Polyhedral Oligomeric Silsesquioxane as a Cross-linker for Preparation of Inorganic−Organic Hybrid Monolithic Columns

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<ul><li><p>Polyhedral Oligomeric Silsesquioxane as aCross-linker for Preparation of Inorganic-OrganicHybrid Monolithic Columns</p><p>Minghuo Wu,, Renan Wu,*, Ruibing Li,, Hongqiang Qin,, Jing Dong, Zhenbin Zhang,, andHanfa Zou*,</p><p>CAS Key Laboratory of Separation Sciences for Analytical Chemistry, National Chromatographic R &amp; A Center, DalianInstitute of Chemical Physics, Chinese Academy of Sciences (CAS), Dalian 116023, China, and Graduate School ofChinese Academy of Sciences, Beijing 100049, China</p><p>An inorganic-organic hybrid monolithic capillary col-umn was synthesized via thermal free radical copolym-erization within the confines of a capillary using apolyhedral oligomeric silsesquioxane (POSS) reagentas the inorganic-organic hybrid cross-linker and asynthesized long carbon chain quaternary ammoniummethacrylate of N-(2-(methacryloyloxy)ethyl)-dimethy-loctadecylammonium bromide (MDOAB) as the organicmonomer. The preparation process was as simple aspure organic polymer-based monolithic columns in-stead of using POSS as the nanosized inorganic-organichybrid blocks (cross-linker) of the monolithic matrix.The pore properties and permeability could be tunedby the composition of the polymerization mixture. Thecharacterization and evaluation results indicated thatthe synthesized MDOA-POSS hybrid monolith pos-sessed the merits of organic polymer-based monolithsand silica-based monoliths with good mechanical andpH (pH 1-11) stabilities, which may be attributed tothe incorporation of the rigid nanosized silica core ofPOSS. Column efficiencies of 223 000 and 50 000N/m were observed in capillary electro-driven chroma-tography (CEC) and -HPLC, respectively. Peptides andstandard proteins were baseline separated by thishybrid column in CEC and -HPLC, respectively, aswell. The separation of bovine serum albumin (BSA)tryptic digest was also attempted to show its potentialapplication in proteome analysis.</p><p>Monolithic columns can be described as the integratedcontinuous porous separation media for separation sciences. Inthe past decade, monolithic columns, the novel state-of-the-artstationary phases, have been given comprehensive attention andapplied widely in microscale chromatographic separations, suchas capillary liquid chromatography (CLC), capillary electro-drivenchromatography (CEC), and microfluidic devices. Microscalemonolithic columns are usually prepared within the confines of</p><p>capillaries, and with no need of supporting frits. Monolithiccolumns possess advantages that include easy preparation,versatility in surface modification, great permeability, and goodpeak capacity. These unique merits have made monolithiccolumns the attractive alternative to the packed and open-tubularcolumns in the analytical separation sciences.1-4 In recent years,with the rapid development of nanoscale chromatographic separa-tion systems coupled to mass spectrometry, the use of capillarymonolithic columns have emerged as a promising choice for theseparation of complex biological samples to provide a lowerbackpressure drop, better column stability, and better resolutionand sensitivity.5-7</p><p>On the basis of the chemical nature of monoliths, monolithiccolumns can be mainly classified into organic polymer-based andsilica-based monolithic columns.8,9 However, the mechanical andsolvent instability of polymer-based monoliths, and the pHsensitivity of silica-based monoliths are the inherent drawbacksfor polymer-based and silica-based monoliths, respectively.10-12</p><p>Recently, the emerging organic-inorganic hybrid monolithiccolumns, incorporating organic moieties into inorganic (usuallysilica) monolithic matrices via the co-condensation of organofunc-tional trialkoxysilanes [(RO)3Si-R: where R represents theorganofunctional group] and conventional tetra-alkoxysilanes(i.e., TMOS or TEOS) by the sol-gel method, seem to be thepromising choices for polymer- or silica-based monolithiccolumns.10-14 Organic-inorganic hybrid monolithic columns aresupposed to combine the merits of the organic polymer and</p><p>* To whom correspondence should be addressed. Tel.: +86-411-84379610.Fax: +86-411-84379620. E-mail: hanfazou@dicp.ac.cn (H.Z.). Tel.: +86-411-84379576. Fax: +86-411-84379620. E-mail: wurenan@dicp.ac.cn (R.W.).</p><p> Chinese Academy of Sciences. Graduate School of Chinese Academy of Sciences.</p><p>(1) Svec, F. J. Sep. Sci. 2004, 27, 14191430.(2) Zou, H. F.; Huang, X. D.; Ye, M. L.; Luo, Q. Z. J. Chromatogr. A 2002,</p><p>954, 532.(3) Tanaka, N.; Kobayashi, H.; Ishizuka, N.; Minakuchi, H.; Nakanishi, K.;</p><p>Hosoya, K.; Ikegami, T. J. Chromatogr. A 2002, 965, 3549.(4) Vlakh, E. G.; Tennikova, T. B. J. Sep. Sci. 2007, 30, 28012813.(5) Wu, R. A.; Hu, L. G.; Wang, F. J.; Ye, M. L.; Zou, H. J. Chromatogr. A 2008,</p><p>1184, 369392.(6) Kasicka, V. Electrophoresis 2008, 29, 179206.(7) Sandra, K.; Moshir, M.; DHondt, F.; Verleysen, K.; Kas, K.; Sandra, P.</p><p>J. Chromatogr. B 2008, 866, 4863.(8) Gusev, I.; Huang, X.; Horvath, C. J. Chromatogr. A 1999, 855, 273290.(9) Guiochon, G. J. Chromatogr. A 2007, 1168, 101168.</p><p>(10) Hayes, J. D.; Malik, A. Anal. Chem. 2000, 72, 40904099.(11) Colon, H.; Zhang, X.; Murphy, J. K.; Rivera, J. G.; Colon, L. A. Chem.</p><p>Commun. 2005, 28262828.(12) Yan, L. J.; Zhang, Q. H.; Zhang, H.; Zhang, L. Y.; Li, T.; Feng, Y. Q.; Zhang,</p><p>L. H.; Zhang, W. B.; Zhang, Y. K. J. Chromatogr. A 2004, 1046, 255261.(13) Xu, L.; Lee, H. K. J. Chromatogr. A 2008, 1195, 7884.</p><p>Anal. Chem. 2010, 82, 54475454</p><p>10.1021/ac1003147 2010 American Chemical Society 5447Analytical Chemistry, Vol. 82, No. 13, July 1, 2010Published on Web 05/28/2010</p></li><li><p>inorganic silica-based monoliths, such as easy fabrication, widepH range tolerance, good mechanical stability, and high perme-ability. Since Hayes and Malik10 prepared an organic-inorganicporous capillary monolithic column using N-octadecyldimethyl[3-(trimethoxysilyl)propyl] ammonium chloride as the organofunc-tional alkoxysilane for CEC, a number of organic-inorganic hybridcolumns have been reported by using different organofunctionalalkoxysilanes including phenyltriethoxysilane, 3-aminopropyltri-ethoxysilane, C8-triethoxysilane, and methyltrimethoxysilane.11-19</p><p>Bridged silane monomers such as 1,2-bis(trimethoxysilyl)ethaneand 1,2-bis(triethoxysilyl)ethane have also been used to preparehybrid monoliths.20 Dulay et al.21 also prepared an organic-silicahybrid monolithic column, named the photopolymerized sol-gel(PSG) monolith, using methacryloxypropyl trimethoxysilane (MPT-MS) as the monomer via polycondensation and photoinitiatedpolymerization. Different from the above-mentioned organic-in-organic hybrid monolithic columns, we have previously developeda one-pot approach to prepare the hydrophilic and hydrophobicorganic-inorganic hybrid monolithic capillary columns via the insitu co-condensation and copolymerization between the organicpolymerization precursors and the inorganic alkoxysilanes, whichcan be developed as a versatile method to synthesize theorganic-silica hybrid monoliths by using a variety of organicmonomers.22 Nevertheless, the use of the alkoxysilanes in theabove-mentioned approaches would probably result in the residualsilanol groups on monolith surface, which would possibly causethe peak tailing, broadening, or nonspecific adsorption in prac-tice.23</p><p>Polyhedral oligomeric silsesquioxane (POSS) is a type ofcagelike silsesquioxane, which embodies a truly inorganic-organichybrid architecture containing an inner inorganic framework madeup of silicon and oxygen.24-26 It refers to the structures with theempirical formula Rn(SiO1.5)n, where R represents a range oforganofunctional groups, while n is an even integer g4. POSSchemical reagents are thought to be the smallest silica particleswith sizes of 1-3 nm, which can be easily incorporated intocommon polymers via copolymerization, grafting, or blending.Using POSS reagents as the monomers in copolymerizationprocesses is convenient with no dramatic change in reaction</p><p>conditions, as long as the POSS monomers are soluble in themonomer mixture.25 The POSS reagents also offer a uniqueopportunity to prepare truly molecularly dispersed nanocom-posites, which can be used as rigid hard building blocks inpolymer for various hybrid materials.27 The interest in POSSmaterials is based on the facts that the rigid silicon and oxygenframework could greatly enhance the mechanical and thermalstability of the resulted POSS-containing nanohybrid poly-mers.24,28</p><p>In this work, we applied a POSS reagent of POSS-methacrylsubstituted (POSS-MA) as the cross-linker to prepare an inorganic-organic hybrid monolithic column, for the first time to our bestknowledge, via the copolymerization with a functional monomerof N-(2-(methacryloyloxy)ethyl)-dimethyloctadecylammonium bro-mide (MDOAB) in a toluene-dodecanol porogen system. Theresulting inorganic-organic hybrid monolithic capillary column(MDOA-POSS) was systematically investigated, which exhibitedgood mechanical stability and good pH stability. This approachwould represent a versatile method to prepare the inorganic-organic hybrid monolithic column by using a variety of organicfunctional monomers for copolymerization.</p><p>EXPERIMENTAL DETAILSMaterials. POSS-methacryl substituted (cage mixture, N )</p><p>8,10,12, POSS-MA) was purchased from Acros (NJ, USA). 2,2-(Dimethylamino)ethyl methacrylate (DEMA) was purchased fromNanjing Chemlin Chemical Industry Co., Ltd. (Nanjin, China).-Methacryloxypropyltrimethoxysilane (-MAPS) and trifluoro-acetic acid (TFA) were purchased from Sigma (St Louis, MO,USA). Ribonuclease B from bovine pancreas, bovine serumalbumin (BSA), cytochrome C from bovine heart, enolase fromyeast, insulin, and ovalbumin were all purchased from Sigma (StLouis, MO, USA). Lysozyme from chicken egg white was obtainedfrom Sino-American Biotechnology Co. (Beijing, China). Azobi-sisobutyronitrile (AIBN) was purchased from Shanghai ChemicalPlant (Shanghai, China) and recrystallized in ethanol before use.A fused-silica capillary with 75 m i.d. and 375 m o.d. waspurchased from Reafine Chromatography Ltd. (Hebei, China).Dithiothreitol (DTT), iodoacetamide, and the protease inhibitorscocktail were all purchased from Sino-American BiotechnologyCo. (Beijing, China). Daisogel ODS-AQ (5 m, 120 pore) waspurchased from Daiso (Osaka, Japan). HPLC-grade acetonitrile(ACN) from Merck (Darmstadt, Germany) was used for thepreparation of mobile phases. Water used in all experiments wasdoubly distilled and purified by a Milli-Q system (Millipore Inc.,MA, USA). Other chemical reagents were of analytical grade.</p><p>Synthesis of N-(2-(Methacryloyloxy)ethyl)dimethy-loctadecylammonium Bromide (MDOAB). The synthesis ofMDOAB was similar to that of allyldimethyldodecylammoniumbromide (ADDAB) as in a previous work.22 DEMA (4 mL, 23.8mmol) was added dropwise to a solution of 1-bromooctadecane(9 mL, 26.5 mmol) in ethanol (20 mL), which was stirred for 10min at room temperature. Then, the obtained solution was heatedto 55 C and stirred for 36 h. After removal of the solvent by arotary evaporator under vacuum, the resulting yellow liquid was</p><p>(14) Roux, R.; Puy, G.; Demesmay, C.; Rocca, J. L. J. Sep. Sci. 2007, 30, 30353042.</p><p>(15) Yan, L. J.; Zhang, Q. H.; Zhang, W. B.; Feng, Y. Q.; Zhang, L. H.; Li, T.;Zhang, Y. K. Electrophoresis 2005, 26, 29352941.</p><p>(16) Yan, L. J.; Zhang, Q. H.; Feng, Y. Q.; Zhang, W. B.; Li, T.; Zhang, L. H.;Zhang, Y. K. J. Chromatogr. A 2006, 1121, 9298.</p><p>(17) Tian, Y.; Zhang, L. F.; Zeng, Z. R.; Li, H. B. Electrophoresis 2008, 29, 960970.</p><p>(18) Constantin, S.; Freitag, R. J. Sol-Gel Sci. Technol. 2003, 28, 7180.(19) Kanamori, K.; Yonezawa, H.; Nakanishi, K.; Hirao, K.; Jinnai, H. J. Sep. Sci.</p><p>2004, 27, 874886.(20) Hutanu, D. Synthesis and characterization of novel stationary phases for</p><p>small scale liquid chromatographic separations of proteins and nanopar-ticles. Ph.D. Thesis, Oregon State University, 2008.</p><p>(21) Dulay, M. T.; Quirino, J. P.; Bennett, B. D.; Kato, M.; Zare, R. N. Anal.Chem. 2001, 73, 39213926.</p><p>(22) Wu, M. H.; Wu, R. A.; Wang, F. J.; Ren, L. B.; Dong, J.; Liu, Z.; Zou, H. F.Anal. Chem. 2009, 81, 35293536.</p><p>(23) Allen, D.; Rassi, Z. E. Analyst 2003, 128, 12491256.(24) Li, G. Z.; Wang, L. C.; Toghiani, H.; Daulton, T. L.; Koyama, K.; Pittman,</p><p>C. U. Macromolecules 2001, 34, 86868693.(25) Li, G.; Wang, L.; Ni, H., C. U. P., Jr J. Inorg. Organomet. Polym Mater. 2001,</p><p>11, 123154.(26) Pielichowski, K.; Njuguna, J.; Janowski, B.; Pielichowski, J. Adv. Polym. Sci.</p><p>2006, 201, 225296.</p><p>(27) Livage, J. Curr. Opin. Solid State Mater. Sci. 1997, 2, 132138.(28) Tanaka, K.; Inafuku, K.; Adach, S.; Chujo, Y. Macromolecules 2009, 42,</p><p>34893492.</p><p>5448 Analytical Chemistry, Vol. 82, No. 13, July 1, 2010</p></li><li><p>recrystallized in 40 mL ethyl acetate/hexane (20/80, v/v). Afterincubated at 4 C for 1 h, followed by filtration and vacuum drying,9.2 g white solid product (MDOAB) was obtained with 95% yield.The product was characterized by matrix assisted laser desorptionionization time-of-flight (MALDI-TOF), m/z at 410.33 [M+].</p><p>Preparation of the MDOA-POSS Hybrid MonolithicColumn. Before the preparation of the MDOA-POSS hybridmonolithic column, the fused-silica capillary was respectivelyrinsed by 1.0 M NaOH for 6 h, water for 30 min, 1.0 M HCl for6 h, and water for 30 min, and was dried by nitrogen stream atroom temperature. A 50% -MAPS methanol (v/v) solution wasused to introduce the CdC bonds onto the inner surface ofcapillary to anchor monolithic matrix as previously reported.29</p><p>First, the solution was injected into the capillary, and then thecapillary was submerged in a water bath at 50 C overnight withthe both ends sealed with silicone rubbers. After that, the capillarywas rinsed by methanol to flush out the residuals and dried bynitrogen stream again for further use.</p><p>The polymerization mixture consisting of monomer MDOAB,cross-linker POSS-MA, initiator AIBN, and binary porogen oftoluene-dodecanol were ultrasonicated in a bath sonicator (90W) for 15 min to degas. The obtained homogeneous solution wasmanually introduced into the -MAPS pretreated capillary to anappropriate length with a syringe. By sealing the both ends ofthe capillary with silicone rubbers, the capillary was incubated at55 C for 12 h. Finally, the prepared monolith capillary columnwas washed by methanol to remove unreacted monomers andporogens. The structures of POSS-MA and MDOAB and theschematic synthesis of the MDOA-POSS hybrid monolith areillustrated in Figure 1.</p><p>Preparation of BSA Tryptic Digest. The tryptic digestprocedure of BSA was according to that previously reported byus with minor modifications.22 BSA (2 mg) was dissolved in 1mL of denaturing buffer containing 8 M urea. After the additionof 10 L DTT solution, the mixture was incubated at 60 C for1 h to reduce the disulfide bonds of the protein. After that, 3.7mg IAA was added, and the mixture was incubated at roomtemperature in the dark for 40 min. Finally, the mixture wasdiluted 8-fold with 100 mM ammonium bicarbonate buffer (pH8.2) and digested at 37 C for 20 h with trypsin at the enzyme-to-substrate ratio of of 1:...</p></li></ul>

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