Synthesis of 1D Silica Nanostructures with Controllable Sizes Based

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Synthesis of 1D Silica Nanostructures with Controllable Sizes Based on Short Anionic Peptide Self-Assembly Shengjie Wang,* Qingwei Cai, Mingxuan Du, Junyi Xue, and Hai Xu* State Key Laboratory of Heavy Oil Processing and Center for Bioengineering and Biotechnology, China University of Petroleum (East China), No. 66 Changjiang West Road, Qingdao 266580, P. R. China

Downloaded by UNIV OF PENNSYLVANIA on September 3, 2015 | http://pubs.acs.org Publication Date (Web): August 31, 2015 | doi: 10.1021/acs.jpcb.5b06455

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ABSTRACT: Artificial synthesis of silica under benign conditions is usually achieved by using cationic organic matrices as templates while the anionic analogues have not received enough consideration, albeit they are also functioning in biosilica formation. In this work, we report the design and self-assembly of an anionic peptide amphiphile (I3E) and the use of its self-assemblies as templates to synthesize 1D silica nanostructures with tunable sizes. We show that short I3E readily formed long nanofibrils in aqueous solution via a hierarchical self-assembly process. By using APTES and TEOS as silica precursors, we found that the I3E nanofibrils templated the production of silica nanotubes with a wide size distribution, in which the silica size regulation was achieved by tuning the interactions among the peptide template and silicon species. These results clearly illustrate a facile method for generating silica nanomaterials based on anionic matrices. agents for directing polyamines and silaffins assembly.19 Recent research indicated that silacidins can also affect the silicifying rate and sizes of the resulting silica nanospheres, and play more important roles especially in silicon starvation conditions.20 In situ atomic force microscopy (AFM) investigation on silica nucleation indicated that silica preferred to nucleate on substrates patterned with cationic and anionic groups rather than those with pure cationic groups, and silica nucleation was most rapid when oppositely charged species were proximal.21 These results show that anionic molecules may go beyond a single function of cross-linking in the silica production in organisms. On the whole, however, what we know about anionic species in biosilica formation is rather limited in comparison with cationic analogues. Silica with well-defined morphologies has been constructed by using biotemplates with a negatively charged surface such as proteins, viruses, bacterial flagella, bacteria, and fungi.22,23 Relative to these large biotemplates and bioextracts such as silacidins that typically have complex structures and properties, small and simple synthetic molecules that can readily selfassemble into well-defined nanostructures will be particularly attractive. This is because they allow us to fabricate silica nanomaterials from molecular engineering and with better control over their nanostructures, sizes, and morphologies.24−27 In this work, we designed an anionic amphiphilic short peptide Ac-IIIE-CONH2 (I3E), which readily underwent self-assembly in aqueous solution to form long nanofibrils with a negatively

1. INTRODUCTION Biogenic silica (biosilica) produced by living organisms such as diatoms and sponges exhibits evident superiority over artificial silica in its multiscale ordered structures, species-specific characteristics and functionalities, and mild formation conditions, and has therefore attracted great attention to investigate its formation mechanism(s). During the past few decades, considerable efforts have been made to explore the biosilicification process and identify the components involved in this process. Certain organic bioextracts such as silicateins, silaffins, and long-chain polyamines have been found to be implicated in biosilica formation, and their mechanistic roles such as catalyzing and templating mineralization and even phase separation during the silicification process have been argued.1−6 These investigations have greatly inspired researchers to construct novel inorganic nanomaterials with natural or synthetic organic molecules, and as a result, a variety of silica nanostructures and morphologies have been synthesized in vitro.7−18 However, the artificial synthesis of silica nanomaterials with intricate and ordered hierarchical structures that are comparable to biosilica is still in its infancy. Apart from our limited understanding of the genetic control mechanism, there are likely to be other as yet unknown factors. Except for the widely investigated cationic molecules such as polyamines and silaffins isolated from diatoms, certain anionic groups or bioextracts (e.g., phosphates, sulfates, and silacidins) should be given more attention, although they lack the ability to precipitate silica directly. For example, silacidins rich in glutamic acid, aspartic acid, and phosphate groups have been suggested to regulate the in vivo silica formation, in which they serve as cross-linking © XXXX American Chemical Society

Received: July 6, 2015 Revised: August 15, 2015

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DOI: 10.1021/acs.jpcb.5b06455 J. Phys. Chem. B XXXX, XXX, XXX−XXX

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The Journal of Physical Chemistry B


Scheme 1. Schematic Illustration of the Self-Assembly Process of I3E via β-sheet Formation, Lateral Stacking, and Twisting into Nanofibrils

2.3. Silica Nanostructures Preparation. Silica nanostructures were prepared at neutral pH and room temperature. In a typical reaction, APTES was introduced into a mixture of the peptide solution and an equal volume of ethanol, followed by stirring at room temperature for 0.5 h. Then, TEOS was added drop by drop with stirring. The reaction was carried out at room temperature under static conditions for 4 days. Silica hybrids were obtained by centrifuging, washed alternately with ethanol and water, and then freeze-dried. The concentrations of I3E and the silica precursors (TEOS and APTES) in the reaction mixture were fixed at 1 and 45 mM, respectively. To regulate the silica nanostructures, the relative concentration of TEOS and APTES and the solution pH were changed. 2.4. Instrumental Characterizations. Circular dichroism (CD) measurements were performed on a MOS-450 spectrometer (Biologic, France) at 25 °C with a 1 mm quartz cuvette. Spectra were recorded from 190 to 260 nm with a 0.5 nm step and a 1 s collection time per step. The presented CD signal was the average of five individual measurements, expressed as [θ] (deg cm2 dmol−1). AFM imaging was performed on a commercial Nanoscope IVa MultiMode AFM (Digital Instruments, Santa Barbara, CA) in tapping mode. Samples were prepared by dropping a drop of peptide solution (2.0 mM, pH 7) onto a freshly cleaved mica surface and adsorbing for about 30 s. Then, the mica surface was rinsed with water and dried gently with nitrogen gas for imaging. For transmission electron microscopy (TEM) characterizations, the mineralized samples were dispersed in ethanol and transferred on to the carbon-coated copper grid. After drying under ambient conditions, the samples were observed on a JEOL JEM-2100UHR electron microscope operated at 200 kV. As for the peptide samples, negative staining with 2% uranyl acetate was performed prior to TEM observation. Scanning electron microscopy (SEM) imaging was performed on a Hitachi S-4800 field emission SEM (FE-SEM) with an acceleration voltage of 5.0 kV. Samples were prepared by placing a droplet of a suspension of hybrid silica powder in ethanol on an aluminum stub and allowing ethanol to evaporate. Samples were coated with a thin Au layer to

charged surface. Using 3-aminopropyltriethoxysilane (APTES) and tetraethoxysilane (TEOS) as silica precursors, the peptide nanofibrils templated the synthesis of uniform silica nanotubes, whose diameters (from 10 to 60 nm) could be well tuned by simply adjusting the reaction conditions. To the best of our knowledge, this is the first time one-dimensional (1D) silica nanostructures with controllable sizes have been constructed by using small self-assembling anionic molecules.

2. EXPERIMENTAL SECTION 2.1. Materials. The materials for peptide synthesis including Fmoc-protected amino acids, O-(1H-benzotriazole1-yl)-N,N,N′,N′-tetramethyluronium hexafluorophosphate (HBTU), N-hydroxybenzotriazole anhydrous (HOBT), N,N′diisopropyl ethylamine (DIEA), trifluoroacetic acid (TFA), triisopropylsilane (TIS), and Rink amide-MBHA resin were purchased from GL Biochem Ltd. (Shanghai) and used as received. Piperidine, dichloromethane (DCM), and N,N′dimethylformamide (DMF) were purchased from Bo Maijie Technology (Beijing), and were redistilled prior to use. TEOS, APTES, TEOA, anhydrous ethanol, and other chemicals were purchased from Sigma-Aldrich and used as received. Water was from a Millipore water purification system with a minimum resistivity of 18.0 MΩ cm. 2.2. I3E Synthesis and Self-Assembly. Blocked I3E peptide was synthesized on a CEM microwave peptide synthesizer according to Fmoc solid-phase chemistry and the detailed synthetic procedures have been given in our previous publications.25,28 The crude product was collected through repeat precipitation (at least five times) in cold diethyl ether and then subjected to preparative liquid chromatography. After purification, the resulting product has a high purity (>98%), indicated by its mass spectrometry (MS) and reverse-phase high performance liquid chromatography (RP-HPLC) characterizations (Figure S1 and Figure S2). The purified I3E powder was dissolved in water at a concentration of 2.0 mM. The solution pH was first adjusted to 9.0 using dilute NaOH solution to accelerate the peptide dissolution, and then adjusted to pH 7.0. The peptide solution was kept at ambient temperature for at least 1 week before characterization or use for silica mineralization. B


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increase the contrast and quality of the image before SEM observation. Zeta potential measurements were performed on a Malvern Zetasizer Nano-ZS with laser wavelength of 633 nm. Fluorescence measurements of thioflavin-T (ThT, 75 μM) in the presence and absence of I3E (2.0 mM) were carried out on an F-2500 FL spectrometer at ambient temperature. Emission spectra were collected from 450 to 550 nm with an exciting wavelength of 440 nm. The excitation and emission slits were set to 5 and 10 nm, respectively.

3. RESULTS AND DISCUSSION 3.1. I3E Self-Assembly. Because of ease of synthesis, structural stability, and easier establishment of the relationship between molecular structures and assembled nanostructures, self-assembly of short peptides and their applications have attracted considerable interest.29−31 Our previous studies have shown that short cationic peptides rich in hydrophobic isoleucine residues have a strong propensity for β-sheet structuring and self-assembling into 1D nanostructures.25,28,32,33 I3E is composed of three consecutive isoleucine residues and one hydrophilic glutamic acid (Scheme 1). Its Nand C-terminals were capped during synthesis, thus making the charge on the peptide arise purely from the glutamic acid side chain. The secondary structure of 2.0 mM I3E was characterized with CD spectroscopy and there produced two negative peaks at 218 and 196 nm (Figure 1a), representing β-sheet and coil

Figure 2. (a) TEM image of I3E nanofibrils and (b) the statistical analysis of their diameters determined from TEM. I3E was dissolved in ultrapure water to form a 2.0 mM solution (pH 7.0) and kept at room temperature (20 ± 1 °C) for at least 1 week before TEM characterization.

clear from AFM imaging (Figure 3a and Figure S3). The prevailing occurrence of twisted fibrils in the AFM character-

Figure 1. (a) CD spectrum of the I3E aqueous solution (2.0 mM, pH 7.0) and (b) fluorescence emission spectra of 75 μM ThT in aqueous solution with or without I3E (2.0 mM). The peptide solution was prepared by directly dissolving the peptide powder into water and then adjusting the solution pH to 7.0. The peptide solution was incubated at room temperature for at least 1 week prior to instrumental characterizations and use for ThT binding.

Figure 3. (a) AFM height image of I3E nanofibrils, (b) cross-section profile of I3E nanofibrils, and (c) height variation of a twisted nanofibril.

ization most likely results from the substrate (mica) interference and/or the sample preparation procedures (e.g., water rinsing), which readily leads to the reversal of peptide self-assemlby.32 The heights derived from the AFM measurement were mostly in the range of 8−11 nm (Figure 3b), consistent with those derived from TEM imaging. The heights along the twisted nanofibrils oscillated with a periodicity of around 80 nm (Figure 3c). In addition, some thin I3E nanofibrils were observed to assemble into thick ones (black arrows in Figure 3). A hypothesis about the self-assembly of I3E can be proposed on the basis of the above experimental observations. As shown in Scheme 1, monomers (strands) move close to each other driven by the hydrophobic attractions of isoleucine chains. Hydrogen bonds are easily formed between adjacent strands and actuate fast growth into long β-sheets. The resulting βsheets are twisted owing to the intrinsic molecular chirality of the constituent amino acid residues.32,35 Despite the inherent

(or unfolded structures) secondary structures, respectively. This suggests that there is partial coil or unfolded structures in the peptide solution as well as regular β-sheet arrays. Figure 1b shows the fluorescence emission spectra of benzothiol dye ThT in aqueous solution in the presence of I3E or not. There was an obvious increase in the fluorescence emission at ∼485 nm when ThT was mixed with the I3E solution. This indicated that considerable I3E molecules formed β-sheet secondary structures and subsequently self-assembled into amyloid-like fibrils, in which the β-strands are arranged perpendicular to both the fibrillar axis and the lamination of multiple β-sheets.28,32,34 As shown in Figure 2a, I3E underwent self-assembly in water to form uniform nanofibrils of some 10 nm in width and a few micrometers in length (Figure 2b). Some left-handed twisted nanofibrils were also observed in addition to smooth ones, as labeled with red arrows. The left-handed twisting feature was C


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Figure 4. TEM images of silica nanostructures templated by I3E at pH 7.0 (a), pH 8.0 (b), and pH 9.0 (c). APTES was introduced into a mixture of the peptide solution (2.0 mM) and an equal volume of ethanol, followed by stirring for 0.5 h. Then, TEOS was added drop by drop with stirring. The concentrations of I3E, APTES, and TEOS were 1.0, 1.0, and 44 mM, respectively. The silicification reaction was carried out at room temperature under static conditions for 4 days. After centrifugation, the collected silica hybrids were washed with alternately water and ethanol, and freeze-dried.

twisting of β-sheets, their hydrophobic regions remain significantly exposed to water. To shield them from water, there is also a growth along the lateral direction, which leads to the lateral stacking of the β-sheets. However, such a lateral stacking is constrained by the intrinsic twisting of β-sheets and the electrostatic repulsions between glutamic acid side chains. In this case, thin I3E fibrils, with limited stacking and significant twisting of β-sheets, are finally formed. Note that in order to decrease the electrostatic and steric repulsions, charged carboxyl groups always tend to extend outward from the hydrophobic core of the aggregates, which further favor curvature and adds stabilization in the aqueous environment. 3.2. Construction of Silica Nanostructures. Similar to silacidins, the anionic I3E monomer and assemblies also lack a catalytic function for the sol−gel reaction of silicic acid or TEOS. However, in the absence of other catalysts, the I3E nanofibrils can act as a template and manipulate APTES to perform such a function and regulate silica morphogenesis. In addition to catalyzing the sol−gel process of TEOS by its amine group, APTES can also serve as a costructure-directing agent to ensure precise transcription of the template morphology to the resulting silica: (1) the protonation of the amine group at near neutral pH can make APTES adsorb onto the I3E nanofibril surface as well as interact with the deprotonated silanol groups of silicic monomer and polysiloxanes via electrostatic interactions (−NH3+···−OOC− and −NH3+···−OSi) and hydrogen bonding; (2) its alkoxysilane moiety is capable of condensing with TEOS by forming Si−O−Si bonds.22,23,36,37 We expect to construct silica nanostructures with high control by regulating the interactions among the peptide template, APTES and TEOS, and silica intermediates. The silicification reactions were performed over the pH range of 7 to 9 to make the best use of the interfacial interactions among the peptide template and silicon species

and, at the same time, to avoid acid or base becoming the primary catalyst. Silica precipitate could not be observed over this pH range in the absence of APTES, and no silica precipitate can be found in the absence of TEOS because the hydrolyzed APTES forms a six- or five-membered chelate ring that sterically hinders its self-condensation.38 It was not controllable in silica morphogenesis, and only spherical silica can be obtained if we mixed only the two silica precursors (Figure S4). However, in the copresence of I3E/APTES/TEOS, very long and uniform silica nanotubes were obtained with thin inner diameters, slightly less than the template diameter as a result of silica penetration into the peptide layer (Figure 4). It is interesting that their outer diameters or the silica layer thicknesses increased with pH. The silica nanotubes formed at pH 7.0, 8.0, and 9.0 have outer diameters of around 14, 28, and 46 nm, respectively, which means that regular silica nanotubes with controllable thickness can be obtained by slight adjustment of the solution pH. The pKa of the amine group of APTES is around 10.3, while that of the carboxyl group in the glutamic acid side chain is around 4.5. Thus, APTES and I3E are respectively protonated and deprotonated over the pH range of 7−9. Strong electrostatic interactions should be expected between the anionic peptide assemblies and cationic APTES molecules besides hydrogen bonding. As shown in Figure 5a, the Zeta potential value is about −30 mV for the 1.0 mM I3E solution (equal volume of ethanol and water), and an obvious increase in the zeta potential value with the addition of APTES, suggesting the binding of APTES onto the surface of the peptide assemblies with the consequent neutralization of the surface negative charges. Note that the zeta potential value remained constant when the concentration of APTES was greater than 1 mM, suggesting that excessive APTES might dissolve in solution, rather than adsorb on the peptide D


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the mineralization rate in solution by significantly raising the APTES concentration or the solution pH, the nontemplated silica deposition became obvious (Figure S5). Up until now, most artificial 1D silica nanostructures have been prepared based on cationic templates with TEOS or silicic acid as silica precursors,24−27,40,41 in which their sizes are often regulated to a limited extent (usually a few nanometers) by surface functional groups that are trapped into the silica coating during the mineralization. In comparison, anionic I3E assemblies are not a catalyst but rather a template for attracting cationic APTES through electrostatic interaction and hydrogen bonding. In addition, APTES adsorbed on the template can act as the catalyst for the sol−gel reaction. It may be this change that leads to the easy regulation of mineral size. Thus, following the schematic illustration in Scheme 2, the accumulation of APTES around I3E assemblies can be expected at a pH of 7−9, followed by silica formation and rapid growth along the peptide assemblies upon TEOS introduction. Unlike the amine groups immobilized on the templates via covalent bonds, those in APTES are more mobile and can continue adsorbing onto the growing silica nanostructures during the mineralization process. They therefore play a catalytic role for the subsequent silicification and also decrease the electrostatic repulsion for silica deposition owing to the neutralization of the negative charges on the template surface. The I3E assemblies act as templates only at the beginning of the reaction and then APTES takes over the structure directing function during the succeeding mineralization. This shows that we can adopt various noncationic nanostructures as templates to construct silica materials or hybrid materials with novel structures and designable properties.


Figure 5. (a) Zeta potentials of the 1.0 mM I3E solution (equal volume of water and ethanol) at pH 7.0 with different concentrations of APTES and (b) influence of pH on the charge per silanol group for silicic acid and polysiloxane.

assemblies. Therefore, 1.0 mM APTES was used in our silicification experiments to avoid more nontemplating silica precipitation in the bulk solution. The APTES adsorbed on the template will interact with TEOS, followed by condensation onto the silica layer (Scheme 2). During the subsequent growth of the silica layer, the interfacial electrostatic interactions between the amine group and silica intermediates play a crucial role. It is well-known that during the sol−gel process, the Si-OCH2CH3 groups of TEOS and APTES first undergo hydrolysis to form silanol groups and this is followed by the condensation reaction. The pKa values of the silanol groups of silica intermediates are dependent on their degree of polymerization, for instance being around 9.0 for silicic acid and partly hydrolyzed monomers and around 6.5 for polysiloxanes.39 Using the Henderson− Hasselbach equation, their deprotonated states or charge properties can be predicted (Figure 5b). It can be seen that the charge property of silicic acid varies significantly over the pH range of 7 to 9, in contrast to that of polysiloxanes. For instance, silicic acid carries little negative charge at pH 7.0, thus disfavoring its electrostatic interaction with the protonated amine group on the preformed silica layer around the template, while at pH 9.0, silicic acid is mostly deprotonated and a strong electrostatic interaction will favor its adsorption onto the preformed silica layer resulting in a thick silica layer (Figure 4a,c). Accordingly, if we decrease the mineralizing rate to ensure complete adsorption of the resulting silicic acid or the partly hydrolyzed monomers, we can obtain thicker silica nanostructures. As expected, we observed silica nanotubes thicker than 60 nm with the addition of a hydrolysis inhibitor triethanolamine (TEOA) (Figure 6). However, if we increased

4. CONCLUSIONS On the basis of the self-assembly of a designed short peptide I3E, we have demonstrated an effective approach for preparing 1D silica nanostructures by using anionic organic matrices. Driven by hydrogen bonding and hydrophobic interactions, anionic I3E underwent self-assembly in aqueous solution to form long nanofibrils with a negatively charged surface. By using APTES and TEOS as silica precursors, silica nanotubes with controllable sizes were templated by the anionic selfassemblies. The size regulation of silica nanotubes was achieved by tuning the interactions among the peptide template and silicon species. This work sheds new light on how to construct intricate and novel silica or hybrid nanostructures with anionic organic matrices in biomineralization.

Scheme 2. Schematic Illustration of the Formation Process of Silica Nanostructures of Controlled Sizes by the Cooperation of I3E Nanofibril Template, APTES, and TEOS. In this Reaction, APTES Serves as the Catalyst and the Bridging and StructureDirecting Agent

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Figure 6. (a) SEM image of silica nanostructures in the presence of TEOA and (b) their diameter distribution. The molar concentrations of I3E, APTES, TEOS, and TEOA were 1.0, 1.0, 44, and 90 mM, respectively, and the silicification reaction was performed at pH 9.0. The scale bar is 100 nm.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpcb.5b06455. ESI-MS spectrum and HPLC profile of I3E, AFM image of I3E nanofibrils, TEM image of silica spheres prepared from TEOS and APTES in the absence of peptide templates, and TEM images of silica hybrids obtained when the APTES concentration and solution pH were significantly increased (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*(Shengjie Wang) Tel.: +86 532 8698 3455. E-mail: sjwang@ upc.edu.cn. *(Hai Xu) Tel: +86 532 8698 1569. E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work is supported by the National Natural Science Foundation of China (21103229) and the Fundamental Research Funds for the Central Universities (15CX05017A). HX acknowledges the support by the Program for New Century Excellent Talents in University (NCET-11-073).

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REFERENCES

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