COMMUNICATION DOI: 10.1002/asia.201301213
Bioinspired, Cysteamine-Catalyzed Co-Silicification of (1 H, 1 H, 2 H, 2 HPerfluorooctyl)triethoxysilane and Tetraethyl Orthosilicate: Formation of Superhydrophobic Surfaces Ji Hun Park,[a] Ji Yup Kim,[a] Woo Kyung Cho,*[b] and Insung S. Choi*[a]
Abstract: Bioinspired silicification attracts a great deal of interest because of its physiologically relevant, mild conditions for hydrolysis and condensation of silica precursors, which makes the bioinspired approach superior to the conventional sol–gel process, particularly when dealing with biological entities. However, the morphological control of silica structures with incorporation of functional groups in the bioinspired silicilication has been unexplored. In this work, we co-silicificated (1 H, 1 H, 2 H, 2 H-perfluorooctyl)triethoxysilane and tetraethyl orthosilicate to investigate the morphological evolution of fluorinated silica structures in the cetyltrimethylammonium bromide-mediated, cysteamine-catalyzed silicification. The generated micrometer-long wormlike and spherical silica structures display superhydrophobicity after film formation. Interestingly, the measurement of dynamic water contact angles shows that the morphological difference leads to a different wetting state, either the selfcleaning or the pinning state of the superhydrophobic surface.
compatible conditions for bioinspired silicification allowed for the encapsulation of individual living cells within silica shells with maintenance of cell viability.[3] The same approach has been applied to single-cell encapsulation with abiological titanium oxide under cytocompatible conditions.[4] On the other hand, the structural intricacy found in biogenic silica has also inspired researchers to chemically emulate the biological processes for fabrication of silica structures that were not realized by the sol–gel processes in the synthesis of nanomaterials.[5] The mechanistic studies revealed that the catalytic biotemplates and their self-assembled structures played a pivotal role in generating the hierarchical silica morphologies in nature: for example, silicateins, isolated from glass sponges, are silica-forming enzymes that possess the catalytic activity for the hydrolysis of silicate and the condensation of silicic acid.[6] Tetraethyl orthosilicate (TEOS), which is stable in aqueous solution at neutral pH, has widely been employed to achieve the in vitro silicification with silicateins, and silica nanospheres and thin films have been synthesized with silicateins and their derivatives for applications in nanometric insulators and semiconductors.[7] Besides the use of the naturally occurring silicateins as a catalyst for bioinspired silicification, the active site of silicateins, which is composed of histidine and serine, was mimicked chemically for the in vitro silicification; in comparison, complicated and time-consuming processes are required to prepare intact or recombinant silicateins, and the obtained amount of silicateins was limitedly small.[8] Morse et al. reported that cysteamine, which bears an amine and a thiol group, acts as a biomimetic small catalyst for the hydrolysis and condensation of TEOS under biologically relevant, mild conditions (pH 7.0; room temperature).[9] Compared with the sol–gel processes, the substrate scope has not been investigated yet in the bioinspired silicification that uses silicateins or small molecules as a catalytic template, probably due to the fact that previous studies so far mainly focused on the mechanism of biosilicification in glass sponges.[10] However, the incorporation of organosilicates into silica structures in bioinspired processes would greatly widen the application areas of siliceous materials with physicochemical properties that are tailored further by organic groups in organosilicates[11] and/or their post-modifications after the structure construction.[12] It could also be envi-
Despite the low concentration of silicic acid (< 100 ppm) in the ocean, some marine organisms construct siliceous exoskeletons under physiological conditions, exemplified well by diatoms[1] and glass sponges.[2] The mechanisms of biogenic silica formation (biosilicification) have been investigated, perhaps because they could provide a bioinspired alternative to the conventional sol–gel silica synthesis that generally does harm to living cells in cell-surface engineering and biotechnological applications. For example, the cyto[a] J. H. Park, J. Y. Kim, Prof. Dr. I. S. Choi Center for Cell-Encapsulation Research and Molecular-Level Interface Research Center Department of Chemistry KAIST Daejeon 305-701 (Korea) Fax: (+ 82) 42-350-2810 E-mail: [email protected] [b] Prof. Dr. W. K. Cho Department of Chemistry Chungnam National University Daejeon 305-764 (Korea) E-mail: [email protected] Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/asia.201301213.
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sioned that different structures would be generated with organosilicates because their reaction rates pertaining to hydrolysis and condensation are different from those of TEOS. We have previously reported that the addition of cetyltrimethylammonium bromide (CTAB) to the bioinspired, cysteamine-catalyzed reaction made it possible to control the morphologies of silica from TEOS, generating micrometersized and discrete silica particles in a water/ethanol co-solvent system.[13] In this study, we expanded the substrate scope to (1 H, 1 H, 2 H, 2 H-perfluorooctyl)triethoxysilane (FTES) and investigated the morphological evolution of silica structures with the co-condensation of FTES and TEOS under the bioinspired conditions of the cysteaminecatalyzed reaction (Figure 1 a). FTES had been co-con-
Figure 2. FE-SEM micrographs of fluorinated silica with different morphologies: particulate morphologies were observed at the water-to-ethanol ratios of 4:1, 2:1, and 1:1, and worm-like morphologies were observed at the water-to-ethanol ratio of 0.3:1.
by field-emission scanning electron microscopy (FE-SEM) and transmission electron microscopy (TEM). For the elemental mapping and imaging, we used scanning transmission electron microscopy combined with energy-dispersive X-ray spectroscopy (STEM-EDS). Figure 2 shows the FE-SEM micrographs of the silica structures generated. We observed that a higher water content in the co-solvent system generally led to more discrete and smaller silica structures. Discrete nanoparticles 54 7 nm and 281 35 nm in diameter were formed when the water-to-ethanol ratios were 4 and 2, respectively (Figure 2 a, b). This trend could be explained by the stability of the self-assembled CTAB structures (e.g., micelles) and/or the solubility of the silicates (and hydrolysis rate), and is in a good agreement with our previous report.[13] When the solvent ratio decreased to 1, the size of silica particles increased further to about 1 mm, but some of them were fused together (Figure 2 c). Gigantic silica spheres or their broken structures were observed for solvent ratios of 0.9 and 0.8 (see the Supporting Information, Figure S1 g, h). However, lower solvent ratios (0.35–0.7) yielded particulate silica structures of about 1 mm in diameter (see the Supporting Information, Figure S1 b–f). With a water-to-ethanol ratio of 0.3, a completely new, micrometer-long, worm-like structure was formed, which has been rarely found in the synthesis of silica. The effects of the solvent ratio on the morphological changes were observed to be the same, when the total concentration of the silica precursors was changed to 80 mm or 120 mm (see the Supporting Information, Figure S2). Owing to the dramatic morphological change at the solvent ratio of around 0.3, we also implemented a systematic change of the TEOS-to-FTES ratio for water-to-ethanol ratios of 0.3 and 0.4, respectively (total concentration of silica precursors: 100 mm; see the Supporting Information, Figure S3). No pre-
Figure 1. a) Chemical structures of FTES and TEOS. b) Representative synthetic procedures for the co-silicification by co-condensation of FTES and TEOS. The total concentration of silica precursors was fixed at 100 mm (40 mm of FTES and 60 mm of TEOS), and the water-to-ethanol ratio was varied from 4:1 to 0.3:1 (v/v).
densed with TEOS in the sol–gel process for superhydrophobic coating by taking advantage of the low surface free energy of the fluorinated compounds;[14] we also examined the morphological effect on the water contact angle and surface free energy after forming thin films on silicon substrates. The deposition of as-prepared structures on the solid substrate provided a simple way to the generation of superhydrophobic surfaces. The synthetic procedure is depicted in Figure 1 b. The cocondensation of FTES and TEOS was performed in a water/ ethanol co-solvent system containing 50 mm of cysteamine and 5 mm of CTAB. Our previous report showed that this condition led to the formation of discrete silica particles with TEOS only.[13] The total concentration of silicates was fixed at 100 mm (TEOS/FTES = 3:2). The water-to-ethanol ratio was varied to investigate morphological changes of fluorinated silica, as we thought that the relative solubility of silicates in water and ethanol would affect the morphologies of silica formed. After incubation for 6 hours without stirring, white precipitates were collected by centrifugation and washed with absolute ethanol three times to remove unreacted silicates. The morphologies of silica were examined
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ing), and 802 cm1 (SiOSi symmetric stretching).[16] X-ray photoelectron spectroscopy (XPS) was also used to corroborate the fluorine incorporation after film formation on the silicon substrate (see the Supporting Information, Figure S5). The area integrations of the XPS peaks were obtained for fluorine (F), carbon (C), oxygen (O), and silicon (Si). The elemental compositions for the worm-like silica were 39.89 % (F), 31.15 % (C), 17.53 % (O), and 11.42 % (Si). The values for the spherical silica were 39.53 % (F), 28.26 % (C), 19.64 % (O), and 12.57 % (Si). The percentages of fluorine were almost the same between the two silica structures, although their morphologies were significantly different. These data implied that the formation of the two different silica structures shared a common mechanism in which hydrolysis and co-condensation were affected greatly by the water-to-ethanol ratio. However, the detailed mechanism on the formation of the worm-like structure remains to be investigated. The micro/nanoscopic heterogeneity of worm-like and spherical fluorinated silica structures was found to yield superhydrophobicity when they were simply drop-casted onto silicon substrates. The static water contact angles were measured to be 158 2.68 for the worm-like silica and 153 5.48 for the spherical one, showing that the superhydrophobic surfaces (static water contact angle > 1508) were generated by the surface roughness along with fluorine incorporation (see the Supporting Information, Figure S6 and Table S1). While both silica-coated surfaces displayed superhydrophobicity, their wetting states were found to be different based on dynamic water contact angle measurements, which were carried out by using the tilting-plate method (Figure 4).[17] The advancing (qadv) and receding (qrec) water contact angles were similar to each other for the worm-like surface, resulting in a relatively low hysteresis (see the Supporting Information, Table S2). A water droplet on that surface rolled off at a tilting angle of 188, thus showing a self-cleaning property.[18a,b] By contrast, a water droplet on the spherical silicacoated surface exhibited a high hysteresis with a difference between qadv and qrec of 59.68, and the water droplet rolled down at a tilting angle of 788, showing the pinning effect to some extent.[18c] We thought that these differences in the dynamic wetting properties were caused by the difference in the surface roughness and heterogeneity because the chemical compositions were the same or at least similar. It was found that the worm-like surface generally displayed a higher root-mean-square roughness (Rq) than the spherical one (see the Supporting Information, Figure S7). We thus supposed that the water droplet on the worm-like surface was at the more heterogeneous wetting state (Cassie–Baxter state),[19a] while the water droplet on the spherical surface was at the more homogeneous wetting state (Wenzel state).[19b] This assumption was supported by calculating the fraction of air in contact with a water droplet at each surface. The air fraction values, which were calculated with the self-assembled monolayers of FTES on Si/SiO2 as a reference (see the Supporting Information, Figure S8), were 0.910 and 0.865 for the worm-like and spherical surfaces, respectively
cipitates were observed when the amount of FTES exceeded that of TEOS. Particulate morphologies were found in most cases of different TEOS-to-FTES ratios, but worm-like morphologies were only observed at TEOS-to-FTES ratios of 1:1 and 3:2 with the solvent ratio of 0.3. Notably, worm-like structures were not formed with the solvent ratio of 0.4. The worm-like structure was characterized further by TEM because such an analysis gave the information on the interior and, more importantly, the spatial distribution of fluorine molecules. We used the silica structures (fused particles) generated at the water-to-ethanol ratio of 0.4 as a comparison because the subtle change of the solvent ratio in this range greatly altered the silica morphologies under the same conditions. The TEM images showed that both worm-like and particulate structures had closely packed interiors (Figure 3 a, b). The STEM-EDS elemental mapping
Figure 3. a, b) TEM micrographs of worm-like and particulate silica structures. c, d) STEM-EDS elemental mapping of worm-like silica structures and particulate silica structures.
diagrams showed a uniform distribution of fluorine in both structures, thus indicating that FTES was hydrolyzed and incorporated into the siloxane network homogeneously without segregation (Figure 3 c, d). The fluorine incorporation in the as-prepared silica was confirmed by attenuated total reflection infrared (ATR-IR) spectroscopy (see the Supporting Information, Figure S4). In the ATR-IR spectrum of wormlike silica, the peaks at 900 and 1200 cm1 were assigned to the CF stretching of CF2 and CF3, respectively,[15] and the characteristic peaks of silica were found at 1135 and 1073 (SiOSi asymmetric stretching), 962 (SiO stretch-
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estingly, the surface free energies of the silica surfaces were lower than that of polytetrafluoroethylene (Teflon; 19.1 mJ m2)[21] and even lower than that of a smooth surface composed of hexagonally close-packed CF3 groups (6.7 mJ m2),[22] probably because of the structural heterogeneity of the deposited silica structures.[23] In summary, we reported the morphological evolution of fluorinated silica for the bioinspired co-silicification of FTES and TEOS under cysteamine-catalyzed silicification conditions. In particular, at the water-to-ethanol ratio of 0.3 and TEOS-to-FTES ratio of 3:2 unprecedented worm-like fluorinated structures were obtained, which were successfully used to generate a superhydrophobic surface with low water contact angle hysteresis and low surface free energy. We believe that other types of organosilicates, such as amine- and thiol-containing ones, could be incorporated into the siloxane network for the fabrication of functionalized silica with regulated morphologies, which is our next research objective. Those organosilicates will be particularly useful in the manipulation of cell–material interfaces once incorporated into the silica network in the bioinspired cytocompatible fashion.
Figure 4. Top: FE-SEM micrographs of worm-like (a) and spherical (b) fluorinated silica drop-casted on silicon substrates. Bottom: dynamic water contact angles that were measured by the tilting-plate method. Optical photographs of water droplets were taken just before rolling-off for the two surfaces.
Experimental Section Representative Procedures The stock solutions of cysteamine (250 mm) and CTAB (40 mm) were prepared in deionized water. The cysteamine stock solution (0.268 mL) was combined with the CTAB stock solution (0.335 mL), and to the mixture was added ethanol (2.0 mL). The water-to-ethanol ratio was set as 0.3:1 (v/v). Subsequently, FTES (40 mm, 41.1 mL) and TEOS (60 mm, 35.8 mL) were added to the mixture of cysteamine (final conc.: 50 mm) and CTAB (final conc.: 5 mm) under vigorous stirring. The resulting mixture was then allowed to stand without disturbance at room temperature for 6 h. The silica precipitates were collected by centrifugation, washed several times with ethanol, and finally dispersed in ethanol. The water-toethanol ratio was adjusted from 0.3:1 (v/v) to 4:1 (v/v) by adding extra water while the volume of ethanol was fixed (2 mL). The superhydrophobic surface was generated by drop-casting of worm-like fluorinated silica (concentrated in ethanol) onto silicon substrates. Contact angle measurements were performed using a Phoenix 300 goniometer (for static contact angles) and a Phoenix 300 goniometer equipped with a tilting stage (for dynamic contact angles) (Surface Electro Optics Co., Ltd., Korea). The resulting silica was characterized with a Bruker ALPHA FT-IR spectrometer equipped with a ZnSe ATR crystal. XPS study was performed with a K-Alpha system (Thermo VG, U.K.) using a monochromated Al X-ray source (AlKa line: 1486.6 eV). The FE-SEM micrographs were obtained using an Inspect F50 microscope (FEI-Philips Co., Netherlands) with an accelerating voltage of 10 keV. The TEM images and the STEMEDS elemental mapping diagrams were acquired by using a JEOL FB2100F instrument operated at 200 kV. The size of particles was measured based on SEM images by averaging the diameters of at least 300 particles.
(see the experimental details in the Supporting Information). The larger air fraction indicated more heterogeneous wetting where a water droplet highly interacted with both the silica surface and air pockets rather than the silica surface only. Additionally, the surface free energies of two silica surfaces were calculated to further confirm the watersliding property. The surface free energy (gS) was calculated by the Owens–Wendt geometric mean equation, which divides the surface tension into the dispersive (D) and polar (P) ones.[20] ð1 þ cos qÞgL ¼ 2
qffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi qffiffiffiffiffiffiffiffiffiffiffiffiffiffi D ðgD g Þ þ 2 ðgPS gPL Þ S L
where q is the measured contact angle of a liquid on the surface, and gL is the surface tension of the liquid. P In our system, the value of gS (= gD S þ gS ) was determined by measuring the contact angles with water and diiodomethane. The surface free energy (gS) of the silicon substrate coated with the worm-like fluorinated silica was calculated to be 0.54 mJ m2. In comparison, the gS value for the substrate with spherical silica (ca. 1 mm in diameter) was 1.80 mJ m2, and increased greatly to 42.33 mJ m2 in the case of ~ 100 nm particles (see the Supporting Information, Figure S6 and Table S1). This calculation indicated that a water droplet could slide down more easily on the wormlike silica surface than on the spherical silica surface. Inter-
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Acknowledgements This research was supported by the Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Science, ICT & Future Planning (MSIP) (2012R1A3A2026403 and 2013-042216 to I.S.C., and NRF2013R1A1A1059642 to W.K.C.)).
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Keywords: cysteamine · fluorination · glass sponge · silicification · superhydrophobicity
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[1] a) D. Losic, J. G. Mitchell, N. H. Voelcker, Adv. Mater. 2009, 21, 2947 and references therein; b) N. Krçger, S. Lorenz, E. Brunner, M. Sumper, Science 2002, 298, 584. [2] R. L. Brutchey, D. E. Morse, Chem. Rev. 2008, 108, 4915 and references therein. [3] a) W. Xiong, Z. Yang, H. Zhai, G. Wang, X. Xu, W. Ma, R. Tang, Chem. Commun. 2013, 49, 7525; b) S. H. Yang, E. H. Ko, Y. H. Jung, I. S. Choi, Angew. Chem. 2011, 123, 6239; Angew. Chem. Int. Ed. 2011, 50, 6115; c) S. H. Yang, K.-B. Lee, B. Kong, J.-H. Kim, H.-S. Kim, I. S. Choi, Angew. Chem. 2009, 121, 9324; Angew. Chem. Int. Ed. 2009, 48, 9160. [4] a) E. H. Ko, Y. Yoon, J. H. Park, S. H. Yang, D. Hong, K.-B. Lee, H. K. Shon, T. G. Lee, I. S. Choi, Angew. Chem. Int. Ed. 2013, 52, 12279; b) S. H. Yang, E. H. Ko, I. S. Choi, Langmuir 2012, 28, 2151. [5] a) R. Andre, M. N. Tahir, F. Natalio, W. Tremel, FEBS J. 2012, 279, 1737; b) N. Hassan, A. Soltero, D. Pozzo, P. V. Messina, J. M. Ruso, Soft Matter 2012, 8, 9553; c) M. Sumper, Angew. Chem. 2004, 116, 2301; Angew. Chem. Int. Ed. 2004, 43, 2251; d) M. Sumper, Science 2002, 295, 2430. [6] a) J. N. Cha, K. Shimizu, Y. Zhou, S. C. Christiansen, B. F. Chmelka, G. D. Stucky, D. E. Morse, Proc. Natl. Acad. Sci. USA 1999, 96, 361; b) Y. Zhou, K. Shimizu, J. N. Cha, G. D. Stucky, D. E. Morse, Angew. Chem. 1999, 111, 825; Angew. Chem. Int. Ed. 1999, 38, 779; c) K. Shimizu, J. Cha, G. D. Stucky, D. E. Morse, Proc. Natl. Acad. Sci. USA 1998, 95, 6234 – 6238. [7] a) L. A. Bawazer, M. Izumi, D. Kolodin, J. R. Neilson, B. Schwenzer, D. E. Morse, Proc. Natl. Acad. Sci. USA 2012, 109, E1705; b) S. Pagliara, A. Polini, A. Camposeo, H. C. Schrçder, W. E. G. Mller, D. Pisignano, Appl. Phys. Lett. 2012, 101, 193702; c) A. Polini, S. Pagliara, A. Camposeo, A. Biasco, H. C. Schrçder, W. E. G. Mller, D. Pisignano, Adv. Mater. 2011, 23, 4674. [8] a) D. H. Adamson, D. M. Dabbs, C. R. Pacheco, M. V. Giotto, D. E. Morse, I. A. Aksay, Macromolecules 2007, 40, 5710; b) D. Kisailus, M. Najarian, J. C. Weaver, D. E. Morse, Adv. Mater. 2005, 17, 1234;
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Bioinspired, Cysteamine-Catalyzed Co-Silicification of (1 H, 1 H, 2 H, 2 HPerfluorooctyl)triethoxysilane and Tetraethyl Orthosilicate: Formation of Superhydrophobic Surfaces Ji Hun Park,[a] Ji Yup Kim,[a] Woo Kyung Cho,*[b] and Insung S. Choi*[a]
Abstract: Bioinspired silicification attracts a great deal of interest because of its physiologically relevant, mild conditions for hydrolysis and condensation of silica precursors, which makes the bioinspired approach superior to the conventional sol–gel process, particularly when dealing with biological entities. However, the morphological control of silica structures with incorporation of functional groups in the bioinspired silicilication has been unexplored. In this work, we co-silicificated (1 H, 1 H, 2 H, 2 H-perfluorooctyl)triethoxysilane and tetraethyl orthosilicate to investigate the morphological evolution of fluorinated silica structures in the cetyltrimethylammonium bromide-mediated, cysteamine-catalyzed silicification. The generated micrometer-long wormlike and spherical silica structures display superhydrophobicity after film formation. Interestingly, the measurement of dynamic water contact angles shows that the morphological difference leads to a different wetting state, either the selfcleaning or the pinning state of the superhydrophobic surface.
compatible conditions for bioinspired silicification allowed for the encapsulation of individual living cells within silica shells with maintenance of cell viability.[3] The same approach has been applied to single-cell encapsulation with abiological titanium oxide under cytocompatible conditions.[4] On the other hand, the structural intricacy found in biogenic silica has also inspired researchers to chemically emulate the biological processes for fabrication of silica structures that were not realized by the sol–gel processes in the synthesis of nanomaterials.[5] The mechanistic studies revealed that the catalytic biotemplates and their self-assembled structures played a pivotal role in generating the hierarchical silica morphologies in nature: for example, silicateins, isolated from glass sponges, are silica-forming enzymes that possess the catalytic activity for the hydrolysis of silicate and the condensation of silicic acid.[6] Tetraethyl orthosilicate (TEOS), which is stable in aqueous solution at neutral pH, has widely been employed to achieve the in vitro silicification with silicateins, and silica nanospheres and thin films have been synthesized with silicateins and their derivatives for applications in nanometric insulators and semiconductors.[7] Besides the use of the naturally occurring silicateins as a catalyst for bioinspired silicification, the active site of silicateins, which is composed of histidine and serine, was mimicked chemically for the in vitro silicification; in comparison, complicated and time-consuming processes are required to prepare intact or recombinant silicateins, and the obtained amount of silicateins was limitedly small.[8] Morse et al. reported that cysteamine, which bears an amine and a thiol group, acts as a biomimetic small catalyst for the hydrolysis and condensation of TEOS under biologically relevant, mild conditions (pH 7.0; room temperature).[9] Compared with the sol–gel processes, the substrate scope has not been investigated yet in the bioinspired silicification that uses silicateins or small molecules as a catalytic template, probably due to the fact that previous studies so far mainly focused on the mechanism of biosilicification in glass sponges.[10] However, the incorporation of organosilicates into silica structures in bioinspired processes would greatly widen the application areas of siliceous materials with physicochemical properties that are tailored further by organic groups in organosilicates[11] and/or their post-modifications after the structure construction.[12] It could also be envi-
Despite the low concentration of silicic acid (< 100 ppm) in the ocean, some marine organisms construct siliceous exoskeletons under physiological conditions, exemplified well by diatoms[1] and glass sponges.[2] The mechanisms of biogenic silica formation (biosilicification) have been investigated, perhaps because they could provide a bioinspired alternative to the conventional sol–gel silica synthesis that generally does harm to living cells in cell-surface engineering and biotechnological applications. For example, the cyto[a] J. H. Park, J. Y. Kim, Prof. Dr. I. S. Choi Center for Cell-Encapsulation Research and Molecular-Level Interface Research Center Department of Chemistry KAIST Daejeon 305-701 (Korea) Fax: (+ 82) 42-350-2810 E-mail: [email protected] [b] Prof. Dr. W. K. Cho Department of Chemistry Chungnam National University Daejeon 305-764 (Korea) E-mail: [email protected] Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/asia.201301213.
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sioned that different structures would be generated with organosilicates because their reaction rates pertaining to hydrolysis and condensation are different from those of TEOS. We have previously reported that the addition of cetyltrimethylammonium bromide (CTAB) to the bioinspired, cysteamine-catalyzed reaction made it possible to control the morphologies of silica from TEOS, generating micrometersized and discrete silica particles in a water/ethanol co-solvent system.[13] In this study, we expanded the substrate scope to (1 H, 1 H, 2 H, 2 H-perfluorooctyl)triethoxysilane (FTES) and investigated the morphological evolution of silica structures with the co-condensation of FTES and TEOS under the bioinspired conditions of the cysteaminecatalyzed reaction (Figure 1 a). FTES had been co-con-
Figure 2. FE-SEM micrographs of fluorinated silica with different morphologies: particulate morphologies were observed at the water-to-ethanol ratios of 4:1, 2:1, and 1:1, and worm-like morphologies were observed at the water-to-ethanol ratio of 0.3:1.
by field-emission scanning electron microscopy (FE-SEM) and transmission electron microscopy (TEM). For the elemental mapping and imaging, we used scanning transmission electron microscopy combined with energy-dispersive X-ray spectroscopy (STEM-EDS). Figure 2 shows the FE-SEM micrographs of the silica structures generated. We observed that a higher water content in the co-solvent system generally led to more discrete and smaller silica structures. Discrete nanoparticles 54 7 nm and 281 35 nm in diameter were formed when the water-to-ethanol ratios were 4 and 2, respectively (Figure 2 a, b). This trend could be explained by the stability of the self-assembled CTAB structures (e.g., micelles) and/or the solubility of the silicates (and hydrolysis rate), and is in a good agreement with our previous report.[13] When the solvent ratio decreased to 1, the size of silica particles increased further to about 1 mm, but some of them were fused together (Figure 2 c). Gigantic silica spheres or their broken structures were observed for solvent ratios of 0.9 and 0.8 (see the Supporting Information, Figure S1 g, h). However, lower solvent ratios (0.35–0.7) yielded particulate silica structures of about 1 mm in diameter (see the Supporting Information, Figure S1 b–f). With a water-to-ethanol ratio of 0.3, a completely new, micrometer-long, worm-like structure was formed, which has been rarely found in the synthesis of silica. The effects of the solvent ratio on the morphological changes were observed to be the same, when the total concentration of the silica precursors was changed to 80 mm or 120 mm (see the Supporting Information, Figure S2). Owing to the dramatic morphological change at the solvent ratio of around 0.3, we also implemented a systematic change of the TEOS-to-FTES ratio for water-to-ethanol ratios of 0.3 and 0.4, respectively (total concentration of silica precursors: 100 mm; see the Supporting Information, Figure S3). No pre-
Figure 1. a) Chemical structures of FTES and TEOS. b) Representative synthetic procedures for the co-silicification by co-condensation of FTES and TEOS. The total concentration of silica precursors was fixed at 100 mm (40 mm of FTES and 60 mm of TEOS), and the water-to-ethanol ratio was varied from 4:1 to 0.3:1 (v/v).
densed with TEOS in the sol–gel process for superhydrophobic coating by taking advantage of the low surface free energy of the fluorinated compounds;[14] we also examined the morphological effect on the water contact angle and surface free energy after forming thin films on silicon substrates. The deposition of as-prepared structures on the solid substrate provided a simple way to the generation of superhydrophobic surfaces. The synthetic procedure is depicted in Figure 1 b. The cocondensation of FTES and TEOS was performed in a water/ ethanol co-solvent system containing 50 mm of cysteamine and 5 mm of CTAB. Our previous report showed that this condition led to the formation of discrete silica particles with TEOS only.[13] The total concentration of silicates was fixed at 100 mm (TEOS/FTES = 3:2). The water-to-ethanol ratio was varied to investigate morphological changes of fluorinated silica, as we thought that the relative solubility of silicates in water and ethanol would affect the morphologies of silica formed. After incubation for 6 hours without stirring, white precipitates were collected by centrifugation and washed with absolute ethanol three times to remove unreacted silicates. The morphologies of silica were examined
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ing), and 802 cm1 (SiOSi symmetric stretching).[16] X-ray photoelectron spectroscopy (XPS) was also used to corroborate the fluorine incorporation after film formation on the silicon substrate (see the Supporting Information, Figure S5). The area integrations of the XPS peaks were obtained for fluorine (F), carbon (C), oxygen (O), and silicon (Si). The elemental compositions for the worm-like silica were 39.89 % (F), 31.15 % (C), 17.53 % (O), and 11.42 % (Si). The values for the spherical silica were 39.53 % (F), 28.26 % (C), 19.64 % (O), and 12.57 % (Si). The percentages of fluorine were almost the same between the two silica structures, although their morphologies were significantly different. These data implied that the formation of the two different silica structures shared a common mechanism in which hydrolysis and co-condensation were affected greatly by the water-to-ethanol ratio. However, the detailed mechanism on the formation of the worm-like structure remains to be investigated. The micro/nanoscopic heterogeneity of worm-like and spherical fluorinated silica structures was found to yield superhydrophobicity when they were simply drop-casted onto silicon substrates. The static water contact angles were measured to be 158 2.68 for the worm-like silica and 153 5.48 for the spherical one, showing that the superhydrophobic surfaces (static water contact angle > 1508) were generated by the surface roughness along with fluorine incorporation (see the Supporting Information, Figure S6 and Table S1). While both silica-coated surfaces displayed superhydrophobicity, their wetting states were found to be different based on dynamic water contact angle measurements, which were carried out by using the tilting-plate method (Figure 4).[17] The advancing (qadv) and receding (qrec) water contact angles were similar to each other for the worm-like surface, resulting in a relatively low hysteresis (see the Supporting Information, Table S2). A water droplet on that surface rolled off at a tilting angle of 188, thus showing a self-cleaning property.[18a,b] By contrast, a water droplet on the spherical silicacoated surface exhibited a high hysteresis with a difference between qadv and qrec of 59.68, and the water droplet rolled down at a tilting angle of 788, showing the pinning effect to some extent.[18c] We thought that these differences in the dynamic wetting properties were caused by the difference in the surface roughness and heterogeneity because the chemical compositions were the same or at least similar. It was found that the worm-like surface generally displayed a higher root-mean-square roughness (Rq) than the spherical one (see the Supporting Information, Figure S7). We thus supposed that the water droplet on the worm-like surface was at the more heterogeneous wetting state (Cassie–Baxter state),[19a] while the water droplet on the spherical surface was at the more homogeneous wetting state (Wenzel state).[19b] This assumption was supported by calculating the fraction of air in contact with a water droplet at each surface. The air fraction values, which were calculated with the self-assembled monolayers of FTES on Si/SiO2 as a reference (see the Supporting Information, Figure S8), were 0.910 and 0.865 for the worm-like and spherical surfaces, respectively
cipitates were observed when the amount of FTES exceeded that of TEOS. Particulate morphologies were found in most cases of different TEOS-to-FTES ratios, but worm-like morphologies were only observed at TEOS-to-FTES ratios of 1:1 and 3:2 with the solvent ratio of 0.3. Notably, worm-like structures were not formed with the solvent ratio of 0.4. The worm-like structure was characterized further by TEM because such an analysis gave the information on the interior and, more importantly, the spatial distribution of fluorine molecules. We used the silica structures (fused particles) generated at the water-to-ethanol ratio of 0.4 as a comparison because the subtle change of the solvent ratio in this range greatly altered the silica morphologies under the same conditions. The TEM images showed that both worm-like and particulate structures had closely packed interiors (Figure 3 a, b). The STEM-EDS elemental mapping
Figure 3. a, b) TEM micrographs of worm-like and particulate silica structures. c, d) STEM-EDS elemental mapping of worm-like silica structures and particulate silica structures.
diagrams showed a uniform distribution of fluorine in both structures, thus indicating that FTES was hydrolyzed and incorporated into the siloxane network homogeneously without segregation (Figure 3 c, d). The fluorine incorporation in the as-prepared silica was confirmed by attenuated total reflection infrared (ATR-IR) spectroscopy (see the Supporting Information, Figure S4). In the ATR-IR spectrum of wormlike silica, the peaks at 900 and 1200 cm1 were assigned to the CF stretching of CF2 and CF3, respectively,[15] and the characteristic peaks of silica were found at 1135 and 1073 (SiOSi asymmetric stretching), 962 (SiO stretch-
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estingly, the surface free energies of the silica surfaces were lower than that of polytetrafluoroethylene (Teflon; 19.1 mJ m2)[21] and even lower than that of a smooth surface composed of hexagonally close-packed CF3 groups (6.7 mJ m2),[22] probably because of the structural heterogeneity of the deposited silica structures.[23] In summary, we reported the morphological evolution of fluorinated silica for the bioinspired co-silicification of FTES and TEOS under cysteamine-catalyzed silicification conditions. In particular, at the water-to-ethanol ratio of 0.3 and TEOS-to-FTES ratio of 3:2 unprecedented worm-like fluorinated structures were obtained, which were successfully used to generate a superhydrophobic surface with low water contact angle hysteresis and low surface free energy. We believe that other types of organosilicates, such as amine- and thiol-containing ones, could be incorporated into the siloxane network for the fabrication of functionalized silica with regulated morphologies, which is our next research objective. Those organosilicates will be particularly useful in the manipulation of cell–material interfaces once incorporated into the silica network in the bioinspired cytocompatible fashion.
Figure 4. Top: FE-SEM micrographs of worm-like (a) and spherical (b) fluorinated silica drop-casted on silicon substrates. Bottom: dynamic water contact angles that were measured by the tilting-plate method. Optical photographs of water droplets were taken just before rolling-off for the two surfaces.
Experimental Section Representative Procedures The stock solutions of cysteamine (250 mm) and CTAB (40 mm) were prepared in deionized water. The cysteamine stock solution (0.268 mL) was combined with the CTAB stock solution (0.335 mL), and to the mixture was added ethanol (2.0 mL). The water-to-ethanol ratio was set as 0.3:1 (v/v). Subsequently, FTES (40 mm, 41.1 mL) and TEOS (60 mm, 35.8 mL) were added to the mixture of cysteamine (final conc.: 50 mm) and CTAB (final conc.: 5 mm) under vigorous stirring. The resulting mixture was then allowed to stand without disturbance at room temperature for 6 h. The silica precipitates were collected by centrifugation, washed several times with ethanol, and finally dispersed in ethanol. The water-toethanol ratio was adjusted from 0.3:1 (v/v) to 4:1 (v/v) by adding extra water while the volume of ethanol was fixed (2 mL). The superhydrophobic surface was generated by drop-casting of worm-like fluorinated silica (concentrated in ethanol) onto silicon substrates. Contact angle measurements were performed using a Phoenix 300 goniometer (for static contact angles) and a Phoenix 300 goniometer equipped with a tilting stage (for dynamic contact angles) (Surface Electro Optics Co., Ltd., Korea). The resulting silica was characterized with a Bruker ALPHA FT-IR spectrometer equipped with a ZnSe ATR crystal. XPS study was performed with a K-Alpha system (Thermo VG, U.K.) using a monochromated Al X-ray source (AlKa line: 1486.6 eV). The FE-SEM micrographs were obtained using an Inspect F50 microscope (FEI-Philips Co., Netherlands) with an accelerating voltage of 10 keV. The TEM images and the STEMEDS elemental mapping diagrams were acquired by using a JEOL FB2100F instrument operated at 200 kV. The size of particles was measured based on SEM images by averaging the diameters of at least 300 particles.
(see the experimental details in the Supporting Information). The larger air fraction indicated more heterogeneous wetting where a water droplet highly interacted with both the silica surface and air pockets rather than the silica surface only. Additionally, the surface free energies of two silica surfaces were calculated to further confirm the watersliding property. The surface free energy (gS) was calculated by the Owens–Wendt geometric mean equation, which divides the surface tension into the dispersive (D) and polar (P) ones.[20] ð1 þ cos qÞgL ¼ 2
qffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi qffiffiffiffiffiffiffiffiffiffiffiffiffiffi D ðgD g Þ þ 2 ðgPS gPL Þ S L
where q is the measured contact angle of a liquid on the surface, and gL is the surface tension of the liquid. P In our system, the value of gS (= gD S þ gS ) was determined by measuring the contact angles with water and diiodomethane. The surface free energy (gS) of the silicon substrate coated with the worm-like fluorinated silica was calculated to be 0.54 mJ m2. In comparison, the gS value for the substrate with spherical silica (ca. 1 mm in diameter) was 1.80 mJ m2, and increased greatly to 42.33 mJ m2 in the case of ~ 100 nm particles (see the Supporting Information, Figure S6 and Table S1). This calculation indicated that a water droplet could slide down more easily on the wormlike silica surface than on the spherical silica surface. Inter-
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Acknowledgements This research was supported by the Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Science, ICT & Future Planning (MSIP) (2012R1A3A2026403 and 2013-042216 to I.S.C., and NRF2013R1A1A1059642 to W.K.C.)).
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Keywords: cysteamine · fluorination · glass sponge · silicification · superhydrophobicity
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