Free-Standing Silica Colloidal Nanoporous Membranes - Langmuir

Feb 4, 2009 - We prepared robust free-standing 200 μm-thick colloidal ... flux through these membranes is of the order of 3.6 × 10−10 mol s−1 cm...
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Langmuir 2009, 25, 3096-3101

Free-Standing Silica Colloidal Nanoporous Membranes Andrew K. Bohaty,† Joanna J. Smith, and Ilya Zharov* Department of Chemistry, UniVersity of Utah, Salt Lake City, Utah 84112 ReceiVed June 18, 2008. ReVised Manuscript ReceiVed December 17, 2008 We prepared robust free-standing 200 µm-thick colloidal membranes (nanofrits) with a relatively large area and no mechanical defects by sintering silica colloidal films. The silica spheres used to prepare the nanofrits were 338, 300, or 251 nm in diameter, leading to 25, 22.5, and 19 nm nanopore sizes, respectively. The room-temperature diffusional flux through these membranes is of the order of 3.6 × 10-10 mol s-1 cm-2 for a Fe(bpy)32+ ion in acetonitrile test solution in the absence of applied pressure and is in good agreement with the calculated diffusional flux for colloidal crystals of the same thickness. To evaluate the feasibility of nanofrit surface modification, we treated them with 3-aminopropyltriethoxysilane after rehydroxylation. We found, by measuring the surface coverage for dansyl amide on the surface, that the number of the amines on the nanofrit surface is lower as compared to that observed for colloidal films not treated with heat. As a result, the selectivity for the transport of Fe(bpy)32+ through the aminated nanofrits in the presence of acid is lower than the selectivity observed for amine-modified colloidal films.

Introduction Nanoporous membranes are important in fundamental studies of molecular transport and molecule-surface interactions in confined spaces,1 as well as in applications such as separations2,3 and sensing.4 Nanoporous membranes have been prepared using a variety of methods including lithography,5 anodic oxidation of aluminum films,6 track etching of polymers,7 surfactant-directed self-assembly,8 and by templating colloidal crystals.9,10 Novel molecular square membranes have been developed using transition-metal corners and difunctional bridging ligands as edges to feature well-defined nanosized cavities11 and demonstrated size-selective transport of various probe molecules through these novel molecular square membranes.11 A 2.6 nm thin membrane was synthesized from a polymerized calix[6]arene monolayer on a polymer support and has shown gas permeation selectivity.12 Membranes composed of an aligned array of open-ended carbon nanotubes have shown diffusive transport of aqueous ionic transport through carbon nanotube cores, in which the transport through the membrane can be enhanced by introducing charged functional groups at the carbon nanotube openings.13 A new class of polymeric materials for nanofiltration membranes has been developed using lyotropic liquid-crystal (LLC) assem* Corresponding author. E-mail: [email protected]. † Present address: Department of Chemistry, Carnegie-Mellon University. (1) Tanev, P. T.; Butruille, J.-R.; Pinnavaia, T. J. In Chemistry of AdVanced Materials; Interrante, L. V., Hampden-Smith, M. J., Eds.; Wiley-VCH: New York, 1998; p 329. (2) Davis, M. E. Nature 2002, 417, 813. (3) Artu, G.; Roosmasari, I.; Richau, K.; Hapke, J. Sep. Sci. Technol. 2007, 42, 2947–2986. (4) Bayley, H.; Martin, C. R. Chem. ReV. 2000, 100, 2575. (5) Tong, H. D.; Jansen, H. V.; Gadgil, V. J.; Bostan, C. G.; Berenschot, C. G. E.; van Rijn, C. J. M.; Elwenspoek, M. Nano Lett. 2004, 4, 283. (6) Toh, C.-S.; Kayes, B. M.; Nemanick, E. J.; Lewis, N. S. Nano Lett. 2004, 5, 767. (7) Yoshida, M.; Asano, M.; Suwa, T.; Reber, N.; Spohr, R.; Katakai, R. AdV. Mater. 1997, 9, 757. (8) Liu, N. G.; Dunphy, D. R.; Atanassov, P.; Bunge, S. D.; Chen, Z.; Lopez, G. P.; Boyle, T. J.; Brinker, C. J. Nano Lett. 2004, 4, 551. (9) Xu, H.; Goedel, W. A. Angew. Chem., Int. Ed. 2003, 42, 4694. (10) Xu, H.; Goedel, W. A. Langmuir 2002, 18, 2363. (11) Czaplewski, K. F.; Hupp, J. T.; Snurr, R. Q. AdV. Mater. 2001, 13, 1895– 1897. (12) Yan, X.; Janout, V.; Hsu, J. T.; Regen, S. T. J. Am. Chem. Soc. 2002, 124, 10962–10963. (13) Majumder, M.; Keis, K.; Zhan, S.; Meadows, C.; Cole, J.; Hinds, B. J. J. Membr. Sci. 2008, 316, 89–96.

blies,14,15 which possess ordered nanometer-scale aqueous domains and can be used to affect gas solubility and diffusivity through the materials.15 Several different zeolite membranes have been prepared and investigated for separations based on differences in molecular size, adsorption, and diffusion properties.16,17 Self-assembly of block copolymers has also been used to prepare nanoporous membranes.18,19 Many of the aforementioned techniques are quite demanding. Colloidal crystals, on the other hand, form via self-assembly of silica nanospheres into a close-packed face-centered cubic (fcc) lattice20 and contain ordered arrays of three-dimensional interconnected pores. The nanopore size can be easily controlled in the 5-100 nm range by changing the silica nanosphere diameter, while the diffusive flux of small molecules normal to the (111) plane of an infinitely thick colloidal crystal is only ca. 10 times smaller relative to the free solution, independent of the size of the nanopores used.21 Thus, colloidal crystals constitute promising candidates for nanoporous membranes. Recently, we demonstrated that silica colloidal crystals can be suspended across openings in silicon wafers.22 However, those membranes were small (∼7 × 10-4 cm2) and thus not particularly suitable for preparative scale separations, and they eventually fell out of the wafer openings. To avoid these drawbacks, we decided to develop free-standing colloidal membranes (nanofrits), which would be mechanically robust and would possess a larger surface area. To prepare such membranes, the silica spheres used to create the colloidal membrane need to be physically bonded together. One way to accomplish this task is by sintering, which causes the silica spheres (14) Zhou, M.; Kidd, T. J.; Noble, R. D.; Gin, D. L. AdV. Mater. 2005, 17, 1850–1853. (15) Gin, D. L.; Bara, J. E.; Noble, R. D.; Elliott, B. J. Macromol. Rapid Commum. 2008, 29, 367–389. (16) Bowen, T. C.; Noble, R. D.; Falconer, J. L. J. Membr. Sci. 2004, 245, 1–33. (17) Arruebo, M.; Falconer, J. L.; Noble, R. D. J. Membr. Sci. 2006, 269, 171–176. (18) Thurn-Albrecht, T.; Schotter, J.; Kastle, C. A.; Emley, N.; Shibauchi, T.; Krusin-Elbaum, L.; Guarini, K.; Black, C. T.; Tuominen, M. T.; Russell, T. P. Science 2000, 290, 2126. (19) Liu, G.; Ding, J. AdV. Mater. 1998, 10, 69–71. (20) Wong, S.; Kitaev, V.; Ozin, G. A. J. Am. Chem. Soc. 2003, 125, 15589. (21) Newton, M. R.; Morey, K. A.; Zhang, Y.; Snow, R. J.; Diwekar, M.; Shi, J.; White, H. S. Nano Lett. 2004, 4, 875. (22) Bohaty, A. K.; Zharov, I. Langmuir 2006, 22, 5533–5536.

10.1021/la801922a CCC: $40.75  2009 American Chemical Society Published on Web 02/04/2009

Free-Standing Silica Colloidal Nanoporous Membranes

to fuse to one another.23 This can be achieved by heating at a temperature >1000 °C, which causes silica to flow at the surface fusing the spheres together. Sintered colloidal films on solid support have been recently used as a media for in-plane bioseparations.24,25 In the present article, we describe the preparation of novel self-assembled nanoporous membranes, robust free-standing nanofrits comprised of sintered silica spheres ranging in size from 250 to 340 nm. The molecular fluxes through these membranes were measured and compared to calculated fluxes to demonstrate the absence of mechanical defects. To evaluate the feasibility of nanofrit surface modification, the surface of the membranes was modified with an aminecontaining silane, and the transport of positively charged ions through the aminated nanofrits was studied.

Experimental Section Materials. Butylamine (99%, Aldrich), dansyl chloride (99%, Aldrich), 3-aminopropyltriethoxysilane, APTES (98%, Aldrich), 2,2′dipyridyl (99%, Aldrich), ammonium iron(II) sulfate hexahydrate (99%, Aldrich), ammonium hexafluorophosphate (99.5%, Acros), ammonium hydroxide, NH4OH (30%, Mallinckrodt), tetrabutylammonium hydroxide (99%, Aldrich), trifluoroacetic acid (99%, Aldrich), and tetraethyl orthosilicate, TEOS (99.9%, Alfa Aesar) were all used as received. The 18 MΩ cm water used in all experiments was obtained from a Barnstead “E-pure” water purification system. All ethanol used was 200 proof. Acetonitrile (HPLC grade, Mallinckrodt), triethylamine, TEA (100%, J. T. Baker), and dichloromethane were freshly distilled from calcium hydride. Iron trispipyridine hexafluorophosphate was synthesized according to the literature procedures,26,27 as well as dansyl derivative 1.28,29 Column chromatography was carried out using silica gel (Silicycle) 60 Å, 230-400 mesh under slight pressure. TLC was performed using aluminum foil plates coated with 0.1 mm Merck silica gel 60 F254. TLC plates were visualized using UV or potassium permanganate. NMR spectra were recorded using a Varian VXL-300 MHz spectrometer in CDCl3 at 300 MHz (1H NMR) or 75 MHz (13C NMR) using CDCl3 as the solvent. 1H NMR spectra were referenced to residual CHCl3 (δ 7.27 ppm), and 13C NMR spectra were referenced to CHCl3 (δ 77.23 ppm). Scanning electron microscopy (SEM) images were obtained using a Hitachi S3000N instrument. A Branson 1510 sonicator was used for all sonications. UV/vis measurements were performed using an Ocean Optics USB2000 or USB4000 instrument.

Preparation of Silica Spheres. All silica spheres were prepared according to the previously reported procedures.30-32 All glassware (23) Chabanov, A. A.; Jun, Y.; Norris, D. J. Appl. Phys. Lett. 2004, 84, 3573– 3575. (24) Le, T. V.; Ross, E. E.; Velarde, T. R. C.; Legg, M. A.; Wirth, M. J. Langmuir 2007, 23, 8554–8559. (25) Zheng, S.; Ross, E.; Legg, M. A.; Wirth, M. J. J. Am. Chem. Soc. 2006, 128, 9016–9017. (26) Sarkar, D.; Subbarao, P. V.; Begum, G.; Ramakrishna, K. J. Colloid Interface Sci. 2005, 288, 591–596. (27) Braga, T. G.; Wahl, A. C. J. Phys. Chem. 1985, 89, 5822–5828. (28) Gennaro, M. C.; Mentasti, E.; Sarzanini, C.; Porta, V. Chromatographia 1988, 25, 117–124. (29) Summers, W. A.; Lee, J. Y.; Burr, J. G. J. Org. Chem. 1975, 40, 1559– 1561. (30) Bogush, G. H.; Tracy, M. A.; Zukoski, C. F., IV J. Non-Cryst. Solids 1988, 104, 95–106. (31) Sto¨ber, W.; Fink, A.; Bohn, E. J. Colloid Interface Sci. 1968, 26, 62–69. (32) Wang, W.; Gu, B.; Liang, L.; Hamilton, W. J. Phys. Chem. B 2003, 107, 3400–3404.

Langmuir, Vol. 25, No. 5, 2009 3097 was cleaned with 18 MΩ cm water and Alconox, followed by a thorough rinse with 18 MΩ cm water to remove any residual detergent left on the glassware. TEOS (48 g, 0.20 mol) was placed in a 500 mL volumetric flask, and absolute ethanol was added to obtain 500 mL of 0.20 M solution. NH4OH (70 mL, 1.1 mol) and water (257 g, 14.3 mol) were placed into another 500 mL volumetric flask, and absolute ethanol was added to obtain 500 mL of 1.1 M ammonia and 17.0 M water solution. These two solutions were simultaneously poured into a 2 L Erlenmeyer flask and vigorously stirred for 24 h. After about 30 min of being stirred, the solution became cloudy, indicating silica sphere formation. After 24 h, the silica spheres were centrifuged in 15 mL centrifuge tubes (Corning) at 1163g for 15 min. After all of the spheres were collected as pellets at the bottom of the centrifuge tubes, the supernatant was decanted, and the silica spheres were purified by suspending the spheres in 10 mL of ethanol by sonication for 30 min, during which the tubes were periodically shaken by hand to free any pieces of the pellet stuck to the sides of the tubes. The colloidal suspension in the centrifuge tubes was then centrifuged for 15 min at 1163g. The supernatant was decanted, and the purification steps were repeated five more times. After the final centrifugation, the supernatant was decanted, and the silica spheres were dried in a stream of nitrogen for 2 h. The dried silica spheres were then transferred to a scintillation vial and placed in an oven at 120 °C for 24 h. SEM images were taken of the silica spheres, and the size was determined from 100 individually measured silica spheres to be 363 ( 16 nm. The silica spheres were calcinated by placing them in a Petri dish and breaking any large aggregates with a spatula. The Petri dish with the silica spheres was placed in a furnace programmed to heat the spheres for 4 h at 600 °C.23-25 The furnace was heated at a rate of 20 °C a minute. After 4 h, the furnace was turned off, and the spheres were allowed to cool to room temperature while in the furnace. SEM images of the spheres were obtained, and 100 individual silica spheres were measured to determine the average size of 338 ( 15 nm. Silica spheres 300 ( 20 nm in diameter were prepared and calcinated following the above procedure but using different concentrations of the reagents, that is, TEOS (48 g, 0.20 mol, 0.20 M final concentration), NH4OH (38 mL, 0.60 mol of NH3, 0.60 M NH4OH final concentration), and water (280 g, 15.5 mol, 17.0 M final concentration). Silica spheres 251 ( 21 nm in diameter were prepared and dried following the above procedure with the following reagent concentrations: TEOS (48 g, 0.20 mol, 0.20 M final concentration), NH4OH (25 mL, 0.40 mol of NH3, 0.40 M NH4OH final concentration), and water (288 g, 16 mol, 17 M). Surface Modification of Silica Spheres. Amines were attached to the surface of the dried silica spheres as follows. APTES (0.20 mL, 0.85 mmol), silica spheres (0.50 g), and 15 mL of dry acetonitrile were placed in a 20 mL scintillation vial under the atmosphere of nitrogen. The vial was capped and wrapped with Parafilm. The solution was stirred for 17 h. The colloidal solution was then centrifuged at 1163g for 15 min in 15 mL centrifuge tubes (Corning). The silica spheres formed pellets at the bottom of the tubes, and the supernatant was removed. The silica spheres were purified by sonicating the centrifuge tubes containing the silica spheres and 10 mL of fresh acetonitrile for 30 min to disperse the spheres into acetonitrile. The dispersed silica spheres were then centrifuged again at 1163g for 15 min. These purification steps were repeated four times. The amine-modified silica spheres were then dried using a stream of nitrogen. Calcinated silica spheres were rehydroxylated prior to their surface modification with amines using the procedure described below for nanofrits. Dansyl moieties were attached to the surface of amine-modified silica spheres following the previously described procedure.33 Aminemodified silica spheres were placed in a 20 mL scintillation vial containing 10 mL of a 0.004 M solution of dansyl chloride in dry acetonitrile with three drops of 2,6-lutidine under the nitrogen atmosphere. The vial was capped, wrapped with Parafilm, and the (33) White, H. S.; Murray, R. W. Anal. Chem. 1979, 51, 236–239.

3098 Langmuir, Vol. 25, No. 5, 2009 reaction mixture was stirred for 24 h at room temperature. The silica spheres solution was placed in a 15 mL centrifuge tube (Corning), and the tube was centrifuged at 1163g for 15 min. A pellet formed at the bottom of the tube, the supernatant was removed, and the modified spheres were sonicated for 30 min to disperse the silica spheres into 10 mL of acetonitrile. The dispersed silica spheres were then centrifuged again at 1163g for 15 min. This process was repeated four times. The dansyl-modified spheres were then dried in a stream of nitrogen and stored in a 20 mL scintillation vial. The coverage of the dansyl moiety on the surface of the silica spheres was determined using the previously reported procedure33 as outlined below. The dansyl-modified silica spheres (0.002 g) prepared as described above were placed in a 20 mL scintillation vial containing 3 mL of a 0.1 M KOH solution in ethanol for 17 h. The resulting solution was filtered through a 20 nm pore Anodisc membrane (Whatman), and the absorbance at 323 nm was measured. The concentration of the dansyl was determined using the extinction coefficient of 7550 M-1 cm-1. The latter was determined by allowing 0.20 mL of butylamine (0.0020 mol) to react with 0.010 g (0.037 mmol) of dansyl chloride in 10 mL of dry acetonitrile in a 20 mL round-bottom flask equipped with a stir bar and gas inlet valve. The solution was stirred under the nitrogen atmosphere for 3 h. The pale green reaction mixture was then diluted 1/1000 with 0.1 M KOH in absolute ethanol. This solution was then diluted again with 0.1 M KOH in absolute ethanol to prepare five solutions with concentrations between 1 × 10-8 and 5 × 10-7 M dansyl amide. These solutions were used to obtain a linear calibration curve that gave an extinction coefficient of 7550 M-1 cm-1. The concentration of the dansyl was used to determine the number of dansyl moieties on the surface. The surface area of one 338, 300, or 251 nm silica sphere (359 000, 280 000, or 198 000 nm2, respectively) was used to calculate the number of dansyl molecules per nm2. Amine Surface Coverage of Unsintered Silica Colloidal Crystals. Silica spheres from above that were not treated with any form of heat (363 nm) were self-assembled on a piece of a cut precleaned microscope slide (Fisher). The 3 mm wide and 75 mm long piece of glass was placed vertically into a 3 DR vial (19 mm outer diameter, 65 mm height, 11.1 mL capacity, Fisher) that contained 5 mL of a 1.5 wt % colloidal solution of 363 nm silica spheres dispersed in absolute ethanol. The piece of glass was kept vertical by wrapping a piece of copper wire around the mouth of the vial and around the top of the piece of glass. The glass vertically orientated in the vial was placed on a benchtop in a hood under a crystallization dish raised 6 cm off the bench. The hood used for the deposition process was free of any major vibrations. The piece of glass slide remained in the solution until all of the solvent had evaporated, usually 6-7 days. Next, the silica colloidal crystals on glass slides were placed under the nitrogen atmosphere in a 20 mL scintillation vial containing a solution of 15 mL of dry acetonitrile and 0.20 mL (0.85 mmol, 0.056 M) of 3-amino-propyltriethoxysilane for 17 h at room temperature. After the glass slide was immersed in the silane solution, the opening in the vial was wrapped in Parafilm. The treated colloidal films were then placed in 20 mL of pure acetonitrile for 3 h to remove any unreacted silane. The amine-modified silica colloidal crystals were then placed under the nitrogen atmosphere in a 20 mL scintillation vial containing 10 mL of 0.004 M solution of dansyl chloride in dry acetonitrile with three drops of 2,6-lutidiene. After the glass slide was immersed in the solution of dansyl chloride, the opening in the vial was wrapped with Parafilm, and the solution was slowly stirred for 24 h. The modified colloidal films were then soaked in 15 mL of acetonitrile for 4 h two times. The dansyl-modified silica colloidal crystals were dried by allowing the solvent to evaporate overnight. The colloidal crystals were scraped off the surface of the glass slides and weighed. The surface coverage of dansyl amide on the surface of the silica colloidal crystals was determined following the procedures outlined above for the dansylmodified silica spheres. The surface area of the colloidal crystals was estimated on the basis of the diameter of the silica spheres

Bohaty et al. comprising the crystal with the assumption that the entire surface of the spheres has been modified. Preparation of Nanofrits. Colloidal frits were prepared by placing a glass slide vertically into a 10 mL beaker containing a sonicated ∼12 wt % ethanol colloidal solution of calcinated silica spheres (338, 300, or 251 nm in diameter). The glass slide was held vertically using copper wire. The filled beaker with no lid was placed under a 190 × 100 crystallization dish that was elevated 6 cm off the bench in a hood free of any major vibrations. The solvent was allowed to evaporate for 1-2 days, leaving ∼200 µm thick colloidal films on the glass slide and sides of the beaker. Pieces of the colloidal films were removed from the glass slide by gently breaking the colloidal films using a razor blade and handling the pieces with forceps. The pieces of the colloidal films were placed on a silicon wafer, which was then placed in a furnace and heated at 1050 °C for 12 h to sinter the silica spheres. The furnace was heated at a rate of 20 °C a minute. After 12 h, the furnace was turned off, and the colloidal pieces were allowed to cool to room temperature inside the furnace.24,25 The resulting sintered silica colloidal membranes were very durable. Surface Modification of Nanofrits. The sintered colloidal membranes were first rehydroxylated by placing the pieces in a 20 mL polyethylene bottle containing 10 mL of a pH 9.5 solution of tetrabutylammonium hydroxide in water. The bottle was placed in a 60 °C oil bath and heated for 24 h.24 The rehydroxylated pieces were washed with deionized water (100 mL × 2), 1 M nitric acid (100 mL × 2), methanol (100 mL × 2), deionized water (100 mL), and acetonitrile (100 mL × 2) in succession. The pieces were stored in acetonitrile until further modification. Nanofrits were modified with amines by placing the rehydroxylated sintered nanofrit pieces in a 20 mL scintillation vial containing a mixture of 15 mL of dry acetonitrile and 0.20 mL of APTES (0.85 mmol, 0.056 M) for 24 h at room temperature. After the sintered pieces were placed in the mixture, the vial was covered with a lid and wrapped with Parafilm. After 24 h, the modified pieces were soaked in 20 mL of pure acetonitrile for 2 h two times and either used to construct the colloidal frit membrane or the amine modification procedure was repeated a second time. In the latter case, the length of time for the APTES modification following the above procedures was 24 h as well. The nanofrit pieces were then soaked in 20 mL of pure acetonitrile for 2 h two times and used in the colloidal frit membrane construction. Amine Surface Coverage for Nanofrit. The coverage of the dansyl moiety on the surface of the colloidal frits was determined using the previously reported procedure.33 Amine-modified nanofrits were treated with dansyl chloride as described above for silica spheres. The dansyl-modified colloidal frits were rinsed with 20 mL of acetonitrile two times and then placed in 15 mL of acetonitrile for 24 h. A dansyl-modified nanofrit piece (0.002 g) was placed in a 20 mL scintillation vial containing 3 mL of a solution of 0.1 M KOH in ethanol for 17 h. The resulting solution was filtered through a 20 nm pore Anodisc membrane (Whatman), and the absorbance at 323 nm was measured. The concentration of the dansyl was determined using the extinction coefficient of 7550 M-1 cm-1 (see above) and was used to determine the number of dansyl moieties on the surface. Diffusion Measurements through Nanofrits. Nanofrit pieces were placed between two PTFE flat washers (5.16 mm inner diameter, 14.27 mm outer diameter, and 1.02 mm thickness (Small Parts, Inc.)) coated with Loctite Hysol 0151 Epoxy on the inner lip of the washer, so that the epoxy came in contact with the nanofrits. The newly constructed membranes were allowed to cure for 24 h before any diffusion measurements were performed. The colloidal membranes ranged from 0.10 to 0.17 cm2 in area. Room-temperature diffusion measurements through the colloidal frits were conducted by placing a nanofrit comprised of 251, 300, or 338 nm silica spheres between two connected 1 cm quartz cuvettes. One of the cuvettes contained 4 mL of either 9.55 mM solution of Fe(bpy)32+ in acetonitrile or 8.53 mM solution of 1 in acetonitrile, and the other cuvette contained 4 mL of pure acetonitrile. The nanofrit was placed between two Kalrez o-rings to prevent leaking, and a clamp was used to hold the cuvettes, o-rings, and colloidal membrane

Free-Standing Silica Colloidal Nanoporous Membranes

Figure 1. SEM images of nanofrits comprised of 338 nm silica spheres. Left: SEM image showing no major cracks or defects (size bar ) 10 µm). Right: Close-up image displaying the fcc lattice (size bar ) 5 µm).

in place. Each cuvette contained a stir bar and was covered with Parafilm to prevent evaporation. The cuvette containing the receiving solution was then placed in the cuvette holder between two fiber optics cables, and the solution was blanked. Both solutions were stirred, and the dye diffusion rate was measured by recording the absorbance in the receiving cuvette at a wavelength of 520 nm for Fe(bpy)32+ and 255 or 283 nm for 1 for at least 18 h while stirring both solutions. When selectivity measurements were conducted, the solutions in both cuvettes contained 50 mM TFA. For diffusion measurements of 1 with solutions containing 50 mM TFA, the wavelength at 255 nm was monitored. For measurements conducted without TFA present, the wavelength at 283 nm was monitored. Data points were recorded every 150 s with an initial delay of 150 s. All measurements were repeated in triplicate.

Results and Discussion Preparation and Characterization of Nanofrits. For highquality colloidal frits to be prepared, the silica spheres have to be calcinated. Calcinating silica spheres is a known method to prevent cracks in large area colloidal crystalline films.23 When silica spheres are calcinated, the solvents (water and ethanol) trapped inside the silica spheres are released, and the spheres shrink. This process leads to the more dense spheres (with the density of ca. 2.17 g cm-3) as compared to as-prepared silica spheres (with ca. 1.97 g cm-3 density).23 Attempts to sinter colloidal films prepared from as-made silica spheres typically lead to cracks in the films and even to their disintegration.23,24 Thus, colloidal frits were prepared using calcinated silica spheres as described in the Experimental Section. The nanopore size (the distance from the center of the three-dimensional nanopore to the nearest silica sphere surface) of the resulting membranes depends on the size of the silica spheres used to assemble the membrane (it is calculated as ∼15% of the sphere radius), and for 338, 300, or 251 nm diameter spheres it is 25, 22.5, and 19 nm, respectively. The resulting sintered silica colloidal membranes were very durable; that is, they could be handled, sonicated, and sealed into Teflon washers (see below). Mechanical strength testing of the sintered colloidal membranes is planned in the future. Figure 1 shows SEM images of a colloidal frit. The images show that there are no cracks visible in the frit over a large area, but there are point defects in the assembled fcc lattice. These point defects are small and appear to only penetrate one or two layers of the spheres. Side views of the frits (a representative image is shown in Figure 2) reveal no cracks or defects that extend across the entire membrane. The frits appear to slightly increase in thickness from the top to the bottom in the direction of the solvent evaporation. This variation is not significant for small area nanofrits, and an average thickness of 200 µm has been used in the diffusional flux calculations below. However, this feature limits the area of the frits that can be prepared with

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Figure 2. SEM image side view of a nanofrit comprised of 338 nm silica spheres.

Figure 3. Photographs of nanofrits. Top: Without PTFE washers showing the sintered colloidal frit in the epoxy. Bottom: With PTFE washers.

the presently described methodology. To create larger frits with uniform thickness, other techniques such as spin coating,34 centrifugation,35 or sedimentation36 will be used in the future. To perform diffusion measurements for the nanofrits, suitable membranes had to be constructed. Figure 3 shows photographs of colloidal frit membranes shown next to a dime for size comparison. The top image shows the colloidal frit without the washers, while the bottom image shows the frit between the two PTFE washers. Diffusion measurements were then used to detect the presence of major defects in the nanofrits. This was accomplished by calculating the molecular flux of a salt solution of known concentration through a nanofrit of a known thickness and area to a reservoir of a pure solvent on the other side of the nanofrit, and comparing this value to the observed flux. If there were any major defects, there should be a significant difference between the observed and calculated flux with the same solutions. The flux of Fe(bpy)32+ in acetonitrile was calculated as:

Jcolloid )

∆C ε · ·D L τ sol

(1)

where ∆C is 9.55 mM, L is 0.020 cm, ε is 0.26, τ is 3.0, and Dsol is 1.0 × 10-5 cm2 s-1 for Fe(bpy)32+ in acetonitrile.37 The values of ε and τ are the void fraction and tortuosity of the membrane, respectively. The values used here have previously been determined for silica colloidal crystals.37 This calculation gave the flux of 4.14 × 10-10 mol s-1 cm-2. The observed fluxes for six different nanofrits were (3.6 ( 0.2) × 10-10 mol s-1 cm-2, corresponding to a membrane thickness of 0.023 cm, which is in good agreement with the calculated value, confirming that no mechanical defects are present in the (34) Jiang, P.; McFarland, M. J. J. Am. Chem. Soc. 2004, 126, 13778–1378. (35) Zhang, X. Y.; Wang, T. W.; Jiang, W. Q.; Wu, D.; Liu, L.; Duan, A. H. Chin. Chem. Lett. 2005, 16, 1109–1112. (36) My´guez, H.; Meseguer, F.; Lopez, C.; Mifsud, A.; Moya, J. S.; Vazquez, L. Langmuir 1997, 13, 6009–6011. (37) Newton, M. R.; Morey, K. A.; Zhang, Y.; Snow, R. J.; Diwekar, M.; Shi, J.; White, H. S. Nano Lett. 2004, 4, 75–880.

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nanofrits. We were unable to compare the observed fluxes for intact frits to those for the frits containing mechanical defects, as our attempts to create pinholes in the frits led to their cracking into smaller pieces. However, our previously reported studies of suspended colloidal membranes22 showed that colloidal crystals containing major mechanical defects possess fluxes several orders of magnitude higher than those measured for the intact crystals. Amine-Modified Nanofrits. Several possible applications can be envisioned for the colloidal nanofrits, including size-selective filtration, preparation of responsive membranes, and separations based on nanofrit surface modification. For the latter two applications, the ability to surface-modify the nanofrits is critical. Earlier, we described surface-aminated thin nanoporous colloidal films permselective for the transport of charged versus neutral species.38,39 Specifically, we demonstrated that a thin colloidal film can be modified with surface amino groups. At pH lower than the pKa of the surface-bound amines,40 the transport of positively charged species in the colloidal films was almost completely impeded, while the transport of neutral species was unaltered. We demonstrated that the permselectivity is based on electrostatic repulsion and can be modulated by adjusting the Debye screening length of the electric field within the pores of the colloidal film. Moreover, we found that the degree of permselectivity was larger than that anticipated on the basis of electrostatic interactions alone. For instance, for the colloidal films comprising 440 nm silica spheres, the distance from the center of the pore to the nearest sphere surface is ca. 35 nm. Within this structure, a ∼50% blocking of diffusion was observed at an electrolyte concentration of 0.05 M, where the Debye screening length (κ-1) is only ca. 1.5 nm. Assuming that the electric field extends ∼5κ-1 from the surface (∼7.5 nm, corresponding to the distance where the potential decays to ∼1% of the surface potential), the electric field extends over only a small fraction (∼22%) of the effective pore width. We speculated that the tortuous path that molecules take to diffuse through the colloidal crystal, the finite thickness of the amine layer, and the high surface area of the colloidal crystal contributed to enhancing the electrostatic permselectivity. Thus, we decided to perform the nanofrit amination to compare the permselectivity of the resulting surface-modified membranes to that of the aminated thin colloidal films. Unlike the latter, for the nanofrits to become modified with amines, their surface needs to be rehydroxylated because most of the surface hydroxyl groups were lost during the sintering.23 We regenerated the hydroxyl groups on the nanofrit surface using a procedure developed by Le et al.24 The rehydroxylated sintered colloidal pieces were first subjected to the surface amination procedure used previously for the colloidal films because of the selectivity results observed for the aminated colloidal films along with no evidence of a thick polymer film formation.37,38 The amine-modified frit pieces were also subjected to an additional 24 h amination step to potentially increase the number of amines present in the nanofrit. After the surface of the frits was modified with amines using APTES for 24 h either once or twice, the frits were still free of any major defects, and there was no presence of a thick polymeric film on the frit or inside its nanopores (Figure 4). Diffusion rates were then used to determine if any transport selectivity can be achieved for positively charged species (38) Newton, M. R.; Bohaty, A. K.; White, H. S.; Zharov, I. J. Am. Chem. Soc. 2005, 127, 7268–7269. (39) Newton, M. R.; Bohaty, A. K.; Zhang, Y.; White, H. S.; Zharov, I. Langmuir 2006, 22, 4429–4432. (40) Shyue, J.-J.; De Guire, M. R.; Nakanishi, T.; Masuda, Y.; Koumoto, K.; Sukenik, C. N. Langmuir 2004, 20, 8693–8698.

Bohaty et al.

Figure 4. SEM image of a nanofrit modified twice with APTES for 24 h (size bar ) 5 µm).

Figure 5. Diffusion rate of Fe(bpy)32+ through 3-aminopropyltriethoxysilane-modified (24 h) colloidal frit (338 nm silica spheres) with 50 mM TFA (red) and without the acid (blue).

Figure 6. Diffusion rates of Fe(bpy)32+ through amine-modified (24 h × 2) colloidal frit (338 nm silica spheres) with 50 mM TFA (red) and without the acid (blue).

(Fe(bpy)32+) through the aminated nanofrits with and without acid present in solution. The diffusion rate is defined as:

RD ) Jcolloid · S

(2)

where Jcolloid is the molecular flux across the colloidal membrane and S is its area.41 The diffusion rate through the membrane can be determined by measuring the number of moles of the molecule diffusing through the membrane as a function of time. Figures 5 and 6 show the diffusion rates of Fe(bpy)32+ in acetonitrile solution through amine-modified nanofrits with and without 50 mM trifluoroacetic acid (TFA) present (in the latter case, the acetonitrile solution was unbuffered). Although the exact pKa of amines on the silica surface in acetonitrile solution is not known, we assume that it is similar to that in aqueous solution,40 and that at the above acid (41) Cussler, E. L. Diffusion: Mass Transfer in Fluid Systems, 2nd ed.; Cambridge University Press: New York, 1997.

Free-Standing Silica Colloidal Nanoporous Membranes

concentration given the low pKa of TFA the majority of the surface amines become protonated. For the nanofrit that underwent a single 24 h amination, the experiment showed no change in diffusion rate of Fe(bpy)32+ when TFA was present in solution. This suggests that that either the surface was not modified with amines at all or there were not enough surface amines to create a high enough positive surface charge to repel the positively charged ions from the colloidal nanopores. This lack of selectivity was observed for two additional membranes modified for 24 h. To examine these two possible explanations, the aminemodified nanofrits were treated with dansyl chloride. The resulting materials fluoresced when irradiated with 365 nm light. This qualitative experiment showed that there are amines present on the surface. Indeed, when rehydroxylated nanofrits not modified with amines were subjected to dansyl chloride, their surface did not fluoresce when irradiated with 365 nm light. Next, the aminemodified nanofrits were subjected to a second APTES treatment to introduce more amines on the surface. Figure 6 shows the diffusion rates of Fe(bpy)32+ through nanofrits comprised of 338 nm silica spheres that have been modified twice with APTES for 24 h each time. In this case, when acid was present the diffusion rate of Fe(bpy)32+ decreased by 22.4% (from (3.3 ( 0.6) × 10-11 to (2.5 ( 0.4) × 10-11 mol s-1 cm-1). This decrease was observed for two additional modified colloidal frits. Diffusion through nanofrits modified twice with APTES for 24 h was measured for 8.53 mM solution of a neutral molecule 1 in acetonitrile to establish if the selectivity results from the electrostatic repulsion of Fe(bpy)32+ from the positively charged aminated silica surface when acid was present. The diffusion rate of 1 did not change when 50 mM TFA was added to the solution. This result indicates that the selectivity observed for Fe(bpy)32+ is indeed due to electrostatic effects (and not sterics or specific binding), as has been shown to be the case for aminemodified colloidal films. The relatively small decrease in diffusion rate observed for the amine-modified nanofrits, as compared to the thin colloidal films,37,38 could be due to the low number of amines on the surface of the sintered colloidal frit as compared to that on the surface of unsintered colloidal films. This, in turn, may result from the lower number of hydroxyl groups on the surface of the colloidal frit sintered at high temperature.23 To determine if this is indeed the case, the dansyl surface coverage for sintered nanofrits was compared to that for colloidal films not treated with heat. The colloidal films that were not treated with heat were allowed to react with APTES for 17 h, as was done for amine-modified colloidal films deposited on platinum microdisk electrodes,37,38 and rehydroxylated nanofrits that were sintered were allowed to react with APTES for 24 h or 2 × 24 h. Both materials were then treated with dansyl chloride. To determine the surface coverage, the newly formed dansyl amide was removed from the surface using 0.1 M solution of potassium hydroxide in absolute ethanol, and the absorbance at 323 nm was measured. This value was then used to estimate the surface coverage of dansyl amide.

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For the colloidal films not exposed to heat, the surface coverage was 1.3 dansyl groups per nm2. This number was only 0.45 dansyl groups per nm2 for the rehydroxylated nanofrits treated once with APTES for 24 h. The dansyl coverage increased to 0.8 groups per nm2 when the amine-modified nanofrits were treated with APTES for 24 h for a second time. Because the dansyl surface coverage is proportional to the number of amines on the silica surface, the low amine coverage could indeed be the cause of the small change in the diffusion rate for aminemodified sintered nanofrits. The difference in surface coverage for sintered colloidal frits and regular colloidal films is ∼60%, which is similar to the difference between the selectivity for the amine-modified colloidal films37,38 and amine-modified colloidal frits. We also investigated if the pore size in the aminated nanofrits is a factor that affects the selectivity. Frits with pore size of 54 and 40 nm were compared. The selectivity for both nanofrits was found to be the same, indicating that changing the pore size in this range does not affect the diffusion rate. Given that the Debye screening length is only ∼1.2 nm for the experimental conditions used (9.55 mmol of Fe(bpy)32+), the selectivity is likely due to the tortuous path diffusing molecules take through the nanopores,37,38 and thus a 25% change in the relatively large nanopore size is not sufficient to become a factor.

Conclusions We demonstrated that robust free-standing colloidal membranes (nanofrits) can be prepared with a relatively large area and no mechanical defects by sintering silica colloidal films. The diffusional flux of solvated Fe(bpy)32+ ion in acetonitrile solution through these nanofrits in the absence of applied pressure is on the order of 3.6 × 10-10 mol s-1 cm-2 and is in good agreement with the diffusional flux calculated for colloidal crystals of the same thickness (200 µm) based on the diffusivity of the Fe(bpy)32+ ion in the same solvent. The nanofrits can be modified with amines using APTES after rehydroxylation, but the number of the surface amines is lower as compared to colloidal films not treated with heat. As a result, the selectivity for the transport of Fe(bpy)32+ through the aminated nanofrits in the presence of TFA is lower than the selectivity observed for amine-modified thin colloidal films. These results show that the nanofrits could potentially be used for separations, but their surface modification has to be further optimized. Our present work in this direction includes optimizing the APTES modification conditions as well as preparing amine-containing polymer brushes inside the frit nanopores. We are also currently working on preparing thinner (ca. 60 µm) nanofrits to achieve a flux of 1 × 10-8 mol s-1 cm-1. Finally, we are studying the size-selective transport of biomacromolecules through the nanofrits. Acknowledgment. This work was supported by the NSF CAREER Award (CHE-0642615). LA801922A