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Fabrication of fluorescent SiO2@ zeolitic imidazolate framework-8 nanosensor for Cu2+ detection
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DOI: 10.1039/C4AN01773K
Yonghai Song, Dongqin Hu, Fenfen Liu, Shouhui Chen and Li Wang∗
Key Laboratory of Functional Small Organic Molecule, Ministry of Education, College of Chemistry and Chemical Engineering, Jiangxi Normal University, Nanchang 330022, People’s Republic of China
∗ Corresponding author: Tel/Fax: +86 791 88120861. E-mail: [email protected] (L. Wang). 1
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Abstract
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DOI: 10.1039/C4AN01773K
A simple strategy to fabricate fluorescent SiO2@zeolitic imidazolate framework-8 (ZIF-8) core-shell nanosensor for Cu2+ detection was demonstrated in this work. The nanosensor was synthesized using carboxyl-functionalized SiO2 nanoparticles (SiO2 NPs) as a template to induce the growth of ZIF-8 on its surface. The porous SiO2@ZIF-8 exhibited extremely good adsorption property and large specific surface area to accumulate Cu2+ and the pyridyl nitrogen sites in imidazole played vital roles in the recognition of Cu2+. The fluorescent intensity decreased linearly with the increasing of Cu2+ concentration in the range of 10-500 nM and the detection limit was estimated to be 3.8 nM. The SiO2@ZIF-8 nanosensor could be further used to determine a trace amount of Cu2+ in real water samples, while some previous sensors had to be dispersed in organic solution for use, such as DMSO and MeCN. The core-shell nanostructures of SiO2@ZIF-8 made it possible to disperse directly in aqueous solution and prevented ZIF-8 from aggregation, which enhanced the sensing performances of the SiO2@ZIF-8 nanosensor. Keywords: Fluorescent nanosensor; SiO2@ZIF-8; Core-shell nanostructure; Cu2+ detection
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1. Introduction
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DOI: 10.1039/C4AN01773K
Metal-organic frameworks (MOFs) have emerged as interesting functional materials due to their tunable structures with high thermal stability, organic functionality, large pore sizes, open metal sites in the skeleton and high surface area.
1-5
Great efforts have been made to prepare MOFs with
different shapes and sizes for special purposes.6-12 Among them, luminescent MOFs originated from the luminescent ability of metal nodes or the intrinsic luminescence of organic ligands is attracting extensive interesting. Particularly, luminescent nanoscale MOFs (NMOFs) takes advantage of the excellent properties of both MOFs and nanomaterials and shows enhanced properties for practical applications. 13,14 Therefore, luminescent NMOFs have been found applications for detecting metal ions, molecules and temperature. 15-17 Core-shell MOFs have received intensive attention due to their new or enhanced physicochemical properties as compared with their counterparts.18-20 A few core-shell MOFs structures such as MOF@MOF porous structures, SiO2@MOF architectures, polystyrene (PS)@MOF films, Fe3O4 nanoparticles (NPs)@MOF and qutumn dots (QDs) or metal NPs@MOFs have been developed. 20-29
Template-assisted strategy is an effective approach to synthesize core-shell MOFs. 26 Generally,
the templates are stabilized by surfactant, capping agents or ions before the introduction of MOF precursor solution, and then the functional groups initiate the binding with metal ions to confine the growth of MOFs on the surface of the templates. For example, Oh and co-workers successfully prepared uniform SiO2@coordination polymers (CPs) by introducing the carboxyl-terminated SiO2 NPs (SiO2-COOH NPs) into the CPs precursor solution. The shell thickness could be easily adjusted by changing the amount of reactants.30 Qiu and Yan used mercaptoacetic acid-functionalized
Fe3O4
and
carboxyl-terminated
PS
to
fabricate
Fe3O4@Cu3(benzene-1,3,5-tricarboxylate)2 and PS@zeolitic imidazolate framework-8 (ZIF-8) with 3
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step-by-step strategy to adjust the shell thickness. 23,24
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DOI: 10.1039/C4AN01773K
Copper is widely used in our daily life. However, excessive Cu2+ will result in pathological and physiological events, such as Alzeimer’s disease and Wilson disease.31, 32 Therefore, it is of great significance to develop a facile and efficient method for a trace amount of Cu2+ detection. Inductively coupled plasma atomic emission spectrometry, atomic absorption spectrometry and fluorescent sensors have been employed to detect a trace amount of Cu2+。33-40 Recently, luminescent MOFs sensors for metal ions detection have attracted more and more attention due to their good photostability, high sensitivity, good selectivity and fast response. 41,42 In this work, the SiO2-COOH NPs was used as a template to construct SiO2@ZIF-8 nanosensor for a trace amount of Cu2+ detection for the first time. Scheme 1 demonstrated the synthetic route of SiO2@ZIF-8 and its sensing mechanism. The optical transparency, narrow size distribution and easy preparation of SiO2-COOH NPs made it possible to act as a template to construct core-shell nanostructures. Then the interaction between carboxyl and Zn2+ induced the growth of ZIF-8 on the surface of SiO2-COOH NPs to construct homogeneous core-shell nanostructures. The SiO2@ZIF-8 nanosensor exhibited strong fluorescence due to linker-based emission of imidazole. Upon the addition of Cu2+, the fluorescence was significantly quenched. The quenching effect could be ascribed to the coordination of the pyridyl nitrogen in imidazole of SiO2@ZIF-8 with Cu2+, which could be used to selectively sense a trace amount of Cu2+.
41,43
What’s more, the porous
SiO2@ZIF-8 exhibited extremely good adsorption property and large specific surface area to accumulate Cu2+.44 The SiO2@ZIF-8 nanosensor could be further used to determine Cu2+ in real water samples, while some sensors had to be dispersed in organic solution for use, such as DMSO, MeCN. The core-shell nanostructures of SiO2@ZIF-8 made it possible to disperse directly in aqueous solution and prevented ZIF-8 from aggregation, which enhanced the sensing performances 4
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of the nanosensor.
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DOI: 10.1039/C4AN01773K
2. Materials and methods 2.1 Materials and solutions Zn(NO3)2·6H2O, 2-methylimidazole, succinic anhydride, ethylenediamne tetraacetic acid disodium (EDTA), tetraethoxysilane (TEOS) and (3-aminopropyl)triethoxysilane (APTES) were purchased from Shanghai Aladdin Chemistry Co. Ltd. (Shanghai, China). Ethanol, ammonium hydroxide and N, N-dimethylformamide (DMF) were purchased from Xilong Chemical Co. Ltd. (Guangdong, China). All reagents were of analytical grade and used without further purification. All solutions were prepared with ultra-pure water, purified by a Millipore-Q system (18.2 MΩ cm-1). 2.2 Instrumentation and determinations The measurement of excitation and emission spectra was performed on a Hitachi F-7000 fluorescent spectrophotometer. The emission spectra were monitored using a 381 nm excitation wavelength, while the excitation spectra were recorded at an emission wavelength of 488 nm. Avatar 360 Fourier transform infrared spectra (FTIR) spectrometer (Nicolet, USA) was applied to record FTIR spectra. X-ray powder diffraction (XRD) data were collected on a D/Max 2500 V/PC X-ray powder diffractometer using Cu Kα radiation (λ=0.154056 nm, 40 kV, 200 mA). Transmission electron microscopy (TEM) was carried out on a JEM-20f10 (HR) microscope. The scanning electron microscopy (SEM) image was taken using a XL30 ESEM-FEG SEM at an accelerating voltage of 20 kV equipped with a Phoenix energy dispersive X-ray analyzer. 2.3 Synthesis of SiO2-COOH NPs. SiO2 NPs were synthesized according to a modified sol-gel Stöber approach. 45 Typically, 5.0 mL ethanol (99.9%), 5.0 mL ultra-pure water and 5.0 mL ammonium hydroxide (99.8%) were mixed by 5
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stirring for 20 min. TEOS (200 μL, 97%) dissolved in ethanol (2.0 mL, 99.9%) was added toArticle theOnline View DOI: 10.1039/C4AN01773K
mixed solvents and stirred for 5 h in a water bath (30 °C). Then the solid was formed and collected by centrifuging at 10000 rpm for 10 min and washed with ethanol for 5 times. Finally, the solid was dried in vacumm at 60 °C to obtain SiO2 NPs. Succinic anhydride (2.5 g) and APTES (5.0 mL, 97%) were dissolved with DMF (150 mL) in a 250 mL round-bottom flask. After the above solution was stirred for 3 h in a water bath (30 °C), a DMF solution of SiO2 NPs (40 mL, 0.012g mL-1) was added. After stirring for 12 h, the solid was collected by centrifuging at 10000 rpm for 10 min and washed with ethanol for 5 times. Finally, the solid was dried in vacumm at 60 °C to obtain SiO2-COOH NPs for use in the following experiments. 2.4 Synthesis of SiO2@ZIF-8 core-shell nanostructure The 0.5 mg SiO2-COOH NPs was dispersed in DMF (1000 μL) in a 50 mL three-necked bottle and then a DMF solution of Zn(NO3)2·6H2O (50 mM, 4.0 mL) was added. After stirring for 1 h in an oil bath (150 °C), a DMF solution of 2-methylimidazole (50 mM, 4.0 mL) was added the above solution. Then the mixture was stirred for 2 h and brownish yellow SiO2@ZIF-8 with core-shell nanostructure was obtained. After being cooled to room temperature, the product was collected by centrifugation at 3000 rpm for 10 min and washed with ethanol for three times. Finally, the product was dried in vacumm at 60 °C for 24 h, and the solid was dispersed in 5.0 mL of pure water to form SiO2@ZIF-8 suspension. The synthetic route of SiO2@ZIF-8 was shown in Scheme 1. 2.5 Fluorescent measures The SiO2@ZIF-8 and ZIF-8 suspensions were prepared by dispersing 10 mg SiO2@ZIF-8 or ZIF-8 powder in 100 mL ultra-pure water under ultrasonication for 5 min. Then 10 μL SiO2@ZIF-8 or ZIF-8 supensions and 90 μL HEPES buffer (pH 7, 100 mM) were incubated at room temperature for 30 min and the final concentration of SiO2@ZIF-8 or ZIF-8 was 10 μg mL-1. For quantization of 6
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Cu(NO3)2 concentration, 80 μL HEPES buffer (pH 7, 100 mM) was added into 10 μL SiO2@ZIF-8 View Article Online DOI: 10.1039/C4AN01773K
(100 μg mL-1), and then 10 μL Cu(NO3)2 in concentration the range of 0-10 μM were added to the above solution. The final volume of the reaction solution was 100 μL. The final concentration of SiO2@ZIF-8 was 10 μg mL-1 and the concentration of Cu(NO3)2 in the reaction solutions were in the range of 0-1.0 μM. For interference experiments, the 10 μL 10 μM NaCl, Hg(NO3)2, Cd(NO3)2, NiCl2, MgCl2, AgNO3, FeCl2, FeCl3 and MnCl2 were added to SiO2@ZIF-8 solution (10μL, 100 μg mL-1), respectively. The final concentrations of these substances were 1.0 μM. Finally, the emission spectra at 381 nm excitation of these reaction solutions were measured. 3. Results and discussion 3.1 Characterization of SiO2-COOH NPs, ZIF-8 and SiO2@ZIF-8 NPs. The morphology and structures of as-prepared SiO2@ ZIF-8 NPs were investigated by different characterization techniques. The SEM image showed that the carboxyl-functionalized SiO2 NPs had an average diameter of 380 nm and the surface was smooth (Fig. 1A). The formation of core-shell nanostructures were characterized by SEM and TEM. The SEM image demonstrated that the surface became rough after uniform ZIF-8 grew on the surface of SiO2-COOH NPs at the reaction time of 30 min (Fig. 1B). However, as the reaction time was prolonged to 2 h, more ZIF-8 nanocrystals grew around SiO2-COOH NPs, thus the core-shell nanostructure was obtained (Fig. 1C). The TEM image of SiO2@ZIF-8 showed the particles with an average size of about 460 nm accompanied by the ZIF-8 shell thickness of 40 nm (Fig. 1D). The interaction between carboxyl and Zn2+ was essential to induce the growth of ZIF-8 on the surface of SiO2-COOH NPs and to construct homogeneous core-shell nanostructure. After the 2-methylimidazole and Zn(NO3)2 were mixed in the absence of SiO2-COOH NPs, the SEM image showed some big particles were full of ZIF-8 crystals with about several micrometers’ size (Fig. S1, Supporting Information). 7
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FTIR spectra were used to characterize the SiO2-COOH NPs, ZIF-8 and SiO2@ZIF-8. The bandOnline View Article DOI: 10.1039/C4AN01773K
at 1108 cm-1 was the characteristic peak of Si-O-Si and the band at 1182 cm-1 resulted from the stretching vibration of C-O.
46
The peak at 1635 cm-1 corresponded to the stretching vibration of
C=O in –COOH and –CO-NH-. The peak at 1364 cm-1 was the bending vibration of N-H band and stretching vibration of C-N band. Besides, the characteristic band of –OH stretching vibration appeared at about 3500 cm-1.
26
The results indicated the existing of carboxyl in SiO2-COOH NPs
and successful functionalization of SiO2. For SiO2@ZIF-8, the appearance of characteristic peaks of ZIF-8 nanocrystals further confirmed the successful growth of ZIF-8 on the surface of SiO2-COOH NPs (Fig. 2A)
47
. For example, the bands in the spectra region of 500-1350 cm-1 and 1350-1500
cm-1 were the bending and stretching of imidazole ring, respectively. Furthermore, crystal structure analysis from XRD patterns demonstrated that SiO2 NPs was amorphous, and the characteristic peaks of (011), (002), (112), (022), (013) and (222) were indexed to the pure ZIF-8 nanocrystals. 48 The XRD pattern of SiO2@ZIF-8 also showed the characteristic peaks of ZIF-8 (Fig. 2B), indicating the ZIF-8 nanocrystals were involved in the core-shell nanostructures. 3.2 Fluorescent spectra of core-shell SiO2@ZIF-8 nanostructures and sensing mechanism to Cu2+ The fluorescent spectra of ZIF-8 and SiO2@ZIF-8 were studied. As shown in Fig. 3A, the emission spectrum of SiO2@ZIF-8 showed the maximum emission peak at 485 nm upon excitation at 381 nm. The emission was mainly ascribed to the π−π* transition of 2-methylimidazole in ZIF-8 (Fig. S2, Supporting Information). And the coordination of Zn2+ with 2-methylimidazole did not interrupt the fluorescence of 2-methylimidazole since Zn2+ had filled d-orbital and accordingly the ZIF-8 could produce ligand-centered emission. However, the fluorescence of 2-methylimidazole was enhanced due to the formation of the porous structure and the coordination of Zn2+ with 2-methylimidazole (Fig. S2, Supporting Information).43 As shown in Fig. 3A, it was obvious that 8
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the fluorescence of SiO2@ZIF-8 was higher than that of ZIF-8 at the same concentration. TheOnline View Article DOI: 10.1039/C4AN01773K
stronger emission could be also ascribed to the core-shell nanostructure and accordingly provided higher sensitivity for the detection of analytes. It was noticeable that after 1.0 μM Cu2+ was added into the SiO2@ZIF-8 solution, the fluorescent intensity dramatically decreased. The quenching mechanism might be ascribed to the binding between Cu2+ and pyridyl nitrogen in imidazole instead of Zn2+, which reduced ligand-centered charge transfer and led to the decrease of intraligand luminescent efficiency (Fig S2, Supporting Information). To further prove the binding between Cu2+ and pyridyl nitrogen, 1.0 μM chelating agent EDTA was added into the mixing solution containing SiO2@ZIF-8 and Cu2+, and the efluorescence of 2-methylimidazole was recovered (Fig S3, Supporting information). It was because that EDTA had stronger coordination ability with Cu2+ as compared with that of pyridyl nitrogen sites, leading to the dissociation of Cu2+ from SiO2@ZIF-8-Cue and accordingly enhanced intraligand luminescent efficiency.49 Therefore, the pyridyl nitrogen sites in SiO2@ZIF-8 played a vital role in the recognition of Cu2+. 3.3 Optimal conditions for fluorescent sensing Since Cu2+ could quench the fluorescence of SiO2@ZIF-8, the optimal conditions for the sensing of Cu2+ were investigated in the following experiments. Firstly, it was necessary to find out the suitable SiO2@ZIF-8 concentration. The fluorescence of SiO2@ZIF-8 with concentrations ranged from 1.0 μg mL-1 to 50 μg mL-1 was measured. As shown in Fig.3B, there was a good linear relationship between the fluorescent intensity and the concentration of SiO2@ZIF-8 in the range of 1.0 μg mL-1 to 30 μg mL-1. However, the fluorescent intensity deviated from linear relationship in the higher concentration. The lower concentration would lead to the sensitive quenching effect of fluorophore, however, too low concentration of fluorophore would result in the increase of noise. 9
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Therefore, 10 μg mL-1 was ultimately chosen in the following experiments in order to obtain theOnline View Article DOI: 10.1039/C4AN01773K
maximum signal-to-noise and the lowest detection limit for Cu2+ detection. Then fluorescent response of SiO2@ZIF-8 at different pH was investigated. As shown in Fig. 3C, the pH of the reaction media had obvious effects on the fluorescence of SiO2@ZIF-8. When the pH of the media was smaller than 5, the ligand was protonated to dissociate with the metal ions and accordingly the fluorescence was quenched.17 It was also confirmed by the lower fluorescence of 2-methylimidazole than that of ZIF-8 (Fig. S2, Supporting Information). When the pH values were between 5 and 7, the fluorescence became stable and had a strongest intensity at pH value of 7. The fluorescent intensity decreased as the pH value of media was increased to 8 due to the formation of precipitation.50 We also found that after Cu2+ was added into the reaction media, the fluorescent intensity dramatically decreased. However, the fluorescence showed little changes at various pH values in the presence of Cu2+ due to strong coordination interaction between Cu2+ and 2-methylimidazole. Therefore, pH 7 was chosen as the optimum pH condition. The time-dependent fluorescent response of SiO2@ZIF-8 to Cu2+ was then investigated. As shown in Fig. 3D, the fluorescent intensity of SiO2@ZIF-8 was quenched by 82 % in 15 min after 1.0 μM Cu2+ was added into the aqueous solution of SiO2@ZIF-8. It was maintained in the next 30 min, indicating the reaction could be completed within 15 min. Thus, the solution was kept for 15 min for the fluorescence detection in the following experiments. 3.3 Detection sensitivity to Cu2+ To evaluate the sensitivity of SiO2@ZIF-8 nanosensor to detect Cu2+, different concentrations of Cu2+ were added into HEPES buffer (pH 7, 100 mM) containing 10 μg mL-1 SiO2@ZIF-8 under the above-mentioned optimal experimental conditions. The fluorescence of SiO2@ZIF-8 was obviously quenched as the concentrations of Cu2+ increased (Fig. 4A). The quenching effect was calculated 10
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according to Stern-Volmer equation (1):
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DOI: 10.1039/C4AN01773K
I0/I=1+Ksv [M]
(1)
where I0 and I were the fluorescent intensity of SiO2@ZIF-8 before and after the incorporation of metal ions, respectively. [M] was the concentration of metal ion and Ksv was the coefficient of quenching. As shown in Fig. 4B, there was a good linear relationship between I0/I and Cu2+ concentration in the range of 10-500 nM with a correlation coefficient of 0.9914. Ksv was calculated to be 1.83×106 M-1 which was higher than that of Eu(PDC)1.5 (89.4 M-1) and Eu2(FMA)2(OX)(H2O)4 (528.7 M-1) for Cu2+ detection in DMF solution.
42,51
The detection limit
was calculated to be 3.8 nM based on the ratio of signal-to-noise of 3. The result was much lower than the allowed concentration of Cu2+ (32 μM or 2.0 mg L−1) in drinking water permitted by the World Health Organization (WHO), suggesting the SiO2@ZIF-8 e could be used to detect Cu2+ in drinking water.
52,53
Besides, a comparison of the sensing performances of different fluorescent
probes for Cu2+ was listed in Table, demonstrating that SiO2@ZIF-8 nanostructures sensing system exhibited superior sensitivity to previously reported sensing systems.36-38,40,54-57 Taking C-mpg-C3N4 as an example,40 the detection limit was higher (12.3 nM) than that of the SiO2@ZIF-8, and the linear range was narrower (10-100 nM). As for MB-GO, 57 the linear range was wider (53.3-1333 nM), but the detection limit was higher (53.3 nM). By comparing, it could be clearly seen that the linear range was common, but the detection limit was the lowest. The SiO2@ZIF-8 could be directly used to detect Cu2+ in aqueous solutions, while some sensors mentioned in Table 1 had to be dispersed in organic solution for use, such as DMSO and MeCN. Therefore, the SiO2@ZIF-8 had an advantage in the sensing performance when compared to other works. The good performance of this sensor was ascribed to core-shell nanostructures of SiO2@ZIF-8 and extremely good adsorption property and large specific surface area to accumulate Cu2+. The nanostructures of core-shell 11
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SiO2@ZIF-8 made it possible to disperse directly in aqueous solution and prevented ZIF-8 fromOnline View Article DOI: 10.1039/C4AN01773K
aggregation, which enhanced the sensing performances of the nanosensor. 3.4 Detection selectivity to Cu2+ Selectivity is an important parameter to assess the performance of the nanosensor. In order to evaluate the selectivity of SiO2@ZIF-8, the as-prepared samples were dispersed in aqueous solution containing M(NO3)x or MClx, respectively (M= Na+, Ag+, Ni2+, Mg2+， Fe2+, Fe3+, Mn2+, Hg2+, Cd2+ and Cu2+). The fluorescent intensities of the nanosensor in the presence of interference metal ions were studied under the same conditions. As shown in Fig. 5, no obvious change was observed in the presence of Na+, Ag+, Ni2+, Mg2+， Fe2+, Fe3+, and Mn2+ as compared with that of nanosensor. Besides, there was only a little quenching effect for Hg2+, and slight enhancing effect for Cd2+. The quenching effect for Hg2+ might be ascribed to the fact that the weak binding between Hg2+ and imidazole nitrogen atoms weakened the intraligand luminescent efficiency. While the enhancing effect for Cd2+ was mainly due to the ligand-centered charge transfer to increase the luminescent efficiency.43 However, the little change in the presence of Hg2+ and Cd2+ did not significantly influence Cu2+ detection. All these observations indicated a small influence of other metal ions on this Cu2+ sensing system. The highly specificity of SiO2@ZIF-8 nanosensor is ascribed to a faster chelating process and higher thermodynamic affinity of Cu2+ with nitrogen of ZIF-8 over other transition metal ions.58 Thus, the SiO2@ZIF-8 nanosensor exhibited good selectivity and was expected to be used for Cu2+ detection. 3.5 Sensing application in practical water samples The core-shell SiO2@ZIF-8 nanosensor was used to detect Cu2+ in tap water and Yaohu River for assessing the practicability. The water samples were filtered with a membrane (0.22 μm) and then the Cu2+ with different concentrations was spiked. The added standard Cu2+ could be measured 12
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with good recoveries (about 102%) (Table S1, Supporting Information). It suggested that theView results Article Online DOI: 10.1039/C4AN01773K
obtained by using the core-shell SiO2@ZIF-8 nanosensor were accurate and credible. Therefore, the SiO2@ZIF-8 core-shell nanosensor was capable to monitor a trace amount of Cu2+ in water samples. 4．Conclusion In summary, a fluorescent SiO2@ZIF-8 core-shell nanosensor for a trace amount of Cu2+ detection was fabricated. The SiO2-COOH NPs acted as a template to induce the growth of ZIF-8 on the surface to construct SiO2@ZIF-8 with core-shell nanostructure. The pyridyl nitrogen sites in imidazole played vital roles in the recognition of Cu2+. The porous core-shell nanostructure of SiO2@ZIF-8 resulted in larger surface areas as compared with pure ZIF-8, providing more binding sites to interact with Cu2+. The core-shell SiO2@ZIF-8 could well disperse in aqueous solution and showed high sensitivity as well as good selectivity as nanosensor for Cu2+ detection. The core-shell SiO2@ZIF-8 could be also applied for a trace amount of Cu2+ sensing in real water samples with good recoveries. Acknowledgements This work was financially supported by National Natural Science Foundation of China (21165010, 21465014 and 21465015), Young Scientist Foundation of Jiangxi Province (20122BCB23011), Foundation of Jiangxi Educational Committee (GJJ13244), Natural Science Foundation of Jiangxi Province (20142BAB203101 and 20143ACB21016), The Ministry of Education by the Specialized Research Fund for the Doctoral Program of Higher Education (20133604110002) and the Open Project Program of Key Laboratory of Functional Small organic molecule, Ministry of Education, Jiangxi Normal University (No. KLFS-KF-201214 and KLFS-KF-201218). 13
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References
View Article Online
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1.
J. Lee, O. K. Farha, J. Roberts, K. A. Scheidt, S. T. Nguyen and J. T. Hupp, Chem. Soc. Rev. 2009, 38, 1450-1459.
2.
M. P. Suh, H. J. Park, T. K. Prasad and D.-W. Lim, Chem. Rev. 2011, 112, 782-835.
3.
P. Horcajada, T. Chalati, C. Serre, B. Gillet, C. Sebrie, T. Baati, J. F. Eubank, D. Heurtaux, P. Clayette and C. Kreuz, Nat. Mater. 2010, 9, 172-178.
4.
J. An, S. J. Geib and N. L. Rosi, J. Am. Chem. Soc. 2009, 131, 8376-8377.
5.
A. Cadiau, C. D. Brites, P. M. Costa, R. A. Ferreira, J. Rocha and L. D. Carlos, ACS Nano 2013, 7, 7213-7218.
6.
Y. Zhang, Y. Luo, J. Tian, A. M. Asiri, A. O. Al-Youbi and X. Sun, PloS one, 2012, 7, e30426.
7.
J. Tian, S. Liu, Y. Luo and X. Sun, Catal. Sci. & Technol. 2012, 2, 432-436.
8.
W. Lu, X. Qin, A. M. Asiri, A. O. Al-Youbi and X. Sun, Analyst 2013, 138, 429-433.
9.
H. Li, J. Zhai and X. Sun, RSC Adv. 2011, 1, 725-730.
10.
H. Li, L. Wang, J. Zhai, Y. Zhang, J. Tian and X. Sun, Macromol. Rapid Commun. 2011, 32, 899-904.
11.
H. Li and X. Sun, Chem. Commun. 2011, 47, 2625-2627.
12.
X. Sun, S. Dong and E. Wang, J. Am. Chem. Soc. 2005, 127, 13102-13103.
13.
J. Li, S. Guo and E. Wang, RSC Adv. 2012, 2, 3579-3586.
14.
H. Wei, B. Li, Y. Du, S. Dong and E. Wang, Chem. Mater. 2007, 19, 2987-2993.
15.
H. Tan and Y. Chen, Chem. Commun. 2011, 47, 12373-12375.
16.
Y. Yuan, W. Wang, L. Qiu, F. Peng, X. Jiang, A. Xie, Y. Shen, X. Tian and L. Zhang, Mater. Chem. Phys. 2011, 131, 358-361. 14
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1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
Page 14 of 26
Page 15 of 26
17.
H. Tan, C. Ma, L. Chen, F. Xu, S. Chen and L. Wang, Sens. Actuators. B. 2014, 190,Online View Article DOI: 10.1039/C4AN01773K
621-626. 18.
M. Zhao, K. Deng, L. He, Y. Liu, G. Li, H. Zhao and Z. Tang, J. Am. Chem. Soc. 2014, 136-.
19.
J. Zheng, C. Cheng, W. Fang, C. Chen, R.-W. Yan, H.-X. Huai and C.-C. Wang, CrystEngComm 2014, 16, 3960-3964.
20.
H. Lee, J. Park, S. Choi, J. Son and M. Oh, Small 2013, 9, 490-490.
21.
K. Koh, A. G. Wong-Foy and A. J. Matzger, Chem. Commun. 2009, 6162-6164.
22.
S. Furukawa, K. Hirai, K. Nakagawa, Y. Takashima, R. Matsuda, T. Tsuruoka, M. Kondo, R. Haruki, D. Tanaka and H. Sakamoto, Angew. Chem. Int. Ed. 2009, 48, 1766-1770.
23.
F. Ke, L. Qiu, Y. Yuan, X. Jiang and J. Zhu, J. Mater. Chem. 2012, 22, 9497-9500.
24.
H. Lee, W. Cho and M. Oh, Chem. Commun. 2012, 48, 221-223.
25.
W. Zhan, Q. Kuang, J. Zhou, X. Kong, Z. Xie and L. Zheng, J. Am. Chem. Soc. 2013, 135, 1926-1933.
26.
Y. Fu, C. Yang and X. Yan, Chem. Eur. J. 2013, 19, 13484-13491.
27.
C. Wang, K. deKrafft and W. Lin, J. Am. Chem. Soc. 2012, 134, 7211-7214.
28.
S. Wei, Q. Wang, J. Zhu, L. Sun, H. Lin and Z. Guo, Nanoscale 2011, 3, 4474-4502.
29.
G. Lu, S. Li, Z. Guo, O. K. Farha, B. G. Hauser, X. Qi, Y. Wang, X. Wang, S. Han and X. Liu, Nat. Chem. 2012, 4, 310-316.
30.
C. Jo, H. J. Lee and M. Oh, Adv. Mater. 2011, 23, 1716-1719.
31.
S. VanáSnick, Chem. Commun. 2010, 46, 6329-6331.
32.
K. J. Barnham, C. L. Masters and A. I. Bush, Nat. Rev. Dru. Discovery. 2004, 3, 205-214.
33.
J. Chen and K. C. Teo, Anal. Chim. Acta. 2001, 450, 215-222.
34.
K. Sreenivasa Rao, T. Balaji, T. Prasada Rao, Y. Babu and G. Naidu, Spectrochim. Acta, Part 15
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Analyst
Analyst
B 2002, 57, 1333-1338.
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DOI: 10.1039/C4AN01773K
35.
M. Liu, H. Zhao, S. Chen, H. Yu, Y. Zhang and X. Quan, Biosens. Bioelectron. 2011, 26, 4111-4116.
36.
G.-Y. Lan, C.-C. Huang and H.-T. Chang, Chem. Commun. 2010, 46, 1257-1259.
37.
Y.-H. Chan, J. Chen, Q. Liu, S. E. Wark, D. H. Son and J. D. Batteas, Anal. Chem. 2010, 82, 3671-3678.
38.
J. Zheng, C. Xiao, Q. Fei, M. Li, B. Wang, G. Feng, H. Yu, Y. Huan and Z. Song, Nanotechnology 2010, 21, 045501.
39.
S. Liu, J. Tian, L. Wang, Y. Zhang, X. Qin, Y. Luo, A. M. Asiri, A. O. Al‐Youbi and X. Sun, Adv. Mater. 2012, 24, 2037-2041.
40.
E. Z. Lee, Y. S. Jun, W. H. Hong, A. Thomas and M. M. Jin, Angew. Chem. Int. Ed. 2010, 49, 9706-9710.
41.
B. Chen, L. Wang, Y. Xiao, F. R. Fronczek, M. Xue, Y. Cui and G. Qian, Angew. Chem. Int. Ed. 2009, 48, 500-503.
42.
Y. Xiao, Y. Cui, Q. Zheng, S. Xiang, G. Qian and B. Chen, Chem. Commume. 2010, 46, 5503-5505.
43.
S. Liu, Z. Xiang, Z. Hu, X. Zheng and D. Cao, J. Mater. Chem. 2011, 21, 6649-6653.
44.
Y. Dong, R.Wang, G. Li, C. Chen, Y. Chi and G. Chen, Anal. Chem. 2012, 84, 6220-6224.
45.
C. J. Brinker and G. W. Scherer, Sol-gel Sci Gulf Professional Publishing, 1990.
46.
D. Buso, K. M. Nairn, M. Gimona, A. J. Hill and P. Falcaro, Chem. Mater. 2011, 23, 929-934.
47.
J. Cravillon, R. Nayuk, S. Springer, A. Feldhoff, K. Huber and M. Wiebcke, Chem. Mater. 2011, 23, 2130-2141. 16
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Page 16 of 26
Page 17 of 26
48.
A. Schejn, L. Balan, V. Falk, L. Aranda, G. Medjahdi and R. Schneider, CrystEngComm View Article Online DOI: 10.1039/C4AN01773K
2014, 16, 4493-4500. 49.
J. K. Yang and A. P. Davis, J. Colloid. Interface. Sci. 1999, 216, 77-85.
50.
H. Tan, C. Ma, Y. Song, F. Xu, S. Chen and L. Wang, Biosens. Bioelectron. 2013, 50, 447-452.
51.
G. Yang, M. Li, S. Li, Y. Lan, W. He, X. Wang, J.-S. Qin and Z. Su, J. Mater. Chem. 2012, 22, 17947-17953.
52.
M. Araya, M. Olivares, F. Pizarro, A. Llanos, G. Figueroa and R. Uauy, Environ. Health. Perspect. 2004, 112, 1068.
53.
O. Akoto and J. Adiyiah, Int. J. Environ. Sci. Technol. 2007, 4, 211-214.
54.
Z. Xu, L. Zhang, R. Guo, T. Xiang, C. Wu, Z. Zheng and F. Yang, Sens. Actuators. B. 2011, 156, 546-552.
55.
L. Tang and M. Cai, Sens. Actuators, B. 2012, 173, 862-867.
56.
J. Liu, L. Lin, X. Wang, S. Lin, W. Cai, L. Zhang and Z. Zheng, Analyst 2012, 137, 2637-2642.
57.
J. Huang, Q. Zheng, J. Kim and Z. Li, Biosens. Bioelectron. 2013, 43, 379-383.
58.
A. P. De Silva, H. N. Gunaratne, T. Gunnlaugsson, A. J. Huxley, C. P. McCoy, J. T. Rademacher and T. E. Rice, Chem. Rev. 1997, 97, 1515-1566.
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Figure caption
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Scheme 1. Synthetic route for core-shell SiO2@ZIF-8 nanostructures and sensing mechanism of core-shell SiO2@ZIF-8 nanostructures for Cu2+. Fig.1. SEM images of SiO2 NPs (A), SiO2@ZIF-8 reacted for 30 min (B), 2 h (C) and TEM image of SiO2@ZIF-8 reacted for 2 h (D). Fig.2. FTIR spectra of SiO2, ZIF-8 and SiO2@ZIF-8 core-shell nanostructure (A); XRD patterns of SiO2 NPs, ZIF-8 and SiO2@ZIF-8 core-shell nanostructure (B). Fig.3. Excitation (left) and emission (right) spectrum of ZIF-8 and SiO2@ZIF-8 core-shell nanostructures. The concentration was 10 μg mL-1 (A). Concentration-dependent fluorescent response of SiO2@ZIF-8 (B). The fluorescent response of SiO2@ZIF-8 in the presence (red) and absence (black) of 1.0 μM Cu2+ at different pH values (C). Time-dependent fluorescence response of SiO2@ZIF-8 to 1.0 μM Cu2+ at pH 7 HEPES buffer (100 mM) (D). Fig.4. Fluorescent emission spectra of SiO2@ZIF-8 sensor exposed to various concentrations of Cu2+ 0, 10, 50, 100, 200, 300, 400, 500 nM from top to bottom (A). Fluorescent intensity ratio before (I0) and after (I) the addition of Cu2+ versus the concentration of Cu2+ (B). The concentration of SiO2@ZIF-8 was 10 μg mL−1. The excitation wavelength was 381 nm. Fig.5. The fluorescent response of SiO2@ZIF-8 (10 μg mL-1) in pH 7 HEPES buffer towards various metal ions (1.0 μM).
18
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Table 1. Comparison of sensing performance of different fluorescent probes for Cu2+ detection. View Article Online DOI: 10.1039/C4AN01773K
a
Fluorescent probes
Detection limit (nM)
Linear range (nM)
Ref.
DNA-AgNCsa
8
10-200
36
16-MHA-capped CdSe QDsb
5
5-1×105
37
Peptide-coated ZnS QDs
500
0-2.6×104
38
c-mpg-C3N4c
12.3
10-100
40
Rhodamine-based derivative
3.42×103
10×103-3×105
54
Benzimidazole-based molecular
18.2
0-3×103
55
CDs-BSAd
5.8×10-4
2.0×10-3-1.5
56
MB-GOe
53.3
53.3-1333
57
SiO2@ZIF-8
3.8
10-500
this work
DNA modified Ag nanoclusters
b
16-mercaptohexadecanoic capped CdSe quantumn dots
c
c-mpg-C3N4 Cubic mesopprous graphite carbon nitride
d
CDs-BSA bovine serum albumin modified carbon dots
e
MB molecular beacons GO graphene oxide
19
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Scheme 1
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Fig. 1
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Fig. 2
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Fig. 3
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Fig. 4
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Table of Contents
Text: Formation of core-shell SiO2@ZIF-8 nanostructures for Cu2+ detection
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Page 1 of 26
Fabrication of fluorescent SiO2@ zeolitic imidazolate framework-8 nanosensor for Cu2+ detection
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DOI: 10.1039/C4AN01773K
Yonghai Song, Dongqin Hu, Fenfen Liu, Shouhui Chen and Li Wang∗
Key Laboratory of Functional Small Organic Molecule, Ministry of Education, College of Chemistry and Chemical Engineering, Jiangxi Normal University, Nanchang 330022, People’s Republic of China
∗ Corresponding author: Tel/Fax: +86 791 88120861. E-mail: [email protected] (L. Wang). 1
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Abstract
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A simple strategy to fabricate fluorescent SiO2@zeolitic imidazolate framework-8 (ZIF-8) core-shell nanosensor for Cu2+ detection was demonstrated in this work. The nanosensor was synthesized using carboxyl-functionalized SiO2 nanoparticles (SiO2 NPs) as a template to induce the growth of ZIF-8 on its surface. The porous SiO2@ZIF-8 exhibited extremely good adsorption property and large specific surface area to accumulate Cu2+ and the pyridyl nitrogen sites in imidazole played vital roles in the recognition of Cu2+. The fluorescent intensity decreased linearly with the increasing of Cu2+ concentration in the range of 10-500 nM and the detection limit was estimated to be 3.8 nM. The SiO2@ZIF-8 nanosensor could be further used to determine a trace amount of Cu2+ in real water samples, while some previous sensors had to be dispersed in organic solution for use, such as DMSO and MeCN. The core-shell nanostructures of SiO2@ZIF-8 made it possible to disperse directly in aqueous solution and prevented ZIF-8 from aggregation, which enhanced the sensing performances of the SiO2@ZIF-8 nanosensor. Keywords: Fluorescent nanosensor; SiO2@ZIF-8; Core-shell nanostructure; Cu2+ detection
2
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1. Introduction
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Metal-organic frameworks (MOFs) have emerged as interesting functional materials due to their tunable structures with high thermal stability, organic functionality, large pore sizes, open metal sites in the skeleton and high surface area.
1-5
Great efforts have been made to prepare MOFs with
different shapes and sizes for special purposes.6-12 Among them, luminescent MOFs originated from the luminescent ability of metal nodes or the intrinsic luminescence of organic ligands is attracting extensive interesting. Particularly, luminescent nanoscale MOFs (NMOFs) takes advantage of the excellent properties of both MOFs and nanomaterials and shows enhanced properties for practical applications. 13,14 Therefore, luminescent NMOFs have been found applications for detecting metal ions, molecules and temperature. 15-17 Core-shell MOFs have received intensive attention due to their new or enhanced physicochemical properties as compared with their counterparts.18-20 A few core-shell MOFs structures such as MOF@MOF porous structures, SiO2@MOF architectures, polystyrene (PS)@MOF films, Fe3O4 nanoparticles (NPs)@MOF and qutumn dots (QDs) or metal NPs@MOFs have been developed. 20-29
Template-assisted strategy is an effective approach to synthesize core-shell MOFs. 26 Generally,
the templates are stabilized by surfactant, capping agents or ions before the introduction of MOF precursor solution, and then the functional groups initiate the binding with metal ions to confine the growth of MOFs on the surface of the templates. For example, Oh and co-workers successfully prepared uniform SiO2@coordination polymers (CPs) by introducing the carboxyl-terminated SiO2 NPs (SiO2-COOH NPs) into the CPs precursor solution. The shell thickness could be easily adjusted by changing the amount of reactants.30 Qiu and Yan used mercaptoacetic acid-functionalized
Fe3O4
and
carboxyl-terminated
PS
to
fabricate
Fe3O4@Cu3(benzene-1,3,5-tricarboxylate)2 and PS@zeolitic imidazolate framework-8 (ZIF-8) with 3
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step-by-step strategy to adjust the shell thickness. 23,24
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Copper is widely used in our daily life. However, excessive Cu2+ will result in pathological and physiological events, such as Alzeimer’s disease and Wilson disease.31, 32 Therefore, it is of great significance to develop a facile and efficient method for a trace amount of Cu2+ detection. Inductively coupled plasma atomic emission spectrometry, atomic absorption spectrometry and fluorescent sensors have been employed to detect a trace amount of Cu2+。33-40 Recently, luminescent MOFs sensors for metal ions detection have attracted more and more attention due to their good photostability, high sensitivity, good selectivity and fast response. 41,42 In this work, the SiO2-COOH NPs was used as a template to construct SiO2@ZIF-8 nanosensor for a trace amount of Cu2+ detection for the first time. Scheme 1 demonstrated the synthetic route of SiO2@ZIF-8 and its sensing mechanism. The optical transparency, narrow size distribution and easy preparation of SiO2-COOH NPs made it possible to act as a template to construct core-shell nanostructures. Then the interaction between carboxyl and Zn2+ induced the growth of ZIF-8 on the surface of SiO2-COOH NPs to construct homogeneous core-shell nanostructures. The SiO2@ZIF-8 nanosensor exhibited strong fluorescence due to linker-based emission of imidazole. Upon the addition of Cu2+, the fluorescence was significantly quenched. The quenching effect could be ascribed to the coordination of the pyridyl nitrogen in imidazole of SiO2@ZIF-8 with Cu2+, which could be used to selectively sense a trace amount of Cu2+.
41,43
What’s more, the porous
SiO2@ZIF-8 exhibited extremely good adsorption property and large specific surface area to accumulate Cu2+.44 The SiO2@ZIF-8 nanosensor could be further used to determine Cu2+ in real water samples, while some sensors had to be dispersed in organic solution for use, such as DMSO, MeCN. The core-shell nanostructures of SiO2@ZIF-8 made it possible to disperse directly in aqueous solution and prevented ZIF-8 from aggregation, which enhanced the sensing performances 4
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of the nanosensor.
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DOI: 10.1039/C4AN01773K
2. Materials and methods 2.1 Materials and solutions Zn(NO3)2·6H2O, 2-methylimidazole, succinic anhydride, ethylenediamne tetraacetic acid disodium (EDTA), tetraethoxysilane (TEOS) and (3-aminopropyl)triethoxysilane (APTES) were purchased from Shanghai Aladdin Chemistry Co. Ltd. (Shanghai, China). Ethanol, ammonium hydroxide and N, N-dimethylformamide (DMF) were purchased from Xilong Chemical Co. Ltd. (Guangdong, China). All reagents were of analytical grade and used without further purification. All solutions were prepared with ultra-pure water, purified by a Millipore-Q system (18.2 MΩ cm-1). 2.2 Instrumentation and determinations The measurement of excitation and emission spectra was performed on a Hitachi F-7000 fluorescent spectrophotometer. The emission spectra were monitored using a 381 nm excitation wavelength, while the excitation spectra were recorded at an emission wavelength of 488 nm. Avatar 360 Fourier transform infrared spectra (FTIR) spectrometer (Nicolet, USA) was applied to record FTIR spectra. X-ray powder diffraction (XRD) data were collected on a D/Max 2500 V/PC X-ray powder diffractometer using Cu Kα radiation (λ=0.154056 nm, 40 kV, 200 mA). Transmission electron microscopy (TEM) was carried out on a JEM-20f10 (HR) microscope. The scanning electron microscopy (SEM) image was taken using a XL30 ESEM-FEG SEM at an accelerating voltage of 20 kV equipped with a Phoenix energy dispersive X-ray analyzer. 2.3 Synthesis of SiO2-COOH NPs. SiO2 NPs were synthesized according to a modified sol-gel Stöber approach. 45 Typically, 5.0 mL ethanol (99.9%), 5.0 mL ultra-pure water and 5.0 mL ammonium hydroxide (99.8%) were mixed by 5
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stirring for 20 min. TEOS (200 μL, 97%) dissolved in ethanol (2.0 mL, 99.9%) was added toArticle theOnline View DOI: 10.1039/C4AN01773K
mixed solvents and stirred for 5 h in a water bath (30 °C). Then the solid was formed and collected by centrifuging at 10000 rpm for 10 min and washed with ethanol for 5 times. Finally, the solid was dried in vacumm at 60 °C to obtain SiO2 NPs. Succinic anhydride (2.5 g) and APTES (5.0 mL, 97%) were dissolved with DMF (150 mL) in a 250 mL round-bottom flask. After the above solution was stirred for 3 h in a water bath (30 °C), a DMF solution of SiO2 NPs (40 mL, 0.012g mL-1) was added. After stirring for 12 h, the solid was collected by centrifuging at 10000 rpm for 10 min and washed with ethanol for 5 times. Finally, the solid was dried in vacumm at 60 °C to obtain SiO2-COOH NPs for use in the following experiments. 2.4 Synthesis of SiO2@ZIF-8 core-shell nanostructure The 0.5 mg SiO2-COOH NPs was dispersed in DMF (1000 μL) in a 50 mL three-necked bottle and then a DMF solution of Zn(NO3)2·6H2O (50 mM, 4.0 mL) was added. After stirring for 1 h in an oil bath (150 °C), a DMF solution of 2-methylimidazole (50 mM, 4.0 mL) was added the above solution. Then the mixture was stirred for 2 h and brownish yellow SiO2@ZIF-8 with core-shell nanostructure was obtained. After being cooled to room temperature, the product was collected by centrifugation at 3000 rpm for 10 min and washed with ethanol for three times. Finally, the product was dried in vacumm at 60 °C for 24 h, and the solid was dispersed in 5.0 mL of pure water to form SiO2@ZIF-8 suspension. The synthetic route of SiO2@ZIF-8 was shown in Scheme 1. 2.5 Fluorescent measures The SiO2@ZIF-8 and ZIF-8 suspensions were prepared by dispersing 10 mg SiO2@ZIF-8 or ZIF-8 powder in 100 mL ultra-pure water under ultrasonication for 5 min. Then 10 μL SiO2@ZIF-8 or ZIF-8 supensions and 90 μL HEPES buffer (pH 7, 100 mM) were incubated at room temperature for 30 min and the final concentration of SiO2@ZIF-8 or ZIF-8 was 10 μg mL-1. For quantization of 6
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Cu(NO3)2 concentration, 80 μL HEPES buffer (pH 7, 100 mM) was added into 10 μL SiO2@ZIF-8 View Article Online DOI: 10.1039/C4AN01773K
(100 μg mL-1), and then 10 μL Cu(NO3)2 in concentration the range of 0-10 μM were added to the above solution. The final volume of the reaction solution was 100 μL. The final concentration of SiO2@ZIF-8 was 10 μg mL-1 and the concentration of Cu(NO3)2 in the reaction solutions were in the range of 0-1.0 μM. For interference experiments, the 10 μL 10 μM NaCl, Hg(NO3)2, Cd(NO3)2, NiCl2, MgCl2, AgNO3, FeCl2, FeCl3 and MnCl2 were added to SiO2@ZIF-8 solution (10μL, 100 μg mL-1), respectively. The final concentrations of these substances were 1.0 μM. Finally, the emission spectra at 381 nm excitation of these reaction solutions were measured. 3. Results and discussion 3.1 Characterization of SiO2-COOH NPs, ZIF-8 and SiO2@ZIF-8 NPs. The morphology and structures of as-prepared SiO2@ ZIF-8 NPs were investigated by different characterization techniques. The SEM image showed that the carboxyl-functionalized SiO2 NPs had an average diameter of 380 nm and the surface was smooth (Fig. 1A). The formation of core-shell nanostructures were characterized by SEM and TEM. The SEM image demonstrated that the surface became rough after uniform ZIF-8 grew on the surface of SiO2-COOH NPs at the reaction time of 30 min (Fig. 1B). However, as the reaction time was prolonged to 2 h, more ZIF-8 nanocrystals grew around SiO2-COOH NPs, thus the core-shell nanostructure was obtained (Fig. 1C). The TEM image of SiO2@ZIF-8 showed the particles with an average size of about 460 nm accompanied by the ZIF-8 shell thickness of 40 nm (Fig. 1D). The interaction between carboxyl and Zn2+ was essential to induce the growth of ZIF-8 on the surface of SiO2-COOH NPs and to construct homogeneous core-shell nanostructure. After the 2-methylimidazole and Zn(NO3)2 were mixed in the absence of SiO2-COOH NPs, the SEM image showed some big particles were full of ZIF-8 crystals with about several micrometers’ size (Fig. S1, Supporting Information). 7
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FTIR spectra were used to characterize the SiO2-COOH NPs, ZIF-8 and SiO2@ZIF-8. The bandOnline View Article DOI: 10.1039/C4AN01773K
at 1108 cm-1 was the characteristic peak of Si-O-Si and the band at 1182 cm-1 resulted from the stretching vibration of C-O.
46
The peak at 1635 cm-1 corresponded to the stretching vibration of
C=O in –COOH and –CO-NH-. The peak at 1364 cm-1 was the bending vibration of N-H band and stretching vibration of C-N band. Besides, the characteristic band of –OH stretching vibration appeared at about 3500 cm-1.
26
The results indicated the existing of carboxyl in SiO2-COOH NPs
and successful functionalization of SiO2. For SiO2@ZIF-8, the appearance of characteristic peaks of ZIF-8 nanocrystals further confirmed the successful growth of ZIF-8 on the surface of SiO2-COOH NPs (Fig. 2A)
47
. For example, the bands in the spectra region of 500-1350 cm-1 and 1350-1500
cm-1 were the bending and stretching of imidazole ring, respectively. Furthermore, crystal structure analysis from XRD patterns demonstrated that SiO2 NPs was amorphous, and the characteristic peaks of (011), (002), (112), (022), (013) and (222) were indexed to the pure ZIF-8 nanocrystals. 48 The XRD pattern of SiO2@ZIF-8 also showed the characteristic peaks of ZIF-8 (Fig. 2B), indicating the ZIF-8 nanocrystals were involved in the core-shell nanostructures. 3.2 Fluorescent spectra of core-shell SiO2@ZIF-8 nanostructures and sensing mechanism to Cu2+ The fluorescent spectra of ZIF-8 and SiO2@ZIF-8 were studied. As shown in Fig. 3A, the emission spectrum of SiO2@ZIF-8 showed the maximum emission peak at 485 nm upon excitation at 381 nm. The emission was mainly ascribed to the π−π* transition of 2-methylimidazole in ZIF-8 (Fig. S2, Supporting Information). And the coordination of Zn2+ with 2-methylimidazole did not interrupt the fluorescence of 2-methylimidazole since Zn2+ had filled d-orbital and accordingly the ZIF-8 could produce ligand-centered emission. However, the fluorescence of 2-methylimidazole was enhanced due to the formation of the porous structure and the coordination of Zn2+ with 2-methylimidazole (Fig. S2, Supporting Information).43 As shown in Fig. 3A, it was obvious that 8
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the fluorescence of SiO2@ZIF-8 was higher than that of ZIF-8 at the same concentration. TheOnline View Article DOI: 10.1039/C4AN01773K
stronger emission could be also ascribed to the core-shell nanostructure and accordingly provided higher sensitivity for the detection of analytes. It was noticeable that after 1.0 μM Cu2+ was added into the SiO2@ZIF-8 solution, the fluorescent intensity dramatically decreased. The quenching mechanism might be ascribed to the binding between Cu2+ and pyridyl nitrogen in imidazole instead of Zn2+, which reduced ligand-centered charge transfer and led to the decrease of intraligand luminescent efficiency (Fig S2, Supporting Information). To further prove the binding between Cu2+ and pyridyl nitrogen, 1.0 μM chelating agent EDTA was added into the mixing solution containing SiO2@ZIF-8 and Cu2+, and the efluorescence of 2-methylimidazole was recovered (Fig S3, Supporting information). It was because that EDTA had stronger coordination ability with Cu2+ as compared with that of pyridyl nitrogen sites, leading to the dissociation of Cu2+ from SiO2@ZIF-8-Cue and accordingly enhanced intraligand luminescent efficiency.49 Therefore, the pyridyl nitrogen sites in SiO2@ZIF-8 played a vital role in the recognition of Cu2+. 3.3 Optimal conditions for fluorescent sensing Since Cu2+ could quench the fluorescence of SiO2@ZIF-8, the optimal conditions for the sensing of Cu2+ were investigated in the following experiments. Firstly, it was necessary to find out the suitable SiO2@ZIF-8 concentration. The fluorescence of SiO2@ZIF-8 with concentrations ranged from 1.0 μg mL-1 to 50 μg mL-1 was measured. As shown in Fig.3B, there was a good linear relationship between the fluorescent intensity and the concentration of SiO2@ZIF-8 in the range of 1.0 μg mL-1 to 30 μg mL-1. However, the fluorescent intensity deviated from linear relationship in the higher concentration. The lower concentration would lead to the sensitive quenching effect of fluorophore, however, too low concentration of fluorophore would result in the increase of noise. 9
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Therefore, 10 μg mL-1 was ultimately chosen in the following experiments in order to obtain theOnline View Article DOI: 10.1039/C4AN01773K
maximum signal-to-noise and the lowest detection limit for Cu2+ detection. Then fluorescent response of SiO2@ZIF-8 at different pH was investigated. As shown in Fig. 3C, the pH of the reaction media had obvious effects on the fluorescence of SiO2@ZIF-8. When the pH of the media was smaller than 5, the ligand was protonated to dissociate with the metal ions and accordingly the fluorescence was quenched.17 It was also confirmed by the lower fluorescence of 2-methylimidazole than that of ZIF-8 (Fig. S2, Supporting Information). When the pH values were between 5 and 7, the fluorescence became stable and had a strongest intensity at pH value of 7. The fluorescent intensity decreased as the pH value of media was increased to 8 due to the formation of precipitation.50 We also found that after Cu2+ was added into the reaction media, the fluorescent intensity dramatically decreased. However, the fluorescence showed little changes at various pH values in the presence of Cu2+ due to strong coordination interaction between Cu2+ and 2-methylimidazole. Therefore, pH 7 was chosen as the optimum pH condition. The time-dependent fluorescent response of SiO2@ZIF-8 to Cu2+ was then investigated. As shown in Fig. 3D, the fluorescent intensity of SiO2@ZIF-8 was quenched by 82 % in 15 min after 1.0 μM Cu2+ was added into the aqueous solution of SiO2@ZIF-8. It was maintained in the next 30 min, indicating the reaction could be completed within 15 min. Thus, the solution was kept for 15 min for the fluorescence detection in the following experiments. 3.3 Detection sensitivity to Cu2+ To evaluate the sensitivity of SiO2@ZIF-8 nanosensor to detect Cu2+, different concentrations of Cu2+ were added into HEPES buffer (pH 7, 100 mM) containing 10 μg mL-1 SiO2@ZIF-8 under the above-mentioned optimal experimental conditions. The fluorescence of SiO2@ZIF-8 was obviously quenched as the concentrations of Cu2+ increased (Fig. 4A). The quenching effect was calculated 10
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according to Stern-Volmer equation (1):
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DOI: 10.1039/C4AN01773K
I0/I=1+Ksv [M]
(1)
where I0 and I were the fluorescent intensity of SiO2@ZIF-8 before and after the incorporation of metal ions, respectively. [M] was the concentration of metal ion and Ksv was the coefficient of quenching. As shown in Fig. 4B, there was a good linear relationship between I0/I and Cu2+ concentration in the range of 10-500 nM with a correlation coefficient of 0.9914. Ksv was calculated to be 1.83×106 M-1 which was higher than that of Eu(PDC)1.5 (89.4 M-1) and Eu2(FMA)2(OX)(H2O)4 (528.7 M-1) for Cu2+ detection in DMF solution.
42,51
The detection limit
was calculated to be 3.8 nM based on the ratio of signal-to-noise of 3. The result was much lower than the allowed concentration of Cu2+ (32 μM or 2.0 mg L−1) in drinking water permitted by the World Health Organization (WHO), suggesting the SiO2@ZIF-8 e could be used to detect Cu2+ in drinking water.
52,53
Besides, a comparison of the sensing performances of different fluorescent
probes for Cu2+ was listed in Table, demonstrating that SiO2@ZIF-8 nanostructures sensing system exhibited superior sensitivity to previously reported sensing systems.36-38,40,54-57 Taking C-mpg-C3N4 as an example,40 the detection limit was higher (12.3 nM) than that of the SiO2@ZIF-8, and the linear range was narrower (10-100 nM). As for MB-GO, 57 the linear range was wider (53.3-1333 nM), but the detection limit was higher (53.3 nM). By comparing, it could be clearly seen that the linear range was common, but the detection limit was the lowest. The SiO2@ZIF-8 could be directly used to detect Cu2+ in aqueous solutions, while some sensors mentioned in Table 1 had to be dispersed in organic solution for use, such as DMSO and MeCN. Therefore, the SiO2@ZIF-8 had an advantage in the sensing performance when compared to other works. The good performance of this sensor was ascribed to core-shell nanostructures of SiO2@ZIF-8 and extremely good adsorption property and large specific surface area to accumulate Cu2+. The nanostructures of core-shell 11
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SiO2@ZIF-8 made it possible to disperse directly in aqueous solution and prevented ZIF-8 fromOnline View Article DOI: 10.1039/C4AN01773K
aggregation, which enhanced the sensing performances of the nanosensor. 3.4 Detection selectivity to Cu2+ Selectivity is an important parameter to assess the performance of the nanosensor. In order to evaluate the selectivity of SiO2@ZIF-8, the as-prepared samples were dispersed in aqueous solution containing M(NO3)x or MClx, respectively (M= Na+, Ag+, Ni2+, Mg2+， Fe2+, Fe3+, Mn2+, Hg2+, Cd2+ and Cu2+). The fluorescent intensities of the nanosensor in the presence of interference metal ions were studied under the same conditions. As shown in Fig. 5, no obvious change was observed in the presence of Na+, Ag+, Ni2+, Mg2+， Fe2+, Fe3+, and Mn2+ as compared with that of nanosensor. Besides, there was only a little quenching effect for Hg2+, and slight enhancing effect for Cd2+. The quenching effect for Hg2+ might be ascribed to the fact that the weak binding between Hg2+ and imidazole nitrogen atoms weakened the intraligand luminescent efficiency. While the enhancing effect for Cd2+ was mainly due to the ligand-centered charge transfer to increase the luminescent efficiency.43 However, the little change in the presence of Hg2+ and Cd2+ did not significantly influence Cu2+ detection. All these observations indicated a small influence of other metal ions on this Cu2+ sensing system. The highly specificity of SiO2@ZIF-8 nanosensor is ascribed to a faster chelating process and higher thermodynamic affinity of Cu2+ with nitrogen of ZIF-8 over other transition metal ions.58 Thus, the SiO2@ZIF-8 nanosensor exhibited good selectivity and was expected to be used for Cu2+ detection. 3.5 Sensing application in practical water samples The core-shell SiO2@ZIF-8 nanosensor was used to detect Cu2+ in tap water and Yaohu River for assessing the practicability. The water samples were filtered with a membrane (0.22 μm) and then the Cu2+ with different concentrations was spiked. The added standard Cu2+ could be measured 12
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with good recoveries (about 102%) (Table S1, Supporting Information). It suggested that theView results Article Online DOI: 10.1039/C4AN01773K
obtained by using the core-shell SiO2@ZIF-8 nanosensor were accurate and credible. Therefore, the SiO2@ZIF-8 core-shell nanosensor was capable to monitor a trace amount of Cu2+ in water samples. 4．Conclusion In summary, a fluorescent SiO2@ZIF-8 core-shell nanosensor for a trace amount of Cu2+ detection was fabricated. The SiO2-COOH NPs acted as a template to induce the growth of ZIF-8 on the surface to construct SiO2@ZIF-8 with core-shell nanostructure. The pyridyl nitrogen sites in imidazole played vital roles in the recognition of Cu2+. The porous core-shell nanostructure of SiO2@ZIF-8 resulted in larger surface areas as compared with pure ZIF-8, providing more binding sites to interact with Cu2+. The core-shell SiO2@ZIF-8 could well disperse in aqueous solution and showed high sensitivity as well as good selectivity as nanosensor for Cu2+ detection. The core-shell SiO2@ZIF-8 could be also applied for a trace amount of Cu2+ sensing in real water samples with good recoveries. Acknowledgements This work was financially supported by National Natural Science Foundation of China (21165010, 21465014 and 21465015), Young Scientist Foundation of Jiangxi Province (20122BCB23011), Foundation of Jiangxi Educational Committee (GJJ13244), Natural Science Foundation of Jiangxi Province (20142BAB203101 and 20143ACB21016), The Ministry of Education by the Specialized Research Fund for the Doctoral Program of Higher Education (20133604110002) and the Open Project Program of Key Laboratory of Functional Small organic molecule, Ministry of Education, Jiangxi Normal University (No. KLFS-KF-201214 and KLFS-KF-201218). 13
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References
View Article Online
DOI: 10.1039/C4AN01773K
1.
J. Lee, O. K. Farha, J. Roberts, K. A. Scheidt, S. T. Nguyen and J. T. Hupp, Chem. Soc. Rev. 2009, 38, 1450-1459.
2.
M. P. Suh, H. J. Park, T. K. Prasad and D.-W. Lim, Chem. Rev. 2011, 112, 782-835.
3.
P. Horcajada, T. Chalati, C. Serre, B. Gillet, C. Sebrie, T. Baati, J. F. Eubank, D. Heurtaux, P. Clayette and C. Kreuz, Nat. Mater. 2010, 9, 172-178.
4.
J. An, S. J. Geib and N. L. Rosi, J. Am. Chem. Soc. 2009, 131, 8376-8377.
5.
A. Cadiau, C. D. Brites, P. M. Costa, R. A. Ferreira, J. Rocha and L. D. Carlos, ACS Nano 2013, 7, 7213-7218.
6.
Y. Zhang, Y. Luo, J. Tian, A. M. Asiri, A. O. Al-Youbi and X. Sun, PloS one, 2012, 7, e30426.
7.
J. Tian, S. Liu, Y. Luo and X. Sun, Catal. Sci. & Technol. 2012, 2, 432-436.
8.
W. Lu, X. Qin, A. M. Asiri, A. O. Al-Youbi and X. Sun, Analyst 2013, 138, 429-433.
9.
H. Li, J. Zhai and X. Sun, RSC Adv. 2011, 1, 725-730.
10.
H. Li, L. Wang, J. Zhai, Y. Zhang, J. Tian and X. Sun, Macromol. Rapid Commun. 2011, 32, 899-904.
11.
H. Li and X. Sun, Chem. Commun. 2011, 47, 2625-2627.
12.
X. Sun, S. Dong and E. Wang, J. Am. Chem. Soc. 2005, 127, 13102-13103.
13.
J. Li, S. Guo and E. Wang, RSC Adv. 2012, 2, 3579-3586.
14.
H. Wei, B. Li, Y. Du, S. Dong and E. Wang, Chem. Mater. 2007, 19, 2987-2993.
15.
H. Tan and Y. Chen, Chem. Commun. 2011, 47, 12373-12375.
16.
Y. Yuan, W. Wang, L. Qiu, F. Peng, X. Jiang, A. Xie, Y. Shen, X. Tian and L. Zhang, Mater. Chem. Phys. 2011, 131, 358-361. 14
Analyst Accepted Manuscript
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1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
Page 14 of 26
Page 15 of 26
17.
H. Tan, C. Ma, L. Chen, F. Xu, S. Chen and L. Wang, Sens. Actuators. B. 2014, 190,Online View Article DOI: 10.1039/C4AN01773K
621-626. 18.
M. Zhao, K. Deng, L. He, Y. Liu, G. Li, H. Zhao and Z. Tang, J. Am. Chem. Soc. 2014, 136-.
19.
J. Zheng, C. Cheng, W. Fang, C. Chen, R.-W. Yan, H.-X. Huai and C.-C. Wang, CrystEngComm 2014, 16, 3960-3964.
20.
H. Lee, J. Park, S. Choi, J. Son and M. Oh, Small 2013, 9, 490-490.
21.
K. Koh, A. G. Wong-Foy and A. J. Matzger, Chem. Commun. 2009, 6162-6164.
22.
S. Furukawa, K. Hirai, K. Nakagawa, Y. Takashima, R. Matsuda, T. Tsuruoka, M. Kondo, R. Haruki, D. Tanaka and H. Sakamoto, Angew. Chem. Int. Ed. 2009, 48, 1766-1770.
23.
F. Ke, L. Qiu, Y. Yuan, X. Jiang and J. Zhu, J. Mater. Chem. 2012, 22, 9497-9500.
24.
H. Lee, W. Cho and M. Oh, Chem. Commun. 2012, 48, 221-223.
25.
W. Zhan, Q. Kuang, J. Zhou, X. Kong, Z. Xie and L. Zheng, J. Am. Chem. Soc. 2013, 135, 1926-1933.
26.
Y. Fu, C. Yang and X. Yan, Chem. Eur. J. 2013, 19, 13484-13491.
27.
C. Wang, K. deKrafft and W. Lin, J. Am. Chem. Soc. 2012, 134, 7211-7214.
28.
S. Wei, Q. Wang, J. Zhu, L. Sun, H. Lin and Z. Guo, Nanoscale 2011, 3, 4474-4502.
29.
G. Lu, S. Li, Z. Guo, O. K. Farha, B. G. Hauser, X. Qi, Y. Wang, X. Wang, S. Han and X. Liu, Nat. Chem. 2012, 4, 310-316.
30.
C. Jo, H. J. Lee and M. Oh, Adv. Mater. 2011, 23, 1716-1719.
31.
S. VanáSnick, Chem. Commun. 2010, 46, 6329-6331.
32.
K. J. Barnham, C. L. Masters and A. I. Bush, Nat. Rev. Dru. Discovery. 2004, 3, 205-214.
33.
J. Chen and K. C. Teo, Anal. Chim. Acta. 2001, 450, 215-222.
34.
K. Sreenivasa Rao, T. Balaji, T. Prasada Rao, Y. Babu and G. Naidu, Spectrochim. Acta, Part 15
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1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
Analyst
Analyst
B 2002, 57, 1333-1338.
View Article Online
DOI: 10.1039/C4AN01773K
35.
M. Liu, H. Zhao, S. Chen, H. Yu, Y. Zhang and X. Quan, Biosens. Bioelectron. 2011, 26, 4111-4116.
36.
G.-Y. Lan, C.-C. Huang and H.-T. Chang, Chem. Commun. 2010, 46, 1257-1259.
37.
Y.-H. Chan, J. Chen, Q. Liu, S. E. Wark, D. H. Son and J. D. Batteas, Anal. Chem. 2010, 82, 3671-3678.
38.
J. Zheng, C. Xiao, Q. Fei, M. Li, B. Wang, G. Feng, H. Yu, Y. Huan and Z. Song, Nanotechnology 2010, 21, 045501.
39.
S. Liu, J. Tian, L. Wang, Y. Zhang, X. Qin, Y. Luo, A. M. Asiri, A. O. Al‐Youbi and X. Sun, Adv. Mater. 2012, 24, 2037-2041.
40.
E. Z. Lee, Y. S. Jun, W. H. Hong, A. Thomas and M. M. Jin, Angew. Chem. Int. Ed. 2010, 49, 9706-9710.
41.
B. Chen, L. Wang, Y. Xiao, F. R. Fronczek, M. Xue, Y. Cui and G. Qian, Angew. Chem. Int. Ed. 2009, 48, 500-503.
42.
Y. Xiao, Y. Cui, Q. Zheng, S. Xiang, G. Qian and B. Chen, Chem. Commume. 2010, 46, 5503-5505.
43.
S. Liu, Z. Xiang, Z. Hu, X. Zheng and D. Cao, J. Mater. Chem. 2011, 21, 6649-6653.
44.
Y. Dong, R.Wang, G. Li, C. Chen, Y. Chi and G. Chen, Anal. Chem. 2012, 84, 6220-6224.
45.
C. J. Brinker and G. W. Scherer, Sol-gel Sci Gulf Professional Publishing, 1990.
46.
D. Buso, K. M. Nairn, M. Gimona, A. J. Hill and P. Falcaro, Chem. Mater. 2011, 23, 929-934.
47.
J. Cravillon, R. Nayuk, S. Springer, A. Feldhoff, K. Huber and M. Wiebcke, Chem. Mater. 2011, 23, 2130-2141. 16
Analyst Accepted Manuscript
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1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
Page 16 of 26
Page 17 of 26
48.
A. Schejn, L. Balan, V. Falk, L. Aranda, G. Medjahdi and R. Schneider, CrystEngComm View Article Online DOI: 10.1039/C4AN01773K
2014, 16, 4493-4500. 49.
J. K. Yang and A. P. Davis, J. Colloid. Interface. Sci. 1999, 216, 77-85.
50.
H. Tan, C. Ma, Y. Song, F. Xu, S. Chen and L. Wang, Biosens. Bioelectron. 2013, 50, 447-452.
51.
G. Yang, M. Li, S. Li, Y. Lan, W. He, X. Wang, J.-S. Qin and Z. Su, J. Mater. Chem. 2012, 22, 17947-17953.
52.
M. Araya, M. Olivares, F. Pizarro, A. Llanos, G. Figueroa and R. Uauy, Environ. Health. Perspect. 2004, 112, 1068.
53.
O. Akoto and J. Adiyiah, Int. J. Environ. Sci. Technol. 2007, 4, 211-214.
54.
Z. Xu, L. Zhang, R. Guo, T. Xiang, C. Wu, Z. Zheng and F. Yang, Sens. Actuators. B. 2011, 156, 546-552.
55.
L. Tang and M. Cai, Sens. Actuators, B. 2012, 173, 862-867.
56.
J. Liu, L. Lin, X. Wang, S. Lin, W. Cai, L. Zhang and Z. Zheng, Analyst 2012, 137, 2637-2642.
57.
J. Huang, Q. Zheng, J. Kim and Z. Li, Biosens. Bioelectron. 2013, 43, 379-383.
58.
A. P. De Silva, H. N. Gunaratne, T. Gunnlaugsson, A. J. Huxley, C. P. McCoy, J. T. Rademacher and T. E. Rice, Chem. Rev. 1997, 97, 1515-1566.
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Figure caption
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Scheme 1. Synthetic route for core-shell SiO2@ZIF-8 nanostructures and sensing mechanism of core-shell SiO2@ZIF-8 nanostructures for Cu2+. Fig.1. SEM images of SiO2 NPs (A), SiO2@ZIF-8 reacted for 30 min (B), 2 h (C) and TEM image of SiO2@ZIF-8 reacted for 2 h (D). Fig.2. FTIR spectra of SiO2, ZIF-8 and SiO2@ZIF-8 core-shell nanostructure (A); XRD patterns of SiO2 NPs, ZIF-8 and SiO2@ZIF-8 core-shell nanostructure (B). Fig.3. Excitation (left) and emission (right) spectrum of ZIF-8 and SiO2@ZIF-8 core-shell nanostructures. The concentration was 10 μg mL-1 (A). Concentration-dependent fluorescent response of SiO2@ZIF-8 (B). The fluorescent response of SiO2@ZIF-8 in the presence (red) and absence (black) of 1.0 μM Cu2+ at different pH values (C). Time-dependent fluorescence response of SiO2@ZIF-8 to 1.0 μM Cu2+ at pH 7 HEPES buffer (100 mM) (D). Fig.4. Fluorescent emission spectra of SiO2@ZIF-8 sensor exposed to various concentrations of Cu2+ 0, 10, 50, 100, 200, 300, 400, 500 nM from top to bottom (A). Fluorescent intensity ratio before (I0) and after (I) the addition of Cu2+ versus the concentration of Cu2+ (B). The concentration of SiO2@ZIF-8 was 10 μg mL−1. The excitation wavelength was 381 nm. Fig.5. The fluorescent response of SiO2@ZIF-8 (10 μg mL-1) in pH 7 HEPES buffer towards various metal ions (1.0 μM).
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Table 1. Comparison of sensing performance of different fluorescent probes for Cu2+ detection. View Article Online DOI: 10.1039/C4AN01773K
a
Fluorescent probes
Detection limit (nM)
Linear range (nM)
Ref.
DNA-AgNCsa
8
10-200
36
16-MHA-capped CdSe QDsb
5
5-1×105
37
Peptide-coated ZnS QDs
500
0-2.6×104
38
c-mpg-C3N4c
12.3
10-100
40
Rhodamine-based derivative
3.42×103
10×103-3×105
54
Benzimidazole-based molecular
18.2
0-3×103
55
CDs-BSAd
5.8×10-4
2.0×10-3-1.5
56
MB-GOe
53.3
53.3-1333
57
SiO2@ZIF-8
3.8
10-500
this work
DNA modified Ag nanoclusters
b
16-mercaptohexadecanoic capped CdSe quantumn dots
c
c-mpg-C3N4 Cubic mesopprous graphite carbon nitride
d
CDs-BSA bovine serum albumin modified carbon dots
e
MB molecular beacons GO graphene oxide
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Scheme 1
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Fig. 1
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Fig. 2
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Fig. 3
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Fig. 4
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Fig. 5
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Table of Contents
Text: Formation of core-shell SiO2@ZIF-8 nanostructures for Cu2+ detection
Analyst Accepted Manuscript
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