Triple-Interpenetrated Lanthanide-Organic Framework as Dual Wave

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Triple-Interpenetrated Lanthanide-Organic Framework as Dual Wave Bands Self-Calibrated pH Luminescent Probe Sheng-Li Hou, Jie Dong, Meng-Hua Tang, Xiao-Lei Jiang, Zhuo-Hao Jiao, and Bin Zhao Anal. Chem., Just Accepted Manuscript • DOI: 10.1021/acs.analchem.9b00848 • Publication Date (Web): 29 Mar 2019 Downloaded from http://pubs.acs.org on March 29, 2019

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Analytical Chemistry

Triple-Interpenetrated Lanthanide-Organic Framework as Dual Wave Bands Self-Calibrated pH Luminescent Probe Sheng-Li Hou, Jie Dong, Meng-Hua Tang, Xiao-Lei Jiang, Zhuo-Hao Jiao, and Bin Zhao* Department of Chemistry, Key Laboratory of Advanced Energy Material Chemistry, MOE, Nankai University, Tianjin 300071, China. E-mail: [email protected]. Fax: 86-022-23502458. ABSTRACT: pH value is a key parameter in reflecting the health status, reaction process, and water quality. The construction of highly sensitive pH luminescent ratiometric is important but challenging. Herein we designed and synthesized a unique triple-interpenetrated luminescent lanthanide-organic framework {[Eu(PPTA)0.5(NO3)(DMF)2]·H2O}n (V104) based on an amphoteric ligand 4,4’,4’’,4’’’-(1,4-phenylenebis(pyridine-4,2,6triyl))tetrabenzoic acid (H4PPTA). Compound V104 possesses high solvent and acid/alkaline stabilities. Luminescent investigations reveal that V104 exhibits dual emission peaks at 390 and 480 nm, and these two peaks can separately detect OH– and H+ among various anions and cations. Importantly, V104 can serve as a self-calibrated pH ratiometric to quantitatively detect pH value, and the sensitivity can reach 403.2% per pH for OH–, and 129.5% per pH for H+. Furthermore, by encapsulating magnetic γ-Fe2O3 nanoparticles in V104, the pH sensor can be readily separated from the analyte by external magnet and recycled at least four times, suggesting as-synthesized γ-Fe2O3@V104 has potential for monitoring pH fluctuations in water. To our knowledge, this is the first self-calibrated ratiometric pH-sensor based on two responsive wave bands which can separately detect OH– and H+.

pH value is a fundamental parameter in cytology, environmentology, biomedicine, and chemistry, and only minor change of pH value may cause devastating damages in creatures or chemical reactions.1,2 On the other hand, real-time detecting of pH value can be applied in monitoring the process of cell metabolism, the activity of enzyme, and the quality of water.3-5 Therefore, the precise sensing of fluctuating pH value is highly significant. Up to now, many methods have been developed for pH-sensing,6-9 and in these explorations, the self-calibrated luminescent pH sensors based on organic dyes,10 supramolecules,11 quantum dots,12 nanoparticles,13 block copolymers,14 and coordination compounds15 have been extensively investigated because they can provide their own internal standard by employing the ratio of two different peaks as the detecting signals. Hence, these pH sensors have high interference rejection and offer more accurate analysis results without additional calibration.16-19 Nevertheless, They are mono wave band pH ratiometrics performed by change of one emission peak. Dual wave bands pH ratiometrics based on two responsive emission peaks which can individually detect OH– and H+ have never been reported so far.20-23 Luminescent MOFs are a new class of hybrid materials which are constructed from organic ligands and metal ions.24-27 Owing to their adjustable porous structures, modifiable luminescent response sites, and tailored frameworks, they have been widely explored as luminescent sensors in detection of explosives,28 corrosive gases,29 small organic molecules,30,31 metal ions,32,33 antibiotics,34 cancer markers,35 temperature,36,37 and amino acids.38 Compared with other luminescent

sensors, MOFs based sensors often exhibit excellent sensitivity, high selectivity and reusability. In fact, luminescent MOFs are also promissing candidates to serve as self-calibrated pH ratiometrics because they generally possess two luminescent centers: metal ions and the large-conjugation π-system of ligands. However, MOF-based self-calibrated pH sensors are rarely reported.39-41 Hence, it is important but challenging to develop self-calibrated MOFs-based sensors for detecting the fluctuating pH value. In this contribution, we selected an amphoteric ligand 4,4’,4’’,4’’’-(1,4-phenylenebis (pyridine-4,2,6-triyl)) tetrabenzoic acid (H4PPTA) with two pyridine rings as the strut to synthesize a unique triple-interpenetrated framework {[Eu(PPTA)0.5(NO3)(DMF)2]·H2O}n (V104). Compound V104 exhibits good solvent stabilities and acid/base stabilities within a wide pH ranging from 2 to 12. Luminescent explorations indicate that V104 displays two clear and distinguishable emission peaks at 390 and 480 nm in water, and the luminescent intensity at these dual wave bands can be enhanced by OH– and H+, respectively. Interestingly, the relationship between the intensity ratio (I480/390 or I390/480) and pH value can be fitted as linear functions from pH = 2 to 7 and 7 to 10.5, revealing V104 can serve as a self-calibrated pH ratiometric to quantitatively detect pH value. Moreover, magnetic γ-Fe2O3 nanoparticles can be readily loaded in V104 via one-pot strategy, and the luminescent character of V104 is almost unaffected. The introduced γ-Fe2O3 can be exploited to separate the sensors from analyte after placing in a magnetic field, which is convenient for reusing the pH sensors. Importantly, the synthesized γ-Fe2O3@V104 can keep stable and accurate

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even after four cycles in the responsive window, suggesting this pH sensor has potential to apply in monitoring the quality of water. To our knowledge, this is the first example of MOF-based pH ratiometric for separately detecting OH– and H+ via two independentresponsive wave bands.

EXPERIMENTAL SECTION Characterization. Crystallographic measurement was collected on a Super-Nova Single Crystal Diffractometer (Oxford). Powder X-ray diffraction (PXRD) was measured by Ultima IV (Rigaku) with Cu Kα radiation, and all samples were grinded sufficiently before tested. The PXRD patterns were smoothed by jade 6 using Savitzky-Golay method (5 points). Luminescent detection was tested on Cary Eclipse (Agilent) fluorospectro photometer. Field emission scanning electron microscope (FESEM) and Energy-dispersive Xray spectroscopy (EDS) elemental mapping images were captured by MERLIN Compact (ZEISS). Thermogravimetric analysis (TGA) was performed on TG 209 (NETZSCH) under N2 atmosphere, and the heating rate was 10 °C/min. Transmission electron microscope (TEM) images were captured through Tecnai G2 F20 (FEI). Diffuse reflectance UV-Vis spectroscopy was recorded on UV-3600 (SHIMADZU). Inductively coupled plasma mass spectrometer (ICP-MS) was determined by IRIS Advantage (Thermo). X-ray photoelectron Spectrometer (XPS) analysis was performed on an Axis Ultra DLD (Kratos Analytical Ltd.) spectrometer. Synthesis of {[Eu(PPTA)0.5(NO3)(DMF)2]·H2O}n (V104). Eu(NO3)3·6H2O (100 mg) and H4PPTA (50 mg) were dissolved in 15 mL DMF, and transferred to a teflon-lined stainless autoclave. After adding 1 mL HNO3 (10 M) as the modifier, the autoclave was heated to 120 °C and kept for 24 hours under autogenous pressure. Then, light yellow block crystals were obtained, and washed three times with DMF before drying. Elemental analysis calculated for compound V104: C 45.9%, H 3.82%, N 7.65%. Found: C 46.2%, H 3.93%, N 7.83%. CCDC number: 1833869. Synthesis of γ-Fe2O3 nanoparticles. γ-Fe2O3 nanoparticles were synthesized according to literature with slight modifications.42 Under argon atmosphere, 1.04 g FeCl3 and 0.4 g FeCl2 were dissolved in an HCl solution (0.4 M, 5 mL). The resulting solution was added into NaOH solution (1.5 M, 50 mL) under vigorous mechanical stirring, and black precipitate was generated instantly. After ten minutes, the precipitate was isolated, and washed with water and HCl solution (0.01 M). Finally, the obtained precipitate was dried and calcined at 210 °C for 3 hours to produce γ-Fe2O3. Synthesis of γ-Fe2O3@V104. The obtained γ-Fe2O3 was firstly immersed in DMF at 100 °C for 12 hours and dried in vacuum. Then, 15 mg of the resulting γ-Fe2O3, 100 mg Eu(NO3)3·6H2O, 50 mg H4PPTA, 15 mL DMF, and 1 mL HNO3 (10 M) were added into a flask (50 mL). After ultrasonic dispersion for 30 min, the flask was heated at 120 °C under vigorous mechanical stirring for 12 hours. The composite material γ-Fe2O3@V104 was obtained after washing with DMF three times.

Luminescent experiment. In a typical experiment, 10 mg of V104 was added in 10 mL HCl or NaOH solution with the value of pH ranging from 2 to 7 and 7 to 10.5, respectively. After ultrasonic treatment for 10 minutes, 3 mL of these solutions was placed in a quartz cuvette, and the luminescent spectra were recorded using an excitation wavelength of 300 nm. Reversibility of γ-Fe2O3@V104. After luminescent experiments, the ratiometric sensor γ-Fe2O3@V104 was collected by external magnet, and washed with a buffer solution of pH = 7 and water until the eluate turned neutral. After that, the resulting sensor was re-dispersed in corresponding pH solution and measured by luminescence spectrometer.

RESULTS AND DISCUSSION Block crystals were obtained through solvothermal reaction between organic ligand H4PPTA and Eu(NO3)3•6H2O in the DMF/H2O mixed solvent at 120 °C (Figures S1 and S2). The crystal structure was determined by single crystal X-ray diffraction analysis, and the structural factors are given in Tables S1 to S3. V104 crystallizes in the monoclinic system with P21/c space group. The foundational unit of V104 consists of one Eu ion, one half of deprotonated PPTA4–, two coordinated DMF molecules, one coordinated NO3– and one free water molecule (Figure S3). Each Eu ion coordinates to nine oxygen atoms from two DMF molecules, two NO3–, and three PPTA4–. The nine Eu−O bond distances range from 2.371 to 2.587 Å (Table S2). Two adjacent Eu ions are bridged by two carboxyls from PPTA4– ligands and two oxygen atoms from two NO3– with the μ3-η2(O, O)-coordinated mode, forming a binuclear cluster [Eu2] as secondary unit building (Figure 1A). Each [Eu2] cluster in V104 is bridged by four PPTA4– ligands, while each ligand is linked to four [Eu2] clusters with two coordination modes (bidentate chelating to one Eu ion and bidentate cis-bridging to two Eu ions). The binding energy of Eu 3d5/2 is observed at 1134.6 eV from XPS analysis, which indicates the oxidation state of Eu ion is +3 (Figure S4). IR spectra display that the stretching vibrations of −OH and C=O in

Figure 1. (A) The coordination environment of Eu, H4PPTA, and [Eu2] cluster in V104. (B) Ball−stick models of a single network in compound V104. (C) Simplified topology of the triple-interpenetrated framework.


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ligand are disappeared in as-synthesized V104, suggesting the oxygen atoms of ligand coordinate with Eu ions (Figure S5). Thus a neutral tripleinterpenetrated three-dimensional framework is formed (Figure 1B and Figure S6). Although some doubleinterpenetrated Ln–MOFs have been reported, the structures with three-fold interpenetration or more are rather rare in Ln–MOFs (Table S4).43-45 Considering the [Eu2] cluster and ligand as nodes separately, the framework of V104 can be simplified as a 4,4-connected framework with a sqc-type topology (Figure 1C and Figure S7). The interpenetrated motifs of V104 along different directions were also shown in Figure S8. The powder X-ray diffraction (PXRD) analysis of assynthesized V104 displays that the experimental PXRD pattern is in good agreement with the simulated one, revealing the obtained crystals are pure phase (Figure S9). Variable-temperature PXRD patterns were measured to explore the thermal stability of V104, and the consistent PXRD patterns among simulated and experimental ones indicate that compound V104 can keep crystalline state even at 200 °C (Figure S10). The thermal stability can also be confirmed by thermogravimetric analysis. A weight loss of 2.1% is observed from 40 °C to 140 °C (Figure S11), which belongs to the removal of water molecules in the channel (calculated 2.46%). The experimental PXRD patterns are conform to the simulated one after separately immersing in water, dioxane, methanol, toluene, dichloromethane, and N-methyl pyrrolidone for ten hours, suggesting the good stabilities of V104 in common solvents (Figure S12). Futhermore, V104 samples were soaked in different acidic/alkaline solutions for ten hours, and the PXRD patterns are still consistent with the simulated one, indicating that V104 can keep stable between pH = 2 and 12 (Figure 2).

transitions of Eu ions, respectively (Figure S14).46 Encouraged by the good luminescent performances and acid/base stabilities, we investigated whether V104 can be applied in monitoring pH value in water through luminescent detection. In a typical experiment, 10 mg assynthesized V104 were dispersed in 10 mL different pH solutions at room temperature, and formed a suspension liquid by ultrasonication. Then, 3 mL resulting sample was measured by fluorescence spectrophotometer, and the results were shown in Figure 3. The intensity of emission peak at 480 nm (I480) increases with pH changing from 7.0 to 2.0, while the intensity at 390 nm (I390) keeps almost unchanged (Figure S15). These two emission peaks at 480 and 390 nm are well separated and easily distinguished in luminescent spectra. Interestingly, the relationship between I480/I390 and pH value can be fitted as linear function (1) with a correlation coefficient (R2) of 0.96 (Figure 3C), and the sensitivety (section 2 in supporting information) is 129.5% per pH based on calculation. On the other hand, the I390 rises dramatically along with pH value elevating from 7 to 10.5, while the I480 remains almost constant (Figure S16). The relation beween pH value and I390/I480 also can be fitted as function (2) with R2 = 0.99, and the calculated sensitivity is 403.2% per pH from 7 to 10.5. These aforementioned experimental results undoubtedly indicate that V104 is a sensitive pH ratiometric from pH = 2 to 10.5. Although many luminescent pH sensors have been reported in recent years, they often works in a narrow pH-response window. It should be noted that the pH sensors with high sensitivity in both acid and basic solutions are rather rare (Table S5).47,48 Importantly, traditional self-calibrated ratiometrics are based on the

pH = 13 pH = 12 pH = 11 pH = 3 pH = 2 pH = 1 As-synthesized

5

10

15

2 ()

20

25

Figure 2. The PXRD patterns of compound V104 after immersing in various acidic/ alkaline solutions for ten hours.

Under excitation at 300 nm, V104 exhibits two broad emission peaks with the maximum values at 390 and 480 nm, which are attributed to π* → π and π* → n transitions of H4PPTA, and four sharp peaks at 590, 616, 649, and 695 nm belong to 5D0-7FJ (J = 1, 2, 3, 4)

Figure 3. pH dependent luminescent spectra of V104 in aqueous solution with pH ranging from 2 to 7 (A) and 7 to 10.5 (B). (C) the linear relationship between pH value and luminescent intensity ratio. I390 or I480 is the luminescent intensity of emission peak at 390 nm or 480 nm.



changes of mono-wave band in their detection range (enhancement or quenching).49-51 Luminescent pH ratiometrics based on dual responsive wave bands which can separately detect OH– and H+ ions have never been reported. pH = 7.207 – 0.772 I480/I390 (1) pH = 6.895 + 0.248 I390/I480 (2) The luminescent selectivity of V104 to H+ among various cations was explored (Figure S17). As shown in Figure 4, only Li+ has a slightly enhancement on the emission, the remaining cations keep unchanged or decreased to some extent, revealing V104 has good selectivity to H+. The influences of some common anions on V104 were also studied (Figure S18), and these anions have negligibly influence on the luminescent intensity when comparing with the blank solution, indicating the good interference rejection of V104 to anions when detecting OH– (Figure 4). To evaluate the analytical efficiency and accuracy of the self-calibrated luminescent ratiometric in practical applications, we set the response curve of V104 as the calibrated curve, several typical acids and bases, such as nitric acid, sulfuric acid, and potassium hydroxide solutions with determined pH values were tested (Figure S19). To our surprise, the obtained results by V104 are closed to the measured value by commercial pH meter under basic conditions, and the corresponding error value is between ±0.17. Even in acidic conditions, the error value is also in the range of –0.26 ~ +0.39, indicating the proposed method has high reliability and sensitivity in real applications.

patterns of as-synthesized γ-Fe2O3@V104 are consistent with the simulated one, revealing that V104 keeps crystalline state after compositing with γ-Fe2O3 (Figure S20). The broadened absorption band (Figure S21) and emerged magnetism (Figure S22) of γ-Fe2O3@V104 indicate that γ-Fe2O3 is introduced in the resulting samples. The binding energy of Fe 2p3/2 is found to be 710.8 eV in XPS spectra, which is consistent with the standard Fe2O3 (710.7 eV), suggesting the oxidation state of Fe in the loaded γ-Fe2O3 is unchanged (Figure S23).55 In addition, transmission electron microscopy (TEM) and high-angle annular dark-field (HAADF) images obviously indicate that γ-Fe2O3 nanoparticles are encapsulated in V104 (Figure 5A and B). The energydispersive X-ray spectroscopy (EDS) elemental mapping images display that Eu and Fe elements are evenly distributed (Figure 5C and Figure S24), which are also observed in the line scan profiles (Figure S25), revealing as-synthesized γ-Fe2O3@V104 is a pitaya type composite material. In addition, under excitation wavelength at 300 nm, the luminescent spectra of γ-Fe2O3@V104 exhibit two broad emission peaks at 390 and 480 nm and four sharp peaks at 591, 615, 650, and 696 nm (Figure 5E), which are well consistant with bulk crystal V104. These characterizations demonstrate that magnetism and luminescence can be simultaneously maintained in the composite material γ-Fe2O3@V104 without interference. The composite material γ-Fe2O3@V104 can disperse in water without initial agglomeration (Figure 5D and Figure S26). Their magnetism can be “switched on” while exposed in a magnetic field, which can be exploited to separate the sensors from analyte. They will lose (switch off) their magnetism after removing the external magnetic field, and there is no remanent magnetization.

Figure 4. Influences of various anions and cations on the luminescent intensity of V104.

As a luminescent pH sensor, the reversibility is an important parameter in practical applications. Filtration, centrifugation or gravitational separation are commonly used to separate the sensors from analyte in past decades, but these methods are time consuming and energy wasting.52,53 Magnetic separation is an ideal candidate to overcome these obstacles presenting in traditional ways, which is more convenient and efficient in separation and redispersion process.54 Based on the above considerations, magnetic γ-Fe2O3 nanoparticles were rationally synthesized and encapsulated in the compound V104, and a croci material γ-Fe2O3@V104 were obtained via one-pot strategy (Figure 5). The PXRD

Figure 5. Electron microscope images of as-synthesized γFe2O3@V104. A: TEM, inset is HRTEM image of encapsulated γ-Fe2O3; B: HAADF; C: EDS elements mapping. D: Photographs (under 365 nm UV lamp) of magnetic separation of γ-Fe2O3@V104 form the analyte and its re-dispersion. E: Liquid photoluminescence spectra, the spectra of γ-Fe2O3@V104 and V104 were normalized. Inset is the color photographs under natural light.


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Moreover, the precision of obtained γ-Fe2O3@V104 as pH ratiometric was also investigated in water with different pH values. The corresponding error values are between –0.26 ~ 0.39 in acid conditions and –0.17 ~ – 0.01 in basic conditions when compared with V104, revealing the γ-Fe2O3@V104 can replace V104 as a selfcalibrated pH ratiometric (Figure S27). Then, the regenerated experiments of γ-Fe2O3@V104 were investigated. After measuring in neutral solution, luminescent sensor γ-Fe2O3@V104 was collected via a magnet and washed with water. The resulting γFe2O3@V104 was re-dispersed in acidic solution (pH = 2) and measured by fluorescence spectrophotometer. The following recycled experiments were operated as described above, and the results were recorded in Figure 6. Remarkably, γ-Fe2O3@V104 can be used at least four cycles, indicating it is a re-generable sensor for monitoring pH value. Meanwhile, the PXRD patterns were in good agreement with the simulated one, revealing γ-Fe2O3@V104 still keeps stable after four cycles (Figure S28). The SEM image of regenerated γFe2O3@V104 indicate the morphologies are almost unchanged (Figure S29). XPS spectra (Figure S23), EDS elemental mapping images (Figure S29) and scanning lines (Figure S30) were clearly displayed the unvaried valence state and homogeneous elements distribution, which further confirmed γ-Fe2O3@V104 are stable enough during experiments. Furthermore, the Eu and Fe ions leaching were analyzed via inductively coupled plasma mass spectrometer, and no Eu or Fe ion (below detection limit) was detected in the eluate after each cycle. All these results certify the good reusability of γFe2O3@V104 as a luminescent pH ratiometric in water.

I390/ I480

15

pH = 10

10 5

pH = 7 0

I480/ I390

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


5

pH = 2 0

1

2

3

Number of recycle

4

Figure 6. Reversibility of pH tests using γ-F2O3@V104.

The possible mechanism is proposed according to aforementioned experimental results and published references. Firstly, the consistent PXRD patterns indicate that V104 still remains stable after luminescent detection, demonstrating the enhancement of luminescent intensity is not caused by the framework collapse. Sencondly, the luminescent intensity of V104 can keep unchanged over time in water (Figure S31). Thirdly, the Na+ and Cl– have negligible influence on the luminescent intensity (Figure 4), indicating the enhancement of luminescent intensity is mainly

attributed to the changing concentration of H+ or OH–. To further investigate the enhancement phenomena of H+/OH– on the luminescent emission, IR spectroscopy was applied to monitor the influence of different pH on the samples (Figure S32). With the pH changing from 10 to 2, two peaks belong to the N–H+(Cl–) vibration of protonated pyridines were emerged between 2250 and 2700 cm–1,56 indicating the pyridine groups of ligands were protonated in acidic environment (Scheme S1). The protonated pyridines will lower the energy of n orbit, resulting in the emission transition state change from n → π* to π → π*, and the luminescent intensity at 480 nm significantly increased.57,58 The enhanced luminescent intensity is also observed in free ligand H4PPTA (Figure S33), which further confirms that the enhancement of luminescent intensity is attributed to protonated pyridines. With the pH value increasing from 7 to 10.5, the luminescent intensities of emission peaks belonging to Eu3+ were gradually declined (Figure S33C), indicating the energy transferred from ligands to Eu ions was suppressed. Hence, the luminescent intensity at 390 nm was enhanced (Scheme S1). The stepwise quenching luminescent intensity of Eu3+ under alkaline environment was also demonstrated by previous works.59,60 Comparably, in the absence of Eu3+ or only mixing the ligand and Eu(NO3)3 in basic solutions, the luminescent intensity at 390 nm only exhibits slightly increase (Figure S33), which also suggests the enhanced intensity at 390 nm mainly ascribed to the stepwise quenching luminescent intensity of Eu3+.

CONCLUSION In summary, a triple-interpenetrated framework {[Eu(PPTA)0.5(NO3)(DMF)2]·H2O}n with good solvent and acid/base stabilities was constructed and characterized. Luminescent results indicate that compound V104 exhibits dual emission peaks at 390 and 480 nm, which can separately detect OH– and H+ according to the luminescent enhancement. Interestingly, the relationship between intensity ratio (I480/I390 or I390/I480) and pH value can be fitted as linear function, and the calculated sensitivity is 129.5% per pH from 2 to 7 and 403.2% per pH from 7 to 10.5, suggesting V104 can be employed as a self-calibrated pH ratiometric to sensitively monitor pH value in water without additional references. Importantly, this luminescent sensor is immune to common anions and cations, implying the good selectivity of V104 in quantitative detect pH value. Furthermore, by encapsulating magnetic γ-Fe2O3 nanoparticles in V104, a composite material γ-Fe2O3@V104 was fabricated via one-pot strategy. The magnetism and luminescence property can be maintained simultaneously without mutual interference according to experimental results. Recycled investigations reveal that γ-Fe2O3@V104 can be readily separated from the analyte through an external magnetic field and reused at least four cycles, indicating γ-Fe2O3 nanoparticles can be used as a promising platform for magnetic separation, and the proposed γ-Fe2O3@V104 can serve as a re-generable sensor for monitoring pH fluctuations. Markedly, V104 represents the first self-calibrated luminescent pH sensor based on two responsive wave bands to separately detect OH– and H+, which may bring new inspiration to



construct more sensitive and reversible luminescent ratiometrics.

ASSOCIATED CONTENT Supporting Information The Supporting Information is available free of charge on the ACS Publications website. X-ray crystallographic data in CIF format, Synthesis procedure, IR spectra, TGA and PXRD results.

AUTHOR INFORMATION Corresponding Author * E-mail: [email protected]. Fax: 022-23502458.

Notes

The authors declare no competing financial interest.

ACKNOWLEDGMENT This work was supported by the NSFC (21625103, 21571107, and 21421001), 111 project from the Ministry of Education of China (B12015), National Programs of the Nano-Ket Project (2017YFA0206700). The authors also thank Dr. AnFei Yang and Xue-Jing Che for discussing and commenting on the manuscript.

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