Copper(I) Metal–Organic Framework: Visual Sensor for Detecting

Communication pubs.acs.org/IC

Copper(I) Metal−Organic Framework: Visual Sensor for Detecting Small Polar Aliphatic Volatile Organic Compounds Yang Yu,†,‡ Jian-Ping Ma,† Chao-Wei Zhao, Jing Yang, Xiao-Meng Zhang, Qi-Kui Liu, and Yu-Bin Dong* College of Chemistry, Chemical Engineering and Materials Science, Collaborative Innovation Center of Functionalized Probes for Chemical Imaging in Universities of Shandong, Key Laboratory of Molecular and Nano Probes, Ministry of Education, Shandong Normal University, Jinan 250014, P. R. China S Supporting Information *

H2O⊂CuI-MOF (1) was recently reported by us.5 As shown in Scheme 1, compound 1 contains rhombuslike open channels

I

ABSTRACT: A porous Cu -MOF [H2O⊂Cu2(L)2I2; L = 1-benzimidazolyl-3,5-bis(4-pyridyl)benzene], which can be a visual and luminescent sensor for detecting small polar aliphatic volatile organic compounds (VOCs), such as alcohols, ketones, and halocarbons, is reported. The nakedeye and luminescent detection limitations for these VOCs are 5 and 1 ppm, respectively.

Scheme 1. Synthesis and Crystal Structure of 1

T

he development of a convenient, low-cost volatile organic compound (VOC) identification and detection system with a high sensitivity is in high demand for many applications, such as environmental monitoring, medical diagnosis, chemical workplace, and even homeland security.1 The current VOC detection methods, including gas chromatography (GC) and GC−mass spectrometry, are either expensive or nonportable. On the other hand, the selectivity and sensitivity of some portable sensors based on metal oxide, conductive polymers, and a quartz crystal microbalance are relatively low. Therefore, a real need for the development of high-performance portable VOC sensors remains. In this context, solvatochromism and vapochromism are promising phenomena. The solvatochromic and vapochromic species change color upon exposure to analyte liquid and vapor phases, and therefore detection of analytes can often occur even by the naked eye. Metal−organic frameworks (MOFs) are a rapidly growing class of materials with attractive various potential applications. Compared to gas storage, separation, and magnetic, luminescent, and catalytic properties, chemical sensing, especially visual colorimetric sensing based on MOFs, has not yet been extensively studied.2 The high thermal stability and speciestypical selectivity endow MOFs to be congenital chemical sensors and are able to respond to selective uploaded analyte molecules. Compared to visual molecular organometallic and coordination complex sensors,3 MOFs that exhibit a guestresponsive visual colorimetric property after the incorporation of certain guest species are extremely rare.4 Beyond a doubt, the most practical and convenient sensors are naked-eye colorimetric ones. In this contribution, we report on a porous CuI-MOF, which is able to upload various small polar VOCs, such as alcohols, ketones, and halocarbons, and, furthermore, acts as a sensitive naked-eye colorimetric sensor to perceive these VOCs under ambient conditions. © XXXX American Chemical Society

(dimensions ∼10 × 11 Å) running along the crystallographic c axis. The water (H2O) guests are located in the pores as a water cluster, and no obvious interactions between the water cluster and framework are found based on single-crystal analysis. This structural feature would allow external guest molecules to reversibly replace the water clusters inside without destroying the framework integrity.5 Indeed, the solvent-exchange experiment demonstrates that the encapsulated water molecules can be easily exchanged by some small aliphatic VOCs (C1−C5) under ambient conditions. The aliphatic VOCs tested in this study include alcohols [methanol (MeOH), ethanol (EtOH), 2-propanol, and npropanol), ketone (acetone, 2-butanone, 2-pentanone, and 3pentanone), and chlorinated hydrocarbons (CH2Cl2 and CHCl3). After immersion in the above organic solvents at ambient temperature, the crystals of 1 undergo a rapid naked-eye detectable color change. As shown in Figure 1, the color of 1 changes from dark brown (1) to deep yellow (2, MeOH⊂CuIL), yellow (3, EtOH⊂CuIL), deep yellow (4, n-propanol⊂CuIL),

Figure 1. Photographs showing the color change of the bulk samples of 1 in the liquid phase. Received: September 22, 2015

A

DOI: 10.1021/acs.inorgchem.5b02150 Inorg. Chem. XXXX, XXX, XXX−XXX

Communication

Inorganic Chemistry bright yellow (5, 2-propanol⊂CuIL), light yellow (6, acetone⊂CuIL), orange-red (7, 2-butanone⊂CuIL; 8, 2-pentanone⊂CuIL), orange (9, 3-pentanone⊂CuIL), light yellow, (10, CH 2 Cl 2 ⊂CuIL), and deep yellow (11 (CHCl 3 ⊂CuIL), respectively. In addition, the times of the color change response are different. It seems that the guest dimension is the dominating factor for the color response rate. As shown in Figure 1, the analytes with smaller size would facilitate guest exchange and, consequently, the color change. Logically, smaller analytes could enter the pores more easily, which would lead to a faster color response. On the other hand, this difference in color response demonstrates that the analytes were uploaded in the pores instead on the external surface adsorption. The adsorption of these aliphatic VOCs by 1 was further confirmed by the 1H NMR spectra. As shown in Figure 2, the 1H

Figure 3. Single-crystal diffraction showing that the MeOH, EtOH, npropanol, acetone, CH2Cl2, and CHCl3 guest species are located in 1 through host−guest hydrogen-bonding interactions.

guests, it is possible because of its inner polar environment caused by the embedded I ions and N atoms in the framework. Besides this, the water guests in 1 cannot be replaced by CCl4 (SI), which might be additional evidence for this guest selectivity. So, 1 could be considered an adsorbent to selectively adsorb polar aliphatic VOCs over nonpolar and weak polar aromatic VOCs. The UV−vis diffuse-reflectance spectra of aliphatic VOCs⊂CdL2 display a gradual broadening of the absorption band in the visible region (Figure 4), and they are well consistent

Figure 2. 1H NMR spectra recorded for the CDCl3 (for the uploaded alcohols and acetones) and DMSO (for the uploaded halocarbons) extracts. The corresponding VOCs are marked for clarity.

NMR spectra recorded for the CDCl3 (for the uploaded alcohols and acetones) and dimethyl sulfoxide (DMSO; for the uploaded halocarbons) extracts clearly evidence that the corresponding VOCs existed in 1. In addition, 1 retained its powder X-ray diffraction (PXRD) patterns (Supporting Information, SI) after the addition of analyte molecules, indicating the retention of crystallinity after the incorporation of guest molecules in the framework. The generation of no additional peaks in the PXRD patterns suggests that the VOCs are not bound to the CuI centers, and presumably they are located in the cavities and weakly interacting with the framework walls to generate the necessary analyte response. Thanks to the unusually stable nature of the MOF crystals, such a speculation can be well demonstrated by single-crystal Xray analysis (SI). As shown in Figure 3, all of the guest molecules are not coordinated to the metal ions but are fixed inside pores by weak hydrogen-bonding interactions. For alcohol-type guests, the interhost−guest hydrogen bonds (N−H···O) consist of a hydroxyl group on the alcohol and an imidazolyl moiety of the ligand, whereas the hydrogen bonds (C−H···X, where X = O, Cl) between the ketone or halocarbon type and framework are composed of guest heteroatoms and phenyl H atoms on the ligands. In addition, the CuIL framework of 1 is rigid. Compared to 1, the dCu−Cu contact in VOCs⊂CuIL varied in a range of 0.0218−0.0991 Å, and the cell volume varied in a range of 5−32.5 Å3 (SI). It is worth pointing out that 1 cannot upload volatile aromatic molecules such as benzene and its alkyl-substituted derivatives under the same conditions. Although we distance ourselves from any type of explanation and say forthright that we do not know why 1 only preferred the small polar aliphatic

Figure 4. UV−vis diffuse-reflectance spectra of 1−11.

with their colors. Together with the single-crystal X-ray structure, the color change herein could be attributed to an intermolecular electron-transfer transition between the host framework and encapsulated guests. The band-gap energies of these VOCincorporated host−guest complexes were calculated from UV− vis data, which show that the band-gap energies6 of VOCs⊂CuIL range from 2.24 to 2.32 eV (2, 2.31; 3, 2.32 eV; 4, 2.24; 5, 2.28; 6, 2.26; 7, 2.30; 8, 2.24; 9, 2.24; 10, 2.26; 11, 2.30 eV), which are larger than that of 1 (1.95 eV). However, they are all in the visible-light region (SI). Besides a naked-eye color change, VOCs encapsulation mentioned above also caused a dramatic change in their photoluminescence (PL) spectrum (Figure 5). Compared to H2O⊂CuIL (1), the emission intensities of the VOC-loaded samples from the liquid phase are much enhanced, which might be attributed to the structural rigidity enhancement imposed by the host−guest interactions. In addition, the emission of

Figure 5. PL spectra of H2O⊂CuIL (1) and alcohols⊂CuIL (2−5), ketones⊂CuIL (6−9), and halocarbons⊂CuIL (10 and 11). B

DOI: 10.1021/acs.inorgchem.5b02150 Inorg. Chem. XXXX, XXX, XXX−XXX

Communication

Inorganic Chemistry

change for CH2Cl2 was observed at 10 ppm. Such an emission difference was also reflected by their color change in the vapor phase (Figure 7) and was further supported by the corresponding 1 H NMR spectra (SI). In summary, we report a solvatochromic CuIL-MOF that can be a visual color and luminescence response sensor for small polar VOCs in the solid state. It might be one of the most sensitive solvatochromic MOF sensors. Work is in progress to obtain new naked-eye or luminescent colorimetric MOF sensors of this type in our laboratory.

VOCs⊂CuIL is slightly blue-shifted from that of 1. As shown in Figure 5, H2O⊂CuIL (1) exhibits a maxium emission at 601 nm upon excitation at 418 nm. alcohols⊂CuIL (2−5), ketones⊂CuIL (6−9), and halocarbons⊂CuIL (10 and 11) show emission maxima at 591−599 nm (2−5), 598−601 nm (6−9), and 581− 585 nm (10 and 11) (λex = 418 nm), respectively. With decreasing solvent polarity (H2O > VOCs), a blue shift of λem was observed, which correlates with the band-ap energies of the solvent⊂CuIL MOFs, showing that a CuIL MOF has a positive solvatochromic effect, which is different from some reported negative solvatochromic MOFs.4a,d As a useful sensor, it should respond to analytes at low concentrations, for example, in the vapor phase. Further study demonstrated that 1 showed a visual colorimetric response to these saturated VOC vapors at ambient temperature but took a longer time (Figure 6), which makes it a potential practical

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ASSOCIATED CONTENT

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.5b02150. Synthesis of 2−11, 1H NMR spectra, single-crystal measurement, and experiment for VOC detection (PDF) X-ray crystallographic data in CIF format for 2 (CIF) X-ray crystallographic data in CIF format for 3 (CIF) X-ray crystallographic data in CIF format for 4 (CIF) X-ray crystallographic data in CIF format for 6 (CIF) X-ray crystallographic data in CIF format for 10 (CIF) X-ray crystallographic data in CIF format for 11 (CIF)

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Figure 6. Photographs showing the color change of the bulk samples of 1 in the vapor phase.

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. sensor. The naked-eye detection limit for alcohols and ketones is ca. 5 ppm (SI); however, there is no visual detectable color change for halocarbons at this concentration. The sensing of these small VOC molecules based on the PL spectrum is much more sensitive than that of naked-eye observation, especially at low concentrated vapors. The corresponding detection limits are summarized in Table S3. The representative emission spectra for alcohols, ketones, and halocarbons are shown in Figure 7. For MeOH and acetone analytes, the caused emission intensity changes are obvious enough for sensing at 1 ppm, whereas a comparable intensity

Present Address ‡

Y.Y.: School of Chemistry and Chemical Engineering, Jiangsu Normal University, Xuzhou, Jiangsu 221116, P. R. China. Author Contributions †

These authors contributed equally.

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work is supported by the National Science Foundation of China (Grants 21475078, 21271120, and 21201112), 973 Program (Grants 2012CB821705 and 2013CB933800), and Taishan scholar’s construction project.

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REFERENCES

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Figure 7. PL spectra of representative analytes of MeOH, acetone, and CH2Cl2 vapors at 10, 5, and 1 ppm. The corresponding photographs at 6 h are inserted. C

DOI: 10.1021/acs.inorgchem.5b02150 Inorg. Chem. XXXX, XXX, XXX−XXX