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Ratiometric and Turn-on Luminescence Detection of Water in Organic Solvents Using a Responsive Europium-Organic Framework You Zhou, Denan Zhang, Wenzhe Xing, Jing Cuan, Yuhua Hu, Yuting Cao, and Ning Gan Anal. Chem., Just Accepted Manuscript • DOI: 10.1021/acs.analchem.9b00493 • Publication Date (Web): 05 Mar 2019 Downloaded from http://pubs.acs.org on March 8, 2019
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Ratiometric and Turn-on Luminescence Detection of Water in Organic Solvents Using a Responsive Europium-Organic Framework You Zhou,†,* Denan Zhang,† Wenzhe Xing,† Jing Cuan‡, Yuhua Hu,† Yuting Cao,† Ning Gan†,* †
Faculty of Materials Science and Chemical Engineering, Ningbo University, Ningbo 315211, Zhejiang, China Institute for Superconducting & Electronic Materials, School of Mechanical, Materials and Mechatronics Engineering, University of Wollongong, Wollongong, NSW 2522, Australia ‡
ABSTRACT: The development of simple, rapid-response sensors for water detection in organic solvents is highly desirable in the chemical industry. Here we demonstrate a unique luminescence water sensor based on a dual-emitting Europium-Organic Framework (Eu-MOF), which is assembled from a purposely selected 2-aminoterephthalic acid ligand with responsive fluorescence inherent in its intramolecular charge transfer (ICT) process. This ICT process can be rapidly switched-on in the presence of water owing to its ability to boost and stabilize the ICT state. In contrast, the Eu3+ emission within the framework is insensitive to water and can serve as a reference, thus enabling highly sensitive water detection in a turn-on and ratiometric way. In addition, the significant ratiometric luminescence response induced by water makes Eu-MOF undergo a distinct change of emititng color from red to blue, which is favorable for visual analysis with the naked eye. Sensitive determination of water content (0.05-10% v/v) in various organic solvents is achieved in multiple readouts including ratiometric emission intensity, emission color, or the Commission Internationale de l’Eclairage (CIE) chromaticity coordinate. The present Eu-MOF sensor featuring high sensitivity and reusability, self-calibration, simple fabrication and operation, and capability for real-time and in-situ detection is expected to have practical applications in water analysis for industrial processes.
As one of the most common impurities in organic solvents, water is seriously detrimental to laboratory chemistry and industrial manufacturing. Therefore, the development of simple, fast and reliable sensors for detecting water in organic products is highly desired. The traditional and broadly used techniques for the determination of water contents are Karl-Fisher titration and gas chromatography. Unfortunately, these methods have certain limitations, involving requirements of specialized instruments, well-trained personnel, time-consuming processes, and inability of in-situ and real time monitoring.1 In order to circumvent these limitations, much effort has been devoted to developing luminescent sensors for water detection.2,3 Luminescent water sensors featuring simple operation, fast response, high sensitivity, ease of fabrication and capability for non-invasive and in-situ detection have been considered as attractive potential alternatives to traditional analysis methods.2,4,5 Luminescent water sensors have been primarily explored based on organic fluorescence molecules.6-8 However, the organic molecular water probes usually can hardly be recovered and reused,4,8 and are not suitable for long-term water content monitoring owing to their poor stability upon strong and continuous ultraviolet light irradiation. In addition to organic fluorophores, a number of other fluorescence materials have also been exploited as water sensors, such as polymers,9 metal complexes,10,11 Cu nanoclusters,12-14 carbon dots,15,16 lanthanide hybrids17-20 and upconversion nanoparticles.21,22 Luminescent metal-organic frameworks (MOFs) combining the intrinsic properties of MOFs and intense fluorescence have emerged as particularly exciting materials for chemical sensors
in the past decade.23-26 In comparison with traditional luminescent materials, luminescent MOFs show three major advantages in chemical sensing: 1) their hybrid nature gives rise to multiple emissions, because all the component metal cations, organic linkers, and guest species can potentially serve as photonic units; 2) their high and permanent porosity facilitates the preconcentration of analytes and the interactions between analytes and photonic units within the framework; 3) the structural and chemical tunability on the molecular level is extremely beneficial to the high detection selectivity through pore sieving functions or specific host-guest interactions. Luminescent MOFs have been intensively investigated to detect various targets, including metal cations,27,28 small molecules,29.30 gases,26 biomarkers31-33 and temperature.34,35 Of particular interest is that luminescent MOFs have demonstrated considerable potential for water detection.36-46 By taking advantage of various water-dependent processes, such as intramolecular proton-transfer (ESIPT),4,5,36 structure transformation,37-40 and energy transfer,41-43 a number of water sensors based on MOF materials have been developed. However, the existing MOF water sensors are mainly relied on a single emission intensity,37-41 which is susceptible to the external errors introduced by optical occlusion, excitation power fluctuation, and concentration inhomogeneity. Fortunately, these complications can be circumvented by ratiometric luminescent detection strategy. Recently, Dong et al.45 and Yin et al.46 reported two ratiometric water sensors based on dual-emitting MOF hybrids, which are fabricated by encapsulating photoactive carbon dots or tris(2,2′-bipyridine) ruthenium (2+) (Ru(bpy)32+) in MOFs, respectively. These
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ratiometric water sensing platforms based on MOF hybrids are sensitive and attractive. However, the synthesis of MOF hybrids is usually more complicated and costly when compared to that of isolated MOFs. In addition, the incorporated photoactive guests in MOF hybrids may leak into the solution during the sensing process, thus interfering the analysis, and accompanying by the poor reusability of the sensor and the pollution of the samples. What’s more, despite the recent remarkable progress, the researches on MOF water sensors are still in their infancy. It is highly desired to explore novel MOF water sensors based on new sensing mechanisms. Herein, we present an efficient ratiometric water sensor based on an isolated Eu-MOF (Eu(atpt)1.5(H2O)n). The Eu-MOF is simply assembled from a purposely selected 2aminoterephthalic acid ligand (atpt) with water-responsive fluorescence.46,47 The as-prepared Eu-MOF features two emitting centers: Eu3+ and the atpt ligand. The Eu3+ emission is insensitive to water, while the emission of atpt is significantly switched on in the presence of water, thus allowing sensitive water detection in a turn on and self-calibrated way. The waterdriven ratiometric luminescence response makes Eu-MOF exhibit a distinct change in emitting color, which is favorable for visual analysis with the naked eye. Highly sensitive realtime and in-situ determination of water content (0.05-10% v/v) in various organic solvents has been successfully achieved in multiple readouts involving the ratiometric emission intensity and emission color. What’s more, this work demonstrates an unprecedented intramolecular charge transfer (ICT) mechanism in MOFs for water sensing, which is expected to open a new way for MOF materials to develop water sensors.
EXPERIMENTAL SECTION Materials and methods. 2-aminoterephthalic acid (atpt) and 1,10-phenanthroline (phen) were purchased from Aldrich. EuCl3·6H2O (99.9%) was purchased from Adamas. All the chemicals were commercially available and used without further purification. Scanning electron microscopy (SEM) images were obtained with a Hitachi S-4800 scanning electron microscope. Thermo-gravimetric (TG) analysis was performed on a SII TG/DTA7300 thermal analyzer at a 10 °C min−1 heating rate. Fourier transform infrared spectrum (FTIR) was conducted with the specimen enclosed within KBr slices using a Nicolet 7600 FTIR spectrometer. Powder X-ray diffraction (PXRD) patterns were recorded with a D8 Advance X-Ray Powder Diffractometer employing CuKα radiation (40 kV and 40 mA). The photoluminescence spectra, luminescent decay curves and outer luminescent quantum efficiency were collected using an Edinburgh FLS920 phosphorimeter equipped with an integrating sphere (150 mm diameter, BaSO4 coating). Synthesis of [Eu(atpt)1.5(H2O)]n (Eu-MOF). EuCl3∙6H2O (0.511 g), atpt (0.385 g), phen (0.280 g) and deionized water (35 mL) were mixed in a 100 mL beaker for ultrasonic dissolution. 4.2 mL NaOH aqueous solution (0.65 M) was added to the mixture and dissolved evenly under ambient conditions. Then the resultant mixture was transferred to a 50 mL Teflon-lined stainless steel vessel and heated at 160 °C for 12 h. After cooling to room temperature, the obtained brown solid was collected by centrifugation, followed by thorough washing with deionized water and ethanol. Finally, the products were dried at 60 °C under vacuum. Sensing of water in organic solvents. Typically, for the water sensing experiments, 10 mg Eu-MOF powder was
dispersed in 10 mL N,N-Dimethylformamide (DMF), followed by ultrasonication for 10 min to ensure the homogeneity of the suspension. Then 0.2 mL of the prepared Eu-MOF suspension was transferred to quartz cells, followed by addition of different volumes of water using precision micropipette. Subsequently, DMF was added to the quartz cells to keep the final volume of the mixtures of water/DMF at 2 mL. These mixtures in quartz cells were keep ultrasound to maintain the uniform dispersion during the experiment. The photoluminescence spectra were recorded using an excitation wavelength of 377 nm. The sensing experiments towards water in ethanol and acetone were performed in a similar way, and the final concentration of EuMOF in the mixed solution remained at 0.1 mg mL−1.
RESULTS AND DISCUSSION Eu-MOF solid was prepared by the hydrothermal reaction of atpt, phen and EuCl3 in alkaline aqueous solution (Figure 1a). The PXRD pattern of the obtained Eu-MOF is shown in Figure 1b. It is well matched with that simulated from the Crystallographic Information File (CIF) file (CCDC number: 222296).48 Eu-MOF crystallizes in the triclinic P1 group and features one crystallographically unique Eu3+ ion, which is coordinated with eight atoms: two nitrogen atoms from a phen ligand, five carboxylate oxygen atoms from atpt ligands and one terminal water molecule (Figure 1c). Every two Eu3+ ions are bridged through carboxylate groups of atpt ligands to form the secondary building unit (SBU) of the Eu-MOF topological structure (Figure 1c). The SBUs are further linked by carboxylate Atpt ligands, thus resulting in a porous framework (Figure 1d).20 Figure 1e presents a typical SEM image of EuMOF, which reveals that the product is composed of uniform rhomboid microcrystals. Dynamic light-scattering (DLS) study of these microcrystals showed a single distinct population with a size distribution peak at 2.5 μm (Figure S1). Figure S2 depicts a)
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Figure 1. (a) Schematic illustration of the hydrothermal synthesis of Eu-MOF. (b) Experimental PXRD pattern of Eu-MOF and that simulated from the CIF file (CCDC number: 1522098). (c) Secondary building unit (SBU) of Eu-MOF topological structure. Magenta represents Eu atoms, while wine, red and light blue represent C, O, and N atoms, respectively. H atoms are omitted for clarity. (d) View of topological framework of Eu-MOF assembled from SBUs and atpt ligands running through b axis. e) Typical SEM image of Eu-MOF rhomboid microcrystals.
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Figure 2. Photoluminescence properties of Eu-MOF: (a) solid state excitation (λem = 614 nm) and emission spectra (λex = 377 nm); (b) solid state luminescence decay curve of the Eu3+ 5D0→7F2 transition (614 nm); (c) photostability under continuous illumination with a high power xenon lamp (450 W) for 30 min, with the time bin 5 s; (d) emission spectra of Eu-MOF suspensions (0.1 mg/mL) of organic solvents (DMF, acetone, ethanol) and water. The inset contains the corresponding photographs under 365 nm irradiation of a laboratory UV lamp, from left to right: water, ethanol, DMF, and acetone.
the N2 adsorption-desorption isotherm of Eu-MOF. The Brunauer-Emmett-Teller (BET) surface area was calculated to be 383 m2 g−1. The free amino groups (-NH2) of the atpt ligand within Eu-MOF can be reflected by FTIR analysis (Figure S3), in which νs and νas of -NH2 are observed at 3467 and 3342 cm−1, respectively. Thermo-gravimetric (TG) analysis demonstrates that Eu-MOF is stable up to about 350 °C, above which the crystalline framework is decomposed, leading to a weight loss of 45.7 wt% (Figure S4). Figure 2a shows the solid state photoluminescent excitation and emission spectra of Eu-MOF. Upon excitation at 377 nm, Eu-MOF exhibits characteristic sharp emissions of Eu3+ at 592, 614, 652 and 700 nm, which correspond to the 5D0→7FJ (J = 1, 2, 3, 4) transitions of Eu3+, respectively. The Eu3+ centered excitation spectrum (λem = 614 nm) displays a strong broad band ranging from 240 to 400 nm, indicating that the emission energy is mainly emigrated from the electronic levels of organic chromophores in Eu-MOF. The weak sharp band at 465 nm of
the excitation spectrum is ascribed to the 7F0→5D2 transition of Eu3+. The luminescence decay curve (5D0→7F2) of Eu-MOF is plotted in Figure 2b, which can be fitted with a single exponential function, yielding a lifetime value of 0.602 ms. The absolute external luminescence quantum yield of Eu-MOF was determined as 21.3%, which is reasonably high in comparison with other Eu-containing MOF materials.49,50 Furthermore, EuMOF shows excellent photostability under continuous illumination with a xenon lamp (450 W). Photobleaching is barely detected after 30 min of continuous exposure to a high power xenon lamp (Figure 2c). We next examined the luminescence of Eu-MOF in several of the most common used organic solvents (DMF, acetone, ethanol) and water. As illustrated in Figure 2d, the emission spectra of Eu-MOF in the organic solvents present characteristic sharp Eu3+ emission bands, which are similar to that recorded in the solid state. A completely different scenario, however, occurs in the emission spectrum of Eu-MOF in water. A strong broad emission centered at 430 nm emerges in the emission spectrum of Eu-MOF aqueous suspension, while the Eu3+ emission intensity basically remains constant. The dramatic turn-on behavior of luminescence can be readily visualized by the naked eye using a portable UV lamp. As demonstrated in the photographs of Eu-MOF suspensions under 365 nm irradiation by a UV lamp (inset of Figure 2d), the red emission color of Eu-MOF in organic solvents turns into bright blue in water. The significant luminescence response of EuMOF towards water suggests the practical feasibility of EuMOF for water sensing in a ratiometric and turn on way. To clarify the origin of the strong blue emission peaking at 430 nm from Eu-MOF, the luminescent spectra of atpt and phen ligands in DMF with a concentration of 10-7 M were collected (Figure S5). The emission band of atpt ligand is identical to the blue emission band of Eu-MOF, which reveals that this blue emission is derived from the atpt ligand. In addition, Eu-MOF possesses remarkable photostability in the organic solvents and water, as verified by the time-dependent emission spectra (Figure S6-9). The retention of the crystalline integrity of EuMOF samples after soaking in organic solvents and water is confirmed by PXRD measurements (Figure S10). The merits of chemical- and photo-stability in organic solvent and water of Eu-MOF are crucial for the water detection application in organic solvents. To quantitatively assess the sensing performance of Eu-MOF toward water, emission spectra of a Eu-MOF DMF suspension (0.1 mg/mL) were measured upon addition of aliquots of water
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Figure 3 Color-coded three-dimensional (3D) contour and projection maps showing emission spectra (λex = 377 nm) of Eu-MOF in DMF upon addition of aliquots of water (0.05-10% v/v) in the range of 390-720 nm (a) and 390-520 nm (b). (c) Plot of the intensity ratio (Δ) of the ligand emission to the Eu3+ emission versus water contents (0.05-6% v/v) in DMF. Error bars represent ±1 standard deviation, n = 3.
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(0.05-10% v/v) (Figure 3a). With increasing water contents, the emission intensity from atpt ligands of Eu-MOF rises sharply (Figure 3b), whereas the Eu3+ emission intensity is insensitive to the water content. Therefore, the intensity ratio (Δ) of the ligand emission to the Eu3+ emission highly depends on the concentration of water, and can serve as the ratiometric detection parameter. Figure 3c plots the variation of Δ as a function of water content. We found a good linear correlation for Δ versus water concentration in the range of 0.05-6% v/v, which can be fitted by the following equation Δ = 0.156 + 0.198 C
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with a correlation coefficient (R ) of 0.9905, where C is the water content in DMF. The limit of detection (LOD) is estimated as 0.02 % v/v based on the standard deviation of ten blank measurements and the calibration curve’s slope. Table S1 summarizes the LOD of the Karl-Fisher titration method and other reported luminescence water sensors based in various mechanisms, indicating that the LOD of the Eu-MOF water sensor is competitive. In addition, the detection performance of the present Eu-MOF and the recently reported ratiometric water sensors based on MOF hybrids18b-d were compared using relative sensitivity (Sr), which is independent of the probes that operate by different mechanisms. Here, Sr represents the relative change in Δ per % v/v change in the water content (C): 𝑆 =
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As illustrated in Figure S11, Eu-MOF exhibits a considerably high Sr with a maximum value of 1.14 (% v/v)-1, which is better than those documented by Yin et al.18b and Wu et al.18d To evaluate the effect of the content of Eu-MOF probe on the sensing performance, the ratiometric luminescence responses of Eu-MOF DMF suspensions with three different concentrations (0.05, 0.1, and 0.2 mg/mL) toward water were determined (Figure S12). We found that the intensity ratio Δ is independent of the Eu-MOF concentration in a given water content, showing the robustness of this Eu-MOF sensor. The time-dependent ratiometric luminescence response of Eu-MOF toward water is shown in Figure S13, demonstrating the fast response rate. The feasibility of Eu-MOF sensor in real samples was also examined. Specifically, the Eu-MOF was employed to analyse
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Figure 4. (a) Photographs of Eu-MOF in DMF containing different water contents (0.05-10% v/v) under 365 nm irradiation by a UV lamp. (b) The CIE chromaticity coordinates calculated from the emission spectra that are presented in Figure 3a, showing the linear shift from red to blue region with increasing water content from 0 to 10% v/v.
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three DMF specimens spiked with different amounts of water. The detection parameters (Δ) were collected and converted to water contents by eqn (1), as listed in Table S2. The recovery of water in DMF samples ranges from 101.6% to 112% with tolerable relative standard deviations (RSD, n = 3). These results indicate that this Eu-MOF sensor is competent to detect water in real samples. The changes of water content in DMF suspension of EuMOF also trigger significant variations in the luminescence color. Eu-MOF presents two emissions in the visible range: the blue one of atpt and the red one of Eu3+. The intensity ratio of these two emissions is greatly influenced by the water concentration, thus leading to continuous emission color shifts from red to blue (Figure 4a). The emission color change is very sensitive, as demonstrated by the fact that Eu-MOF shows emission color change that is observable by the naked eye from red to magenta in the presence of a low water content of 1% v/v (Figure 4a). To further evaluate the emitting color change of Eu-MOF toward the water content, the Commission Internationalede l’Eclairage (CIE) coordinates of the emission spectra of Eu-MOF suspensions with different water contents were calculated (Figure 4b). It can be seen that the calculated chromaticity progressively moves from red to blue, and a linear relationship between the CIE coordinate and water content can be established, which suggests the feasibility of quantitative water sensing by monitoring the luminescence color by the naked eye or with a CCD camera. This colormetric strategy is quite attractive due to the capability of visualizing water content in-situ and in real-time, althrough the sensitivity and accuracy of this colormetric strategy might not be as good as those of the ratiometric spectroscopic method. The applicability of Eu-MOF for quantitative water determination in ethanol and acetone was also demonstrated (Figure S14,15). Water titration in both ethanol and acetone leads to substantial enhancement of the blue emission of EuMOF and concomitant distinct change in the emission color. The luminescence responses of Eu-MOF to water in ethanol and acetone are on the same order as that in DMF. These results provide an indication that Eu-MOF is able to detect water in various organic solvents. The reusability of this Eu-MOF sensor was also checked by recycling experiments showing the ratiometric luminescence response of Eu-MOF to water. The hydrated Eu-MOF sample was regenerated by treating it with MeOH and then drying at 60 °C. We recorded the ratiometric emission profiles (Δ) of three cycles of Eu-MOF in anhydrous DMF ↔ the mixture of DMF and water (10% v/v). As shown in Figure S16, the values of Δ in the three cycles are basically the same, revealing that Eu-MOF is reusable for water detection, therefore affording an important advantage over molecular probes. The good reusability can be ascribed to the robustness of Eu-MOF, which is evidenced by that the PXRD pattern of the sample after three cycles is identical to that of the pristine one (Figure S17). To shed some light on the turn-on mechanism of the blue emission from Eu-MOF towards water, the luminescence response of isolated atpt ligands to water was examined. As shown in Figure 5a, the emission intensity of atpt gradually increases upon addition of aliquots of water. Isolated atpt and Eu-MOF follow the same trend in their luminescence changes to water, indicating that turn-on luminescence of Eu-MOF is generated by the interactions between atpt linkers and water molecules. However, the water-induced luminescence response
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thereby promoting the flow of charge from the donor to the acceptor (amino group) and favoring the ICT process. Similar results have also been reported in other aminobenzoic acids.5153 In addition, the water-induced enhancement of the ICT efficiency can be verified by the emission (Figure 3b) and UVvis absorption (Figure S19) spectra of Eu-MOF, in which the intensities of ICT band increased largely upon the addition of water. Based on the above discussions, the luminescence sensing mechanism of Eu-MOF toward water is reasonably proposed (Figure 5c). Initially, the pores of Eu-MOF are filled by organic solvents, and the ICT emission of the atpt linkers is buried underneath the Eu3+ emission because of the low ICT rate. With the titration of water, the molecules of organic solvents accommodated in the pores are progressively replaced by water, leading to a water surrounding environment of atpt linkers. The ICT emission is subsequently switched on, due to the significant enhancement of the ICT efficiency of atpt linkers resulting from the boosting formation and effective stabilization of ICT state by water.
CONCLUSION
Figure 5. (a) Color-coded contour map showing emission spectra of atpt ligand in DMF with various water contents (0-15% v/v). (b) Emission spectra of atpt (1×10-7 M) in water and different organic solvents (acetone, DMF, ethanol). The emission spectra have been normalized to the same intensity. (c) Schematic illustration of luminescent sensing mechanism of Eu-MOF towards water.
of atpt is less significant than that of Eu-MOF (Figure S18). The LOD was calculated to be 0.11 % v/v, which is much worse than that of Eu-MOF (0.02 % v/v). The superior LOD of the Eu-MOF sensor should be attributed to the preconcentration of water molecules within the channels of EuMOF, which is most likely originated from the strong hydrogen bonding ability of water with Eu-MOF. Aminobenzoic acids and their derivatives are known to feature intramolecular charge transfer (ICT) fluorescence,47,51-53 we therefore reasoned that the broad structureless emission band of atpt is derived from the ICT state formed by charge transfer from the electron donating amino group to the ortho electron-withdrawing carboxyl group. To substantiate our assumption, the emission spectra of atpt in various solvents (acetone, DMF, ethanol, water) were recorded (Figure 5b). With the increase of solvent polarity from cyclohexane to water, the emission of atpt undergoes a distinct red shift, which supports the charge transfer nature of the emissive state. Given the large dipole moment of the ICT state resulting from charge separation, water with the highest polarity facilitates the dipoledipole interactions with atpt in the ICT state, and therefore gives rise to a stable ICT state.54,55 The ICT state could be further stabilized by the efficient hydrogen bonding with water, since water presents better capability towards the formation of hydrogen bonds, compared to the organic solvents, owing to its smaller size and low steric hindrance.54,56 The effective stabilization of the ICT state is reflected by the maximum emission wavelength that occurs in water (Figure 5b). In addition to the stabilization, hydrogen bonding plays a vital role in boosting the formation of the ICT state. Specifically, the hydrogen bonding of water with carboxylic oxygen increases the electron affinity of the electron acceptor (carboxyl group),
In summary, we have developed a turn-on and ratiometric luminescent water sensor based on a robust Eu-MOF with dual visible emissions originating from Eu3+ and the atpt ligand. The water detection depends on a usual luminescence switch of atpt that is inherent in the ICT process. The conversion between the presence and absence of water can turn the ligand emission on or off, while the Eu3+ emission is independent of water, thus giving rise to a significant ratiometric response and emission color change over cycles. The water induced turn-on luminescence of atpt ligand can be attributed to the substantial enhancement of ICT efficiency resulting from the effective formation and stabilization of the ICT state promoted by water. Multi-readout (ratiometric emission intensity, emission color, or CIE chromaticity coordinate) determination of water content (0.05-10% v/v) in various organic solvents is achieved, showing the following characteristics: high sensitivity and reusability, self-calibration, simple fabrication and operation, and capability for real-time and in-situ detection. It is expected that this present Eu-MOF sensor will have practical applications in water analysis for industrial processes.
ASSOCIATED CONTENT Supporting Information The Supporting Information is available free of charge on the ACS Publications website. Characterization of Eu-MOF samples including FT-IR spectrum, TG curve, PXRD patterns and emission spectra, and several metrics for evaluating sensing capability involving relative sensitivity, reusability and contour maps of luminescence titration (PDF)
AUTHOR INFORMATION Corresponding Author *Email:
[email protected],
[email protected] Notes The authors declare no competing financial interest.
ACKNOWLEDGMENTS This work was supported by the National Natural Science Foundation of China (21701092), the Zhejiang Provincial Natural Science Foundation of China under Grant Nos. LQ17B010002, the
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Natural Science Foundation of Ningbo (2018A610138), and K. C. Wong Magna Fund in Ningbo University.
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Table of Contents (for TOC only) 10
6
25.0k
20.0k
15.0k
5
4
D0 7F2 (Eu3+)
8
30.0k
ICT (Ligand)
Water Content (v/v%)
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
Analytical Chemistry
10.0k
2 0
5.0k
0.0
400 450 500 550 600 650 700
Wavelength (nm)
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