一锅法室温聚合生成氧化石墨烯纳米层整体住进行毛细电色谱法.docx

Graphene oxide石墨烯氧化物 Fabrication of Graphene Oxide Nanosheets Incorporated Monolithic Column via One-Step Room Temperature Polymerization for Capillary Electrochromatography一锅法室温聚合生成氧化石墨烯纳米层整体住进行毛细电色谱法Man-Man Wang, and Xiu-Ping Yan*,State Key Laboratory of Medicinal Chemical Biology, and Research Center for Analytical Sciences, College of Chemistry,Nankai University, Tianjin 300071, ChinaCollege of Public Health, Hebei United University, Tangshan 063000, Hebei, ChinaABSTRACT: Graphene oxide (GO) has received great interest for its unique properties and potential diverse applications.Here, we show the fabrication of GO nanosheets incorporated monolithic column via one-step room temperature polymerization for capillary electrochromatography (CEC). GO is attractive as the stationary phase for CEC because it provides only ionized oxygen-containing functional groups to modify electroendoosmotic flow (EOF) but also aromatic macromolecule to give hydrophobicity and electrostatic stacking property. Incorporation of GO into monolithic column greatly increased the interactions between the tested neutral analytes (alkyl benzenes and polycyclic aromatics) and the stationary phase and significantly improved their CEC separation. Baseline separation of the tested neutral analytes on the GO incorporated monolithic column was achieved on the basis of typical reversedphase separation mechanism. The precision (relative standard deviation (RSD), n = 3) of EOF was 0.3%, while the precision of retention time, peak area, and peak height for the tested neutral analytes were in the range of 0.43.0%, 0.84.0%, and 0.84.9%,respectively. In addition, a set of anilines were well separated on the GO incorporated monolith. The GO incorporated monolithicGraphene oxide (GO), as a two-dimensional nanomaterial,has attracted tremendous attention for its novel properties and potential applications in eletronics, energy research,catalysis, and biomedical research.110 GO is a chemically modified graphene sheet with a giant aromatic macromolecule containing reactive oxygen functional groups on their basal planes and edges such as epoxide, hydroxyl, and carboxylic acid. Owing to its unique structure, GO exhibits outstanding physicochemical properties such as exceptional thermal and mechanical properties, high electrical conductivity, superiordispersibility, and facile modification,1,1114 which renders andpromotes its popularity extensively. In particular, the ultrahighspecific surface area and electrostatic stacking property ofGO given by its electron-rich double-sided polyaromatic scaffoldmakes it a promising candidate as an extraordinarily wonderfuladsorbent.1517 GO has been reported as a high efficient preconcentration and matrix in direct surface-enhanced laserdesorption/ionization analysis or mass spectrometry.15,16 Mostrecently, a platform of graphene and GO sheets supported on silica as versatile and high-performance adsorbents for solidphase extraction has been reported for various analytes rangingfrom small molecules of pollutants to biomolecules such as proteins and peptides.17 In addition, graphene and GO have been explored for solid-phase microextraction of pyrethroid pesticides in natural water samples18 and polycyclic aromatic hydrocarbons in water and soil samples.1Considering the superior qualities of GO, novel stationaryphase materials based on GO for chromatography are expected. However, there is difficulty in the packing of nanothickness GO into the cartridge/column format.17 Furthermore, it is also difficult to use GO directly to fabricate uniform separation matrix and to retain it in the column, especially under high pressure in chromatographic systems. To avoid the abovementioned problems and still take advantage of the specific features of GO, fabrication of GO incorporated monolithic column is a good choice.Monolithic materials with a porous “single particle” structure have been extensively evolved as the legitimate member of the large family of separation media because of their available preparation, fast mass transfer, and enhanced efficiency.2022 Monolithic columns not only overcome the difficulties associated with standard packed column technology but also eliminate the need for end frits to retain the stationary phase in capillary electrochromatography (CEC).23,24 CEC is a hybrid separation technique that combines the capillary column format and electroendoosmotic flow (EOF) typical of high-performance capillary electrophoresis with the use of a solid stationary phase and a separation mechanism based on specific interactions of solutes with a stationary phase characteristic ofHPLC.2527 The stationary phase in CEC plays a dual role:providing sites for the desired interactions as in HPLC andgenerating EOF.26Here, we report the fabrication of GO incorporated capillarymonolithic column for CEC. GO not only possesses the stabilityand large surface area necessary for separation media but alsoprovides ionized oxygen-containing functional groups to modifyEOF in CEC and aromatic macromolecule to give hydrophobicityand electrostatic stacking property. In this work,GO incorporated capillary monolithic column was fabricated viaa room temperature strong inorganic acid initiated methacrylatepolymerization strategy28 and evaluated as a new stationaryphase for CEC application. EXPERIMENTAL SECTIONMaterials and Chemicals. All reagents used were of analytical grade unless otherwise stated. Ultrapure water(Wahaha, Hangzhou, China) was used throughout all experiments.-Methacryloxypropyltrimethoxysilane (-MAPS), ethylene glycol dimethacrylate (EDMA), and methacrylic acid (MAA)were purchased from Acros (Geel, Belgium), Alfa Aesar (Ward Hill, MA, USA), and Tianjin Guangfu Fine Chemical ResearchInstitute (Tianjin, China), respectively. Cyclohexanol was fromMedical Material Supplier of the Academy of Military MedicalSciences (Beijing, China), and HNO3 was obtained from TianjinNo. 3 Chemical Reagent Plant (Tianjin, China). Thiourea,benzene, toluene, ethylbenzene, and isopropylbenzene weresupplied by Tianjin Guangfu Fine Chemical Research Institute(Tianjin, China). Naphthalene, acenaphthene, fluorene, andanthrancene were from Tianchang Chemical Co. Ltd. (Anshan,China). Acetanilide, aniline, 4-methylaniline, 2-nitroaniline, and1-naphthylamine were purchased from Tianjin No. 1 ChemicalReagent Plant (Tianjin, China), Tianjin Huayue ChemicalReagent Plant (Tianjin, China), Beijing Changcheng ChemicalReagent Plant (Beijing, China), Shanghai No. 3 Chemical ReagentPlant (Shanghai, China), and Beijing Chemical Plant (Beijing,China), respectively. HPLC-grade of CH3OH and CH3CN werepurchased from Burdick & Jackson (Muskegon, MI, USA).Fused silica capillary (375 m o.d 75 m i.d.) was supplied byYongnian Optic Fiber Plant (Handan, China).Preparation of GO Sheets Incorporated Monolithic Materials. GO was synthesized from natural graphite powder based on a modified Hummers method.29,30 The resulting purified GO powders was collected by centrifugation and air-dried.For the fabrication of GO incorporated monolith, GO sheetswere dissolved in cyclohexanol to create a brown and homogeneousdispersion at a concentration of 0.2 mg mL1 underultrasonication for 0.5 h. Cyclohexanol was used as the porogenin the preparation of the monolithic stationary phase. MAA(10 L), EDMA (100 L), and HNO3 (0.14 mmol) were added into the resulting mixture (600 L). After further sonication for 15 min, the monophasic solution was introduced into the preconditioned fused silica capillary to an appropriate length by syringe injection. Both ends of the filled capillary were plugged, and the capillary was placed at room temperature for 24 h. The obtained column was then rinsed with CH3OH to remove the unreacted monomers and porogen. For comparison,monolithic column without adding GO was also prepared in thesame way. The materials were also synthesized in stainless steelcolumns (4.1 mm i.d.) and were dried under vacuum for 12 hafter Soxhlet extraction with CH3OH for solid-UV spectrophotometricand TEM characterization. CEC Separation. Electrochromatographic experiments were carried out on a P/ACE MDQ capillary electrophoresissystem (Beckman, Fullerton, CA, USA) equipped with a DADdetector. Data acquisition and processing was controlled byBeckman ChemStation software. The mobile phases preparedfrom the mixture of CH3CN and 12.5 mM acetic acid (HAc)sodium acetate (NaAc) buffer at pH 5.6 (70/30, v/v) for CECseparation of neutral analytes and from the mixture of CH3CNand 5 mM phosphate buffer solution (PBS) at pH 8.0 (55/45,v/v) for the separation of polar compounds were filtered beforeuse. Prior to CEC experiments, a detection window was createdby burning off a 2 mm segment of the protecting polymer layerat the end of the monolithic bed. The monolithic capillarycolumn (total length, 31.2 cm; effective length, 20.0 cm) wasthen installed in the CEC instrument and equilibrated at 15 kVuntil a stable current and baseline was achieved. To avoid thegeneration of bubbles during separation, a pressure of 20 psiwas applied to both inlet and outlet vials simultaneously and15 kV voltage was performed if not otherwise stated. DADdetection was set at 214 nm for neutral compounds and235 nm for anilines. The temperature was kept at 25 C. Thesamples were degassed under ultrasonication and injectedelectrokinetically by applying a voltage of 1 kV for 3 s.Characterization. Tapping-mode atomic force microscopy(AFM) was conducted on a Multimode SPM with a NanoscopeIIIa Controller from Digital Instruments. Fourier transforminfrared(FT-IR) spectra (4000400 cm1) were obtainedusing a Magna-560 spectrometer (Nicolet, Madison, WI, USA) inKBr plate. Solid UV spectra (200800 nm) were recorded on aV-550 spectrometer (JASCO, Japan). The morphologies andmicrostructures of the GO and GO incorporated monolithwere characterized on a JEOL-100CXII transmission electronmicroscopy (TEM) with an accelerating voltage of 100 kV andon a SS-550 scanning electron microscope (Shimadzu, Japan)at 15.0 kV. RESULTS AND DISCUSSIONCharacterization of GO. GO is a layered material with awide range of oxygen functional groups including hydroxyl,epoxy, and carboxylic acid groups located on their basal planesand the sheet edges (Figure 1A).1 The FT-IR spectra of theprepared GO material reveal the characteristic bands of thecarboncarbon double bonds at 1620 cm1 and hydroxyl inGO appearing at 3430 cm1 (Figure 1B). The band around1725 cm1 corresponds to CO stretching vibrations fromcarbonyl and carboxylic groups. The bands around 1230 cm1and 1070 cm1 are attributed to COH stretching vibrationsand CO stretching vibrations, respectively. The FT-IR spectraprovide the evidence of the presence of different types ofoxygen functionalities on the GO material. AFM image of GOillustrates their flakelike shapes with the thickness of GO beingabout 1.15 nm (Figure 1C).Fabrication of GO Incorporated Monolithic CapillaryColumn Based on HNO3 Initiated Polymerization atRoom Temperature. The presence of functional groups inGO sheets improves their solubility,11 thus providing conveniencefor the fabrication of monolithic materials. At the sametime, the oxygen-containing functional groups provide activesites for the desired interactions for separation as in HPLC andmodify EOF in CEC. A good dispersion of GO in the polymerizationmixture is required to ensure a uniform monolithic matrixin the capillary column. The GO sheets without any treatmentwere well dispersed in cyclohexanol via gentle ultrasonication, forming brown transparent solution (Figure 2A-b). Atconcentrations lower than 0.2 mg mL1, the dispersions arevery stable, and no precipitation was observed under a microscopeeven after storage for 5 weeks (Figure 2A-c). Figure 2Bshows the TEM image of GO suspension in cyclohexanoldeposited for 5 weeks (Figure 2A-c), revealing the goodsolubility and stability of GO in cyclohexanol.Addition of the monomer mixture and HNO3 into thecyclohexanol suspension of GO resulted in a room temperaturepolymerization; thus, GO incorporated monolith was prepared.Figure 3A shows the brown GO incorporated monolith afterCH3OH washing in comparison with the white rod of thepolymer of MAAEDMA without incorporation of GO (4.1 mmin diameter). The TEM image of GO incorporated monolithafter extraction with CH3OH obviously shows the presenceof GO sheets in the monolith (Figure 3B). Furthermore,comparison of the solid state UV spectra of GOMAAEDMApolymers (before and after extraction with CH3OH) and MAAEDMA polymers shows that a significant absorbance of GOappears at 270 nm in the GO incorporated polymers while themain absorbance in the polymers is still present in the polymersafter extraction (Figure 3C). The above results confirmthe successful incorporation of the GO into the polymers.Comparison of the FT-IR spectra of the GO, the polymersproduced from MAA and EDMA, and the GO incorporated polyMAAEDMA monolith shows that immobilization of GO intopoly MAAEDMA monolith did not result in new absorptionpeaks and significant peak shifts, indicating that GO wasimmobilized via physical adsorption rather than covalentbonding (Figure 3D). Figure 4 shows the SEM images of theGO incorporated poly MAAEDMA monolithic column. Thepolymer matrix with the incorporation of GO sheets exhibited aporous and uniform structure.CEC Separation on GO Incorporated CapillaryMonolithic Column. CEC is a powerful separation techniquethat essentially combines the advantages of HPLC and capillaryelectrophoresis. As in HPLC, solutes partition between themobile and stationary phases. However, instead of a pressuregradient to pump the mobile phase, application of an electricfield to the capillary induces EOF, which is responsible for bulktransport.27,31 Thus, the elution order of the analytes dependsnot only on their partition between the mobile phase andstationary phase but also on their mobility. For neutral species,their separation is only controlled by their partition between thestationary phase and mobile phase. It is well-known that EOFgenerated by the ionized functionalities located on the surfaceof the stationary phase is the driving force in CEC. Generally,neutral compounds are used as test models to evaluate theperformance of GO incorporated column in CEC. The migrationvelocity (umig) of neutral component is given as follows, whereueo is the migration velocity of a neutral unretained marker and kis the chromatographic retention factor of the test neutralcompound.27umig = ueo/(1 + k)The chromatographic behaviors of neutral compounds inCEC depend on their k values, which can be regarded asthe same results performed in HPLC. In other words, thechromatographic behaviors of neutral compounds on theconditioned column reflect the interactions between the GOincorporated polymeric monolith matrix and the solutesdirectly.To study the influence of GO on column retention, thioureawas used as the EOF marker, and a series of alkyl benzenesand polycyclic aromatic hydrocarbons were separated on a GOincorporated monolithic column and a poly(MAAEDMA)monolithic column with 70% CH3CN/30% HAcNaAc buffer(12.5 mM, pH 5.6) as the mobile phase. Alkyl benzenes ofbenzene, toluene, ethylbenzene, isopropylbenzene, and polycyclicaromatic hydrocarbons of acenaphthene and fluorene werealmost coeluted on the poly MAAEDMA column (Figure 5A).In contrast, GO incorporated monolithic column gave a baselineseparation of all the compounds with much longer retentiontimes but a reduction of the retention time for the EOF markerthiourea (Figure 5B). Chromatographic parameters includingretention time (t), retention factor, and EOF mobility on GOincorporated monolithic column and poly MAAEDMAmonolithic column are compared in Table 1 to examine therole of incorporated GO in CEC separation.There was an enhanced EOF mobility (EOF) on the GOincorporated monolith which was attributed to the increasingionized groups of GO in monolithic matrix. The intrinsiccharacteristics of GO such as carbon-based ring structures andoxygen functional groups affected CEC separation in view ofthe changes in EOF and interactions with solutes. Both EOFand the interaction between solutes and column matrix decide theretention behaviors of the test neutral components. EOF is thedriving force in CEC separation, and higher EOF should giveshorter retention time. However, the retention times andretention factors of all the test compounds on the monolithiccolumn incorporated with GO were significantly higher thanthose on the monolithic column without GO. The above resultsReproducibility and Loading Capacity of GO IncorporatedMonolithic Column. Figure 8 shows the reproducibilityfor three replicate CEC separations of the tested neutralcompounds on the GO incorporated monolith in terms of electrochromatograms.The precision (relative standard deviationRSD), n = 3) of EOF was 0.3%. For the selected modelcompounds, the precision of retention time, peak area, and peakheight were in the range of 0.43.0%, 0.84.0%, and 0.84.9%,respectively. The loading capacity of the GO-incorporatedmonolithic column was 0.45 mg mL1 for alkyl benzenes, 0.30mg mL1 for naphthalene and acenaphthene, and 0.26 mg mL1for fluorene and anthracene. CONCLUSIONSIn summary, we have fabricated a GO incorporated monolithiccolumn via one-step room temperature polymerization forCEC. The prepared GO incorporated monolithic column givesexc