Direct Observation of Siloxane Chirality on Twisted ... - ACS Publications

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Direct observation of siloxane chirality on twisted and helical nanometric amorphous silica Yutaka Okazaki, Thierry Buffeteau, Elise Siurdyban, David Talaga, Naoya Ryu, Ryohei Yagi, Emilie Pouget, Makoto Takafuji, Hirotaka Ihara, and Reiko Oda Nano Lett., Just Accepted Manuscript • DOI: 10.1021/acs.nanolett.6b02858 • Publication Date (Web): 01 Sep 2016 Downloaded from http://pubs.acs.org on September 5, 2016

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Direct observation of siloxane chirality on twisted and helical nanometric amorphous silica Yutaka Okazaki1, Thierry Buffeteau2, Elise Siurdyban2, David Talaga2, Naoya Ryu3, Ryohei Yagi1, Emilie Pouget4, Makoto Takafuji1,5, Hirotaka Ihara1,5& Reiko Oda4,* 1

Department of Applied Chemistry and Biochemistry, Kumamoto University, 2-39-1 Kurokami, Chuo-ku Kumamoto 860-8555, Japan

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Institut des Sciences Moléculaires, UMR5255 - CNRS, CNRS, University of Bordeaux, 33405 Talence, France 3

Materials Development Department, Kumamoto Industrial Research Institute, 3-11-38 Higashimachi, Higashi-ku Kumamoto 862-0901, Japan

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Institute of Chemistry & Biology of Membranes & Nanoobjects (UMR5248 CBMN), CNRS Universite Bordeaux - Bordeaux INP, 2 rue Robert Escarpit, 33607 Pessac, France

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Kumamoto Institute for Photo-Electro Organics (PHOENICS), 3-11-38 Higashimachi, Higashiku Kumamoto 862-0901, Japan

KEYWORDS

ACS Paragon Plus Environment

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chiral nanostructures, chiral transcription, origin of inorganic chirality, chiral enhancement, VCD

ABSTRACT.

Synthesis of chiral inorganic or hybrid nanomaterials through sol-gel transcription of chiral organic templates has attracted a great deal of interest since more than a decade. However, the chiral nature of these inorganic matrices has never been directly observed. For the first time, we report a direct evaluation of chirality on non-crystalline silica chiral nanoribbons by vibrational circular dichroism (VCD) measurements. Strong Cotton effect around 1150–1000 cm-1, from SiO-Si asymmetric stretching vibration was observed. Surprisingly, calcination of these hybrid nanoribbons doubled the intensity of Cotton effects. On the basis of TEM observations, IR, VCD, NMR, and Raman spectroscopies, we demonstrate that the silica chirality originates from twisted siloxane network composed of chiral arrangement of the Si-O-Si bonds. Our findings clearly prove the presence of chiral organization of amorphous silica network, making them very promising chiral platforms for chiral recognition, optical applications or asymmetric catalysis.

TEXT Nanometric chiral objects based on molecular assembly such as twisted or helical nanoribbons represent a new class of objects having important potential in a large panel of applications, taking advantage for example of 1) electromechanical or optical chirality,1-3 2) local chiral environment for catalysis,4-6 and 3) chiral recognition7,8 as described in recent reviews by Wang et al.9 or Zhang et al.10 Sol-gel transcription to silica materials, using these nanometric self-assemblies as chiral templates offers them polymorphisms with further structural stability.11,12 Chiral mesoporous silica having helical holes (several nanometers) were studied using X-ray diffraction and


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diffractograms by Che et al.13,14 Helical mesoporous silica can be used as effective templates for various attractive inorganic nanomaterials. Sanchez et al. reported helical mesoporous organosilica fibers functionalized with fluorophore and chromophore for sensor and catalytic applications.15 Qi et al. described the potential of helical mesoporous silica as template for the creation of SnO2 nanotubes, and their applications in lithium-ion batteries.16 The evaluation of the detailed internal chirality of silica is essential for using silica nanostructures as effective chiral platforms. Often, they are studied by non-direct observations such as enantioselective molecular recognition or catalysis, and induced circular dichroism of achiral dyes.17-21 However, with these approaches, it is difficult to evaluate the origin of the observed chirality. In this paper, direct and quantitative chirality evaluation of nanohelices made from amorphous silica is performed by vibrational circular dicroism (VCD) spectroscopy. In addition, chirality enhancement of silica nanohelices upon calcination was analyzed by TEM observations, IR, VCD, Raman and NMR spectroscopies. Based on these results, we discuss about the origin of the chiral signal of silica network. We previously reported the fabrication of individualized silica chiral nanoribbons with controlled pitch and handedness by sol-gel reaction of tetraethoxysilane (TEOS) using molecular assemblies of chiral surfactant (having the formula C2H4-1,2-((CH3)2N+C16H33)2 with a tartrate counterion, and denoted “16-2-16 tartrate”) as organic template (Figure 1a).22-26 Interestingly, the organization of these self-assemblies are sensitive to composition and also varies with time giving us access to various nanostructures from twisted ribbons (with Gaussian curvature) (Figure 1c), helical ribbons (with cylindrical curvature) (Figure 1d) as well as tubes.23,24 For the present study, we selected eight types of silica nanoribbons by modifying three parameters: right handed (transcribed from 16-2-16 L tartrate) or left handed (from 16-2-16 D tartrate) twisted


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(silica transcription after 1 hour aging of the organic template at 20 ˚C) or helical (3 days aging at 20 ˚C) nanoribbons and before or after calcination. Calcination was performed by heating the silica-lipid hybrid nanoribbons after transcription at 600 ˚C for 2 hours under air. After the calcination process, both twisted and helical morphologies were maintained (Figures 1c,d) but they shrank slightly. The detailed study of their sizes was performed using TEM images with more than 200 helices both for twisted and helical ribbons (Figure 2). For the helical ribbons, the helical pitch decreased in the average from 69.4 nm to 58.6 nm (approximately 84%), the width of the ribbon decreased from 33.6 nm to 29.1 nm (approximately 87%) and the thickness of silica wall decreased from 3.5 nm to 2.9 nm (approximately 83%) (Figure 2b). Similar tendencies were observed in silica twisted nanoribbons (Figure S1b).


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Figure 1. Morphological transcription from lipidic nanoribbons to silica nanoribbons. (a) Chemical structure of the 16-2-16 L-tartrate. (b) Schematic illustrations of cross-sectional surfaces of lipidic nanoribbons (left), silica-lipid hybrid nanoribbons before (middle) and after (right) calcination. (c) TEM images of lipidic twisted nanoribbons prepared by 1 hour aging of 16-2-16 L-tartrate (1 mM) aqueous solution at 20 ˚C (left), silica-lipid hybrid twisted nanoribbons prepared by sol-gel reaction of TEOS using lipidic twisted nanoribbons as template (middle) and silica twisted nanoribbons calcinated at 600 ˚C for 2 hours (right). (d) TEM images of lipidic helical nanoribbons prepared by 3 days aging of 16-2-16 L-tartrate (1 mM) aqueous solution at 20 ˚C (left), silica-lipid hybrid helical nanoribbons prepared by sol-gel reaction of TEOS using lipidic helical nanoribbons as template (middle) and silica helical nanoribbons calcinated at 600 ˚C for 2 hours (right).

Figure 2. Morphological characterization of silica-lipid hybrid nanoribbons before and after calcination. (a) Schematic illustration of a silica helical nanoribbon for quantitative evaluation of morphological change. (b) Pitch (left), width (middle) and thickness (right) distribution of silicalipid hybrid helical nanoribbons before (top) and after (bottom) calcination.


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Infrared (IR) measurements of these silica nanoribbons exhibit specific vibrational signals of silica in the 900 - 1250 cm-1 spectral range (Figures 3a,b, bottom). The band located at 1090 cm-1 and its shoulder at 1240 cm-1 are associated with the transverse optic (TO) and longitudinal optic (LO) modes of the Si-O-Si asymmetric stretching vibration (νaSi-O-Si), respectively. The band at 960 cm-1 is assigned to the Si-OH stretching vibration of free silanol groups (Figures 3a, b, bottom). Surprisingly, vibrational circular dichroism (VCD) measurements clearly demonstrated Cotton effect both for the LO and TO modes. The VCD spectra of L-silica-lipid hybrid twisted nanoribbons (L-tw) presented a strong negative-positive (from long to short wavenumbers) Cotton effect for the TO mode at 1090 cm-1 and a much weaker negative Cotton effect for the LO mode (Figure 3a, top, dotted line). Opposite spectra were obtained for D-silica-lipid hybrid twisted nanoribbons (D-tw). The two components of the 1090 cm-1 band are probably due to the coupling of the different νaSi-O-Si vibrators which are distributed in a chiral arrangement within the silica network. Similar phenomenon was observed with the νaCH2 vibration of 16-2-16 alkyl chains.27 These results unambiguously indicate that chiral information of organic template was transcribed to the silica nanoribbons. Although a few VCD spectra of siloxane-based materials have been reported in the literature for ladder-like silsesquioxane polymers28 and for micrometric chiral mesoporous silica particles,29 no such strong and clearly opposite VCD signals for silica enantiomeric nanostructures have been reported before, which enable quantitative evaluation of the chiral information of silica nanoribbons.


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Figure 3. IR and VCD spectra of silica nanoribbons. (a) IR and VCD spectra of silica-lipid hybrid twisted nanoribbons before (dotted line) and after (solid line) calcination. (b) IR and VCD spectra of silica-lipid hybrid helical nanoribbons before (dotted line) and after (solid line) calcination. All VCD spectra were normalized for an absorbance intensity of the 1090 cm-1 band equals to one. After the calcination at 600 ˚C, the intensity of the Si-OH stretching band 960 cm-1 reduced (Figures 3a,b, bottom, solid line with respect to dotted line (before calcination)) in agreement with the Si-O-Si bond formation (silanol condensation reaction) during calcination. Strikingly, an important increase in VCD signal for the bands at 1100 cm-1 and 1240 cm-1 was observed upon calcination (Figure 3a, top, solid line). To compare the chiral information of these silica nanoribbons, the dimensionless anisotropic ratio, g, was calculated according to the following equation (∆A being the differential absorbance of left and right circularly polarized light).

∆

The g-factor of calcinated silica twisted or helical nanoribbons (gtw-calc or ghel-calc) is about 1.7 or 1.9 times larger than that of uncalcinated silica-lipid hybrid twisted or helical nanoribbons (gtw


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or ghel), respectively. The VCD spectra of helical ribbons are slightly stronger than those of twisted ribbons (approximately 10 % before calcination and 25% after calcinaion). The structural modification of the silica network through the calcination process was also followed by solid state

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Si dipolar decoupling/magic-angle spining (DD/MAS) NMR spectra.

As shown in Figure 4a, silica-lipid hybrid twisted nanoribbons showed two peaks at -102.1 ppm and -110.9 ppm, which are assigned to SiO3(OH) unit (Q3) and SiO4 unit (Q4), respectively. The composition of Q3 and Q4 obtained from curve fitting using Lorentzian function varied from 13.6% (Q3) and 86.4% (Q4), before calcination, to 10.4% (Q3) and 89.6% (Q4) after calcination. These results indicate that about 25% of silanol group was dehydrated to create Si-O-Si bond through the calcination process, in good agreement with IR observation.

Figure 4. Mechanistical considerations of chiral enhancement. (a) 29Si DD/MAS NMR spectrum of silica-lipid hybrid twisted nanoribbons. (b)

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Si DD/MAS NMR spectrum of silica twisted


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nanoribbons calcinated at 600 ˚C for 2 hours. (c) Raman spectra of silica-lipid hybrid twisted nanoribbons (top) and 16-2-16 (D) tartrate (bottom). (d) Raman spectrum of silica twisted nanoribbons calcinated at 600 ˚C for 2 hours. (e) Schematic illustration of local ordered structures formed by four- and three-membered SiO2 quasiplanar rings, known as D1 and D2 defect modes, respectively. Raman spectra provides us other complementary information. Silica-lipid hybrid twisted nanoribbons (Figure 4c, top) show an intense and broad band located at about 440 cm-1, related to the Si-O-Si bending vibration (motion of the O atom along a line bisecting the Si-O-Si angle, R band);30 a narrower band at 484 cm-1, attributed to the same vibration for Si bearing hydroxyl groups (D0 band)31 and/or to the breathing mode of four membered rings (D1 band, Figure 4e);32 two weak and broad bands at about 800 and 1065 cm-1, attributed to the symmetric and asymmetric stretching vibrations of Si-O-Si linkages;30 and a band at 977 cm-1, related to the stretching Si-O vibration of Si-OH groups.31 Note that the bands at 1126, 1060, and 890 cm-1 come from 16-2-16 tartrate entrapped in the silica matrix as confirmed by the Raman spectrum of the organic compound (Figure 4c, bottom) and disappear after calcination of the silica-lipid hybrid nanoribbons (Figure 4d). On the other hand, after the calcination a new narrow band appears at 602 cm-1 (D2 band). Similar spectral modifications have been reported for porous silica nanoparticles,31 and was attributed to the breathing modes of three membered rings (Figure 4e).32 The strained 3-fold siloxane ring is the product of endothermic reaction after condensation reactions through heating process, principally between vicinal silanol groups, and is generally associated with a decrease of the Si-OH content. Ensemble of the Raman spectra modification upon calcination suggests that these condensation reactions produce structural changes of the silica network involving shrinking which corroborate with NMR, IR, and TEM observations.


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Based on these results, the origin of the chirality observed with the silica nanohelices by VCD can be discussed. The siloxane network with chiral curvature is constructed through a sol-gel reaction of TEOS on the surface of self-assemblies of 16-2-16 L- (or D-) tartrate as chiral organic templates. The chirality leading to the VCD signals can either be at molecular level, i.e. molecular chirality from tartrate is imprinted in the siloxane network (molecular imprinting, Figure 5a), or siloxane network itself expressing chiral arrangement of the Si-O-Si bonds by distortion (Figure 5b). The presence of unreacted silanol groups indicates the defects on the network. The calcination reduces defects by creation of new siloxane bonds (three-membered SiO2 quasiplanar rings) as observed by IR, NMR and Raman spectroscopies. Therefore, if the chirality originates from the molecular imprinting, it should not increase through the calcination process, but rather decrease during surface modification through bond formation in the absence of tartrate. On the other hand, the global shrinkage of nanoribbons is observed after calcination as described above, leading to the decrease in the pitch (92% for twisted ribbons, 84% for helical ribbons), in the silica wall thickness (86% for twisted ribbons, 83% for helical ribbons), and in the width of the ribbon (84% for twisted ribbons, 87% for helical ribbons). Therefore, chirality probably originates from the siloxane network distortion, and the resulting chirality enhancement through shrinkage of the network and decrease in pitch.


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Figure 5. Schematic illustration of possible origins of silica chirality. (a) Molecular chirality derived from chiral center of tartaric acid. (b) Mechanistic model of chiral enhancement of siloxane network. (c) Schematic illustration of Gaussian curvature versus cylindrical curvature. The higher values of g-factors of helical ribbons with respect to twisted ribbons can in part be explained by their smaller pitch. However, as shown in Figure 5c, the nature of the network of the two morphologies is clearly different; i.e. twisted ribbons have Gaussian curvature, whereas helical ribbons have cylindrical curvature; therefore they cannot be compared directly. In summary, we have reported, for the first time, strong VCD signals of siloxane network forming right-handed vs left-handed twisted and helical ribbon-like nanostructures. An original chiral enhancement phenomenon was observed by simple calcination process. Based on the ensemble of data from TEM images,

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Si DD/MAS NMR, IR and Raman spectra, it was

demonstrated that the small structural changes due to dehydration upon calcination induces the large enhancement of VCD signals. The ensemble of this study reveals that the observed silica chirality originates from a chirally arranged siloxane network expressed from chiral organization of Si-O-Si bonds. Such an investigation on nanometric chiral network helps elucidating the mechanism of formation of inorganic chiral materials, and designing promising chiral platforms.

Materials and Methods All chemicals were reagent grade and purchased from chemical suppliers. N,N’-DihexadecylN,N,N’,N’-tetramethylethylene diammonium L- (D-) tartrate (16-2-16 L- (D-) tartrate) was synthesized by the previously reported procedure.24 IR and VCD measurements. KBr disks were prepared by mixing the powder of silica helical or twisted ribbons (0.2-0.3 mg) with KBr powder (120 mg) and pressing at a pressure of 10


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ton/cm2 in vacuo. These experimental conditions give KBr disks of about 0.4 mm thick, exhibiting an absorbance intensity for the 1090 cm-1 band ranged between 0.65 and 0.75. The infrared and VCD spectra were recorded with a ThermoNicolet Nexus 670 FTIR spectrometer equipped with a VCD optical bench.33 In this optical bench, the light beam was focused on the sample by a BaF2 lens (191 mm focal length), passing an optical filter (depending on the studied spectral range), a BaF2 wire grid polarizer (Specac), and a ZnSe photoelastic modulator (Hinds Instruments, Type II/ZS50). The light was then focused by a ZnSe lens (38.1 mm focal length) onto a 1x1 mm2 HgCdTe (ThermoNicolet, MCTA* E6032) detector. IR absorption and VCD spectra were recorded at a resolution of 4 cm-1, by coadding 50 scans and 24000 scans (8h acquisition time), respectively. Since the sample preparation may induce molecular orientation of the helical or twisted ribbons, vibrational linear dichroism (VLD) spectra were systematically measured before each VCD experiment. Only KBr disks exhibiting very low or no VLD were used for VCD experiments. The KBr disks of powdered materials were placed on a rotating sample holder and the VCD spectra were obtained with the KBr disks rotated at four angles around the light beam axis (0°, 45°, 90° and 135°). VCD experiments were also performed with sample rotated 180° along the sample plane (changing from the front-side to the back-side of the KBr disk). The VCD spectra of the two enantiomers of silica helical and twisted ribbons were obtained by averaging the eight spectra. Baseline corrections of the VCD spectra were performed by subtracting the raw VCD spectra of a KBr disk without silica powders. The photoelastic modulator was adjusted for a maximum efficiency in the mid-IR region at 1400 cm-1. Calculations were performed via the standard ThermoNicolet software, using Happ and Genzel apodization, de-Haseth phase-correction and a zero-filling factor of one. Calibration spectra were


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recorded using a birefringent plate (CdSe) and a second BaF2 wire grid polarizer, following the experimental procedure previously published.34 Raman measurements. Raman spectra were recorded with a LabRam HR800 confocal spectrometer (HORIBA), using for excitation 514.5 nm radiation from an argon ion laser and equipped with an air-cooled CCD detector (ANDOR) cooled to -90°C. The laser power at the samples was 1 mW and the spectra were accumulated 8 times during 900 s for the silica twisted nanoribbon samples and 8 times during 120 s for the 16-2-16 (D) sample. The spectral resolution given by the entrance slit of 100 µm and the 600 lines/mm grating was 6.8 cm-1. A DuoScan device composed of two mirrors was activated in order to prevent any sample degradation. Using this device, the excitation beam scan a 10x10 µm sized area with a 50x objective (NA 0.75) in the confocal regime. Transmission electron microscopy observations. Transmission electron microscopy (TEM) images were observed by JEM1400plus (JEOL). A surface of triacetylcellulose-coated cupper grids, coated on the surface by carbon, was hydrophilized by irradiating them with UV lamp. A drop of 16-2-16 L-tartrate aqueous solution was casted on the grid. After removal of excess solution by filter paper, grids were air-dried at room temperature. The specimens were poststained with osmium tetroxide using a Filgen osmium plasma coater OPC60A. In the case of silica nanoribbons, an aqueous suspension of silica nanoribbons was casted on the grid. After removal of excess solution by filter paper, grids were air-dried at room temperature. After vacuum drying, these grids were used for TEM observation. 29

Si dipolar decoupling/magnetic-angle spinning (DD/MAS) NMR

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Si DD/MAS were measured by Varian Unity INOVA AS400 (Inova) equipped with a Varian

7 mm VT CP/MAS probe. The powder of silica nanoribbons was put into a 5 mm ZrO2 rotor and


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the rotor was spun at 4000 Hz. The relaxation time was 300 s. Chemical shifts (δ) were expressed in parts per million (ppm) relative to the polydimethylsilane (-34.4 ppm) as a standard. Curve fitting of the 29Si DD/MAS spectra were carried out using Lorentzian equation as follows: ℎ⁄ 1 + − ⁄ + with ℎ: peak height, : peak position, : half-value width and : background.

ASSOCIATED CONTENT Supporting Information The Supporting Information is avalaible free of charge on the ACS Publications website at DOI: Morphological characterization of silica-lipid hybrid twisted nanoribbons before and after calcination. This material is available free of charge via the Internet at http://pubs.acs.org.

AUTHOR INFORMATION Corresponding Author *E-mail: [email protected] Author Contributions Y.O. and E.P. synthesized chiral surfactants. Y.O., N.R. and R.Y. prepared silica nanoribbons. Y.O. carried out TEM observations, NMR measurements and all calculations. T.B., E.S. and D.T. measured IR, VCD and Raman spectra. R.O. and T.B. designed the study. H.I., M.T. and E.P. involved in study design. Y.O., R.O. and T.B. wrote the paper. All authors discussed the results, commented on the manuscript, and contributed to the interpretation of the data.


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ACKNOWLEDGMENTS This work was supported by Strategic Young Researcher Overseas Visits Program for Accelerating Brain Circulation from the Japan Society for the Promotion of Science, Centre National de la Recherche Scientifique, and the Bordeaux University.

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Graphical TOC

A direct evaluation of chirality on non-crystalline silica chiral nanoribbons by vibrational circular dichroism (VCD) measurements showing strong Cotton effect from Si-O-Si asymmetric stretching vibration


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Direct Observation of Siloxane Chirality on Twisted ... - ACS Publications

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