Double Solvent Sensing Method for Improving Sensitivity and

Aug 4, 2017 - Department of Chemistry, Faculty of Sciences, Tarbiat Modares University, P.O. Box 14117-13116, Tehran, Islamic Republic of Iran ..... B...
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Double Solvent Sensing Method for Improving Sensitivity and Accuracy of Hg(II) Detection Based on Different Signal Transduction of a Tetrazine-Functionalized Pillared Metal−Organic Framework Sayed Ali Akbar Razavi,† Mohammad Yaser Masoomi,† and Ali Morsali* Department of Chemistry, Faculty of Sciences, Tarbiat Modares University, P.O. Box 14117-13116, Tehran, Islamic Republic of Iran S Supporting Information *

ABSTRACT: To design a robust, π-conjugated, low-cost, and easy to synthesize metal−organic framework (MOF) for cation sensing by the photoluminescence (PL) method, 4,4′-oxybis(benzoic acid) (H2OBA) has been used in combination with 3,6-di(pyridin-4-yl)-1,2,4,5-tetrazine (DPT) as a tetrazine-functionalized spacer to construct [Zn(OBA)(DPT)0.5]·DMF (TMU-34(-2H)). The tetrazine motif is a π-conjugated, water-soluble/stable fluorophore with relatively weak σ-donating Lewis basic sites. These characteristics of tetrazine make TMU-34(-2H) a good candidate for cation sensing. Because of hydrogen bonding between tetrazine moieties and water molecules, TMU-34(-2H) shows different PL emissions in water and acetonitrile. Cation sensing in these two solvents revealed that TMU-34(-2H) can selectively detect Hg2+ in water (by 243% enhancement) and in acetonitrile (by 90% quenching). The contribution of electron-donating/accepting characteristics along with solvation effects on secondary interactions of the tetrazine motifs inside the TMU-34(-2H) framework results in different signal transductions. Improved sensitivity and accuracy of detection were obtained using the double solvent sensing method (DSSM), in which different signal transductions of TMU-34(-2H) in water and acetonitrile were combined simultaneously to construct a double solvent sensing curve and formulate a sensitivity factor. Calculation of sensitivity factors for all of the tested cations demonstrated that it is possible to detect Hg2+ by DSSM with ultrahigh sensitivity. Such a tremendous distinction in the Hg2+ sensitivity factor is visualizable in the double solvent sensing curve. Thus, by application of DSSM instead of one-dimensional sensing, the interfering effects of other cations are completely eliminated and the sensitivity toward Hg(II) is highly improved. Strong interactions between Hg2+ and the nitrogen atoms of the tetrazine groups along with easy accessibility of Hg2+ to the tetrazine groups lead to a shorter response time (15 s) in comparison with other MOF-based Hg2+ sensors.

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INTRODUCTION Sensing of Hg2+ is of special importance because this ion is the most stable inorganic form of mercury with good solubility in water that, even at low concentrations, can cause many serious problems like digestive, kidney, and specially neurological complications for humans.1,2Release of Hg2+ in nature through industrial waste has dramatically increased. Considering the toxicity and widespread release of Hg2+, a fast, facile, cheap, and effective method to detect Hg2+ is needed.3,4 Traditional detection methods for Hg2+, like gas chromatography, ultrasensitive stripping voltammetry, atomic emission/ absorption spectrometry, inductively coupled plasma mass spectrometry, cold-vapor atomic fluorescence spectrometry, and anodic stripping voltammetry are quite sensitive and effective, but they are very expensive and slow.5−15 Recently, photoluminescence (PL) methods have drawn a lot of interest to themselves for sensing applications because these methods are very selective, sensitive, low-cost, and visualizable while offering rapid response times and real-time monitoring.16 Metal−organic frameworks (MOFs) are a class of crystalline porous materials that have received a lot of attention in sensing © 2017 American Chemical Society

applications because of their unique characteristics, especially for porosity and tunable chemical functionalization. The PL behaviors of MOFs originate from the conjugated organic linkers and/or metal ions or clusters as well as some other factors like host−guest/solvent interactions, hydrogen bonding, and π−π interactions in their crystalline structures. In linkerbased luminescence behavior, π-extended conjugated MOFs have excellent signal transduction. Moreover, immobilization of functional groups as fluorophores has a significant effect on the secondary interactions and PL behavior of MOFs.17,18 Many studies of Hg2+ detection have encountered challenges like long response times, the poor dispersion of MOFs in water, framework collapsr, and noticeable effects of other cations on the PL spectra.11,19−28 These problems lead to lower sensitivity, precision, and accuracy of detection. The 1,2,4,5-tetrazine motif shows unique characteristics like reversible redox activity, significant color change, and low-lying π* orbitals that make it usable for applications in the fields of Received: May 6, 2017 Published: August 4, 2017 9646

DOI: 10.1021/acs.inorgchem.7b01155 Inorg. Chem. 2017, 56, 9646−9652

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Inorganic Chemistry Scheme 1. Structure of TMU-34(-2H) and Tetrazine-Functionalized Pores

mg of TMU-34(-2H) powder into 4.00 mL of water or 4.00 mL of acetonitrile. (III) Cation-incorporated TMU-34(-2H) emulsions were prepared by adding 1−5 mL of target analyte solution (1.0 × 10−3 M) into the emulsions of step (II) in an incremental fashion to achieve target analyte concentrations of (2−10) × 10−4 M. (IV) The mixed solutions prepared in step (III) were shaken for a few seconds. After these steps, the fluorescence properties of all cations incorporated in TMU-34(-2H) emulsion solutions were measured. The sensitivity factor (S) is formulated as follows:

sensors, electrochemistry, and conducive materials, but its applications in supramolecular chemistry are rare.29 Here, by applying the electron-donating/accepting characteristics of 1,2,4,5-tetrazine, we suggest a strategy for ultrahigh-sensitivity sensing of Hg(II). Also, secondary interactions of the tetrazine group are completely discussed.

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EXPERIMENTAL SECTION

Materials and Characterization. All of the required chemicals were obtained from commercial suppliers and used without further purification unless otherwise stated. Ultrasonic generation was carried out in a SONICA-2200 EP ultrasonic bath (frequency of 40 kHz). Powder X-ray diffraction (PXRD) measurements were performed using a Philips X’pert diffractometer with monochromatized Cu Kα radiation. Elemental analyses were carried out on a CHNS Thermo Scientific Flash 2000 elemental analyzer. Sorption studies were performed with a TriStar II 3020 surface area analyzer from Micromeretics Instrument Corporation using N2 at 77 K. The samples were characterized using energydispersive X-ray spectroscopy (EDAX) on a CamScan MV2300 instrument with gold coating. Thermal behavior was measured with a PL-STA 1500 apparatus at a heating rate of 10 °C min−1 in a static atmosphere of argon. Synthesis. 3,6-Di(pyridin-4-yl)-1,2,4,5-tetrazine (DPT). The DPT spacer was synthesized according to reported procedures (Scheme S1).30−32 [Zn(OBA)(DPT)0.5]·DMF (TMU-34(-2H)). A powdered sample of TMU-34(-2H) was synthesized by mixing Zn(CH3COO)2·2H2O (0.22 g, 1 mmol), H2OBA (0.26 g, 1 mmol), and DPT (0.24 g, 1 mmol) in 30 mL of DMF/acetonitrile (1:1) and sonicating the mixture for 60 min at ambient temperature and atmospheric pressure. The mixture was then centrifuged, and the resulting powder was washed with DMF and dried at 80 °C for 48 h. Yield: 0.37 g (85% based on OBA). IR data for selected bands (KBr pellet, ν/cm−1): 656(w), 780(w), 874(w), 1091(w), 1160(m), 1231(m), 1399(vs), 1500(s), 1629(s), 1672(s), 3429(m). Elemental analysis (%) found for [Zn(C14O5H8)(C12N6H8)0.5]·(C3ONH7): C, 54.0; N, 10.9; H, 3.9. Fluorescence Measurements. Luminescence spectra were recorded with a PerkinElmer LS-55 fluorescence spectrometer at room temperature. Both the excitation and emission slit widths were 10 nm. Fluorescence measurements were carried out in a 1 cm quartz cuvette. For fluorescence measurements, the following steps were taken: (I) Stock solutions in water of nitrate salts of metal ions (Na+, Mg2+, Sr2+, Al3+, Co2+, Ni2+, Cu2+, Zn2+, Cd2+, Pb2+, Hg2+, Li+, K+, Fe2+, Fe3+, Ca2+, and Cr3+) at a concentration of 10−3 M were prepared. (II) Highly dispersed emulsions of TMU-34(-2H) were prepared by introducing 2

S=

Rf − Ri Ri

where Ri and Rf are the initial sensor response and the response of the sensor in the presence of the desired cation, respectively. This response is defined as the ratio of the intensities of the TMU-34(-2H) PL emission peaks in water and acetonitrile. The quenching or enhancement effect was calculated using the Stern−Volmer equation:

I0 = 1 + KSV[M] I where I0 and I are the luminescence intensities of the suspension of TMU-34(-2H) without and with the addition of cation, respectively, [M] is the molar concentration of the cation, and KSV is the Stern− Volmer quenching constant.26

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RESULTS AND DISCUSSION Characterization. TMU-34(-2H) was synthesized in a fast and facile manner via a sonochemical reaction using zinc acetate, H2OBA as the oxygen donor ligand, and DPT as the nitrogen donor spacer (Scheme 1). The PXRD pattern of the sonochemically synthesized TMU-34(-2H) is identical to simulated pattern reported earlier (Figure S1).32,33 TMU-34(-2H) is constructed from paddle-wheel Zn2(COO)4 clusters pillared by DPT spacer ligands.32 It contains three-dimensional interconnected pores (aperture size: 4.4 Å × 8.1 Å, including van der Waals radii) that are functionalized with tetrazine groups (Figures S2 and S3).32 To study the porosity of TMU-34(-2H), N2 adsorption/desorption was carried out at 77 K and 1 bar on the sonochemically synthesized sample. The results showed that TMU-34(-2H) can adsorb 201 cm3·g−1 N2 with a Brunauer−Emmett−Teller (BET) specific surface area of 667 m2·g−1 (Figure S4). Thermogravimetric analysis showed that there is a weight loss between 110 and 220 °C that is attributed to the loss of two 9647

DOI: 10.1021/acs.inorgchem.7b01155 Inorg. Chem. 2017, 56, 9646−9652

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Inorganic Chemistry

emission peak in water (Figure 1). Moreover, contrary to the general trend for the other solvents, the red shift in the PL emission peak and decrease in the emission peak intensity reveals that there is a specific interaction between water molecules and the TMU-34(-2H) framework. To explore this behavior of the PL spectrum of TMU-34(2H), mixtures of water and acetonitrile were examined. Acetonitrile was selected because TMU-34(-2H) is dispersed very well and has relatively intense emission in acetonitrile. As represented in Figure 2, as the water content in the mixture increases, the intensity of the emission peak decreases upon excitation at either 458 and 504 nm. This is in very good agreement with intensities of TMU-34(-2H) in pure acetonitrile and water. Thus, this PL behavior of TMU-34(-2H) in water/acetonitrile mixtures shows that water molecules interact with the TMU-34(-2H) framework. TMU-34(-2H) is functionalized with tetrazine moieties which are nitrogen-rich. Because the four N(sp2) lone pairs can act as Lewis basic sites and electron-donating motifs, it is able to interact through hydrogen bonding with two acidic hydrogens in water molecules. Thus, because of this hydrogen bonding, electron density is transferred from the tetrazine moieties to the two hydrogens in water molecules, and consequently, the quantum efficiencies of the π → π* and/or n → π* transitions are reduced, and the fluorescence emission intensity decreases.34−36 1,2,4,5-Tetrazine contains four π-accepting and σ-donating N(sp2) atoms, and therefore, it has simultaneous electrondonor/acceptor characteristics.37 The contribution of the electron-donating/accepting characteristics along with solvation effects (hydrogen bonding in water) on secondary interactions of the tetrazine motifs inside the TMU-34(-2H) framework cause the unique behavior and different signal transduction in cation sensing. Hence, because of the different behaviors of TMU-34(-2H) in water and acetonitrile, sensing of cations was investigated in both solvents. Sensing of Hg2+ in Acetonitrile and Water. The PL spectra of TMU-34(-2H) in acetonitrile were recorded under ambient conditions in the presence of various cations as mentioned in the experimental section. None of the cations except Hg2+ have a significant effect on the PL spectrum of TMU-34(-2H), whereas the PL spectrum shows 90% quenching and 243% enhancement in the presence of Hg2+in acetonitrile and water, respectively (Figures 3, S15, and S16).

guest DMF molecules (calculated 14.42% and found 16.43%). A second weight loss was observed in the range of 290−400 °C corresponding to decomposition of the framework (Figure S5). EDAX analysis was used to determine the Hg(II) adsorption after sensing tests. The results for Hg(II)@TMU-34(-2H) in water and acetonitrile showed that each tetrazine moiety interacts with one Hg(II) ion (Figures S6−S9). Also, the element mapping data indicated the distribution of Hg(II) and Zn(II) inside the framework (Figures S10−S13). Photoluminescence Properties of TMU-34(-2H). Here, different solvents (water, methanol, ethanol, acetonitrile, DMF, toluene, and n-hexane) were tested to find the best one for use in cation sensing applications. For all of the solvents except water, the excitation and emission wavelengths are centered at 458 and 618 nm, respectively. The emission peak of TMU-34(2H) is enhanced with increasing of solvent polarity (Figures 1

Figure 1. Effect of the solvent on the TMU-34(-2H) fluorescence emission peak.

and S14 and Table S1). TMU-34(-2H) demonstrates different PL behavior in water, and its excitation and emission wavelengths are centered at 504 and 648 nm, respectively. Water is the most polar solvent among these selected solvents, and on the basis of the trend for the other solvents, TMU-34(2H) was expected to have the highest emission intensity in water. However, it actually has the lowest intensity for the

Figure 2. PL emission of TMU-34(-2H) in water (W.)/acetonitrile (A.) mixtures upon excitation at (a) 504 nm (the excitation wavelength of pure water) and (b) 458 nm (the excitation wavelength of pure acetonitrile). 9648

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Figure 3. (a) Effects of cations on the PL peak at 618 nm upon excitation at 458 nm in acetonitrile. (b) Effects of cations on the PL peak at 648 nm upon excitation at 504 nm in water.

Table 1. Evaluated Sensing Parameters for TMU-34(-2H) in Water and Acetonitrile solvent

λex (nm)

emission band (nm)

λem max (nm)

response time (s)

signal change

KSV (M−1)

detection limit (M)

water acetonitrile

504 458

610−710 580−680

648 618

15 15

243% enhancement 90% quenching

3737 63618

1.8 × 10−6 6.9 × 10−6

Scheme 2. Representation of Electron Transfer between Hg2+ and TMU-34(-2H) in Water and Acetonitrile

density from the tetrazine moieties toward Hg2+ ions chelating to the N(sp2) atoms of the DPT ligands. As mentioned above, this different behavior in water compared with acetonitrile may be attributed to the hydrogen bonding between water molecules and tetrazine moieties in the TMU-34(-2H) framework in the presence of water as a solvent, causing the transfer of electron density from the tetrazine ring toward the hydrogens in water molecules. As a result, contrary to the electron transfer in acetonitrile, the electrons are transferred from Hg2+ toward the tetrazine ring, which is most likely the reason for the enhancement of the PL emission peak of TMU-34(-2H) in water (Scheme 2).34−36,44 Double Solvent Sensing Method (DSSM). Most luminescent MOFs for Hg2+ detection are based on quenching or enhancement of the emission intensity with one-dimensional (1D) signal transduction. However, one of the most important factors for an ideal sensor is sensitivity, which cannot be achieved by this type of signal transduction.45 The double solvent sensing method (DSSM) is based on simultaneous combination of two independent and different signal transductions of TMU-34(-2H) in water and acetonitrile for construction of a double solvent sensing curve and formulation of a sensitivity factor that improves the sensitivity and accuracy of Hg2+ detection. Sensitivity is defined as the change in response in the presence of the analyte divided by the initial response.45 The

The emission intensity clearly decreases gradually with increasing Hg2+ concentration in acetonitrile, but different behavior occurs in water (Figure S17). As TMU-34(-2H) shows exceptional selectivity for Hg2+, we aimed to test it in the presence of other ions. The results show that there is no significant change in the quenching behavior of TMU-34(-2H) in the presence of Hg2+ upon addition of other ions (Figure S18). The evaluated sensing parameters for TMU-34(-2H) in water and acetonitrile are summarized in Tables 1 and S2. The value of KSV in acetonitrile is among the highest values reported for Hg2+ detection (Table 1 and Figure S19).38,39 The detection limit of TMU-34(-2H) for Hg2+ was calculated on the basis of the 3δ IUPAC criterion (Table 1 and section 7 in the Supporting Information). Lots of research has explored the detection of Hg2+ using nitrogen- and sulfur-functionalized MOFs because mercury ions have high affinity toward N and S atoms. Hg2+ ions are detected by Hg2+−S or Hg2+−N interactions leading to the quenching effect. It seems that after binding of Hg2+ to S or N atoms, electrons are transferred from the conjugated π system of the framework to unoccupied Hg2+ orbitals, causing the significant fluorescence quenching.19−21,25,27,28,40−43 The tetrazine moiety is a relatively weak σ donor and can act as a Lewis base. Therefore, the quenching of TMU-34(-2H) in acetonitrile upon addition of Hg2+ may be attributed to transfer of electron 9649

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other cations have been effectively eliminated and consequently sensitivity and accuracy of detection have been highly improved The response time was recorded for acetonitrile- and waterdispersed TMU-34(-2H) as a fluorescence probe in the presence of 2 × 10−4 M Hg2+. The results show a fast response of 15 s for acetonitrile and water at the mentioned concentration of Hg2+. This very fast response along with the high value of KSV in comparison with other MOFs may be attributed to the highly dispersible nature of TMU-34(-2H), causing greater and easier accessibility of tetrazine groups to Hg2+ and strong interactions between Hg2+ and these groups (Table S4).23,24,45,46 Sensor stability and durability through sensing are important for practical applications.47 PXRD patterns show that TMU34(-2H) is stable in both solvents and after sensing experiments (Figure S1). Furthermore, the sensor response remains unaltered after five cycles, indicating the high photostability of TMU-34(-2H) in the presence of Hg2+ cation (Figures S20 and S21). Moreover, comparison of the PXRD patterns of TMU-34(-2H) before and after five repeated tests clearly shows this compound to maintain its original structure (Figure S22).

calculated sensitivity factor in the 2D sensing curve is between 0 and 2 for all of the cations other than Hg2+, whereas for Hg2+ it is equal to 41 (section 8 in the Supporting Information). These calculations show that by changing from 1D signal transduction to the 2D sensing method, the sensitivity and accuracy of Hg2+ detection is highly improved while interfering effects are eliminated effectively (Table S3 and Figure 4).

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CONCLUSION In this work, we used a sonochemical method to synthesize tetrazine-functionalized TMU-34(-2H) as a good candidate for cation sensing because the tetrazine moiety is a π-conjugated and water-soluble/stable fluorophore with relatively weak σdonating Lewis basic sites. TMU-34(-2H) can detect Hg2+ selectively in acetonitrile and water with different 1D signal transductions. By simultaneous combination of the different signal transductions of this MOF in acetonitrile (quenching) and water (enhancement), the double solvent sensing method with 2D signal transduction is used, which can eliminate the effects of other cations and amplify sensitivity toward Hg2+. Calculation of the sensitivity factors shows that by applying the double solvent sensing method instead of 1D sensing, the

Figure 4. Sensitivity factors for all of the cations tested.

Calculations of sensitivity factors for all of the cations demonstrate that it is possible to detect Hg2+ by DSSM with ultrahigh sensitivity. Such a tremendous distinction in the Hg2+ sensitivity factor is visualizable in the double solvent sensing curve (Figure 5). As shown in Figure 5, two 1D detection curves of Hg2+ in water and acetonitrile can be combined together for constructing a 2D double solvent sensing curve. Hence, this method can reduce/eliminate response-influencing environmental factors and specially interfering effects of other cations and hence amplify sensitivity. Figure 4 reveals that by changing 1D curve to 2D curve the interfering effects of

Figure 5. 2D sensing curve for Hg2+ detection in water and acetonitrile. As can be clearly seen, the accuracy and sensitivity are increased because interfering effects are eliminated. 9650

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interfering effects of other cations are completely eliminated and the sensitivity toward Hg(II) is highly improved. Using this two-dimensional sensing curve greatly helps in eliminating interfering effects and thus creates high sensitivity toward Hg(II) with a fast response.

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ASSOCIATED CONTENT

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.7b01155. PXRD patterns, crystal structure images, N2 isotherms at 77 K, TGA profile, EDAX analysis, FE-SEM mapping images, fluorescence spectra, selectivity evaluation, and detection limit calculation (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Tel: (+98) 21-82884416. ORCID

Mohammad Yaser Masoomi: 0000-0003-1329-5947 Ali Morsali: 0000-0002-1828-7287 Author Contributions †

S.A.A.R. and M.Y.M. contributed equally to this work.

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS Support of this investigation by Tarbiat Modares University is gratefully acknowledged. REFERENCES

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DOI: 10.1021/acs.inorgchem.7b01155 Inorg. Chem. 2017, 56, 9646−9652