DNAâSilica Mineralization - American Chemical Society

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DNA−Silica Mineralization: The Formation of Exceptional Two Dimensional-Square p4mm Symmetry by a Structural Transformation Lu Han,* Chenyu Jin,† Ben Liu, and Shunai Che* School of Chemistry and Chemical Technology, State Key Laboratory of Composite Materials, Key Laboratory for Thin Film and Microfabrication Technology of the Ministry of Education, Shanghai Jiao Tong University, Shanghai 200240, People’s Republic of China S Supporting Information *

ABSTRACT: DNA−silica complex (DSC) mesocrystals have been synthesized by the selfassembly of DNA as template, N-trimethoxysilylpropyl-N,N,N-trimethylammonium chloride (TMAPS) as costructure directing agent (CSDA), and tetraethyl orthosilicate (TEOS) as the silica source. A full-scale synthesis-field diagram of DSCs has been constructed, and fibrous products, two-dimensional (2D)-hexagonal p6mm, and 2D-square p4mm platelets have been obtained by varying the synthetic conditions. The rare 2D-square structure possessed an inconsistent hexagonal morphology and appeared as the dominant mesostructure. The combination of X-ray diffraction patterns, scanning electron microscopy images, and highresolution transmission electron microscopy images provided visible evidence for the mesostructural constructions of the 2D-square symmetry that transformed from the 2Dhexagonal symmetry. The driving force for this transformation seems to be the polymerization of the silica species during synthesis, which caused a decrease in the negative charge density from the silicate network. This led to close interactions of the opposing charges along the DNA−DNA interface upon quaternary ammonium phosphate electrostatic “zippers” to facilitate the formation of the 2D-square lattice. KEYWORDS: biomineralization, DNA, liquid crystal, self-assembly, electron microscopy

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optimized for particular requirements.3,4 In vitro, DNA can be condensed into numerous liquid crystal (LC) phases, including cholesteric phase, blue phase, two-dimensional (2D) hexagonal phase (nematic phase), three-dimensional (3D) hexagonal phase, 3D orthorhombic phase, cubic phase, rhombic phase, and supramolecular-directed DNA LCs with lamellar and columnar phases. These phases can be achieved by diverse methods, such as preparing concentrated solutions in the presence of condensation agents (e.g., monovalent/multivalent ions, alcohol, polypeptides or proteins, neutral crowding polymers, and cationic surfactants).5−18 The phase behavior of the DNA LC depends on the concentration of DNA, the type of counterions, the osmotic pressure, and so on. Generally, an increase in the DNA concentration converts the aggregates into more compact structures.4 The condensed DNA often has unique properties, and thus, the in vitro DNA condensation can be viewed as a model system for many physical, biochemical, and biological processes.3 Silicon is the second most abundant element in the Earth’s crust, and through its combination with oxygen, silicates are generated and form the most prevalent group of minerals.19 In many life forms, silicon is an essential structural element.

INTRODUCTION Many biominerals are organized from the mesoscale to the macroscopic scale with state-of-the-art morphologies and structures attributed to the arrangement of the biological molecular templates. Understanding the formation and growth of these biologically relevant materials is essential for revealing the structural components of life and is important for the development of new artificial materials that can mimic the elegance that is observed in nature. Recently, the study of biomineralization has inspired the development of a number of organic/ inorganic hybrid materials that have well-defined crystal shapes and complex structures with superior properties.1 The DNA molecule, which contains genetic information and is central to life science, is a half-rigid molecule that has a righthanded double helix conformation and is 2.2 nm in diameter. The DNA molecule is composed of two highly charged anionic polyelectrolyte helical chains (charge density = 4.17) that are each coiled around the same axis, and each has a pitch length of approximately 34 nm. The persistence length of DNA is around 50 nm.2 Many biological systems, including sperm and viral phage, contain densely packed DNA assemblies, and the DNA packing in chromatin plays an important role in gene regulation. Understanding the mechanism of DNA packing of biological systems, including human systems, is of great importance. In vivo, DNA is several meters long and is naturally packed into compact structures by a variety of methods, © 2012 American Chemical Society

Received: September 23, 2011 Revised: January 3, 2012 Published: January 11, 2012 504

dx.doi.org/10.1021/cm202874w | Chem. Mater. 2012, 24, 504−511

Chemistry of Materials

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diagram, each of the three chemical components was employed in the synthesis, keeping the synthesis temperature consistent at 25 °C. Characterization. Powder XRD patterns were recorded on a Rigaku X-ray diffractometer D/MAX-2200/PC that was equipped with Cu Kα1 radiation (40 kV, 20 mA) and sampled at a rate of 0.1°/min over the range of 2−12° (2θ). The microscopic features of all of the samples were observed using scanning electron microscopy (SEM) on a JEOL JSM-7401F. To observe the genuine external surfaces, the samples were observed without any metal coating. A low accelerating voltage (1 kV with point resolution of ∼1.4 nm) was chosen for all DSC samples. HRTEM was performed using a JEOL JEM-2100 microscope that was equipped with a LaB6 gun operating at 200 kV (Cs 1.0 mm, point resolution 2.3 Å). Images were recorded using a KeenView CCD camera (resolution 1376 × 1032 pixels, pixel size 6.45 × 6.45 μm) at 50 000−120 000 times magnification under lowdose conditions.

The synthesis of silica materials using biomimetic approaches has become an important area of research, and numerous studies have been performed to elucidate the mechanisms for silicate mineral formation by biological organic matrices;20−25 however, more investigation on this subject is required. In general, DNA is incapable of directing the surface deposition of silica to form biominerals because both DNA and silicates are negatively charged and repel each other under pH values of 4.3−11.9, and this range of pH is essential to maintain the DNA double helix configuration.26 However, based on the costructure directing effects of quaternary ammonium silanes, such as N-trimethoxysilylpropyl-N,N,N-trimethylammonium chloride (TMAPS), DNA and DNA superstructures, orderly packed condensing phases or chiral nanometric ribbons and tubules can be transcribed into inorganic materials with different morphologies and structures.27,28 The positively charged quaternary ammonium group of TMAPS acts as a condensing agent for DNA, and the silane site is co-condensed with a silica source (e.g., tetraethoxysilane (TEOS)) for the subsequent assembly of the silica framework. Interestingly, platelet-like DNA−silica complex (DSC) mesocrystals that have exceptional 2D-square p4mm symmetry have been obtained, and this is very rare for the crystal structures.29 In both LCs and DNA condensed phases, it is very rare to encounter such square columnar symmetry30 because the long-range orientational order is always formed by maximizing the interaction energy and minimizing the excluded volume.31 Thus, the 2D-hexagonal columnar packing is the most common mesostructure with the minimum packing volume. The 2D-square mesostructures have been reported in very few cases by the self-assembly/microphase separation of specially designed molecules, such as T-shaped ternary amphiphiles, which are capable of generating 90° rotational symmetry.30,32,33 Another distinct characteristic of a DSC that has a 2D-square structure is that the interaxial separation distance is approximately 25 Å, which is significantly smaller compared to a DSC that has a 2D-hexagonal mesostructure (ca. 29 Å); the effective diameter of DNA in dilute solutions calculated by Rybenkoz et al. (6 nm in the presence of 0.1 M Na+ and 15 nm in the presence of 0.01 M Na+);34 and the interaxial distance of condensed DNA reported (e.g., spermine or spermidine-DNA precipitates) (∼28−32 Å for 2D-hexagonal p6mm or cholesteric phase).35 The 2D-square structure with a small interaxial separation was induced by the use of small counterions, and the structure depends on a specific azimuthal orientation and the electrostatic “zipper” formation to induce close DNA−DNA interactions.35,36 However, the formation of the unique 2D-square DSC structure is still unclear. Herein, we present a full-scale synthesis-field diagram of DSCs that were prepared from TMAPS and the silica source TEOS, and particular attention was focused on the formation of the DSC with a rare 2D-square p4mm mesostructure. The structural solutions were obtained using powder X-ray diffraction (XRD), scanning electron microscopy (SEM), and high-resolution transmission electron microscopy (HRTEM). In framing the DNA by a rigid silica wall, the formation of DSCs facilitates the structural analysis of the DNA LC and the DNA condensation in nature with electron microscopy, and this provides a wealth of information from both reciprocal space (diffraction) and real space (imaging).

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RESULTS AND DISCUSSION Synthesis of DSCs. The DSCs were synthesized by employing the cooperative effects of TMAPS.27−29 The positively charged quaternary ammonium group of TMAPS acted as a condensing agent for DNA, and the alkoxysilane site of TMAPS was capable of co-condensing with TEOS to form the inorganic framework. The trimethylene groups of TMAPS covalently tethered the silicon atoms that were incorporated into the framework to the cationic ammonium groups, regardless of the type of charge on the silicate. We investigated the packing behavior of DNA in DSCs by varying the DNA concentration and the molar ratio of TMAPS and TEOS under ambient conditions (i.e., neutral pH and room temperature). The concentration of DNA and the interaction between DNA and TMAPS are the key factors for DNA condensation and self-assembly, and the amount of TEOS is very important for the subsequent condensation of the framework. The synthesis-field diagram of the DNA/TMAPS/TEOS synthesis system is shown in Figure 1. The determination of

Figure 1. Synthesis-field diagram (mole fraction) of the DNA/ TMAPS/TEOS synthesis system. TEOS and H2O are kept in constant ratio.

mesostructures was performed based on the XRD patterns and HRTEM analyses. The 2D-square p4mm structure appeared in a large area of the synthesis-field diagram, and the 2D-hexagonal p6mm occurred only in the small area where the DNA concentration is very low, suggesting dilute DNA solutions for synthesis. Between the 2D-hexagonal and 2D-square areas was the mix-structure area, which contained both the p6mm and

EXPERIMENTAL SECTION

Preparation of DSCs. The DSCs were synthesized according to ref 22. To prepare the DNA/TMAPS/TEOS triangle synthesis-field 505



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p4mm mesostructure particles. Small amounts of countercations were insufficient to condense the DNA into packed structures, and fibrous products were obtained in the low TMAPS areas. The TEOS concentration variation did not affect the final mesostructure; however, a certain amount of TEOS is necessary to produce DSCs, and too much TEOS results in sticky products that contain impurities. The DSC with 2D-square p4mm structure is favorable compared to the 2D-hexagonal p6mm structure. Of note, the particle size can be controlled by the addition of NaCl (see Supporting Information Figure S1). Reports have shown that alkali, alkali earth metals, and a few transition metal ions are able to interact with the phosphates of DNA by electrostatic interactions.37 These interactions can cause the linkage of two different DNA sites via the interaction of DNA and TMAPS, and the assembly process can be affected. The size of DSCs in well-defined hexagonal morphologies can be controlled from ∼800 nm to ∼1.4 μm by increasing the NaCl/ DNA molar ratio from 0 to 8. Plane-like DSCs that are larger than 2 μm have been formed with a NaCl/DNA molar ratio of 16. However, only amorphous products have been obtained at NaCl/DNA ratios of 32. In addition, small DSCs of approximately 400 nm can be formed by the addition of NaCO3 (see below and ref 29). Figure 2 shows the typical morphology and the structure of the DSCs that have a 2D-square p4mm symmetry. The DSCs

ordering (the highest order of rotation is fourfold), and the axes of reflection are inclined to each other by 45°, which indicates a p4mm plane group. The unit cell parameter calculated from the Fourier diffractogram (FD) is a = ∼24 Å. The DNA molecules and the silica species have a co-assembly feature, and an extremely thin silica wall (or almost no wall) was formed between the two nearest DNA molecules, as shown in Figure 2c. To check the chemical composition of the DSCs, the elemental distribution in the DSC mesocrystals was mapped with X-ray energy dispersive spectroscopy (EDS), and various elements have been confirmed by this measurement. Silicon (from the SiO2 framework), oxygen (from the SiO2 framework and DNA), phosphorus (from the phosphate chain of DNA), and nitrogen (from both the DNA base pair and the quaternary ammonium site of TMAPS) were homogeneously distributed within the DSC particles, indicating the coassembly characteristics of the DNA and silica (see Supporting Information Figure S2 for details). Formation of DSCs. Of note, the DSCs that possess the 2D-square p4mm structures show surprisingly hexagonal morphologies. The morphologies of the crystals should be commensurate with their crystallographic point-group symmetries; however, in this context, the hexagonal morphology of the DSC that had a 6-fold symmetry was inconsistent with the 2D-square lattice that had 4-fold symmetry. Of note, the morphology of the DSCs that had the 2D-square lattice were quite similar to the DSCs that had 2D-hexagonal symmetry, and the well-defined hexagonal shape and the ridge structures can be observed in both cases (see Supporting Information Figure S3). To explore the intrinsic nature of the DSCs and the formation of the unique 2D-square p4mm symmetry, detailed HRTEM observations were carried out. The DSC crystals that have 2D-square structures were generally composed of structural domains with distinct boundaries, and 12-fold quasicrystal-like electron diffraction patterns were often observed. A typical HRTEM image that was taken from one corner of a platelet is shown in Figure 3a and clearly shows three 2Dsquare domains that have 60° relationships with each other. The 12 diffraction spots, which were formed by the three sets of domains, can be observed clearly and are marked with different colors in the FD. Figure 3b shows the HRTEM image that was taken from the central part of a particle, and three 2D-square domains with 60° orientations can also be observed. The arrows show the direction of the ridges, the contrast of which is deeper because of the thickness contrast in the TEM, suggesting that the direction of each ridge is consistent with one ⟨10⟩ axis of the three 2D-square domains. Of note, the domains have generally disordered boundaries; however, some small areas with 2D-hexagonal arrangement can be observed in the adjacent parts, which are indicated and enlarged by white circles. To identify the arrangement of these domains, mosaicked TEM images of the large particles were made by interlinking several HRTEM images (Figure 4a1−c1). The three sets of diffraction spots that correspond to the 2D-square domains were masked with colored rings in the FDs. The corresponding Fourier filtered HRTEM images, using these three sets of diffraction spots, are shown in Figure 4a2−c2. The different DSC crystals have completely different domain arrangements, indicating that the domains were formed randomly. To our surprise, all the domains in the particle have 60° relationships. Thus, a 12-fold electron diffraction pattern was formed due to

Figure 2. Morphology and structure of typical DSC with a 2D-square p4mm symmetry. (a) SEM image showing the microscopic features of the samples. The hexagonal shape can be clearly observed with ridges located in the center. The sample was observed without any metal coating. (b) Top view TEM image of the platelet, the corresponding Fourier diffractogram (FD), and (c) the schematic drawing of the DSC, showing the 2D-square p4mm mesostructure.

were synthesized with molar ratios of DNA:TMAPS:TEOS:H2O = 1:6:15:18333. The SEM image (Figure 2a) revealed that the DSC was composed of hexagonal platelets with ridges that are located in the center. The crystals are uniform in size, with an average diameter of approximately 800 nm and a uniform thickness of approximately 100 nm. The HRTEM image (Figure 2b) shows the well-defined long-range 506



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Figure 4. Mosaicked HRTEM images and the corresponding FDs of the DSC crystal (a1−a3) and the corresponding Fourier-filtered TEM images using three sets of diffraction spots (b1−b3). The colored rings in the FD denote the different sets of 2D-square domains (colors correspond to those shown in the Fourier-filtered TEM images).

Figure 3. HRTEM image taken from (a) a quarter of a platelet and (b) the center of a platelet. Three 2D-square domains with disordered boundaries and small 2D-hexagonal areas can be observed (indicated by white circles).

the combination of the 60° domain connection and the 90° relationship of the fourfold unit cell. Considering the 2D-square domain structure that has 60° relationships and the similar shapes of the DSCs with both 2D-hexagonal and 2D-square mesostructures, we speculate that the hexagonal morphology and the 2D-square lattice of the DSC with 2D-square structures might not be formed simultaneously. It is very likely that the hexagonal lattice is formed at the beginning of the reaction along with the hexagonal platelet-like morphology and, as the synthesis proceeds, is transformed into the 2D-square lattice with the hexagonal morphology. The small 2D-hexagonal domains can be evidence of the incomplete structural transformation. To find proof for the structural transformation, the products were freeze-dried and monitored as a function of reaction time. The XRD pattern of the product shows two peaks at high angles of 2θ = 5.5° and 8.2° (2 h), which is shown in Figure 5a. As the product ages (4−8 h), a reflection at 2θ = 3.5° appeared and grew stronger, and the two reflections at high angles became weaker (Figure 5b,c). The typical XRD pattern of the 2D-square mesostructure appeared after 8 h (Figure 5d,e). At the early stage of the reaction, the ratio of the d-spacing ratio of the first two peaks was larger than √2, which is the expected ratio for the 2D-square p4mm structure, and thus is probably showing a 2D-hexagonal structure or a partially transited 2D rhombic lattice. The reflections that were observed early in the process can be assumed to be the 10, 11, and 20 reflections of the assumed 2D-hexagonal mesostructure.

Figure 5. XRD patterns of the DSC with 2D-square p4mm symmetry at different reaction periods. (a) 2 h, (b) 4 h, (c) 8 h, (d) 12 h, and (e) 24 h.

Of note, the 11 and 20 reflections are stronger than the 10 reflection at the very beginning of the reaction. This behavior is in contrast to what has been previously observed for a series of 507



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mesoporous materials, including the MCM-41 type and the SBA15 type, which are templated by the cylindrical micelles that are formed by simple cationic surfactants and nonionic surfactants, respectively.38−42 In their reports, the intensity of the 11 reflection is initially lower than that of the 20 reflection, but the reverse is observed after longer reaction times. This phenomenon can be explained on the basis of the intensities of the Bragg reflections and suggests that the changes in the degree of intermicellar condensation are responsible for these time-dependent observations.43 However, these XRD data are insufficient to provide detailed information about the structure. The morphological and structural transformations that take place during the synthesis have been further examined by SEM (Supporting Information Figure S4) and HRTEM analyses (Figure 6). Supporting Information Figure S4a, which shows the freeze-dried organogel after 2 h of aging, indicates the aggregated particles in micrometer size. The uniform-sized hexagonal morphology first appeared at 4 h (Supporting Information Figure S4b). More hexagonal platelets formed and separated at 8 h (Supporting Information Figure S4c). Finally, uniform platelets formed, indicating the completion of the silica condensation and structural formation (Supporting Information Figure S4d). However, some impurities formed due to the interruption of the stable conditions during the time-course experiment. The TEM image of the sample at 2 h showed no contrast of the ordered mesostructure because the silicate condensation is too low (not shown). As expected, the TEM of the sample at 4 h showed an ordered 2D-hexagonal p6mm structure with sixfold symmetry in the FD (Figure 6a). At this early stage of the reaction, the silicate walls remained soft and could be easily destroyed and reformed; however, the hexagonal morphology could already be observed with SEM. With further silica condensation, a large number of tiny 2D-square domains appeared at 8 h (Figure 6b1), and they are connected by 2D-hexagonal boundaries or disorder areas. From the FD pattern, the six strongest diffraction spots, which correspond to the original 2D-hexagonal part (marked by white circles), mixed with the weak diffraction spots corresponding to the small 2D-square domains. A Fourier-filtered TEM image that used the three types of diffraction spots is shown in Figure 6b2, in which it can be seen that the tiny 2D-square domains have a 60° relationship and are distributed randomly all over the particle. They share the {10} plane with the 2D-hexagonal structure and are shifted layer by layer to form the 2D-square unit cell. Figure 6c1 shows the TEM image of the product aged for 12 h and depicts the disappearance of the 2D-hexagonal domains and emergence of the 2D-square domains (see also the Fourierfiltered TEM image in Figure 6c2). After 24 h, the platelet consisted of only large 2D-square domains, and the orientation angles between domains remained 60° (Figure 6d1,d2). Figure 6e shows a schematic drawing of the structural transition. The 2D-square mesostructure can be obtained by shifting the lattice planes along one ⟨10⟩ axis, and only three axes of the hexagonal unit cell are available due to the sixfold symmetry. Thus, the 2D-square domains could only have a 60° relationship to each other. Formation Mechanism of DSCs. In a living cell, the DNA molecule, which may be several meters long, is restricted in the nucleus and limited by the nuclear envelope. The DNA molecules undergo alternate condensation and decondensation processes during the cell cycle.4 However, much remains unknown about the consequences of these crowding conditions

Figure 6. HRTEM images of the DCS product freeze-dried at different reaction times. (a) 4 h, (b1) 8 h, (c1) 12 h, and (d1) 24 h. (b2−d2) The Fourier-filtered TEM image using three types of diffraction spots corresponding to (b1−d1). The white circles in (b1) indicate the untransformed 2D-hexagonal domains, and all the 2D-hexagonal domains have the same orientation. (e) A schematic drawing of the relationship of the three possible 2D-square domains with a 60° relationship.

on the functional properties of a DNA molecule and about the organization of a DNA molecule in both the cellular environment and the DNA LC system. Of note, DNA molecules that have large interaxial separations can be modeled as uniformly charged cylinders.44 However, the net charge distribution on the DNA molecules is not homogeneous, and this dramatically changes the interaction potential at intermediate distances.45 An axial charge separation that is due to counterion binding in helical grooves allows the close approach of opposite charges along the DNA−DNA contact and forms an electrostatic “zipper” that “fastens” the molecules together.35,36 The phase behavior of the columnar DNA packing depends both on the interaxial distance and on the special requirements for the azimuthal orientation and would lead to the formation of a 2D-square structure with a small interaxial separation, as conjectured by Harreis et al.36 508



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Figure 7. Schematic illustration of the 2D-hexagonal to 2D-square transformation. (a) Cooperative organization of the DNA/TMAPS/silica assemblies. At the beginning of synthetic reaction, the alkoxysilane sites of TMAPS will be hydrolyzed by small amounts of water in the grooves, will move to the DNA surface gradually, and then will co-condensed with TEOS. (b) The self-assembly of the DNA/TMAPS/silica species forms a silicatropic DNA liquid crystal 2D-hexagonal mesophase in a cooperative manner, and the hexagonal platelet morphologies are also formed in accordance with a 6mm point-group symmetry. (c) The transformation from the 2D-hexagonal to 2D-square. The cationic quaternary ammonium helical strands and negative phosphate backbone of DNA emerge on the surface of DNA/TMAPS/silica complex columns due to the decrease of charge density of the silicates. Therefore, the negatively charged strands come close to the positively charged quaternary ammonium helical strands of the opposing molecule to form an electrostatic “zipper”, which leads to the formation of the 2D-square p4mm structure.

Thus, a mechanism is proposed that is based on the DNA condensation theory and the current experimental data, that is, a 2D-hexagonal to 2D-square structural transition. When TMAPS, which contains a positively charged quaternary ammonium group, is added to the DNA solution, it acts as a condensing agent for DNA, and a helical arrangement of the cationic quaternary ammonium groups could form in the minor and major grooves of the DNA via hydrophobic interactions between the silanes and the grooves. This is similar to the interactions between DNA and cations (Figure 7a).46,47 As the synthetic reaction proceeds, the alkoxysilane sites of TMAPS are hydrolyzed, move to the DNA surface gradually, and cocondense with TEOS. The self-assembly of the DNA/TMAPS/ silica species forms a silicatropic DNA liquid crystal 2Dhexagonal mesophase in a cooperative manner, and the hexagonal platelet morphologies are also formed in accordance with a 6mm point-group symmetry (Figure 7b). Because of the partially condensed silicate layer, the DNA molecules are surrounded by negative charges on their surfaces, and the net charge distribution of the DNA molecules is hindered. The formation of the hexagonal arranged ridges is also related to the formation of the hexagonal crystal morphology. Judging from the SEM and TEM images, the ridge is thicker than the average thickness of the platelets, indicating that the self-assembly process of the axes is much faster than that of the other parts. At this stage of the reaction, the silicate is very soft and the DNA/TMAPS/silica can move and readily deform. In the silica polymerization step, the silica condensation would cause the negative charge density of the silicate network to decrease. The negative charges of the DNA phosphate chains and the positive

charges of TMAPS are gradually exposed, and the inhomogeneous net charge distribution of the charges are screened. In this case, the highly charged anionic polyelectrolyte phosphate chains strongly interact with each other, and the DNA molecules need to align in a certain direction so that the closely opposing stripes have complementary charges that line the length of DNA−DNA contact. This alignment creates a “zipper” that pulls the molecules together via electrostatic attraction, making the DNA molecules more densely packed (Figure 7c). The small interaxial separation facilitates the silicate condensation, which makes the structure more stable. This “zipper” structure can induce the DNA molecules nearby to form square unit cells and join the 2D-square domains and drive the transformation from the 2D-hexagonal to the 2Dsquare mesostructure. It seems that the energy of the large domain is lower than that of the small domains; thus, the small 2D-square domain can be swallowed into the large domain, resulting in the presence of only a few large domains. Of note, before the copolymerization is complete, the silica framework is flexible enough that the DNA/TMAPS/silica units can be rearranged while keeping the initially formed hexagonal morphology. This is similar to our previous observation of the structural transformation of the hollow silica mesoporous crystals.48 However, the DSC with 2D-square single crystal features can be achieved when the crystal is small enough. These small crystals were synthesized by the addition of 0.1 M Na2CO3 to the synthesis gel. The hexagonal morphology and the ridges was deformed during the structural transformation process (Supporting Information Figure S5). 509



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We also investigated the formation of the DSCs with a 2D-hexagonal p6mm symmetry. We found that the mesostructure remains the same after the original hexagonal mesostructure is fabricated after 2 h (See Supporting Information Figure S6). This observation can be explained by the concentration and the relative amount of the silica source to the DNA. At low DNA concentrations, the DNA molecules are more separated and the molar ratio of TMAPS/DNA and TEOS/DNA is 10 times larger than the synthesis conditions for the 2D-square DSC. The formation of the condensed silica that surrounds their surface would make the DNA covered by thicker silicate in the early stages, making the DNA/TMAPS/silica units impossible to align and reform and resulting in hexagonal close packing. Furthermore, the unit cell areas for both of the 2D-hexagonal and 2D-square structures have been calculated, and of note, the A2D‑hex = (29 Å)2 sin 2π/3 = 728 Å2 is larger than the A2D‑sq = (25 Å)2 = 625 Å2. As discussed, the long-range orientational order is always formed by maximizing the interaction energy and minimizing the excluded volume. Thus, the excluded volume of the 2D-square mesostructure is even smaller due to the unique azimuthal orientation and the “zipper” formation. The DSCs with the rare 2D-square structure are formed as a predominating phase in the synthesis-field diagram of the DSCs. We speculate that the 2D-square may also exist in the DNA condense structures in nature.

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CONCLUSIONS

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ASSOCIATED CONTENT

S Supporting Information *

SEM and TEM images, time-course experiment, and the EDS mapping results of the DSCs (PDF). This material is available free of charge via the Internet at http://pubs.acs.org.

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected] (L.H.); [email protected] (S.C.). Fax: +86-21-5474-5365. Tel: +86-21-5474-2852. Present Address †

Max Planck Institute of Colloids and Interfaces, Science Park Golm, 14476 Potsdam, Germany.

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REFERENCES

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Learning from nature is the best way to initiate the development of new materials. The exciting results described in this article clearly demonstrate that significant progress has been made in the self-assembly of DNA. A full-scale synthesisfield diagram of the DSCs, prepared with TMAPS and TEOS, is given, and the detailed structural study and formation mechanisms are also revealed. The formation process provides a way to generate new materials with exceptional structure and morphology through a structural transformation or recrystallization of the initially formed architecture. This expands the horizons for the study of the physical theory of DNA assembly, biological macromolecule interactions, and the assembly of functional materials.

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ACKNOWLEDGMENTS

We acknowledge support from the National Natural Science Foundation of China (Grant No. 20890121), the 973 Project (2009CB930403), and the Grand New Drug Development Program of China (No. 2009ZX09310-007). 510



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