Ratiometric Fluorescence Sensing and Real-Time Detection of Water

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Ratiometric Fluorescence Sensing and Real-Time Detection of Wa-ter in Organic Solvents with One-Pot Synthesis of Ru@MIL-101(Al)-NH2 Hua-Qing Yin, Jichun Yang, and Xue-Bo Yin Anal. Chem., Just Accepted Manuscript • DOI: 10.1021/acs.analchem.7b03723 • Publication Date (Web): 17 Nov 2017 Downloaded from http://pubs.acs.org on November 18, 2017

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Analytical Chemistry

Ratiometric Fluorescence Sensing and Real-Time Detection of Water in Organic Solvents with One-Pot Synthesis of Ru@MIL-101(Al)-NH2 Hua-Qing Yin,† Ji-Chun Yang,† and Xue-Bo Yin*,†,‡ †

State Key Laboratory of Medicinal Chemical Biology and Tianjin Key Laboratory of Biosensing and Molecular Recognition, College of Chemistry, Nankai University, Tianjin, 300071, China ‡

Collaborative Innovation Center of Chemical Science and Engineering (Tianjin), Nankai University, Tianjin, 300071, China

*Fax: +86-22-23503034, Email: [email protected].

ABSTRACT: Ratiometric fluorescence detection attracts much attention because of its decreased environment influence and easy-to-differentiate color and intensity change. Herein, a guest-encapsulation metal organic framework (MOF), Ru@MIL-NH2, is prepared with 2-aminoterephthalic acid, AlCl3, and Ru(bpy)32+ by simple one-pot method for ratiometric fluorescence sensing of water in organic solvents. The rational selection of excitation wavelength provides dual emission at 465 and 615 nm from Ru@MIL-NH2 under single excitation of 300 nm. High sensitivity, low detection limit (0.02% v/v), wide response range (0-100%), and fast response (less than 1 min) are obtained for ratiometric fluorescence sensing of water under single excitation with Ru@MIL-NH2 as probe. Moreover, the result of water content is independent on the concentration of Ru@MIL-NH2 as the merit of ratiometric fluorescence detection. Response mechanism reveals that the protonation of nitrogen atom of MIL-NH2, π-conjugation system, and the stable fluorescence of Ru(bpy)32+ achieve the ratiometric fluorescence. The analysis of real spirit samples confirms the proposed method. Test strip is prepared with Ru@MIL-NH2 for convenient use. We believe that such turn-on ratiometric host-guest MOFs and the rational selection of excitation wavelength will offer the guidance for ratiometric fluorescence detection with wide applications. fluorophores and the detection condition should be designed and optimized carefully.

Water is generally considered as contaminant and impurity for dry products and some chemical products. Trace water influences not only the yield of chemical and drugs but also their activity and application.1, 2 A simple and fast analytical method for accurate quantification of water in organic solvents is of high importance for pharmaceutical manufacturing and some chemical products, and has attracted great interest in fundamental analytical chemistry.3 Karl Fischer titration was extensively used for liquid and solid samples.4, 5 However, such approach requires specialized instrument, water-free titration cell, and welltrained personnel. Capacitive and impedance water sensors show the advantages of low detection limit, broad response range, the robustness and ease of calibration, while the methods suffer from low precision and the requirement of specialized equipment.6, 7

Luminescence metal organic frameworks (MOFs),14-17 which combine inherent photophysical property and porosity for guest-encapsulating ability,18 are promising for fluorescence sensing applications.19-24 Mg-MOF has been designed to detect trace water (0.05–5% v/v) in organic solvents with fast response.2 Lanthanide MOFs (Ln-MOFs) are important branch of fluorescence materials. The dual fluorescence from Eu/Tb-MOFs combining with superparamagnetism achieved sensitive water detection with good reusability as [email protected] A red lightemitting Eu-MOF was used to encapsulate blue-emitting carbon dots (Cdots) to form two colors emitting nanocomposite.26 When the composite was dispersed in water, the encapsulated Cdots were released while the red emission of Eu-MOFs was quenched for ratiometric fluorescence detection of water. Zn-MOF from a purposely designed (5-(2-(5-fluoro-2-hydroxyphenyl)-4,5-bis(4fluorophenyl)-1H-imidazol-1-yl)isophthalic acid showed dual emission behavior originated from intramolecular proton transfer for selective response to water on a molecular level.27 However, the water response range of the sensors was limited no more than 30%. The development of water determination method with high sensitivity, low detection limit, high reusability, fast response, and wide response range in organic solvents is therefore critical.

Fluorescence method is simple and sensitive.8 Different to the intensity change of single-color fluorescence detection, ratiometric fluorescence decreases the environment influence and vision tiredness to provide easy-todifferentiate color and intensity changes.9-11 One of the keys to realize ratiometric fluorescence detection is the design of ratiometric fluorescence systems. Some organic fluorophores have been developed to determine water content using ratiometric fluorescence.12, 13 While the preparation of organic fluorophores was complex, the

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Instrumentation and Characterization. UV-Vis absorption spectrum was recorded by a UV-2450-visible spectrophotometer, Shimadzu, Japan. The steady-state fluorescence experiments were performed on a FL-4600 Fluorescence Spectrometer, Hitachi, Japan, equipped with a plotter unit and a quartz cell (1cm×1 cm). We use the function of 3D scan of the Fluorescence Spectrometer to get the 3D fluorescence spectra. Infrared spectra were obtained by Bruker TENSOR 27 Fourier transform infrared spectroscopy. Thermogravimetric analysis (TGA) was performed on a PTC-10ATG-DTA analyzer heated from 20 °C at a ramp rate of 15 °C min-1 under air. Transmission electron microscopy (TEM) images was recorded with TecnaiG2 F20, FEI Co. (America) operated at an accelerating voltage of 200 kV. Scanning Electron Microscopy (SEM) images were recorded with JSM-7500F, Japan. XRD patterns were obtained by a D/max-2500 diffractometer (Rigaku, Japan) using Cu-Kα radiation (λ = 1.5418Å). The content of Al and Ru was measured by Inductively Coupled Plasma Atomic Emission Spectroscopy (ICP-AES), IRIS advantage, Thermo, USA. Zeta potential was obtained by Nano-ZS zeta potential analysis meter (Malvern, Britain). Gas chromatography (GC) was recorded with Thermo TRACE 1300 (Italy). N2 adsorption-desorption isotherm was recorded with ASAP2020/Tristar 3000 surface area and pore analyzer at 274 K.

Herein, we introduce a simple strategy to achieve sensitive ratiometric fluorescence detection of water in organic solvents by encapsulating red-emitting Tris(2,2’bipyridyl)ruthenium(II) (Ru(bpy)32+) in the pores of a blue-emitting MOF, MIL-101(Al)-NH2, which was denoted as Ru@MIL-NH2 hereafter. Ru(bpy)32+ shows stable red fluorescence emission in both water and organic solvents. Its high chemical stability and photo-stability make it popular as ideal fluorescence reference.28-32 MIL-NH2, prepared from Al3+ and 2-aminoterephthalic acid (BDCNH2), is an ideal platform to build ratiomatric fluorescence sensor because of its large pores to accommodate guest molecules.33-35 Meanwhile, MIL-NH2 has blue fluorescence itself and the frameworks are stable in water and other solvents.36 Fine adjustment of reaction conditions, selectivity of the precursor ratio and optimal excitation wavelength achieved the efficient blue (465 nm) and red (615 nm) dual-emission fluorescence from Ru@MIL-NH2. The intensity of blue fluorescence enhanced with increased water content in organic solvents, while the fluorescence of the guest molecules, Ru(bpy)32+, kept stable. Thus, turn-on sensing of water content was achieved through the ratiometric fluorescence of the dual-emission under single excitation of 300 nm with Ru@MIL-NH2 as probe. The cage size of MIL-NH2 is suitable to hosting the guest molecule, Ru(bpy)32+ and the leakage of Ru(bpy)32+ is inhibited with stable encapsulation. Therefore, the ratiometric fluorescence shows high stability and reusability for the detection of water content. The efficiency of Ru@MIL-NH2 is validated with the determination of water content in ethanol with broad water response range (0-100%), high sensitivity, and fast response (less than 1 min). The sensor is also confirmed by the determination of water content in N,N-dimethylformamide (DMF) and dimethyl sulfoxide (DMSO) and methanol. Therefore, we report the first example that Ru@MIL-NH2 is used for ratiometric fluorescence and turn-on sensing of water with high sensitivity, low detection limit, fast response, broad range, and high reusability.

Preparation of MIL and MIL-NH2. MIL and MIL-NH2 was prepared with simple one-pot method by a little modification of the previous work for the preparation of MIL-NH2. Aluminum chloride hexahydrate (AlCl3▪6H2O) (37.5 mg, 0.15 mmol) was added to 1,4-terephthalic acid (H2BDC, 32.8mg, 0.28 mmol) or 2-aminoterephthalic acid (BDC-NH2, 40.2 mg, 0.22 mmol) in DMF (6.0 mL). The asobtained mixture was transferred to a stainless steel Teflon-lined autoclave of 30 mL capacity. After sonicated for 5 min until fully dissolved, the mixture maintained at 403 K for 72 h under static conditions. After cooling to room temperature, the solution was removed by centrifugation and the material was washed three times with DMF and ethanol to remove the residual BDC or BDC-NH2. Finally, the samples were dried under room temperature.

EXPERIMENTAL SECTIONS

Preparation of Ru@MIL-NH2. Tris(2,2’-bipyridyl) dichloride ruthenium(II) hexahydrate [Ru(bpy)3Cl2▪6H2O] (15.0 mg, 0.02 mmol) was added to the mixture solution of aluminum chloride hexahydrate (AlCl3▪6H2O, 37.5 mg, 0.15 mmol) and 2-aminoterephthalic acid (BDC-NH2, 36.0 mg, 0.2 mmol) in DMF (6.0 mL). The mixture was transferred to a 30 mL stainless steel Teflon-lined autoclave. After sonicated for 5 min, the mixture maintained at 403 K for 72 h under static condition. After cooling to room temperature, the solvent was removed by centrifugation and the product was washed three times with DMF and ethanol to remove the residual Ru(bpy)32+ and BDC-NH2. Finally, the samples were dried under room temperature.

Chemicals and materials. Tris(2,2-bipyridyl) dichloride ruthenium(II) hexahydrate (Ru(II)(bpy)3Cl2 6H2O) was obtained from Sigma–Aldrich, Shanghai, China. 2Aminoterephthalic acid (BDC-NH2) was purchased from TCI Chemistry Co. Ltd, Shanghai, China. Aluminum chloride hexahydrate (AlCl3 6H2O) was obtained from Fuchen Chemical Reagents Factory, Tianjin. China. N,NDimethylformamide (DMF), dimethyl sulfoxidewith (DMSO), absolute ethyl alcohol and absolute methanol were purchased from Concord Reagent Co., Tianjin, China. All the chemicals were obtained at least of analytical grade and used without further purification. Ultrapure water was prepared with an Aquapro system (18.25 MΩ cm).

UV-visible absorption, fluorescence excitation, and emission spectra of Ru@MIL-NH2. For ultraviolet-

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Analytical Chemistry illustrated with high resolution transmission electron microscopy (HRTEM, Figure 1A). Ru@MIL-NH2 dispersed in water and ethanol easily to form transparent solution (Figure 1B). Fast diffusion kinetics were expected for fast response and high sensitivity of water detection by the nanosize and spherical structure of Ru@MIL-NH2.

visible absorption spectra, the concentration of the free ligand, BDC-NH2, was 0.025 mg mL-1 and other species were 0.05 mg mL-1. For the fluorescence excitation and emission spectra, the concentration of the species was 0.05 mg mL-1, and the slit was 2.5 × 5.0 for free ligand BDC-NH2, and the others were set at 5.0 × 5.0. The fluorescence stability of Ru@MIL-NH2 was operated by incubating Ru@MIL-NH2 in the mixture of water and ethanol, then soaked for 4 days. The fluorescence was detected everyday excited under 300 nm. Ratiometric fluorescence detection of water. Typically, different volumes of water were added to ethanol in a clean quartz colorimetric tube to make a final water concentrations ranged from 0.05 to 100%. Then, Ru@MILNH2 was added to keep the concentration of 0.05 mg·mL-1 in the mixture of methanol/water. These solutions were mixed well, and then, the luminescence spectra of the solution were recorded under an excitation wavelength of 300 nm after 10 s stirring. Reusability of Ru@MIL-NH2 for water detection. Ru@MIL-NH2 was added into ethanol contained of 10% (v/v) water. After detecting fluorescence response, the solution was removed by centrifugation and Ru@MILNH2 was collected after washed with ethanol. After dried under 70 °C, the recycled Ru@MIL-NH2 was used for detecting the mixture of ethanol and water (50% v/v). Using the same procedure, we detected the water content repeatedly for four cycles.

Figure 1. (A) High resolution transmission electron microscopy (HRTEM) image of Ru@MIL-NH2. (B) The photo of 0.5 -1 mg mL Ru@MIL-NH2 solution in 1:1 water/ethanol mixture under daylight. (C) HRTEM elemental mapping of Ru@MILNH2 for aluminum, ruthenium, nitrogen and oxygen elements.

Analysis of real samples. Spirit samples obtained from local supermarket without further purification. We used the ratiometric fluorescence method to detect the water content in spirit samples. 1.0 mg Ru@MIL-NH2 was added to 20 mL of the samples and the final sensor concentration was 0.05 mg mL-1. Then, ratiometric fluorescence was detected after 10 s stirring. We assumed that the spirit samples consisted of only water and ethanol. To validate the practicability of ratiometric fluorescence method, we detected the ethanol content by gas chromatography (GC) method with standard calibration curve strategy.

The composition of Ru@MIL-NH2 was tested with HRTEM elemental mapping and X-ray photoelectron (XPS) spectroscopy. Elemental mapping illustrated that aluminum, ruthenium, nitrogen, and oxygen elements were uniformly distributed in Ru@MIL-NH2 (Figure 1C). Ruthenium and chlorine were observed in XPS spectroscopy of Ru@MIL-NH2 (Figure S3A). The fluorescence stability of Ru@MIL-NH2 was tested to validate the distribution of Ru(bpy)32+. Stable fluorescence was observed for four days (Figure S3B), and no leakage of Ru(bpy)32+ was observed. Ru(bpy)32+, with the size of 11.5 Å,31 was efficiently encapsulated into MIL-NH2, whose pores were 29 Å and 34 Å.34

RESULTS AND DISCUSSION Preparation and characterization of Ru@MIL-NH2. Ru@MIL-NH2 was prepared by simple one-pot method with a little modification of the previous work for the preparation of MIL-NH2. For stable fluorescence emission and high reproducibility, the guest molecules, Ru(bpy)32+, should be embedded into MIL-NH2 in molecular level. We therefore optimized Ru(bpy)32+ content. The stable fluorescence intensity increased with molar ratio of Ru(bpy)32+/BDC-NH2 increased up to 1:10 (Figure S1), so 1:10 was selected as the optimal molar ratio of Ru(bpy)32+/BDC-NH2 to prepare Ru@MIL-NH2. Moreover, the content of Ru(bpy)32+ did not remarkably affect the size and morphology of the products as shown in Figure S2. Ru@MIL-NH2 exhibited spherical structure and uniform distribution with the average diameter of 200 nm as

The experimental PXRD patterns of Ru@MIL-NH2 and MIL-NH2 were similar to that of simulated MIL-NH2 (Figure S4A). The disappearance of Ru(bpy)32+ peak at 7.5° in Ru@MIL-NH2 indicated that Ru(bpy)32+ dispersed in the pores of the host MIL-NH2 but not mixture with MIL-NH2 in its crystalline form. Thus, Ru(bpy)32+ was embedded and distributed in the pores of MIL-NH2 in molecular level rather than simple commixture. 7.0% wt Ru(bpy)32+ was found in Ru@MIL-NH2 according to the result of inductively coupled plasma-atomic emission spectrometry (Table S1). Thanks to the excellent stability and fluorescence property, even little Ru(bpy)32+ was effective enough to rise red emission to achieve ratiometric fluorescence. Thermogravimetric analysis (TGA) revealed that

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MIL-NH2 and Ru@MIL-NH2 were stable up to 413 °C after the loss of ethanol and DMF solvents (Figure S4B). Ru@MIL-NH2 showed more weight loss than MIL-NH2 because of the removal of pyridine group from Ru(bpy)32+. Fourier transform infrared spectra of MIL-NH2, Ru(bpy)32+, and Ru@MIL-NH2 were recorded (Figure S4C). The band centered at 1720 cm-1 in MIL-NH2 and Ru@MIL-NH2 arisen from the stretching vibration of C=O bond. The band at 1595 cm-1 from C=N stretching vibration was observed in Ru(bpy)32+, but disappeared in Ru@MIL-NH2 possibly because it was merged in the strong adsorption of MILNH2. The stretching vibration of O-Al bond at 588 cm-1 demonstrated the coordination of -COOH to Al3+. Brunauer-Emmett-Teller (BET) surface area of MIL-NH2 and Ru@MIL-NH2 was 1503.9 and 1328.5 m3 g-1, respectively (Figure S4D). The lower BET area of Ru@MIL-NH2 indicated that Ru(bpy)32+ was encapsulated in the pores of MIL-NH2.

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300 nm, Ru@MIL-NH2 in pure water and pure ethanol showed obviously blue and red emission at 465 and 615 nm, respectively, as validated with the results in Figure 2D. Red and blue images of pure ethanol and water were clearly observed (inset in Figure 2D). Therefore, 300 nm was selected as single excitation wavelength and provided the possibility to detect water in ethanol with the range of 0-100% with ratiometric fluorescence of Ru@MIL-NH2.

Optical property of Ru@MIL-NH2. The UV-visible absorption spectra of Ru(bpy)32+, BDC-NH2, and Ru@MILNH2 were tested (Figure S5A). The absorption was observed at 290 and 450 nm for Ru(bpy)32+ and at 345 nm for BDC-NH2. The absorption of Ru@MIL-NH2 at 290 nm and 350 nm was consist with the merge of BDC-NH2 and Ru(bpy)32+. Two excitation peaks were observed at 260 nm and 380 nm for BDC-NH2 and MIL-NH2, so the emission of MIL-NH2 originated from BDC-NH2. A broad excitation range was observed for Ru(bpy)32+ and Ru@MIL-NH2 from 290 to 500 nm (Figure S5B). Thus, Ru@MIL-NH2 integrated the optical behavior of Ru(bpy)32+ and MIL-NH2.

Figure 2. 3D fluorescence mapping of Ru@MIL-NH2 in (A) pure ethanol, (B) pure water, and (C) 1:1 (v/v) mixture of water and ethanol. (D) Emission spectra of Ru@MIL-NH2 in pure water (blue) and pure ethanol (red) and the 30% water content in ethanol (black). Inset: The photos of ethanol and water solution of Ru@MIL-NH2 under excitation at 302 nm and daylight.

The emission peak of BDC-NH2 was observed at 455 nm and that of Ru(bpy)32+ at 640 nm (Figure S5C). Ru@MILNH2 showed two emission peaks at 465 and 615 nm with blue and red emissions. The emission of BDC-NH2 red shifted to 465 nm after the coordination between BDCNH2 and Al3+ ions to form Ru@MIL-NH2. The blue-shift emission from 640 to 615 nm illustrated that energy transfer occurs between Ru(bpy)32+ and MIL-NH2. Thus, Ru(bpy)32+ was embedded into the MOFs at molecular level with easy energy transfer in terms of the high stability of fluorescence.

Water detection with Ru@MIL-NH2 as probe. The dual emission of Ru@MIL-NH2 was tested with different water content in ethanol to validate its efficiency. The emission at 465 nm was enhanced gradually but the emission around 615 nm kept stable relatively under singleexcitation of 300 nm with water content increasing from 0 to 100% (Figure 3A). A good linear relationship was observed for the intensity ratio between the emissions at 465 and 615 nm and the water content (Figure 3B). Moreover, the color change was easily identified by naked eyes under excited at 302 nm (inset in Figure 3B). Trace water response was recorded from 0 to 10 % (v/v) to illustrate the sensitivity of the dual-emission probe and good linear response was observed (Figure S6A, B). A low detection limit was observed for water content in ethanol as 0.02% (v/v), which is lower than that obtained from traditional Karl Fischer titration method.4, 25 We also tested the effect of the dose of Ru@MIL-NH2. The linear calibration curve still kept stable even with high dose of Ru@MIL-NH2 (Figure S6). Thus, the influence of probe concentration and environment interference could be eliminated as a merit of ratiometric fluorescence. The color change with increasing water content was illustrated with the CIE

The rational selection of excitation wavelength was one of the merits of fluorescence detection for special application. To detect broad water content, the 3D emission profiles of Ru@MIL-NH2 were recorded in pure ethanol, pure water, and 1:1 mixture of water/ethanol. In pure ethanol, the emission from MIL-NH2 was weaker than that of Ru(bpy)32+ encapsulated in Ru@MIL-NH2 (Figure 2A). However, the emission behavior reversed in pure water as blue fluorescence emission was higher than red one (Figure 2B). If 300 nm was selected as excitation wavelength for Ru@MIL-NH2 in 1:1 mixture of water/ethanol, almost the same emission intensity was observed from MIL-NH2 and Ru(bpy)32+ (Figure 2C). Moreover, at the excitation of

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Figure 3．．The emission spectra of (A) Ru@MIL-NH2 and (D) MIL-NH2 respond to water content in ethanol. The fitting curve between luminescence intensity and water content: (B) Ru@MIL-NH2 with ratiometric fluorescence and (E) MIL-NH2 at single emission at 465 nm under 300 nm excitation. Inset: images of Ru@MIL-NH2 and MIL-NH2 in ethanol with different water content under the excitation of 302 nm. The water content from left to right: 0, 5, 10, 20, 30, 40, 50, 60, 80, and 100% for Ru@MILNH2 and 10, 20, 30, 40, 50, 60, 70, 80, and 90% for MIL-NH2. CIE chromaticity coordinates of (C) Ru@MIL-NH2 and (F) MIL-NH2 in ethanol with different water content under the excitation of 30 nm.

coordinates in Figure 3C, while true red became true blue gradually.

tra observed in Figure S9A. The fluorescence emission intensity was related to water content when BDC-NH2 (Figure S9B) and MIL-NH2 (Figure 3D) were selected as probes. Well linear response was observed as shown in Figure S8C and Figure 3E. The results indicated that the ligand, BDC-NH2, is sensitive to water. Three sp2 hybrid orbitals in –NH2 form three covalent bonds with two hydrogen atoms and the carbon atom in benzene ring. Correspondingly, a delocalized π bond, π1112, forms among benzene ring, two carbonyl groups, and the nitrogen atom in ethanol condition in ethanol (Figure 4A). Water showed stronger capacity than ethanol to protonate BDCNH2 in Ru@MIL-NH2 and the nitrogen atom in Ru@MILNH2 donated its lone-pair electron to H+ to form –NH3+ with increasing water content in ethanol. BDC-NH2 was protonated and the nitrogen in –NH3+ became sp3 hybrid, so the delocalized π1112 bond became π1010 around the benzene ring and two carbonyl groups in water condition. The protonation of nitrogen atom in BDC-NH2 changed the energy of LUMO and HOMO of the π-conjugate system of Ru@MIL-NH2 to result in the emission change. The changed π-conjugate system of MIL-NH2 was validated with 3D fluorescence profiles in Figure 2. The emission was observed at 440 nm with the excitation at 350 and 240 nm in water (Figure 2B). However, the emission at 350 nm excitation became weak, while the other emission red-shifted to 500 nm with the excitation blue-shifted to 200 nm in ethanol (Figure 2A).

To illustrate the superiority of ratiometric fluorescence from Ru@MIL-NH2, we detected water content with MILNH2 as probe as a comparison (Figure 3D). Increasing water content resulted in increased emission intensity, but only blue fluorescence was observed and the intensity change was difficult to distinguish by naked eyes (inset in Figure 3E). The coordinate points were difficult to distinguish to each other observed with MIL-NH2 as probe (Figure 3F). Thus, we built a turn-on ratiometric fluorescence sensor with wide response range, low detection limit, and high sensitivity to detect water in ethanol. To reveal the reusability, the PXRD patterns and ratiometric fluorescence profiles of Ru@MIL-NH2 responding to 10 and 50 % (v/v) water in ethanol were recorded repeatedly (Figure S7). After four cycles, the PXRD patterns and ratiometric fluorescence were still stable to illustrate the practical use of Ru@MIL-NH2 (Figure S7). We also explored the influence of ion strength for the fluorescence stability of Ru@MIL-NH2 with different concentration of NaCl. Even 17 mM of NaCl was added, the fluorescence kept stable as observed in Figure S8. And the ratiometric fluorescence intensity was stable in different ion strength. Water detection mechanism of Ru@MIL-NH2. In order to explore the water-detection mechanism, we detected the response for MIL and BDC-NH2 to water content. MIL did not response to water as its emission spec-

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The protonation of nitrogen atom in Ru@MIL-NH2 was confirmed by the results of zeta potential test. Ru@MILNH2 has lower zeta potential than MIL-NH2 in both water and ethanol (Figure S10) as Cl- ions exist on Ru@MIL-NH2 because chlorine was observed in the XPS pattern of Ru@MIL-NH2 (Figure S3C). The zeta potential of Ru@MIL-NH2 in ethanol was obviously higher than in water. We considered that the counter ions Cl-, on the surface, increased for the lower zeta potential after the protonation of nitrogen atoms in Ru@MIL-NH2. Ru@MIL-NH2 possesses more hydrophilic active sites such as aluminum cluster and free –NH2, so water molecules enter cages easily. Water displaced ethanol molecule and led to the formation of the hydrated form of Ru@MIL-NH2 with increasing water content (Figure 4B). The LUMO and HOMO of π-conjugate system of Ru@MIL-NH2 changed to result in the changed blue emission. The water content in organic solvents could be detected immediately after added Ru@MIL-NH2 for 10 s stirring. Thus, the detection of water content is finished within one minute.

Figure 4. Water-responding mechanism with Ru@MIL-NH2 as probe. (A) The protonation of the nitrogen atom in amino group changed the π-conjugate system of Ru@MIL-NH2. Dash lines illustrate the big π bonds. (B) Trace of water was absorbed in the pores of Ru@MIL-NH2 to result in the turnon blue fluorescence. (C) Fluorescence profile of Ru@MILNH2 in ethanol (EtOH), methanol (MeOH), N,Ndimethylformamide (DMF) and dimethyl sulfoxidewith (DMSO)/water mixture (5.0% v/v). (D) The photos of different solvents contained Ru@MIL-NH2 with water content ranged from 10 to 90% (v/v) under the excitation of 302 nm.

To validate the water exchange response mechanism, ratiometric fluorescence response of Ru@MIL-NH2 to the water content in dimethyl sulfoxide (DMSO), N,Ndimethyl formamide (DMF), and methanol (methanol) was recorded (Figure 4C). The color change from red to blue was clearly observed by naked eyes as illustrated in Figure 4D. Similar to the water response in ethanol, water in the organic solvents protonated the amino groups of Ru@MIL-NH2 and achieved the emission change obviously. Therefore, Ru@MIL-NH2 is a general agent for the ratiometric fluorescence detection of water content in organic solvents by carefully selecting excitation wavelength at 300 nm.

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Detection of water content in real samples. The ratiometric fluorescence strategy was validated with the detection of water content in white spirit samples with the outstanding characteristics of Ru@MIL-NH2. The water content in the samples was 71.6 and 40.7%, respectively. To validate the precision of the proposed method, we detected the ethanol content with gas chromatography (GC) method as a comparison (Figure S11). The relative error of the results between GC-method and the ratiometric fluorescence was 1.6 and 2.0%, respectively (Table S2). Thus, the ratiometric fluorescence method has great potential to practical application. To detect trace of water in organic solvents conveniently and quickly, we prepared fluorescence water-detection test strip by coating Ru@MIL-NH2 on filter paper evenly to form paper-supported film. The testing solvents were added to the test strip to judge the water content rapidly by naked eyes with observable color changing. The redemitting film turned to blue immediately during the water increase from 0 to 5% (Figure 5A). A linear relationship of CIE coordinates versus water content can be established clearly (Figure 5B), which served as a quantitative ratiometric fluorescence sensor for trace of water in solvents.

Figure 5. (A) Photographs showing ratiometric fluorescence color change of the Ru@MIL-NH2 test strip response to different water content. (B) The CIE coordinates of the color obtained from (A). The CIE chromaticity points are related to 0, 0.5, 1.0, 1.5, 2.5, and 5.0% water content from right to left.

CONCLUSION In conclusion, Ru@MIL-NH2 was successfully prepared with simple one-pot procedure. With dual-fluorescence emission under single excitation of 300 nm, Ru@MIL-NH2 acted as an efficient turn-on ratiometric fluorescence probe for real-time and high-sensitivity detection of water content with broad detection range from 0 to 100%. A low detection limit (0.02%) was achieved, which was obviously low than that obtained from Karl-Fischer titration method. Compared with single-emission fluorescence probe, Ru@MIL-NH2 showed obvious color change to differentiate water content without the interference from probe concentration and environment. Mechanism study indicated that the protonation/deprotonation of the nitrogen atoms in Ru@MIL-NH2 changed its emission behavior, while Ru(bpy)32+ kept stable red fluorescence as reference. An additional advantage of Ru@MIL-NH2 is the reusability, which is essential for practical application. Fluorescence water-detection test strip was easily prepared to detect water for convenient use. And the prac-

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Analytical Chemistry (9) Zhou, Y.; Huang, X.; Liu, C.; Zhang, R.; Gu, X.; Guan, G.; Jiang, C.; Zhang, L.; Du, S.; Liu, B.; Han, M.-Y.; Zhang, Z. Anal. Chem. 2016, 88, 6105-6109.

tice use of the sensor to detect water in white spirits was highly consist with the GC-method. Such turn-on ratiometric fluorescence guest-host MOFs will offer the guidance for the development of other ratiometric fluorescence detectors with broad sensing applications.

(10) Hao, J.; Liu, F.; Liu, N.; Zeng, M.; Song, Y.; Wang, L. Sens. Actuators, B 2017, 245, 641-647.

ASSOCIATED CONTENT

(11) Yang, Z.-R.; Wang, M.-M.; Wang, X.-S.; Yin, X.-B. Anal. Chem. 2017, 89, 1930-1936.

Supporting Information The Supporting Information is available free of charge on the ACS Publications site. Figures (Figure S1−S11) and Table 1 and 2. The characterization of Ru@MIL-NH2, including SEM images, size distribution, thermogravimetric analysis, infrared spectra analysis, X-ray photoelectron spectroscopy analysis of Ru(bpy)3Cl2; optical properties of this system, such as UVvis and fluorescence spectrum. Selection of the content of 2+ Ru(bpu)3 and the most appropriate excited wavelength. The real sample analysis with ratiometric detection (PDF)

(12) Fegade, U.; Patil, S.; Kaur, R.; Sahoo, S. K.; Singh, N.; Bendre, R.; Kuwar, A. Sens. Actuators, B 2015, 210, 324-327. (13) Hao, L.; Qiu, Q.-M.; Wang, W.-J.; Gu, L.; Li, H. Chin. J. Chem. 2016, 34, 1109-1113. (14) Furukawa, H.; Cordova, K. E.; O'Keeffe, M.; Yaghi, O. M. Science 2013, 341, 1230444. (15) He, C.; Lu, K.; Liu, D.; Lin, W. J. Am. Chem. Soc. 2014, 136, 5181-5184. (16) He, C.; Liu, D.; Lin, W. Chem. Rev. 2015, 115, 11079-11108. (17) Feng, X.; Feng, Y.; Guo, N.; Sun, Y.; Zhang, T.; Ma, L.; Wang, L. Inorg. Chem. 2017, 56, 1713-1721. (18) Li, B.; Zhang, Y.; Ma, D.; Ma, T.; Shi, Z.; Ma, S. J. Am. Chem. Soc. 2014, 136, 1202-1205.

AUTHOR INFORMATION Corresponding Author

(19) Lu, Y.; Yan, B. Chem Commun (Camb) 2014, 50, 13323-13326.

* Fax: +86-22-23503034, Email: [email protected].

(20) Liu, Y.; Pan, M.; Yang, Q.-Y.; Fu, L.; Li, K.; Wei, S.-C.; Su, C.Y. Chem. Mater. 2012, 24, 1954-1960.

Author Contributions All authors have given approval to the final version of the manuscript.

Notes The authors declare no competing financial interest.

ACKNOWLEDGMENT This work was supported by National Natural Science Foundation of China (Grants 21435001 and 21675090), the National Basic Research Program of China (973 Program, No. 2015CB932001), and Tianjin Natural Science Foundation (15ZCZDSF00060).

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Ratiometric Fluorescence Sensing and Real-Time Detection of Water

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