Analytica Chimica Acta 804 (2013) 240–245
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Metal–organic frameworks of zeolitic imidazolate framework-7 and zeolitic imidazolate framework-60 for fast mercury and methylmercury speciation analysis Fujian Xu a , Lu Kou a , Jia Jia b , Xiandeng Hou a,b , Zhou Long a,b,∗ , Shanling Wang b,∗ a b
College of Chemistry, Key Laboratory of Green Chemistry and Technology of MOE at Sichuan University, Chengdu, Sichuan 610064, China Analytical & Testing Centre, Sichuan University, Chengdu, Sichuan 610064, China
h i g h l i g h t s
g r a p h i c a l
• Speciation analysis of Hg with MOF
Metal–organic frameworks employed as fluorescence sensing probe for fast, sensitive, reproducible and highly selective speciation analysis of Hg.
as fluorescence sensing probe was firstly reported. • The method was characterized with rapidness, high sensitivity and selectivity. • The procedure for preparing smallsized MOF NPs was easy and fast. • Good reproducibility (RSD ≤ 4.5%) and detection limit (≤6 ng mL−1 ) were obtained.
a r t i c l e
i n f o
Article history: Received 25 July 2013 Received in revised form 25 September 2013 Accepted 28 September 2013 Available online 12 October 2013 Keywords: Fluorescence sensing Metal–organic frameworks Speciation analysis Mercury Methylmercury
a b s t r a c t
a b s t r a c t A fluorescence sensing platform based on metal–organic frameworks (MOFs) nanoparticles (NPs) of both zeolitic imidazolate framework-7 (ZIF-7) and zeolitic imidazolate framework-60 (ZIF-60) was developed for speciation analysis of inorganic Hg [Hg(II)] and methylmercury (MeHg+ ). Microwave-ultrasound assisted synthesis was employed for the preparation of ZIF-7 and ZIF-60 NPs, with short reaction time, easy procedure, and small particle size obtained. Based on strict cavity confinement of the ZIF-7 and ZIF60 structures, the proposed method exhibited excellent selectivity for both Hg(II) and MeHg+ , even in the presence of the other Hg species or various cations or anions with the concentration of 50 times high. Effect of pH and ionic strength on sensing behaviour of the ZIF MOF was studied as well. The calculated detection limit is 3 ng mL−1 and 6 ng mL−1 for Hg(II) and MeHg+ , respectively. Furthermore, the application of the developed method to the analysis of local drinking water was demonstrated to be feasible, and the obtained recovery was 102% and 96.2% for Hg(II) and MeHg+ , respectively. © 2013 Elsevier B.V. All rights reserved.
1. Introduction The toxicological and biological effects and characters of mercury (Hg) species highly depend on their chemical forms [1]. For instance, organic forms of Hg, especially methylmercury (MeHg+ ),
∗ Corresponding author. E-mail address: [email protected] (Z. Long). 0003-2670/$ – see front matter © 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.aca.2013.09.058
have much higher toxicity than inorganic and elemental Hg, for its enhanced toxicity, bioaccumulation and volatility [2]. Moreover, inorganic mercury [Hg(II)] can be converted into MeHg+ in the environment after a number of biological processes [3]. Hence, effective speciation analysis of Hg is more important than determination of total Hg, which has received much attention during recent years [4–6]. The most commonly used methods for Hg speciation analysis include atomic absorption spectroscopy (AAS) [4,7], atomic fluorescence spectrometry (AFS) [6,8], liquid chromatography (LC)
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[9,10], gas chromatography (GC) [11,12], capillary electrophoresis (CE) [13–15], and inductively coupled plasma mass spectrometry (ICP-MS) [16,17]. Although these methods are powerful, either complicated pretreatment procedure or expensive instrumentation is needed, which makes it inconvenient for fast and facile determination of more than one Hg species. Recently, because of high sensitivity and simple operation procedure, fluorescence (FL) sensors have become popular [18–23]. Nanoparticles (NPs) such as quantum dots (QDs) or Au NPs are usually employed as FL sensing probe due to their attractive characters including narrow emission bands, stability against photobleaching, high extinction coefficient, and broad absorption spectrum in a visible light [20,24–30]. NP-based FL sensing often follows the mechanism through FL signal changing (e.g. quenching) caused by interaction between target analytes and NPs. Especially for FL sensing of Hg species, Hg(II) has been the mostly concerned analyte and the NP-based probes were usually modified prior to sensing. For instance, Freeman et al. [31] modified QDs with thymine-rich nucleic acid and the FL quenching of QDs were caused by electrontransfer between QDs and the surface Hg(II)-thymine complexes. Yuan et al. [18] modified QDs with 2-hydroxyethyldithiocarbamate (HDTC) which dissociated from the QD surface upon exposure to Hg(II), leading to FL quenching. During the past several years, metal–organic frameworks (MOFs) employed as FL sensing probe have become more attractive because of their unique properties compared with other NPs, such as high porosity, exceptional tunability, tremendous internal surface areas, structural diversity, and robust thermal stability [32–34]. Moreover, intrinsic topology, cavity confinement effect and conformational rigidity enable MOFs to have good size and shape selectivity for the sensing of specific analytes [35–39], even without extra modification or functionalization to accomplish selectivity like other NPs such as QDs or Au NPs. Up to now, various MOFs have been reported as FL sensing probes for specific target analytes such as NO2 − [40], Hg(II) [41–43], Pd2+ [44], Fe3+ [45], Zn2+ [46], Ag+ [47], and MeHg+ [48]. However, all those reported methods involving MOF-based FL sensing focused on only one kind of analyte and no reports concerning Hg speciation analysis have been found to date. In this context, we herein reported a fast and facile method using MOF NPs of zeolitic imidazolate framework-7 (ZIF-7) and zeolitic imidazolate framework-60 (ZIF-60) for speciation analysis of Hg(II) and MeHg+ . ZIF-7 and ZIF-60 were synthesized using microwave-ultrasound assisted method, with short reaction time, easy procedure and small particle size obtained. Aqueous Hg(II) and MeHg+ could be determined by using ZIF-7 and ZIF-60, respectively, in a fast, sensitive, reproducible and highly selective manner. Moreover, the developed method was further explored to real sample analysis, with good results obtained for the determination of both Hg(II) and MeHg+ contained in local drinking water.
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Ltd., Chengdu, China). The Hg(II) solutions were prepared by dissolving HgCl2 in water. All standards and stock solutions were stored at 4 ◦ C in a refrigerator until use. 2.2. Instrumentation The Uwave-1000 microwave reactor (Sineo Microwave Chemistry Technology Co. Ltd., Shanghai, China) was used for synthesis of the ZIF MOF. The FL data were collected from an F-7000 FL spectrometer (Hitachi, Japan) using a 310 nm optical filter. The PXRD patterns were obtained with an X’Pert Pro MPD (Philips, the Netherlands) X-ray diffraction spectrometer using Cuk␣ radiation. The scanning electron microscope (SEM) images were obtained from a JEOL JSM-7500F scanning electron microscope. FT-IR spectra were collected using a Nicolet IS10 FTIR spectrometer (Thermo Inc., USA). The concentration of Hg species was also determined with an inductively coupled plasma optical emission spectrometer (SPECTRO ARCOS, Germany) and an atomic fluorescence spectrometer (Model AFS-9600, Beijing Haiguang Instrument Co. Ltd., Beijing, China). 2.3. Synthesis of ZIF-7 and ZIF-60 For ZIF-7, ZnNO3 ·6H2 O (0.11 g, 0.37 mmol) and bIM (0.06 g, 0.51 mmol) were dissolved in 50 mL of DMF and then stirred for 1 h. The obtained liquid was transferred into the microwave reactor and heated at 100 ◦ C for 10 min. For ZIF-60, ZnNO3 ·6H2 O (0.27 g, 0.91 mmol), IM (0.18 g, 2.65 mmol) and mIM (0.07 g, 0.85 mmol) were dissolved in 30 mL of DMF. The obtained liquid was stirred for 1 h then put in the microwave reactor and heated at 85 ◦ C for 10 min. Subsequently, for both ZIF-7 and ZIF-60, the obtained suspension was centrifuged and the particles were collected from the bottom of the tube, which was then mixed with water under ultrasound for rinsing. The ultimately obtained particles were dried at 80 ◦ C under vacuum until use. Characterization of the obtained ZIF particles was performed by comparing the FT-IR spectra of the particles with according precursors during the synthesis process. 2.4. Speciation analysis and interference studies
2. Experimental
2.0 mg ZIF-7/ZIF-60 was added into 10 mL of water and then kept under ultrasound for 10 min, followed by a 15-min centrifugation at 8000 rpm. The supernatant was collected and aqueous Hg(II)/MeHg+ was subsequently added into the collected suspension for FL measurement. For interference study, solution containing specific cation or anion was added into the aforementioned suspension, followed by the addition of aqueous Hg(II)/MeHg+ and subsequent FL measurement. For the study of the effect of pH and ionic strength (IS), aqueous Hg(II)/MeHg+ was added into the according ZIF suspension with varied pH or IS, followed by FL measurement.
2.1. Chemicals and reagents
3. Results and discussion
All chemicals used in this work are AR grade or better. N,Ndimethylformamide (DMF), imidazole (IM), mercury dichloride (HgCl2 ) and ZnNO3 ·6H2 O were purchased from Kelong Chemical Reagent Co. Ltd. (Chengdu, China). 2-methylimidazole (mIM) and benzimidazole (bIM) were purchased from Chengdu Juhui Chemical Technology Co. Ltd. (Chengdu, China). Methanol was purchased from Amethyst Chemicals (J&K Scientific, China). Standard solutions of MeHg+ in methanol (76.6 ppm) were obtained from National Institute of Metrology China (Beijing, China). De-ionized water (18 M-cm) used all through the experiments was obtained from a water purification system (PCWJ-10, Pure Technology Co.
The size and shape of ZIF MOF particles were examined by SEM, and homogeneously sized ZIF-7 (Fig. 1a) and ZIF-60 (Fig. 1b) particles were observed. Moreover, the dominating peaks of the PXRD spectra of both ZIF-7 and ZIF-60 correlate with the previously reported (Fig. 1c and d) [49,50]. Moreover, the pore size of ˚ ZIF-7 and ZIF-6 NP was previously reported around 3 A˚ and 7 A, respectively [51]. In order to further characterize the obtained MOF particles, according FT-IR spectra were collected. In Fig. 2, for bIM, IM and mIM used for the synthesis of ZIF-7 and ZIF-60, there is a sharp band around 3125 cm−1 due to C H stretch, a weak band near 1820 cm−1 , as well as several strong and broad N-H bands between
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Fig. 1. SEM image of the synthesized NPs of ZIF-7 (a) and ZIF-60 (b), scale bar: 1 m; PXRD spectra (black) of the synthesized ZIF-7 (c) and ZIF-60 (d), and the simulated PXRD curve (red) based on the previously reported. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
3335 and 2500 cm−1 (see The Aldrich Library of FT-IR spectra, ed II, vol. 3). Whereas, for the spectra of ZIF-7 and ZIF-60, all those bands disappear, which indicates that bIM, IM and mIM are all linked into the framework of ZIF-7 or ZIF-60 and fully deprotonated. It should
Fig. 2. FT-IR spectra of bIM (a), ZIF-7 (b), IM (c), mIM (d) and ZIF-60 (e).
also be mentioned that similar spectra comparison between IM and other ZIF MOFs for characterization was reported previously [49]. To investigate the sensitivity of ZIF-7 and ZIF-60 towards Hg(II) and MeHg+ , respectively, aqueous Hg(II) and MeHg+ with different concentrations were added in the aforementioned ZIF-7 and ZIF-60 suspension, respectively, both of which were then measured with the FL spectrometer. It was found out that F/F0 of the ZIF-7 suspension decreased with the increased concentration of Hg(II) (Fig. 3a and b), while F/F0 of the ZIF-60 suspension increased with the concentration of MeHg+ (Fig. 3c and d), both with a linear fashion within certain concentration range of according Hg species (according calibration plot was made as well). F and F0 represents the FL intensities of the ZIF suspension with and without Hg(II)/MeHg+ , respectively. A limit of detection as low as 3 ng mL−1 for Hg(II) and 6 ng mL−1 for MeHg+ was obtained, based on 3 of F/F0 obtained from FL measurements of 11 same ZIF-7 or ZIF60 suspensions, divided by the slope of the according calibration plot. In terms of the sensing mechanism of the proposed method, the high selectivity to Hg(II) and MeHg+ primarily depends on the porous structure of ZIF MOF [48]. The premise lies in successful diffusion of Hg species into the pores of ZIF MOF, based on good size selectivity of ZIF-7 to Hg (II), and ZIF-60 to MeHg+ , respectively (see Scheme 1). The explanation of FL quenching of ZIF-7 towards Hg(II) is most probably similar to that reported previously [42,52,53]. The interaction between the Hg(II) and the free nitrogen atom sites of bIM on the pore surface of ZIF-7 might reduce the energy transfer efficiency from the organic ligand to the Zn atom in the ZIF-7 structure, and thus cause the luminescence decay of the ZIF-7. For ZIF-60, the absorbed MeHg+ most probably inhibited the linker motions (e.g. vibrations, torsional displacements, etc.) of ZIF-60, exerting a rigidifying effect which slowed the nonradiative decay processes and increased the fraction of decaying excited species, thus leading to the increase of FL intensity [33].
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Fig. 3. (a) FL spectra (excited at 277 nm) of the ZIF-7 suspension with the addition of aqueous Hg(II) (0, 1, 5, 10, 25, 100, 150, 200, 600, 800, 1000, 1500 ng mL−1 ). (b) F/F0 changing with respect to Hg(II) concentration, and F0 represents the FL intensities of ZIF-7 suspension at 640 nm and F represents the FL intensities of ZIF-7 suspension containing Hg(II) at 640 nm. Each dot was obtained based on 3 replicated measurements. (c) FL spectra (excited at 277 nm) of the ZIF-60 suspension with the addition of MeHg+ (0, 5, 10, 20, 50, 100, 200, 300, 400, 1000, 1500, 2000, 3000, 4000 and 5000 ng mL−1 ). (d) F/F0 changing with respect to MeHg+ concentration, and F0 represents the FL intensities of ZIF-60 suspension at 670 nm and F represents the FL intensities of ZIF-60 suspension containing MeHg+ at 670 nm. Each dot was obtained based on 3 replicated measurements.
Furthermore, in order to validate the high selectivity of ZIF-7 to Hg(II) and ZIF-60 to MeHg+ , respectively, another experiment was performed. 2.0 mg ZIF NPs and 10 L of 10 g mL−1 according Hg species in methanol was added into a tube with 10 mL of water, which was then put under ultrasound for 10 min. The obtained suspension was then centrifuged at 8000 rpm for 15 min. 1.0 mL of the supernatant on top and 1.0 mL of the suspension at bottom was collected, respectively, with each diluted to 10 mL with water. Both of
Scheme 1. Schematic 3D representation of Hg(II) (a) and MeHg+ (b) sensing based on ZIFs. The figure was made using the Diamond software (version 3.2i, Crystal Impact GbR, Bonn, Germany), and the CIFs were obtained from the Cambridge Crystallographic Data Centre (CCDC).
the diluted suspensions were analyzed in terms of Hg concentration using hydride generation-atomic fluorescence spectrometry (HGAFS). It was found out that FL intensity of Hg for the supernatant was only one fifth and one half of that for the suspension at bottom, for ZIF-7 and ZIF-60, respectively. This measured result illustrates that Hg(II) and MeHg+ are more likely to diffuse into ZIF-7 and ZIF-60, respectively, rather than stay outside. One of the most attractive characters of this proposed method lies in that both ZIF-7 and ZIF-60 demonstrate a high selectivity to Hg(II) or MeHg+ , respectively, even with the presence of the other Hg species. For demonstration, FL signal of the ZIF-7 and ZIF-60 suspension was measured upon the addition of both Hg species with a total Hg concentration of 1.0 g mL−1 . It turned out that F/F0 changed with the concentration of the according Hg species with a linear fashion similar to the measurements involving only one Hg species described above. In order to further evaluate the selectivity of the ZIF MOFs, 15 cations and anions were tested for interference study. In Fig. 4, it can be seen that F/F0 obtained from the according MOF suspension containing Hg(II) or MeHg+ does not obviously change if any cation or anion is added in even with a concentration as high as 50 times. This demonstrates the excellent selectivity of the developed method towards Hg(II) and MeHg+ , respectively. Furthermore, effect of pH and IS of the ZIF suspension was investigated. It should be noted that high F/F0 is desirable for the determination of both Hg(II) and MeHg+ , while lowest FL intensity (best FL quenching) and highest FL intensity is required for the determination of Hg(II) and MeHg+ , respectively. In Fig. 5, it can be seen that for ZIF-7 suspension containing Hg(II) with same concentration, F/F0 does not change obviously with IS or pH higher than
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6, while the highest FL quenching is obtained with pH at 7 or IS at 1 M. For ZIF-60 suspension containing MeHg+ with same concentration, both FL intensity and F/F0 does not change obviously with pH or IS. Therefore, the pH and IS of the ZIF suspension was set as 7 and 1 M, respectively. Moreover, it should be mentioned that good reproducibility is another character of this proposed method. In order to demonstrate that, three batches of the aforementioned ZIF suspension were prepared with the same procedure, each was used for the determination of Hg (II) or MeHg+ with 5 different concentrations. The batch-to-batch reproducibility was good, with RSD (%) of F/F0 no more than 4.5%, and F/F0 changed with the concentration of Hg species also with a linear fashion. In order to test the practical applicability of the developed method, the proposed sensing system was employed for the determination of Hg(II) and MeHg+ contained in local drinking water. Neither Hg(II) nor MeHg+ was detected most probably because of the ultralow concentration of Hg species. Alternatively, the developed method was validated with a standard addition method, and the recovery of spiked Hg(II) and MeHg+ is 102% and 96.2%, respectively. For validation, same spiked samples were also analyzed by inductively coupled plasma optical emission spectrometry (ICPOES), with good agreement obtained (Table 1).
Fig. 4. F/F0 of ZIF-7 (top)/ZIF-60(bottom) containing cation or anion with same concentration (grey bar), in which Hg(II) (a)/MeHg+ (b) with the concentration 2% of the ion was subsequently added in (red bar). (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
Fig. 5. FL intensity (dot) and F/F0 (bar) of ZIF-7 (a and b) and ZIF-60 (c and d) suspension at varied pH (a and c) and IS (b and d), containing Hg (II) (a and b)/MeHg+ (and d) with same concentration.
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Table 1 The recovery of spiked Hg(II) and MeHg+ in local drinking water. Sample
Drinking water a
Added/g mL−1
Detected with FL a /g mL−1 +
+
Hg(II)
MeHg
Hg(II)
MeHg
Hg(II)
MeHg
Hg(II)
MeHg+
0.80
0.80
0.82 ± 0.02
0.77 ± 0.01
102
96.2
0.77 ± 0.02
0.78 ± 0.02
The proposed method herein is fast, sensitive, reproducible and highly selective for speciation analysis of Hg(II) and MeHg+ , with ZIF-7 and ZIF-60 NPs dispersed in water as FL sensing probe, respectively. There was neither Hg(II)-MeHg+ mutual interference nor obvious change of FL signal upon the addition of 15 cations or anions with the concentration 50 times as high as that of Hg species. The effect of IS of ZIF suspension could be ignored, while best sensing performance was obtained with pH of ZIF suspension at 7. Microwave-ultrasound assisted procedure for the synthesis of ZIF-7 and ZIF-60 was fast and easy for preparing ZIF NPs with small sizes. The successful analysis of local drinking water demonstrates the practical utility of this method and its perspective for real analysis in the future. Acknowledgments The authors gratefully acknowledge the financial support from National Natural Science Foundation of China through Grant No. 21205083, Chengdu Bureau of Science and Technology through Grant No. 11DXYB353SF-027, and technical assistance from Analytical & Testing Centre of Sichuan University for obtaining SEM and PXRD data. References
[6] [7] [8] [9] [10] [11]
[12] [13] [14] [15] [16]
+
Mean ± SD, n = 3.
4. Conclusions
[1] [2] [3] [4] [5]
Detected with ICP-OES a /g mL−1
Recovery/%
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Metal–organic frameworks of zeolitic imidazolate framework-7 and zeolitic imidazolate framework-60 for fast mercury and methylmercury speciation analysis Fujian Xu a , Lu Kou a , Jia Jia b , Xiandeng Hou a,b , Zhou Long a,b,∗ , Shanling Wang b,∗ a b
College of Chemistry, Key Laboratory of Green Chemistry and Technology of MOE at Sichuan University, Chengdu, Sichuan 610064, China Analytical & Testing Centre, Sichuan University, Chengdu, Sichuan 610064, China
h i g h l i g h t s
g r a p h i c a l
• Speciation analysis of Hg with MOF
Metal–organic frameworks employed as fluorescence sensing probe for fast, sensitive, reproducible and highly selective speciation analysis of Hg.
as fluorescence sensing probe was firstly reported. • The method was characterized with rapidness, high sensitivity and selectivity. • The procedure for preparing smallsized MOF NPs was easy and fast. • Good reproducibility (RSD ≤ 4.5%) and detection limit (≤6 ng mL−1 ) were obtained.
a r t i c l e
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Article history: Received 25 July 2013 Received in revised form 25 September 2013 Accepted 28 September 2013 Available online 12 October 2013 Keywords: Fluorescence sensing Metal–organic frameworks Speciation analysis Mercury Methylmercury
a b s t r a c t
a b s t r a c t A fluorescence sensing platform based on metal–organic frameworks (MOFs) nanoparticles (NPs) of both zeolitic imidazolate framework-7 (ZIF-7) and zeolitic imidazolate framework-60 (ZIF-60) was developed for speciation analysis of inorganic Hg [Hg(II)] and methylmercury (MeHg+ ). Microwave-ultrasound assisted synthesis was employed for the preparation of ZIF-7 and ZIF-60 NPs, with short reaction time, easy procedure, and small particle size obtained. Based on strict cavity confinement of the ZIF-7 and ZIF60 structures, the proposed method exhibited excellent selectivity for both Hg(II) and MeHg+ , even in the presence of the other Hg species or various cations or anions with the concentration of 50 times high. Effect of pH and ionic strength on sensing behaviour of the ZIF MOF was studied as well. The calculated detection limit is 3 ng mL−1 and 6 ng mL−1 for Hg(II) and MeHg+ , respectively. Furthermore, the application of the developed method to the analysis of local drinking water was demonstrated to be feasible, and the obtained recovery was 102% and 96.2% for Hg(II) and MeHg+ , respectively. © 2013 Elsevier B.V. All rights reserved.
1. Introduction The toxicological and biological effects and characters of mercury (Hg) species highly depend on their chemical forms [1]. For instance, organic forms of Hg, especially methylmercury (MeHg+ ),
∗ Corresponding author. E-mail address: [email protected] (Z. Long). 0003-2670/$ – see front matter © 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.aca.2013.09.058
have much higher toxicity than inorganic and elemental Hg, for its enhanced toxicity, bioaccumulation and volatility [2]. Moreover, inorganic mercury [Hg(II)] can be converted into MeHg+ in the environment after a number of biological processes [3]. Hence, effective speciation analysis of Hg is more important than determination of total Hg, which has received much attention during recent years [4–6]. The most commonly used methods for Hg speciation analysis include atomic absorption spectroscopy (AAS) [4,7], atomic fluorescence spectrometry (AFS) [6,8], liquid chromatography (LC)
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[9,10], gas chromatography (GC) [11,12], capillary electrophoresis (CE) [13–15], and inductively coupled plasma mass spectrometry (ICP-MS) [16,17]. Although these methods are powerful, either complicated pretreatment procedure or expensive instrumentation is needed, which makes it inconvenient for fast and facile determination of more than one Hg species. Recently, because of high sensitivity and simple operation procedure, fluorescence (FL) sensors have become popular [18–23]. Nanoparticles (NPs) such as quantum dots (QDs) or Au NPs are usually employed as FL sensing probe due to their attractive characters including narrow emission bands, stability against photobleaching, high extinction coefficient, and broad absorption spectrum in a visible light [20,24–30]. NP-based FL sensing often follows the mechanism through FL signal changing (e.g. quenching) caused by interaction between target analytes and NPs. Especially for FL sensing of Hg species, Hg(II) has been the mostly concerned analyte and the NP-based probes were usually modified prior to sensing. For instance, Freeman et al. [31] modified QDs with thymine-rich nucleic acid and the FL quenching of QDs were caused by electrontransfer between QDs and the surface Hg(II)-thymine complexes. Yuan et al. [18] modified QDs with 2-hydroxyethyldithiocarbamate (HDTC) which dissociated from the QD surface upon exposure to Hg(II), leading to FL quenching. During the past several years, metal–organic frameworks (MOFs) employed as FL sensing probe have become more attractive because of their unique properties compared with other NPs, such as high porosity, exceptional tunability, tremendous internal surface areas, structural diversity, and robust thermal stability [32–34]. Moreover, intrinsic topology, cavity confinement effect and conformational rigidity enable MOFs to have good size and shape selectivity for the sensing of specific analytes [35–39], even without extra modification or functionalization to accomplish selectivity like other NPs such as QDs or Au NPs. Up to now, various MOFs have been reported as FL sensing probes for specific target analytes such as NO2 − [40], Hg(II) [41–43], Pd2+ [44], Fe3+ [45], Zn2+ [46], Ag+ [47], and MeHg+ [48]. However, all those reported methods involving MOF-based FL sensing focused on only one kind of analyte and no reports concerning Hg speciation analysis have been found to date. In this context, we herein reported a fast and facile method using MOF NPs of zeolitic imidazolate framework-7 (ZIF-7) and zeolitic imidazolate framework-60 (ZIF-60) for speciation analysis of Hg(II) and MeHg+ . ZIF-7 and ZIF-60 were synthesized using microwave-ultrasound assisted method, with short reaction time, easy procedure and small particle size obtained. Aqueous Hg(II) and MeHg+ could be determined by using ZIF-7 and ZIF-60, respectively, in a fast, sensitive, reproducible and highly selective manner. Moreover, the developed method was further explored to real sample analysis, with good results obtained for the determination of both Hg(II) and MeHg+ contained in local drinking water.
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Ltd., Chengdu, China). The Hg(II) solutions were prepared by dissolving HgCl2 in water. All standards and stock solutions were stored at 4 ◦ C in a refrigerator until use. 2.2. Instrumentation The Uwave-1000 microwave reactor (Sineo Microwave Chemistry Technology Co. Ltd., Shanghai, China) was used for synthesis of the ZIF MOF. The FL data were collected from an F-7000 FL spectrometer (Hitachi, Japan) using a 310 nm optical filter. The PXRD patterns were obtained with an X’Pert Pro MPD (Philips, the Netherlands) X-ray diffraction spectrometer using Cuk␣ radiation. The scanning electron microscope (SEM) images were obtained from a JEOL JSM-7500F scanning electron microscope. FT-IR spectra were collected using a Nicolet IS10 FTIR spectrometer (Thermo Inc., USA). The concentration of Hg species was also determined with an inductively coupled plasma optical emission spectrometer (SPECTRO ARCOS, Germany) and an atomic fluorescence spectrometer (Model AFS-9600, Beijing Haiguang Instrument Co. Ltd., Beijing, China). 2.3. Synthesis of ZIF-7 and ZIF-60 For ZIF-7, ZnNO3 ·6H2 O (0.11 g, 0.37 mmol) and bIM (0.06 g, 0.51 mmol) were dissolved in 50 mL of DMF and then stirred for 1 h. The obtained liquid was transferred into the microwave reactor and heated at 100 ◦ C for 10 min. For ZIF-60, ZnNO3 ·6H2 O (0.27 g, 0.91 mmol), IM (0.18 g, 2.65 mmol) and mIM (0.07 g, 0.85 mmol) were dissolved in 30 mL of DMF. The obtained liquid was stirred for 1 h then put in the microwave reactor and heated at 85 ◦ C for 10 min. Subsequently, for both ZIF-7 and ZIF-60, the obtained suspension was centrifuged and the particles were collected from the bottom of the tube, which was then mixed with water under ultrasound for rinsing. The ultimately obtained particles were dried at 80 ◦ C under vacuum until use. Characterization of the obtained ZIF particles was performed by comparing the FT-IR spectra of the particles with according precursors during the synthesis process. 2.4. Speciation analysis and interference studies
2. Experimental
2.0 mg ZIF-7/ZIF-60 was added into 10 mL of water and then kept under ultrasound for 10 min, followed by a 15-min centrifugation at 8000 rpm. The supernatant was collected and aqueous Hg(II)/MeHg+ was subsequently added into the collected suspension for FL measurement. For interference study, solution containing specific cation or anion was added into the aforementioned suspension, followed by the addition of aqueous Hg(II)/MeHg+ and subsequent FL measurement. For the study of the effect of pH and ionic strength (IS), aqueous Hg(II)/MeHg+ was added into the according ZIF suspension with varied pH or IS, followed by FL measurement.
2.1. Chemicals and reagents
3. Results and discussion
All chemicals used in this work are AR grade or better. N,Ndimethylformamide (DMF), imidazole (IM), mercury dichloride (HgCl2 ) and ZnNO3 ·6H2 O were purchased from Kelong Chemical Reagent Co. Ltd. (Chengdu, China). 2-methylimidazole (mIM) and benzimidazole (bIM) were purchased from Chengdu Juhui Chemical Technology Co. Ltd. (Chengdu, China). Methanol was purchased from Amethyst Chemicals (J&K Scientific, China). Standard solutions of MeHg+ in methanol (76.6 ppm) were obtained from National Institute of Metrology China (Beijing, China). De-ionized water (18 M-cm) used all through the experiments was obtained from a water purification system (PCWJ-10, Pure Technology Co.
The size and shape of ZIF MOF particles were examined by SEM, and homogeneously sized ZIF-7 (Fig. 1a) and ZIF-60 (Fig. 1b) particles were observed. Moreover, the dominating peaks of the PXRD spectra of both ZIF-7 and ZIF-60 correlate with the previously reported (Fig. 1c and d) [49,50]. Moreover, the pore size of ˚ ZIF-7 and ZIF-6 NP was previously reported around 3 A˚ and 7 A, respectively [51]. In order to further characterize the obtained MOF particles, according FT-IR spectra were collected. In Fig. 2, for bIM, IM and mIM used for the synthesis of ZIF-7 and ZIF-60, there is a sharp band around 3125 cm−1 due to C H stretch, a weak band near 1820 cm−1 , as well as several strong and broad N-H bands between
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Fig. 1. SEM image of the synthesized NPs of ZIF-7 (a) and ZIF-60 (b), scale bar: 1 m; PXRD spectra (black) of the synthesized ZIF-7 (c) and ZIF-60 (d), and the simulated PXRD curve (red) based on the previously reported. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
3335 and 2500 cm−1 (see The Aldrich Library of FT-IR spectra, ed II, vol. 3). Whereas, for the spectra of ZIF-7 and ZIF-60, all those bands disappear, which indicates that bIM, IM and mIM are all linked into the framework of ZIF-7 or ZIF-60 and fully deprotonated. It should
Fig. 2. FT-IR spectra of bIM (a), ZIF-7 (b), IM (c), mIM (d) and ZIF-60 (e).
also be mentioned that similar spectra comparison between IM and other ZIF MOFs for characterization was reported previously [49]. To investigate the sensitivity of ZIF-7 and ZIF-60 towards Hg(II) and MeHg+ , respectively, aqueous Hg(II) and MeHg+ with different concentrations were added in the aforementioned ZIF-7 and ZIF-60 suspension, respectively, both of which were then measured with the FL spectrometer. It was found out that F/F0 of the ZIF-7 suspension decreased with the increased concentration of Hg(II) (Fig. 3a and b), while F/F0 of the ZIF-60 suspension increased with the concentration of MeHg+ (Fig. 3c and d), both with a linear fashion within certain concentration range of according Hg species (according calibration plot was made as well). F and F0 represents the FL intensities of the ZIF suspension with and without Hg(II)/MeHg+ , respectively. A limit of detection as low as 3 ng mL−1 for Hg(II) and 6 ng mL−1 for MeHg+ was obtained, based on 3 of F/F0 obtained from FL measurements of 11 same ZIF-7 or ZIF60 suspensions, divided by the slope of the according calibration plot. In terms of the sensing mechanism of the proposed method, the high selectivity to Hg(II) and MeHg+ primarily depends on the porous structure of ZIF MOF [48]. The premise lies in successful diffusion of Hg species into the pores of ZIF MOF, based on good size selectivity of ZIF-7 to Hg (II), and ZIF-60 to MeHg+ , respectively (see Scheme 1). The explanation of FL quenching of ZIF-7 towards Hg(II) is most probably similar to that reported previously [42,52,53]. The interaction between the Hg(II) and the free nitrogen atom sites of bIM on the pore surface of ZIF-7 might reduce the energy transfer efficiency from the organic ligand to the Zn atom in the ZIF-7 structure, and thus cause the luminescence decay of the ZIF-7. For ZIF-60, the absorbed MeHg+ most probably inhibited the linker motions (e.g. vibrations, torsional displacements, etc.) of ZIF-60, exerting a rigidifying effect which slowed the nonradiative decay processes and increased the fraction of decaying excited species, thus leading to the increase of FL intensity [33].
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Fig. 3. (a) FL spectra (excited at 277 nm) of the ZIF-7 suspension with the addition of aqueous Hg(II) (0, 1, 5, 10, 25, 100, 150, 200, 600, 800, 1000, 1500 ng mL−1 ). (b) F/F0 changing with respect to Hg(II) concentration, and F0 represents the FL intensities of ZIF-7 suspension at 640 nm and F represents the FL intensities of ZIF-7 suspension containing Hg(II) at 640 nm. Each dot was obtained based on 3 replicated measurements. (c) FL spectra (excited at 277 nm) of the ZIF-60 suspension with the addition of MeHg+ (0, 5, 10, 20, 50, 100, 200, 300, 400, 1000, 1500, 2000, 3000, 4000 and 5000 ng mL−1 ). (d) F/F0 changing with respect to MeHg+ concentration, and F0 represents the FL intensities of ZIF-60 suspension at 670 nm and F represents the FL intensities of ZIF-60 suspension containing MeHg+ at 670 nm. Each dot was obtained based on 3 replicated measurements.
Furthermore, in order to validate the high selectivity of ZIF-7 to Hg(II) and ZIF-60 to MeHg+ , respectively, another experiment was performed. 2.0 mg ZIF NPs and 10 L of 10 g mL−1 according Hg species in methanol was added into a tube with 10 mL of water, which was then put under ultrasound for 10 min. The obtained suspension was then centrifuged at 8000 rpm for 15 min. 1.0 mL of the supernatant on top and 1.0 mL of the suspension at bottom was collected, respectively, with each diluted to 10 mL with water. Both of
Scheme 1. Schematic 3D representation of Hg(II) (a) and MeHg+ (b) sensing based on ZIFs. The figure was made using the Diamond software (version 3.2i, Crystal Impact GbR, Bonn, Germany), and the CIFs were obtained from the Cambridge Crystallographic Data Centre (CCDC).
the diluted suspensions were analyzed in terms of Hg concentration using hydride generation-atomic fluorescence spectrometry (HGAFS). It was found out that FL intensity of Hg for the supernatant was only one fifth and one half of that for the suspension at bottom, for ZIF-7 and ZIF-60, respectively. This measured result illustrates that Hg(II) and MeHg+ are more likely to diffuse into ZIF-7 and ZIF-60, respectively, rather than stay outside. One of the most attractive characters of this proposed method lies in that both ZIF-7 and ZIF-60 demonstrate a high selectivity to Hg(II) or MeHg+ , respectively, even with the presence of the other Hg species. For demonstration, FL signal of the ZIF-7 and ZIF-60 suspension was measured upon the addition of both Hg species with a total Hg concentration of 1.0 g mL−1 . It turned out that F/F0 changed with the concentration of the according Hg species with a linear fashion similar to the measurements involving only one Hg species described above. In order to further evaluate the selectivity of the ZIF MOFs, 15 cations and anions were tested for interference study. In Fig. 4, it can be seen that F/F0 obtained from the according MOF suspension containing Hg(II) or MeHg+ does not obviously change if any cation or anion is added in even with a concentration as high as 50 times. This demonstrates the excellent selectivity of the developed method towards Hg(II) and MeHg+ , respectively. Furthermore, effect of pH and IS of the ZIF suspension was investigated. It should be noted that high F/F0 is desirable for the determination of both Hg(II) and MeHg+ , while lowest FL intensity (best FL quenching) and highest FL intensity is required for the determination of Hg(II) and MeHg+ , respectively. In Fig. 5, it can be seen that for ZIF-7 suspension containing Hg(II) with same concentration, F/F0 does not change obviously with IS or pH higher than
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6, while the highest FL quenching is obtained with pH at 7 or IS at 1 M. For ZIF-60 suspension containing MeHg+ with same concentration, both FL intensity and F/F0 does not change obviously with pH or IS. Therefore, the pH and IS of the ZIF suspension was set as 7 and 1 M, respectively. Moreover, it should be mentioned that good reproducibility is another character of this proposed method. In order to demonstrate that, three batches of the aforementioned ZIF suspension were prepared with the same procedure, each was used for the determination of Hg (II) or MeHg+ with 5 different concentrations. The batch-to-batch reproducibility was good, with RSD (%) of F/F0 no more than 4.5%, and F/F0 changed with the concentration of Hg species also with a linear fashion. In order to test the practical applicability of the developed method, the proposed sensing system was employed for the determination of Hg(II) and MeHg+ contained in local drinking water. Neither Hg(II) nor MeHg+ was detected most probably because of the ultralow concentration of Hg species. Alternatively, the developed method was validated with a standard addition method, and the recovery of spiked Hg(II) and MeHg+ is 102% and 96.2%, respectively. For validation, same spiked samples were also analyzed by inductively coupled plasma optical emission spectrometry (ICPOES), with good agreement obtained (Table 1).
Fig. 4. F/F0 of ZIF-7 (top)/ZIF-60(bottom) containing cation or anion with same concentration (grey bar), in which Hg(II) (a)/MeHg+ (b) with the concentration 2% of the ion was subsequently added in (red bar). (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
Fig. 5. FL intensity (dot) and F/F0 (bar) of ZIF-7 (a and b) and ZIF-60 (c and d) suspension at varied pH (a and c) and IS (b and d), containing Hg (II) (a and b)/MeHg+ (and d) with same concentration.
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Table 1 The recovery of spiked Hg(II) and MeHg+ in local drinking water. Sample
Drinking water a
Added/g mL−1
Detected with FL a /g mL−1 +
+
Hg(II)
MeHg
Hg(II)
MeHg
Hg(II)
MeHg
Hg(II)
MeHg+
0.80
0.80
0.82 ± 0.02
0.77 ± 0.01
102
96.2
0.77 ± 0.02
0.78 ± 0.02
The proposed method herein is fast, sensitive, reproducible and highly selective for speciation analysis of Hg(II) and MeHg+ , with ZIF-7 and ZIF-60 NPs dispersed in water as FL sensing probe, respectively. There was neither Hg(II)-MeHg+ mutual interference nor obvious change of FL signal upon the addition of 15 cations or anions with the concentration 50 times as high as that of Hg species. The effect of IS of ZIF suspension could be ignored, while best sensing performance was obtained with pH of ZIF suspension at 7. Microwave-ultrasound assisted procedure for the synthesis of ZIF-7 and ZIF-60 was fast and easy for preparing ZIF NPs with small sizes. The successful analysis of local drinking water demonstrates the practical utility of this method and its perspective for real analysis in the future. Acknowledgments The authors gratefully acknowledge the financial support from National Natural Science Foundation of China through Grant No. 21205083, Chengdu Bureau of Science and Technology through Grant No. 11DXYB353SF-027, and technical assistance from Analytical & Testing Centre of Sichuan University for obtaining SEM and PXRD data. References
[6] [7] [8] [9] [10] [11]
[12] [13] [14] [15] [16]
+
Mean ± SD, n = 3.
4. Conclusions
[1] [2] [3] [4] [5]
Detected with ICP-OES a /g mL−1
Recovery/%
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