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Langmuir 2008, 24, 9024-9029
Imprinted Nanoporous Organosilicas for Selective Adsorption of Nitroenergetic Targets Brandy J. Johnson,*,†,‡ Brian J. Melde,§ Paul T. Charles,‡ Damaris Concepcio´n Cardona,| Michael A. Dinderman,‡ Anthony P. Malanoski,‡ and Syed B. Qadri⊥ Center for Bio/Molecular Science & Engineering, NaVal Research Laboratory, Washington, D.C. 20375-5348, NOVA Research Incorporated, Alexandria, Virginia 22308, Department of Physics and Chemistry, UniVersity of Puerto Rico at Arecibo, Arecibo, Puerto Rico 00614-4010, and Materials Science and Technology DiVision, NaVal Research Laboratory, Washington, D.C. 20375-5348 ReceiVed February 26, 2008. ReVised Manuscript ReceiVed May 13, 2008 Periodic mesoporous organosilicas incorporating diethylbenzene bridges in their pore walls were applied for the adsorption of nitroenegetic targets from aqueous solution. The materials were synthesized by co-condensing 1,4bis(trimethoxysilylethyl)benzene (DEB) with 1,2-bis(trimethoxysilyl)ethane to improve structural characteristics. Molecular imprinting of the pore surfaces was employed through the use of a novel target-like surfactant to further enhance selectivity for targets of interest (tri- and dinitrotoluenes) over targets of similar structure (p-cresol and p-nitrophenol). The headgroup of the commonly used alkylene oxide surfactant Brij76 was modified by esterification with 3,5-dinitrobenzoyl chloride. This provided a target analogue which was readily miscible with the Brij76 surfactant micelles used to direct material mesopore structures. The impact of variations in precursor ratios and amounts of imprint molecule was evaluated. The use of 12.5% of the modified Brij surfactant with a co-condensate employing 30% DEB was found to provide the best compromise between total capacity and selectivity for nitroenergetic targets.
Introduction In the U.S. there are over 12000 current and former testing and training facilities which are contaminated with compounds related to weapons manufacture, storage, or reclamation.2 Leaching of nitro-energetic compounds, chlorinated hydrocarbons, and their breakdown products into soil and groundwater presents a health hazard to military personnel and their families at these facilities as well as to populations in nearby areas.3,4 Surrounding agricultural regions and neighboring wildlife are also at risk due to migration of these carcinogenic compounds.5,6 Materials for remediation and long-term monitoring of the contaminated and potentially contaminated sites are needed. Periodic mesoporous organosilicas (PMOs) offer the potential to address both monitoring and remediation concerns. PMOs are hybrid organic/inorganic materials which are synthesized by condensing organic-bridged multifunctional silanes (e.g., bis(triethoxysilyl)ethane) around surfactant micelles that act as structure directors.7–10 The resulting materials possess the stability * Corresponding author: e-mail,
[email protected]; phone, 202404-6100; fax, 202-767-9598. † SDC approach for determining author sequence has been employed.1 ‡ Center for Bio/Molecular Science & Engineering, Naval Research Laboratory. § NOVA Research Incorporated. | Department of Physics and Chemistry, University of Puerto Rico at Arecibo. ⊥ Materials Science and Technology Division, Naval Research Laboratory. (1) Tscharntke, T.; Hochberg, M. E.; Rand, T. A.; Resh, V. H.; Krauss, J. PLoS Biol. 2007, 5, e18. (2) Spalding, R. F.; Fulton, J. W. J. Contam. Hydrol. 1998, 2, 139–153. (3) Spain, J. C. Annu. ReV. Microbiol. 1995, 49, 523–555. (4) Spain, J. C.; Hughes, J. B.; Knackmuss, H.-J. Nitroaromatic Compounds and ExplosiVies; CRC Press: Boca Raton, FL, 2000. (5) Valsaraj, K. T.; Qaisi, K. M.; Constant, W. D.; Thibodeaux, L. J.; Ro, K. S. J. Hazard. Mater. 1998, 59, 1–12. (6) Yinon, J. Toxicity and Metabolism of ExplosiVes; CRC Press: Boca Raton, FL, 1990. (7) Asef, T.; MacLachlan, M. J.; Coombs, N.; Ozin, G. A. Nature 1999, 402, 867–71. (8) Inagaki, S.; Guan, S.; Fukushima, Y.; Ohsuna, T.; Terasaki, O. J. Am. Chem. Soc. 1999, 121, 9611–14.
10.1021/la800615y
of silicates with the functionality of organic groups. Narrow pore distributions, large internal surface areas, and ordered pore networks are common after the removal of surfactant. Variations in the organic bridging groups can be used to generate materials for specific functions through alteration of the electrostatic structure of the pore surfaces as well as the flexibility of the reactive moieties.11–15 Variations in the surfactant and the synthesis conditions can be used to alter the structural character of the materials including total and specific surface areas, pore homogeneity, total pore volume, material density, and average pore diameter.16–20 PMOs have been used for the adsorption of a variety of targets from both liquid21–25 and vapor phase samples23,26,27 and have been employed as catalysts.28–31 Surfactant-templated porous organosilicas with diethylbenzene (DEB) bridging groups have previously been shown to bind (9) Melde, B. J.; Holland, B. T.; Blanford, C. F.; Stein, A. Chem. Mater. 1999, 11, 3302–8. (10) Yoshina-Ishii, C.; Asefa, T.; Coombs, N.; MacLachlan, M. J.; Ozin, G. A. Chem. Commun. 1999, 2539–2540. (11) Alauzun, J.; Mehdi, A.; Reye, C.; Corriu, R. New J. Chem. 2007, 31, 911–915. (12) Cho, E. B.; Kim, D.; Jaroniec, M. Langmuir 2007, 23, 11844–11849. (13) Li, C. M.; Liu, J.; Shi, X.; Yang, J.; Yang, Q. H. J. Phys. Chem. C 2007, 111, 10948–10954. (14) Li, J. N.; Qi, T.; Wang, L.; Zhou, Y.; Liu, C. H.; Zhang, Y. Microporous Mesoporous Mater. 2007, 103, 184–189. (15) Xiao, W. X.; Xiao, D.; Yuan, H. Y. Sens. Lett. 2007, 5, 445–449. (16) Chen, Q. R.; Sakamoto, Y.; Terasaki, O.; Che, S. A. Microporous Mesoporous Mater. 2007, 105, 24–33. (17) Kim, J. Y.; Yoon, S. B.; Lee, M. H.; Park, Y. J.; Kim, W. H.; Jee, K. Y. J. Nanosci. Nanotechnol. 2007, 7, 3862–3866. (18) Li, Z.; Chen, D. H.; Tu, B.; Zhao, D. Y. Microporous Mesoporous Mater. 2007, 105, 34–40. (19) Liang, Y. C.; Anwander, R. Dalton Trans. 2006, 1909–1918. (20) Morell, J.; Gungerich, M.; Wolter, G.; Jiao, J.; Hunger, M.; Klar, P. J.; Froba, M. J. Mater. Chem. 2006, 16, 2809–2818. (21) Burleigh, M. C.; Markowitz, M. A.; Spector, M. S.; Gaber, B. P. EnViron. Sci. Technol. 2002, 36, 2515–18. (22) Jayasundera, S.; Burleigh, M. C.; Zeinali, M.; Spector, M. S.; Miller, J. B.; Yan, W.; Dai, S.; Markowitz, M. A. J. Phys. Chem. B 2005, 109, 9198– 9201. (23) Johnson-White, B.; Zeinali, M.; Shaffer, K. M.; Charles, P. T., Jr.; Markowitz, M. A. Biosens. Bioelectron. 2007, 22, 1154–1162.
This article not subject to U.S. Copyright. Published 2008 by the American Chemical Society Published on Web 07/01/2008
Imprinted Nanoporous Organosilicas
several cyclic organics with capacities of 2-20 µg/m2.21,32–34 Unfortunately, these materials were not selective. Adsorption of specific targets is dependent upon establishing favorable interactions with the material surfaces. In addition to providing functional groups for interaction with targets (i.e., DEB), binding affinity and capacity should be increased by the establishment of areas on the surface of the materials that are complementary to targets in size, shape, and/or electronic structure. Molecular imprinting of polymer materials (MIPs) has become a well-established process in which target analogues are used to form specific recognition sites. This process was first applied to sol-gel silica in 1949 and has been investigated since for potential applications in adsorption, separation, catalysis, and sensing.35–37 The use of surfactant headgroups which are (or bear) target analogues would allow PMO materials to be “imprinted” in a similar method but also should control the location of where the imprinting occurs and limit it to pore surfaces. Previously, we incorporated a small amount of surfactant possessing a target-like headgroup into the synthesis of DEB-bridged materials.23,38 The expectation was that upon extraction some target-complement sites would remain on the pore surfaces. Decylaminetrinitrobenzene was used as the target analogue-bearing surfactant, but only marginal success was achieved when it was used for imprinting in combination with Brij76 surfactant micelles. It seems likely that the overall hydrophobic nature and the short chain length of this molecule resulted in inefficient association of the trinitrobenzene headgroup with the hydrophilic alkylene oxide headgroups of Brij76 surfactant. Soon after this work was reported, Walker et al. published work in which silane-modified TNT analogues were used to imprint DEB films using a method that does not depend on surfactant templating.39 The study described here seeks to overcome the shortfalls of the previously described PMOs through the use of a novel imprinting template and co-condensation of two precursors to improve mesoporosity. Brij76 was modified by reaction with 3,5-dinitrobenzoyl chloride for use as both imprint molecule and surfactant template. Incorporation of this compound into the synthesis of the DEB-bridged materials provided marked improvement in the imprinting process. In addition, the DEB bridging groups were combined with ethane bridges for generation of porous materials with enhanced structural characteristics while (24) Matsumoto, A.; Misran, H.; Tsutsumi, K. Langmuir 2004, 20, 7139– 7145. (25) Matsumoto, A.; Yeoh, F. Y.; Fujihara, S.; Tsutsumi, K.; Baba, T. Adsorpt. Sci. Technol. 2006, 24, 451–459. (26) Palaniappan, A.; Su, X. D.; Tay, F. E. H. IEEE Sens. J. 2006, 6, 1676– 1682. (27) Palaniappan, A.; Su, X. D.; Tay, F. E. H. J. Electroceram. 2006, 16, 503–505. (28) Brunel, D.; Fajula, F.; Nagy, J. B.; Deroide, B.; Verhoef, M. J.; Veum, L.; Peters, J. A.; van Bekkum, H. Appl. Catal., A 2001, 213, 73–82. (29) Hoffmann, F.; Cornelius, M.; Morell, J.; Froba, M. J. Nanosci. Nanotechnol. 2006, 6, 265–288. (30) Konovalova, T. A.; Gao, Y. L.; Schad, R.; Kispert, L. D.; Saylor, C. A.; Brunel, L. C. J. Phys. Chem. B 2001, 105, 7459–7464. (31) Weitkamp, J.; Hunger, M.; Rymsa, U. Microporous Mesoporous Mater. 2001, 48, 255–270. (32) Burleigh, M. C.; Jayasundera, S.; Spector, M. S.; Thomas, C. W.; Markowitz, M. A.; Gaber, B. P. Chem. Mater. 2004, 16, 3–5. (33) Burleigh, M. C.; Jayasundera, S.; Thomas, C. W.; Spector, M. S.; Markowitz, M. A.; Gaber, B. P. Colloid Polym. Sci. 2004, 282, 728–733. (34) Burleigh, M. C.; Markowitz, M. A.; Jayasundera, S.; Spector, M. S.; Thomas, C. W.; Gaber, B. P. J. Phys. Chem. B 2003, 107, 12628–12634. (35) Diaz-Garcia, M. E.; Laino, R. B. Microchim. Acta 2005, 149, 19–36. (36) Dickey, F. H. Proc. Natl. Acad. Sci. U.S.A. 1949, 35, 227–229. (37) Holthoff, E. L.; Bright, F. V. Anal. Chim. Acta 2007, 594, 147–161. (38) Johnson-White, B.; Zeinali, M.; Malanoski, A. P.; Dinderman, M. Catal. Commun. 2006, 8, 1052–1065. (39) Walker, N. R.; Linman, M. J.; Timmers, M. M.; Dean, S. L.; Burkett, C. M.; Lloyd, J. A.; Keelor, J. D.; Baughman, B. M.; Edmiston, P. L. Anal. Chim. Acta 2007, 593, 82–91.
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Figure 1. Shown here are the chemical structures of precursors, surfactant, template, and targets relevant to this study.
maintaining some of the binding characteristics provided by the phenyl groups.
Materials and Methods 1,4-Bis(trimethoxysilylethyl)benzene (DEB) and 1,2-bis(trimethoxysilyl)ethane (BTE) were obtained from Gelest, Inc. (Tullytown, PA). Brij76 (polyoxyethylene (10) stearyl ether, C18H37(OCH2CH2)nOH, n ∼ 10), NaOH, HCl, 3,5-dinitrobenzoyl chloride (g98%), dichloromethane (g99.5%), magnesium turnings (98%), p-cresol (pCr), and p-nitrophenol (pNP) were purchased from Sigma-Aldrich (St. Louis, MO). 2,4,6-Trinitrotoluene (TNT), hexahydro-1,3,5-trinitro-1,3,5-triazine (RDX), and 2,4-dinitrotoluene (DNT) were purchased from Pierce Chemical Co. (Rockford, IL). Chemicals were used as received. Figure 1 shows the chemical structures relevant to this study. Water was deionized to 18.2 MΩ cm using a Millipore Milli Q UV-Plus water purification system. The target analogue used for imprinting the PMOs was generated through esterification of Brij76 with 3,5-dinitrobenzoyl chloride.40–42 Briefly, Brij76 (2 g, 2.81 mmol) and 3,5-dinitrobenzoyl chloride (1.3 g, 6 mmol) were dissolved in 60 mL of dichloromethane. Magnesium turnings were added, and the mixture was refluxed for 2 h. The liquid was shaken with 60 mL of 2% NaHCO3 in a separatory funnel. The organic phase was then extracted and evaporated under vacuum. The resulting dinitrobenzne (DNB)-modified Brij76 was orange in color. Our preparation method for the PMOs using Brij76 surfactant in acidic media has been described elsewhere.21,33 Aqueous HCl reaction solution was prepared by adding 13.1 mL of concentrated HCl to 186.9 mL of H2O. Brij76 (4.0 g) or a combination of Brij76 and imprint surfactant was dissolved in the HCl solution with stirring at 50 °C in a closed container. Organosilane (0.0281 mol) (BTE, DEB, or a combination) was added dropwise to the stirring mixture. Stirring was continued at 50 °C for at least 12 h overnight as a white (40) Nozawa, A.; Ohnuma, T. J. Chromatogr. 1980, 187, 261–263. (41) Sun, C.; Baird, M.; Anderson, H. A.; Brydon, D. L. J. Chromatogr., A 1997, 771, 145–154. (42) Sun, C.; Baird, M.; Simpson, J. J. Chromatogr., A 1998, 800, 231–238.
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Table 1. Co-Condensate Material Characteristics and Binding Capacitiesa
material
% DEBd
M-100:0 M-100:0 Imp M-90:10 M-90:10 Imp M-75:25 M-75:25 Imp M-70:30 M70:30 Imp M-60:40 M-60:40 Imp M-50:50 M-50:50 Imp M-0:100 M-0:100 Imp
0 0 10 10 25 25 30 30 40 40 50 50 100 100
single target and mixed samplesb
surface pore pore area volume diameter % modc (m2/g) (cm3/g) (Å) Brij 0 12.5 0 12.5 0 12.5 0 12.5 0 12.5 0 12.5 0 12.5
1180 1157 1071 1077 1056 1075 1004 1095 922 957 813 847 356 317
1.07 1.07 0.75 0.78 0.63 0.64 0.56 0.60 0.52 0.52 0.46 0.44 0.20 0.18
38 39 28 30 22 23 21 22
