Induced Crystallization of Amorphous Biosilica to Cristobalite by

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Induced Crystallization of Amorphous Biosilica to Cristobalite by Silicatein Ido Fuchs,† Yaniv Aluma,‡ Micha Ilan,‡ and Yitzhak Mastai*,† †

Department of Chemistry and the Institute of Nanotechnology, Bar-Ilan University, Ramat-Gan 52900, Israel Department of Zoology, George S. Wise Faculty of Life Sciences, Tel Aviv University, Tel Aviv 6997801, Israel

‡

S Supporting Information *

ABSTRACT: In nature it is known that silicatein (silica protein) controls the mineralization of a wide range of biosilicas. In this paper we present our results on the induced crystallization of biosilica to cristobalite, which is the thermodynamically most stable crystalline form of silica at a relatively low temperature and ambient pressure. The phase transformation of biosilica from marine sponges to cristobalite under thermal treatment was investigated by a variety of methods, e.g., X-ray diffraction, high-resolution electron microscopy−electron diffraction, and optical methods such as Fourier transform infrared (FTIR) spectroscopy. Our results show that biosilica from marine sponges exhibits a direct phase transformation to cristobalite structure at a relatively low temperature (850 °C). Furthermore, it is shown that porous silica templated with silicatein proteins extracted from sponges also exhibits a phase transformation to cristobalite structure at a relatively low temperature. The surprising discovery that silicatein filaments can induce direct crystallization of biosilica to cristobalite highlights the role of silicatein in governing the synthesis and the hierarchical structure control of biosilica minerals.

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INTRODUCTION The formation of crystalline thin films is important for many technological applications such as high performance thin-film transistors (TFT), solar cells, electro-optical devices, and largearea electronics. In an effort to reduce the crystallization temperature and crystallization time of thin films, metalinduced crystallization (MIC) technique has been investigated as an alternative to the crystallization process for thin-film device fabrication. The MIC method was developed and studied mostly for the preparation of polycrystalline silicon thin films from amorphous silicon. In MIC, an amorphous Si film is placed in direct contact with a metal, such as Al, Ni, Cu, or Au,1,2 and this structure is then annealed at temperatures between 200 and 400 °C, which causes the Si films to be transformed into polycrystalline silicon at relative low temperature. In the literature a few mechanisms have been proposed3 to explain metal-induced crystallization such as layer exchange and diffusion-assisted crystallization. It should be mentioned that the phenomenon of induced crystallization is also known to occur under different conditions, for example, flow- and strain-induced crystallization of polymers,4 surface-induced crystallization,5 and solvent-induced crystallization.6 Recently, the concept of template-induced crystallization was demonstrated in few papers; for instance, self-assembled monolayers of ω-substituted alkylthiols on gold surfaces were used to mimic templated crystallization for the three polymorphs of calcium carbonate.7−9 Biomimetics chemistry is an emerging interdisciplinary field that combines information from the study of biological structures and their functions in the development of new strategies for the generation of novel synthetic materials. © 2014 American Chemical Society

Biomineralization process has been one of the main sources for inspiration in materials chemistry. In the biomineralization process, the concept of templateinduced biomineralization is very common and refers to the biological process in which biological macromolecules, proteins, or surfaces are used as the orientation-inducing material in the mineralization of a certain solid phase or morphology.10−13 The basic understanding of the template-induced biomineralization route is that the biological template macromolecules can lower the supersaturation and nucleation energy required for the biomineralization, and as a result, nucleation is induced, resulting in metastable nucleation and crystal growth. Silicatein is a protein used mainly by sponges to capture silicate from the environment in order to build silica structures. Silicatein is one of the well-known examples of biological macromolecules that can induce biomineralization of soluble silicon sources and assemble them into high architecture structures of silica. The biochemistry and molecular mechanism of biosilicification by silicatein have been proposed in several review papers.14,15 Furthermore, silicatein and its derivatives have been used as molecular tools for materials synthesis.16,17 It has been shown that silicatein can be used for the lowtemperature and near-neutral pH synthesis of many interesting and nonbiological metal oxides, e.g., TiO2, Ge2O3, and BaTiF6. The protein silicatein has been reported18,19 to catalyze the production of poly(silicate) from TEOS at neutral pH and Received: November 20, 2013 Revised: February 3, 2014 Published: February 5, 2014 2104

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Figure 1. SEM images: (a) overview of the sponge’s needle-shaped spicules, (b) silicatein filament (red rectangle shows the EDX area taken) located inside the cavity of the sponge spicules and the EDX spectra (inset), (c) high-resolution SEM of the spicule’s surface reveals the 50 nm grains composing this biostructure, and (d) a cross section of the sponge’s spicule reveals the circular direction growth of the silica skeleton.

under ambient temperature.20 Silicatein was also used as an enzymatic catalyst and organic template for biosilicification and for the low-temperature synthesis of various nanomaterials as recently reviewed by Muller and Morse.21−24 In this paper we present our results on the induced crystallization of amorphous biosilica to cristobalite at relatively low temperature and ambient pressure induced by Silicatein. We use multidisciplinary analysis tools such as Fourier transform infrared (FTIR) spectroscopy and electron microscopy techniques to study the phase transformation of the sponge’s amorphous biosilica to cristobalite structure at ambient pressure and low temperature of 850 °C. Furthermore, in this article it is shown that porous silica templated with silicatein proteins extracted from the sponges also exhibit phase transformation to cristobalite structure at a relatively low temperature.

prepared, and the specimen was immersed in it for 12 h, followed by extensive wash in cold Millipore water until it reached a neutral pH level. Isolation of Silicatein Filaments. Specimens of Negombata magnif ica were collected at the Eilat, Red Sea (29°30′07 N; 34°55′02 E) in shallow water (8−12 m) and were kept in seawater until arrival at the laboratory. Sponges were washed with filtered seawater, cut, and then cleaned using 5% NaClO3 solution for 24 h to get clean samples of spicules devoid of cellular and other external organic material. The samples were washed extensively in Millipore water followed by treatment with cold HNO3/H2SO4 1:4 solution for 12 h. The samples were then rinsed with Millipore water, brought to neutral pH, and immersed in cold NH4F/HF 8 M/2 M solution overnight or until a clear solution was achieved. The filaments were washed 5 times with MQ water and dialyzed against 10 L of MQ water at 4 °C overnight. The solution was replaced 3 times. The dialysate was collected using centrifugation at 10 000 rpm for 20 min at 4 °C and kept at 4 °C for further use. Entrapment of Silicatein Filaments. Silica matrix and entrapment of silicatein within was achieved by using the sol− gel process based on the entrapment method of alkaline phosphatase (AIP) proposed by Avnir et al.25 The filaments were transferred from the aquatic solution to Et−OH solution by centrifugation and replacement of the solvent. 0.4 mL of tetraethyl orthosilicate (TEOS) (98%, Sigma) and 0.98 mL of 25 mM HCl solution were stirred in a PP vial for 30 min at 40

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EXPERIMENTAL METHODS Specimen Collection. The silica spicules that were used were isolated from the marine sponge Cinachyrella levantinensis harvested from the Mediterranean Sea at Maagan Michael (32°29′77 N; 34°53′23 E), from a depth of 2 m. Samples were prepared by cutting the sponge to small cubes (4 cm3) which were then gently shaken in 5% NaOCl3 for 24 h, followed by a rinse with water and filtered in 200 μm filters. In several cases, to ensure the absence of organic substance around the spicules, a mixture of cold (4 °C) HNO3:H2SO4 1:4 solution was 2105

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showing the main peaks at 450 cm−1 associated with “rocking” mode of Si−O−Si bond, 874 cm−1 associated with bending of Si−O−Si bond, and 932, 1065 cm−1 associated with νas and νsym, all of which correlate with the FTIR spectra of amorphous silica.27 In view of the above results, it is clear that silicatein filaments play an important role in the organization of the ordered macrostructure/nanostructure of sponge spicules.28−30 As mentioned above, silicatein can induce biomineralization of soluble silicon sources and assemble them into higharchitecture structures of silica, supporting our assumption that silicatein can induce crystallization of amorphous silica to crystalline SiO2 at low temperature. The transformation of amorphous silica to higher SiO2 polymorphs31 is known to occur only at high temperature and/or pressure, sometimes with the assistance of surface directing agent (SDA) and also in the case of highly porous preordered silica (known as zeolites).32 The phase diagram for crystalline SiO2 shows that the first transformation from αquartz to α-tridymite occurs at 870 °C33 and from quartz to βCristobalite at 1600 °C.34 To investigate the phase transformation of amorphous silica in the sponge, we used whole sponge needle-shaped spicules and heated them to 850 °C in a quartz tube furnace for 6 h under air. These thermally treated spicules were characterized by X-ray powder diffraction, electron microscopy HR-TEM, HR-SEM, EDX, and other optical methods. The X-ray diffraction pattern of the thermally treated sponge spicules is shown in Figure 2 (red line) and displays sharp main peaks at

°C. In a separate PP vial 0.93 mL of concentrated silicatein solution was mixed with 0.4 mL of Et−OH and kept cold. The sol solution was cooled using icy water and then transferred to the silicatein solution for 5 min. To the solution 0.25 mL of glycine−NaOH buffer solution (pH 9.5) was added and left at 4 °C for 24 h, followed by transfer to a fume hood in order to speed the aging process. X-ray Diffraction Analyses. Powder X-ray diffraction (XRD) patterns were acquired with a Bruker AXS D8 Advance diffractometer with Cu Kα (λ = 1.5418 Å) operating at 40 kV 40 mA. Samples were mashed with an agate mortar and placed in a PP holder. Data were collected from 5° to 80° with a step size of 0.01° and rotation at 25 rpm. Fourier Transform Infrared Spectroscopy. FT-IR/ATR analyses were taken on a Bruker Alpha spectrometer. A diamond ATR crystal was used as the internal reflection element, and the incident angle was fixed at 45°. The films were scanned 50 times at a 4 cm−1 resolution, in the region of 4000− 450 cm−1. The sample was then placed on the reflection element and carefully pressed, and three spectra were taken and compared to ensure spectra accuracy. Scanning Electron Microscopy. Surface morphology was characterized with a FEI scanning electron microscope (SEM) Model Inspect S. Samples were prepared by sampling a small amount of the spicules on double-sided carbon tape Transmission Electron Microscopy. Samples were examined with a transmission electron microscope (TEM, JEOL 2100, 200 kV) at alternating magnifications. To observe by TEM a small amount of subsample dispersed in Et−OH was dropped onto a conductive holey carbon copper grid, letting the sample to dry out at room temperature. For selected areas electron diffraction patterns (SADP) were acquired. Temperature Treatment. Spicules samples were taken as whole or mashed in a ceramic holder into horizontal tube furnace (Carbolite MTF 12/38/400) set to 850 °C in 5 °C min−1 steps, for 360 min, followed by controlled cooling.

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RESULTS AND DISCUSSION In the first stage of our research we performed a structural and morphology study of the siliceous sponges. Scanning electron microscopy (SEM) images of the native sponge Cinachyrella Levantinensis are shown in Figure 1. As can be seen from Figure 1, the overall structure of the sponge is constructed from needle-like shape SiO2 fibers with dimensions of 4−10 mm long and 40 μm wide in average. In each sponge fiber there is a 4 μm tunnel that holds the silicatein protein and protects it from the outer environment (Figure 1b, marked in the red rectangle). Silicatein is known to be the constructing element of the sponge skeleton.26 Energy dispersive spectrometry (EDS) analysis of the SEM images indicated that the inorganic envelope of native sponge is composed of Si (Kα 1.74 keV) and O (Kα 0.52 keV) (with ratio of 1:2), and the core material (inset in Figure 1b) is composed of C (Kα 0.28 keV) N (Kα 0.39 keV) and overlap with O, indicating organic substance. Moreover, in a closer look using HR-SEM, the architecture of the silica shows circular direction growth of the needle and 50 nm grains forming these circular growing rings, indicating amorphous phase formation (Figure 1c,d). X-ray diffraction (XRD) pattern of the native sponge needles showed a broad peak in the range of 15°−30° (2θ) with no sharp crystallinity peaks (see Supporting Information Figure S.1). Finally the FTIR spectra (Figure 3a) of the native sponge demonstrated the amorphous composition of the silica’s outer layer by

Figure 2. X-ray powder diffraction of natural sponge spicules sample (black line) and thermally treated sample (red line) shows the crystalline diffraction of cristobalite formed as a result of the thermal treatment (main peaks are shown; for full correlation see Supporting Information).

2θ = 22.05°, 28.5°, 31.5°, and 36.4°, which correspond accurately to the diffraction pattern reported for SiO2 αcristobalite,35 with space group of P41212 (cell parameter a = 4.96 Å, b = 6.925 Å). It should be noted that additional weak Xray diffraction peaks at 2θ = 21.0° and 26.7° are observed and correlate with quartz SiO2 (COD #9001578). The FTIR spectra of the native sponge spicules after thermal treatment are shown in Figure 3b. The transformational change of the thermally treated sample is illustrated with the 2106

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appearance of new peaks at 620 cm−1 and a knee at 1200 cm−1 correlated to the structure of cristobalite.27 The transition from amorphous into crystalline structure of the biosilica is also shown by HR-SEM and HR-TEM measurements. Cross-section HR-SEM of the native sponge spicules (Figure 1a,c) shows the circular morphology growth direction of the spicules, which are composed from grains of ca. 50 nm, indicting an amorphous system. On heating the sample, the circular topology of growth vanished and continuous structure was observed (Figure 4a). This crystalline structure of layers was also detected in HR-TEM (Figure 4c) and with dark field measurement (Figure 4d). Selective electron diffraction measurement of the crystalline pattern was conducted (Figure 4b) in order to calculate the d spacing and planes of the crystalline area. The diffraction pattern was compared to the literature and to the sample’s XRD diffraction and found to be cristobalite. Several compositional and structural changes were conducted on the spicules. At first, the needle-like structure was mashed, after external cleaning in acid, to a fine powder and only then exposed to the heat treatment. This was done to explore the possible role of the macrostructure of the spicule on the crystallization process. The XRD and HR-TEM diffraction pattern was unaltered, and crystalline structure was obtained.

Figure 3. FTIR (ATR) spectra: (a) natural sponge spicule sample with characteristic peaks at 450, 874, 932, and 1039 cm−1 corresponding to amorphous silica; (b) thermally treated sponge sample showing additional peaks at 620 and 1200 cm−1 corresponding to the cristobalite crystalline structure.

Figure 4. (a) HR-SEM image of the cross section after thermal treatment showing the laminar dense composition of the surface. (b, c) HR-TEM and diffraction of the crystalline phase obtained from the thermally treated sponge and (scale bar = 2 nm). (d) Dark field image of the sample (scale bar = 50 nm). 2107

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extraction described above and the entrapment of the silicatein within the silica matrix were done carefully so as not to harm the silicatein structure18 (see Supporting Information S.3). In parallel to the silicatein entrapment, a control was prepared without the filaments. After a considerable aging time, the silica matrix was ground and thermally treated at 850 °C. The XRD spectra are shown in Figure 6. In the case before any thermal

Further examination was conducted on the mashed spicules by carefully washing and mixing the mashed sample with an icy mixture of HNO3:H2SO4 1:4 solution for 6−12 h. By this time all of the organic material within the spicules was consumed, and the mash was in its pure inorganic phase. After thermal treatment, XRD, HR-TEM, and diffraction measurements did not show any evidence of crystalline structure. By this we established the intimate role of silicatein in the formation of the crystalline form of cristobalite. In order to see if this attribution for the crystalline structure depends on the sponge type, a different sponge species Negombata magnif ica was taken and cleaned, and spicules were extracted using hypochlorite solution and then icy HNO3:H2SO4 1:4 solution. After achieving neutrality, the sample was dried with air. The spicules of this species were examined by the same experiments. At first, the plain structure of the spicules was heated to 850 °C and examined in XRD and, HR-TEM. This achieved the same transformation to cristobalite (see Supporting Information Figure 2). When mashed and treated by strong acid mixture, the spicules’ inner matter dyed the solution yellow, probably due to the high (∼80%) concentration of the organic material in the spicules of this species. A sample of spicules was divided into two fractions. Both fractions were soaked in the strong acid and thermally treated. However, in the first fraction the spicules were ground before thermally treatment but after being exposed to acid, while the spicules in the second fraction were ground prior to being immersed in acid and thermally treated. It can be seen from Figure 5 that in the case of the fraction that maintained

Figure 6. Sol−gel imbedded with silicatein filament after thermal treatment (red) and blank without the organic filament (black).

activation (see Supporting Information), both the blank and the embedded silica matrix show no sign of crystallinity. After thermal treatment the embedded silica shows a crystalline pattern of cristobalite, with a small amorphous band at 14° (2θ) (Figure 6). This is not the case with the blank, which, as expected, remained amorphous. Conformation for this change and the contribution of silicatein to the formation of cristobalite was obtained from the HR-TEM and pattern diffraction images of the two samples. In the embedded matrix, after calcination of the organic material, the EDX (see Supporting Information S.5) measurement has given strong peaks for oxygen and silicon forming the matrix with negligible amount of Na, C, and Cu from the grid. The diffraction pattern (Figure 7b) showed a crystalline pattern with d spacing correlating to (1̅01), (102), and (201) planes of cristobalite. In a localized focus on the sample we have been also able to see the formation of that crystalline phase using bright field (BF) and dark field (DF) imaging (Figure 7c,d). In summary, the results reported in this paper demonstrate the key role of silicatein in the organization and phase transition of amorphous silica to crystalline form. It is known that silicatein’s filament functions as an enzymatic catalyst and organic template for biosilicification and can direct the growth of many other oxide materials.36 Understanding of our results can be based on the information reported in the literature regarding the role of silicatein as a template for biosilicification. Overall, the main template mechanism of silicatein that governs the synthesis and hierarchical structure control of silica biominerals is based on a slow kinetically controlled catalytic hydrolysis of silica precursors, and the silicatein filaments act as a heterogeneous organic template for silica deposition. We assume that silicatein templates the biosilicification of silica into amorphous silica with some degree of crystallinity at the interface between the silicatein filaments and the silica

Figure 5. Negombata magnif ica sponge samples after thermal treatment. Sample from the first fraction is illustrated in red, and sample from the second fraction (mashed then soaked in acid) is illustrated in black.

the inner organic matter (mashed after acid treatment) a crystalline transformation occurred. But, in the case of the fraction that lacked its original organic core (ground before acid treatment), the thermal treatment did not form any kind of crystalline structure. Silicatein’s role in the transformation process was investigated further by using the organic filament as a surface directing agent in a synthesized silica matrix. Using the high abundance of silicatein filament in spicules of Negombata magnif ica, we were able to extract filaments from the sponge and entrap them in a silica matrix by the Stôber process. The 2108

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Figure 7. (a) HR-TEM (scale bar = 5 nm) and (b) diffraction patterns of the thermally treated sol−gel imbedded with silicatein and the sample location of bright field (BF) (c) and DF (d) of the sample showing the localization of the crystalline position (scale bar = 50 nm).

structure and without their organic filament. The XRD spectra and the diffraction patterns show this transformation to be directly related to the spicules’ organic filament. The samples of spicules whose filament was removed did not produce any crystalline structure and thus gave an amorphous signature. The samples which were ground but left with the filament were able to transform amorphous silica to cristobalite in the same manner as the original sponge sample. This transformation to crystalline structure was also demonstrated for a second sponge species with siliceous spicules. This second species Negombata magnif ica was taken from a different sea with a different natural habitat, but still with inner silicatein filament within its spicules. In this species, the transformation to cristobalite was also achieved, thereby strengthening our first assumption that there is a main role for the protein in directing the formation of such a high polymorph of silica. To measure the extent of this effect on the structural behavior of amorphous silica, we have carefully entrapped the filament without a silica matrix breaking its structure. This plain sol−gel silica with distinct amorphous character was heated to the same temperature as were the spicules, and crystallinity of the sample was tested. The formation of cristobalite occurred as well in this sample, confirming the assumption of the role of the silicatein protein

precursors. Accordingly, those preoriented structured (crystallinity) interfaces can act as a template to induce crystallization of cristobalite at a relatively low temperature. This suggestion can also explain the phase transition to cristobalite of the sol gel silica prepared with silicatein. However, it is clear that future studies are required to better understand the role of silicatein in the phase transition of biosilica to crystalline silica.

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CONCLUSION Structural changes in amorphous biosilica due to thermal activation were investigated in this study, and the role of silicatein protein as a key element in this transformation was examined. We were drawn to the biosilica’s structure as a selfdirected material due to the fact that the formation of the sponge spicules is multiordered on the macro scale, and for that reason it may also be oriented on the nano/micro scale. This was confirmed in the thermal experiment when the amorphous structure of the sponge transformed into the crystalline form of cristobalite at a temperature that is lower than the one reported and even lower with respect to the low-energy transformation to quartz, which lies at 870 °C. In order to better understand the source of this transformation at low energy, we investigated the transformation of sponge spicules without the macro2109

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in the process of producing the highest polymorph of silica at a relative low temperature and ambient pressure.

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ASSOCIATED CONTENT

S Supporting Information *

Additional XRD measurements and blank samples are presented with additional HR-TEM and diffraction pattern of the samples mentioned above. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*Tel 972-3-5317681; e-mail [email protected] (Y.M.). Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS Ido Fuchs acknowledges the BIU Department of Chemistry for funding. This research was supported by the Israel Science Foundation (Grant No. 660/07).

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REFERENCES

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The Journal of Physical Chemistry B

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dx.doi.org/10.1021/jp4114317 | J. Phys. Chem. B 2014, 118, 2104−2111