preparation of polyhedral oligomeric silsesquioxane based hybrid monoliths by ring-opening...

J. Sep. Sci. 2013, 36, 2819–2825 2819

Hui Lin1,2

Zhenbin Zhang1,2

Jing Dong1

Zhongshan Liu1,2

Junjie Ou1

Hanfa Zou1,∗

1CAS Key Laboratory ofSeparation Science forAnalytical Chemistry, NationalChromatographic R & A Center,Dalian Institute of ChemicalPhysics, Chinese Academy ofSciences (CAS), Dalian, China

2University of Chinese Academyof Sciences, Beijing, China

Received April 21, 2013Revised May 28, 2013Accepted May 28, 2013

Research Article

Preparation of polyhedral oligomericsilsesquioxane based hybrid monoliths byring-opening polymerization for capillary LCand CEC

A new organic–inorganic hybrid monolith was prepared by the ring-opening polymer-ization of octaglycidyldimethylsilyl polyhedral oligomeric silsesquioxane (POSS) with 1,4-butanediamine (BDA) using 1-propanol, 1,4-butanediol, and PEG 10 000 as a porogenicsystem. Benefiting from the moderate phase separation process, the resulting poly(POSS-co-BDA) hybrid monolith possessed a uniform microstructure and exhibited excellent per-formance in chromatographic applications. Neutral, acidic, and basic compounds weresuccessfully separated on the hybrid monolith in capillary LC (cLC), and high column ef-ficiencies were achieved in all of the separations. In addition, as the amino groups couldgenerate a strong EOF, the hybrid monolith was also applied in CEC for the separation ofneutral and polar compounds, and a satisfactory performance was obtained. These resultsdemonstrate that the poly(POSS-co-BDA) hybrid monolith is a good separation media inchromatographic separations of various types of compounds by both cLC and CEC.

Keywords: Chromatographic separation / Hybrid monoliths / Polyhedraloligomeric silsesquioxane / POSS / Ring-opening polymerizationDOI 10.1002/jssc.201300417

� Additional supporting information may be found in the online version of this articleat the publisher’s web-site

1 Introduction

Polyhedral oligomeric silsesquioxanes (POSSs), which ex-ist as cage-type nanostructures with the formula Rn(SiO1.5)n

(n = 6, 8, 10, . . . ), can be regarded as organic–inorganic hy-brid materials at a molecular level [1–3]. Due to their uniqueinorganic–organic hybrid nanostructures, facile chemicalmodification, high temperature, and oxidation resistanceproperties, they have become some of the most intriguingexamples of nanostructured organic–inorganic hybrid build-ing blocks, and have been widely used in synthesis and thereinforcement of polymer matrices, heterogeneous catalysts,biomaterials, chemical vapor deposition (CVD) coatings, op-tical or electrical devices, and lithography [4–8].

Their unique physical and chemical properties makePOSSs an ideal alternative to alkoxysilanes for preparation

Correspondence: Dr. Junjie Ou, CAS Key Laboratory of Separa-tion Science for Analytical Chemistry, National ChromatographicR & A Center, Dalian Institute of Chemical Physics, ChineseAcademy of Sciences (CAS), Dalian 116023, ChinaE-mail: [email protected]: +86-411-84379620

Abbreviations: ACN, acetonitrile; BDA, 1,4-butanediamine;cLC, capillary liquid chromatography; POSS-epoxy, octagly-cidyldimethylsilyl polyhedral oligomeric silsesquioxanes

of silica-based materials, such as organic–silica hybrid mono-liths [7,9–13]. As a typical and novel state-of-the-art format ofmonolithic materials, hybrid monolithic columns, combin-ing merits of both organic and inorganic monoliths, such aseasy preparation, high permeability, pH tolerance, and me-chanical strength [14–16], have attracted extensive attentionas separation media in all chromatographic methods to sep-arate small molecules and large biomolecules [11, 17–21]. Byreplacing the alkoxysilanes with a POSS regent, the prepa-ration of POSS-containing hybrid monoliths can be signif-icantly simplified. Besides, ascribed to the unique physicaland chemical properties of POSS, the mechanical and pHstabilities of POSS-based hybrid monoliths were remarkablyimproved [9]. However, the heterogeneous (irregular) mi-crostructure caused by commonly used free-radical polymer-ization still exists in POSS-based hybrid monoliths, leadingto some negative effects, such as low permeability and largereddy diffusion in flow-through systems [22–26].

Recently, we developed a facile approach for the prepara-tion of POSS-based hybrid monoliths by ring-opening poly-merization [11]. A POSS monomer octaglycidyldimethylsilylPOSS (POSS-epoxy) was selected as the precursor to reactwith diamines or polyamines to form the framework of the

∗Additional corresponding author: Professor Hanfa Zou,E-mail: [email protected]

C© 2013 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim www.jss-journal.com

2820 H. Lin et al. J. Sep. Sci. 2013, 36, 2819–2825

monoliths. Benefiting from the moderate phase separationprocess of ring-opening polymerization, the hybrid monolithspossess a well-controlled 3D skeletal structure [11, 25, 27, 28]and are promising for chromatographic separations with ul-trahigh column efficiency [11, 25]. The unique hybrid mono-liths are, of course, of interest to be applied for a myriad of po-tential applications. However, so far, the application of suchmonoliths was concentrated in capillary LC (cLC) and rarelyinvolved CEC analysis. Herein, to further confirm the versatil-ity of this novel preparation approach, an organic–inorganichybrid monolith was prepared by the ring-opening polymer-ization of POSS-epoxy and 1,4-butanediamine (BDA). The ob-tained hybrid monolith was characterized by SEM and evalu-ated systematically in cLC. In addition, the monolith was alsoinvestigated in CEC for a comprehensive understanding ofits chemistry and efficiency.

2 Materials and methods

2.1 Chemicals and materials

POSS-epoxy, PEG (Mn = 10 000), and (3-aminopropyl)triethoxysilane (APTES) were purchasedfrom Aldrich (Milwaukee, WI, USA). BDA was obtainedfrom J&K Scientific (Beijing, China). 1-Propanol, 1,4-butanediol, thiourea, alkylbenzenes, phenols, and otherstandard compounds were all purchased from Sigma (St.Louis, MO, USA). The fused-silica capillaries with dimen-sions of 50 or 75 �m id and 365 �m od were obtainedfrom Refine Chromatography (Yongnian, Hebei, China).HPLC-grade acetonitrile (ACN) was used for the preparationof mobile phases. The water used in all experiments wasdoubly distilled and purified by a Milli-Q system (Millipore,Milford, MA, USA). Other chemical reagents were all ofanalytical grade.

2.2 Preparation of the poly(POSS-co-BDA) hybrid

monolith

Before the preparation of the monolithic column, the fused-silica capillary was pretreated and rinsed by 1.0 M NaOH for 4h, water for 30 min, 1.0 M HCl for 14 h, and water for another30 min, successively, and then dried by a nitrogen streamat room temperature, according to our previous report [17].A mixture of 3-aminopropyl)triethoxysilane/methanol (50%,v/v) was used to introduce the –NH2 group onto the innersurface of the capillary for anchoring monolith matrices tothe capillary inner wall, as described in a previous report [29].

POSS-epoxy, BDA, 1-propanol, 1,4-butanediol, and PEGwere mixed under ultrasonication to form a homogeneoussolution. For cLC, the mixture was injected into the modified40 cm long capillaries with a syringe. For CEC, the mixturewas introduced in the capillaries (total length 40 cm) to alength of 28 cm. The filled capillaries were then sealed withrubber stoppers and immersed in a water bath at 50�C for

12 h. After that, the capillaries were flushed with methanolto remove the unreacted residuals. Prior to CEC, a detectionwindow was created by burning out a 2–3 mm segment inthe empty section of the capillary at the edge of continuousbed, which was located as close to the monolithic matrix aspossible. The monolithic column was then cut to an effectivelength of 24.5 cm with a total length of 33 cm before use.

2.3 Instrumentation and methods

An Agilent CE system (Hewlett-Packard, Waldbronn,Germany) equipped with a UV detector was used for allelectrochromatographic experiments. The data were acquiredand processed by Agilent ChemStation software (Hewlett-Packard). The mobile phases with different contents of ACNand varying concentrations of phosphate buffer were filteredprior to use. The standard sample mixtures were preparedwith ACN with a concentration of 1.0–10.0 mg/mL. The de-tection wavelength was set at 214 nm, and both the sampleinjection and separation were performed at room tempera-ture. Thiourea was used as the EOF marker. The EOF velocitywas calculated according the equation, �e = l · L/(V · t0), where�e is the effective mobility, l and L are the effective and totallength of the capillary, respectively, V is the applied voltage,and t0 is the migration time of the EOF marker.

The capillary liquid chromatographic evaluation of hy-brid monolithic columns was performed on an LC systemequipped with an Agilent 1100 (Hewlett-Packard) microp-ump and a UV detector (K-2501, Knauer, Germany). Thedetection wavelength was set at 214 or 254 nm. All datawere collected and processed by a chromatography worksta-tion (Beijing Cailu Scientific Instrument, Beijing, China). A7725i injector with a 20 �L sample loop was used for sampleloading. For obtaining a flow rate of nanoliters per minute(nL/min), a T-union connector was used to serve as a splitterwith one end connected to the capillary monolithic columnand the other end to a blank capillary (95 cm long, 50 �mid, and 365 �m od). The split ratio was controlled at about1/280. The outlet of the hybrid monolithic column was con-nected with a Teflon tube to an empty fused-silica capillary(75 �m id and 365 �m od), where a detection window wasmade by removing a 2 mm length of the polyimide coatingin a position 5.5 cm from the separation monolithic columnoutlet.

SEM images were obtained by using a JEOL JSM-5600scanning electron microscope (JEOL, Tokyo, Japan).

3 Results and discussion

3.1 Preparation of the poly(POSS-co-BDA) hybrid

monolith

The poly(POSS-co-BDA) hybrid monolith was prepared byring-opening polymerization (Fig. 1) [11]. As the feed recipeand reaction temperature usually play vitally important


J. Sep. Sci. 2013, 36, 2819–2825 Liquid Chromatography 2821

Figure 1. Preparation of the poly(POSS-co-BDA) hybrid monolith.

roles in the preparation of a monolith, the effect of theseparameters was systematically investigated. Considering theresults of the preparation of POSS-based hybrid monolithsby ring-opening polymerization in our recent report [11], theamounts of POSS-epoxy and BDA were kept at 50 mg and10 �L, respectively, and a solution consisting of propanol,1,4-butanediol, and PEG 10 000 was also chosen as theporogenic solvent mixture. As shown in Table 1, when thecontent of propanol was low, the monolith was loose anddetached from the inner wall of the capillary (column A).As the amount of propanol increased, the monolith becamegradually dense (column B). However, further increasing theamount of propanol would result in a slack monolith, whichcould not be attached to the inner wall of the capillary (columnC). Differently, when the amount of 1,4-butanediol increased,the monolith became increasingly dense and transparent un-der an optical microscope (columns B, D, and E). A similarphenomenon was also observed when the reaction temper-ature increased, which resulted in a decrease of the perme-ability (columns B, H, I, and J). In contrast, a transformationof monoliths from dense to slack occurred as the amount ofPEG 10 000 increased (columns B, F, and G). After careful

evaluation of these results, the conditions used for columnB were adopted for the preparation of poly(POSS-co-BDA)hybrid monoliths for further experiments.

3.2 Characterization of the poly(POSS-co-BDA)

hybrid monolith

Figure 2 presents the cross-section morphology of the hybridmonolith. It was observed that homogenous matrix had uni-form macro through-pores and a well-defined 3D skeletonwell anchored to the inner wall of capillary without any dis-connection. The highly ordered microstructure perhaps ben-efited from the ring-opening polymerization, which wouldresult in a relatively moderate phase separation process, andcould overcome the inherent disadvantages of free-radicalpolymerization to some extent [25, 28].

The obtained hybrid monolith showed good mechani-cal stability as the measured back-pressure was linearly in-creased even over 19.0 MPa when the flow rate increasedfrom 40 to 400 nL/min with a linearity coefficient of R =0.9995 (data not shown). Meanwhile, good permeability

Table 1. Effect of synthesis parameters on the formation of poly(POSS-co-BDA) hybrid monoliths

Column Propanol (�L) 1,4-Butanediol (�L) PEG (mg) T (�C) Morphologya) Permeabilityb) (×10−14 m2)

A 150 40 25 50 Detached from the inner wall Not detectedB 200 40 25 50 Homogenous 4.31

Dark brownC 250 40 25 50 Detached from the inner wall Not detectedD 200 0 25 50 Detached from the inner wall Not detectedE 200 80 25 50 Transparent Too hard to pump throughF 200 40 20 50 Transparent Too hard to pump throughG 200 40 30 50 Nonrigid 40.82H 200 40 25 60 Homogenous 3.45

Dark brownI 200 40 25 70 Homogenous 2.88

BrownJ 200 40 25 80 Homogenous 1.15

Light brown

a) Observed with an optical microscope.b) Permeability, B0 = F�L/(�r2�P), F: linear velocity of mobile phase; �: dynamic viscosity of mobile phase; L: effective column length; r:inner radius of column; �P: pressure drop across the column [30], measured at room temperature.



Figure 2. SEM images of poly(POSS-co-BDA) hybrid monolith (Column B). Magnification: (A) 1000× and (B) 3000×.

at 4.31 × 10−14 m2 of the monolith was also measuredand calculated according to Darcy’s law [30] of permeability,B0 = F�L/(�r2�P), where F is the linear velocity of the mo-bile phase, � is the dynamic viscosity of the mobile phase, Land r are the effective length and inner radius of the column,respectively, and �P is the pressure drop across the column.

The hybrid monolith also showed high reproducibility,which was evaluated through the RSD for the retention fac-tors (k’) of benzene as the model analyte (thiourea as the viodtime marker) in cLC. The run-to-run and day-to-day repeata-bility were 0.17% (n = 3) and 0.24% (n = 3), respectively. Be-sides, the column-to-column and batch-to-batch reproducibil-ity were also investigated, which gave RSD values of 0.36%(n = 3) and 0.51% (n = 3), respectively, indicating satisfactoryreproducibility for the preparation of hybrid monoliths. In ad-dition, no significant decrease of column efficiency or obviouscolumn deterioration was observed even after continuous usefor hundreds of injections under neutral and acidic mobilephases, which demonstrated the high stability and good pHresistance of the poly(POSS-co-BDA) hybrid monoliths.

3.3 Chromatographic evaluation of the

poly(POSS-co-BDA) hybrid monolith

3.3.1 cLC evaluation of the poly(POSS-co-BDA)

hybrid monolith

Due to the hydrophobicity of POSS-epoxy, the preparedpoly(POSS-co-BDA) hybrid monolith was applied for chro-matographic separation in RP mode. Five alkylbenzenes werewell separated with good peak shapes according to their hy-drophobicity in order from low to high (Fig. 3A). The effectof ACN content in the mobile phase on the retention of thesealkylbenzenes was investigated. It could be observed that allretention factors varied inversely with the ACN content in therange of 30–70% (Supporting Information Fig. S1), revealinga typical RP retention mechanism. The relationship of theplate heights of these alkylbenzenes versus the flow rate wasinvestigated. It could be found that a column efficiency of

about 100 000 N/m at the optimal flow velocity was achievedon the hybrid monolith (Fig. 3B). The high column efficiencymay be attributed to the well-defined 3D skeleton and uniformmicrostructure formed by the progressive phase separationprocess in ring-opening polymerization [28], as the hybridmonolith possessed a low surface area of 9.6 m2/g and anaverage pore diameter of 484.1 nm, demonstrating a lack ofmesopores and nanopores (Supporting Information Fig. S2).It has been proved that such a highly ordered microstructurewould accelerate the mass transfer during the separation pro-cess and be beneficial to improve the column efficiency [31].In addition, the column showed a relatively high efficiencywithin a wide range of flow rates from 0.25 to 1.4 mm/s,which means that the hybrid monolith may be promising forrapid separation in cLC applications.

For further validation of the separation ability, polycyclicaromatic hydrocarbons, phenols, benzoic acids, anilines aswell as purines and pyrimidines were separated on the hy-brid monolith. As can be seen, naphthalene, acenaphthene,4,4’-dimethylbiphenyl, p-terphenyl, and pyrene were all base-line separated with good peak shapes (Fig. 4A), and high col-umn efficiencies for these compounds ranged from 68 000 to79 000 N/m. Figure 4B exhibits the chromatogram of separa-tion of three phenols and two benzoic acids derivatives. Fivecompounds were well separated with ultra-high column effi-ciencies, especially for hydroxybenzoic acid and benzoic acid,whose column efficiencies were calculated at 215 000 and226 000 N/m, respectively. Besides, from Fig. 4B, it could befound that hydroquinone and resorcinol were baseline sepa-rated, which revealed that the poly(POSS-co-BDA) hybrid mayhave a special selectivity for phenol isomers. However, onlyhydroxybenzoic acid and benzoic acid were separated, andother benzoic acid derivatives with lower pKa values couldnot be eluted under a mobile phase with 40 mM NH4Ac (pH2.98). It may be ascribed to the electronic interaction betweenthe amino groups on the surface of matrix and the dissociatedcarboxyl group on the analytes. Perhaps a mobile phase witha higher salt concentration or lower pH would improve theseparation performance of benzoic acid derivatives.

Figure 4C presents the separation of six basic anilines,and satisfactory performance was performed with a column



Figure 3. (A) Separation of alkyl-benzenes and (B) dependence ofthe plate heights on the linear ve-locity of the mobile phase on thepoly(POSS-co-BDA) hybrid mono-lith in cLC. Analytes: (1) benzene;(2) toluene, (3) ethylbenzene, (4)propylbenzene, (5) butylbenzene.Experimental conditions: columndimensions, 35.0 cm × 75 �m id;mobile phase, ACN/water (50:50,v/v); flow rate, 100 �L/min (be-fore split) for (A); injection volume,2.5 �L in split mode; detectionwavelength, 214 nm.

Figure 4. Separation of (A) poly-cyclic aromatic hydrocarbons, (B) phe-nols and benzoic acids, (C) anilines,(D) purines and pyrimidines on thepoly(POSS-co-BDA) hybrid monolith.Analytes: (A)—(1) naphthalene, (2) ace-naphthene, (3) 4,4’-dimethylbiphenyl,(4) p-terphenyl, (5) pyrene; (B)—(1)phenol, (2) hydroquinone, (3) resor-cin, (4) 4-hydroxybenzoic acid, (5) ben-zoic acid; (C)—(1) 2,4-diaminotoluene,(2) benzidine, (3) 2-nitroaniline, (4) 2,4-dinitroaniline, (5) 4-aminobiphenyl, (6)2,6-dichloro-4-nitroaniline; (D)—(1) cy-tosine, (2) thymine, (3) uracil, (4) ade-nine. Experimental conditions: columndimensions, 36.9 cm × 75 �m id; mo-bile phase, (A, C) ACN/water (50:50,v/v), (B, D) ACN/40 mM NH4Ac buffer(pH 2.98) (60:40, v/v); flow rate, 100�L/min (before split); injection volume,2.5 �L in split mode; detection wave-length, (A, B, C) 214 nm, (D) 254 nm.

efficiency varying from 35 800 to 52 700 N/m. Figure 4Dshows the chromatographic separation of four purines andpyrimidines on the POSS–BDA hybrid monolith, and highcolumn efficiencies ranging from 73 200 to 82 200 N/m wereachieved. However, the two pyrimidines (thymine and uracil)cannot be separated, maybe due to the low selectivity of thepoly(POSS-co-BDA) hybrid monolith for pyrimidine.

3.3.2 CEC evaluation of the poly(POSS-co-BDA)

hybrid monolith

As a combination of LC and CE, CEC shows unparalleled ad-vantages in low consumption of sample, solvent, and station-ary phases [32–34], and has been widely used in the separationof both small molecules and (bio)macromolecules [19,35,36].The POSS-based monoliths prepared by ring-opening poly-merization have exhibited excellent performance with respectto cLC applications [11], however, such monoliths have notbeen applied in CEC. Here, we evaluated the application of

the poly(POSS-co-BDA) monolith in CEC for the first time.As a number of secondary amine groups are produced by thering-opening reaction, the generation of an anodic EOF wasanticipated. As we expected, the analytes cannot be detectedwhen a positive voltage was applied. In contrast, the sepa-ration could be performed normally when a negative voltagewas applied. Since the pH value of the mobile phase would af-fect the degree of protonation of the amino groups, resultingin a pH-dependent EOF, the influences of the pH value of themobile phase on EOF was investigated. It was observed thatthe EOF decreased from 1.62 × 10−8 to 6.54 × 10−9 m2V−1s−1

with an increase of pH from 3.5 to 8.5.Baseline separation of five alkylbenzenes was obtained

within 11 min by applying a voltage of −28 kV (Fig. 5A), andtheir elution order illustrated a typical RP retention mecha-nism. The effect of ACN content in the mobile phase on theretention of five alkylbenzenes was studied. A decrease of theretention factor could be observed for these compounds whenthe ACN content increased from 30 to 70% (data not shown),which further confirmed the RP retention mechanism of



Figure 5. (A) Electrochromatogram for the separation of alkylbenzenes and (B) dependence of the plate heights of thiourea and benzeneon the linear velocity of mobile phase on the poly(POSS-co-BDA) hybrid monolith. Analytes: (1) thiourea, (2) benzene, (3) toluene, (4)ethylbenzene, (5) propylbenzene, (6) butylbenzene. Experimental conditions: column dimensions, 24.5 cm (effective length) × 75 �m id;mobile phase, ACN/5 mM phosphate buffer (pH 3.5; 40:60, v/v); separation voltage, (A) –28 kV, (B) from –3 to –28 kV; electrokinetic injection,–3 kV for 1 s; detection wavelength, 214 nm.

alkylbenzenes on the poly(POSS-co-BDA) hybrid monolith.By applying a voltage ranging from −3 to −28 kV, the col-umn efficiency of the poly(POSS-co-BDA) hybrid monolithin CEC was also investigated. It should be pointed that theJoule heating is negligible as the current increased linearlyas the voltage increased (−3 to −28 kV; data not shown).Figure 5B shows the Van Deemter curves of the unretainedthiourea and retained benzene, exhibiting the minimum plateheight of 2.5 and 5.9 �m at the optimal flow velocity by corre-sponding to 401 500 and 169 900 N/m, respectively. Besides,no significant decrease of the column efficiencies occurredwithin a wide range of flow rates from 0.23 to 1.35 mm/s, andshowed typical monolithic behavior (efficiency being main-tained at higher velocities), promising for rapid separation inCEC applications. In addition, it was found that the columnefficiencies in CEC were obviously much higher than thosein cLC, which may be due to the effective suppression of eddydiffusion in CEC.

The phenols were also separated on the hybrid mono-lith in CEC. Six phenols were all well separated with goodpeak shapes (Fig. 6), the highest column efficiency was calcu-lated at 183 300 N/m for hydroquinone. Particularly, three po-sitional isomers, hydroquinone, pyrocatechol, and resorcin,were almost baseline separated, which demonstrated the highseparation ability of the poly(POSS-co-BDA) hybrid monolith.Furthermore, pyrocatechol and phenol were well separatedin CEC, in comparison, the baseline separation of pyrocate-chol and phenol could not be achieved in cLC even after op-timizing the operating conditions (Supporting InformationFig. S3). These results demonstrated the higher separationability of the poly(POSS-co-BDA) hybrid monolith in CEC.

4 Concluding remarks

A new poly(POSS-co-BDA) organic–inorganic hybrid mono-lithic column was prepared by the ring-opening polymeriza-tion of POSS-epoxy and BDA. The obtained hybrid monolith

Figure 6. Electrochromatogram for the separation of phenolson the poly(POSS-co-BDA) hybrid monolith. Analytes: (1) hydro-quinone, (2) resorcinol, (3) pyrocatechol, (4) phenol, (5) 4-cresol,(6) 4-tert-butylphenol. Experimental conditions: column dimen-sion, 24.5 cm (effective length) × 75 �m id; mobile phase, ACN/5mM phosphate buffer (pH 3.5; 40:60, v/v); separation voltage,–10 kV; electrokinetic injection, –3 kV for 2 s; detection wave-length, 214 nm.

possessed a well-defined 3D skeleton and high mechanicalstability. Benefiting from the uniform microstructure, thehybrid monolith exhibited excellent performance in cLC, inwhich alkylbenzenes, polycyclic aromatic hydrocarbons, phe-nols, anilines, and purines and pyrimidines were all success-fully separated with high column efficiencies. In addition, thehybrid monolith was applied for the separation of neutral andpolar compounds in CEC, and a satisfactory performance wasobtained. It was demonstrated that the poly(POSS-co-BDA)hybrid monolith is a good chromatographic media for cLCas well as CEC. In order to further assess the breadth of thishybrid monolith in CEC application, more characterizationand separation should be explored in the future.

Financial support is gratefully acknowledged from theChina State Key Basic Research Program Grant (2013



CB-911203,2012CB910601, 2012CB-910101), the NationalNatural Sciences Foundation of China (21235006), the CreativeResearch Group Project of NSFC (21021004), and the Knowl-edge Innovation program of DICP to H.Z., the National NaturalSciences Foundation of China (No. 21175133) and the Hun-dred Talents Program of the Dalian Institute of Chemical Physicsof Chinese Academy of Sciences to J.O. as well as the NationalNatural Sciences Foundation of China (No. 21105101) to J.D.

The authors have declared no conflict of interest.

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