity of 43 as the concentration of CN⁻ increases. After 77.5 × 10⁻⁷ mol L⁻¹, the linearity would mainly stay smooth. A linear relationship existed between the fluorescence intensity of the FFIC MNPs–Zn and the concentration of CN⁻ over the range of 2.5 × 10⁻⁷ mol L⁻¹ to 77.5 × 10⁻⁷ mol L⁻¹. The correlation coefficient was R² = 0.9906 and the detection limit was 7.7 × 10⁻⁷ mol L⁻¹. Therefore the FFIC MNPs–Zn can be used to respond to CN⁻ efficiently and specifically, just as the chemosensor in ref. 18, due to the complex constant of CN⁻ and Zn²⁺ being larger than that of the FFIC MNPs and Zn²⁺ with the decrease of fluorescence intensity.

Fig. 4 Fluorescence spectra of the 0.1 g L⁻¹ FFIC MNPs–Zn in the absence and presence of CN⁻ from 0 to 77.5 × 10⁻⁷ mol L⁻¹. The inset shows the fluorescence intensity dependence on the concentration of CN⁻ (CH₃CN–H₂O, 1 : 4, v/v, buffered at pH = 7.1 with tris–HCl, excitation was at 390 nm, emission was monitored at 485 nm). Slit 2.50 × 5.00 mm.

Anion competition studies

The sensitivity of the fluorescence turn-on response to other biologically relevant anions was examined. Aqueous solutions of Cl⁻, I⁻, SCN⁻, Br⁻, HPO₄²⁻, F⁻, HSO₃⁻, and S₂O₃²⁻ were titrated into a suspension solution of the FFIC MNPs–Zn and changes in the emission and absorbance spectra were monitored. Interfering ions were tested at 25 μmol L⁻¹, while the target analyte, CN⁻, was tested at 5 μmol L⁻¹. Fig. 5 shows the changes in fluorescence intensity caused by the other anions and CN⁻ and it can be seen that there is slight fluorescence quenching after adding the interfering ion dropwise, but only CN⁻ can change the fluorescence intensity obviously, which means other anions remain entirely silent at the same conditions. As a result, the selectivity of the FFIC MNPs–Zn toward the CN⁻ ions over other anions is remarkably high.

Fig. 5 Blue bars: fluorescence emission of the FFIC MNPs–Zn (0.1 g L⁻¹) in the presence of the different anions at 25 μmol L⁻¹. Red bars: the fluorescence response of the FFIC MNPs–Zn upon addition of the CN⁻ ions at a concentration of 5 μmol L⁻¹ (CH₃CN–H₂O, 1 : 4, v/v, buffered at pH = 7.1 with tris–HCl (0.1 M), excitation was at 390 nm, emission was at 485 nm).

Regeneration properties

Recycling of the switchable FFIC MNPs was conducted with respect to the response to zinc ions. After responding to zinc ions, we allowed the residual MNPs to adsorb zinc to saturation, separated them magnetically, dried them under vacuum and reused them to detect CN⁻. After that we let EDTA chelate and adsorb residual Zn²⁺ on the surface of the MNPs–Zn and then the MNPs regained the capacity to respond to Zn²⁺. This circulation exhibits that the same FFIC MNPs possess the capacity to respond to Zn²⁺ and CN⁻ repeatedly as shown in Fig. 6.

Fig. 6 Recycling and reusing of the same FFIC MNP sensors that show fluorescence enhancement for Zn²⁺ and fluorescence quenching for CN⁻. Slit 2.50 × 5.00 mm.

The fluorescent functional moieties can also be washed off the fluorescent functional molecules from the assembled MNPs in CH₃CN via ultrasonication and then reassembled with new fluorescent functional molecules to detect zinc again. The recycling process was easy and high yielding. Fig. 7 illustrates the fluorescence intensity of the FFIC MNPs after each cycle. After 7 cycles, the ability to respond to Zn²⁺ ions is still active and efficient. Therefore the MNPs have outstanding reproducibility and practicability.

Fig. 7 Using the same “Host” (MNPs), we change the “Guest” (fluorescent molecules). Slit 2.50 × 5.00 mm.

Conclusions

In summary, we have prepared novel and functionalized solid magnetic nanoparticles which act as a new type of colorimetric and switchable chemosensor for sensing and separating Zn²⁺ and CN⁻ efficiently in aqueous solutions. These multifunctional nanoparticles exhibit high selectivity and sensitivity for targeting Zn²⁺ and CN⁻ over a number of other metal ions and anions tested, with the detection limit of 4.5 × 10⁻⁷ mol L⁻¹ for Zn²⁺ and 7.7 × 10⁻⁷ mol L⁻¹ for CN⁻. The FFIC MNPs show a good ability of adsorbing Zn²⁺ ions and can separate zinc ions from aqueous solution with a commercial magnet easily and quickly. Furthermore, the switched FFIC MNPs–Zn could be separated and collected after responding to CN⁻ within 60 s. The recycling of the FFIC MNPs or FFIC MNPs–Zn is easy and high-yielding in the described approach (based on a sol–gel grafting reaction and simple self-assembly techniques). We believe that this technique would provide a very promising alternative for developing high-performance magnetic sensing materials for Zn²⁺ and CN⁻ detection and separation from aqueous solution.

Acknowledgements

We thank the National Natural Science Foundation of China (no. 21174052), the Natural Science Foundation of Jilin Province of China (no. 20130101024JC) and Jilin Province Science and Technology research plan (no. 20140204054GX) for their generous financial support.

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c5ra12258a

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