RSC Advances

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aAustralian Centre for Research on Separ

Chemistry, University of Tasmania, Privat

E-mail: mirek.macka@utas.edu.aubRegional Centre of Advanced Technologies

Chemistry, Faculty of Science, Palacky Un

Olomouc, Czech Republic, CZ-77146cAustralian Centre for Research on Separ

Pharmacy, University of Tasmania, Private

† Electronic supplementary informa10.1039/c2ra23417c

Cite this: RSC Adv., 2013, 3, 24927

Received 19th December 2012Accepted 13th September 2013

DOI: 10.1039/c2ra23417c

www.rsc.org/advances

This journal is ª The Royal Society of

Porous layer open tubular monolith capillary column:switching-off the reaction kinetics as the governingfactor in their preparation by using an immiscible liquid-controlled polymerization†

Radim Knob,ab Michael C. Breadmore,a Rosanne M. Guijt,c Jan Petrb

and Mirek Macka*a

A new approach for preparation of monolithic porous layer open-

tubular capillary columns that eliminates the need for incomplete

polymerization is presented. By expelling monomers from the

capillary with a completely immiscible liquid a thin layer of poly-

merization mixture that wets the wall is left and polymerized by

UV light.

Heterogeneous phases have found widespread application in anumber of areas of chemistry, from extraction and separationmaterials through to supports for immobilised catalysts andorganic synthesis. The solid-phase is typically a packed bed, butopen tubular (OT) columns in which the wall of a narrowdiameter capillary is coated with the material are an attractivealternative. A major advantage is that an OT column can beushed with the necessary uid more rapidly than a packedcolumn, and practically it behaves like an empty tube. Thishas major implications in their application potential andpractical usage. These columns in particular have found wide-spread use in gas chromatography, as they demonstratesignicant improvements in efficiency and analysis speedover packed beds.1 They are not so attractive for liquid-phaseapplications because improvements over packed columns canonly be obtained when using narrow capillary diameters due toslow mass transfer of liquid components to the phase on thesurface of the column.2,3 The high backpressure of these narrowbore capillary columns and low capacity of the thin lms

ation Science (ACROSS) and School of

e Bag 75, Hobart, TAS 7001, Australia.

and Materials, Department of Analytical

iversity in Olomouc, 17. Listopadu 12,

ation Science (ACROSS) and School of

Bag 26, Hobart, TAS 7001, Australia

tion (ESI) available. See DOI:

Chemistry 2013

together with difficulties in preparation of the capillary haslimited their use.

Porous layer open tubular (PLOT) columns were introducedto overcome some of these limitations especially in heteroge-neous phase capacity because the surface morphology of porousmaterials offers a higher surface area while minimizing theissue of slow mass transfer.4,5 Although the solid to liquid ratiofor PLOT columns is lower compared to packed columns, theircapacities are still sufficient for a wide range of catalytic andchromatographic applications.6–9

More recently, monolithic, or continuous porous stationaryphases, have attracted signicant interest due to their relativelysimple preparation in capillaries, lack of frits and the ability tocontrol both the physical and chemical properties of thematerial.10,11 These same properties make them very attractivefor PLOTs. While there have been a number of reports onthe preparation of monolithic PLOT columns, in all casesthe monolith is only formed on the wall by interruption of thereaction kinetics thus preventing formation of a completepolymer over the entire cross section of the capillary. This hasbeen reported with both thermal and UV-initiators. Eeltinket al.12 were the rst to use UV polymerization to create amonolithic PLOT and found that it was necessary to rotate thecapillary below the light source to produce an even surfacecoating. Evanescent wave-initiated photopolymerization usingan axially-oriented UV-LED (ref. 6) was also utilized for thefabrication of a short PLOT column, which was used as anenzymatic microreactor. Moving a UV source along the capillarywas used by Nesterenko et al.13 followed by Collins et al.14 whopassed the capillary through the centre of a UV chamber con-taining several circular arrays of UV LEDs. As the thickness ofthe monolith in all of these approaches is controlled by inter-rupting the polymerization process, repeatability can be aproblem if the reagents and experimental conditions are notexactly the same each time. Also, irregular thickness is likely tooccur due to preferential growth at the thicker regions.14

An alternative way to restrict monolith formation to thecapillary wall is to completely switch-off the reaction kinetics as

RSC Adv., 2013, 3, 24927–24930 | 24927

Fig. 1 Optical microscope images of the g-MAPS functionalized fused silicacapillary. (A) Boundary between polymerization mixture (on left) and FC-770 (onright) showing different wettability of the capillary surface. (B) Capillary afterpumping first the polymerization mixture and then FC-770 showing remaininglayer of the polymerization mixture on the capillary wall.

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the governing factor rather than interrupting it. This can beachieved by removing the polymerization mixture from thecentre of the capillary prior to polymerization. Essential to thisapproach is that the wetting of the capillary surface of thepolymerization mixture should be much greater than for theother phase. Here, the capillary wall was treated with g-meth-acryloxypropyltrimethoxysilane for optimal wettability with thepolymerization mixture. The polymerization mixture15 wasreplaced in the capillary with FC-770, a fully uorinated solvent(for properties see ESI†) that is immiscible with the polymeri-zation mixture. Importantly, based on existing knowledge ofuorohydrocarbon solvents and their immiscibility with othernon-uorinated solvents, they can be likely used as an immis-cible liquid for other pre-polymer solutions. Our work thereforepresents a general new approach to create polymer layer such asmonolith for separation method on inside surfaces of micro-uidic channels. When using FC-770 to replace the polymeri-zation mixture, no mixing between the two uids was observedand the desired concentric two-phase system was formed. Ahomogeneous PLOT layer is then formed by UV-initiated poly-merization of this thin lm wetting the capillary wall. The effectof the difference in wettability of the capillary surface betweenthe two uids is illustrated with the optical microscope image inFig. 1A, showing the difference in wettability of the surface atthe boundary between the polymerization mixture and the FC-770. As illustrated by the photograph in Fig. 1B, the superiorwettability of the surface with the polymerization mixtureresulted in a thin lm of polymerization mixture remaining onthe capillary surface when pumping FC-770 through the capil-lary. The boundary between the two immiscible liquids can beobserved close to the capillary wall due to the different refractiveindexes of the uids.

Exposure of this thin layer of polymerization mixture withUV light forms a layer of porous monolith covalently attached tothe surface of the capillary. In Fig. 2B, a SEM image of a PLOTmonolith formed inside a fused-silica capillary aer 5 replicatesof the thin layer formation and polymerization process isshown.

When compared with an image of an uncoated capillary(Fig. 2A) there is an evenly distributed monolith layer attachedto the inner surface of the capillary, with a thickness of 0.25 mm.SEM images show the monolith is present as a single layer, nointermittent layers were performed. SEM images of two separatecapillaries are included in ESI,† demonstrating the excellentrepeatability of this method. Control of the PLOT monolithlayer thickness to obtain a desirable thickness is believed to bepossible through adjustment of the experimental parametersand is subject of ongoing research.

We believe that this new approach provides an attractivealternative for the fabrication of thin layer PLOT monolithcolumns because the reported issues of inhomogeneity andirreproducibility of thin lms could be addressed by elimi-nating the dependence on kinetics. Further, the fact that exactlythe same polymerization mixture is used for formation of thePLOTmeans that its porous properties will be the same as thoseof the full monolith, something that not guaranteed bypremature termination of the polymerization process. This

24928 | RSC Adv., 2013, 3, 24927–24930

means that previously developed and well knownmonoliths canbe easily implemented in the PLOT format without signicantand time-consuming reoptimisation. The fact that exchange ofthe uids can be easily achieved using syringe pumps and thepolymerization initiated using a simple low-cost UV LEDarray,16–18 makes this a simple, elegant and low-cost approachfor formation of these columns.

As one demonstration of the applicability of the new PLOTmonolith, the epoxide groups were ring-opened19 and modiedwith quaternary ammonia latex nanoparticles to provide acationic surface charge for anion exchange capillary electro-chromatography. Quaternary ammonium latex particles havepreviously been used for the electrochromatographic separationof inorganic anions20–22 and this provide a suitable means tobenchmark the performance of these columns.

Fig. 3 shows a comparison of the separation of bromide,nitrate and iodide performed in an uncoated capillary and alatex-coated monolithic PLOT column. The separation in thePLOT column shows a number of differences to that in the non-

This journal is ª The Royal Society of Chemistry 2013

Fig. 2 Scanning electron microscopy images of uncoated capillary (A), PLOTmonolith capillary (B), and a detailed view on the PLOT monolith capillary wall (C).

Fig. 3 Separation of 0.1 mM bromide, nitrate, and iodide by capillary electro-phoresis using an uncoated capillary and PLOT modified anion exchange PLOTcolumn. Conditions: 10 mM HClO4 Tris pH 8, effective length 8.5 cm, total length33.5 cm, detection at 200 nm, voltage �20 kV, injection for 3 s at 50 mbar.

Table 1 Effective mobilities and capacity factors of bromide, iodide, and nitratein an uncoated capillary, open-tubular and PLOT modified columns

Bromide Iodide Nitrate

meff[� 10�9

m2 V�1 s�1]

Uncoated �81.2 �79.6 �75.2Open-tubular22 �71.6 �64.7 �66.2PLOT �70.5 �61.7 �65.4

k0 Open-tubular22 0.118 0.187 0.120PLOT 0.132 0.225 0.130

Efficiency[platesper m]

Uncoated 194 212 221 038 175 560Open-tubular22 266 557 55 161 120 058PLOT 150 812 122 528 232 953

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PLOT capillary. First, the peaks are earlier, suggesting a muchhigher value of the electroosmotic ow, which can be directlyattributed to an increased number of positive charges on thesurface of the capillary, most likely due to the increased surfaceare of the monolithic PLOT. Second, that there is a differentselectivity, with the position of iodide changing relative to theother two ions in the monolithc PLOT. Iodide has a muchstronger affinity for the cationic groups on the latex particles,and its change in position further indicates an increasedamount of positive charge on the monolithic PLOT whencompared to the non-PLOT capillary.

A comparison of an uncoated capillary, the prepared PLOTand open-tubular22 columns was conducted to demonstrate the

This journal is ª The Royal Society of Chemistry 2013

increase of the surface area for attachment of the latex nano-particles. The effective mobilities (meff) of anions on threedifferent capillaries and corresponding capacity factors (k0) inthe background electrolyte composed of 0.01 mol L�1 perchlo-rate Tris pH 8 are shown in Table 1. For equations, see ESI.†Under these conditions, the increase in k0 for the PLOT columncompared with the open-tubular system was 11.5% for bromide,20.1% for iodide, and 8.9% for nitrate.

The repeatability of migration times on the PLOT columnwere less than 0.49% intraday and less than 1.41% interday (fordetails, see ESI†). The separation efficiency under the usedconditions was comparable to previously reported perfor-mances for OT columns directly coated with latex particles.22

While efficiency values of packed ion-exchange columns in CECgenerally supersede values for OTC columns of comparablecapacity, and this is also the case here, the principal advantageand practical strong point of all OTC columns is that they are‘open’ that is open to ushing when ever needed at much higherow rates, thusmaking the resulting usage easier and giving themethods a potential to be more rapid.

Conclusions

A new approach for the preparation of monolithic porous layeropen-tubular columns in fused silica capillaries has beendeveloped, for the rst time eliminating the need for

RSC Adv., 2013, 3, 24927–24930 | 24929

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incomplete polymerization. This is achieved by expelling thepolymerization mixture from the centre of the capillary with acompletely immiscible non-wetting liquid, leaving a thin layerof polymerization mixture wetting the capillary wall. This thinlayer can then be fully polymerized, overcoming repeatabilityproblems encountered with incomplete polymerization as wellas the associated change in physical and chemical properties. Incontrast with alternative methods, this method is fundamen-tally more robust because it is not based on incomplete poly-merization. As this approach does not change thepolymerization mixture, it will allow the direct transfer ofalready published monolith formulations already publishedmany of which have been well characterised. This will enable awide range of monolithic PLOT columns to be made by simpleand inexpensive means, and to be used in an even wider rangeof applications.

Acknowledgements

R.K. and J.P. acknowledge support from Operational ProgramResearch and Development for Innovations European RegionalDevelopment Fund (project CZ.1.05/2.1.00/03.0058) andProgram Education for Competitiveness European Social Fund(project CZ.1.07/2.3.00/20.0018). M.C.B. acknowledges receiptof an ARC QEII Fellow from the Australian Research Council(DP0984745). M.M. acknowledges a UTAS start-up grant andARC Future Fellowship (FT120100559). The authors would liketo thank Dr Karsten Gomann (Central Science Laboratory,UTAS) for assistance with the SEM images.

Notes and references

1 M. J. E. Golay, et al., in Gas Chromatography, ed. V. J. Coates,Academic Press, New York, 1958.

2 J. H. Knox and M. T. Gilbert, J. Chromatogr., 1979, 186, 405.3 G. Gulochon, Anal. Chem., 1981, 53, 1318.4 Y. Guo and L. A. Colon, Anal. Chem., 1995, 67, 2511.

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5 R. Swart, S. Brouwer, J. C. Kraak and H. Poppe,J. Chromatogr., A, 1996, 732, 201.

6 S. Abele, P. Smejkal, O. Yavorska, F. Foret and M. Macka,Analyst, 2010, 135, 477.

7 S. A. Zaidi, S. M. Lee and W. J. Cheong, J. Chromatogr., A,2011, 1218, 1291.

8 Z.-H. Wei, X. Wu, B. Zhang, R. Li, Y.-P. Huang and Z.-S. Liu,J. Chromatogr., A, 2011, 1218, 6498.

9 M. Rosberg, S. R. Wilson, T. Greibrokk and E. Lundanes,J. Chromatogr., A, 2010, 1217, 2782.

10 S. Eeltink and F. Svec, Electrophoresis, 2007, 28, 137.11 R. D. Arrua, M. Talebi, T. J. Causon and E. F. Hilder, Anal.

Chim. Acta, 2012, 738, 1.12 S. Eeltink, F. Svec and J. M. J. Frechet, Electrophoresis, 2006,

27, 4249.13 E. Nesterenko, O. Yavorska, M. Macka, A. Yavorsky and

B. Paull, Anal. Methods, 2011, 3, 537.14 D. A. Collins, E. P. Nesterenko, D. Brabazon and B. Paull,

Anal. Chem., 2012, 84, 3465.15 J. A. Deverell, T. Rodemann, J. A. Smith, A. J. Canty and

R. M. Guijt, Sens. Actuators, B, 2011, 155, 388.16 S. Abele, F.-Q. Nie, F. Foret, B. Paull and M. Macka, Analyst,

2008, 133, 864.17 Z. Walsh, S. Abele, B. Lawless, D. Heger, P. Klan,

M. C. Breadmore, B. Paull and M. Macka, Chem. Commun.,2008, 6504.

18 Z. Walsh, P. A. Levkin, B. Paull, F. Svec and M. Macka, J. Sep.Sci., 2010, 33, 61.

19 N. P. Dinh, Q. M. Cam, A. M. Nguyen, A. Shchukarev andK. Irgum, J. Sep. Sci., 2009, 32, 2556.

20 M. C. Breadmore, M. Macka, N. Avdalovic and P. R. Haddad,Anal. Chem., 2001, 73, 820.

21 J. P. Hutchinson, P. Zakaria, A. R. Bowie, M. Macka,N. Avdalovic and P. R. Haddad, Anal. Chem., 2005, 77,407.

22 M. C. Breadmore, M. C. Boyce, M. Macka, N. Avdalovic andP. R. Haddad, Analyst, 2000, 125, 799.

This journal is ª The Royal Society of Chemistry 2013