Fa-Shuo Shana,
Jia-Ping Lai*a,
Hui Sun*b,
Ping Zhangc,
Chong Luod,
Yan-Hui Hea and
Huan-Ran Fenga
aGuangzhou Key Laboratory of Analytical Chemistry for Biomedicine, School of Chemistry & Environment, South China Normal University, Guangzhou 510006, Guangdong, China. E-mail: laijp@scnu.edu.cn; Fax: +86-20-39310187; Tel: +86-20-39310257
bCollege of Environmental Science & Engineering, Guangzhou University, Guangzhou 510006, Guangdong, China. E-mail: cherrysunhui@aliyun.com
cSchool of Chemistry & Chemical Engineering, Guangzhou University, Guangzhou 510006, Guangdong, China. E-mail: zhangping@gzhu.edu.cn
dPublic Monitoring Center for Agro-product, Guangdong Academy of Agricultural Sciences, Guangzhou 510640, Guangdong, China. E-mail: 407628926@qq.com
First published on 8th July 2016
A fast responsive and highly selective mercury(II) sensor was developed using the fluorescence quenching mechanism of 2,9-dimethyl-1,10-phenanthroline (2,9-DMP) towards mercury(II). Furthermore, two kinds of metal–organic frameworks (MOFs) of 2,9-DMP–Hg(II) have been synthesized and characterized by X-ray diffraction. The probe 2,9-DMP exhibited a fast and highly selective response to mercury(II) ions in aqueous solution, showing a good linear relationship over the concentration range of 0.050–2.0 μM for Hg(II) with R2 = 0.9994. The established procedure was used to monitor the concentration of mercury(II) in drinking water and Zhujiang River water samples with satisfactory recoveries of 85.42–101.5% and 81.72–96.09%, respectively. Furthermore, the fluorescence quenching mechanism of 2,9-DMP by mercury(II) was investigated and clarified by the data of single crystal X-ray diffraction of MOFs and the Job's plot analyses of 2,9-DMP towards Hg(II).
The conventional methods for the determination of Hg(II) ions include atomic absorption/emission spectroscopy (AAS/AES),1,2 cold vapor atomic fluorescence spectrometry,3 inductively coupled plasma mass spectrometry (ICP-MS),4–6 and anodic stripping voltammetry.7,8 Although these methods allow sensitive and quantitative determination of Hg(II) ion levels, they require expensive and sophisticated instrumentation, along with tedious sample preparation procedures. So those methods are not suitable for real-time and in situ measurement in the field. Thus, there is a strong incentive to develop a fast, simple, selective, sensitive, reliable and convenient approach for real-time monitoring of Hg(II) in the environment.
In the past decade, much effort has been made to develop portable sensors, including fluorescent sensors9–13 and colorimetric sensors.14–18 Especially, the fluorescent Hg(II) probes or/and sensors have attracted a huge interest because of its simple operation, high sensitivity, and adaptability for Hg(II) determination in-field. Recently, a number of fluorometric probes or/and sensors using organic chromophores,19,20 conjugated polymers,21–25 quantum dots,26–28 carbon nanodots29,30 and metal nanoparticles31 have been developed to detect Hg(II) ions in environmental and biological samples. Although these approaches have made great contributions toward Hg(II) assay, there still existed some limitations such as insufficient water-solubility, irreversible Hg(II) complex, poor selectivity, long response-time and sophisticated synthesis of the probe or sensor materials.
Herein, we report a facile, fast responsive, highly selective probe for monitoring Hg(II) in water samples based on the fluorescence quenching of 2,9-dimethyl-1,10-phenanthroline (2,9-DMP) by Hg(II). Meanwhile, the binding model of complex 2,9-DMP–Hg(II) was explored and characterized by the Job's plot analyses and the single crystal X-ray diffraction for two new metal–organic frameworks (MOFs) of 2,9-DMP–Hg(II).
Fluorescence spectra were recorded with a Hitachi FL-2700 fluorescence spectrophotometer (Hitachi, Japan). The pH measurements were performed on a PHS-3C digital pH-meter (INESA, China). X-ray diffraction data of single crystals were collected using a Bruker APEX II diffractometer (Bruker, Switzerland).
For selectivity and interference investigation, the solution of 2,9-DMP and 19 metal ions were mixed to obtain the final concentration of 10 μM for 2,9-DMP and 20 μM for 19 metal ions in the mixed solution, respectively. Meanwhile, in order to evaluate the interference of the mixture of 18 metal ions on the fluorescence intensity, 10 μM 2,9-DMP and 1 μM Hg2+ were mixed. Then 18 reference metal ions (the concentration of each one was 1 μM) were added to above mixture. The resulted solution was shaken well and analyzed immediately. Then all the fluorescence spectra studies were performed on a Hitachi FL-2700 fluorescence spectrophotometer. The excitation wavelength of fluorescence spectrometry was set at 275 nm. Excitation and emission slits were set at 10 nm and the voltage of the photomultiplier was set at 400 V in the fluorescence test except for the quantitative determination (700 V).
In order to investigate the possibility of the quantitative detection of Hg(II) with the 2,9-DMP fluorescent probe, a series standard concentrations of Hg(II) were added to 3 μM 2,9-DMP solution at pH 2.0 adjusted with Tris-HNO3 buffer solution. The fluorescence emission spectra of the standard solution were recorded under the excitation wavelength of 275 nm.
To investigate the binding stoichiometry of 2,9-DMP–Hg(II) complex, firstly, 10−4 M of 2,9-DMP and 10−4 M of Hg(II) stock solution at pH 2.0 were prepared respectively. Then the stock solution of 2,9-DMP and Hg(II) were mixed with ratio from 9:
1 (v/v) to 1
:
9 (v/v) in test tubes to keep a total volume of 1 mL. Finally, the mixtures were diluted to 10 mL with a Tris-HNO3 buffer solution at pH 2.0 to get the test solution with a total concentration of [2,9-DMP] + [Hg(II)] = 10 μM.
To test the applicability of the proposed fluorescence probe for the analysis of practical samples, the Hg(II) in drinking water and Zhujiang River water was detected by the proposed method. To estimate the detection accuracy, the water samples were spiked with Hg(II) standard solution to the concentrations of 0.50, 1.0 and 2.0 μM. The recoveries of spiked Hg(II) standard solution in drinking water were calculated by the following formulate:
The evaluation and confirmation of MOF 1 and MOF 2 were performed by single crystal X-ray diffraction with a Bruker APEX II diffractometer (Bruker, Switzerland).
In order to explore the possibility to use 2,9-DMP as probe for sensing Hg(II) in real water sample, the influence of coexisting metal ions on the fluorescence of 2,9-DMP–Hg(II) was also characterized in detail (Fig. 1B). As can be seen from Fig. 1B, the fluorescence quenching of 2,9-DMP–Hg(II) is not affected by most metal ions except for several metal ions such as Fe(II), Cr(III), Mn(II), Ba(II), Ca(II) and Co(II). Nevertheless, the interference of above six metal ions on the fluorescence of 2,9-DMP–Hg(II) is slight. Amongst them, only the relative error produced by Fe(II) is over 10%. This is possibly ascribed to that 2,9-DMP has the similar skeleton structure to phenanthroline which can complex with Fe(II). In fact, there is almost no Fe(II) in air-exposed water due to the high reduction capacity of Fe(II). In order to simulate the water sample, the cooperative influence of the mixture of 18 reference metal ions on the fluorescence of 2,9-DMP–Hg(II) was also investigated. It can be seen from Fig. 1C that the fluorescence of complex 2,9-DMP–Hg(II) was slightly enhanced by 7.5% due to the addition of 18 reference metal ions (Fig. 1C, blue line), which demonstrated that the selectivity of the probe 2,9-DMP for detection of Hg(II) in complicated water sample is acceptable.
First of all, the rapid response capacity is very important for an on-site or on-line monitoring method in field. Unfortunately, a lot of literature did not mention the responsive time of the probes for sensing related targets. In some other literatures, the reaction time between probe and target metal ions has been reported to be more than half or one hour32,33 For this purpose, the response time of 2,9-DMP to Hg(II) was monitored on-line with a Hitachi FL-2700 fluorescence spectrophotometer. As shown in Fig. 2, the complex reaction of 2,9-DMP and Hg(II) is completed within about 20 s, which is much faster than that of most literature reported.32–38 Hence the probe 2,9-DMP can be utilized to rapid monitor Hg(II) in-site or/and on-line in environmental water.
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Fig. 2 The fluorescence response process of the reaction of 2,9-DMP (10 μM) and Hg(II) (10 μM) monitored by a fluorescence spectrophotometer on-line. |
On the other hand, the pH value of the environment usually shows an effect on the performance of probe toward target metal ions due to the protonation or deprotonation reaction of the fluorophore and the hydrolysis reaction of the metal ions. Thus, it is necessary to investigate the effect of pH value of aqueous solution on the fluorescence of probe 2,9-DMP. Accordingly, a series of 2,9-DMP solutions with pH value ranging from 0.0 to 6.0 adjusted with Tris-HNO3 were prepared, since precipitation shall produce when pH is above 7.0. The variation of the fluorescence intensity of the probe (10 μM) in the absence and presence of Hg(II) (10 μM) with the pH value was studied and is outlined in Fig. 3. As depicted in Fig. 3, the fluorescence intensity of the probe 2,9-DMP is slightly affected by pH value of solution in the range of 0.5–3.5 while the fluorescence intensity of 2,9-DMP is decreased rapidly when pH > 4.0. On the other hand, the fluorescence intensities of complex 2,9-DMP–Hg are almost not affected by the pH value in the range from 0.0 to 6.0, indicating that the complex 2,9-DMP–Hg(II) is very stable in pH < 6.0 media. In consideration of the stability of 2,9-DMP, the following experiments were all performed in pH 2.0 solution adjusted with Tris-HNO3.
To test the applicability of the proposed fluorescence probe for the analysis of practical samples, the Hg(II) in drinking water and digested Zhujiang River water samples were detected by the proposed method. Since the concentrations of Hg(II) in drinking water and Zhujiang River water samples are too low to detect, the drinking water samples were spiked directly with Hg(II) standard solution to concentrations of 0.50, 1.0 and 2.0 μM. And the Zhujiang River water samples were spiked with Hg(II) standard solution to concentrations of 0.50, 1.0 and 2.0 μM and then digested with concentrated HNO3. Then the residues were dissolved in deionized water and detected using the proposed method. The recoveries were listed in Table S1.† As can be seen from Table S1,† the satisfactory recoveries of addition Hg2+ standard solution were obtained at 85.42–101.5% and 81.72–96.09% for drinking water and Zhujiang River water, respectively. In summary, the fluorescence probe 2,9-DMP provides a very highly selective and accurate monitoring method for determination of Hg(II) in drinking water and Zhujiang River water samples within a practical concentration range.
Firstly, the Job's plot analysis was used for determination the stoichiometry between 2,9-DMP and Hg(II). All the analyses were performed with the total concentration of [2,9-DMP] and [Hg(II)] set at 10 μM. As shown in Fig. 5, the related fluorescence intensity (F0 − F) of 2,9-DMP is increased firstly and then is decreased gradually with the increase of the ratio of Hg(II) to [2,9-DMP] + [Hg(II)]. The fluorescence intensity reaches the maximum value when the ratio of Hg(II) to [2,9-DMP] + [Hg(II)] is about 0.5, which suggests a 1:
1 complex between 2,9-DMP and Hg(II) has been formed.
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Fig. 5 Job's plot for determining the stoichiometry of the probe and Hg(II) ions (the total concentration of 2,9-DMP and Hg(II) ions was 10 μM, λex = 275 nm). |
On the other hand, the 1:
1 complexation mode of 2,9-DMP with Hg(II) has been further confirmed by the single crystal X-ray diffraction results of the two new 2,9-DMP–Hg(II) MOFs which were synthesized by hydro–solvothermal synthesis in the existence of Cl− and NO3− media, respectively. The two similar yellow block crystals were obtained and the appearance of MOF 1 was depicted in Fig. S1.†
The two new MOFs were further characterized by single-crystal X-ray diffraction measurement with a Bruker APEX II diffractometer at 298 K using graphite monochromatic Mo-Kα radiation (λ = 0.71073 A). The structures were solved by direct methods and refined by full-matrix least squares on F2 with SHELXS-97 and SHELXL-97 program. All non-hydrogen atoms were refined with anisotropic displacement parameters. Hydrogen atoms were included in calculated positions and refined with isotropic thermal parameters riding on the parent atoms. The single-crystal X-ray diffraction studies reveal that the Hg–MOF 1 synthesized in NO3− media is a 1-D MOF. Hg–MOF 1 crystallized in the monoclinic space group P21/c with an asymmetric unit containing two Hg(II) ions, one 2,9-DMP and two nitrate ions. As shown in Fig. 6A, Hg1 is tetra-coordinated to two pyridine nitrogen atom (N1, N2), one nitrate oxygen atoms (O6a), exhibiting a distorted tetrahedral coordination geometry. Another Hg2 ion is hexa-coordinated with five nitrate oxygen atoms (O2, O3, O3a, O5, O6), showing distorted octahedral coordination geometry. The bond lengths of Hg–O are in the range from 2.239(12) Å to 3.032(12) Å while that of Hg–N are in the range from 2.263(13) Å to 2.369(13) Å. Every neighboring N center is linked together with an Hg⋯Hg bond with the distance of 2.5205(10) Å to generate a MOF unit which is further connected by the oxygen atoms of the nitrate, giving rise to a 1-D Hg-chain along with a axis (Fig. 6B). The more detailed crystal data and structure refinement for Hg–MOF 1 are shown in Table S2.† The selected bond lengths and angles for Hg–MOF 1 are listed in Table S3.†
Meanwhile, another yellow block Hg–MOF 2 was also obtained in the Cl− media and crystallized in monoclinic, C2/c space group. The X-ray diffraction data of Hg–MOF 2 (Fig. 7A) suggests that its asymmetric unit contain three Hg(II) ions, two 2,9-DMPs and four chlorides. Hg1 is tetra-coordinated to four chlorides (Cl1, Cl2, one μ2-bridged and the other terminal coordinated), exhibiting a distorted tetrahedral coordination geometry. Hg2 ion is tetra-coordinated with one μ2-bridged chlorides (Cl1a), two nitrogen atoms (N1, N2) on 2,9-DMP, and another Hg2 ion, showing distorted tetrahedral coordination geometry. The bond lengths of Hg–N are in the range from 2.348(12) Å and 2.351(12) Å while that of Hg–Cl are in the range from 2.391(13) Å to 2.619(13) Å. Hg–MOF 2 exhibits a 1-D chain with an Hg⋯Hg distance of 2.5523(8) Å (Fig. 7B). The detailed crystal data and structure refinement for Hg–MOF 2 are shown in Table S4† and the selected bond lengths and angles for this MOF are listed in Table S5.†
In a word, both the Job's plot analyses and the results of single crystal X-ray diffraction of two new Hg–2,9-DMP MOFs demonstrate that the complexation mode of 2,9-DMP and Hg(II) is 1:
1 type. And the data of single crystal X-ray diffraction indicate that 1-D chain structures of two new Hg–2,9-DMP MOFs are connected by the bridges of Hg(NO3)3− or HgCl42− in NO3− or Cl− media, respectively.
Footnote |
† Electronic supplementary information (ESI) available. CCDC 1479671 and 1479677. For ESI and crystallographic data in CIF or other electronic format see DOI: 10.1039/c6ra13514e |
This journal is © The Royal Society of Chemistry 2016 |