Composite MOF mixture as volatile organic compound sensor – A new approach to LMOF sensors

Materials Letters 190 (2017) 33–36

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Composite MOF mixture as volatile organic compound sensor – A new approach to LMOF sensors Christopher J. Balzer, Mitchell R. Armstrong, Bohan Shan, Bin Mu ⇑ School for Engineering of Matter, Transport and Energy, Arizona State University, 501 East Tyler Mall, Tempe, AZ, USA

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Article history: Received 23 September 2016 Received in revised form 28 December 2016 Accepted 29 December 2016 Available online 30 December 2016 Keywords: Metal-organic frameworks Luminescence Fluorescent sensor

a b s t r a c t Five luminescent metal–organic frameworks (LMOFs) were synthesized solvothermally to investigate fluorescent enhancement and/or quenching effects in the solid state in the presence of a series of volatile organic compounds (VOCs). Each of the MOFs was characterized with powder X-ray diffraction directly after synthesis, activation, and with emission/excitation studies. Three of the LMOFs, LaBTB (BTB = 1,3,5-tris(4-carboxyphenyl)benzene acid), ZrPDA (PDA = 1,4-phenyldiacrylic acid), and UiO-66NH2, were mixed and exposed to a series of VOCs to show the feasibility of solid mixtures in creating fingerprint-like spectroscopic regions for detection and identification of VOCs. This new strategy increases signal discrimination and dilutes effects of p-stacking on quantum yields. Ó 2016 Elsevier B.V. All rights reserved.

1. Introduction The study of metal-organic frameworks (MOFs), a class of hybrid porous materials, has flourished in the past two decades due to their diverse physical and chemical properties. Built from metal-ion centers coordinated with organic ligands, their tunability allows for various applications, including gas separation and storage [1], catalysis [2], magnetism [3], biomedical imaging [4], chemical sensing [5], etc. MOFs have shown promise over other porous materials in chemical sensing due to their nearly limitless ligand-metal combinations, high surface area, and well-defined pore size. Host-guest interactions can alter the luminescence of a MOF with the most studied observed change being in the fluorescence intensity, whereby analytes either quench or enhance the luminescence. These ‘‘turn on/off” mechanisms for sensing are easily observable with spectroscopic equipment and visible to the eye in some cases. Volatile organic compounds (VOCs) pose serious threats in today’s world due to their toxicity and frequent use. MOFs have been shown to exhibit quenching and enhancement effects when treated with VOCs. For example, the luminescence of an Eu3+ based MOF showed an eight-fold increase in luminescence intensity in the presence of DMF – known for its hepatotoxicity [6]. MOF-5 has also been shown to have sensitive enhancement to ethanol at concentrations as low as 5 ppm [7]. To the best of our knowledge, there are no reports documenting the use of composite ⇑ Corresponding author. E-mail address: [email protected] (B. Mu). http://dx.doi.org/10.1016/j.matlet.2016.12.111 0167-577X/Ó 2016 Elsevier B.V. All rights reserved.

mixtures of MOFs as sensors. This type of sensor can circumvent two setbacks in current LMOF sensors: (1) signal discrimination, with multiple compounds possibly quenching or enhancing fluorescence and (2) lower quantum yields/self-quenching in the solid state due to p-stacking. By selecting MOFs based on their luminescence mechanism, or max emission/excitation locations, the combination of responses can filter fluorescence intensity effects of analyte size, polarity and functional group for detection; meaning, as more MOFs are added, it is not as crucial to have strong response signals, effectively covering up weak solid-state emission. Here, we report this new strategy for designing MOF based sensors by using three different luminescent MOFs with distinct spectral locations. To this end, five different MOFs were screened in the solid state for their intensity responses to VOCs, including three MOFs that haven’t been screened in previous literature – ZrPDA, LaBTB, and ZnBTC. 2. Materials and methods 2.1. Chemicals and characterization All reagents and solvents were commercially available and used without further modification. Powder XRD was recorded on a PANalytical X’Pert Pro using Cu-Ka radiation (k = 1.5406 Å) with Goniometer = PW3050/65 (h/2h). All luminescence spectrum were recorded by a Horiba Jobin Yvon model Nanolog equipped with a 450 W xenon lamp with a standard InGaAs multi-channel array detector. Luminescence data was compiled using Horiba FluorEssence software. As-synthesized, activated, and simulated

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PXRD and emission/excitation spectra for MOFs and ligands can be found in the Supplementary Information Figs. S1–S10. 2.2. Luminescence measurements All luminescence measurements were conducted in the solid state with fixed emission/excitation slits (1 nm, 1 nm). VOCs investigated include methanol, ethanol, 2-propanol, dimethylformamide (DMF), chloroform, dimethyl methylphosphate (DMMP), and acetone. To investigate the effect of various pure VOCs on fluorescence intensity, 50 lL of each VOC was dropped on 5 mg powder samples of partially activated MOF (SI). The solvent was allowed 30 min to diffuse into MOF pores before drying at room temperature. Trials were run in triplicate for each MOF/VOC combination.

3. Results and discussion 3.1. Fluorescence and VOC screening Fig. 1 shows the effects of VOCs on fluorescent intensity each MOF. Error bars represent ±1r. For LaBTB, the emission peak is centered at k = 364 nm when excited at k = 328 nm, while free 1, 3,5-tris(4-carbonxyphenyl)benzene acid (BTB) has peak emission centered at k = 392 when excited at k = 340 nm (Fig. S6). Both emission transitions are attributed to p⁄ ? p transitions. The blue shift can be attributed to decreased intermolecular interactions, leading to less self-quenching and less stabilization of the excited state when BTB is in coordination with lanthanum metal sites [8]. LaBTB showed partial quenching in the presence of acetone and DMF, while enhancement with DMMP. Acetone quenching can be attributed to wide absorbance bands that cause energy transfer between BTB and acetone molecules as reported in similar systems [9]. The role of DMF and DMMP in quenching and enhancement are likely polarity dependent.

TbBTC showed typical lanthanide-based luminescence as the emitting state is the triplet state. Free 1,3,5-benzenetricarboxylic (BTC) demonstrated peak emission at k = 370 nm when excited at k = 334 nm (p⁄ ? p). TbBTC shows distinct emission at k = 488, 544, 581, and 620 nm when excited at k = 295 nm as shown in Fig. S7. The most prominent peak (544 nm) is ascribed to the 5 D4 ? 7F5 transition. TbBTC showed quenching when exposed to acetone and enhancement with DMF. Previous reports explain this due to acetone’s wide absorption and DMF’s high polarity to facilitate more efficient charge/energy transfer from S1 ? T1 [9]. As shown in Fig. S7, ZnBTC displays strong emission centered at k = 419 nm when excited at k = 320 nm (p⁄ ? p). Exposure to chloroform resulted in slight fluorescent enhancement, while DMF resulted in quenching. Because DMF’s absorbance above 300 nm is negligible, the quenching must be due to a form of electronically unfavorable bond pathway, raising the HOMO. Free 2-aminoterephthalic acid (BDC-NH2) features wide emission centered at k = 556 nm when excited at 400 nm. UiO66-NH2 has a significant blue shift as maximum emission occurs at k = 455 nm when excited at k = 328 nm (Fig. S8). A large Stokes shift in BDC-NH2 indicates significant relaxation before fluorescing. When in coordination, the Stokes shift narrows and reflects the influence of the rigid, symmetric Zr-O clusters in inhibiting said energy transfers between ligand molecules. Known to be sensitive to phosphate ions [10], BDC-NH2 showed relatively weak interactions with the VOCs studied. Quenching due to acetone is not as strong as the other MOFs that had significant absorbance overlap with acetone. Mechanisms for enhancement due to 2-propanol were not studied further. Free 1,4-phenlydiacrylic acid has peak emission at k = 405 nm when excited at k = 386 nm, a p ? p⁄ transition. In coordination with zirconium, the maximum emission shifts to k = 432 nm when excited at k = 378 nm. However, a shoulder peak at k = 405 nm is maintained (Fig. S9). indicating ligand to metal charge transfer (LMCT) in the MOF. ZrPDA exhibits enhancement under exposure to alcohol compounds. Enhancement likely stems from hydrogen

Fig. 1. Ratiometric representation of fluorescence enhancement/quenching under exposure to listed VOCs. The MOFs tested include LaBTB (A), TbBTC (B), UiO-66-NH2 (C), ZnBTC (D), and ZrPDA (E). ‘‘Blank” refers to the standard fluorescent intensity of a partially activated MOF sample.

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Fig. 2. Intensity of composite MOF powder fluorescence with respect to excitation and emission wavelength. The three spectral regions behave according to screening results as acetone (B), 2-propanol (C), and DMF (D) are added in a stepwise fashion. Intensities are normalized to a standard value from the blank composite sample (A). (1) UiO-66NH2 kmax = 328, 455 nm (excitation, emission) (2) ZrPDA region kmax = 378, 432 nm (3) LaBTB region kmax = 328, 364 nm.

bonding with the ligand complex, adding electron density to the conduction band. 3.2. Three MOF mixture Equal masses of ZrPDA, LaBTB and UiO-66-NH2 were mixed into a composite powder to measure responses of the bulk powder to VOCs (5 mg total). These MOFs were chosen because of their shared luminescence properties: LaBTB shares a strong excitation peaks with UiO-66-NH2 at 328 nm and UiO-66-NH2 shares strong emitting regions in the range of 450 nm. Mixed together, these MOFs create three distinct spectroscopic regions in a contour plot of excitation and emission (Fig. 2). The peaks from LaBTB and ZrPDA are clearly visible in Fig. 2A. Initially, the region from UiO66-NH2 is hidden because LaBTB and ZrPDA have stronger emission per mass. 50 lL of acetone, 2-propanol, and DMF were added stepwise to the MOF mixture. The intensity in all three regions decreased as a result of acetone addition within the error ranges from the bar charts (Fig. 2B). Upon addition of 2-propanol, the peak from LaBTB decreases slightly, while UiO-66-NH2 and ZrPDA show fluorescent intensity enhancement (Fig. 2C). Finally, upon addition of DMF, the LaBTB region is suppressed, along with reduction in the intensity of the ZrPDA peak (Fig. 2D). These results closely resemble the responses from the VOC screenings done previously. All of the ratiometric responses for each addition of solvent fall within the error bars of the individual VOC screenings, indicating the MOFs act independently of one another. With independent action, any number of MOFs in a composite mixture could be used to har-

ness the sensitivities and selectivities of each individual MOFs with minimal mixture effects. We also see how weak quenching or enhancement marginally contributes less as the number of MOFs increases; as with LaBTB, the poor ratiometric luminescent responses (Fig. 1A) could not be used to identify a compound on its own, but was valuable as a spectroscopic region in the MOF mixture. With this approach, high selectivity and sensitivity are less important than picking MOFs with favorable emission/excitation bands. 4. Conclusions Even when MOFs show lackluster sensitivity and intensity changes in the solid state, their fluorescence in conjunction with other MOFs creates fingerprint-like spectra that can be used to sense and identify a particular analyte. With the large number of spectral regions from already studied luminescent MOF literature, a wide array of choices exists for a MOF mixture. Moreover, this shows the viability of using MOFs as solid state sensors, despite lower quantum yields as compared to luminescence of MOF in solutions – an important step toward industrial applications. Funding This research was financed by the start-up package provided to Dr. Bin Mu and Fulton Undergraduate Research Initiative (FURI) to Christopher J. Balzer by Arizona State University.

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Acknowledgments We gratefully acknowledge the use of the facilities within the Leroy Eyring Center for Solid State Science at Arizona State University. Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.matlet.2016.12. 111. References [1] J. Li, R. Kuppler, H. Zhou, Selective gas adsorption and separation in metal– organic frameworks, Chem. Soc. Rev. 38 (2009) 1477–1504, http://dx.doi.org/ 10.1039/b802426j. [2] J. Liu, L. Chen, H. Cui, J. Zhang, L. Zhang, C.-Y. Su, Applications of metal–organic frameworks in heterogeneous supramolecular catalysis, Chem. Soc. Rev. 43 (2014) 6011–6061, http://dx.doi.org/10.1039/c4cs00094c. [3] M. Kurmoo, Magnetic metal–organic frameworks, Chem. Soc. Rev. 38 (2009) 1353–1379, http://dx.doi.org/10.1039/b804757j.

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