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Photothermal-Enhanced Detoxification of Chemical Warfare Agent Simulants Using Bio-Inspired Core-Shell DopamineMelanin@Metal-Organic Frameworks and Their Fabrics Aonan Yao, Xiuling Jiao, Dairong Chen, and Cheng Li ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b19445 • Publication Date (Web): 28 Jan 2019 Downloaded from http://pubs.acs.org on January 29, 2019
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Photothermal-Enhanced Detoxification of Chemical Warfare Agent Simulants Using Bio-Inspired CoreShell Dopamine-Melanin@Metal-Organic Frameworks and Their Fabrics Aonan Yao, Xiuling Jiao, Dairong Chen, and Cheng Li* National Engineering Research Center for Colloidal Materials and School of Chemistry and Chemical Engineering, Shandong University, 250100 Jinan, China
KEYWORDS: self-detoxifying materials, metal-organic frameworks, photothermal catalysis, electrospun nanofibers, toxic organophosphates
ABSTRACT: Self-detoxifying materials capable of both capture and destruction of chemical warfare agents (CWAs) are highly desirable for efficient personal protection and safe handling of contaminated materials. Developing new strategies to improve CWA removal efficiency of these materials is highly relevant for CWA purification technology. Herein, we present novel photothermal-enhanced catalytic detoxification of CWA simulants and its application in selfdetoxifying gas filters. The material design features a well-defined core-shell nanostructure (CSN) consisting of inner photothermal material and outer microporous catalyst. As a
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demonstration, the CSN was obtained by growing a Zr-based metal-organic frameworks (MOFs), UiO-66-NH2, onto bio-inspired dopamine-melanin nanoparticles (Dpa) via heterogeneous nucleation induced by metal chelation. The resultant Dpa@UiO-66-NH2 CSN has increased the turnover frequency (TOF) of a nerve agent simulant 4-nitrophenyl phosphate (DMNP) by 2.9 and 1.7 fold in the presence of NIR laser and simulated solar light, respectively. Further incorporation of Dpa@UiO-66-NH2 CSN into polymer fibers by electrospinning has led to even greater photothermal enhancement effect (5.8- and 3.2-fold TOF increase), achieving faster DMNP degradation rate than the corresponding pure MOF powder for the first time and the shortest half-life of DMNP (1.8 min) among reported MOF-based self-detoxifying fabrics. The significant photothermal enhancement in the detoxification ability of Dpa@UiO-66-NH2 fabrics is attributed to the instantaneous heat transfer from the photothermal core to the catalytic shell and effective heat retention enabled by the surrounding polymer matrix. The Dpa@UiO-66-NH2 fabrics can be easily prepared on a large scale and demonstrate efficient protection against DMNP aerosols as stand-alone gas filters. This strategy of photothermal-enhanced catalytic detoxification can be feasibly extended to other catalytic detoxification systems and holds promise for next-generation gas masks.
1. INTRODUCTION Highly toxic chemical warfare agents (CWAs) are still stockpiled in thousands of tons and being released accidentally or intentionally towards military and civilian personnel, which constitutes a continuing grave threat to human beings and the environment.1 The need to find and develop environmentally friendly CWA purification technologies without secondary toxicity for immediate personal protection and chemical weapon destruction remains urgent.2
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Currently used technology employs activated carbon or its impregnated form as a broadspectrum filter to eliminate these deadly chemicals.3 However, due to their limited interaction with adsorbates and low removal efficiencies, they are often made bulky and heavy to ensure proper protection and have serious disposal issues. In comparison, self-detoxifying materials capable of both capture and destruction of CWAs are more desirable for efficient personal protection and safe handling of contaminated materials.4 Among others, metal-organic frameworks (MOFs), which are endowed with high porosity and crystallinity by combining metal centers and organic linkers, have recently become one of the most promising materials that can act as both effective adsorbents and heterogeneous catalysts for CWA removal owing to their ultrahigh surface area, rich chemical functionality, and abundant catalytic active sites.5-7 For example, several bulk Zr-based MOF crystals such as UiO-66-NH2, MOF-808 and NU-1000, have achieved excellent effects on the degradation of organophosphate with a minimum half-life of 0.5 min.8-13 Even so, CWA purification technology based on self-detoxifying materials is still in its infancy, and new strategies needs to be developed to improve CWA removal efficiency in these systems. An alternative approach to promote catalytic reactions is to use catalysts with photothermal effect (PTE).14-17 By converting photon to heat, they can act as a heater at the catalyst/reactant interface, which promotes efficient collisions of molecules and the mass transfer rate, particularly for catalysts with micropore, thereby accelerating the rate of reaction.18,
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PTE-
assisted catalysis has the advantage of instantaneous temperature rise and local heating effect, and is a highly feasible and versatile strategy with low requirements on the surface properties of the catalyst. Since the detoxification rate of CWA (usually expressed in terms of half-life) is the paramount performance parameter after their adsorption, the introduction of PTE into catalytic
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MOFs is well suited to accelerate CWA detoxification. Based on these considerations, we propose herein a novel strategy to improve the catalytic detoxification of CWAs by using PTEassisted catalysis. The catalyst design features a well-defined core-shell nanostructure (CSN) consisting of inner photothermal core and outer MOF shell. This structure provides a high contact interface between the catalyst and the reactants as well as direct heat transfer from the nanoheater to the reactive layer, which can efficiently promote the catalytic reaction. The practical application of MOFs in CWA removal requires not only rapid detoxification, but also appropriate forms to simplify processing and deployment in emergency situations. However, MOFs are typically present in the form of isolated crystalline powders, making the fabrication of their gas filters quite challenging. Recently, many studies have focused on immobilizing MOF crystals on textile or polymer fibers for the removal of CWAs.20-28 Nonetheless, pore blockage, poor distribution of particles, and shielding of the inert matrix has often led to reduced the removal abilities. Increasing the mass loading of MOFs has only resulted in limited improvements.23, 26, 27Adjusting the attachment of MOF crystals to the support can lead to greater improvement but requires complex preparation processes, which typically involve special equipment.21, 25 So far, the fabrication of MOF fabrics with better catalytic properties than MOF powders in a scalable route has remained a significant challenge. In view of the above considerations and the need for advanced CWA purification technology, this study aims to achieve photothermal-enhanced catalytic detoxification of CWA simulants and its application in self-detoxifying gas filters. To attain these goals, UiO-66-NH2 was grown onto bio-inspired dopamine-melanin nanoparticles (Dpa) to form Dpa@UiO-66-NH2 CSNs. Here Dpa was chosen as the inner core because it has recently been reported with excellent PTE for photothermal therapy,29 and can easily induce heterogeneous nucleation of MOFs via metal
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chelation due to its richness in amine, imine, and catechol groups.30-32 The degradation rate of a nerve agent simulant 4-nitrophenyl phosphate (DMNP) has been significantly improved using Dpa@UiO-66-NH2 CSNs when irradiated by NIR laser or simulated solar light. To obtain selfdetoxifying gas filters with novel photothermal-enhanced catalytic properties, Dpa@UiO-66NH2 CSNs have been further incorporated into polymeric fibers by electrospinning. The resultant fabric with 60 wt% Dpa@UiO-66-NH2 CSN has exhibited faster degradation rate of DMNP than pure UiO-66-NH2 powder under the irradiation of NIR laser. The structure-function correlation was investigated by comparing Dpa@UiO-66-NH2 CSN with pure UiO-66-NH2 and the physical mixture of Dpa and UiO-66-NH2 in terms of catalytic efficiency in both powder and fabric form. It has been revealed that Dpa@UiO-66-NH2 CSN is most efficient for photothermal-enhancing the catalytic reaction due to instantaneous heat transfer from the nanoheater to the catalytic layer and high contact interface between the catalyst and the reactants. The fabric containing Dpa@UiO-66-NH2 can be conveniently prepared on a large scale, and its direct use as standalone gas filters for efficient protection against DMNP aerosols was also demonstrated. 2. EXPERIMENTAL SECTION 2.1. Chemicals and Materials. 2-aminoterephthalic acid (H2BDC-NH2, 99%) and dopamine hydrochloride (99%) were purchased from Alfa Aesar. Zirconium (IV) chloride (ZrCl4, 98%) was purchased from Acros Organics. Polyacrylonitrile (PAN, Mw=150 000, 99%) was purchased from Tianjin Bodi Chemical Co., Ltd. N-ethylmorpholine (99%), N,Ndimethylformamide (DMF, 99.5%), dimethyl 4-nitrophenyl phosphate (DMNP, 99.8%), methanol (99.5%) and aqueous ammonium hydroxide (NH3·H2O, 25-28%) were purchased from Shanghai Macklin Biochemical Co., Ltd. All raw materials were used without further purification.
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2.2. Preparation of Dpa@UiO-66-NH2 CSN. First, Dpa were synthesized using the synthetic route described in the previous literature.29 Specifically, NH3·H2O (2 mL) was mixed with water (90 mL) and ethanol (40 mL) by mild stirring at 30 °C for 0.5 h. An aqueous solution (10 mL) with dopamine hydrochloride (0.5 g) is then poured into the above mixture. After stirring at 30 °C for 24 h, the product was gathered by centrifugation and washed thoroughly with water. To prepare Dpa@UiO-66-NH2 CSN, Dpa (7 mg) were dispersed in 50mL of DMF containing 1.3 mmol of ZrCl4, and stirred at room temperature for 5 h to adsorb Zr ions on the surface of Dpa. Then 1.3 mmol of H2BDC-NH2 was added and stirred for another 1 h. After that, the mixture was poured into a 100 mL autoclave, sealed, and incubated at 140 °C for 24 h. After the reaction was completed and cooled to room temperature, the product was collected by centrifugation and washed three times with DMF. Before catalytic reactions, the contained solvent molecules were removed by solvent exchange with methanol in a Soxhlet extractor for 12 h, and then vacuum-dried at 110°C for 24 h. 2.3. Electrospinning. Dpa@UiO-66-NH2 CSN (0.044, 0.17, or 0.60 g for different mass loading), UiO-66-NH2 (0.49 g), or the mixture of Dpa and MOF (0.11 g of Dpa and 0.49 g of UiO-66-NH2) were dispersed in 3.6 g of DMF under ultrasonication. Then 0.4 g of PAN was dissolved in this dispersion and stirred for 12 h. The above solution was charged into a 10 mL syringe with a metal needle (inner diameter = 0.6 mm) and placed on a programmable electrospinning machine. The pushing rate was set at 1.0 mL h-1 and the electric field voltage was set at 16 kV. The fibers were collected on an aluminum foil at a distance of 18 cm from the needle under ~10% relative humidity at 30 °C, and then dried at 40 °C for 24 h before peeled off. 2.4. Characterization. The morphology and microstructure of the samples was characterized by field emission scanning electron microscope (Hitachi-SU8010) and transmission electron
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microscope (JEOL JEM-1011). HAADF-STEM elemental EDS mapping and line scan data were carried out using Titan G2 60-300 with image corrector. The crystalline structure was analyzed using a Rigaku D/Max 2200 PC diffractometer (Cu Kα radiation at 40 kV, 20 mA and λ = 0.15418 nm). FTIR were collected with a Bruker Alpha spectroscopy meter. UV-visible diffuse reflectance spectra were measured on an integrating sphere-equipped UV-vis-IR spectrometer (Agilent Cary 500) using BaSO4 reference. Inductively coupled plasma-optical emission spectroscopy (ICP-OES) data were acquired on a Optima 5300DV. TGA measurements were performed on a Mettler Toledo TGA/SDTA 851E analyzer by heating the samples to 800 °C at a rate of 10 °C min-1 under air atmosphere. The N2 physisorption isotherms and specific surface area were acquired from a Quadrasorb SI instrument at liquid nitrogen temperature (77 K). UV−visible absorption spectra were obtained using a Hitachi U-4100 spectrophotometer. 2.5. Photothermal-Enhanced Catalytic Degradation of DMNP. 2.5.1. Batch Reactions. DMNP degradation using the MOF-based catalyst was conducted in a similar way to the previously described method,21 except that light irradiation from a NIR laser (808 nm, 2 W cm-2) or AM 1.5 simulated solar light (SSL, 0.6 W cm-2) was applied during the reaction. Specifically, a certain amount of powder sample or shredded fiber sample was dispersed in a Nethylmorpholine aqueous buffer (1 mL, 0.45 M), and the suspension was stirred at 1100 rpm for 30 min in a 7 mL quartz tube at room temperature. The quartz tube was then placed in a dark box with the light source, and 4 μL of DMNP was added into the suspension under vigorous stirring. At each time interval, a 20 μL aliquot was taken from the reaction and added in a Nethylmorpholine aqueous buffer (10 mL, 0.15 M). The absorbance at 407 nm corresponding to the degradation product of DMNP, p-nitrophenoxide was measured and used to monitor the reaction course according to the Lambert-Beer Law. The DMNP conversion percentage was
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obtained by dividing the concentration of p-nitrophenol by the initial DMNP concentration. The pseudo-first-order rate constant k was obtained by plotting the natural log of DMNP conversion percentage as a function of time. The half-life (t1/2, 50% conversion) was calculated by ln2/k. The regeneration of the catalysts was performed by washing them repeatedly with the buffer solution, soaking in ethanol at 50 °C for 12 h, and drying in an oven. 2.5.2. Aerosol Filtration Test A DMNP solution with a concentration of 0.023 mol L−1 was obtained by adding DMNP (4 μL) to 1 mL of N-ethylmorpholine aqueous solution (0.45 M). At a pressure drop of 8 kPa, the atomized DMNP solution was discharged from the commercial atomizer and passed through the fibrous filter sandwiched between a glass tube and a collecting bottle at a flow velocity of 0.11 mL min-1 cm-2. The diameter of the exposed filter is 1.5 cm. The filter is illuminated by NIR laser or SSL during filtration. After the atomization process (about 5 min), the product was collected by thoroughly washing the collecting bottle and the fibrous filter using 50 mL of N-ethylmorpholine buffer solution (0.15 M). 3. RESULTS AND DISCUSSION Fig. 1 describes the procedure for preparing Dpa@UiO-66-NH2 CSNs and their fabrics. In the first step, Dpa with excellent colloidal stability are synthesized in an mixed solvent of ethanol and water under alkaline conditions according to a reported method.29 In the second step, the metal-chelating activity of the catechol groups of Dpa drives the heterogeneous nucleation and growth of UiO-66-NH2 on the surface of Dpa, yielding exclusively Dpa@UiO-66-NH2 CSNs. In the last step, polymeric fibrous mats with varying mass loadings of Dpa@UiO-66-NH2 CSNs are easily prepared by electrospinning under proper conditions. The representative SEM image shows that the synthesized Dpa are uniform spheres with an average diameter of 200 nm and a relatively smooth surface (Fig. 2a). The powder XRD pattern
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displays only one wide reflection associated with their amorphous structure (Fig. 2c). The FTIR spectrum is consistent with those reported in the literature (Fig. 2d),29 confirming the successful synthesis of Dpa. After the solvothermal reaction with MOF precursors under optimized conditions, the individual particle size was increased to 400 nm and the particle surface became rough due to the encapsulation of interconnected nanocrystallites with a discernable size of 3080 nm (Fig. 2b). The sharp diffraction peaks agree well with the simulated data and that of UiO66-NH2 synthesized under identical conditions in the absence of Dpa (Fig. S1, Fig. 2c), indicating that the right phase with good crystallinity was obtained. The appearance of carboxyl groups (characteristic bands at 1653, 1571, 1434 and 1383 cm-1), Zr-O bond (763 and 685 cm-1) and C-N bond (1260 cm-1) in the FTIR spectra provides further evidence for the formation of UiO-66-NH2 (Fig. 2d). The broad absorption peak around 3500 cm-1 could be assigned to -OH in Dpa and/or -OH in any surface-bonded water in the MOF and Dpa@MOF. Two well-defined peaks at 3357 and 3461 cm-1 are assigned to the N-H stretching modes for primary amines in UiO-66-NH2.33,34 In addition, the stretching vibration of phenolic -OH at 1287 cm-1 in Dpa disappeared after UiO-66-NH2 encapsulation, indicating a strong interaction between the Zr ion and the catechol group. The HAADF-STEM image displays a clear brightness contrast between the middle and edge regions of a single particle (Fig. 2e), indicating a possible core-shell structure. Further EDS elemental mapping and line scanning analysis show that C and N elements are more distributed in the core region, while the Zr and O elements are more distributed in the peripheral region (Fig. 2e and f), verifying the core–shell structure formed by MOF encapsulating Dpa. It is worth noting that almost no unencapsulated Dpa and loose UiO66-NH2 were observed in the product, indicating the dominant heterogeneous nucleation of UiO66-NH2 on Dpa, which can be attributed to the metal-chelating activity of Dpa and the
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appropriate crystallization conditions. These results demonstrate that the target Dpa@UiO-66NH2 CSNs were successfully synthesized by our method. To determine the MOF content in the synthesized CSN, TGA analysis was performed on the samples. As shown in Fig. 3a, Dpa loses 98% of its mass at 650 °C, while UiO-66-NH2 and Dpa@MOF CSN have similar two-step weight loss with 48% and 43% residual mass, respectively. For pure UiO-66-NH2, the escape of solvent from the pores and dehydration of the Zr6O4(OH)4 nodes results in the first weight loss below 300 °C, and the collapse of the framework due to the elimination of structural 2-aminoterephthalic acid linkers induces the second weight loss. By comparing the weight loss between samples, the mass fraction of UiO66-NH2 in CSN was estimated to be 82%, which is in agreement with the mass fraction calculated based on ICP-OES analysis (Table S1, calculation details shown in supporting information). The porosity of the samples was analyzed by nitrogen physisorption measurements. As shown in Fig. 3b, Dpa displays a V-type isotherm with an H3 loop, while UiO-66-NH2 and Dpa@UiO66-NH2 CSN show almost identical type IV isotherms. The Brunauer-Emmett-Teller (BET) surface area of Dpa, UiO-66-NH2, and Dpa@UiO-66-NH2 CSN is calculated to be 21, 851, and 730 m2 g-1, respectively. The results indicate good accessibility of MOF pores in the CSN, which is beneficial for their further catalytic application. The measured values are slightly higher than the theoretical geometric surface area (700 m2 g-1) of the ideal UiO-66-NH2 with 12 linkers per node,35 indicating that low levels of structural defects such as missing linkers exist in the product although no special modulators are used in the synthesis. The optical properties of the samples were studied by UV-vis-NIR spectra measurement. As shown in Fig. 3c, Dpa exhibits broad absorption from UV to NIR wavelengths, which is
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responsible for its dark brown color and photothermal property. UiO-66-NH2 displays an absorption band at 230 nm and a shoulder near 300 nm, which corresponds to the characteristic π →π* and n→π* transitions of aromatic C−C bonds and C=O bonds, respectively.36 In addition, a strong absorption band between 300 to 440 nm peaked at 360 nm originates from the presence of amino groups, and is the cause for the yellow color of the solid. Dpa@UiO-66-NH2 CSN has the same absorption band as pure UiO-66-NH2 in the UV region, but due to the incorporation of Dpa, the absorbance in the visible to NIR region is significantly increased, resulting in a yellowish gray color of the powder. It should be mentioned that the much higher NIR absorption of Dpa than Dpa@UiO-66-NH2 is due to the larger amount of Dpa used in the measurement. This result indicates that the light absorbing properties of Dpa remain well in the CSNs, making them useful for photothermal materials. In order to study PTE of the samples, Dpa, UiO-66-NH2, and Dpa@UiO-66-NH2 CSN were dispersed in water and exposed to NIR laser and simulated solar light (SSL), respectively, and the temperature rise with irradiation time was recorded with pure water as a negative control. (Fig. 4) It can be seen that Dpa@UiO-66-NH2 CSN shows the highest temperature rise among the samples, and the temperature rise after 900 s of NIR and SSL irradiation is 48.5 and 40.2 °C, respectively. Under the same conditions, Dpa shows a slightly lower temperature rise of 44.8 and 34.2 °C, respectively. In contrast, pure water and UiO-66-NH2 solution displays very small temperature increase of 2.4 and 5.6 °C under NIR irradiation, respectively, but more prominent temperature rise of 13.1 and 24.6 °C under SSL irradiation, respectively. Therefore, the slightly higher PTE of Dpa@UiO-66-NH2 CSN than Dpa can be attributed to the presence of UiO-66NH2. In addition, Dpa@UiO-66-NH2 CSN shows good photostability, with the initial photothermal efficiency maintained for at least 5 cycles of irradiation (Fig. S2). In addition,
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Dpa@UiO-66-NH2 CSN shows much higher PET in dry conditions than in water, with a temperature rise of 137 and 87 °C after 900 s of NIR and SSL irradiation, respectively (Fig. S3). These results demonstrate that both NIR laser and SSL are good candidates for photothermalenhanced catalysis in this system. Next, we explore Dpa@UiO-66-NH2 CSN for photothermal-enhanced detoxification of CWA simulants. In order to reduce the risk of direct handling of highly toxic CWAs such as VX, GB (sarin), and GD (soman), methyl paraoxon containing phosphate ester bonds has been used in routine laboratories as stimulants of nerve agents.5-7,
10
The reaction scheme for the
photothermal-enhanced catalytic conversion of DMNP to p-nitrophenoxide is shown in Fig. 5. A common mechanism for the hydrolysis of DMNP is that the oxygen on the phosphorus-oxygen bond first binds to the coordination unsaturated site on the Lewis acidic metal cation, accompanied by an attenuation of the phosphorus-oxygen bond.9 Then, the phosphate receives a metal-bound or free hydroxide anion. Finally, the catalyst is regenerated by dissociation of nontoxic products from the active site. The Zr6O4(OH)4 node contains enzyme-like bimetallic Lewisacidic metal centers bridged by a hydroxide, which are effective for the cleavage of P–O bonds, and the amine moiety in UiO-66-NH2 as a Brønsted base can enhance the catalytic activity by the transfer of a proton during the catalytic cycle.8,9 Blank experiment shows that long-term irradiation of NIR laser or SSL has negligible effect on the stability of DMNP (Fig. S4). When catalyzed by UiO-66-NH2 under room light, DMNP is hydrolyzed and converted to p-nitrophenoxide with a half-life of 2.6 min (Fig. S5, Fig. 6a, Table 1). Under the NIR laser irradiation, the catalytic hydrolysis rate of DMNP was almost unchanged from that under room light (Fig. S6, Fig. 6a, Table 1), indicating that UiO-66-NH2 has no photothermal catalytic effect under such illumination conditions. Under the irradiation of SSL,
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the catalytic hydrolysis rate of DMNP was slightly increased, the turnover frequency (TOF) increased by 0.2-fold, and the half-life was shortened to 2.1 minutes (Fig. S7, Fig. 6a, Table 1).These results are consistent with the optical absorption range of UiO-66-NH2 and demonstrate that UiO-66-NH2 alone does not result in notable photothermal-enhanced catalysis. It is also noted that due to low levels of structural defects in the UiO-66-NH2 synthesized here, the resultant half-life of DMNP is much slower compared to previously reported (Fig. S8, Table S2).21 In order to introduce PTE into the catalytic reaction, the simplest way may be to add a photothermal reagent in addition to the catalyst to heat the reaction upon light absorption. However, we believe that a physical mixture of the two components is unlikely to be as effective as a well-designed core-shell structure in terms of uniform and localized heating and accelerated reaction thereof. To prove this point, a physical mixture of UiO-66-NH2 and Dpa was used as the control sample and compared to Dpa@UiO-66-NH2 CSN in terms of catalytic performance (Fig. 6b and c). The two samples display almost the same degradation rate and kinetics of DMNP in 30 min of reaction under room light, with corresponding t1/2 being 2.7 and 2.8 min, respectively (Fig. S9, Fig. S10, Table 1). These values are also similar to that of the equivalent UiO-66-NH2 powder. This result indicates that under room light the presence of Dpa in catalysts has no effect on the progress of the reaction, and the catalytic efficiency depends only on the amount of catalytic UiO-66-NH2. When irradiated with NIR laser, the degradation kinetics of DMNP catalyzed by Dpa@UiO-66-NH2 CSN significantly increases, with t1/2 reduced to 0.7 min and TOF increased by 2.9 fold (Fig. 6c, Fig. S11, Table 1). In contrast, the mixture shows much poorer enhancement effect under identical conditions, giving t1/2 of 1.7 min and a 0.7-fold TOF increase (Fig. 6b, Fig. S12, Table 1). The catalytic results using the physical mixture and the
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Dpa@UiO-66-NH2 CSN under the irradiation of SSL shows a similar trend (Fig. 6b, Fig. 6c, Fig. S13, Fig. S14, Table 1). The promoted degradation of DMNP upon NIR laser and SSL illumination could be related to the generation of PTE, which enables photothermal-enhanced catalysis. As depicted in Fig. 6d, the higher photothermal-enhanced catalytic efficiency of Dpa@UiO-66-NH2 CSN than the physical mixture could be attributed to the direct and instantaneous heat transfer from the photothermal Dpa core to the adjacent catalytic MOF shell upon light absorption and a high contact interface between the catalytic active sites and the reactants. It is also noted that NIR laser appears to be more effective than SSL in enhancing catalysis of both cases. This could be attributed to the high heat conversion efficiency of Dpa in the NIR region.29 In addition, we found that Dpa is more catalytically active under NIR laser than under SSL irradiation. (Fig. S15-17, Table 1). The cycle performance of the Dpa@UiO-66-NH2 CSN catalyst under different light illumination conditions was tested. After five cycles, the conversion of DMNP for 30 min of reaction decreased from 100% to 56%, 75% and 70% under room light, NIR laser and SSL irradiation, respectively (Fig. S18). XRD patterns of the catalysts after five cycles showed the same amorphization of the structure, which was independent of the illumination conditions (Fig. S19). The degradation of catalytic performance may be due to clogging of the active site and destruction of the MOF crystal structure by interaction with reactants and products. These results demonstrate that photothermal-enhanced catalysis can improve the cycling stability of catalytic performance compared to conventional ones. For further application to gas filters, Dpa@UiO-66-NH2 CSN was incorporated into polymeric fibrous mats by electrospinning. Polyacrylonitrile (PAN) fibrous mats with different mass load of Dpa@UiO-66-NH2 CSN (i.e. 10, 30, and 60 wt%) were prepared and referred to as
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Dpa@UiO-66-NH2/PAN-x%, where x% stands for the mass load of CSN. Representative SEM images show that as CSN increases, more and more clusters appeared in the view field compared to the pure PAN fibrous mat (Fig. S20). The corresponding swatches gradually approached the yellowish gray color of Dpa@UiO-66-NH2 CSN. XRD patterns and FTIR spectra show that the crystal structure and functional group of UiO-66-NH2 remain intact after electrospinning (Fig. S21, Fig. S22). The actual UiO-66-NH2 content in the fibrous mats was estimated to be 9, 30, and 56 wt% based on TGA, respectively (Fig. S23, calculation details are shown in supporting information). The BET surface area of the fibrous mats increases proportionally with the increasing MOF content (Fig. S24), indicating that the MOF pores are still accessible in the polymer matrix. The reduced BET surface area may be due to the blockage of MOF pores by the polymer matrix. While PAN fibers display no activity towards the degradation of DMNP, Dpa@UiO-66NH2/PAN fibers show increasingly higher catalytic activity as the CSN loading increases, yielding t1/2 of 138, 36, and 13 min under room light for Dpa@UiO-66-NH2/PAN-10%, -30%, and -60%, respectively (Fig. S25, Table S4). However, the catalytic properties of the Dpa@UiO66-NH2-60% is still much lower than the equivalent UiO-66-NH2 powder (t1/2 = 13 vs. 2.6 min). The reduced catalytic activity may arise from aggregation of particles in the polymer matrix and pore blockage of the MOF, which is also commonly encountered in previous studies.25, 28, 31, 32 Further increase of catalyst loading is not suitable for the production of fibers by electrospinning and results in a sharp decrease in the mechanical strength of the fibrous mats (Fig. S26). Here by using the strategy of photothermal-enhanced catalysis the detoxification ability of Dpa@UiO-66-NH2/PAN can be greatly improved. Fibrous mats incorporating only UiO-66-NH2 (referred to as UiO-66-NH2/PAN) or the mixture of UiO-66-NH2 and Dpa (referred to as
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mixture/PAN) were also prepared and used as control samples. As shown in Fig.7a-c, the three fibers show similar conversion profiles under room light, with 80% DMNP degraded over 0.5 h of reaction and t1/2 around 14 min (Fig. S27-29, Table 1). When irradiated by NIR laser, Dpa@UiO-66-NH2/PAN exhibits the fastest degradation rate (t1/2 = 1.8 min) and the highest conversion of DMNP (100%) among the three fibers, while mixture/PAN reduces t1/2 to 6.7 min and increases the conversion to 97% compared to UiO-66-NH2/PAN (Fig. S30-32, Table 1). In addition, TOF is increased by 5.8 folds from 0.0039 to 0.0264 s-1 using Dpa@UiO-66-NH2/PAN upon NIR irradiation, which is much higher than the 1.1- and 0.06-fold increase of TOF using mixture/PAN and UiO-66-NH2/PAN (Table 1). More importantly, the degradation rate of DMNP using Dpa@UiO-66-NH2/PAN-60% under NIR irradiation is even faster than using pure UiO66-NH2 powder (t1/2 = 1.8 vs. 2.6 min), which has rarely been achieved with previously reported MOF-based fabrics.20, 21, 23, 25-28 The catalytic results under the irradiation of SSL shows a similar trend for UiO-66-NH2/PAN, mixture/PAN, and Dpa@UiO-66-NH2/PAN (Fig. 7a-c, Fig. S33-35, Table 1). While the NIR laser is more effective than SSL for photothermal-enhanced catalysis in the cases of mixture/PAN and Dpa@UiO-66-NH2/PAN, SSL is more effective than the NIR laser in the case of UiO-66-NH2/PAN, which agrees well with the results obtained from the corresponding powder samples. These results indicate that photothermal-enhanced catalysis can significantly improve the detoxification properties of the fibers, wherein the core-shell structure of PTE material and catalyst functions more efficiently than the physical mixture of the two components. The cycle test of the Dpa@UiO-66-NH2/PAN catalyst shows that the conversion rate after five cycles maintained 62%, 75%, and 67% of the initial value under the illumination of room light, NIR laser, and SSL, respectively (Fig. S36). Similar to the powder samples, the crystallinity of
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UiO-66-NH2 in the fiber catalysts decreased significantly after five cycles (Fig. S37). To demonstrate the heterogeneity of the catalytic process, the Dpa@UiO-66-NH2/PAN catalyst was filtered at t = 5 min of the reaction. The DMNP conversion immediately stopped rising after catalyst filtration (Fig. S38), indicating that the Dpa@UiO-66-NH2/PAN catalyst is heterogeneous and no leakage of particles to the solution. It is noted that the photothermal-enhanced catalytic detoxification is more pronounced in the fibrous mats than in the powders. For instance, TOF is increased by 2.9 times for Dpa@UiO-66NH2 whereas 5.8 times for Dpa@UiO-66-NH2/PAN-60% under the irradiation of NIR laser (Table 1). This may be due to the encapsulation of the photothermal catalytic particles by the fibrous matrix, which prevents rapid heat dissipation of the catalytic system. Therefore, the significant improvement in the detoxification ability of Dpa@UiO-66-NH2/PAN under NIR laser or SSL irradiation can be attributed to: (i) the core-shell structure that enables efficient heat transfer from the photothermal core to the catalytic shell, and (ii) the surrounding fibrous matrix that helps in heat retention. As a material must be able to first ab/adsorb the agent before reaction can occur on timescales of a filter, the DMNP adsorption was tested by placing the samples in a sealed bottle with a vial containing 4 μL DMNP at room temperature. The weight uptake of DMNP has been measured to be 155 and 79 mg g-1 for Dpa@UiO-66-NH2 CSN and Dpa@UiO-66-NH2/PAN-60% after adsorption for 24 h, respectively. Assuming that DMNP adsorbs very little on Dpa and/or PAN, the weight uptakes per gram of Zr is estimated to be 470 and 430 mg for Dpa@UiO-66-NH2 CSN and Dpa@UiO-66-NH2/PAN-60%, respectively. These values are slightly higher than catalytic detoxifying materials with no microspores but significantly lower than those developed with mesoporosity.24 Therefore, we believe that DMNP can probably diffuse in through defects,
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but mostly adsorb on the MOF surface due to the small pore openings (smaller than 6 Å) of UiO66-NH2.37 The Dpa@UiO-66-NH2/PAN were further exposed to DMNP aerosol as a self-standing gas filter in a homemade device to demonstrate their possible function as a self-detoxifying protective media (Fig. 7d and e). Due to the toxicity of DMNP, we are not allowed to handle large amount of it at a time under normal laboratory conditions. So a small amount of DMNP was diluted in the buffer solution and atomized to form an aerosol. At the same time, the buffer offers water and alkaline conditions to facilitate the hydrolysis of DMNP.38 The amount of pnitrophenoxide collected was divided by the initial amount of DMNP to give the total percent conversion. As shown in Fig. 7f, UiO-66-NH2/PAN, mixture/PAN, and Dpa@UiO-66-NH2/PAN displayed the same DMNP conversion of 37% under room light. Under the irradiation of NIR laser, the conversion of DMNP increased to 77% and 54% for Dpa@UiO-66-NH2/PAN and mixture/PAN, respectively, while that for UiO-66-NH2/PAN changed very little. When irradiated with SSL, the DMNP conversion increased in all three cases from 37% to 42%, 48%, and 69% for UiO-66-NH2/PAN, mixture/PAN, and Dpa@UiO-66-NH2/PAN, respectively. The trend of enhancement is consistent with that of the batch experiments. The total conversion percentage may be largely underestimated due to the loss of filtrate under vacuum conditions and the brief filtration process (5 min). Besides using aerosol of DMNP buffer solution, DMNP was spread directly onto the filter, which was previously impregnated with the buffer solution. The conversion after reaction for 10 min show a similar trend to the case of using DMNP buffer solution as the aerosol, that is, Dpa@UiO-66-NH2/PAN outperforms the other filters under three light irradiation conditions (Fig. S39). The fibrous mats show good mechanical robustness during and after the filtration. The particles co-electrospun with the polymer are wholly or partially
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embedded in the polymer matrix, which results in the mechanical stability. SEM images showed that the Dpa@UiO-66-NH2/PAN morphology remained after filtration, with no signs of detachment of the particles from the fibers (Fig. S40). Crimping or folding Dpa@UiO-66NH2/PAN for several results in little mass losses (Fig. S41). These results demonstrate that Dpa@UiO-66-NH2/PAN can effectively resist and degrade CWA as a filter with the assistance of photothermal-enhanced catalytic detoxification. 4. CONCLUSIONS In summary, we have successfully synthesized a rationally designed core–shell Dpa@UiO-66NH2 with a photothermal core and a microporous catalytic shell, which exhibits substantially enhanced catalytic efficiency toward CWA stimulant degradation under the irradiation of NIR laser or simulated solar light in both powder and fabric form. This work addresses the longstanding challenge of reduced catalytic performance of MOF fabrics, leading to gas filters with better detoxification capability than pure MOF powders for the first time. This novel photothermal-enhanced catalytic detoxification strategy can be feasibly extended to other catalytic detoxification systems and holds promise for CWA purification technologies and nextgeneration gas masks. In addition, the bio-inspired Dpa@UiO-66-NH2 nanostructure synthesized here with its excellent biocompatibility and host-guest chemistry may be of great interest for a wider range of applications like chemo-photothermal therapy.39
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Figure 1. Schematic illustration of the fabrication procedures for Dpa@MOF CSNs and their fabrics for photothermal-enhanced detoxification of CWA simulants.
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Figure 2. (a, b) SEM images of Dpa and Dpa@UiO-66-NH2 CSN. (c) XRD patterns of Dpa, UiO-66-NH2, Dpa@UiO-66-NH2 CSN, and the simulated data. (d) FTIR spectra of Dpa, UiO66-NH2, and Dpa@UiO-66-NH2 CSN. (e) HAADF-STEM image and the corresponding EDS elemental mapping of an individual Dpa@UiO-66-NH2 CSN. (f) HAADF-STEM image and the corresponding EDS line scan of one Dpa@UiO-66-NH2 CSN.
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Figure 3. TGA curves (a), N2 sorption-desorption isotherms (b), and UV-vis-NIR spectra (c) of Dpa, UiO-66-NH2, and Dpa@UiO-66-NH2 CSN. Insets show the corresponding optical photographs.
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Figure 4. Temperature rise of the Dpa@UiO-66-NH2 CSN, Dpa, and UiO-66-NH2 aqueous solution as a function of NIR laser (a) and simulated solar light (b) irradiation time, compared to blank water.
Figure 5. Photothermal-enhanced catalytic hydrolysis of DMNP using Dpa@MOF/fiber catalysts.
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Table 1. Catalyst properties and performance toward the degradation of a CWA simulant DMNP under different illumination conditions.
MOF wt %
BET SA (m2 g-1)
UiO-66-NH2 (6.7 mg)
100
Dpa
Sample
TOF (s-1)c
t1/2 (min)
TOF increase fold
Room light
NIR laser
Simulated solar light
Room light
NIR laser
Simulated solar light
NIR laser
Simulated solar light
851
2.6
2.6
2.1
0.0184
0.0184
0.0227
0
0.2
0
21
624
130
420
—
—
—
—
—
Mixture of UiO-66-NH2 and Dpa (7.7 mg)
87
697
2.8
1.7
1.8
0.0170
0.0281
0.0265
0.7
0.6
Dpa@UiO66-NH2 (7.7 mg)
82a, 87b
730
2.7
0.7
1.0
0.0176
0.0681
0.0477
2.9
1.7
UiO-66NH2/PAN (12 mg)
56a
178
14
13
10
0.0034
0.0036
0.0046
0.1
0.4
Mixture/PAN (12 mg)
55a
148
14
6.7
7.7
0.0034
0.0072
0.0063
1.1
0.9
Dpa@UiO66-NH2/PAN (12 mg)
56a
254
13
1.8
2.9
0.0039
0.0264
0.0162
5.8
3.2
Powders
Fibers
a
The MOF content is estimated based on TGA. b The MOF content is estimated based on ICP-OES. c TOF was calculated per Zr6
cluster at t1/2. .
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Figure 6. Percentage conversion of DMNP to p-nitrophenoxide as a function of reaction time using UiO-66-NH2 (a), mixture of UiO-66-NH2 and Dpa (b), and Dpa@UiO-66-NH2 CSN (c) as the catalyst. (d) Schematic illustration of photothermal-enhanced catalytic detoxification of DMNP using Dpa@UiO-66-NH2 CSN. Error bars are the standard deviation in triplicated readings.
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Figure 7. Percentage conversion of DMNP to p-nitrophenoxide as a function of reaction time obtained in batch experiment using UiO-66-NH2/PAN (a), mixture/PAN (b), and Dpa@UiO-66NH2/PAN (c) as the catalyst. (d) Photographs showing stand-alone flexible UiO-66-NH2/PAN fibrous mat in large area and its corresponding SEM image. (e) Photograph of the filter test device with self-detoxifying fibrous mats as gas filters against the attack of a DMNP aerosol under NIR laser irradiation. (f) Percentage conversion of DMNP to p-nitrophenoxide using UiO66-NH2/PAN, mixture/PAN, and UiO-66-NH2/PAN fibrous mats in the filter test under different light irradiation. Error bars are the standard deviation in triplicated readings. ASSOCIATED CONTENT Supporting Information. TEM and SEM images, temperature variation versus time, UV-vis spectra and kinetic analyses, percent conversion of DMNP using fibrous catalysts, cycle performance of the catalyst, XRD patterns, FTIR spectra, TGA curves, N2 sorption-desorption isotherms, tensile stress-strain curves, photographs and weight of folded and crimpled fibrous samples, and comparison of material properties.
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AUTHOR INFORMATION Corresponding Author *C. Li Email:
[email protected] Notes The authors declare no competing financial interest. ACKNOWLEDGMENT The authors acknowledge support from the National Natural Science Foundation of China (NSFC 21771118), and the Taishan Scholars Climbing Program of Shandong Province (Grant tspd20150201). REFERENCES (1) Szinicz, L. History of Chemical and Biological Warfare Agents. Toxicology 2005, 214, 167-181. (2) Chemical Warfare Agents: Toxicology and Treatment; Marrs, T. C., Maynard, R. L., Sidell, F. R., Eds.; John Wiley & Sons, Ltd 2007. (3) Truong, Q.; Wilusz, E. 13: Advances in Chemical and Biological Protective Clothing A2. In Smart Textiles for Protection; Chapman, R. A., Ed.; Woodhead Publishing: 2013; pp 364-377. (4) Raushel, F. M. Catalytic Detoxification. Nature 2011, 469, 310-311. (5) DeCoste, J. B.; Peterson, G. W. Metal-Organic Frameworks for Air Purification of Toxic Chemicals. Chem. Rev. 2014, 114, 5695-5727.
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(12) Peterson, G. W.; Moon, S. Y.; Wagner, G. W.; Hall, M. G.; DeCoste, J. B.; Hupp, J. T.; Farha, O. K. Tailoring the Pore Size and Functionality of UiOType Metal-Organic Frameworks for Optimal Nerve Agent Destruction. Inorg. Chem. 2015, 54, 9684-9686. (13) Liu., Y.; Moon., S. Y.; Hupp., J. T.; Farha., O. K. Dual-Function Metal Organic Framework as a Versatile Catalyst for Detoxifying Chemical Warfare Agent Simulants. ACS Nano 2015, 9, 12358-12364. (14) Meng, X.; Wang, T.; Liu, L.; Ouyang, S.; Li, P.; Hu, H.; Kako, T.; Iwai, H.; Tanaka, A.; Ye, J. Photothermal Conversion of CO(2) into CH(4) with H(2) over Group VIII Nanocatalysts: an Alternative Approach for Solar Fuel Production. Angew. Chem., Int. Ed. 2014, 53, 11478-11482. (15) Zhu, L.; Gao, M.; Peh, C. K. N.; Ho, G. W. Solar-Driven Photothermal Nanostructured Materials Designs and Prerequisites for Evaporation and Catalysis Applications. Mater. Horiz. 2018, 5, 323-343. (16) Yang, M.; Gao. M. Hong, M.; Ho, G. W. Visible-to-NIR Photon Harvesting: Progressive Engineering of Catalysts for Solar-Powered Environmental Purification and Fuel Production, Adv. Mater. 2018, 30, 1802894. (17) Gao, M.; Zhu, L.; Peh, C. K. N.; Ho, G. W. Solar Absorber Material and System Designs for Photothermal Water Vaporization towards Clean Water and Energy Production. Energy Environ. Sci. 2019, DOI:10.1039/C8EE01146J. (18) Yang, Q.; Xu, Q.; Yu, S.; Jiang, H. Pd Nanocubes@ZIF-8: Integration of Plasmon-Driven Photothermal Conversion with a Metal–Organic Framework
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for Efficient and Selective Catalysis. Angew. Chem., Int. Ed. 2016, 55, 36853689. (19) Wang, F.; Huang, Y.; Chai, Z.; Zeng, M.; Li, Q.; Wang, Y.; Xu, D. Photothermal-Enhanced Catalysis in Core-Shell Plasmonic Hierarchical Cu7S4 Microsphere@Zeolitic Imidazole Framework-8. Chem. Sci. 2016, 7, 68876893. (20) Lõpez-Maya, E.; Montoro, C.; Rodríguez-Albelo, L. M.; Aznar Cervantes, S. D.; Lozano-Pérez, A. A.; Cenís, J. L.; Barea, E.; Navarro, J. A. R. Textile/MetalOrganic-Framework Composites as Self-Detoxifying Filters for ChemicalWarfare Agents. Angew. Chem., Int. Ed. 2015, 54, 6790-6794. (21) Zhao, J.; Lee, D. T.; Yaga, R. W.; Hall, M. G.; Barton, H. F.; Woodward, I. R.; Oldham, C. J.; Walls, H. J.; Peterson, G. W.; Parsons, G. N. Ultra-Fast Degradation of Chemical Warfare Agents Using MOF-Nanofiber Kebabs. Angew. Chem., Int. Ed. 2016, 55, 13224-13228. (22) Moon, S. Y.; Proussaloglou, E.; Peterson, G. W.; DeCoste, J. B.; Hall, M. G.; Howarth, A. J.; Hupp, J. T.; Farha, O. K. Detoxification of Chemical Warfare Agents Using a Zr6-Based Metal-Organic Framework/Polymer Mixture. Chem. Eur. J. 2016, 22, 14864-14868. (23) Lu, A. X.; McEntee, M.; Browe, M. A.; Hall, M. G.; DeCoste, J. B.; Peterson, G. W. MOFabric: Electrospun Nanofiber Mats from PVDF/UiO-66NH2 for Chemical Protection and Decontamination. ACS Appl. Mater. Interfaces 2017, 9, 13632-13636.
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(24) Giannakoudakis, D. A.; Hu, Y.; Florent, M.; Bandosz, T. J. Smart Textiles of MOF/g-C3N4 Nanospheres for the Rapid Detection/Detoxification of Chemical Warfare Agents. Nanoscale Horiz. 2017, 2, 356-364. (25) Lee, D. T.; Zhao, J.; Peterson, G. W.; Parsons, G. N. Catalytic “MOF-Cloth” Formed via Directed Supramolecular Assembly of UiO-66-NH2 Crystals on Atomic Layer Deposition-Coated Textiles for Rapid Degradation of Chemical Warfare Agent Simulants. Chem. Mater. 2017, 29, 4894-4903. (26) Peterson, G. W.; Lu, A. X.; Epps, T. H., III. Tuning the Morphology and Activity of Electrospun Polystyrene/UiO-66-NH2 Metal-Organic Framework Composites to Enhance Chemical Warfare Agent Removal. ACS Appl. Mater. Interfaces 2017, 9, 32248-32254. (27) McCarthy, D. L.; Liu, J.; Dwyer, D. B.; Troiano, J. L.; Boyer, S. M.; DeCoste, J. B.; Bernier, W. E.; Jones, W. E., Jr. Electrospun Metal–Organic Framework Polymer Composites for the Catalytic Degradation of Methyl Paraoxon. New J. Chem. 2017, 41, 8748-8753. (28) Dwyer, D. B.; Lee, D. T.; Boyer, S. M.; Bernier, W. E.; Parsons, G. N.; Jones, W. E., Jr. Toxic Organophosphate Hydrolysis Using NanofiberTemplated UiO-66-NH2 Metal-Organic Framework Polycrystalline Cylinders. ACS Appl. Mater. Interfaces 2018, 10, 25794-25803. (29) Liu, Y.; Ai, K.; Liu, J.; Deng, M.; He, Y.; Lu, L. Dopamine-Melanin Colloidal Nanospheres: An Efficient Near-Infrared Photothermal Therapeutic Agent for In Vivo Cancer Therapy. Adv. Mater. 2013, 25, 1353-1359.
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Stable
Zr-Benzenedicarboxylate
Metal-Organic
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Photocatalysts for Hydrogen Generation. Chem. Eur. J. 2010, 16, 11133– 11138. (37) Plonka, A. M.; Wang, Q.; Gordon, W. O.; Balboa, A.; Troya, D.; Guo, W.; Sharp, C. H.; Senanayake, S. D.; Morris, J. R.; Hill, C. L.; Frenkel, A. I. In Situ Probes of Capture and Decomposition of Chemical Warfare Agent Simulants by Zr-Based Metal Organic Frameworks. J. Am. Chem. Soc. 2017, 139, 599602. (38) Ploskonka, A. M.; DeCoste, J. B. Tailoring the Adsorption and Reaction Chemistry of the Metal−Organic Frameworks UiO-66, UiO-66-NH2, and HKUST-1 via the Incorporation of Molecular Guests. ACS Appl. Mater. Interfaces 2017, 9, 21579−21585. (39) Nazari, M.; Rubio-Martinez, M.; Tobias, G.; Barrio, J. P.; Babarao, R.; Nazari, F.; Konstas, K.; Muir, B. W.; Collins, S. F.; Hill, A. J.; Duke, M. C.; Hill, M. R. Metal-Organic-Framework-Coated Optical Fibers as LightTriggered Drug Delivery Vehicles. Adv. Funct. Mater. 2016, 26, 3244-3249.
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