Assembly of Uniform Hollow Nanospheres and Helical Nanotubes

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Chiral Hybrid Mesoporous Silicas: Assembly of Uniform Hollow Nanospheres and Helical Nanotubes with Tunable Diameters Xiaowei Wu, and Cathleen Marie Crudden Chem. Mater., Just Accepted Manuscript • DOI: 10.1021/cm3012815 • Publication Date (Web): 31 Aug 2012 Downloaded from http://pubs.acs.org on September 7, 2012

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Chemistry of Materials

Chiral Hybrid Mesoporous Silicas: Assembly of Uniform Hollow Nanospheres and Helical Nanotubes with Tunable Diameters Xiaowei Wu, Cathleen M. Crudden * Department of Chemistry, Queen’s University, Chernoff Hall, 90 Bader Lane, Kingston, Ontario, K7L 3N6 (Canada) Keywords: Mesoporous material, Nanocapsules, Nanotubes, Chirality, Helical. Supporting Information Placeholder ABSTRACT: Uniform helical silica nanotubes and hollow silica nanostructures with adjustable diameters have been prepared through the self-assembly of sodium dodecyl sulfate (SDS) as the surfactant, N-trimethoxysilylpropyl-N,N,Ntrimethylammoniumchloride (TMAPS) as a co-structure directing agent (CSDA), a binapthyl-based chiral dopant and TEOS (Si(OEt)4) as the bulk silica constituent. Depending on the ratio of anionic surfactant to cationic co-structure directing agent, the morphology can be tuned from hollow spheres to hollow nanotubes. At a 1:1 ratio of TMAPS/SDS, in the presence of the axially chiral dopant molecule, uniformly helical structures are obtained. The chirality of the dopant is shown to affect the sense of helicity. Under identical conditions, a mono-silylated chiral dopant only leads to the formation of well dispersed uniform hollow spheres rather than helical nanotubes, which further demonstrates the importance of incorporating the chiral dopant as an integral component of the siloxane network, rather than merely as a surface group.

1. Introduction Hybrid silica materials with various nanostructures, such as nanospheres, nanotubes and nanorods are currently attracting considerable attention.1 When prepared using surfactant templates, materials with low densities, high surface areas, and highly ordered walls and pores can be obtained. Such uniform materials have been shown to have advantages in terms of mass transport and catalysis vs disordered materials.2-7 In particular, chiral hybrid silica nanospheres and tubes with tunable, narrow diameters could be of considerable interest for applications in the development of confined nanocatalysts,5 biosensors, targeted drug delivery, enzyme encapsulation and as nanoreactors.8-15 Synthetic routes to well-defined hollow nanostructures include soft-template-based routes, 16-34 hard-template approaches,35-47 and template-free syntheses.48, 49 In particular, hollow silica nanoparticles are desirable for many applications and have been prepared employing such diverse templates as polyelectrolyte nanoparticles,35-37 emulsions,22, 24, 25 lyotropic phases exhibing a multilamellar vesicular structures, 18 preformed vesicles, 26-28 fluorinated surfactants29, 30 and cationic surfactants.23, 31, 32 In a unique type of soft-templating approach, uniform hollow silica nanospheres can be prepared employing droplets as the templates.22, 23, 33 Building on their seminal work with co-structure directing agents and anionic surfactants, the Che group has recently shown that hollow spheres of various diameters can be prepared by the use of carboxylic acid oil drops as the template and the surfactant, along with amine-containing co-structure directing agents.34, 50, 51 In all cases, spherical particles with varying degrees of mesoporosity and diameter were obtained.

Structurally defined nanotubes are also of significant importance, especially since these also open up the possibility of designing helical structures. 19, 20 Hybrid silica nanotubes with walls that contain chiral aromatic groups are obtained by using self-assemblies of appropriate amphiphiles,21 and mesoporous silica nanotubes with helical channels can be accessed by the self-assembly of surfactants by using hollow-out methods. 48, 49 We report herein a novel approach for the synthesis of nanoporous helical materials, where a co-structure directing agent (CSDA) is used in combination with a chiral monomer that is directly incorporated into the walls of the material. This chiral “dopant” molecule can be used to control the helicity of the resulting particles. This builds on previous work from our group in which we demonstrated that the introduction of chiral dopant molecules into the framework of silicate materials can influence the structure and ordering of the resulting organosilica materials at the pico and nanometer scales.52, 53 Importantly, through careful control over the conditions, hollow nanostructures of various morphologies, ranging from nanospheres to hollow straight nanorods to hollow helical nanotubes, can be prepared. Furthermore, by chemical modification of the structure of the dopant, we are able to demonstrate conclusively the importance of incorporating the chiral dopant as an integral component of the siloxane network, rather than merely as a surface group. The procedure developed is robust and scalable, with ordered uniform materials obtained even after scaling the procedure up by a factor of thirty. 2. Experimental

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General Procedure: In a typical synthesis of anionic surfactant-templated hollow nanoparticles, 0.028 g of sodium dodecyl sulfate (SDS) is dissolved in a solution of 25.5 g of deionized water and 2.5 g sodium borate buffer solution at room temperature to keep the pH value of the solution constant at about 9. To the solution was added 0.019 g of N-trimethoxysilylpropyl-N,N,Ntrimethylammoniumchloride (TMAPS) and a mixture of 0.16 g of TEOS and 0.037 g 1 (2,2’-dimethoxy-1,1’binapthyl-3,3’-bis(triethoxysilane)) or 0.028 g 2 (2,2’dimethoxy-1,1’binapthyl-3-triethoxysilane) (using TEOS as the solvent) with stirring. After 10 minutes, the stirring was stopped and the reaction mixture was aged in a sealed glass vial with a teflon-lined cap at room temperature for 6 h and then 85°C for 1 day. The precipitate was filtered, washed three times alternatively with water and ethanol, dried at 80 °C, and extracted in an HCl/ethanol solution (3.2 mL concentrated HCl in 1000 mL ethanol) at 75 °C overnight, centrifuged, washed with ethanol and recentrifuged. Drying of the resulting material was accomplished in a vacuum oven at 80 °C overnight to give surfactant-free chiral amino-functionalized nanoporous organosilica material. The molar composition of the reaction mixture was: 1 SDS/X TMAPS /7.7 TEOS/1556 H2O/0.6 1 or 2. To obtain chiral, hollow nanotubes of tunable particle size, the TMAPS/SDS molar ratio (X) was varied (0.4, 0.6, 0.8, 1) without or with dopant 1 while keeping the other reaction conditions constant. These samples without dopant are named TEOS-4, TEOS-6, TEOS-8 and TEOS-10 while those with dopant 1 are named Bis-4, Bis-6, Bis-8 and Bis-10 respectively in this text. When dopant 2 was employed, spherical particles with adjustable diameters are obtained by changing the molar ratio of TMAPS to SDS from 0.4 to 1 and the resulting samples are denoted Mono-4, Mono-6, Mono-8 and Mono-10 respectively in the text. Synthesis of 2,2'-dimethoxy-1,1'-binaphthyl-3,3'bis(triethoxysilane) 1 and 2,2’-dimethoxy-1,1’-binapthyl3-triethoxysilane 254: To a dry argon filled 25 mL Schlenk tube containing a Teflon coated stir bar, was added 3,3'-diiodo-2,2'-dimethoxy-1,1'-binaphthyl 55, 56 (847 mg, 2 mmol) and chloro (1,5-cyclooctadiene) rhodium (I) dimer (74 mg, 0.15 mmol) under an inert atmosphere of nitrogen. The flask was then capped and flushed with argon before being charged with dry, degassed DMF (4 mL) followed by freshly distilled NEt3 (1.2 ml, 9 mmol). The resulting mixture was stirred at room temperature for 10 mins. Through the Schlenk valve, triethoxysilane was added dropwise (1.1 ml, 6 mmol) at 0 °C. The Schlenk valve was then closed under argon, stirred at room temperature for 2 h then 80 °C and monitored by TLC. After 6 hours, the flask was allowed to cool to room temperature and the mixture was concentrated in vacuo. The crude product was dissolved in dichloromethane (1.5 mL) and purified over silica gel column using a gradient elution of 12 % - 20 % THF in hexanes. Pure fractions were collected, while mixed fractions were subjected to preparatory TLC developed at 12 % THF/hexanes, to afford the products 1 and product 2 as clear viscous oils. 1: 35% yield, 1H NMR (CDCl3, 400 MHz,) δ ppm 8.28 (s, 2H), 7.83 (d,2H), 7.29 (t, 2H), 7.18 (d, 2H), 7.14(d, 2H), 3.9 (q, 12H), 3.15 (s, 6H), 1.18 (t, 18H). 13C NMR

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(100 MHz, CDCl3,) δ ppm 160.6 (2C), 139.6 (2C), 136.2 (2C), 130.2 (2C), 128.6 (2C), 127.2 (2C), 126.0 (2C), 125.7 (2C), 124.4 (2C), 122.5 (2C), 60.4 (2C), 58.8 (6C), 18.27 (6C). LRMS (EI) m/z (rel intensity) 638 (100) HRMS calcd. for C34H46O8Si2: 638.2731, found TOF MS EI+ 638.2748. 2, 45% yield, 1H NMR (CDCl3, 400 MHz) δ ppm 8.26 (s, 1 H), 7.89 (d, 1 H), 7.82 (d, 1 H), 7.76 (d, 1 H), 7.34 (d, 1 H), 7.26 (t, 1 H), 7.21 (t, 1 H), 7.13 (q, 2 H), 7.07 (q, 2 H), 3.89 (q, 6 H) 3.68 (s, 3 H), 3.24 (s, 3 H), 1.20 (t, 9 H) 13C NMR (CDCl3, 400 MHz) 160.53 (1C), 155.03(1C), 139.26(1C), 135.86(1C), 134.08(1C), 130.41(1C), 129.83(1C), 129.13(1C), 128.66(1C), 127.93(1C), 127.04(1C), 126.56(1C), 125.47(1C), 125.37(1C), 125.28(1C), 124.36(1C), 123.65(1C), 122.89(1C), 119.48(1C), 113.60(1C), 60.66(1C), 58.87(1C), 56.41(3C), 18.20(3C). LRMS (EI) m/z (rel intensity) 476 (100) HRMS calcd. for C28H32O5Si: 476.2019, found TOF MS EI+ 476.2023. Characterization: The microscopic features of all samples were observed with a LEO/ZEISS 1530 FE-SEM (field-emission scanning electron microscope) without gold coating. TEM images were taken from thin edges of particles supported on a porous carbon grid using a JEOL 2010 STEM instrument operating at 200 keV. The sample was suspended in ethanol using ultrasound, after which a droplet of the suspension was dried on this grid. The nitrogen adsorption/desorption isotherms were measured at 77 K with a Micromeritics ASAP 2010. The surface area was calculated by the Brunauer-Emmett-Teller (BET) method and the pore size was obtained from the maxima of the pore size distribution curve calculated by the density functional theory (DFT) method using the adsorption branch of the isotherm. Solid-state CP MAS 13C and 29Si NMR measurements were recorded on a Bruker Avance 600 spectrometer operating at 150.9 and 119.2 MHz for 13C and 29Si, respectively, and using a Bruker 5 mm CP MAS probe. A typical spinning rate for CP MAS experiments is 11 kHz. A cross polarization contact time of 2 ms was used to acquire 29Si and 13C CP MAS spectra with a repetition delay of 2 s. The number of scans was in excess of 600 and 2400 for 13C and 29Si respectively to obtain sufficient signal to noise ratios. Elemental analysis was carried out on a CEI flash 2000 CHNS at temperature 950 °C, the oxygen was released for 5 s at the rate of 140 mL / min. Circular dichroism spectroscopy was carried out by dispersing 0.3 mg of mesoporous silica in 3 mL of ethanol in a clean vial. The sample was sonicated at room temperature for 30 s. The solution was transferred by pipette to a clean quartz cuvette (path length of 1 cm) and a spectrum obtained using a JASCO J-715 spectrometer under the following conditions: 0.2 nm resolution, 5 nm band width, 500 nm / min speed and 1 accumulation. The suspension is diluted if necessary to obtain an appropriate absorbance reading. Solution measurements were prepared in ethanol as well and diluted to appropriate absorbance values. Zeta potential and size distribution of the nanoparticles measurements were performed on Zetasizer Nano using DTS0012 disposable sizing polystyrene cuvettes at2


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25 °C. The samples were prepared, centrifuged, washed with water-free ethanol three times and directly extracted in an HCl/ethanol solution (3.2 mL concentrated HCl in 1000 mL ethanol) at 75 °C overnight, centrifuged, washed with ethanol, recentrifuged and dispersed directly into water-free ethanol for the measurements. These samples were not vacuum dried prior to the measurements, but instead the samples were filtered through a PTFE 0.22 µm syringe filter (Dikma Technologies Inc). The concentrations of the samples were 0.02 g/L. TG - DTA analysis was performed on a TGA Q 500. The temperature was increased from 30 oC to 800 oC with a temperature increase rate of 10 oC / min. The carrier gas is air at a flow rate of 90 ml / min. 3. Results and discussion Chiral dopants 1 and 2 were prepared from the same starting material by a Rh-catalyzed Masuda coupling as shown in equation 1.54 The starting material for this synthesis, 3,3'-diiodo-2,2'-dimethoxy-1,1'-binaphthyl, is prepared in two steps from commercially available materials by orthometalation and iodination of the corresponding methylated binol derivative.53, 55, 56 The Masuda coupling yields both the bis-silylated monomer 1 along with partially reduced species 2, which can be separated by column chromatography. The presence of two polymerizable groups in 1, and only a single siloxane in 2 provides the interesting opportunity to test the importance of including the chiral dopant molecule directly in the backbone of the material.

I

OMe

OMe

(EtO)3Si

MeO

I

OMe

(EtO)3Si

HSi(OEt)3

chiral dopant 1, at different molar ratios of TMAPS / SDS. At low molar ratios of TMAPS/SDS (0.4), the materials consist entirely of spherical particles with diameters of approximately 30 nm (Figure 1a1, Figure 1a2). As the TMAPS/SDS molar ratio is increased, the aspect ratio of the particles also increases such that eventually they adopt a hollow rod-like morphology (Figure 1b2, Figure 1c2). At a perfect 1:1 ratio of TMAPS to SDS, the rods take on a twisted helical morphology uniformly throughout the sample, Figure 1d2. The overall handedness of the chiral nanotubes obtained under these conditions was estimated by counting characteristic morphologies from 250 randomly chosen particles in the SEM images, and the enantiomeric excess ee (ee = {(R - L)/(R + L)} × 100%), where L and R are the number of left- and righthanded rods) was shown to be 20%. When R-1 was used as the dopant, an excess of rods with the opposite handedness was observed. TEM images of the extracted samples confirmed the hollow spherical and tubular morphologies observed by SEM. The diameter of the hollow spherical particles at a TMAPS / SDS molar ratio of 0.4 was determined to be 33 ± 3 nm with a wall thickness of 7 ± 0.5 nm leaving an internal void diameter of 19 ± 3 nm (Table 1). Disordered pores were observed throughout the walls of the spheres. As the TMAPS / SDS molar ratio increases, the particles lengthen, and their diameter gradually increases from 46 ± 4 nm to 103 ± 9 nm in width (Table 1) (Figure 1b1, Figure 1c1 and Figure 1d1). The length of the particles increases from 58 ± 8 nm at a ratio of 0.6:1 to 234 ± 32 nm at a 1:1 ratio of TMAPS to SDS. Marginal increases in wall thickness are also observed (from 7 ± 0.8 nm to 13 ± 1.5 nm at ratios of 0.4 and 1.0).

+

Rh(COD)Cl (7%)

MeO

Si(OEt)3

MeO

H

NEt3 chiral dopant (1) 35% yield

chiral dopant (2) 45% yield

Equation 1. Synthesis of chiral co-monomer 1 and 2.

With these two chiral dopants in hand, we prepared a variety of materials using the general synthetic scheme shown below. The bulk constituent (Si(OEt)4) was mixed with varying amounts of the co-structure directing agent TMAPS (N-trimethoxysilylpropyl-N,N,Ntrimethylammoniumchloride) along with chiral dopant 1 or 2, and sodium dodecyl sulfate (SDS) as the surfactant with water as the solvent (eq. 2). The largest differences in the morphology of the resulting materials were observed when the amount of TMAPS employed relative to SDS was varied. +Me N 3

Si(OEt)4 (bulk monomer)

+

1 or 2 (co-monomer)

Si(OEt)3 TMAPS (co-structure directing agent) –O SO 3

Chiral material Bis-n (1) or Mono-n (2)

8

SDS (structure directing agent)

(TMAPS/SDS = n)

Equation 2. Synthesis of chiral materials from co-monomers 1 and 2.

Figure 1 shows the electron microscopy (TEM and SEM) images of four extracted samples obtained using


3


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SDS molar ratios (1.20) (Figure 2). The same trends were also observed in the materials without dopant (Figure 1SI) As noted in Figure 2, at these higher ratios of TMAPS to SDS, solid nanorods with the axes of the channels along the length of the particles were obtained. The pitch indicated by the arrow (Figure 2b) clearly reveals the 2-d hexagonal mesoporosity in this type of material.51, 53, 57 Decreasing the molar ratio of TMAPS to SDS below 0.4 : 1 resulted in very low yields of material.

Figure 2. TEM images of extracted chiral hollow particles with TMAPS/SDS molar ratios of 1.2 when add 1 to the silica wall.

Figure 1. TEM (a1, b1, c1, d1) and SEM (a2, b2, c2, d2) images of extracted chiral hollow particles with different TMAPS/SDS molar ratios of 0.4 (a), 0.6 (b), 0.8 (c) 1.0 (d) when add 1 to the silica wall. The higher contrast of the walls compared with the interior of the hollow particles also permits observation of the twisted morphology in the TEM images (Figure 1d1 (highlighted with arrows)).48 Increasing the molar ratio of TMAPS to SDS from 0.4 to 1 also results in an increase in the mesoporosity of the silica walls as revealed by the TEM images in Figure 1a1, Figure 1b1, Figure 1c1 and Figure 1d1. The ordered mesopores in the walls of the nanotubes could be clearly observed at the ends of the nanotubes in Figure 1d1, which is similar to helical nanotubes prepared previously by the Che group.48 Therefore, we reach the conclusion that the morphology of the particles is transformed gradually from nanocapsules to nanotubes, and concurrently the mesostructure in the walls transforms from worm-like to 2D-hexaganol. This trend can be further confirmed by the formation of solid 2Dhexagonal helical rods with fringes at higher TMAPS to

The surface area and porosity of the materials were analyzed by N2 physisorption analysis (Figure 2-SI). The isotherms of Bis-4, Bis-6 and Bis-8 materials displayed type IV isotherms with two capillary condensation steps. The hysteresis loops in the low-pressure range (0.4 < P/P0 < 0.9) can be attributed to defect holes, 58, 59 textural mesoporous associated with vesicular (lamellar) morphology 26, 60 or hollow caves.61 Based on the vesicular morphology assigned by TEM, the hysteresis loops observed herein can be attributed to hollow structures of the materials.61 Adsorptions at P/P0 close to 1.0 are attributed to interparticle voids.62, 63 The isotherm of Bis10 is type IV with an H4 hysteresis loop, which represents two capillary condensations observed at the relative pressures of 0.2 - 0.4 (generated from the mesostructure of the shell) and 0.9 - 1.0. The surface area varied in the narrow range of 287 - 312 m2g-1, and the pore volume decreased from 1.63 cm3g-1 to 0.41 cm3g-1 as the amount of TMAPS increased (Table 1). The 13C CPMAS NMR spectra of the materials containing chiral dopant 1 (Figure 3) are clearly correlated to those of monomer 1 (Figure 3-SI). Although integration of CPMAS NMR spectra is far from straightforward, it appears that the incorporation of 1 in the material decreases slightly as the TMAPS/SDS ratio increases. The loading of 1 estimated by elemental analysis is consistent with this general trend, decreasing from 0.43 mmol/g (13.5 wt%) for Bis-4 to 0.37 mmol/g (11.6 wt%), 0.26 mmol/g (8.1 wt%) and 0.28 mmol/g (8.8 wt%) respectively for materials Bis-6, -8 and -10 respectively. The composition and thermal stability of the surfactant-free Bis-4, -6, -8 and -10 were investigated by TGA and DTA under air (Figure 4-SI). Analysis of the data reveals a decrease in weight between 400–600oC of 14.0%, 10.3%, and 10.2% and 8.7% for Bis-4, -6, -8 and -10 respectively, which is consistent with the results obtained using EA. This is likely due to a faster condensation rate at higher4


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TMAPS content.64, 65 The organic functionality in dopants 1 and 2 has a lower rate of hydrolysis and condensation for two reasons: 1) the bulky aryl substituents are sterically hindering, and slow the inversion at the SN2 transition state;65 2) larger alkyl groups on silicon slow the rate of diffusion of the partially hydrolyzed species through solution and therefore the lower the rate of condensation. All of these factors eventually lead to a lesser incorporation of 1 and 2 as the overall rate of condensation increases.

Figure 3. Figure 1

13

The presence of 1 in the material was also confirmed by circular dichroism spectroscopy, performed of the material as a suspension in EtOH. The expected bissignate curves for the binapthol unit were observed, centered at ca. 238 nm (Figure 4), which corresponds closely with those observed for monomer 1 in solution (Figure 5-SI). Mirror image spectra were obtained for both the monomer and chiral hybrid silica materials upon analysis of the enantiomeric species. In order to assess the effect of the chiral dopant on the overall structure, pure TEOS materials were prepared under identical conditions, with no chiral dopant added. Interestingly, the same basic transformation from spheres to nanotubes is observed as the ratio of TMAPS to SDS approaches unity, indicating the importance of charge matching as the primary factor controlling surface curvature (Figure 1-SI, see Table 1 for exact values of wall thickness and particle size). However, as shown in Figure 5, the main difference between “pure” TEOS materials and those prepared with the chiral dopant is the presence of helicity. While materials prepared with dopant 1 at a 1:1 ratio of TMAPS : SDS are uniformly helical, materials prepared without dopant 1 at the same ratio are largely straight porous nanorods, with only occasional particles displaying helical morphology. This indicates that the chiral dopant is not necessarily required for the observation of helicity, but that its presence gives a strong propensity for the helical morphology and as previously noted, exerts some control over the sense of helicity (handedness) of the particles.

CP MAS NMR spectra of the samples shown in

Table 1. Morphology and physicochemical properties of extracted chiral hollow particles with different TMAPS/SDS molar ratios Samples

TMAPS / SDS

Morphology

Particle size [nm][a]

Wall thickness [nm][a]

Width [nm][a]

Length [nm][a]

Surface area [m2g1][b]

Pore volume [mm3g-1][b]

TEOS-4

0.4

Hollow spheres

35±4

9±0.7

-

-

303

1.42

TEOS-6

0.6

Hollow nanotubes

-

9±0.7

43±5

59±9

278

1.84

TEOS-8

0.8

Hollow nanotubes

-

11±0.9

53±4

69±17

281

1.25

TEOS-10

1.0

Hollow nanotubes

-

11±1.5

78±7

147±25

335

1.13

Bis-4

0.4

Hollow spheres

33±3

7±0.5

-

-

287

1.63

Bis-6

0.6

Hollow nanotubes

7±0.8

46±4

58±8

280

1.30

Bis-8

0.8

Hollow nanotubes

11±1.5

66±7

130±14

296

0.93

Bis-10

1.0

Hollow nanotubes

13±1.5

103±9

234±32

312

0.41

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Chemistry of Materials Mono-4

0.4

Hollow spheres

31±3

7±0.7

-

-

270

1.10

Mono-6

0.6

Hollow spheres

46±12

7±0.6

-

-

287

1.56

Mono-8

0.8

Hollow spheres

73±13

9±0.6

-

-

266

1.25

Mono-10

1.0

Hollow spheres

88±25

11±0.6

-

-

313

0.58

[a] As determined by TEM. [b] As calculated by N2-sorption

8 6 4 CD / mdeg

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S-Bis-4

S-Bis-6

S-Bis-8

S-Bis-10

R-Bis-4

R-Bis-6

R-Bis-8

R-Bis-10

Figure 5. SEM and TEM images of extracted materials with TMAPS / SDS molar ratios of 1.0 without the chiral dopant.

2 0 200 -2

220

240

260

280

300

320

340

360

380

400

-4 -6 -8 λ / nm

The physisorption isotherms of these materials are given in Figure 7-SI, and surface area and pore size distributions are given in Table 1. When a 1.2/1 molar ratio of TMAPS / SDS was used in the reaction, the well ordered mesostructure of the walls was observed (Figure 8-SI). The 13C CP MAS NMR spectra of materials prepared with 2 display signals consistent with the incorporation of monomer 2 (Figure 9-SI, Figure 10-SI), thus we know that this species was incorporated into the material.

Figure 4. CD spectra of Bis-R/S-4, Bis-R/S-6, Bis -R/S-8 and Bis -R/S-10

In order to probe the effect of incorporating the chiral monomer into the backbone of the material, we examined materials made by the incorporation of compound 2, since it has the same basic structure as 1, but will act as an “end-group” in the polymerization. Interestingly, helical structures were never observed with this monomer, and even the transition from spherical to rod-like morphology that was observed in pure TEOS materials was arrested upon the addition of 2. Thus at all ratios of TMAPS/SDS employed, only spherical particles were observed (Figure 6, Table 1). The hollow interior diameter of the particles, estimated from careful TEM measurements, increases from 17 ± 3 nm for Mono-4 to 66 ± 25 for materials prepared with a 1.0 TMAPS to SDS ratio (Table 1). At a 1.2 : 1 molar ratio of TMAPS to SDS, the aspect ratio was finally observed to change, and short hexagonal rods were obtained, however the uniformity was not as high as with dopant 1, (Figure 6-SI). Furthermore, and consistent with our results with 1, at these higher ratios of TMAPS to SDS, the walls of the nanoparticles became much thicker, and in some cases solid particles with an ordered porous structure were observed (Figure 6-SI).


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Chemistry of Materials samples at various reaction times. The initial, as-made mixture was prepared according to the procedure above for TEOS-8. Aliquots were taken at 1 minute, 1 hr, 2 hr, 3 hr and 6 hr from the mixture during aging. These samples were observed under TEM and SEM. Unstructured voids of approximately 50 nm in diameter were observed in the 1 minute sample (Figure 7-1mina, Figure 7-1minb, Figure 7-1minc). After 1 hour, TEM images still indicated the presence of voids of around 50 nm along with little order (Figure 7-1hra and 1hrb). It is not until after 2 hrs that silica-coated vesicles can be clearly observed (Figure 7-2hra, Figure 7-2hrb Figure 7-2hrc, Figure 7-3hra, Figure 7-3hrb and Figure 7-3hrc). The ordered structure of the walls can also be observed at this stage (see inset in Figure 7-3hrb). After further aging of the sample for 6 hours (Figure 7-6hra-c), hollow particles are observed, which resemble the final material in the morphology and structure.

Figure 6. TEM (a1, b1, c1, d1) and SEM (a2, b2, c2, d2) images of extracted chiral hollow particles with different TMAPS / SDS molar ratios of 0.4 (a), 0.6 (b), 0.8 (c) 1 (d) when 2 is employed as the chiral dopant.

The chirality of the materials Mono-4 to Mono-10 was assessed by examination of the CD spectra of the amorphous powders dispersed in alcohol. As expected, even though helical morphology was not observed, mirror image CD spectra due to the presence of 2 in the material were obtained when opposite enantiomers of 2 were employed (Figure 10-SI). The CD spectra of these materials resemble quite closely to those made from 1. The hydrodynamic diameters and colloidal stability in ethanol were determined for TEOS-4, Bis-4, Mono-4 to 8 by dynamic light scattering and zeta potential measurements (Table S1, Figure 11-SI). These data indicate that samples of TEOS-4, Bis-4 and Mono-4 were slightly larger than the corresponding values for the dried particles measured by TEM. They have the same dispersity as their solid counterparts. 71 Further, they displayed relatively high negative zeta potentials (Table S1), indicating good colloidal stability. In order to understand the mechanism of the formation of uniform nanocapsules and nanotubes, the evolution of the nanostructure was studied by freeze drying

Figure 7. TEM and SEM images of calcined samples freeze-dried at different period of reaction of 1 minute (1mina, 1minb, 1minc), 1 hour (1hra, 1hrb, 1hrc) and 2 hours (2hra, 2hrb, 2hrc), 3 hour (3hra, 3hrb, 3hrc) and 6 hours (6hra, 6hrb, 6hrc).

Based on all of the above information, we propose a mechanism for the transformation from hollow spheres to nanotubes with tunable diameters and the formation7



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of nanocapsules with adjustable diameters with an increase in the molar ratio of TMAPS/SDS when 1 or 2 is introduced into the system (Scheme 1). Scheme 1. Schematic illustration of the formation mechanism of tunable uniform nanotubes and nanocapsules and the relation between the morphology and the co-structure directing agent.

The effect of the ratio of the tetraalkylammonium CDSA (TMAPS) to the anionic surfactant can be interpreted with reference to vesicle formation with so-called catanionic surfactants (mixed anionic/cationic surfactants).66, 67 It is known that as the ratio of the two surfactants approaches 1, charge matching at the negative headgroup of the micellar surfactant (in this case SDS) improves, which diminishes the effective head group size of the surfactant in accordance with the charge density matching relationship (g = ν/α0l where ν is the chain volume, α0 is the effective hydrophobic / hydrophilic interfacial area, and l is the chain length).68 The lower charge density is attributed to the strong interaction between TMAPS and the negatively charged sulfate group of SDS, decreasing the electrostatic repulsion between the charged surfactant head groups arranged next to one another in the micelle. This then decreases the effective head group area of surfactant, α0, resulting in an increase in the g value. It is well-known that the g parameter of lyotropic liquid crystal phases increases in the order: micellar, micellar cubic < 2D hexagonal < bicontinuous cubic.69, 70 Thus, as the charge density of the surface is decreased, the materials transition from hollow micellar spheres to hollow nanorods eventually to a 2D hexagonal mesophases of lower curvature. The addition of 1, then, appears to have an influence on the further transformation of the hollow rods to helical hollow rods, since this occurs to a much lesser extent in its absence. Interestingly, incorporation of the monosilylated monomer 2 disrupted the transition from

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spherical to rod like, possibly by affecting the head group size as illustrated in Scheme 1. 4. Conclusions By varying the ratio of anionic surfactant to cationic co-structure directing agent, we have been able to manipulate the morphology and the size of hollow silica nanoparticles. Furthermore, we demonstrate that the inclusion of chiral bis-siloxanes in the synthesis mixture at low levels can be used to bias the transformations towards helical structures. Monodisperse hollow silica spheres with chiral groups contained within the walls, having a diameter of ~20 nm and shell thickness of ~7 nm were obtained at low molar ratios of TMAPS/SDS (ca. 0.4/1). At higher ratios, hollow nanotubes ranging from 46 nm to 102 nm in width and 55 nm to 211 nm in length can be obtained. When a 1/1 ratio of the two structure-directing agents was employed in the presence of chiral dopant 1, the nanotubes were shown to take on a helical twist uniformly throughout the sample. Finally, the use of monosilylated chiral dopant 2 illustrates the importance of incorporating the chiral unit into the backbone of the material, since this “terminating” group did not lead to any helical structures, only well dispersed uniform hollow spheres with adjustable diameters between 20 nm and 100 nm with wall thicknesses of 7-10 nm. This is the first demonstration of the use of chiral dopants in the walls of materials prepared with co-structure directing agents to control structure and morphology and is likely to lead to many novel functional materials, an approach that is currently under study in our laboratory. Supporting Information. More characterization of these materials, including TEM images, nitrogen isotherms, 13C spectrum of monomers, CD spectrums are included in the supporting information. This material is available free of charge via the internet at http://pubs.acs.org

AUTHOR INFORMATION Corresponding Author * Prof. C. M. Crudden, Department of Chemistry, Queen’s University, Chernoff Hall, 90 Bader Lane, Kingston, Ontario K7L 3N6 (Canada) E-mail: E-mail: [email protected] Homepage: www.cruddengroup.ca

ACKNOWLEDGMENT NSERC (the Natural Sciences and Engineering Research Council), CFI (the Canada Foundation for Innovation) and Queen’s University are thanked for support of this research through Discovery, Accelerator, Create and LOF grants to CMC.

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Assembly of Uniform Hollow Nanospheres and Helical Nanotubes

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