Covalent Organic Framework Functionalized with 8-Hydroxyquinoline

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Covalent Organic Framework Functionalized with 8-Hydroxyquinoline as a Dual-mode Fluorescent and Colorimetric pH Sensor Long Chen, Linwei He, Fuyin Ma, Wei Liu, Yaxing Wang, Mark A. Silver, Lanhua Chen, Lin Zhu, Daxiang Gui, Juan Diwu, Zhifang Chai, and Shuao Wang ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b05484 • Publication Date (Web): 25 Apr 2018 Downloaded from http://pubs.acs.org on April 26, 2018

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Covalent Organic Framework Functionalized with 8-Hydroxyquinoline as a Dual-mode Fluorescent and Colorimetric pH Sensor Long Chen,† Linwei He,† Fuyin Ma,† Wei Liu,† Yaxing Wang,† Mark A. Silver,† Lanhua Chen,† Lin Zhu,† Daxiang Gui,† Juan Diwu,† Zhifang Chai,† and Shuao Wang*,† †

State Key Laboratory of Radiation Medicine and Protection, School for Radiological and interdisciplinary Sciences

(RAD-X), Collaborative Innovation Center of Radiation Medicine of Jiangsu Higher Education Institutions and School of Radiation Medicine and Protection, Soochow University, Suzhou 215123, China KEYWORDS: Chemosensor, colorimetric, covalent organic frameworks, fluorescent, low pH, mixed matrix membranes ABSTRACT: Real-time and accurate detection of pH in aqueous solution is of great significance in chemical, environmental, and engineering-related fields. We report here the use of 8-hydroxyquinoline-functionalized covalent organic framework (COF-HQ) for dual-mode pH sensing. In the fluorescent mode, the emission intensity of COF-HQ weakened as the pH decreased, and also displayed a good linear relationship against pH in the range from 1 to 5. In addition, COF-HQ showed discernable color changes from yellow to black as the acidity increased and can be therefore used as a colorimetric pH sensor. All these changes are reversible and COF-HQ can be recycled for multiple detection runs owing to its high hydrolytical stability. It can be further assembled into a mixed matrix membrane for practical applications.

Detecting pH in aqueous solutions is of great importance in water quality monitoring, environmental surveillance, chemical reaction control, and medical diagnosis.1 Up to now, various techniques have been developed to measure pH, including luminescence imaging, optical fiber pH sensors, ion-sensitive field effect transistors (ISFET), as well as various electrochemical methods and so on.2 Luminescence and colorimetric-based chemosensors3 are considered to be one of the most practical installations for evaluating pH, not only because of their high sensitivity and resolution, but also because of their fast response time and facile nature.4 Covalent organic frameworks (COFs) are an emerging class of regular porous polymers constructed from the condensation of organic building blocks containing light elements (e.g. C, H, O, N, and B) into two or three-dimensional crystalline structures. The tunable channel in COFs is a remarkable property which has made them employable in a wide variety of applications, such as heterogeneous catalysis, gas storage and adsorption, chemical sensors, proton conduction, and energy storage.5 Luminescent COFs have been increasingly utilized as perceptive chemosensors, because of their high physicochemical stability, uniform pore size, and fast response time6 in detecting 2,4,6-trinitrophenol (TNP),7 polynitro-aromatic analytes,8 the vapours of electron rich or deficient arene,9 mercury ions,10 and so on. However, COF-based sensors for pH are rarely reported. The only COF-based pH sensor, COF-JLU4 developed by Liu et al., displayed high sensitivity towards acidic solutions based on a “turn-on” manner.11 In this contribution, a pH sensitive fluorescent chemical group, 8-hydroxyquinoline (HQ), was selected and successfully introduced into the channels of a two-dimensional COF. As a result, the terminal COF, termed as COF-HQ, can respond to acidic solutions by

way of a change in both color and fluorescence intensity. Additionally, a COF-HQ-based mixed matrix membrane (MMM) was prepared to monitor pH, implying that the as-synthesized COF material is of excellent machinability. COF-HQ was synthesized by the Schiff-base reaction of 2, 5-bis[2-(quinolin-8-yloxy)ethoxy)terephthalaldehyde (2) with 1,3,5-tri-(4-aminophenyl)benzene (TAPB) in the presence of acetic acid (6 M), as well as n-butyl alcohol and o-dichlorobenzene (1/1, v/v) as the mixed solvent (Figure 1A).12 A variety of characterization methods, including powder X-ray diffraction (PXRD), Fourier transform infrared spectroscopy (FT-IR), thermogravimetric analysis (TGA), elemental analysis, as well as transmission electron microscopy (TEM), were applied to characterize COF-HQ. The PXRD profile of COF-HQ displays multiple peaks in the 2θ range from 2.5° to 30°, which indicates the ordered frameworks of COF-HQ. The characteristic peaks appeared at 2.91, 5.89, 7.80, and 24.6° are assigned to the [100], [110], [200], and [001] facets of COF-HQ, respectively (Figure 1F).12 The existence of the [001] facet at 24.6° is equivalent of a π-π stacking distance of 3.62 Å (Figure 1C).13 The refined structure of COF-HQ resulted in a space group of P6 with a = b = 37.688 Å, c = 3.6847 Å, α = β = 90° and γ = 120°, and Rp and Rwp values of 5.04 and 6.89%, respectively.12 The Pawley refinement patterns of COF-HQ show a negligible difference compared with the experimentally observed curve. The generated PXRD patterns from structural simulation suggest that COF-HQ prefers to arrange according to an eclipsed AA-stacking model instead of the staggered AB-stacking model (Figure 1B, D, and E).14 The FT-IR spectrum of COF-HQ showed the characteristic C=N stretching vibration band at 1,615 cm-1 (Figure 1G), which appeared nearly at the same

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Figure 1. A) Schematic representation of the synthesis of COF-HQ from compound 2 and TAPB. B) Top and C) side views of COF-HQ in AA-stacking model (C, gray; N, blue; O, red; H atoms omitted for clarity). D) AA- and E) AB-stacking models for COF-HQ (adjacent layers shown in green and orange for lucidity). F) PXRD patterns of COF-HQ: (a) experimentally observed, (b) Pawley refined, (c) the difference between (a) and (b), (d) the curves for simulated eclipsed AA-, and (e) staggered AB-stacking models. G) FT-IR spectra of compound 2, TAPB, and COF-HQ. H) N2 adsorption isotherms of COF-HQ under various conditions. I) BET surface areas of COF-HQ after treatment in different conditions.

position as in DhaTph COF (1,613 cm-1).15 In addition, the disappearance of stretching bands for N-H (3,353 cm-1) and C=O (1,663 cm-1) indicates that no starting reactants are present with COF-HQ after the Schiff-base reaction and purification. The elemental analysis of COF-HQ displayed the contents of C, H and N were 77.63%, 6.71%, and 5.87%, respectively, which agrees well with the anticipated proportions of 78.17%, 4.85%, and 7.93%, respectively, in an infinite 2D sheet (Table S1).11 TEM images revealed that COF-HQ particles were spindly in shape, forming belt-shaped or rectangle structures with widths up to 50 nm and lengths in excess of 100 nm (Figure S7).15 Thermogravimetric analysis (TGA) was performed to evaluate the thermostability of COF-HQ. Prior to the experiment, COF-HQ was activated at 100 °C under vacuum for 12 h. COF-HQ exhibits no conspicuous weight loss until about 255 °C as shown in the TGA curve (Figure S8). Gradual weight loss is experienced in COF-HQ after, totaling 31% in the range of 255 900 °C. The loss in this region is due to the decomposition of the framework.15 The permanent porosity of COF-HQ was investigated by nitrogen adsorption isotherm.16 Initially, the sample was freshly activated by degassing in vacuo at 60 °C for 10 h. As shown in Figure 1H, COF-HQ exhibited a classical type-I adsorption curve at 77 K with an uptake at low relative pressure (P/P0 < 0.2), owing to the occupation of nitrogen in the small pores.15 Following the gradual adsorption over the remaining relative pressure (0.2 < P/P0 < 0.8), a steep rise in nitrogen

uptake at relative high pressure (P/P0 > 0.8) was observed and attributed to condensation in the interparticle gap.15 The surface area of COF-HQ was evaluated to be 302 m2 g-1 when applying the Brunauer-Emmett-Teller (BET) method.17 The decreased surface area of COF-HQ compared with TPB-DMTP-COF (2105 m2 g-1) can be considered as the result of the functional group (HQ) occupying the pore space.12 The pore size distribution was calculated by non-local density functional theory (NLDFT), which revealed that the size of the micropores in COF-HQ centered around 1.4 nm (Figure S9), less than that of TPB-DMTP-COF (3.3 nm). In addition, the pore volume for COF-HQ was calculated to be 0.296 cm3 g-1. To test the stability of COF-HQ in water, a certain quantity (40 mg) of COF-HQ was immersed in aqueous solution for 24 h. No significant changes in either the BET surface area (313 m2 g-1) or PXRD pattern indicated good hydrolytic stability of COF-HQ (Figure S10). Moreover, COF-HQ also exhibited good stability in acidic solutions, also confirmed by PXRD and N2 adsorption studies before and after immersion in 0.1 M HCl (Figure 1I). Further immersion studies of COF-HQ both in boiling water and 10 M NaOH solution exemplifies its stability, as a little increase in BET surfaces for both cases were observed. These results may be due to the removal of solvent molecules within the channels under extreme conditions. Encouraged by both the acidic stability of COF-HQ and the fluorophore (HQ)-functionalized framework, we investigated the pH-dependent luminescent property of COF-HQ both in the solid state and in aqueous solution.


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From the solid state spectra shown in Figure 2A, HQ displayed an emission feature at 470 nm (λex = 365 nm), owing to the π-π* charge transfer interaction from the electron-rich phenol ring to the electron-deficient pyridine ring.18 Unlike HQ, COF-HQ exhibits emission at 682 nm when excited with 365 nm light. The red shift could be attributed to the π-π stacking interaction between adjacent 8-hydroxyquinoline species.19 As in aqueous solution, the fluorescence intensity of COF-HQ is strongly correlated with the pH of the solution, in which the intensity gradually decreases with increasing acidity (Figure 2B and C). When pH = 1, the fluorescence intensity of COF-HQ was completely quenched. Considering that the pores within COF-HQ might be conducive to incorporation and diffusion of protons, this distinct phenomenon could be induced by the protonation of HQ grafted within the framework (Figure 2D).20 This would damage the π-electron conjugated system of HQ and lead to the noticeable fluorescence quenching.21 Moreover, the relationship between the emission intensity at 644 nm versus pH is established in Figure 2E. Quantitatively, the linear response between these can be described using the equation, y = 12,137x – 12,816 with a correlation coefficient (R2) of 0.979. This correlation could be used directly for determining the pH of aqueous solutions from 1 to 5. Until now, only one COF-based pH sensor (COF-JLU4) has been reported displaying such an interesting fluorescence response. The emission intensity of COF-JLU4 at 428 nm gradually decreased with the basicity enhancing.11 Unlike COF-JLU4, COF-HQ can be slowly quenched by acidic solution. To evaluate the recyclability of COF-HQ used as a fluorescent sensor to probe pH, pH-dependent luminescence spectra were recorded within a pH range of 1 ≤ pH ≤ 5 for five continuous cycles.22 As shown in Figure 2F, the fluorescence at 644 nm nearly quenched after adjusting the pH of the solution to 1 with conc. HNO3. Meanwhile, the color of the solution quickly turned black. However, after regulating the pH to 5 using triethylamine, the color rapidly converted back to orange and the emission could be recovered to nearly similar intensities for five consecutive cycles. These experiments demonstrably support the satisfactory integrity and reproducibility of COF-HQ. Interestingly, when COF-HQ was immersed in a solution of pH = 1, the color of COF-HQ turned from orange to black immediately, which inspired us to apply COF-HQ as a colorimetric-based pH sensor. To investigate the hypothesis, 2 mg of COF-HQ was immersed into 2 mL of various acidic solutions with pH ranging from 1 to 5. As shown in Figure 3A, the colors of these suspensions gradually evolved from dark to light orange as the acidity of the solution decreased. However, when the pH of the solutions exceeded 5 (Figure S11), the color remained orange and showed no

Figure 2. A) Solid state fluorescence spectra of HQ and COF-HQ. B) Fluorescence spectra of COF-HQ in various acidic conditions (λex = 365 nm). C) Images of the fluorescence of COF-HQ after treatment with various acidic solutions. D) Protonation and deprotonation process of HQ within the COF-HQ framework in acidic and basic media. E) The linear relationship between pH and fluorescence intensity. F) The repeatability of COF-HQ for five cycles.

visible difference to the naked eye. This interesting phenomenon further supports the use of COF-HQ as an optical chemosensor to distinguish various acidic solutions within the pH scope from 1 to 5. These results indicate that COF-HQ not only demonstrates good correlation between emissions in low pH, but also could be regarded as a novel nano-sized pH chemosensor both by changes in the color and fluorescence intensity. Compared with pH meters, the advantage of COF-HQ is the fast response time and easy operation.23,24 In order to prove the potential practical application and machinability of COF-HQ, a mixed matrix membrane (MMM) was prepared. Nowadays, the construction of MOF- or COF-based MMMs is a remarkable strategy to accesses functional materials,25,26 which have been widely researched in gas separation,27 proton conductivity,28 membrane reactors,29 gaseous HCl sensors,30 etc. First, the COF-HQ-based MMM was fabricated by combination of COF-HQ with aqueous solutions of polyvinyl alcohol (PVA). After stirring the mixed orange solution for 4 h at room temperature,



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ORCID Shuao Wang: 0000-0002-1526-1102 Author Contributions

The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Notes

The authors declare no competing financial interest. ACKNOWLEDGMENT

Figure 3. A) Eye naked color changes of COF-HQ after immersion in varieties of acidic solutions from pH 1 to pH 5. B) COF-HQ-based mixed matrix membrane with the diameter of 7 cm. C) Changes in color of COF-HQ-based MMM after dropping of assorted acidic solutions.

water from the viscous solution was evaporated under vacuum to finally obtain the desired MMM. As shown in Figure 3B, the prepared MMM was flexible, mechanically robust, and free of macroscopic deficiencies. Compared with the free nanoscale COF-HQ, the MMM exhibited a similar color change after contacting various low pH solutions. As shown in Figure 3C, the color of COF-HQ-based MMM gradually became black with increasing acidity within 1 min. The benefit of this fast response is a direct result of the large contact area on the surface of the membrane. In conclusion, an 8-hydroxyquinoline-functionalized covalent organic framework (COF-HQ) has been synthesized and characterized by multiple techniques, including PXRD, FT-IR, TGA, and so on. After treatment with low pH solutions, COF-HQ showed remarkable changes both in color and in fluorescence intensity, indicating it can be used as a colorimetric and fluorescent nano-sized chemosensor. Furthermore, the as-prepared COF-HQ-based mixed matrix membrane showed gradual color changes after immersion in gradient acidic solutions, suggesting that the material is of great potential for practical applications. Considering the high chemical stabilities of and convenient modifications that can be made to COFs, our work provides a new strategy to fabricate dual responsive pH sensors within this promising platform. ASSOCIATED CONTENT Supporting Information

Supporting Information is available free of charge via the Internet at http://pubs.acs.org. Synthesis and characterization of compound 2, elemental analysis, TEM image, TGA curve, and atomic coordinates for AA- and AB-stacking model of COF-HQ. (PDF) AUTHOR INFORMATION Corresponding Author

* E-mail: [email protected]

We are grateful for the financial support from the National Natural Science Foundation of China (21790370, 21790374, 21761132019, U1532259), the China Postdoctoral Research Foundation (32317532 and 32317551), a project funded by the Priority Academic Program Development of the Jiangsu Higher Education Institutions (PAPD), and the “Young Thousand Talented Program” in China. In particular, we appreciate Dr. Qi Sun and Dr. Yang Li in simulating the structure of COF-HQ.

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SYNOPSIS TOC

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Figure 1. A) Schematic representation of the synthesis of COF-HQ from compound 2 and TAPB. B) Top and C) side views of COF-HQ in AA-stacking model (C, gray; N, blue; O, red; H atoms omitted for clarity). D) AA- and E) AB-stacking models for COF-HQ (adjacent layers shown in green and orange for lucidity). F) PXRD patterns of COF-HQ: (a) experimentally observed, (b) Pawley refined, (c) the difference between (a) and (b), (d) the curves for simulated eclipsed AA-, and (e) staggered AB-stacking models. G) FT-IR spectra of compound 2, TAPB, and COF-HQ. H) N2 adsorption isotherms of COF-HQ under various conditions. I) BET surface areas of COF-HQ after treatment in different conditions. 444x221mm (300 x 300 DPI)



Figure 2. A) Solid state fluorescence spectra of HQ and COF-HQ. B) Fluorescence spectra of COF-HQ in various acidic conditions (λex = 365 nm). C) Images of the fluorescence of COF-HQ after treatment with various acidic solutions. D) Protonation and deprotonation process of HQ within the COF-HQ framework in acidic and basic media. E) The linear relationship between pH and fluorescence intensity. F) The repeatability of COF-HQ for five cycles. 206x331mm (300 x 300 DPI)


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Figure 3. A) Eye naked color changes of COF-HQ after immersion in varieties of acidic solutions from pH 1 to pH 5. B) COF-HQ-based mixed matrix membrane with the diameter of 7 cm. C) Changes in color of COFHQ-based MMM after dropping of assorted acidic solutions. 349x247mm (300 x 300 DPI)


Covalent Organic Framework Functionalized with 8-Hydroxyquinoline

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