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Self-Assembled Metal-Organic Frameworks Crystals for Chemical Vapor Sensing Chenlong Cui, Yayuan Liu, Hongbo Xu, Shaozhou Li, Weina Zhang, Ping Cui, and Fengwei Huo* Metal-organic frameworks (MOFs),[1,2] also known as porous coordination polymers (PCPs),[3] which are built through the coordination between metal ions/clusters and organic ligands, afford rigid structures, permanent high porosity and great chemical tunabilities.[4,5] Endowed with numerous superior characteristics, MOFs have been garnering considerable attentions in the field of gas storage/separation,[6–8] catalysis,[9–11] biomedicine,[12,13] sensing,[14–19] etc.[20–22] However, the study of MOFs as sensors is still in its infancy, in spite of their distinguished properties that render them promising candidates for sensing applications. As a result, research on MOFs as potential sensing materials can be of great importance and necessity. Thus far, a handful of MOF-based sensors have been constructed either by employing luminescent framework[16] or by taking the advantage of photonic MOFs structures.[17,18] Compared to luminescent MOFs, which rely on luminescent quenching for signal transduction, photonic MOFs structures require neither molecular level functionalization nor complicated signal detection, and thus representing a more attractive alternative in the field of sensing. Nevertheless, existing reports on photonic structures based sensors focused mainly on thin films,[17] hybrid and template structures,[18,23–26] which limits their generalization to MOFs-based sensors. On one hand, since the report of ZIF-8 thin film that works as a photonic sensor from Hupp’s group,[17] the studies in MOFs thin films have been rapidly developed.[27] However, most of the films in literature were prepared based on methods highly related to their own properties.[27] Thus, general strategies towards MOFs thin film preparation are still immature. On the other hand, the efficiency of fabricating template structures is not high due to the necessity of template fabrication and removal.[18] Moreover, in order to improve the detection performance, other materials are usually incorporated into the system, which brings complexity into fabrication.[24,28] From what introduced above, apparently a simple and general route towards fabricating MOFs sensors is still in great demand. Herein,
C. Cui, Y. Liu, H. Xu, S. Li, W. Zhang, P. Cui, F. Huo School of Materials Science and Engineering Nanyang Technological University 50 Nanyang Avenue, Singapore 639798, Singapore E-mail: [email protected] DOI: 10.1002/smll.201302983
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we present the fabrication of a photonic sensor obtained through the self-assembly of MOFs crystals via LangmuirBlodgett (LB) technique (Scheme 1).[29] The self-assembly of nanoparticles of various sizes and shapes, including that of semiconductor nanoparticles (NPs), organic microspheres, metal NPs and metal oxide NPs has been thoroughly investigated in recent years.[30,31] The studies on the applications of self-assembled structures, such as biosensor, catalysis and data storage, are getting more and more attractive.[31] MOFs crystals are good candidates for the self-assembled structures. However, to the best of our knowledge, only a few studies have been carried on the self-assembly of MOFs crystals and none of them had investigated the applications of self-assembled MOFs structures.[32–34] It is worthwhile noting that compared to other bottom-up strategies available, LB technique, which has always been playing a crucial role in directing the self-assembly of nanoparticles at air-liquid interface, is a fast and facile way to achieve the self-assembled MOFs structures. LB technique makes it possible to get assembled films consisting of MOFs crystals that cannot be directly prepared as thin films. Hence, LB technique was chosen for demonstrating the strategy of fabricating MOFs particle based photonic sensors. The basic synthetic procedure is illustrated in Scheme 1. The self-assembled MOFs 3D film, formed after several times of transfer process from water-air interface to solid platform, was used as a photonic sensor of chemical vapors. The color of MOFs crystals film would change upon exposing to different vapour environments, which could be further tested by UV-vis spectrum. Monodispersed UIO-66 particles were synthesized using ZrCl4 and 1,4-benzendicarboxylic acid (BDC) under the modulation of acetic acid at 120 °C according to previous work.[34] After washing with N,N-dimethylformamide (DMF) and then methanol, the surfaces of UIO-66 particles were modified with polyvinylpyrrolidone (PVP, Mw∼55000). The modified UIO-66 particles were further washed with water to remove the excess PVP and finally redispersed in mixed water and ethanol solution (volume ratio 1:1). The fabrication of LB film was conducted in a more accessible way to common labs since it requires no specific equipments. Briefly, approximately 50 µl concentrated UIO-66 crystals solution was dropped on a glass slide so that a loose monolayer would form spontaneously on the water surface. Then ∼2 wt% sodium dodecyl sulfate (SDS) was added onto the water surface to afford a condensed film.
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though the particle arrangement was not well controlled in long range, which could probably be attributed to the fast consolidation process. Nevertheless, as shown in Figure 1b, the thickness of each layer was determined solely by the particle size since monodispersed particles were used. Such method guarantees a controllable layer thickness, making it more convenient over fabrication.[35] The orientations of UIO-66 crystals film were confirmed by X-ray diffraction (XRD) (Figure 1c). In another aspect, the robust thermal stability (Figure S1) as well as the high porosity (Figure 1d) of UIO-66 particles renders them a promising candidate for Scheme 1. Schematic illustration the fabricating self-assembled MOFs nanoparticles for chemical vapour sensing. chemical vapor sensing. After multiple transfer processes, the self-assembly of MOFs structure with certain color could be formed on glass The condensed film could subsequently be transferred to dif- slides. Figure 2a shows that the color of the UIO-66 crysferent substrates such as silica, glass, etc through the simple tals film varied when viewed from different angles. Since dip coating process. The process can be repeated facilely to UIO-66 is a white powder, the exhibition of colors shall be obtain multi-layered MOFs crystal LB film. Figure 1a shows attributed to the ordered photonic structure. The photonic that most of the octahedral MOFs particles were situated properties of UIO-66 crystals films were further characwith their triangular facets up even after 5 repeated transfer, terized using UV-vis spectrum. Four samples consisted of
Figure 1. a) SEM images of monodispersed UIO-66 particles transferred on glass substrate b) and its cross section. c) XRD patterns of simulated, powder and LB film of UIO-66. d) Nitrogen adsorption-desorption isotherm for monodispersed UiO-66 octahedral particles at 77 K up to 1 bar. small 2014, 10, No. 18, 3672–3676
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Figure 2. a) Photos for 5-layer UIO-66 LB film on glass substrate viewed from different angle. b) Absorbance UV-vis spectra of 1-layer, 2-layer, 3-layer, 5-layer UIO-66 LB film transferred on glass. c) UIO-66 LB film consisting large particles shown a red shift in Bragg peak. d) Reflectance UV-vis spectra of UIO-66 LB film on silicon.
UIO-66 particles with same size but different numbers of layers were tested and as can be seen from Figure 2b, the Bragg diffraction peaks became stronger and narrower with the increasing layer number. The result was in accordance with previous studies on LB SiO2 structures.[36–39] One of the advantages of such property is that by simply repeating the transfer process, we will be able to get a very sharp peak for more precise detection. Another interesting phenomenon of the UIO-66 crystals film was the relationship between particle size and peak position: comparing with a film made from smaller particles, film built up by larger particles showed a red-shifted peak when other parameters were fixed (Figure 2c). Importantly, if peak position could be easily modulated, overlap between the structure peak and the molecules’ own absorbance peaks can be avoided by choosing MOFs particles of proper size. UIO-66 crystals film on silicon wafer was also fabricated and the reflective spectra are shown in Figure 2d. However, the multiple peaks of UIO-66 crystals film on silicon makes it less suitable for sensing since it is more difficult to judge the peak shift after chemical exposure. Hence, transparent glass is a better platform for UIO-66 crystals films. As a proof-of-concept, a 5-layer UIO-66 crystals LB film on glass substrate was chosen to demonstrate its sensing capability towards various chemical vapors. Here, several commonly used solvents such as water, methanol, DMF were
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used as analytes. The UV-vis spectra of UIO-66 crystals film after analytes exposure were shown in Figure 3a. Obvious peak shift of UIO-66 crystals film can be easily observed upon exposing to toluene, DMF, chloroform, ethanol, methanol and acetone but not water under ambient conditions. The peak shift was due to the absorption of guest molecules inside UIO-66 crystals film, which changed the effective reflective index (ne) of whole film when other parameters were given. Therefore, the more guest molecules absorbed, the greater the increase in the ne of the film and hence, the larger the red shift. Correspondingly, the low affinity between UIO-66 and water resulted in the minimal change observed. With a significant red shift as large as nearly 30 nm after exposure to DMF and ethanol, it is possible to “read” the signal directly by naked eyes, indicating the promising potential of self-assembled MOFs film. In addition, the interactions between guest molecules and MOFs sensor are designable given the great flexibility of MOFs.[40–42] For instance, a narrow pore aperture will block the guest molecules from entering the MOFs pores, which will result in smaller peak shift. Moreover, it is obvious that MOFs containing carboxylic acid as ligands are not stable when dispersed in alkaline solution and hence, the sensing of alkaline materials with MOFs can hardly be operated in liquid phase. However, by exposing to vapor phase, the self-assembled UIO-66 crystals films was less restricted by the properties of the analytes.
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Figure 3. a) Absorbance spectra of glass supported 5-layer UIO-66 crystals film upon exposing to various chemical vapors b) and corresponding peak shifts.
For example, when the UIO-66 crystals film was used for testing the aqueous ammonia vapor (Figure S3), absorbance peak shifted 16 nm compared to the control group. Though UIO-66 particles can decompose quickly in aqueous ammonia, the film exhibited good recoverability up to 5 cycles (Figure S4) when exposed to 1% aqueous ammonia vapor. The dynamic response of UIO-66 self-assembled structure was also measured under the vapor of 1% aqueous ammonia (Figure S5). Thanks to the hierarchical structures, the response was fast and stabilized after 200 s. In summary, we reported a general route for the fabrication of self-assembled photonic MOFs sensors via Langmuir-Blodgett technique. UIO-66 nanoparticles were chosen as the building blocks of the MOFs sensor as a proof-of-concept. Circumventing the complex signal transduction of luminescent MOFs sensors, the photonic UIO-66 sensor serves as a potent candidate for “readable” sensors by exhibiting significant and selective absorbance peak shift toward several chemical vapors. Furthermore, the system possesses great flexibility since both its absorbance peak position and intensity can be fine tuned. Considering the tailorability of MOFs, we believe that more properties could be imparted into photonic MOFs sensors, especially the size selectivity of guest molecules derived from the uniform pore structure in MOFs. Different from MOFs thin film and reverse opal structures, our strategy is not only general but also facile to be operated since MOFs particles are easy to be synthesized and the self-assembled films can be obtained without specific equipment. Thus, the study opens a promising way to combine self-assembly methods with MOFs to create distinguished functional structures for wide range applications.
Experimental Section Materials: All the chemicals are commercial available, purchased from Sigma/Aldrich and used as received. Synthesis of PVP-functionalized monodispersed UIO-66: Firstly 0.0233 g ZrCl4 and 0.0166 g BDC were dissolved separately in 5 ml DMF. Then the two solutions were fully mixed in a 30 ml glass vial (fisher) and 1.5 ml acetic acid was added into the mixture. After sonication for 10 min, the mixture was transferred to an oven preheated at 120 °C. Product was collected after 24 hours by small 2014, 10, No. 18, 3672–3676
centrifugation, washed with DMF for 3 times and soaked overnight in methanol at 60 °C for 3 times. At last, the product was washed in methanol for another 3 times. A point shall be highlighted here is that the size of the UIO-66 crystals varied slightly from batch to batch. To functionalize the surface of UIO-66 particles with PVP, the as-synthesized particles were dissolved in 10 ml 2 wt% PVP (Mw∼55000) aqueous solution and then settled on Vortex agitator overnight. Finally, the PVP-functionalized particles were collected by centrifugation, washed with DI water 3 times to remove excess PVP and redispersed in 1 ml water: ethanol = 1:1 solution. Self-assembly of MOF crystals via Langmuir-Blodgett technique: Glass slides and silicon wafers were treated with Piranha solution (H2SO4/H2O2, 70:30(v/v)) at 70 °C for 50 min and rinsed with DI water several times prior to drying under nitrogen flow. The 2D selfassembly of MOFs crystals was carried out on water-air interface according to a previously reported procedure with minor modification.[43] Briefly, an 8 mm*35 mm glass slide was leaned against the rim of a Petri dish (14 cm in diameter and 1.5 cm in depth) and the Petri dish was then carefully filled up with DI water. Subsequently, 100 ul UIO-66 suspension (10 mg/ml) was dropped on the glass slide which spread quickly on water surface. After 5 min, 2–3 drops of 2 wt% aqueous solution of sodium dodecyl sulfate (SDS) was added to consolidate the UIO-66 film so that a close-packed monolayer was formed. Afterwards, the monolayer was transferred onto various substrates, such as glass or silicon. Finally, the transferred film was dried under ambient conditions. Characterization: X-ray diffraction (XRD) patterns were obtained using Bruker AXS D8 Advance diffractometer (CuKα radiation, λ = 1.5406 Å). Scanning electron microscope (SEM) images were taken using a JEOL JSM-7600 field-emission SEM. Nitrogen adsorption isotherms of UIO-66 samples were measured with a Micromeritics ASAP 2020 adsorption apparatus at 77 K up to 1 bar. Thermogravimetric analyses of UIO-66 powder were performed on a Q500 TGA (TA Instruments). UV-vis spectra were recorded on Shimadzu UV-2501PC. Performance of UIO-66 sensor detected by UV-vis spectra: A 5-layer UIO-66 film on glass slide was cut into small pieces 7 mm*15 mm in size and purged thoroughly with N2 to remove absorbed molecules. A small piece of cotton was placed at the bottom of a quartz cuvette and the as-purged film was then placed on top of it. To collect spectra of UIO-66 film exposing to chemical vapors, 10 ul analyte was dropped on the cotton and the quartz cuvette was quickly sealed. UIO-66 film was recovered under N2 flow prior to each test to obtain corresponding spectra.
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Supporting Information Supporting Information is available from the Wiley Online Library or from the author.
Acknowledgements F.H. acknowledges financial support from Nanyang Technological University (start-up grant), the AcRF Tier 1 and AcRF Tier 2 from the Ministry of Education, Singapore, and the Singapore National Research Foundation under the Campus for Research Excellence and Technological Enterprise (CREATE) programme Nanomaterials for Energy and Water Management.
[1] O. M. Yaghi, M. O’Keeffe, N. W. Ockwig, H. K. Chae, M. Eddaoudi, J. Kim, Nature. 2003, 423, 705. [2] G. Ferey, Chem. Soc. Rev. 2008, 37, 191. [3] S. Horike, S. Shimomura, S. Kitagawa, Nat. Chem. 2009, 1, 695. [4] N. Stock, S. Biswas, Chem. Rev. 2012, 112, 933. [5] M. O’Keeffe, O. M. Yaghi, Chem. Rev. 2012, 112, 675. [6] Y. Y. Liu, Z. Y. U. Wang, H. C. Zhou, Greenhouse Gases 2012, 2, 239. [7] K. Sumida, D. L. Rogow, J. A. Mason, T. M. McDonald, E. D. Bloch, Z. R. Herm, T. H. Bae, J. R. Long, Chem. Rev. 2012, 112, 724. [8] D. Tanaka, A. Henke, K. Albrecht, M. Moeller, K. Nakagawa, S. Kitagawa, J. Groll, Nat. Chem. 2010, 2, 410. [9] L. Q. Ma, C. Abney, W. B. Lin, Chem. Soc. Rev. 2009, 38, 1248. [10] M. Yoon, R. Srirambalaji, K. Kim, Chem. Rev. 2012, 112, 1196. [11] H. L. Jiang, T. Akita, T. Ishida, M. Haruta, Q. Xu, J. Am. Chem. Soc. 2011, 133, 1304. [12] P. Horcajada, R. Gref, T. Baati, P. K. Allan, G. Maurin, P. Couvreur, G. Ferey, R. E. Morris, C. Serre, Chem. Rev. 2012, 112, 1232. [13] P. Horcajada, T. Chalati, C. Serre, B. Gillet, C. Sebrie, T. Baati, J. F. Eubank, D. Heurtaux, P. Clayette, C. Kreuz, J. S. Chang, Y. K. Hwang, V. Marsaud, P. N. Bories, L. Cynober, S. Gil, G. Ferey, P. Couvreur, R. Gref, Nat. Mater. 2010, 9, 172. [14] Y. J. Cui, Y. F. Yue, G. D. Qian, B. L. Chen, Chem. Rev. 2012, 112, 1126. [15] L. E. Kreno, K. Leong, O. K. Farha, M. Allendorf, R. P. Van Duyne, J. T. Hupp, Chem. Rev. 2012, 112, 1105. [16] H. L. Jiang, Y. Tatsu, Z. H. Lu, Q. Xu, J. Am. Chem. Soc. 2010, 132, 5586. [17] G. Lu, J. T. Hupp, J. Am. Chem. Soc. 2010, 132, 7832. [18] Y. N. Wu, F. T. Li, W. Zhu, J. C. Cui, C. A. Tao, C. X. Lin, P. M. Hannam, G. T. Li, Angew. Chem. 2011, 153, 12726; Angew. Chem. Int. Ed. 2011, 50, 12518.
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[19] B. L. Chen, S. C. Xiang, G. D. Qian, Acc. Chem. Res. 2010, 43, 1115. [20] R. Demir-Cakan, M. Morcrette, F. Nouar, C. Davoisne, T. Devic, D. Gonbeau, R. Dominko, C. Serre, G. Ferey, J. M. Tarascon, J. Am. Chem. Soc. 2011, 133, 16154. [21] W. Zhang, R. G. Xiong, Chem. Rev. 2012, 112, 1163. [22] L. Zhang, H. B. Wu, S. Madhavi, H. H. Hng, X. W. Lou, J. Am. Chem. Soc. 2012, 134, 17388. [23] G. Lu, O. K. Farha, L. E. Kreno, P. M. Schoenecker, K. S. Walton, R. P. Van Duyne, J. T. Hupp, Adv. Mater. 2011, 23, 4449. [24] G. Lu, O. K. Farha, W. N. Zhang, F. W. Huo, J. T. Hupp, Adv. Mater. 2012, 24, 3970. [25] Y. N. Wu, F. T. Li, Y. X. Xu, W. Zhu, C. A. Tao, J. C. Cui, G. T. Li, Chem. Commun. 2011, 47, 10094. [26] R. Li, Y. P. Yuan, L. G. Qiu, W. Zhang, J. F. Zhu, Small. 2012, 8, 225. [27] A. Betard, R. A. Fischer, Chem. Rev. 2012, 112, 1055. [28] F. M. Hinterholzinger, A. Ranft, J. M. Feckl, B. Ruhle, T. Bein, B. V. Lotsch, J. Mater. Chem. 2012, 22, 10356. [29] M. Tsotsalas, A. Umemura, F. Kim, Y. Sakata, J. Reboul, S. Kitagawa, S. Furukawa, J. Mater. Chem. 2012, 22, 10159. [30] M. R. Jones, K. D. Osberg, R. J. Macfarlane, M. R. Langille, C. A. Mirkin, Chem. Rev. 2011, 111, 3736. [31] Z. H. Nie, A. Petukhova, E. Kumacheva, Nat. Nanotechnol. 2010, 5, 15. [32] N. Yanai, M. Sindoro, J. Yan, S. Granick, J. Am. Chem. Soc. 2013, 135, 34. [33] N. Yanai, S. Granick, Angew. Chem. 2012, 124, 5736. Angew. Chem. Int. Ed. 2012, 51, 5638. [34] G. Lu, C. L. Cui, W. N. Zhang, Y. Y. Liu, F. W. Huo, Chem-Asian. J. 2013, 8, 69. [35] L. D. Bonifacio, B. V. Lotsch, D. P. Puzzo, F. Scotognella, G. A. Ozin, Adv. Mater. 2009, 21, 1641. [36] P. Masse, S. Ravaine, Colloid. Surface. A. 2005, 270, 148. [37] S. G. Romanov, M. Bardosova, M. Pemble, C. M. S. Torres, Appl. Phys. Lett. 2006, 89, 043105. [38] S. G. Romanov, M. Bardosova, D. E. Whitehead, I. M. Povey, M. Pemble, C. M. S. Torres, Appl. Phys. Lett. 2007, 90, 133101. [39] M. Bardosova, M. E. Pemble, I. M. Povey, R. H. Tredgold, Adv. Mater. 2010, 22, 3104. [40] G. Lu, S. Z. Li, Z. Guo, O. K. Farha, B. G. Hauser, X. Y. Qi, Y. Wang, X. Wang, S. Y. Han, X. G. Liu, J. S. DuChene, H. Zhang, Q. C. Zhang, X. D. Chen, J. Ma, S. C. J. Loo, W. D. Wei, Y. H. Yang, J. T. Hupp, F. W. Huo, Nat. Chem. 2012, 4, 310. [41] M. Meilikhov, S. Furukawa, K. Hirai, R. A. Fischer, S. Kitagawa, Angew. Chem. 2013, 125, 359; Angew. Chem. Int. Ed. 2013, 52, 341. [42] D. Buso, J. Jasieniak, M. D. H. Lay, P. Schiavuta, P. Scopece, J. Laird, H. Amenitsch, A. J. Hill, P. Falcaro, Small. 2012, 8, 80. [43] C. Li, G. S. Hong, P. W. Wang, D. P. Yu, L. M. Qi, Chem. Mater. 2009, 21, 891.
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Received: September 14, 2013 Revised: January 13, 2014 Published online: March 3, 2014
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Self-Assembled Metal-Organic Frameworks Crystals for Chemical Vapor Sensing Chenlong Cui, Yayuan Liu, Hongbo Xu, Shaozhou Li, Weina Zhang, Ping Cui, and Fengwei Huo* Metal-organic frameworks (MOFs),[1,2] also known as porous coordination polymers (PCPs),[3] which are built through the coordination between metal ions/clusters and organic ligands, afford rigid structures, permanent high porosity and great chemical tunabilities.[4,5] Endowed with numerous superior characteristics, MOFs have been garnering considerable attentions in the field of gas storage/separation,[6–8] catalysis,[9–11] biomedicine,[12,13] sensing,[14–19] etc.[20–22] However, the study of MOFs as sensors is still in its infancy, in spite of their distinguished properties that render them promising candidates for sensing applications. As a result, research on MOFs as potential sensing materials can be of great importance and necessity. Thus far, a handful of MOF-based sensors have been constructed either by employing luminescent framework[16] or by taking the advantage of photonic MOFs structures.[17,18] Compared to luminescent MOFs, which rely on luminescent quenching for signal transduction, photonic MOFs structures require neither molecular level functionalization nor complicated signal detection, and thus representing a more attractive alternative in the field of sensing. Nevertheless, existing reports on photonic structures based sensors focused mainly on thin films,[17] hybrid and template structures,[18,23–26] which limits their generalization to MOFs-based sensors. On one hand, since the report of ZIF-8 thin film that works as a photonic sensor from Hupp’s group,[17] the studies in MOFs thin films have been rapidly developed.[27] However, most of the films in literature were prepared based on methods highly related to their own properties.[27] Thus, general strategies towards MOFs thin film preparation are still immature. On the other hand, the efficiency of fabricating template structures is not high due to the necessity of template fabrication and removal.[18] Moreover, in order to improve the detection performance, other materials are usually incorporated into the system, which brings complexity into fabrication.[24,28] From what introduced above, apparently a simple and general route towards fabricating MOFs sensors is still in great demand. Herein,
C. Cui, Y. Liu, H. Xu, S. Li, W. Zhang, P. Cui, F. Huo School of Materials Science and Engineering Nanyang Technological University 50 Nanyang Avenue, Singapore 639798, Singapore E-mail: [email protected] DOI: 10.1002/smll.201302983
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we present the fabrication of a photonic sensor obtained through the self-assembly of MOFs crystals via LangmuirBlodgett (LB) technique (Scheme 1).[29] The self-assembly of nanoparticles of various sizes and shapes, including that of semiconductor nanoparticles (NPs), organic microspheres, metal NPs and metal oxide NPs has been thoroughly investigated in recent years.[30,31] The studies on the applications of self-assembled structures, such as biosensor, catalysis and data storage, are getting more and more attractive.[31] MOFs crystals are good candidates for the self-assembled structures. However, to the best of our knowledge, only a few studies have been carried on the self-assembly of MOFs crystals and none of them had investigated the applications of self-assembled MOFs structures.[32–34] It is worthwhile noting that compared to other bottom-up strategies available, LB technique, which has always been playing a crucial role in directing the self-assembly of nanoparticles at air-liquid interface, is a fast and facile way to achieve the self-assembled MOFs structures. LB technique makes it possible to get assembled films consisting of MOFs crystals that cannot be directly prepared as thin films. Hence, LB technique was chosen for demonstrating the strategy of fabricating MOFs particle based photonic sensors. The basic synthetic procedure is illustrated in Scheme 1. The self-assembled MOFs 3D film, formed after several times of transfer process from water-air interface to solid platform, was used as a photonic sensor of chemical vapors. The color of MOFs crystals film would change upon exposing to different vapour environments, which could be further tested by UV-vis spectrum. Monodispersed UIO-66 particles were synthesized using ZrCl4 and 1,4-benzendicarboxylic acid (BDC) under the modulation of acetic acid at 120 °C according to previous work.[34] After washing with N,N-dimethylformamide (DMF) and then methanol, the surfaces of UIO-66 particles were modified with polyvinylpyrrolidone (PVP, Mw∼55000). The modified UIO-66 particles were further washed with water to remove the excess PVP and finally redispersed in mixed water and ethanol solution (volume ratio 1:1). The fabrication of LB film was conducted in a more accessible way to common labs since it requires no specific equipments. Briefly, approximately 50 µl concentrated UIO-66 crystals solution was dropped on a glass slide so that a loose monolayer would form spontaneously on the water surface. Then ∼2 wt% sodium dodecyl sulfate (SDS) was added onto the water surface to afford a condensed film.
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though the particle arrangement was not well controlled in long range, which could probably be attributed to the fast consolidation process. Nevertheless, as shown in Figure 1b, the thickness of each layer was determined solely by the particle size since monodispersed particles were used. Such method guarantees a controllable layer thickness, making it more convenient over fabrication.[35] The orientations of UIO-66 crystals film were confirmed by X-ray diffraction (XRD) (Figure 1c). In another aspect, the robust thermal stability (Figure S1) as well as the high porosity (Figure 1d) of UIO-66 particles renders them a promising candidate for Scheme 1. Schematic illustration the fabricating self-assembled MOFs nanoparticles for chemical vapour sensing. chemical vapor sensing. After multiple transfer processes, the self-assembly of MOFs structure with certain color could be formed on glass The condensed film could subsequently be transferred to dif- slides. Figure 2a shows that the color of the UIO-66 crysferent substrates such as silica, glass, etc through the simple tals film varied when viewed from different angles. Since dip coating process. The process can be repeated facilely to UIO-66 is a white powder, the exhibition of colors shall be obtain multi-layered MOFs crystal LB film. Figure 1a shows attributed to the ordered photonic structure. The photonic that most of the octahedral MOFs particles were situated properties of UIO-66 crystals films were further characwith their triangular facets up even after 5 repeated transfer, terized using UV-vis spectrum. Four samples consisted of
Figure 1. a) SEM images of monodispersed UIO-66 particles transferred on glass substrate b) and its cross section. c) XRD patterns of simulated, powder and LB film of UIO-66. d) Nitrogen adsorption-desorption isotherm for monodispersed UiO-66 octahedral particles at 77 K up to 1 bar. small 2014, 10, No. 18, 3672–3676
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Figure 2. a) Photos for 5-layer UIO-66 LB film on glass substrate viewed from different angle. b) Absorbance UV-vis spectra of 1-layer, 2-layer, 3-layer, 5-layer UIO-66 LB film transferred on glass. c) UIO-66 LB film consisting large particles shown a red shift in Bragg peak. d) Reflectance UV-vis spectra of UIO-66 LB film on silicon.
UIO-66 particles with same size but different numbers of layers were tested and as can be seen from Figure 2b, the Bragg diffraction peaks became stronger and narrower with the increasing layer number. The result was in accordance with previous studies on LB SiO2 structures.[36–39] One of the advantages of such property is that by simply repeating the transfer process, we will be able to get a very sharp peak for more precise detection. Another interesting phenomenon of the UIO-66 crystals film was the relationship between particle size and peak position: comparing with a film made from smaller particles, film built up by larger particles showed a red-shifted peak when other parameters were fixed (Figure 2c). Importantly, if peak position could be easily modulated, overlap between the structure peak and the molecules’ own absorbance peaks can be avoided by choosing MOFs particles of proper size. UIO-66 crystals film on silicon wafer was also fabricated and the reflective spectra are shown in Figure 2d. However, the multiple peaks of UIO-66 crystals film on silicon makes it less suitable for sensing since it is more difficult to judge the peak shift after chemical exposure. Hence, transparent glass is a better platform for UIO-66 crystals films. As a proof-of-concept, a 5-layer UIO-66 crystals LB film on glass substrate was chosen to demonstrate its sensing capability towards various chemical vapors. Here, several commonly used solvents such as water, methanol, DMF were
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used as analytes. The UV-vis spectra of UIO-66 crystals film after analytes exposure were shown in Figure 3a. Obvious peak shift of UIO-66 crystals film can be easily observed upon exposing to toluene, DMF, chloroform, ethanol, methanol and acetone but not water under ambient conditions. The peak shift was due to the absorption of guest molecules inside UIO-66 crystals film, which changed the effective reflective index (ne) of whole film when other parameters were given. Therefore, the more guest molecules absorbed, the greater the increase in the ne of the film and hence, the larger the red shift. Correspondingly, the low affinity between UIO-66 and water resulted in the minimal change observed. With a significant red shift as large as nearly 30 nm after exposure to DMF and ethanol, it is possible to “read” the signal directly by naked eyes, indicating the promising potential of self-assembled MOFs film. In addition, the interactions between guest molecules and MOFs sensor are designable given the great flexibility of MOFs.[40–42] For instance, a narrow pore aperture will block the guest molecules from entering the MOFs pores, which will result in smaller peak shift. Moreover, it is obvious that MOFs containing carboxylic acid as ligands are not stable when dispersed in alkaline solution and hence, the sensing of alkaline materials with MOFs can hardly be operated in liquid phase. However, by exposing to vapor phase, the self-assembled UIO-66 crystals films was less restricted by the properties of the analytes.
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Self-Assembled Metal-Organic Frameworks Crystals for Chemical Vapor Sensing
Figure 3. a) Absorbance spectra of glass supported 5-layer UIO-66 crystals film upon exposing to various chemical vapors b) and corresponding peak shifts.
For example, when the UIO-66 crystals film was used for testing the aqueous ammonia vapor (Figure S3), absorbance peak shifted 16 nm compared to the control group. Though UIO-66 particles can decompose quickly in aqueous ammonia, the film exhibited good recoverability up to 5 cycles (Figure S4) when exposed to 1% aqueous ammonia vapor. The dynamic response of UIO-66 self-assembled structure was also measured under the vapor of 1% aqueous ammonia (Figure S5). Thanks to the hierarchical structures, the response was fast and stabilized after 200 s. In summary, we reported a general route for the fabrication of self-assembled photonic MOFs sensors via Langmuir-Blodgett technique. UIO-66 nanoparticles were chosen as the building blocks of the MOFs sensor as a proof-of-concept. Circumventing the complex signal transduction of luminescent MOFs sensors, the photonic UIO-66 sensor serves as a potent candidate for “readable” sensors by exhibiting significant and selective absorbance peak shift toward several chemical vapors. Furthermore, the system possesses great flexibility since both its absorbance peak position and intensity can be fine tuned. Considering the tailorability of MOFs, we believe that more properties could be imparted into photonic MOFs sensors, especially the size selectivity of guest molecules derived from the uniform pore structure in MOFs. Different from MOFs thin film and reverse opal structures, our strategy is not only general but also facile to be operated since MOFs particles are easy to be synthesized and the self-assembled films can be obtained without specific equipment. Thus, the study opens a promising way to combine self-assembly methods with MOFs to create distinguished functional structures for wide range applications.
Experimental Section Materials: All the chemicals are commercial available, purchased from Sigma/Aldrich and used as received. Synthesis of PVP-functionalized monodispersed UIO-66: Firstly 0.0233 g ZrCl4 and 0.0166 g BDC were dissolved separately in 5 ml DMF. Then the two solutions were fully mixed in a 30 ml glass vial (fisher) and 1.5 ml acetic acid was added into the mixture. After sonication for 10 min, the mixture was transferred to an oven preheated at 120 °C. Product was collected after 24 hours by small 2014, 10, No. 18, 3672–3676
centrifugation, washed with DMF for 3 times and soaked overnight in methanol at 60 °C for 3 times. At last, the product was washed in methanol for another 3 times. A point shall be highlighted here is that the size of the UIO-66 crystals varied slightly from batch to batch. To functionalize the surface of UIO-66 particles with PVP, the as-synthesized particles were dissolved in 10 ml 2 wt% PVP (Mw∼55000) aqueous solution and then settled on Vortex agitator overnight. Finally, the PVP-functionalized particles were collected by centrifugation, washed with DI water 3 times to remove excess PVP and redispersed in 1 ml water: ethanol = 1:1 solution. Self-assembly of MOF crystals via Langmuir-Blodgett technique: Glass slides and silicon wafers were treated with Piranha solution (H2SO4/H2O2, 70:30(v/v)) at 70 °C for 50 min and rinsed with DI water several times prior to drying under nitrogen flow. The 2D selfassembly of MOFs crystals was carried out on water-air interface according to a previously reported procedure with minor modification.[43] Briefly, an 8 mm*35 mm glass slide was leaned against the rim of a Petri dish (14 cm in diameter and 1.5 cm in depth) and the Petri dish was then carefully filled up with DI water. Subsequently, 100 ul UIO-66 suspension (10 mg/ml) was dropped on the glass slide which spread quickly on water surface. After 5 min, 2–3 drops of 2 wt% aqueous solution of sodium dodecyl sulfate (SDS) was added to consolidate the UIO-66 film so that a close-packed monolayer was formed. Afterwards, the monolayer was transferred onto various substrates, such as glass or silicon. Finally, the transferred film was dried under ambient conditions. Characterization: X-ray diffraction (XRD) patterns were obtained using Bruker AXS D8 Advance diffractometer (CuKα radiation, λ = 1.5406 Å). Scanning electron microscope (SEM) images were taken using a JEOL JSM-7600 field-emission SEM. Nitrogen adsorption isotherms of UIO-66 samples were measured with a Micromeritics ASAP 2020 adsorption apparatus at 77 K up to 1 bar. Thermogravimetric analyses of UIO-66 powder were performed on a Q500 TGA (TA Instruments). UV-vis spectra were recorded on Shimadzu UV-2501PC. Performance of UIO-66 sensor detected by UV-vis spectra: A 5-layer UIO-66 film on glass slide was cut into small pieces 7 mm*15 mm in size and purged thoroughly with N2 to remove absorbed molecules. A small piece of cotton was placed at the bottom of a quartz cuvette and the as-purged film was then placed on top of it. To collect spectra of UIO-66 film exposing to chemical vapors, 10 ul analyte was dropped on the cotton and the quartz cuvette was quickly sealed. UIO-66 film was recovered under N2 flow prior to each test to obtain corresponding spectra.
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Supporting Information Supporting Information is available from the Wiley Online Library or from the author.
Acknowledgements F.H. acknowledges financial support from Nanyang Technological University (start-up grant), the AcRF Tier 1 and AcRF Tier 2 from the Ministry of Education, Singapore, and the Singapore National Research Foundation under the Campus for Research Excellence and Technological Enterprise (CREATE) programme Nanomaterials for Energy and Water Management.
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© 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Received: September 14, 2013 Revised: January 13, 2014 Published online: March 3, 2014
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