Discriminative Molecular Detection Based on Competitive Absorption

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Discriminative Molecular Detection Based on Competitive Absorption by a Luminescent Metal-Organic Framework Sunhwi Eom, Han Geul Lee, Dong Won Kang, Minjung Kang, Hyojin Kim, Youngseo Kim, Sungnam Park, Dohyun Moon, and Chang Seop Hong ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b16926 • Publication Date (Web): 09 Nov 2018 Downloaded from http://pubs.acs.org on November 12, 2018

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ACS Applied Materials & Interfaces

Discriminative Molecular Detection Based on Competitive Absorption by a Luminescent Metal-Organic Framework Sunhwi Eom,†, ‡ Han Geul Lee,†, ‡ Dong Won Kang,† Minjung Kang,† Hyojin Kim,† Youngseo Kim,† Sungnam Park,† Dohyun Moon, § and Chang Seop Hong†,* †Department of Chemistry, Korea University, Seoul 136-713, Korea.

Beamline Division, Pohang Accelerator Laboratory, Pohang, Kyungbuk 790-784, Korea.

§

ABSTRACT: Distinguishing specific molecules from similar chemical species with minor structural differences is challenging, and differentiation has typically been based on analyte-dependent hostguest interactions upon irradiation with a single wavelength. In this study, we prepared a Cd-based metal-organic framework exhibiting nearly constant emission intensity over a wide range of excitations. Due to its unique emission characteristics, this material facilitated the differentiation of specific molecules amidst structurally similar chemical species via competitive absorption. Such discriminative identification was uniquely achieved based on the use of different excitation wavelengths, and is demonstrated to be applicable to the recognition of a target analyte in sensory applications.

Keywords: discriminative detection, photoluminescence, constant emission intensity, metal-organic framework, competitive absorption

additional luminescence quenching or enhancement of a target analyte compared with those of other competing molecules.7, 17 A drawback of this approach is that mere changes in luminescence intensity of a host framework often fail to achieve discriminative recognition of a single target analyte, especially when it is to be distinguished from analytes with similar structural motifs. Scheme 1. Schematic diagram representing discriminative recognition of a target analyte. Signal readouts correspond to emission responses including a) wavelength changes, b) pattern changes, and c) intensity changes, upon irradiation at excitation wavelength ex. d) A unique and facile method for molecular differentiation utilizes different excitation wavelengths (ex1 and ex2) enabling competitive absorption quenching.

Luminescent metal-organic frameworks (MOFs), which are used in chemical sensing and toxic gas removal applications, are essentially composed of organic ligands, d10 transition metal ions, some lanthanide elements, and the guest molecules that are incorporated into the framework.1-6 The diffusion of guest molecules into the pores of a host framework promotes interaction with the framework backbone, thereby altering the emission characteristics from those of the guest-free framework. The host-guest coupling results in quenching or enhancement of the resulting emission, depending on the relative energy levels of the target molecules with respect to the host system.7 Quenching typically occurs in luminescent MOFs following the capture of electron-deficient analytes or metal ions in cavities of the frameworks.8-10 Furthermore, spectral overlap between the absorption bands of the analyte molecules and the emission band of the host material enables luminescence quenching via an energy transfer process.7 In addition, luminescence quenching in an Eu-MOF was observed when nitroaromatic explosives were added, and was attributed to competitive absorption of the irradiated light by both an analyte and a ligand of the MOF.11-12 Along with sensing capabilities, the discriminative recognition of target analytes is essential for the utilization of smart sensory materials in chemical sensing, molecular separation, biotechnology, and environmental monitoring applications.2, 13-16 A common strategy for detection of an analyte of interest is based on variations in emission intensity induced by

To specify an analyte in the presence of chemical substances with only minor structural differences, emission patterns can be used as identification codes for specific molecules.18-19 In this system, however, the patterns are rather irregular and difficult to predict. Another successful protocol for the decoding of structurally similar target molecules entails direct visualization of the target molecule via elaborate guest-induced structural modulation in an interpenetrated porous coordination polymer.20 Detection of and differentiation between aromatic species are facilitated by signal readouts due to the variation in emission wavelength. Thus, previous sensory systems for discriminative detection of a specific analyte were based on complex

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analyte-dependent host-guest interactions after irradiation with a single wavelength, closely correlating emission outputs with the size, shape, and active groups of a target molecule (Scheme 1a - c). However, this approach often provides rather vague readouts that hamper precise discrimination of specific analytes. Thus, it is essential that a promising platform for discriminative detection applications is developed which exhibits distinct output signals in response to different target molecules. Herein, we prepared a Cd-based MOF, [Cd3(L)1.5(DMF)(H2O)2] [1; H4L = 4,4,4,4-(ethene-1,1,2,2-tetrayl)tetrakis(([1,1-biphenyl]-4carboxylic acid], comprising a typical fluorophore ligand with a tetraphenylethylene moiety. Tetraphenylethylene is a promising luminophore moiety that has been widely used in the construction of luminescent MOFs.8, 21-23 In this work, we detected selective quenching in chlorinated benzenes. Polychlorinated benzenes are a class of persistent organic pollutants that linger in living organisms and resist biodegradation.24-25 Accordingly, we investigated the quenching behaviors of a series of chlorinated benzenes such as chlorobenzene (CB), 1,2-dichlorobenzene (DCB), 1,2,4-trichlorobenzene (TCB), 1,2,3,4-tetrachlorobenzene (1,2,3,4-TeCB), 1,2,4,5tetrachlorobenzene (1,2,4,5-TeCB), and pentachlorobenzene (PeCB), and found that the absorption intensities of the chlorinated benzenes influenced their quenching efficiencies. More importantly, 1 exhibited a strong and flat emission response over a wide range of excitation wavelengths. Taking advantage of the unique emission features of 1, we report the facile discriminative sensing of a target molecule via a quenching mechanism, which has not yet been demonstrated (Scheme 1d). Notably, the discriminative identification is associated with competing absorption of incident radiation by a specific analyte and the host MOF.

Figure 1. a) Chain structure of 1 showing core coordination spheres around Cd atoms. One of the disordered Cd atoms (Cd3 and Cd4) was omitted for clarity. The dotted line represents a weak bonding interaction. b) Coordination mode of L4-. c) Three-dimensional network structure of 1 represented in the ab plane.

A solvothermal reaction of Cd2+ and H4L in DMF yielded transparent block-shaped crystals of 1. Using synchrotron X-ray diffraction, we determined the single crystal structure of 1 containing four types of Cd atoms in the asymmetric unit. Three Cd centers (Cd2, Cd3, and Cd4; Figure 1a) adopt a common distorted octahedral geometry with a bond length ranging between 2.2 – 2.5 Å, while the remaining Cd atom (Cd1) is uniquely coordinated by four oxygen atoms derived from the carboxylate moieties (Cd-O length = 2.2 – 2.3 Å) and two weak bonding interactions (Cd-O length = 2.652 Å). Interestingly, Cd3 and Cd4 are connected by the oxygen of the solvent DMF, and with the aid of double carboxylate bridges. Overall, the structure features a Cd chain oriented in a one-dimensional zigzag pattern along the a direction, bridged by carboxylate moieties and DMF (Figure 1a). The L4- ligand links six Cd atoms and acts as a 8bridge (Figure 1b). In the extended view, the Cd chains are interconnected by the ligand, resulting in the construction of a threedimensional framework (Figure 1c and S1). A closer analysis of the extended framework reveals straight rhombus-shaped channels along the c axis, and the opening of the channel shows diagonal dimensions of 6.0 x 17.0 Å2, considering the van der Waals surface of the window. The solvent-accessible void volume of 1 was 51.1%. We performed a thermogravimetric analysis (TGA) of 1 to establish its thermal stability. After the loss of water molecules, steady weight loss over the temperature range of 130 – 390 °C corresponded to the slow elimination of DMF from the framework structure (Figure S2). Powder X-ray diffraction (PXRD) data revealed that the profile of 1 was consistent with the simulated pattern (Figure S3). Before measuring the luminescent properties of 1, its excitation wavelength was determined. The UV-vis absorption spectrum of the ligand H4L showed maximum absorption by the organic ligand at 290 nm and a broad absorption until 400 nm (Figure S4). Accordingly, we measured the photoluminescence of both the ligand and 1 at an excitation wavelength (ex) of 290 nm (Figure S5). The emission peak (509 nm) of 1 was similar to that of the ligand (512 nm), indicating that the fluorescence in 1 was a ligand-based emission. Quantum yields in the solid state were 10.3% for the ligand and 52.5% for 1, while the average lifetimes were 1.6 ns for the ligand and 2.4 ns for 1 (Figure S6). To analyze the sensing capabilities of 1, the luminescence properties were determined in various organic solvents (Figure S7). The structure of 1 was stable in DMF, while it was not stable in water (Figure S8). So, we conducted the luminsecence experiments under DMF conditions. After the addition of DMF (1 mL) to 1 (2 mg), the resulting slurries were sonicated for 30 min and mixed with different organic solvents (1 mL) to yield the suspensions. As shown by the emission spectra, fluorescence in TCB was completely quenched. These results demonstrate that 1 was super-selective and sensitive toward TCB amongst the organic molecules tested. To investigate the concentration dependence of this phenomenon, we used various concentrations of TCB, from 0 mM to 42.9 mM. As the concentration of TCB increased, the luminescence intensity gradually decreased (Figure S9). From the emission data, the quenching efficiency (QE), calculated as (I0 - I)/I0 × 100%, where I0 is the luminescence intensity before the addition of TCB and I is the luminescence intensity after addition, was estimated to be 95% at a TCB concentration of 13 mM (Figure S10). The Stern-Volmer (S-V) equation, I0/I = Ksv[C] + 1, where I0 is the luminescence intensity without the analyte, I is the luminescence intensity with the analyte, [C] is the concentration of the analyte, and Ksv is the quenching constant (units M-1), was used to determine the quenching mechanism. In the dilute concentration region (0 – 2 mM), the luminescence data was fitted to I0/I = (3.23  102)[C] + 1 (Figure S10). As the concentration increased, the


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luminescence behavior deviated from linearity. Such nonlinear behavior has been found in systems exhibiting energy transfer or selfabsorption pathways.12, 26 Remarkably, the relative intensity plot for 1 yielded two nonlinear curves at different concentration regions (Figure S11). To fit the quenching data, we employed an exponential equation, I0/I = Aexp(k[C]) + B, where A, B, and k are constants. A, B, and k were 2.52, -2.71, and 0.166 in the low concentration range, respectively, and 30.7, -17.9, and 0.027 in the high concentration range, respectively. The TCB molecule is smaller than the passage window of 1 (6.0  17.0 Å2), facilitating the diffusion of TCB into the channel (Figures 1c and S12). In comparison, a single nonlinear curve was observed in the emission spectrum of Eu-MOF, in which the analytes only interact with the surface of the framework.27 Therefore, the presence of the two curves in the spectrum of 1 may suggest surface contact with TCB, as well as the presence of the channels which are wide enough to allow entry of the analytes.

light (Figure S29). Meanwhile, energy transfer is not feasible in this system because the absorption bands of the analytes did not overlap the emission band of 1 (Figures S30 and S31). To further probe the quenching behaviors of the chlorinated benzenes, we collected their UV-vis absorption spectra (Figure S32). The absorption bands of the chlorinated benzenes containing more than three Cl atoms overlapped with the excitation wavelength ( = 290 nm), while the peaks of CB and DCB were out of the absorption range. These results explain the quenching behavior of 1 which originates from self-absorption (inner filter effect) of irradiation by the chlorinated benzenes and/or electron transfer upon co-absorption of the incident light by 1 and the analytes, as expected from the DFT calculations.11-12 For instance, CB and DCB showed moderate QEs because these analytes failed to absorb the light and the ligand-based fluorescence was emitted with only a minor loss in intensity (Figure 2). In comparison, other chlorinated benzenes exhibited significant QEs due to the significant absorption of irradiated light by the analytes. Furthermore, the QE of PeCB was lower than that of 1,2,4,5-TeCB despite the presence of additional Cl atoms on the benzene ring of PeCB (Figure 2). The intensity of the absorption band for 1,2,4,5TeCB was greater than that of PeCB, explaining the unexpected trend in QE. 0.8

Interaction of guest molecules with the framework leads to quenching via electron transfer from the conduction band of the framework to the lowest unoccupied molecular orbital (LUMO) of a guest molecule.8 Moreover, spectral overlap between the absorption band of an analyte and the emission band of the host framework plays a role in fluorescence quenching.7 Thus, to probe the quenching mechanism in 1, we calculated the energies of the highest occupied molecular orbitals (HOMOs) and LUMOs of the series of chlorinated benzenes and the H4L ligand using density functional theory (DFT) (Figure S28). We conjectured that the HOMOs and LUMOs of the framework and the ligand were similar, based on the nearly identical positions of the luminescence bands. From the DFT results, the energy of the LUMO of H4L is lower than that of the chlorinated benzenes, suggesting that an electron transfer process may be viable between the host framework and analyte molecules after co-absorption of irradiated

Absorbance / a.u.

200 0 250 300 350 400 450 Excitation wavelength / nm

600 (c) 400 200

CB DCB 1,2,3,4-TeCB PeCB

 = 270 nm

0 300 400 500 600 700 800 Wavelength / nm

600 (e) 400 200


 = 290 nm

0.6 0.4

PeCB



270

310 nm

0.2 0.0 260

400 200

280 300 320 Wavelength / nm

340


 = 280 nm

0 300 400 500 600 700 800 Wavelength / nm

600 (f)

0 300 400 500 600 700 800 Wavelength / nm

CB DCB 1,2,3,4-TeCB

(b)

600 (d)

Intensity / a.u.

The reusability of 1 in a TCB sensing application was examined. In the first cycle, we measured the emission spectrum of 1 dispersed in DMF without TCB present. After addition of TCB to the DMF suspension, the fluorescence spectra were collected. To restore 1 to its original state in preparation for subsequent measurements, 1 was filtered and washed three times with fresh DMF. The quenching cycles were repeated five times by maintaining the luminescence intensity (Figure S13). The PXRD data indicates that the structure of 1 remained intact after the cycling (Figure S14). To further understand the quenching behavior, we also measured the emission spectra of 1 in the presence of other chlorinated benzenes such as CB, DCB, 1,2,3,4TeCB, 1,2,4,5-TeCB, and PeCB. The spectra were then analyzed for quenching efficiency and relative intensity variations (Figures 2 and S15 – S27). The quenching behaviors of TeCB and PeCB were overall similar to that of TCB. While QE increased with an increase in concentration, the number of Cl atoms in the analyte also affected the QE.

400

Intensity / a.u.

Figure 2. Quenching efficiency of 1 with different chlorinated benzenes at concentrations of 5.2 and 9.0 mM.

Intensity / a. u.

600 (a)

Intensity / a.u.

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ACS Applied Materials & Interfaces

Intensity / a.u.

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400 200


 = 310 nm

0 300 400 500 600 700 800 Wavelength / nm

Figure 3. a) Emission intensity variation of 1 dispersed in DMF at 509 nm as a function of excitation wavelength. b) UV/Vis spectra of 1 with the corresponding chlorinated benzenes. c) – f) Emission spectra of 1 with the corresponding chlorinated benzenes at the indicated excitation wavelengths. The emission data for 1 suggests that the quenching mechanism is governed by the excitation wavelength. To analyze the emission behavior of 1 with respect to variations in ex, ex was changed from 260 to 470 nm, and the emission intensity was measured at 509 nm (Figure 3a). The intensity abruptly increased and then remained almost flat over the range of 280 – 410 nm, beyond which point it decreased sharply. This result suggests that the strong emission intensity of 1 is retained over a wide range of excitation wavelengths



(280 – 410 nm). In the UV-vis spectra of CB, DCB, 1,2,3,4-TeCB, and PeCB (Figure 3b), absorption bands were broad and separated from each other. For example, the tail of the absorption band for CB disappeared around 278 nm, while that of PeCB reached 320 nm. With ex = 270 nm, no significant emission was observed for any of the tested compounds (Figure 3c). Only CB showed luminescence when the same compounds were excited at ex = 280 nm (Figure 3d). Increasing ex to 290 nm induced emissions for CB and DCB (Figure 3e), while emission peaks were observed for all chlorinated benzenes except PeCB when ex increased further to 310 nm (Figure 3f). The absorption of radiation by the analytes accounts for the quenching mechanism. When ex was set to 280 nm, the absorption band of CB was outside the absorption range while those of the other analytes were within the region. The overlap in the absorption bands of the three chlorinated benzenes (DCB, 1,2,3,4-TeCB, and PeCB) facilitates absorption of the incident light by the analytes, which shields the excitation of 1 and is followed by fluorescence quenching.28 In the case of CB, excitation was not screened by its absorption and emission occurred after excitation of 1. Thus, CB was recognized by irradiating the sample at 270 nm (quenching) and 280 nm (emission). The discriminative recognition of the other chlorinated benzenes was also readily achieved by the self-absorption characteristics of the analytes at specific wavelengths (Figure 3 and Table S1). Further, the extinction coefficients of 1,2,3,4-TeCB and PeCB were much smaller than those of CB and DCB (Figure S33). While CB and DCB might effectively filter the excitation radiation due to their relatively higher extinction coefficients, the self-absorption of TeCB and PeCB could be weak. Therefore, in combination with the inner filter effect, electron transfer between 1 and the analytes upon co-absorption of incident radiation plays a key role in the overall fluorescence quenching mechanism (Figures S28 and S29). Previous methods for distinguishing target molecules have been associated with altered intensity or emission wavelength or pattern when excited with a single excitation wavelength.7, 18, 20 In comparison, 1 facilitates the discriminative sensing of specific molecules via the use of different excitation wavelengths, providing distinct signal readouts. Similar absorptionbased quenching behaviors were also observed in bromobenzenes, which is due to the overlap of the absorption bands of the bromobenzenes with the excitation wavelength (Figure S34). When 1 was applied in the detection of nitroaromatics, a quenching process occurred via both electron transfer from the LUMO of 1 to the HOMO of the electron-deficient nitroaromatic molecule and energy transfer, as judged by the spectral overlap (Figures S35 – 37). Several works on selective sensing for organic molecules with similar structures were reported. For example, the fluorescence intensities were unchanged upon organic molecules, while the fluorescence intensity varied when the target nitroaromatic analyte was added.29 Moreover, selective emission enhancement occurred in electron-rich organic amines due to electron transfer to the framework.30 Volatile organic compounds with similar structures were also successfully recognized on the basis of the intensity ratio of ligand-based emission to Eu3+ emission or wavelength variations.16, 20 In comparison, the discriminative method for sensing chlorinated benzenes with similar structures using 1 is ascribed to the emission change upon excitation wavelength. We carried out the selectivity test for the chlorinated benzenes by slightly modifying the reported method.16 We randomly selected one of the chlorinated benzenes and measured the photoluminescence at various excitation wavelengths. The emissions were quenched at the excitation wavelengths of 270 and 280 nm, while the fluorescence

signals appeared upon the excitations at 290 and 310 nm. With reference to Table S1, we could determine that the randomly selected sample was DCB (Figure S38 and Table S2) In summary, we have introduced a facile yet efficient sensing system for the differentiation of target molecules with small structural variations using a coordination polymer platform. Remarkably, the sensing platform exhibited flat and strong emission intensities over a wide range of excitation wavelengths. The unique emissive properties of this framework enabled the discriminative recognition of specific analytes via self-absorption and/or electron transfer quenching mechanism. The findings are remarkable due to the unique absorptionbased discriminative recognition of target molecules by different irradiation wavelengths, and the technique is distinct from previously reported strategies for the detection of specific molecules which are related to excitation by a single wavelength.

ASSOCIATED CONTENT Supporting Information. Detailed preparations and additional experimental data (PDF). These materials are available free of charge via the Internet at http://pubs.acs.org.

AUTHOR INFORMATION Corresponding Author *E-mail: [email protected]

Author Contributions ‡These

authors contributed equally.

ACKNOWLEDGMENT This work was supported by the Korea CCS R&D Center (KCRC) grant funded by the Korean government (the Ministry of Science, ICT, & Future Planning (MSIP)) (NRF-2014M1A8A1049253), the Basic Science Research Program (NRF-2018R1A2A1A05079297), and PAL.

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Discriminative Molecular Detection Based on Competitive Absorption by a Luminescent MetalOrganic Framework A Cd-based metal-organic framework exhibits almost constant emission intensities across a range of wide excitations, enabling discriminative detection of target molecules with similar structures via competitive absorption.


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Discriminative Molecular Detection Based on Competitive Absorption

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