Letter pubs.acs.org/Langmuir
Synergy between Polyamine and Anionic Surfactant: A Bioinspired Approach for Ordered Mesoporous Silica Shaoxin Deng, Chengxiang Shi, Xueyan Xu, Hui Zhao, Pingchuan Sun, and Tiehong Chen* Institute of New Catalytic Materials Science, Key Laboratory of Advanced Energy Materials Chemistry (MOE), College of Chemistry, Collaborative Innovation Center of Chemical Science and Engineering (Tianjin), Nankai University, Tianjin 300071, China S Supporting Information *
ABSTRACT: A novel bioinspired approach for ordered mesoporous silica was developed on the basis of the synergic coassembly between polyamine and an anionic surfactant as a template. With the help of cationic polyamine, anionic surfactant micelles could be utilized as a mesostructure template, whereas with the aid of the anionic surfactant micelles the cationic polyamine chains underwent aggregation to exert their ability to induce silica condensation. Mesoporous silicas with well-ordered mesostructure of Fd-3m symmetry and 3D hexagonal close-packed mesostructure (hcp) were fabricated. Because of the abundant types of anionic surfactants and polyamines, the synthesis approach can be regarded as a general method for anionic-surfactant-templated mesoporous silica, and new mesostructures and morphologies are expected.
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INTRODUCTION The elaborate morphologies and ornate surface patterns of the external silica skeletons in unicellular organisms, such as diatoms and radiolaria, are fascinating and have attracted a great deal of attention. It has been well established that organic biomolecules control the formation of hierarchically ordered, well-defined biosilicas under mild conditions. An organic matrix has been isolated from biosilicas and identified as peptides containing lysine and serine residues (such as silaffins) and long-chain polyamines.1 It has been found that silaffins contain not only positive quaternary ammonium groups but also negative phosphorylated serine residues. Because of this zwitterionic nature, individual silaffins or silaffin mixtures could form supramolecular assemblies and only in this aggregated form could the silaffins induce silica formation in vitro.2 Polyamines could induce silica precipitation if polyanions or silaffins with an acidic domain were present to allow their electrostatic assembly. Silica precipitation in vitro guided by long-chain polyamines has been shown to depend on polyvalent anion phosphate, and these anions induced the electrostatic supramolecular self-assembly of silaffins and polyamines, which were effective in silica precipitation only in their accumulated forms.3−6 The importance of polyanions in biosilica was recently proved by the finding of a new class of aspartate/glutamate-rich and serine phosphate-rich peptides, highly acidic phosphopeptides named silacidins isolated from diatom shells, which could assist polyamines in silica precipitation in vitro.7 Other phosphorylated saccharides were also identified as part of the organic matrix in biosilica.8 It could be regarded that the formation of organic matrices by electrostatic assembly between polyanionic © 2014 American Chemical Society
phosphoproteins and cationic polyamine may be a general mechanism for controlling silica formation in diatoms. The study of the biosilicification mechanisms has inspired novel and facile strategies to produce complex, hierarchically structured materials. Mesoporous materials with hierarchical structures and well-defined morphologies have attracted much
Figure 1. XRD patterns of the calcined samples prepared at (a) RT (20−25 °C) and (b) 80 °C at pH 5.3 (denoted as SPS-1) and (c) RT (20−25 °C) and (d) 80 °C at pH 6.0 (denoted as SPS-2).
Received: November 20, 2013 Revised: January 28, 2014 Published: February 26, 2014 2329
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precursors to form mesostructured silica. Well-ordered mesoporous silicas were first prepared by using cationic quaternary ammonium surfactants.13 Later on, neutral nonionic surfactants were also used to template mesoporous silica under very acidic conditions.14,15 Although double hydrophilic block copolymer was reported as a template after coassembly with polyions, the templating mechanism was similar to the case of nonionic PEO-based surfactants.16 Anionic surfactants were used as templates to synthesize mesostructured materials via interaction with mediated positive inorganic species or metal cations, and lamellar and disordered mesostructures were obtained.17 Che et al. proposed a costructure directing method using organosilanes (with amine or quaternary ammonium end groups) as the costructure directing agent (CSDA) to bridge the interactions between the surfactants and inorganic species, and this method has been widely used in the synthesis of anionic-surfactanttemplated mesoporous silicas.18,19 Inspired by the effect of the electrostatic supramolecular selfassembly of silaffins (or polyamines) with the polyanions, here we report a novel bioinspired approach to synthesize mesoporous silica by using an anionic surfactant as a template. Different from the CSDA method in which organosilanes were used to bridge the surfactant micelles and silica precursors,18,19 here, on the basis of the electrostatic self-assembly between cationic polyamine and anionic surfactant, the anionic micelles were covered with cationic polyamine chains, which were meanwhile gathered by accumulated anionic micelles. The aggregated cationic polyamine chains could thus effectively guide the condensation of silica precursor to form well-ordered silica mesostructures under mild conditions. Performing in a bioinspired fashion under mild conditions and without using any organosilanes, this approach could not only shed light on the study of biosilicification but also serve as a general method for the anionic-surfactant-templated mesoporous materials.
Figure 2. (a, b) SEM and (c−f) TEM images of the calcined sample (SPS-1) synthesized at 80 °C. The TEM images were recorded along the (c) [100], (d) [110], (e) [111], and (f) [211] directions. The corresponding FT diffractograms were inserted into the HRTEM images.
attention, and their synthesis approaches are currently actively studied.9,10 Besides possible practical applications, studies on mesoporous materials may also shed light on fundamental mechanisms of biomineralization.11,12 Generally, surfactant micelles were utilized as templates to assemble with inorganic
Figure 3. (a) SEM and (b−d) TEM images of the calcined samples synthesized at 80 °C (SpS-2). The corresponding FT diffractograms were inserted into the HRTEM images. 2330
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Letter
EXPERIMENTAL SECTION
All chemicals were commercially available and were used as received. N-Lauroylsarcosine sodium (Sar-Na) and poly(diallyldimethylammonium chloride) (PAC, average molecular weight 200−350K, 20 wt % solution in water) were purchased from Aladdin Chemical Company, China. Tetraethylsiloxane (TEOS) and hydrochloric acid (HCl, 36−38%) were obtained from Tianjin Chemical Reagent Company (Tianjin, China). In a typical synthesis, 0.29 g of Sar-Na was completely dissolved in 30.0 mL of deionized water under stirring at room temperature. Then, 2.0 g (or 1.0 g) of a 0.1 M HCl solution was added to the surfactant solution under vigorous stirring. After the solution was stirred for 15 min at room temperature, 5.0 g (20 wt % solution) of PAC was added. Immediately, the solution became a milky suspension as a result of the formation of Sar-PAC complex colloids. After the solution was stirred for another 20 min, 1.5 g of TEOS was added to the above solution. The mixture was stirred at room temperature for another 2 h and then was transferred to a glass tube, which was left at room temperature or was put into an 80 °C oven for 24 h under static conditions. The solid products were collected by centrifugation, washed with deionized water, and dried at 60 °C. The as-synthesized samples were calcined at 550 °C for 6 h to remove the templates. Powder X-ray diffraction (XRD) patterns were recorded using a Bruker diffractometer with Cu Kα radiation (D8 Advance X-ray diffractometer, Cu Kα, λ = 1.5406 Å, 40 kV and 40 mA). TEM observations were performed on a Philips Tecnai F20 microscope working at 200 kV. All samples subjected to TEM measurements were dispersed in methanol ultrasonically and were dropped on copper grids. SEM images were obtained with a field-emission scanning electron microscopy (FESEM; JEOL, JSM-7500F, 8 kV) instrument. Nitrogen adsorption and desorption isotherms were measured on a BELSORP-mini II sorption analyzer at 77 K. The specific surface area was calculated by the BET (Brunauer−Emmett−Teller) method, the pore-size distribution was calculated from the adsorption branch using the BJH (Barett−Joyner−Halenda) method, and the total pore volume was obtained at p/p0 = 0.99. 13C CPMAS NMR spectra were recorded on a Varian Infinity Plus 400 instrument at a spin rate of 8 kHz, a 0.8 ms contact time, and a 3 s recycle delay.
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RESULTS AND DISCUSSION The synthesis was facile and could be performed at room temperature or 80 °C, with anionic surfactant N-lauroylsarcosine sodium (Sar-Na) and poly(diallyldimethylammonium chloride) (PAC) as cotemplates. Because the series of samples consisted of surfactant and polyelectrolyte cotemplated mesoporous silicas, they were denoted as the SPS-n series. The smallangle XRD patterns of the calcined samples synthesized at room temperature or 80 °C are shown in Figure 1. For XRD patterns (Figure 1a,b) of the calcined samples synthesized at pH of 5.3 (with the addition of 2.0 g of 0.1 M HCl solution during synthesis), two well-resolved diffraction peaks were observed and were indexed to the (220) and (311) characteristic diffractions of the 3D cubic Fd-3m mesostructure,20,21 and this sample was denoted as SPS-1. For XRD patterns (Figure 1c,d) of the calcined samples synthesized at pH of 6.0 (with the addition of 1.0 g of 0.1 M HCl solution during synthesis), the clearly resolved diffraction peaks could be indexed to (100), (002), and (101) characteristic diffractions of 3D hexagonal close-packed mesostructure (hcp) with P63/mmc symmetry.22,23 The unit cell parameters of the calcined sample synthesized at 80 °C were a = 6.29 nm and c = 10.26 nm with a c/a ratio of 1.631, which is very close to the ideal c/a ratio (1.633) of the hexagonal close-packed structure. The sample with a 3D hexagonal close-packed mesostructure was denoted as SPS-2. SEM images showed that sample SPS-1 generally consisted of submicrometer particles with irregular facets (Figure 2a,b).
Figure 4. Nitrogen adsorption−desorption isotherms and pore diameter distribution curves (insets) of the calcined samples synthesized at 80 °C: (a) SPS-1 and (b) SPS-2.
TEM images of calcined SPS-1 observed along directions [100], [110], [111], and [211], respectively, are shown in Figure 2c−f, together with their corresponding FT diffractograms. The highly ordered mesostructure of SPS-1 as revealed by TEM images implies that each particle shown in the SEM images could be a single-crystalline mesocrystal. From the SEM images shown in Figure 3a, sample SPS-2 was composed of disclike particles, and some of the discs clearly exhibited the morphology of a hexagonal plate (as the red-dashed-line contour). TEM images observed along different directions with corresponding FT diffractograms of calcined SPS-2 are shown in Figure 3. The mesostructure was well ordered, as can be derived from the uniform areas of the TEM images. The nitrogen adsorption−desorption isotherms and pore size distribution curves of calcined SPS-1 and SPS-2 are shown in Figure 4, and they all exhibited type IV isotherms with distinct adsorption steps at relative pressures p/p0 of 0.3−0.7, corresponding to nitrogen capillary condensation in the cagelike mesopores. The t-plot micropore analyses (Figure S1) indicated that there were almost no micropores in the two samples. The structural and texture parameters of the samples are listed in Table 1. 2331
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formation of mesostructured silica with the anionic surfactant micelles as a template. 13C CPMAS NMR spectra of Sar-Na, PAC (mixed with Al2O3 and dried) and as-synthesized SPS-1 are shown in Figure 5. By comparison of the spectra, it is clear that both Sar− and PAC were present in as-synthesized SPS-1. It should be noted that the signals corresponding to the aliphatic carbons in the alkyl chain of Sar-Na remained relatively sharp whereas the signal of the carbonyls of Sar-Na at around 175 ppm became broadened. The alkyl chains within the anionic surfactant micelles were relatively flexible and thus gave rise to relatively sharp NMR signals. PAC chains interacted with the silica precursors and finally were fixed by the silica wall so that the signals of the PAC carbon groups exhibited relatively broad peaks. The carbonyls of anionic surfactants were electrostatically interacted with the fixed PAC chains; therefore, their signal at around 175 ppm became broadened in the as-synthesized sample. The charge of Sar-Na could be tuned at different pH values, and this would influence the aggregated packing mesophase of the Sar-PAC complexes, giving rise to different mesostructure of the as-synthesized silica. Here in our synthesis, only with the help of PAC could the anionic surfactant micelles play the role of template for mesostructured silica. However, it must be noted that without the addition of Sar-Na to the synthesis, PAC alone was not effective enough to induce silica precipitation (Figure S4). Profiting from the coassembly with anionic surfactant micelles, the PAC chains were aggregated and thus were endowed with the silica precipitation function. This reminds us of the case that negative phosphorylated serine residues in silaffins promoted their effect via supramolecular assembly to induce silica formation in vitro.3−6 Herein, the synergy between anionic surfactants and cationic polyamine just mimics the scene in silica biomineralization. On the basis of the above experimental results, the mechanism of the synthesis method is illustrated in Scheme 1.
Table 1. Structural Parameters of Mesoporous Silicas Synthesized at 80 °C sample
structure
SPS-1
3D-cubic (Fd-3m) 3D-hexagonal (P63/mmc)
SPS-2
unit cell (nm)a
surface area (m2 g−1)b
pore volume (cm3 g−1)c
pore diameter (nm)d
a0 = 16.9
846
0.97
3.6
a = 6.29, c = 10.26
867
0.78
3.2
a
Calculated from the XRD patterns. bCalculated by the BET method. Obtained at a relative pressure p/p0 of 0.99. dCalculated from the adsorption branch of the N2 isotherm by the BJH method.
c
It is well known that the ionic self-assembly of polyelectrolyte and oppositely charged surfactants can form highly ordered mesomorphous liquid-crystalline phases.24−27 During the synthesis, the electrostatic coassembly between anionic Sar-Na micelles and polycationic PAC chains resulted in Sar-PAC complex colloids and gave rise to a milky suspension (Figure S2). The Sar-PAC organic complex colloids could be collected by centrifugation and were subjected to XRD measurement to monitor their mesophase. As shown in Figure S3, low-angle XRD pattern of the soft Sar-PAC complex exhibited diffraction peaks, indicating the presence of ordered mesophases. However, the diffraction peaks in Figure S3 could not be assigned to any single mesophase and they were probably due to a mixture of different mesophases. It has been reported recently that phase transition occurred in polyelectrolyte−surfactant complexes with different hydration states,28 and the soft Sar-PAC complex solid sample could undergo mesophase transformation and exhibited different mesophases as a result of the drying effect before and during the XRD measurement. Upon addition of the silica precursor to the suspension of the mesomorphous Sar-PAC complex, the outer-layer PAC chains would control the condensation of the silicate precursors to induce the
Figure 5. 13C CPMAS NMR spectra of Sar-Na, PAC (mixed with Al2O3 and dried), and as-synthesized SPS-1. 2332
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Scheme 1. Schematic Illustration of the Formation Process of Mesostructured Silica SPS-n
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CONCLUSIONS A novel bioinspired approach for ordered mesoporous silica with anionic surfactants as templates was developed. Because of the electrostatic self-assembly between the cationic polyamine and anionic surfactant, the organic complex colloid mesophase was formed and served as a template in which the anionic micelles were covered with cationic polyamine chains, which were meanwhile gathered by the accumulated anionic micelles. With the help of cationic polyamine, anionic surfactant micelles could be utilized as a mesostructure template whereas with the aid of the anionic surfactant micelles the cationic polyamine chains underwent aggregation to possess the ability to induce silica condensation. Mesoporous silicas with well-ordered mesostructure, SPS-1 (with cubic Fd-3m mesostructure) and SPS-2 (with hcp mesostructure) were fabricated. Performing in a bioinspired fashion under mild conditions and without using any organosilanes, this approach could not only shed light on the study of biosilicification but also serve as a general method for the anionic-surfactant-templated mesoporous materials. Because of the abundant types of anionic surfactants and polyamines, new mesostructures and morphologies are also expected.
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ASSOCIATED CONTENT
S Supporting Information *
t-plot micropore analysis, XRD pattern, and experimental photographs. This material is available free of charge via the Internet at http://pubs.acs.org/.
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This work was supported by NSFC (no. 21373116), the Tianjin Natural Science Research Fund (13JCYBJC18300), RFDP (20120031110005), and the MOE Innovation Team (IRT13022) of China.
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REFERENCES
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(22) Che, S. A.; Lim, S.; Kaneda, M.; Yoshitake, H.; Terasaki, O.; Tatsumi, T. The effect of the counteranion on the formation of mesoporous materials under the acidic synthesis process. J. Am. Chem. Soc. 2002, 124, 13962. (23) Yuan, P.; Sun, J.; Xu, H.; Zhou, L.; Liu, J.; Zhang, D.; Wang, Y.; Jack, K.; Drennan, J.; Zhao, D.; Lu, G.; Zou, X.; Zou, J.; Yu, C. Extensive inspection of an unconventional mesoporous silica material at all length-scales. Chem. Mater. 2011, 23, 229. (24) Antonietti, M.; Conrad, J.; Thünemann, A. Polyelectrolyte− surfactant complexes: a new type of solid, mesomorphous material. Macromolecules 1994, 27, 6007−6011. (25) Faul, C. F. J.; Antonietti, M. Ionic self-assembly: facile synthesis of supramolecular materials. Adv. Mater. 2003, 15, 673−683. (26) Piculell, L.; Norrman, J.; Svensson, A.; Lynch, I.; Bernardes, J.; Loh, W. Ionic surfactants with polymeric counterions. Adv. Colloid Interface Sci. 2009, 147−148, 228−236. (27) Ikkala, O.; Brinke, G. Functional materials based on selfassembly of polymeric supramolecules. Science 2002, 295, 2407−2409. (28) Leonard, M.; Strey, H. H. Measurement of phase transition free energies in polyelectrolyte−surfactant complexes. Macromolecules 2010, 43, 4379−4383.
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