Architecture of novel periodic mesoporous organosilicas based on the flexible skeleton of aspartic acid-bridged organosilane

Materials Letters 193 (2017) 299–304

Contents lists available at ScienceDirect

Materials Letters journal homepage: www.elsevier.com/locate/mlblue

Architecture of novel periodic mesoporous organosilicas based on the flexible skeleton of aspartic acid-bridged organosilane Wang Jianqiang a,⇑, Zhou Man a, Gu Changqing a, Zhang Wenqi a, Lv Mengyuan a, Guo Cheng a, Sun Linbing b,⇑, Zhang Hailu c,⇑ a b c

College of Chemistry and Molecular Engineering, Nanjing Tech University, 30 Puzhu South Road, Nanjing, Jiangsu 211816, China College of Chemical Engineering, State Key Laboratory of Materials-Oriented Chemical Engineering, Nanjing Tech University, Nanjing 210009, China Laboratory of Magnetic Resonance Spectroscopy and Imaging, Suzhou Institute of Nano-tech and Nano-bionics, Chinese Academy of Sciences, Suzhou 215123, China

a r t i c l e

i n f o

Article history: Received 2 December 2016 Received in revised form 13 January 2017 Accepted 28 January 2017 Available online 4 February 2017 Keywords: Aspartic acid Periodic mesoporous organosilicas (PMOs) Bridged organosilane Biomaterials Nanocomposites

a b s t r a c t Novel periodic mesoporous organosilicas materials (PMOs) based on the flexible skeleton of aspartic acid were synthesized by the co-condensation of aspartic acid-bridged organosilane (Asp-BOSP) and tetraethyl orthosilicate (TEOS) in an acidic medium, using the Pluronic P123 surfactant as a template. Furthermore, a new amino acid organosilane was developed by the simple reaction between traditional organosilicone ((3-aminopropyl) trimethoxysilane) and aspartic acid. The small-angle XRD and N2 adsorption-desorption isotherms demonstrate that these PMOs materials possess ordered 2D hexagonal mesostructures in the region of low molar concentrations of Asp-BOSP (610%). Analysis of FTIR and 29Si MAS solid-state NMR confirm that the aspartic acid is incorporated into the framework of the PMOs materials. Ó 2017 Elsevier B.V. All rights reserved.

1. Introduction Periodic mesoporous organosilicas (PMOs) because of organic groups or functional moieties as the part of framework components have attracted extensive research interest since they were first prepared by three research groups of Stein, Ozin and Inagaki [1–3]. Thereby, they eliminate disadvantages (such as pore blocking, disordered/amorphous structure, etc.) of the other functionalized mesoporous silicas (grafting and co-condensation) [4,5]. As PMOs-based materials can offer unique physical and chemical properties, including high surface areas, large pore volume, high hydrothermal or mechanical stability and diversity of framework components [6], they are widely employed in various fields such as catalysis, sensors, optics, chemical separation, drug release or controlling and electronic devices [7]. PMOs materials are generally synthesized by hydrolysis and condensation reactions of bridged organosilane molecules ((R0 O)3Si-R-Si(OR0 )3; R: bridged organic groups, R0 = methyl or ethyl) as precursors via the selfassembly process in the presence of a structure-directing agent [8]. R in the bridged organosilane molecules play a pivotal role in creating the flexibility, rigidity and functionality of the framework ⇑ Corresponding authors. E-mail addresses: [email protected] (J. Wang), [email protected] (L. Sun), [email protected] (H. Zhang). http://dx.doi.org/10.1016/j.matlet.2017.01.133 0167-577X/Ó 2017 Elsevier B.V. All rights reserved.

of PMOs, and previous study of PMOs shows that simple bridged organosilanes can construct different frameworks. Some researchers subsequently created PMOs materials utilizing more advanced functional precursors of bridged organosilane molecules with heteroatoms (N, S, P, O, etc.) as precursors [9]. Moreover, metal complexes, metal nanoparticles, and chiral bridges were also introduced into the framework of PMOs for different applications [10–12]. Therefore, exploring bridged organosilane molecules as organic moieties is one of the most significant factors to prepare flexible skeleton of highly ordered PMOs. Amino acids as important biological molecules are widely applied in numerous fields such as solid phase peptide synthesis, agrochemical compounds and biomedical sensors [13]. The aspartic acid, which is well-known acidic amino acids and play crucial roles in memory, learning, biodegradation and corrosion inhibitors [14], is firstly investigated in this work as the reactant to form a new aspartic acid-bridged organosilane precursor through its condensation with (3-aminopropyl) trimethoxysilane (APTES). Based on the amino acid-bridged organosilane precursor, novel PMOs materials containing aspartic acid as part of framework were developed. And in this co-condensation method, the P123 triblock copolymer surfactant and mixed organosilanes were employed to obtain the PMOs materials, taking advantage of the flexible skeleton of aspartic acid-bridged organosilane. The creation of ordered pore structures and composites of PMOs materials were further

300

J. Wang et al. / Materials Letters 193 (2017) 299–304

confirmed by crucial spectra characterizations such as FTIR and 29 Si MAS solid-state NMR. 2. Experimental

(m,4H), 0.56–0.51 (m,4H). ESI-MS: m/z = 678.28 (M + H+), m/z = 700.26 (M + Na+). FT-IR (KBr, m cm1): 3296, 3067, 2942, 2840, 1692, 1651, 1538, 1450, 1301, 1270, 1243, 1194, 1085, 816, 737, 621, 449. (1H-NMR, FTIR and MS spectroscopy analyses were shown in the Supplementary Material).

2.1. Reagents and materials Surfactant Pluronic P123, ethylene oxide (EO)/propylene oxide (PO) triblock copolymer, composition of EO20PO70EO20, with the average molecular weight of 5800 was purchased from Aldrich. Other reagents such as (3-Aminopropyl) trimethoxysilane, tetraethyl orthosilicate (TEOS), aspartic acid, and organic solvents were purchased from the Chemical Reagent Crop. in China. All reagents and solvents were of AR grade. 2.2. Preparation of aspartic acid-bridged organosilane precursor (AspBOSP) Typically (as shown in Scheme 1), 3.55 g of N-(9Fluorenylmethoxy- carbonyl)-L-aspartic acid (Fmoc-Asp, 10 mmol) which was prepared according to the references [15] (given in the supporting information), 9.13 g of 2-(7-Aza-1H-benzotriazole-1-y l)-1,1,3,3-tetramethy-luronium hexafluorophosphate (HATU, 20 mmol) and 7.76 g of N, N-Diisopropylethylamine (DIEPA, 60 mmol) were dissolved in 100 mL of anhydrous tetrahydrofuran to obtain clear solutions. The above mixtures were stirred for 30 min at 0 °C in an ice bath. Then, 3.59 g of (3-Aminopropyl) trimethoxysilane (APTES, 20 mmol) was slowly dropped into the mixtures. The reaction system was maintained at 0 °C in an ice bath and continuously stirred for 2 h, and 200 mL of dichloromethane was directly added into the mixtures. Following this step, 100 mL of saturated ammonium chloride solution, saturated sodium bicarbonate and saturated brines were used to extract the organic layers, respectively. The organic extraction solutions were dried with the anhydrous sodium sulfate. The final crude products of white solid powder were obtained by distilling the solvents under reduced pressure condition. The relatively pure substance of the above solid was recovered by pulping and washing thoroughly with n-hexane and then dried under vacuum conditions. The product of aspartic acid-bridged organosilane compound, denoted as Asp-BOSP, was obtained with a yield of 76%. 1 H NMR (400 MHz,DMSO-d6, TMS) d7.90–7.88 (d, J = 7.5 Hz, 2H), 7.85–7.80 (m, 2H), 7.71–7.69 (d, J = 7.5 Hz, 2H), 7.45–7.40 (m, 3H), 7.34–7.31 (t, J = 7.4 Hz, 2H), 4.32–4.20 (m, 4H), 3.43 (s, 18H), 3.01–2.99 (d, J = 6.0 Hz, 4H), 2.48–2.35 (m, 2H), 1.44–1.38

2.3. Preparation of periodic mesoporous organosilicas with aspartic acid in the framework (Asp-PMOs) The periodic mesoporous organosilicas materials of aspartic acid bridged organosilane (Asp-PMOs) were prepared by the cocondensation of TEOS and Asp-BOSP with the P123 template, as shown in Scheme 2. In a typical synthesis, Pluronic P123 (2.0 g) was firstly dissolved in the mixed solution of distilled water (15 g) and HCl (60 g 2 M) at room temperature to obtain a clear solution. Then, 9.36 g of sodium chloride was added into the above-mentioned solution. After dissolution, the mixtures were continuously stirred in a water bath and were heated up to 40 °C. Subsequently, a certain amount of TEOS and the Asp-BOSP in DMF solution were added to the above homogenous solution under vigorous stirring for 24 h at 40 °C. The reactant mixtures were thereafter transferred into a stainless steel reactor with a polytetrafluorethylene liner, and then were kept at 100 °C for another 24 h. Finally, the product was recovered by filtration, washed with distilled water and dried in air under ambient conditions. The template was extracted by refluxing 1.0 g of the aboveprepared products in the mixed solutions of 250 mL of absolute ethanol and 5 mL of hydrochloric acid (2 M) for 48 h. The materials obtained were denoted as Asp-PMOs-n, where n (n = 5%, 10%, 15%, 20%) was the molar percent of Asp-BOSP/(Asp-BOSP + TEOS). 2.4. Sample characterization 1 H NMR (400 MHz) spectra were obtained using a Bruker AC400 Avance spectrometer and all chemical shifts (d) are reported in parts per million (ppm) downfield from TMS; J values are given in hertz (Hz). FTIR spectra were recorded on a Thermo iS5 FT instrument in the region from 4000 to 400 cm1 at 298 K, and the sample was mixed with KBr at the ratio of 3:97 (mol/mol) and then pressed as transparent disc. High resolution mass spectra were run on Aglient 6550 iFunnel Q-TOF. Small-angle X-ray diffraction (XRD) detection of samples were performed on ARL XTRA diffractometer using Cu Ka radiation (k = 1.5418 Ǻ) at 40 kV and 20 mA in the 2h range of 0.5–8°. Nitrogen adsorptiondesorption isotherms were measured on a Micrometrics

Scheme 1. Preparation of aspartic acid-bridged organosilane precursor (Asp-BOSP).

301

J. Wang et al. / Materials Letters 193 (2017) 299–304

Scheme 2. Preparation of aspartic acid bridged periodic mesoporous organosilicas (Asp-PMOs).

ASAP2020, and the samples were degassed at 373 K overnight in the degassing port of the adsorption analyzer prior to testing. Pore size distributions were calculated using the Barret-Joyner-Halenda (BJH) algorithm on the adsorption branch. The most probable pore size was obtained from peak positions of the distribution curves. The pore volume was taken at the P/P0 of 0.973 single point. Solid-state NMR experiments were carried out using a Bruker AVANCE III-500 spectrometer (BrukerBioSpin, Karlsruhe, Germany) operating at a magnetic field strength of 11.7 T, equipped with a 4 mm double-resonance MAS probe. The 29Si magic angle spinning (MAS) NMR spectra with 1H decoupling were collected at an 8 kHz MAS spinning speed with a p/4 pulse width of 2.3 ls and a recycle delay of 90 s. The chemical shifts were externally referenced to tetramethylsilane (TMS).

XRD pattern of Asp-PMOs-20% shows no apparent diffraction peaks. These results prove that the mesostructural order of AspPMOs-n is closely connected with the concentrations of bridged organosilane precursor. The mesostructural organizations maintain in the addition of 5% and 10% of the aspartic acid-bridged organosilane precursor (Asp-BOSP). Moreover, the diffraction angle 2h of (1 0 0) reflection peak decreases as the amount of ASP-BOSP precursor increases, which shows the lattice expansion owing to the incorporation of organic groups in the mesoporous framework. The intensity of (1 0 0) peak gradually reduces with increasing the

100 3. Results and discussion

The periodic mesoporous organosilicas with different amounts of aspartic acid groups in the framework were synthesized by the direct co-condensation method as described in the experimental section. The small angle powder XRD patterns of the solventextracted PMOs samples are presented in Fig. 1, including AspPMOs-5%, Asp-PMOs-10%, Asp-PMOs-15%, Asp-PMOs-20%. The sample of Asp-PMO-5% displays three well-resolved diffraction peaks corresponding to (1 0 0), (1 1 0) and (2 0 0) reflections in the 2h range of 0.5°–2.5°, characteristic of mesoporous material with well-ordered 2D hexagonal (P6mm) structures. A sharp (1 0 0) diffraction peak and a broad (1 1 0) reflection peak are also observed in the XRD patterns of Asp-PMOs-10%. Only a relatively broad (1 0 0) reflection peak appears in the XRD pattern of AspPMOs-15%. When the precursor content is increased to 20%, the

110

200

Asp-PMOs-5%

Intensity(a.u)

3.1. Structural characterization of periodic mesoporous organosilicas Asp-PMOs

Asp-PMOs-10% Asp-PMOs-15%

Asp-PMOs-20% 1

2

3

4

2θ degree Fig. 1. Small-angle powder XRD patterns of periodic mesoporous organosilicas AspPMOs-n (n = 5%, 10%, 15%, 20%).

302

J. Wang et al. / Materials Letters 193 (2017) 299–304

molecular size and the bulk of organic groups occupy more space of the mesoporous channels, thus resulting in the reduction of the pore diameter and the pore volume. The other textural parameters (d100, a0) of Asp-PMOs-n are also summarized in Table 1further confirming that the Asp-PMOs-n samples with low concentrations of Asp-BOSP are mesoporous materials with uniform pore distributions.

Asp-BOSP concentration in the synthetic mixtures. All the results of XRD patterns are partly due to the disturbance of the AspBOSP with large molecular size in the formation and selfassembly of surfactant aggregates during the co-condensation process. The N2 adsorption–desorption isotherms and the pore size distribution curves of Asp-PMOs-n are displayed in Fig. 2, and their corresponding structural parameters are listed in Table 1. As shown in Fig 2A, the materials Asp-PMOs-5% and Asp-PMOs-10% show the type IV isotherms with a clear H1 type hysteresis loop at high relative pressures (P/P0) from 0.65 to 0.9 according to the IUPAC classification, which is due to the uniform pore size and structure of these materials. With the increasing amounts of AspBOSP, the material Asp-PMOs-15% and Asp-PMOs-20% exhibit the type IV with a clear H4 and H3 type hysteresis loop, respectively. The H4 type hysteresis loop demonstrates that mesopore and micropore are simultaneously formed within the channels of the Asp-PMOs-15%. The relative pressure P/P0 of the steep increase in the adsorption is reduced in the samples with high amounts of Asp-BOSP, which shows that the Asp-PMOs-5% and Asp-PMOs10% samples have larger pore sizes than the other two samples do, as shown in Fig 2B. BET surface areas of the materials Asp-PMOs-n range from 162 to 107 m2 g1, and the total pore volumes vary from 0.40 to 0.29 cm3 g1 (Table 1). The pore diameter and the pore volume of Asp-PMO-5%, Asp-PMO-10% and Asp-PMO-15% decline with the increasing concentration of Asp-BOSP in the mixtures. This result was reasonable because of the incorporation of bridged organosilane groups in the framework of the Asp-PMOs materials; to illustrate, the aspartic acid-bridged organosilane precursor has a large

3.2. Spectroscopic analysis of periodic mesoporous organosilicas AspPMOs The chemical bonds and the presence of organic moieties in the framework of Asp-PMOs were confirmed by FTIR spectroscopy. The FTIR spectra of solvent extracted Asp-PMOs-n samples are shown in Fig. 3. In all the samples, a broad and strong peak at 3367 cm1 was attributed to the stretching vibration of N–H. A sharp and intense band at 1695 cm1 is ascribed to the C@O stretching vibration of the amide. The weak vibration bands at 2900–3000 cm1 and 1457 cm1 are assigned to the CAH stretching vibrations and the CAH bending vibrations of surfactant P123, respectively, confirming that the existence of trace residual surfactants. The band from 1100 to 1000 cm1 can be attributed to the SiAOASi stretching vibration, and the band at 757 and 459 cm1 should be associated with the bending vibration of SiAOASi bond. The results demonstrate the hydrolysis and co-condensation of the SiAOCH2CH3 groups of aspartic acid-bridged organsilanes (AspBOSP). The intensity of 1750–1500 cm1 bands increases gradually from the Asp-PMOs-5% to the Asp-PMOs-20% samples, which signifies the increased concentration of amide in the resultant materials.

2.5

A

B

Adsorption Desorption

a

dV/ dlog (D) (cm3/g)

3 -1

Volume adsorbed (cm g )

2.0 a

b c

1.5

b 1.0

0.5

d 0.0

d

0.2

0.4 0.6 Relative Pressure (P/P0)

0.8

c 4

1.0

6

8

10

12

14

16

18

20

Pore Diameter (nm)

Fig. 2. (A) Nitrogen adsorption-desorption isotherms and (B) Pore size distribution of periodic mesoporous organosilicas Asp-PMOs-n (n = (a) 5%, (b) 10%, (c) 15%, (d) 20%).

Table 1 Textual parameters of periodic mesoporous organosilicas Asp-PMOs-n.

a b c d

Sample

d100a (nm)

a0b (nm)

SBET (m2/g)

Pore volumec (cm3/g)

Pore sized (nm)

Asp-PMOs-5% Asp-PMOs-10% Asp-PMOs-15% Asp-PMOs-20%

4.7 5.0 5.3 –

5.4 5.8 6.1 –

162 166 119 107

0.40 0.27 0.15 0.29

7.3 6.0 5.0 –

Interplanar spacing of the (1 0 0) plane, as obtained from the pffiffiffi XRD analysis. The a0 is unit cell parameter, calculated from a0 ¼ 2d100 = 3. Pore volume is determined from the N2 adsorption and desorption isotherm. Pore diameter is calculated using the BJH model based on the desorption branch of the isotherm.

J. Wang et al. / Materials Letters 193 (2017) 299–304

C=O

303

related to the materials Asp-PMOs-n (where n = 5%, 10%, 15%, 20%), respectively.

Si-O-Si

Asp-PMO-20%

Transimittance(%)

4. Conclusions Asp-PMO-15%

Asp-PMO-10%

Asp-PMO-5% Si-OH -NH

4000

3500

P123 3000

2500

2000

1500

1000

500

Wavelength( cm-1) Fig. 3. FTIR spectra of periodic mesoporous organosilicas Asp-PMOs-n (n = 5%, 10%, 15%, 20%)

Periodic mesoporous organosilicas with the framework incorporated by aspartic acid (Asp-PMOs) have been first developed through the co-condensation of TEOS and aspartic acid-bridged organosilane (Asp-BOSP) in the presence of the triblock copolymer Pluronic P123 as a template. The ordered 2D hexagonal mesostructure is constructed in the Asp-PMOs materials with the molar concentration being less than or equal to 10% of Asp-BOSP. Characterization of FTIR and 29Si MAS solid-state NMR characterization confirm successful corporation of aspartic acid into the PMOs framework. Moreover, for the first time, aspartic acidbridged organosilane is prepared by the reaction between amino acid and conventional organosilane. We anticipate that this method to build POMs with new architectures would inspire peer researchers to configure designer POMs-based materials for practical applications. Acknowledgement Financial supported by National Natural Science Foundation of China (No. 21106069) and the State Key Laboratory of MaterialsOriented Chemical Engineering (KL15-08) are gratefully acknowledged. Appendix A. Supplementary data Detailed synthetic procedures of Fmoc-Asp and The 1H-NMR, IR, ESI-MS spectroscopy of Fmoc-Asp and Asp-BOSP are available in the supplementary materials. Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.matlet.2017.01. 133. References

Fig. 4. The 29Si MAS NMR spectra of periodic mesoporous organosilicas Asp-PMOsn (n = (a) 5%, (b) 10%, (c) 15%, (d) 20%).

The 29Si magic-angle spinning (MAS) solid-state NMR spectra of the surfactant extracted Asp-PMOs-n with the different concentration of Asp-BOSP are provided in Fig. 4, showing that both Q and T sites corresponding to Qn (Qn = Si(OSi)n(OH)4n, n = 2–4) and Tm(Tm = RSi(OSi)m(OEt)3m, m = 2, 3) [16] appear in the 29Si MAS NMR spectra of the Asp-PMOs-n samples. The prominent signal at 114 ppm was assigned to Q4, and two shoulders at 101 ppm and -92 ppm were attributed to Q3 and Q2, respectively. The results imply a high degree of TEOS cross-linking under the employed synthetic condition. The T signals arising from the organic functional parts of bridged organosilane were obtained in the range of 50 ppm to 70 ppm, which were assigned to the T3 at 66 ppm and T2 at 58 ppm [17,18]. This result illustrates that the organic groups are completely incorporated in the silica network with a high degree condensation of silanol groups, confirming the stability of SiAC bond under synthetic conditions [19]. Moreover, increasing the amounts of Asp-BOSP intensifies the T signals. Based on normalized peak areas, the ratio of Tm/(Tm + Qn) is around 0.453, 0.339, 0.215 and 0.063, which is

[1] B.J. Melde, B.T. Holland, C.F. Blanford, A. Stein, Mesoporous sieves with unified hybrid inorganic/organic frameworks, Chem. Mater. 11 (1999) 3302–3308. [2] T. Asefa, M.J. MacLachan, N. Coombs, G.A. Ozin, Periodic mesoporous organosilicas with organic groups inside the channel walls, Nature 402 (1999) 867–871. [3] S. Inagaki, S. Guan, Y. Fukushima, A.T. Ohsuna, O. Terasaki, Novel mesoporous materials with a uniform distribution of organic groups and inorganic xxide in their frameworks, J. Am. Chem. Soc. 121 (1999) 9611–9614. [4] M.C. Burleigh, M.A. Markowitz, M.S. Spector, B.P. Gaber, Direct synthesis of periodic mesoporous organosilicas: functional incorporation by cocondensation with organosilanes, J. Phys. Chem. B. 105 (2001) 9935–9942. [5] L.Y. Guan, B. Di, M.X. Su, J. Qian, Immobilization of b-glucosidase on bifunctional periodic mesoporous organosilicas, Biotechnol. Lett. 35 (2013) 1323–1330. [6] B. Xiao, J. Zhao, X. Liu, P.Y. Wang, Q.H. Yang, Synthesis of 1,10-phenanthroline functionalized periodic mesoporous organosilicas as metal ion-responsive sensors, Microporous. Mesoporous. Mater. 199 (2014) 1–6. [7] S.S. Park, M.S. Moorthy, C.S. Ha, Periodic mesoporous organosilicas for advanced applications, NPG Asia Mater. 6 (2014) e96–e116. [8] W.J. Hunks, G.A. Ozin, Challenges and advances in the chemistry of periodic mesoporous organosilicas (PMOs), J. Mater. Chem. 15 (2005) 3716–3724. [9] P.V.D. Voort, D. Esquivel, E.D. Canck, F. Goethals, I.V. Driessche, F.J. RomeroSalguero, Periodic mesoporous organosilicas: from simple to complex bridges; a comprehensive overview of functions, morphologies and applications, Chem. Soc. Rev. 42 (2013) 3913–3955. [10] X.A. Liu, P.Y. Wang, L. Zhang, J. Yang, C. Li, Q.H. Yang, Chiral mesoporous organosilica nanospheres: effect of pore structure on the performance in asymmetric catalysis, Chem. Eur. J. 16 (2010) 12727–12735. [11] A. Corma, D. Das, H. Garcia, A. Leyva, A periodic mesoporous organosilica containing a carbapalladacycle complex as heterogeneous catalyst for Suzuki cross-coupling, J. Catal. 229 (2005) 322–331. [12] S. Polarz, A. Kuschel, Preparation of a periodically ordered mesoporous organosilica material using chiral building blocks, Adv. Mater. 18 (2006) 1206–1209.

304

J. Wang et al. / Materials Letters 193 (2017) 299–304

[13] J.H. Shin, S.S. Park, M. Selvaraj, C.S. Ha, Adsorption of amino acids on periodic mesoporous organosilicas, J. Porous Mater. 19 (2012) 29–35. [14] X. Zhao, R.S. Wei, L.G. Chen, D. Jin, X.L. Yan, Glucosamine modified nearinfrared cyanine as a sensitive colorimetric fluorescent chemosensor for aspartic and glutamic acid and its applications, New J. Chem. 38 (2014) 4791– 4798. [15] L.A. Carpino, G.Y. Han, 9-Fluorenylmethoxycarbonyl amino-protecting group, J. Org. Chem. 37 (1972) 3404–3409. [16] M.S. Moorthy, S.S. Park, F.P. Dong, S.H. Hong, M. Selvaraj, C.S. Ha, Step-up synthesis of amidoxime-functionalised periodic mesoporous organosilicas

with an amphoteric ligand in the framework for drug delivery, J. Mater. Chem. 22 (2012) 9100–9108. [17] C. Li, Identifying the highly isolated transition metal ions/oxides in molecular sieves and on oxide supports by UV resonance Raman spectroscopy, J. Catal. 216 (2003) 203–212. [18] W. Whitnall, T. Asefa, G.A. Ozin, Hybrid periodic mesoporous organosilicas, Adv. Funct. Mater. 15 (2005) 1696–1702. [19] M.P. Kapoor, Q.A. Yang, S. Inagaki, Organization of phenylene-bridged hybrid mesoporous silisesquioxane with a crystal-like pore wall from a precursor with nonlinear symmetry, Chem. Mater. 16 (2004) 1209–1213.