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Ultrasensitive Detection of Hg(II) Ions in Aqueous Medium Using Zinc-Based Metal−Organic Framework Asha Pankajakshan,† Denis Kuznetsov,‡ and Sukhendu Mandal*,† †

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School of Chemistry, Indian Institute of Science Education and Research Thiruvananthapuram, Thiruvananthapuram, Kerala 695551, India ‡ Department of Functional Nanosystems and High Temperature Materials, National University of Science and Technology, MISIS, Leninsky, pr. 4, Moscow 119049, Russia S Supporting Information *

ABSTRACT: Here we describe, a simple solution and solidstate sensor for the ultrasensitive detection of Hg(II) ions in both standardized and environmental samples through changes of fluorescence intensity of a zinc-based metal− organic framework (MOF). The MOF is highly water stable in a wide pH range from 4 to 11. The lower detection limit of Hg2+ is 10−11 M with a very high binding constant of 1.011 × 109 M−1 s−1. It also exhibits a high selectivity toward mercury ion in the presence of other interfering metal ions. The MOF is found to retain its efficiency in detecting mercury ion in spiked environmental water samples.

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and cations.16−18 On the contrary, relatively fewer MOF sensors are reported for the mercury(II) ions compared to other means such as self-assembled materials (SAMs),19 nanoparticles (NPs),20,21 carbon-dots,22 biosensors,23,24 and dye bound optical sensors.25,26 Quantitative detection of Hg(II) ion in an aqueous medium using MOFs faces challenges due to the poor dispersion of MOFs in an aqueous medium. Another critical factor that affects selective and sensitive detection of Hg(II) ions using MOF is the interference of the coexisting ions like Cu2+, Zn2+, Cd2+, Pb2+, etc. Not only that, most of the MOF based Hg(II) ion sensors do not exhibit a lower detection limit of 10 nM, which is the maximum permitted level of Hg(II) ion in drinking water as per USA EPA (Table S1). Mercury ion has high complexation affinity to N atoms, and it has a larger ionic radius compared to the coexisting interfering ions. On the basis of these criteria, we have taken the strategy to incorporate nitrogen-containing ligands for the construction of the porous structure. Hence, we have used both 5-amino isophthalic acid and 4,4′-azopyridine with the amino and the azo (−N=N−) groups for larger space and specific interaction sites to enhance the sensitivity and selectivity of Hg(II) ion. Herein, we have reported the synthesis of a zinc-based porous structure {[Zn(4,4′-AP)(5-AIA)]. (DMF)0.5}n, (1) [4,4’ AP = 4,4′-azopyridine, 5-AIA = 5-amino isophthalic acid, and DMF = N,N′-dimethylformamide] for the swift,

INTRODUCTION The rapid industrialization expels a considerable sum of contaminants including the deadly poisonous mercury(II) ion into the environment.1 In the environment, ionic mercury is converted into neurotoxic methylmercury by bacteria. Even minor quantity of bioaccumulated mercury can cause health problems related to vital body organs like the spinal cord, kidney, etc.2 The United States Environmental Protection Agency (US EPA) has set up the standard for the maximum permissible amount of Hg(II) ion in drinking water as 10 nM. Therefore, it is very much essential to detect Hg(II) ion in water for environmental observation and health protection. Several sophisticated techniques are available for the detection and quantification of Hg(II) ion including chromatography,3 spectrofluorimetry,4 and atomic absorption spectroscopy,5 but all of these are time-consuming and expensive. Hence, quick and facile detection and quantification of trace Hg(II) ion remain a challenge in the field of environmental study and protection.6,7 The fluorescence-based methods possess marked nobleness because of their benefits like extraordinary sensitivity, simplicity in operation, and cost-effectiveness.8,9 Luminescent metal−organic frameworks (MOFs) are in the limelight for over a decade because of their high porosity, facile synthesis, and remarkable thermal and mechanical stability.10 The luminescent nature of MOFs depends on the structure, bonding, interactions (like H-bonding, π−π interaction), etc.;11 hence, by wisely selecting the metal ions and ligands, the luminescence can be tuned for a strategic application.12 Several luminescent MOFs are constructed for the selective sensing of solvent molecules,13 pharmaceuticals,14 anions,15 © XXXX American Chemical Society

Received: October 15, 2018

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DOI: 10.1021/acs.inorgchem.8b02898 Inorg. Chem. XXXX, XXX, XXX−XXX

Article

Inorganic Chemistry

S1). The disordered DMF molecules reside in the pores of the onedimensional channel (Figure S1d). The topological analysis identified that the structure has a new topological network with a uninodal 7-c net, which can be presented as a Schläfli symbol {36.48.56.6} (TD10 = 1946) (see Supporting Information). The experimental powder X-ray diffraction pattern was matching with the simulated XRD patterns, showing crystalline phase purity (Figure S2). The PXRD pattern of compound 1 was collected after soaking the compound in water and some organic solvents (Figures S3 and S4). The pattern was matching with the simulated one, indicating the structural stability in water and some organic solvents. The ICP-AES data of the supernatant were also collected after 24 h soaking in water, and it was found that compound 1 was stable without degradation (Table S5). The infrared spectrum of compound 1 exhibited all the characteristics peaks for aromatic ligands (Figure S5). The thermogravimetric analysis showed the initial weight loss up to 80 °C, and it may be due to the loss of DMF molecules from the pores. It was stable up to 115 °C; after that, it started to lose the organic struts (Figure S6). The pore volume and BET surface area of compound 1 were calculated as 1.668 cm3/g and 173 m2/g, respectively (Figure S7).

selective, and sensitive detection of the toxic mercury ion in water. The soft acid Hg(II) ion has a higher affinity toward the soft base like N; hence the incorporation of the nitrogencontaining ligands 4,4′-AP and 5-AIA in the structure guides the Hg(II) ion for specific interaction and corresponding variation in the ligand-based luminescence property.27,28 This as-synthesized compound exhibits excellent water stability and can selectively probe Hg(II) ion in the presence of other interfering metal ions in picomolar (femtogram) limit. On the basis of our knowledge, this is one of the rare examples that senses Hg(II) ion in an aqueous medium in picomolar (femtogram) limit with very high binding constant (Table S1).

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EXPERIMENTAL SECTION

Synthesis, Characterization and Structure. Compound 1 was synthesized solvothermally using Zn(NO3)2·6H2O (0.1 mmol), 4,4′azopyridine (0.1 mmol), and 5-amino-isophthalic acid (0.1 mmol) in DMF solvent at 100 °C for 20 h. The dark brown colored block crystals were characterized using single crystal X-ray diffraction, powder X-ray diffraction, TGA, IR, etc. The single crystal X-ray diffraction studies revealed that compound 1 crystallizes in the P21/c space group (Table S2). The Zn ion adopts distorted tetrahedral geometry with two oxygen atoms of the 5-AIA and one nitrogen atom from 4,4′-AP molecule and another nitrogen atom from the 5-AIA ligand, respectively (Tables S3 and S4). As a result of two monodentate carboxylate groups and a monodentate amino group, each 5-AIA connects three Zn centers to form two-dimensional sheetlike structure along bc plane (Figure 1a). The angle between the

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RESULTS AND DISCUSSION Solution-state luminescence studies were conducted for the compound 1, 4,4′-azopyridine, and 5-amino isophthalic acid at room temperature. The compound 1 shows a blue emission peak at 405 nm when excited at 300 nm. This peak corresponds to the 5-AIA ligand-centered electronic transitions (Figure S8 and S9). The quantum yield (QY) of the compound 1 is calculated to be 11% (see Supporting Information). The high QY and stability of compound 1 in aqueous phase provoked us to explore its sensing properties in the aqueous medium. To carry out the sensing study, we have done fluorescence titration of compound 1 in an aqueous medium with the addition of different mono- and divalent metal ions like Li+, K+, Ca2+, Cd2+, Pb2+, Hg2+, etc. Results showed that there was a substantial change in fluorescence intensity in the case of Hg(II) ion, while it was negligible for other analytes (Figures 2 and 3). This demonstrates the high selectivity of 1 toward Hg(II) ion over other cations in the aqueous phase. The effect of water on the luminescence intensity of compound 1 was

Figure 1. (a) Two-dimensional layered architecture along the bc plane, (b) three-dimensional structure of compound 1, where layered architecture is pillared by 4,4′-azopyridine ligand. tetrahedral zinc centers is ∼136°, which is similar to the zeolitic imidazolate framework (ZIF) and Si−O−Si angle in the zeolite structure. This particular structural arrangement may provide stability in an aqueous media like ZIF or zeolite structure.29 The sheets are pillared by the 4,4′-AP ligand in the longitudinal axes with the distance approximately 13 Å. This connectivity produces one-dimensional rectangular-shape channels, with a dimension of 8.25 × 13.02 Å2 without considering van der Waals radii (Figures 1b and

Figure 2. Graph shows the quenching of fluorescence by adding Hg(II) ion. Photograph in the inset is the color change under UV light. B

DOI: 10.1021/acs.inorgchem.8b02898 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry

Figure 4. Percentage of quenching of fluorescence by adding Hg(II) ion solutions of different concentrations.

mono- and di-valent cations (Li+, Na+, K+, Mg2+, Ca2+, Sr2+, Ba2+, Mn2+, Ag+, Fe2+, Co2+, Ni2+, Cu2+, Zn2+, and Cd2+) (Figure S17). The experiments with interfering cations revealed that the Hg(II)-induced fluorescence response was unaffected by other metal ions, which exhibited that compound 1 has selective interaction with Hg(II) ion (Figure S18). We have investigated the luminescence stability of compound 1, before and after Hg(II) ion addition, in a wide pH range from 2 to 12. At a pH below 4, the compound 1 is found to be degrading by the broadness and shift in the emission peak. The emission is almost unchanged in the pH range of 8−12. At pH 7, compound 1 exhibits the detectable quenching after Hg(II) ion addition. A negligible quenching was observed in the range of pH 5−7 (Figure S19). The high selectivity of compound 1 for Hg(II) ion is probably ascribed to a combination of several factors such as the larger radius and coordination diversity of Hg(II), the binding ability of azo group with Hg(II) ion to form a stronger Hg−N bond. The presence of the soft donor atom nitrogen in the MOF framework increases its selectivity and affinity toward the Hg(II) ions, which is a soft acid. In compound 1, 5-AIA is bonded with Zn centers to form the layered architecture. The 4,4′-AP acts as a pillar to the layers to form the threedimensional structure with the one-dimensional rectangular channel, which can be readily accessed by the Hg(II) ions. The interaction of Hg(II) ions with free-standing −NN− group affects the electron delocalization in the compound 1 and subsequently changes the fluorescence intensity significantly.30,31 To support our hypothesis, we have carried out FTIR and XPS studies of compound 1 before and after the sensing experiments. The slight shift in the FTIR peaks corresponding to the C−N stretching, after the addition of Hg(II), supports the mechanism (Figure S20). The N 1s XPS spectrum of compound 1 exhibits a broad peak at around ∼400 eV, which fits well with the three peaks at ∼399.10 eV, ∼402.36 eV, and ∼403.57 eV, respectively, assigned to the N atoms from pyridine group, azo group, and coordinated −NH2 group. After addition of Hg(II) ion, all the peaks red-shifted slightly, and a new peak appeared at ∼405.53 eV, which indicated the interaction of N atoms of the azo group in the compound 1 with the Hg(II) ion. The binding of Hg2+ and −N=N− leads to loss of electron density at the azo group, which in turn raises its binding energy (Figure 5).28,32 More importantly, the space in the one-dimensional channel between −N=N− groups of the 4,4′-AP molecules is more suitable to interact with the larger ionic radius of Hg(II) ion than other metal ions. The compound 1 was thoroughly characterized after the sensing studies to prove the binding of Hg(II) ion. The scanning electron microscopic image after the

Figure 3. Percentage of fluorescence quenching for different metal ions in an aqueous medium at room temperature.

checked as well which exhibited insignificant change (Figure S10). We have carried out the control experiments with 5-AIA in water, and it showed no visual detection of color change after Hg(II) ions addition (Figure S11). This proved that the framework of Zn, 5-AIA, and 4,4′-AP has a key role in the selective detection of Hg(II) ion. We carried out steady-state and time-resolved emission studies to underscore the fluorescence quenching mechanism. The relative change in the fluorescence intensity as the function of the concentration of the Hg(II) ion was estimated via Stern−Volmer (SV) plot, and SV constant was calculated to be 1.011 × 109 M−1 (Figure S12). According to our knowledge, this is one of the highest among those reported for Hg(II) ion sensing using MOF/ coordination polymer or nanomaterials or self-assembled materials (Table S1). The plot in figure S12 suggests an excellent linear relationship (R2 = 0.953) between the change in intensity and the concentration of Hg(II) ion. The rate of quenching (kq) was found to be high around 0.54 × 1014 M−1 s−1 (see Supporting Information). It was also observed that the absorbance peak for compound 1 has been shifted to a wavelength of 330 nm after the addition of Hg2+ and it may be due to the formation of a ground state complex (Figure S13). The excitation spectra were recorded for compound 1 after the addition of Hg(II) ion and displayed no peak at 330 nm, which indicated the nonemissive nature of the ground state complex and hence acted as subsidiary data for the static mechanism (Figure S14). The temperature dependent quenching experiments were carried out at higher temperature, which gave a lower Ksv value, confirming the disorder in the ground state interaction between the compound 1 and Hg(II) ion as temperature increases (Figure S15). The comparable lifetimes obtained before and after the addition of Hg(II) ion suggest the existence of a static interaction between the fluorophore and Hg(II) ion (Figure S16 and Table S6). The detection limit of compound 1 for the Hg(II) ion sensing is 9.9 × 10−12 M or 0.27 femtogram, which is the lowest value reported so far for a porous hybrid compound (Figures 4 and S17 and Table S1) and even lower than the maximum permitted level of Hg(II) ion in drinking water (10 nM) regulated by USA EPA. The high selectivity and specificity are necessary for real sample detection, and for that we have collected the emission spectra by adding other cations into the aqueous suspension of compound 1 under the same conditions. No significant spectral changes were observed in the presence of other C

DOI: 10.1021/acs.inorgchem.8b02898 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry

1 disturbs the electronic conjugation of the system, which in turn leads to the quenching of fluorescence. Natural water samples analysis revealed that compound 1 could detect Hg(II) ions in natural water even in the presence of other interfering ions. We have also shown that compound 1 can act as a solid state sensor for Hg(II) ion sensing in aqueous media. Presently, we are working on to fabricate a device based on this compound for easy and facile detection of toxic Hg(II) ion in an aqueous medium with record femtogram limit.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.8b02898. Materials and methods, all characterization data, fluorescence spectra, ICP-AES data, natural water analysis (PDF)

Figure 5. XPS plots of compound 1 (a) before and (b) after binding with Hg(II) ion. N1, N2, and N3 represent the nitrogen of azo, pyridine, and amino group nitrogens.

Accession Codes

CCDC 1831742 contains the supplementary crystallographic data for this paper. These data can be obtained free of charge via www.ccdc.cam.ac.uk/data_request/cif, or by e-mailing [email protected], or by contacting The Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ, UK; fax: +44 1223 336033.

quenching studies exhibited no change in morphology (Figure S21). The SEM-EDX spectra illustrate the presence of the element mercury in the compound 1 after the quenching study (Figure S22). The PXRD pattern before and after sensing proved that compound is stable after Hg(II) sensing in aqueous medium (Figure S23). To find the real water sample analysis application, we have carried out the sensing studies using water from four different natural sources such as seawater, river water, tap water, and drinking water. All of the water samples are tested using ICPAES and found no presence of mercury in the water samples except a negligible amount in sea water (Table S7). The sensing analysis was done using the standard addition method. Surprisingly, in three of these, we could detect the mercury ions with more or less similar sensitivity compared to the standard prepared in double distilled water. Even in the seawater, we could detect mercury up to 16.7% (Table S8). These results show that compound 1 can have good accuracy and adequate reproducibility to detect Hg(II) ion in natural water, which also implies that the present probe (compound 1) might be useful to detect Hg(II) ion in environmental samples. Toward real application, we have synthesized a solid-state sensor based on compound 1. For that, we have grown the single crystals of compound 1 on an aluminum foil. Then these single crystals were used for the detection of Hg(II) ion in an aqueous medium (Figures S24 and S25). This solid sensor can detect Hg(II) ion in an aqueous medium in the micromolar level. The efficiency of the solid-state sensor is lesser compared to the solution state, which might be due to the inhomogeneity of the material in the solid state.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Sukhendu Mandal: 0000-0002-4725-8418 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We acknowledge Science and Engineering Research Board (SERB), Govt. of India, (EMR/2016/007501) and the Ministry of Education and Science of the Russian Federation in the framework of increase Competitiveness Program of NUST “MISIS”, implemented by a governmental decree dated March 16, 2013, No. 211 for financial support. A.P. acknowledges CSIR for SRF. We are grateful to Sreehari S R for the pH-dependent study.

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REFERENCES

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CONCLUSIONS In summary, a water stable MOF (compound 1) was constructed for the selective and sensitive detection of Hg(II) ion in the aqueous medium. The compound 1 can detect picomolar (femtogram) concentration of Hg(II) ion with very high quenching constant of 1.011 × 109 M−1 in an aqueous medium. This particular compound can selectively and sensitively detect Hg(II) ions without any interference of coexisting ions in the aqueous medium. The specific interaction of the Hg(II) ion with the freestanding −N=N− group residing on the one-dimensional rectangular channel of D

DOI: 10.1021/acs.inorgchem.8b02898 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry

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DOI: 10.1021/acs.inorgchem.8b02898 Inorg. Chem. XXXX, XXX, XXX−XXX