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One-Pot Construction of Multipodal Hybrid Periodic Mesoporous Organosilica Nanoparticles with Crystal-Like Architectures

Jonas Croissant ,* Xavier Cattoën ,* Michel Wong Chi Man , Philippe Dieudonné , Clarence Charnay , Laurence Raehm , and Jean-Olivier Durand*

Dr. J. Croissant, Dr X. Cattoën, Dr. M. Wong Chi Man, Dr. C. Charnay, Dr. L. Raehm, Dr. J.-O. Durand Institut Charles Gerhardt Montpellier UMR-5253 CNRS-UM2-ENSCM-UM1cc 1701, Place Eugène Bataillon F-34095 , Montpellier Cedex 05 , France E-mail: jonasc@chem.ucla.edu; xavier.cattoen@grenoble.cnrs.fr; durand@um2.fr Dr. P. Dieudonné Laboratoire Charles Coulomb, UMR CNRS 5521 Université Montpellier 2 F-34095 , Montpellier Cedex 5 , France Dr. X. Cattoën, Institut NEEL, Université Grenoble Alpes and CNRS F-38042 , Grenoble , France

DOI: 10.1002/adma.201404226

from bridged organoalkoxysilane precursors so as to maximize the organic content of the material. [ 22 ] The controlled synthesis of PMO NPs, which has recently been achieved by few groups, is still in its infancy. [ 23–27 ] Alike MSN, nanoscaled PMO are attracting much attention due to their wide applicability arising from the versatility of organic moieties that could be periodi-cally organized within porous frameworks. [ 24,28 ] Compared with mesoporous silica NPs, improved properties are foreseen with PMO NPs in catalysis and drug delivery owing to the tunable hydrophobicity and host–guest interactions between the organic fragments and the guests.

In this study, we describe the design of hybrid multipodal PMO (mp-PMO) NPs with crystal-like architectures elaborated in a one-pot two-step process, involving the condensation of benzene-based (B) spherical PMO cores followed by the con-densation of ethylene-based (E) rod-shaped PMO pods on these cores (BE NPs, see Figure 1 ). The mp-PMO nanomaterial mor-phology and composition were fully characterized via different techniques revealing the meso- and molecular periodicity of the organics, as well as very high surface areas of up to 1500 m 2 g −1 . Furthermore, the synthetic process was extended to the growth of ethylene-disulfi de-based PMO pods on the B cores, which highlights the modularity of the procedure to garner a specifi c function in one compartment of the mp-PMO NPs.

The BE mp-PMO NPs were fi rst prepared in a one-pot, two-step process. The sol–gel preparation was performed in an aqueous mixture of cetyltrimethylammonium bromide under basic catalysis at 80 °C. It involved fi rst the controlled addition of 1,4-bis(triethoxysilyl)benzene to afford B PMO nanospheres, followed after 60 min by the growth of E PMO nanorods upon addition of 1,2-bis(triethoxysilyl)ethylene in a near equimolar ratio. The reaction was conducted for an additional 70 min, the resulting BE mp-PMO were collected, and the surfactant was extracted (see the Supporting Information). The stirring speed was found to be a key parameter to control the morphology of the mixed NPs, whereas B@E core–shell spherical structures were obtained when the E precursor was added at the same stir-ring speed used for the preparation of the B core (1400 rpm) (Figure S1, Supporting Information), multipodal NPs were formed when the addition of the E precursor was carried out at lower stirring speed (700 rpm).

The morphology of the BE mp-PMO NPs was then charac-terized by transmission electron microscopy (TEM), revealing astonishing BE mp-PMO NPs composed of one to several pods (see Figure 1 a–d, and NPs representations in Figure 1 e–h). The B PMO cores were spherical particles of 130 ± 20 nm,

The design of morphological and structural complexity at the nanoscale is one of the major challenges of nanotech-nology to mimic the highly functional nanoobjects observed in nature. Indeed, the size, shape, structure, composition, and surface functionalization of nanomaterials directly affect their optical, physicochemical, and electrical properties. [ 1–3 ] The control of elaborated structures of nanomaterials is cru-cial and has launched new areas of research particularly in nanomedicine and catalysis. [ 4,5 ] Multicompartment sophis-ticated nanomaterials such as micelles obtained from block terpolymer, [ 6 ] lipid-protein liquid-crystalline nanoparticles (NPs), [ 7 ] and self-assembled block copolymer have been pre-pared. [ 8 ] Tetrapod-shaped CdSe and CdTe NPs, [ 9,10 ] tri-, tetra-, and tree-like multipodal GaP NPs, [ 11 ] via epitaxial connec-tion of branched heteroonanostructures [ 12 ] have been synthe-sized and the unusual charge separation properties of these materials could have important applications in photovoltaïc energy conversion. NPs with high morphological complexity have been obtained as multipodal colloidal silica clusters, [ 13–15 ] and various kinds of anisotropic nanoassemblies from NPs building block approaches have been investigated. [ 16 ] Of par-ticular interest is the design of porous multipodal nanosys-tems that have been scarcely reported. [ 17 ] One recent example was described by Wiesner and co-workers [ 18 ] in 2013 on mul-ticompartment multipodal mesoporous silica nanoparticles (MSN), synthesized via the epitaxial growth of silica pods of 2D-hexagonal structure on silica cores of cubic porosity. Peri-odic mesoporous organosilicas (PMOs) were fi rst introduced independently by three research groups in 1999 as a valuable alternative to mesoporous silica. [ 19–21 ] They are prepared solely

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whereas the E PMO pods were 100 × 90–150 × 90 nm 3 nanorods. High resolution scanning transmission electron microscopy (HR-STEM) imaging displayed the expected composition-related contrast between the B core and the E pods (Figure 1 i–l). Particularly, in Figure 1 i, where the B and E compartments have comparable diameters, the higher carbon content of the benzene-based core induces a more pro-nounced contrast. Besides, both TEM and HR-STEM revealed the hexagonal shape of the E PMO pods, as clearly seen when the pods were both aligned with the electron beam and situ-ated behind the B cores (see Figure 1 c,k, see Figure S2, Sup-porting Information). Statistical analysis on nearly 180 NPs showed that the majority of the BE mp-PMO contained one or two pods (49% and 27%, respectively, see Figure S3, Sup-porting Information), the quarter left being composed of 3–8 pods (Figure 1 c–l).

To garner better insight into the structure of the BE mp-PMO, B PMO nanospheres and E PMO nanorods were prepared separately (see the Supporting Information). TEM images of B and E showed, as expected, 130 nm spheres and few hundred nanometers rods, respectively ( Figure 2 a,b), consistent with the multiple 100–150 nm pods of BE NPs (Figure 2 c). X-ray diffraction (XRD) patterns of E PMO, B PMO, and BE mp-PMO NPs at low angles displayed peaks at around 2.3°, corresponding to regular repetitions of mesopores (Figure 2 d). While the E PMO nanorods display a 2D-hexagonal struc-ture ( p 6 mm ) as shown by the intense narrow peak at 2.3° with the satellites at 3.9° and 4.6° (corresponding to a d 10 spacing of 38.8 Å), broad peaks are observed in the case of B and BE NPs, which result from the radial organization of the pores. The harmonic peaks at 3.9° and 4.6° can also be distinguished in the diffractogram of BE mp-PMOs (Figure 2 d and Figure S4

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Figure 1. a–d) TEM images of BE mp-PMO containing one to several pods, and e–h) depicted by their schematic NPs representations. i–l) HR-STEM images BE mp-PMO NPs containing one to several pods. c,k) The red hexagons highlight the morphology of the E PMO pods. Scale bars: 100 nm.

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(Supporting Information)). In the TEM images, a radial porosity was observed in the B NPs (see Figure 2 a), while periodicity was readily visible on the E pods by fast Fourier transform (FFT, see Figure S5, Supporting Information). Moreover, a molecular-scale periodicity, fi rst reported by Inagaki et al. [ 19 ] on micron-sized materials, was found in the wide angle (5°–40°) XRD pat-terns of B and BE NPs, as demonstrated by the peaks at 11.7°, 23.6°, and 36.5° corresponding to a lamellar repetition of the aromatic units with a Bragg d-spacing of 7.6 Å (Figure 2 e). Conversely, there is no molecular order in the ethylene frame-work. From the N 2 adsorption–desorption isotherms, both B and E PMO NPs displayed BET (Brunauer–Emmett–Teller) specifi c surface areas (979 and 1042 m 2 g −1 , respectively), pore volumes (0.59 and 0.76 cm 3 g −1 , respectively) with BJH (Bar-rett–Joyner–Halenda) pore size distributions centered around 2 nm that were consistent with a mesoporous internal struc-ture (Figures S6–S8, Supporting Information). However, such PMO NPs exhibited dissimilar shapes of their nitrogen adsorp-tion isotherms that refl ected different pore structure organi-zations. The obtained adsorption isotherm for E nanorods (Figure S7, Supporting Information) can be described as standard type IV curve with a hysteresis loop of type H1 that is typical of a mesoporous templated material exhibiting open and well-defi ned cylindrical-like pore channels, consistent

with the TEM images (Figure 2 b). Otherwise, mesoporous B nanospheres exhibited a sorption isotherm with a shape of type II and a hysteresis loop of type H4 (Figure S6, Supporting Information). This uncommon hysteresis for a mesoporous templated solid is associated with the presence of narrow slit pores [ 29 ] in agreement with the radial porous network of B PMO (Figure 2 a). Moreover, the isotherm for the BE mp-PMO can be depicted as the sum of the isotherms of B and E PMO parts and lead to a surface area of 1413 m 2 g −1 (Figure S8, Sup-porting Information).

The chemical composition of the NPs was then ana-lyzed. First, the comparison of the solid-state NMR, 13 C cross-polarization magic angle spinning (CPMAS) spectra of BE mp-PMO, B PMO, and E PMO nanomaterials demon-strated the presence of both the benzene ( δ = 134 ppm) and the ethylene ( δ = 145 ppm) moieties in BE NPs (Figure 2 f). Second, the 29 Si CPMAS spectra validated the formation of the siloxane network with the major proportion of the T 2 and T 3 signals (Figure 2 g). These observations were supported by the Fourier transform infrared (FTIR) spectrum of BE NPs com-posed of the ν C H and ν Si C stretching vibration modes of the benzene group at 3067 and 1198 cm −1 , respectively, and the out of plane aromatic δ Csp2-H bendings at 554 and 521 cm −1 , as well the ethylene modes: ν Si–C and ν C C at 1193 and

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Figure 2. a,b) TEM images of B PMO nanospheres and E PMO nanorods obtained solely from the bis(triethoxysilyl)benzene or bis(triethoxysilyl)ethylene precursors, respectively. c) TEM images of multipodal BE NPs designed from the one-pot two steps condensation process. d,e) XRD patterns comparison of the BE mp-PMO NPs, B PMO, and E PMO, at low and wide angles. f,g) Solid-state NMR, 13 C and 29 Si CPMAS spectra comparison of the BE mp-PMO, B PMO, and E PMO NPs. For E, cis and trans isomers are present in 13 C CPMAS NMR. *Spinning side-bands.

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1636 cm −1 , and the δ Csp2-H at 926 cm −1 (see Figures S9 and S10, Supporting Information). The ν Si O stretching modes of both moieties were also present from 1060 to 1120 cm −1 . Energy dispersive spectrometry validated the equimolar ben-zene–ethylene proportion of BE NPs when compared with B and E PMO NPs (Figure S11, see Si/C ratios in Figure S12, Supporting Information).

The versatility of the synthetic process was then assessed for the preparation of B PMO core ethylene-disulfi de (EDIS)-based PMO pods (BEDIS) NPs. The design of such NPs involved identical reaction parameters than those used for BE mp-PMO; fi rst the B core was prepared, and second, bis(triethoxysilyl)ethylene was co-condensed with the bis(3-triethoxysilylpropyl)disulfi de precursor in a 1.84:1 molar ratio (see Figure 3 a).

Multipodal BEDIS NPs were readily visible in the TEM images, the B cores being decorated with one to six 50 nm EDIS pods (Figure 3 b,c). The hexagonal mesostructures of the pods were preserved as clearly seen by comparing the FFT transform of the image of the B core (Figure 3 c,e) with the

E pods (Figure 3 d,f). The hexagonal pattern of the diffraction spots is characteristic of long-distance ordered P6mm struc-tures (see Figure 3 f), and the small- and wide-angle XRD pat-terns exhibited as well the occurrence of mesoscopic (38.8 Å) and molecular periodicities (7.6 Å) for BEDIS mp-PMO NPs (see Figure S13a,b, Supporting Information). The small-angle X-ray scattering analyses of BE and BEDIS NPs further vali-dated the d-spacings obtained via the scattering vector q values of 1.6 and 8.2 nm −1 corresponding, respectively, to 39.0 and 7.6 Å (Figure S13c,d, Supporting Information). Addition-ally, the BEDIS pods were extended to a length of 100 nm through a higher amount ethylene-disulfi de mixture (see TEM in Figure S14, Supporting Information). The successful con-densation of the bis(3-triethoxysilylpropyl)disulfi de moiety in the BEDIS NPs was demonstrated in the FTIR spectrum by the aliphatic ν C H vibration modes of the propyl fragments between 2970 and 2860 cm −1 (Figure S15, Supporting Infor-mation), the three different signals of the propyl chain in the 13 C CPMAS spectrum at 10, 27, and 47 ppm (Figure S16a, Supporting Information), and the major T 2 and T 3 signals in

Figure 3. a) Formation of BEDIS mp-PMO NPs, and b) the subsequent TEM images of surfactant-free multipodal NPs. c,d) TEM images, and e,f) the associated FFT confi rmed the expected absence of symmetry in the B core (bright dots arising from the camera), along with the hexagonal symmetry of the mesoporous ethylene-disulfi de-based PMO pods in the [001] incidence.

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the 29 Si CPMAS spectrum (Figure S16b, Supporting Informa-tion). The silicon over sulfur ratio determined by EDS meas-urements was coherently decreased with the higher amount of the bis(3-triethoxysilylpropyl)disulfi de in the nanomaterial with longer EDIS pods (Figure S17, Supporting Information). N 2 adsorption–desorption gave BET surface areas of nearly 1500 m 2 g −1 and pore volumes up to 1.2 cm 3 g −1 for both BEDIS mp-PMO NPs (Figures S18 and S19, Supporting Infor-mation), and the mean BJH pore size of BEDIS remained in the order of 2.1 nm as for BE mp-PMO (see Figure S20a,b, respectively).

In summary, we have described original multipodal benzene core ethylene pod hybrid PMO NPs in a one-pot two-step pro-cess. The nanomaterial possesses a crystal-like organization, with hexagonally-shaped E pods of P6mm mesostructure, and B PMO cores periodically organized at the molecular scale. Furthermore, the procedure could be modifi ed to design eth-ylene-disulfi de-based PMO nanopods, which are most likely biodegradable. [ 27 ] The ability to garner various functions solely on the pods is very attractive, since it would pave the way for various applications. The introduction of one function in the B core that could interact with a second function in the E pods is envisioned for multistep catalysis, as well as photonic and biomedical applications. Indeed, all mp-PMO NPs display very high surface areas and pore volumes of up to 1500 m 2 g −1 and 1.20 cm 3 g −1 , which render them very attractive multicompart-ment nanocontainers. If selective drug adsorption could be tailored, the mp-PMO could be applied for multidrug delivery. Finally, more complex mp-PMO nanostructures are under investigation with the combination of additional functions in this very promising nanoplatform.

Supporting Information Supporting Information is available from the Wiley Online Library or from the author.

Acknowledgements ANR Mechanano is gratefully acknowledged.

Received: September 12, 2014 Revised: October 6, 2014

Published online: November 6, 2014

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