Effects of Added Organosilanes on the Formation and Adsorption

D.C. 20375, and Center for. Bio Resource Development, George Mason University, Fairfax, Virginia 22030. Received March 31, 2000. In Final Form: Ma...
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Effects of Added Organosilanes on the Formation and Adsorption Properties of Silicates Surface-Imprinted with an Organophosphonate Michael A. Markowitz,*,† Gang Deng,‡ and Bruce P. Gaber† Laboratory for Molecular Interfacial Interactions, Code 6930, Center for Bio/Molecular Science and Engineering, Naval Research Laboratory, Washington, D.C. 20375, and Center for Bio Resource Development, George Mason University, Fairfax, Virginia 22030 Received March 31, 2000. In Final Form: May 17, 2000 Silica particles surface-imprinted with the soman hydrolysis product pinacolyl methylphosphonate (PMP) demonstrated binding selectivity for the hydrolysis product over other phosphonate mono- and diesters. Particle surfaces were imprinted during particle formation by adding pinacolyl methylphosphonate to the microemulsion formed from polyoxyethylene(5) nonylphenyl ether (NP-5), cyclohexane, ammoniated ethanol, and water. A number of trialkoxysilanes with terminal functional groups such as primary amine, quaternary amine, dihydroimidazole, and ethylpyridine were mixed with tetraethoxysilane (TEOS) during particle formation to enhance the binding and selectivity of the resulting surface-imprinted particles. The BET surface area of the silicates was dependent on the specific organosilane added during particle formation. The addition of primary amine- and dihydroimidazole-terminated organosilanes produced high-density, low-pore-volumes colloids while the addition of 2-ethylpyridine- and quatermnary amine-terminated organosilanes produced mesoporous silicates. When pinacolyl methylphosphonate was added during particle formation, a decrease in surface area was observed. Adsorption isotherms revealed that surface-imprinted silicates formed with added quaternary amine-terminated organosilanes had the highest adsorption capacity per gram of silicate. However, more imprinted sites per square meter were formed when the dihydroimidazoleterminated organosilane was used in particle synthesis. Silicates surface-imprinted with PMP were significantly more selective for PMP than for other structurally similar organophosphonates.

Introduction The formation of molecular recognition sites using molecular imprinting is an active field of research.1-3 This approach involves imprinting the shape and functionality of a compound into a metal oxide or polymer matrix. Using molecular imprinting techniques, polymers with selectivity for binding of molecules4-14 and catalysis15-20 have * To whom correspondence should be addressed. † Naval Research Laboratory. ‡ George Mason University. (1) Katz, A.; Davis, M. E. Macromolecules 1999, 32, 4113. (2) Haupt, K.; Mosbach, K. Trends Biotech. 1998, 16, 468. (3) Wulff, G. Angew. Chem., Intl. Ed. Engl. 1995, 34, 1812. (4) Saurez-Rodriguez, J. L.; Diaz-Garcia, M. E. Anal. Chim. Acta 2000, 405, 67. (5) Hart, B. R.; Rush, D. J.; Shea, K. J. J. Am. Chem. Soc. 2000, 122, 460. (6) Shi, H. Q.; Tsai, W.; Garrison, M. D.; Ferrari, S.; Ratner, B. D. Nature 1999, 398, 593. (7) Turkewitsch, P.; Wandelt, B.; Darling, G. D.; Powell, W. S. Anal. Chem. 1998, 70, 2025. (8) Wulff, G.; Schonfeld, R. Adv. Mater. 1998, 10, 952. (9) Yano, K.; Tanabe, K.; Takeuchi, T.; Matsui, J.; Ikebukuro, K.; Karube, I. Anal. Chim. Acta 1998, 363, 111. (10) Andersson, L. I.; Muller, R.; Vlatkis, G.; Mosbach, K. Proc. Natl. Acad. Sci., USA 1995, 92, 4788. (11) Spivak, D. A.; Shea, K. J. Macromolecules 1998, 31, 2160. (12) Vidyasanker, S.; Ru, M.; Arnold, F. A. J. Chromatogr. 1997, 775, 51. (13) Liu, X. C.; Mosbach, K. Macromol. Rapid Commun. 1997, 18, 609. (14) Kriz, D.; Ramstom, O.; Svensson, A.; Mosbach, K. Anal. Chem. 1995, 67, 2142. (15) Katz, A.; Davis, M. E. Nature 2000, 403, 286. (16) Ramstrom O.; Mosbach, K. Curr. Opin. Chem. Biol. 1999, 3, 759. (17) Wulff, G.; Gross, T.; Schonfeld, R. Angew. Chem., Int. Ed. Engl. 1997, 36, 1962. (18) Beach, J. V.; Shea, K. J. J. Am. Chem. Soc. 1994, 116, 379. (19) Matsui, J.; Nicholls, I. A.; Karube, I.; Mosbach, K. J. Org. Chem. 1996, 61, 5414. (20) Whitcombe, M. J.; Rodriguez-Villar, P.; Vulfson, E. N. J. Am. Chem. Soc. 1995, 117, 7105.

been formed. To be effective in wide-scale use, imprinted materials must have binding/active sites that are easily accessed by substrate molecules. Consequently, there is increasing interest in developing methods to molecularly imprint colloid surfaces. Surface-imprinting approaches that involve linking complementary hydrogen-bonding functionalized monomers to the imprint molecule through either covalent bonding or self-assembly and then attaching this complex to a silica surface have been reported.15, 21-23 Recently, we described a template-directed molecularimprinting method to form surface-imprinted silica particles that enantioselectively catalyze amide hydrolysis.24 This method uses a surfactant-imprint molecule to stamp silica particle surfaces with an imprint molecule as the particles are synthesized within a water-in-oil microemulsion. However, not all imprint molecules will be amenable to the synthetic chemistry required to produce their surfactant analogues. Therefore, we decided to investigate the efficacy of this surface-imprinting technique using non-surfactant-imprint molecules. Since there is significant interest in developing materials that effectively bind and/or degrade organophosphorus compounds because of their use as nerve agents or pesticides,25-27 we chose pinacolyl methylphosphonate (PMP), the hydrolysis product of the nerve agent soman, as the imprint molecule. Because PMP is water soluble, it will (21) Dai, S.; Burleigh, M. C.; Shin, Y.; Morrow, C. S.; Barnes, C. E.; Xue, Z. Angew. Chem., Int. Ed. 1999, 38, 1235. (22) Hwang, K.-O.; Sasaki, T. J. Mater. Sci. 1998, 8, 2153. (23) Shimada, T.; Kurazono, R.; Morihara, K. Bull. Chem. Soc. Jpn. 1993, 66, 836. (24) Markowitz, M. A.; Kust, P. R.; Deng, G.; Schoen, P. E.; Dordick, J. S.; Clark, D. S.; Gaber, B. P. Langmuir 2000, 16, 1759. (25) Yang, Y. C. Acc. Chem. Res. 1999, 32, 109. (26) Yoshida, M.; Uezu, K.; Goto, M.; Furusaki, S. Macromolecules 1999, 32, 1237. (27) Ember, L. Chem. Eng. News 1997, Sept 15, 26.

10.1021/la000477o CCC: $19.00 © 2000 American Chemical Society Published on Web 06/29/2000

Organosilanes/Silicates with Organophosphonate

be present primarily in the aqueous portion of the microemulsion. However, since no postimprinting particlegrinding processing will be used, only imprinted sites on the particle surface would be accessible to organophosphonate substrates. Key elements in the effectiveness of surface-imprinted binding and catalytic polymers or metal oxides are the efficiency of complementary binding interactions between polymer and imprint molecule functional groups and the effect of imprinting on the surface properties of the imprinted materials. The absence of complementary binding interactions greatly reduces the activity of the imprinted material.1-3 Similarly, the amount of surface area available for imprinting and the heterogeneity of the imprinted surfaces will have a profound impact on the performance of surface-imprinted materials, both of an instrinsic nature and as parts of sensor or catalytic devices. Increasingly, imprinted materials are being tailored to incorporate functional groups that optimize the complementary binding interactions between an imprint molecule and imprint matrix. One example of this approach involves the formation of molecularly imprinted polymers containing pendant phenylamidine groups.8,17 The resulting polymers exhibited improved esterase-like activity and adhesion properties. Molecularly imprinted polystyrene with pendant Eu3+-iminodiacetic acid groups demonstrated selective binding of PMP.28 Therefore, we decided to investigate the effect of a variety of amine functional groups, incorporated during imprinting, on the affinity and selectivity of surface-imprinted silica particles for organophosphonates and on the surface area of the imprinted silicate. Organotrialkoxysilanes with terminal primary amine, quaternary ammonium, dihydroimidazole, or pyridine functional groups were added during particle formation and imprinting. The effect of the added functionalized silanes and imprint molecule on particle morphology and surface area were examined using transmission electron microscopy and nitrogen adsorption-desorption isotherms. The adsorption properties of the functionalized silicates and the relative binding selectivity and affinity of the surface-imprinted particles toward various phosphonates were determined from adsorption isotherms. We chose to use 2-propanol as the solvent because we are interested in the adsorption of analytes from nonaqueous environments. Experimental Section Materials. Unless otherwise noted, all reagent grade chemicals were used as received and deionized water was used for imprinting and binding experiments. Polyoxyethylene(5) nonylphenyl ether (NP-5, Igepal CO-520), ethanol, 2-propanol (anhydrous), glacial acetic acid, l-ascorbic acid, ammonium heptamolybdate tetrahydrate, magnesium nitrate hexahydrate, sulfuric acid, hydrochloric acid, tetraethoxysilane (TEOS), pinacolyl methylphosphonate, isopropyl methylphosphonate, ethyl methylphosphonate, dibutyl butylphosphonate, dimethyl methyl phosphate, diphenyl methylphosphate, (2,6-dinitro-4-trifluoromethyl phenyl)phosphonic acid monobutyl ester, and (2-isopropylphenyl)phosphonic acid were obtained from Aldrich Chemical Co. (Milwaukee, WI). 3-Aminopropyl trimethoxysilane (APTMS), N-trimethoxysilylpropyl-N,N,N-trimethylammonium choride (TMACPTMS), 2-(trimethoxysilylethyl)pyridine (PETMS), and N-(3-triethoxypropyl)-4,5-dihydroimidazole (IPTES) were purchased from Gelest Inc. (Tullytown, PA). A saturated solution of ammonia in ethanol was prepared by passing ammonia gas into denatured ethanol at 20 °C for 5-6 h. UV/vis spectroscopy was performed with a Beckman DU-650 spectrophotometer. (28) Jenkins, A. L.; Uy, O. M.; Murray, G. M. Anal. Chem. 1999, 71, 373-378.

Langmuir, Vol. 16, No. 15, 2000 6149 Silica Particle Synthesis. Silica particles were prepared by stirring a mixture of 37 mL of saturated ammonia solution, 25 mL of ethanol, 0.5 g of Igepal CO-520, 13.5 mL of cyclohexane, and 1.44 mL of water for 30 min at room temperature and then adding 3.6 mL of TEOS. Imprinted silica particles, both with and without functionalized silanes, were prepared by mixing the functionalized silanes and imprinting molecule pinacolyl methylphosphonate in ethanol solution before the addition of TEOS. The mixtures were then stirred overnight at room temperature. The resulting dispersions were separated by centrifugation and then washed sequentially with the following solvents: 20 wt % water in ethanol (5 × 12 mL), acetic acid/ethanol/water (3/3/4, v/v/v) (5 × 12 mL), 20 wt % water in ethanol (5 × 12 mL), and ethanol (5 × 12 mL). The particles were then dried over vacuum for over 10 h at room temperature. With this method, we obtained 1 g of silica particles. Unstained, air-dried samples of the silicates on carbon-coated copper grids were examined by electron microscopy (Zeiss EM-10 transmission electron microscope) to determine silicate morphology. Nitrogen Gas Adsorption-Desorption Isotherms. Gas adsorption experiments were performed using a Micromeritics ASAP 2010 instrument.29 Nitrogen gas was used as the analysis adsorptive and the isotherms were measured at cryogenic temperature (77.4 K). BET surface areas were calculated from the linear part of the adsorption isotherms (i.e., P/Po ) 0.06-0.2) according to IUPAC recommendations.30 The average pore diameter was calculated based on 4V/A by BET and the singlepoint total pore volume was determined at P/Po ) 0.990. Adsorption Isotherm Measurements. One milliliter of pinacolyl methylphosphonate in anhydrous 2-propanol (0.1-10 mM) was added to a microcentrifuge tube containing 15 mg of silica particles. The mixture was shaken vigorously on a vortex mixer for 30 min. After centrifugation, an aliquot of the clear supernatant (5-700 µL) was taken and transferred into a glass test tube to determine the phosphorus content. Phosphorus analysis of supernatants taken as functions of vortex times and substrate concentrations confirmed that adsorption equilibrium was established within 20 min. The phosphorus analysis experiments were carried out as follows: 100 µL of 10 wt % Mg(NO3)2 was added into test tubes containing the supernatant. The test tubes were heated thoroughly over a gas torch to give a white residue. To this residue, 0.5 mL of 0.1 N hydrochloric acid was added and the tubes were shaken vigorously on a vortex mixer and then heated in a boiling water bath (100 °C) for 30 min. Next, 1.7 mL of colorimetric reagent containing one part of 10 wt % of l-ascorbic acid in water and six parts of 0.5 wt % of ammonium heptamolybdate tetrahydrate in 1.0 N sulfuric acid were added. The test tubes were incubated at 45 °C for 30 min and cooled to room temperature and the absorbance was measured at 820 nm. The phosphorus content was calculated based on the absorbance at 820 nm using a calibration curve determined with standard KH2PO4 solutions. Each data point was calculated based on the average of three trials. Selectivity Measurements. The organophosphorus compounds were added (1.0 mL of 1.0 mM stock solutions in 2-propanol) into separate microcentrifuge tubes containing 15 mg of functionalized silica particles imprinted with PMP. The resulting mixtures were shaken vigorously on a vortex mixer for 30 min. After centrifugation (14 000 rpm × 10 min), an aliquot of the clear supernatant (100 µL) was taken from each tube and transferred into separate glass test tubes. The phosphorus content of each sample was determined by phosphorus element analysis. Each data point was calculated based on the average of three trials.

Results and Discussion Effect of Added Functionalized Silanes and Imprinting on Particle Size and Surface Area. The formation properties of silicates have been examined in (29) Webb, P. A.; Orr, C. Analytical Methods in Fine Particle Technology; Micromeritics Instrument Corporation: 1997; Chapter 3. (30) Sing, K. S. W.; Evertt, D. H.; Haul, R. A. W.; Moscou, L.; Pierrotti, R. A.; Rouquerol, J.; Siemieniewska, T. Pure Appl. Chem. 1985, 57, 603.

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Figure 1. Transmission electron micrographs of nonimprinted silicates formed with (a) added APTMS (5% w/w total silica), (b) added IPTES (5% w/w total silica), (c) added PETMS (5% w/w total silica), and (d) added TMACPTMS (5% w/w total silica).

detail.31,32 Silicates formed under basic conditions comprise collapsed polymer clusters formed during the initial stages of alkoxysilane hydrolysis.33,34 These collapsed clusters form a gel network composed of silicate nanoparticles. During evaporation of the solvent used during synthesis, collapse of this gel network occurs to minimize surface tension.32 As the evaporation process occurs, the nanoparticles of the gel network may fuse and sufficiently strengthen the forming silicate against the compressing effects of surface tension, resulting in the formation of porous silicates. For high-density, low-pore-volume silicates, solvent evaporation occurs faster than particle crosslinking. The surface chemistry of the nanoparticles is the main factor determining how they pack as the solvent is evaporated and the extent of particle packing determines the ultimate surface area and porosity of the final silicate. The morphology and porosity of silicates formed with and without added functionalized trialkoxysilanes were characterized by transmission electron microscopy (Figure 1) and nitrogen gas adsortion-desorption isotherms (Figure 2 (Table 1)). Transmission electron microscopy of particles functionalized with 3-aminopropyl trimethoxysilane (APTMS) and N-(3-triethoxypropyl)-4,5-dihydroimidazole (IPTES) revealed that they were spherical (Figure 1a,b). Type II nitrogen gas adsorption-desorption isotherms characteristic of high-density, low-pore-volume particles were observed for these silicates (Figure 2a,b). Particle surface areas and pore volumes for the APTMS and IPTES functionalized particles were similar to those observed for particles formed without added organotri(31) Brinker, C.; Scherer, G. W. Sol-Gel Science; Academic Publishers: San Diego, CA, 1990. (32) Iler, R. K. The Chemistry of Silica; Wiley: New York, 1979. (33) Brinker, C. J.; Keefer, K. D.; Schaefer, D. W.; Assink, R. A.; Kay, B. D.; Ashley, C. S. J. Noncryst. Solids 1984, 63, 45. (34) Brinker, C. J.; Keefer, K. D.; Schaefer, D. W.; Ashley, C. S. J. Noncryst. Solids 1982, 48, 47.

Table 1. Surface Area and Porosity Data for Imprinted and Nonimprinted Silicates

silicate

BET surface area (m2 g-1)

pore volume (cm3/g)

nonimprinted PMP imprinted APTMS APTMS-PMP imprinted IPTES IPTES -PMP imprinted PETMS PETMS -PMP imprinted TMACPTMS TMACPTMS-PMP imprinted

15.41 5.49 33.04 26.90 13.03 6.16 325.31 103.78 285.36 224.31

0.028 0.009 0.044 0.021 0.024 0.010 0.429 0.064 0.69 0.66

alkoxysilanes. TEM revealed that materials formed using added N-trimethoxysilylpropyl-N,N,N-trimethylammonium choride (TMACPTMS) and 2-(trimethoxysilylethyl)pyridine (PETMS) appeared as fused, agglomerated nanoparticulates (Figure 1c,d). Nitrogen gas adsorptiondesorption isotherms for the quaternary ammonium and 2-ethylpyridine functionalized particles are shown in Figure 2c,d. These isotherms and the surface areas and pore volumes derived from them are similar to those obtained for G-xerogels.35 The average pore diameters of the PETMS and TMACPTMS silicates were 5.27 and 9.39 nm, respectively. The surface quaternary ammonium and 2-ethylpyridine groups of the initially formed nanoparticles clearly provide some counter force sufficient to slow the rate of gel network collapse, thereby enabling particle fusion to occur, resulting in the formation of porous silicates. The observed morphology and surface area of the PETMS silicate is likely due to the effects of the 2-ethylpyridine groups on particle surface energy since the most important factor (35) Kiselev, A. V. Discuss. Faraday Soc. 1971, 52, 14.

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Figure 2. Nitrogen gas adsorption-desorption isotherms of silicates formed with added organosilanes. Table 2. Maximum Adsorption Densities, Amax (µmol/g), and Equilibrium Adsorption Constants, K (mM-1), of Pinacolyl Methylphosphonate (PMP) Adsorption onto Imprinted and Nonimprinted Silica Particles in 2-Propanol as a Function of Silicate Mass silicatea

Amax (µmol/g)

K (mM-1)

nonimprinted PMP imprinted TMACPTMS TMACPTMS-PMP imprintedb PETMS PETMS-PMP imprintedb IPTES IPTES-PMP imprintedb APTMS APTMS-PMP imprintedb

4.17 ( 0.23 6.33 ( 0.68 45.10 ( 3.05 91.33 ( 4.20 5.43 ( 0.78 26.91 ( 1.16 12.81 ( 1.58 26.27 ( 2.03 4.81 ( 2.87 4.97 ( 0.43

0.18 ( 0.02 0.39 ( 0.09 3.96 ( 0.97 11.52 ( 1.66 0.37 ( 0.14 1.95 ( 0.30 2.09 ( 0.89 2.88 ( 0.89 2.11 ( 0.61 1.35 ( 0.40

a Functionalized particles synthesized with 5 mol % (total silica) added organotrialkoxysilane. b Silicates surface-imprinted with 20 mol % (total silica) of PMP.

Figure 3. Time dependence of pinacolyl methylphosphonate (PMP) adsorption on PMP imprinted and nonimprinted silicates in 2-propanol. Adsorption (%) refers to the amount of PMP in solutuion that was adsorbed by the IPTES silicate.

contributing to gel collapse is the capillary pressure that arises as evaporation occurs.36 As evaporation progresses, fewer and fewer water molecules are available to solvate the hydrophilic surface of the solid phase, leading to a continual increase in surface tension eventually resulting in gel collapse. Consequently, any surface functionalities that would lower the surface energy of the gel solid phase will reduce the rate of gel collapse, allowing particle crosslinking, leading to the formation of higher surface area porous silicates. For instance, the surface area of silicates (36) Scherer, G. W. In Better Ceramics Through Chemistry III; Brinker, C. J., Clark, D. E., Ulrich, D. R., Eds.; Materials Research Society: Pittsburgh, PA, 1988; pp 179-186.

formed with methyltriethoxysilane increases as the amount of added organosilane increases.37-39 This effect has been attributed to a reduction in surface tension within the gel due to the presence of methyl groups at the surfaces of the gel nanoparticles. As a result, complete collapse of the gel network is prevented during solvent evaporation. Hydroxylated silicon surfaces with chemisorbed 2-[2(trimethoxysilyl)ethyl]pyridine films have a sessile water drop contact angle of 50°, indicating that the 2-ethylpyridine groups have significantly reduced the surface energy (37) Fahrenholtz, W. G.; Smith, D. M. In Better Ceramics Through Chemistry V; Hampden-Smith, M. J., Klemperer, W. G., Brinker, C. J., Eds.; Materials Research Society: Pittsburgh, PA, 1992; p 705. (38) Schwertfegger, F.; Glaubitt, W.; Schubert, U. J. Non-Cryst. Solids 1992, 145, 85. (39) De Witte, B. M.; Commers, D.; Uytterhoeven, J. B. J. Non-Cryst. Solids 1996, 202, 35.

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Figure 4. Langmuir adsorption isotherms for the adsorption of pinacolyl methylphosphonate (PMP) on imprinted and nonimprinted silicates in 2-propanol: (a) TMACPTMS surface-imprinted with PMP (b), nonimprinted TMACPTMS (O), plain surface-imprinted with PMP (]), nonimprinted plain ([); (b) APTMS surface-imprinted with PMP (1), nonimprinted APTMS (3), plain surfaceimprinted with PMP (]), nonimprinted plain ([); (c) PETMS surface-imprinted with PMP (2), nonimprinted PETMS (4), plain surface-imprinted with PMP (]), nonimprinted plain ([); (d) IPTES surface-imprinted with PMP (9), nonimprinted IPTES (0), plain surface-imprinted with PMP (]), nonimprinted plain ([). Functionalized silicates were formed with 5 wt % (total silica) of the added organosilane. Surface-imprinted silicates were formed with 20 mol % of added PMP. Table 3. Maximum Adsorption Densities, Amax (µmol/g), and Equilibrium Adsorption Constants, K (mM-1), of Pinacolyl Methylphosphonate (PMP) Adsorption onto Imprinted and Nonimprinted Silica Particles in 2-Propanol as a Function of Silicate Surface Area

Figure 5. Pinacolyl methylphosphonate (PMP) adsorption on silicates formed with added TMACPTMS (5 wt % of total silica) and imprinted with varying amounts of PMP. PMP substrate concentration ) 5 mM in 2-propanol.

of the native silicon surfaces.40 It may be that the surface 2-ethylpyridine groups of the network nanoparticles are hydrophobic enough to reduce the surface tension of the collapsing network, thereby slowing the rate of gel collapse relative to that of particle fusion. Similarly, the xerogel morphology of the N-trimethoxysilylpropyl-N,N,N-trimethylammonium choride silicate (TMACPTMS) may arise from a reduction of the surface energy of the gel solid phase due to the presence of the methyl groups of the surface quaternary ammonium groups. This explanation is supported by the effects on (40) Dressick, W. J.; Dulcey, C. S.; Georger, J. H., Jr.; Calvert, J. M. Chem. Mater. 1993, 5, 148.

silicatea

Amax (µmol/m2)

K (mM-1)

nonimprinted PMP imprinted TMACPTMS TMACPTMS-PMP imprintedb PETMS PETMS-PMP imprintedb IPTES IPTES-PMP imprintedb APTMS APTMS-PMP imprintedb

0.27 ( 0.02 1.05 ( 0.11 0.16 ( 0.01 0.41 ( 0.02 0.02 ( 0.01 0.26 ( 0.01 0.98 ( 0.12 4.26 ( 0.33 0.15 ( 0.09 0.19 ( 0.02

0.18 ( 0.02 0.39 ( 0.09 3.96 ( 0.97 11.52 ( 1.66 0.37 ( 0.14 1.95 ( 0.30 2.09 ( 0.89 2.88 ( 0.89 2.11 ( 0.61 0.40

a Functionalized particles synthesized with 5 mol % (total silica) added organotrialkoxysilane. b Silicates surface-imprinted with 20 mol % (total silica) of PMP.

the silicate surface area of adding N-trimethoxysilylpropylN,N,N-tributylammonium choride (TBACPTMS, 5 wt % total silica) during silicate synthesis instead of TMACPTMS. The surface area of the TBACPTMS silicate was 579.34 m2/g, considerably larger than that of the TMACPTMS silicate. The increase in surface area clearly derives from the increased hydrophobicity of the propyl-N,N,Ntributylammoniun groups. In addition, repulsive interactions between particles with positively charged surface quaternary ammonium groups may contribute to the xerogel morphology of the TMACPTMS silicate. The mesoporous network of fused particles may have formed in part to minimize contact between neighboring particles as the solvent evaporated. In contrast, the formation of high-density, low surface area particles with added APTMS or IPTES indicate that

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Figure 6. Pinacolyl methylphosphonate PMP adsorption from 2-propanol on (a) nonimprinted APTMS (O), APTMS surfaceimprinted with PMP (b), surface-imprinted plain (2), and nonimprinted plain (4) silica particles; (b) nonimprinted PETMS (O), surface-imprinted PETMS (b), surface-imprinted plain (2), and nonimprinted plain (4) silica particles; (c) nonimprinted TMACPTMS (O), surface-imprinted TMACPTMS (b), surface-imprinted plain (2), and nonimprinted plain (4) silica particles; (d) nonimprinted IPTES (O), surface-imprinted IPTES (b), surface-imprinted plain (2), and nonimprinted plain (4) silicates. Functionalized silicates were formed with 5 wt % (total silica) of the added organosilane. Surface-imprinted silicates were formed with 20 mol % of added PMP.

Figure 7. Structures of organophosphorus compounds used to determine relative affinity of analytes for nonimprinted and surfaceimprinted silicates.

the primary amine and dihydroimidazole groups of the added organosilanes did not provide any chemical barrier to gel network collapse. The dihydroimidazole and amine groups of IPTES and APTMS are stronger bases than the 2-ethylpyridine group, and therefore, they are less likely to reduce the solid-phase surface tension of the gel. Inclusion of the imprint molecule pinacolyl methylphosphonate (PMP) during particle formation resulted in silica particles with lower surface areas and pore volumes (Table 1). Even very small amounts of the added imprint molecule resulted in a reduction of surface area

and porosity for silicates formed with added N-trimethoxysilylpropyl-N,N,N-trimethylammonium choride (TMACPTMS). The surface area of the TMACPTMS silicate decreased from 285.36 to 269.48 m2/g when 1 mol % (total silica) of PMP was added during silicate formation. Increasing the amount of added PMP to 5 and 10 mol % resulted in further reductions in surface area to 239.67 and 225.5 m2/g, respectively. No further reduction in surface area was observed when the amount of added PMP was increased to 20 mol % of total silica (224.31 m2/g). These results indicate that the added imprint molecule

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Table 4. Percentage of Organophosphorus Substrate Solutions Bound to Imprinted and Nonimprinted Silica Particles silicate

EMPa

IPMPb

PMPc

DEMPd

MPTHe

nonfunctionalized nonfunctionalized-PMP imprinted TMACPTMS TMACPTMS-PMP imprinted PETMS PETMS-PMP imprinted IPTES IPTES-PMP

4.06 ( 3.06 2.44 ( 2.74 62.38 ( 2.64 65.55 ( 4.74 9.78 ( 3.62 13.03 ( 3.06 11.16 ( 2.88 17.78 ( 3.06

6.00 ( 3.18 1.00 ( 1.07 59.36 ( 2.42 72.29 ( 1.17 11.65 ( 6.18 14.50 ( 2.27 8.71 ( 3.22 16.48 ( 3.25

1.54 ( 1.36 5.21 ( 4.72 56.51 ( 2.16 81.89 ( 1.17 9.43 ( 3.53 20.21 ( 2.35 8.51 ( 1.87 22.36 ( 1.36

2.91 ( 3.09 1.47 ( 1.05 4.40 ( 2.74 3.01 ( 2.52 1.71 ( 2.30 0.53 ( 2.91 0.51 ( 2.44 0.56 ( 2.21

0.21 ( 1.36 4.50 ( 2.70 0.64 ( 3.36 6.05 ( 5.83 12.03 ( 3.04 10.34 ( 3.10 21.86 ( 5.60 23.77 ( 4.65

a EMP is ethyl methylphosphonate. b IPMP is isopropyl methylphosphonate. c PMP is pinacolyl methylphosphonate. methylphosphonate. e MPTH is methyl parathion.

disrupts cluster cross-linking that occurs during solvent removal, leading to faster collapse of the silicate polymer clusters. Except for the 2-ethylpyridine-functionalized silicate (PETMS), the morphology of the silicates formed with added PMP did not change. The pore size of the mesoporous TMACPTMS silicate increased from 9.39 to 11.42 nm when 20 mol % (total silica) of PMP was added during synthesis. For the PETMS silicate formed with PMP, the reduction in surface area was accompanied by a change in morphology from clusters of fused nanoparticles to larger spherical particles similar to those for the IPTES and APTMS silicates. In addition, the pore volume of the PETMS silicate is similar to the pore volumes for the IPTES and APTES silicates. Effects of Added Organotrialkoxysilanes and Imprinting on the Adsorption Properties of SurfaceImprinted Silica Particles. The effects of added functionalized silanes on the interactions of PMP with silica particles, with and without imprinting, were investigated using adsorption isotherms. 2-Propanol was chosen as the solvent to study the adsorption behavior in a nonaqueous medium. The time dependence of PMP adsorption on the silicates in 2-propanol was measured to determine when adsorption equilibrium was achieved. Data for the dihydroimidazole-functionalized silicate (IPTES) is shown in Figure 3. These data are representative of the timedependent PMP adsorption by all of the silicates. In each case, adsorption equilibrium was reached within 30 min for all of the silicates (PMP concentration ) 10 mM). Therefore, for each sample, PMP and silica particles were equilibrated for at least 45 min prior to centrifugation of the PMP/particles mixture and phosphate analysis of the supernatant. Adsorption isotherms for PMP binding to functionalized and nonfunctionalized silica particles are shown in Figure 4a,b. Each isotherm was fitted to the single-component Langmuir equation,

As ) AmaxKc/(1 + Kc) where As is the concentration of adsorbed PMP, Amax is the maximum adsorption density, c is the equilibrium concentration of PMP in solution, and K is the equilibrium constant for the adsorption of PMP to the silicate. The isotherms were analyzed as functions of the amount of particles used in each experiment and particle surface areas (Table 2). Not surprisingly, the interactions between PMP and the functionalized silica particles were considerably stronger than the interactions between PMP and nonfunctionalized silica particles, with the strongest adsorption occurring with the quaternary ammoniumfunctionalized particles (TMACPTMS). The nonimprinted quaternary ammonium-functionalized particles had much larger adsorption densities than the other particles based on the amount of silica particles used in each experiment. With the exception of the primary amine-functionalized particles (APTES), surface imprinting produced a sig-

d

DEMP is diethyl

nificant increase in adsorption capacity. The effect of surface imprinting on adsorption capacity is evident from the linear increase in adsorption capacity of TMACPTMS silicates formed with increasing amounts of PMP (Figure 5). This is a particularly striking result since the addition of the imprint molecule during synthesis serves to reduce the silicate surface area available for PMP adsorption and indicates that surface imprinting increases the number of effective binding sites on the silica particles. The increase in adsorption capacity per gram of silicate was dependent on the type of organotrialkoxysilane used in silicate synthesis. As a function of silicate mass, the combination of surface imprinting and added quaternary amine-, 2-ethylpyridine-, and dihydroimidazole-terminated organosilanes had significant effects on PMP adsorption. Since both the added organotrialkoxysilanes and imprinting had distinct effects on silicate surface areas, the adsorption data were also plotted as a function of particle surface area (Figure 6) and compared directly to PMP adsorption onto nonfunctionalized silica particles. Adsorption densities and equilibrium adsorption constants were determined from these curves as a function of silicate surface area using the single-component Langmuir equation (Table 3). It was clear from these data that increases in capacity observed as a function of silicate mass for the 2-ethylpyridine- (PETMS) and quaternary ammoniumfunctionalized (TMACPTMS) silicates were largely due to increases in surface areas resulting from the addition of the organosilanes during particle synthesis rather than some intrinsic chemical property of the functional group. In fact, nonfunctionalized surface-imprinted particles had from 2.5 to 4 times greater adsorption capacity than the surface-imprinted PETMS, and TMACPTMS-functionalized silicates. As a function of surface area, the silica particles formed with added dihydroimidazole-terminated organosilane had the largest adsorption capacity for both surface-imprinted and nonimprinted silicates. The dihydroimidazole-functionalized (IPTES) surface-imprinted particles had 4 times greater adsorption capacity than the nonfunctionalized surface-imprinted particles and 10 and 16 times greater adsorption capacity than the TMACPTMS and PETMS surface-imprinted particles, respectively. These data demonstrate that more binding sites per square meter are formed when the dihydroimidazoleterminated organosilane is present during silicate formation than when the other organosilanes are added during synthesis. Since the addition of the PMP during synthesis resulted in TMACPTMS silicates with increased pore diameters, it is unlikely that the relatively smaller adsorption capacity of this silicate results from PMP adsorption at the pore openings, restricting diffusion of additional PMP to unbound imprinted sites. Affinity of Organophosphorus Substrates for Surface-Imprinted Silicates. The differences in adsorption of organophosphate compounds to surface-

Organosilanes/Silicates with Organophosphonate

imprinted silicates and nonimprinted silicates were examined. Specifically, the amounts of ethyl methylphosphonate (EMP), isopropyl methylphosphonate (IPMP), pinacolyl methylphosphonate (PMP), diethyl methylphosphonate (DEMP), and methyl parathion (MPTH) adsorbed from separate 2-propanol solutions onto nonimprinted surfaces and surfaces imprinted with PMP were determined. EMP, IPMP, and PMP are structurally similar phosphonate monoesters while DEMP is a phosphonate diester and MPTH is a thiophosphate (Figure 7). The percentages of the substrates adsorbed from solution are given in Table 4. As expected, silicates with surface quaternary ammmonium groups most strongly adsorbed the phosphonate monoesters. The interactions of the quaternary ammonium-functionalized silicates (TMACPTMS) and phosphonate monoesters are electrostatic in nature while interactions between these substrates and the 2-ethylpyridine- and dihydroimidazolefunctionalized silicates (PETMS and IPTES, respectively) are based on hydrogen bonding. No substrate structural effects on the adsorbance of EMP, IPMP, and PMP to the nonimprinted, functionalized silicates were observed. Essentially no interactions between the TMACPTMS silicates and the phosphonate diester or the thiophosphate were observed. Both the 2-ethylpyridine- and dihydroimidazole-functionalized silicates had greater affinity for the thiophosphate MPTH than the quaternary ammoniumfunctionalized silicates. No enhancement of MPTH adsorption to the surface-imprinted PETMS and IPTES silicates was observed. Even though the structural differences among the phosphonate monoesters are small, significant differences in the affinity of the functionalized surface-imprinted silicates for these compounds were observed. The surfaceimprinted sites of the 2-ethylpyridine-functionalized silicate (PETMS) were the most specific exhibiting enhanced binding for only PMP. The surface-imprinted sites of the quaternary ammonium-functionalized silicate (TMACPTMS) were somewhat less specific. This silicate had enhanced affinity for both IPMP and PMP, although the relative affinity for PMP was significantly higher. The higher level of specific and nonspecific binding observed for the TMACPTMS silicates can be attributed to the combined effects of the added quaternary ammoniumterminated trialkoxysilane on silicate surface area and the relative strength of the binding interactions with the organophosphonate monoesters. The surface-imprinted dihydroimidazolee-functionalized silicate (IPTES) exhibited enhanced affinity for all three phosphonate monoesters to some degree. However, the surface-imprinted

Langmuir, Vol. 16, No. 15, 2000 6155

sites exhibited significantly more affinity for PMP than for IPMP and EMP. The hydrogen-bonding IPTES silicates interact less strongly with the organophosphonate monoesters and had substantially lower surface areas than the TMACPTMS silicates. The relative specificity of the surface-imprinted sites for the phosphonate monoesters was similar to those for the IPTES and TMACPTMS silicates. The 2-ethylpyridine-functionalized silicates (PETMS) exhibited nonspecific binding comparable to the dihydroimidazole-functionalized silicates but the surfaceimprinted sites of the PETMS silicate exhibited greater specificity for PMP. Summary Surface-imprinted, functionalized silicates with specificity for the imprint molecule were prepared in a one-pot procedure and characterized. The formation and adsorption properties of functionalized silicates were dependent on both the terminal functional group of the organosilane added during silicate synthesis and the presence or absence of the imprint molecule pinacolyl methylphosphonate as the silicate formed. Silicates formed with added quaternary amine- and 2-ethylpyridine-terminated organosilanes were mesoporous while synthesis with added primary amine- and dihydroimidazole-terminated organosilanes yielded high-density, low-pore-volume particles. Addition of the imprint molecule during synthesis resulted in silicates with lower surface areas than the nonimprinted silicates. After surface-imprinting, only the quaternary amine-functionalized silicate was mesoporous. Surfaceimprinting improved the adsorption capacity of the quaternary amine-, pyridine-, and dihydroimidazolefunctionalized silicates. The adsorption capacity of the quaternary amine-functionalized silicate increased linearly with the amount of imprint molecule added (0-20 mM range) during silicate formation. Surface-imprinted particles formed with added dihydroimidazole- and 2-ethylpyridine-terminated organosilanes had the best ratio of specific to nonspecific binding. Surface-imprinted quaternary amine-, 2-ethylpyridine-, and dihydroimidazolefunctionalized silicates had a significantly higher degree of specificity for pinacolyl methylphosphonate than for structurally similar organophosphonates. Acknowledgment. We thank the Joint Service CB Defense Technology Technology Base Program for providing funding for this research. Additional funding was provided by the Office of Naval Research through a Naval Research Laboratory Accelerated Research Initiative. LA000477O