The spatially controlled synthesis of poly(glycidyl methacrylate-co-ethylene dimethacrylate) monolithic stationary phases in polyimide coated fused silica capillaries by visible light induced radical polymerisation using a three-component initiator and a 660 nm light emitting diode (LED) as a light source is presented here

Abele

Al-Bokari

Bandari

Beiler

Brett Paull

Brian Lawless

Chatterjee

Chen

Dasgupta

Dominik Heger

Dulay

Eeltink

Gould

Kabatc

McDermott

Michael C. Breadmore

Mirek Macka

Moschou

Murphy

Okanda

Petr Klán

Schuster

Silvija Abele

Svec

Throckmorton

Viklund

Zarah Walsh

English

Walsh, Zarah

Abele, Silvija

Lawless, Brian

Heger, Dominik

Klán, Petr

Breadmore, Michael C.

Paull, Brett

Macka, Mirek

DCU Online Research Access Service

CREATED USING THE RSC COMMUNICATION TEMPLATE (VER. 3.0) - SEE WWW.RSC.ORG/ELECTRONICFILES FOR DETAILS ARTICLE TYPE www.rsc.org/xxxxxx  |  XXXXXXXX This journal is © The Royal Society of Chemistry [year] Journal Name, [year], [vol], 00–00  |  1 Photoinitiated Polymerisation of Monolithic Stationary Phases in Polyimide Coated Capillaries Using Visible Region LEDs Zarah Walsha, Silvija Abelea, Brian Lawlessb, Dominik Hegerc, Petr Klánc, Michael C. Breadmored, Brett Paulla and Mirek Mackaa* Received (in XXX, XXX) 1st January 2007, Accepted 1st January 2007 First published on the web 1st January 2007 DOI: 10.1039/b000000x 5 The spatially controlled synthesis of poly(glycidyl methacrylate-co-ethylene dimethacrylate) monolithic stationary phases in polyimide coated fused silica capillary by visible light induced radical polymerisation using a three-component initiator and a 660 nm light emitting diode (LED) as a light source is presented 10 here.  Since the synthesis of the first organic monolith was reported by Svec and Frechet in 19921 monolithic stationary phases have been recognised as one of most innovative developments since the conception of chromatography by 15 Tswett in the early 1900s2. Initiation by heat1 and ultraviolet (UV) radiation3 are the most common methods of inducing polymerisation, while other methods such as initiation by microwaves4, γ-radiation5 and electron beam6 have been reported more recently. Photoinitiation is of particular interest 20 as it is an excellent method of achieveing sharp plugs of monolith in a specific location within a mold in a short amount of time.   Until recently, photoinitiation could only be carried out using ultraviolet light and was therefore restricted to UV-25 transparent molds such as polytetrafluoroethylene (PTFE) coated fused silica capillaries3 and microfluidic chips made from poly(methyl methacrylate)7, glass8 or cyclic olefin copolymer9. In this work the synthesis of organic polymer monoliths by radical polymerisation using red LEDs with 30 emission max. at 660 nm to initiate polymerisation within polyimide (PI) coated fused silica capillary is investigated. To the authors best knowledge this communication is the first example in the literature of the photoinitiated synthesis of organic polymer monoliths in polyimide (PI) coated fused 35 silica capillaries with visible light in the red region of the spectrum.  The authors are aware of a publication in which Dulay et al10 describe the synthesis of a silica sol-gel monolith in polyimide coated capillary by cationic polymerisation using a 40 cool fluorescent lamp equipped with a bandpass filter to produce 470 nm light. Two recent papers also report the use of UV-LEDs to induce polymerisation both in UV-transparent capillary 7 and in solution11.    Unlike PTFE, polyimide is strongly absorbing below 45 approx. 500 nm, Fig.1, therefore UV radiation cannot be used to induce efficient polymerisation within PI coated fused silica capillaries – light will be absorbed by the coating and will not pass into the capillary to initiate polymerisation. As the transmission of light through PI coated capillaries is 50 already above 50% at 550 nm, photoinitiation of polymerisation can be carried out more efficiently using visible light above 550 nm. The ability to carry out polymerisation in polyimide coated capillaries is particularly useful as they are more durable than their UV-transparent 55 (PTFE-coated) counterparts and are used in the vast majority of capillary separations applications.   Fig. 1 Absorption spectra of PI and PTFE coated fused silica capillaries (spectra measured on Agilent 3D CE instrument, bare fused silica 60 reference, all samples filled with deionised water for measurement), absorption spectrum of the dye sensitiser/borate salt complex and the emission spectrum of the 660 nm LED used for polymerisation.  The photoinitiated polymerisations described here have been carried out using LEDs as the light source. The benefits 65 of using LEDs over classical light sources in both analytical chemistry12 and photointiated polymerisation7, 11 have been reported elsewhere. LEDs are cheap, small, robust and have long lifetimes (up to 100,000 h)11 but their most important feature, with respect to their use in photoinitiated 70 polymerisation, is their relatively small heat generation11. Using a ‘cold’ light source means that there is a remote chance that thermally initiated polymerisation is occuring simultaneously with the photoinitiation so thermal effects, such as enhanced diffusion causing inhomogeneity towards 75 the ends of the monolith are unlikely.  In this study, a novel three-component photoinitiator mixture, consisting of a cyanine dye sensitiser with a borate counter ion radical initiator and an alkoxypyridinium salt, was used to start polymerisation of poly(glycidyl methacrylate-co-80 ethylene dimethacrylate). The initiator system is based on a commercially available n-butyltriphenyl borate salt of a cyanine dye, 3-butyl-2-[5-(1,3-dihydro-3,3-dimethyl-1-propyl-2H-indolylidene)-penta-1,3-dienyl]-1,1-dimethyl-1H- 2  |  Journal Name, [year], [vol], 00–00 This journal is © The Royal Society of Chemistry [year] benzo[e]indolium (HNB 660, Spectra Group Ltd, USA). Efficient single electron transfer from the borate anion to the excited cyanine sensitiser is known to generate the cyanine radical and the butyl radical, along with phenyl borane13-15. The butyl radical species are then able to initiate chain 5 polymerisation. In contrast, the second co-initiator, N-methoxy-4-phenylpyridinium tetrafluoroborate (Sigma-Aldrich, Ireland) can abstract an electron from the excited state of the cyanine dye to produce a methoxy radical and pyridine via reductive cleavage of the N-O bond16†. Such a 10 three-component system, proposed by Kabatc et al17, was shown to initiate polymerisation of 2-ethyl-2-(hydroxymethyl)-1,3-propanediol triacrylate more efficiently than the cyanine/borate salt itself because it is capable of releasing two radical species as polymerisation initiators upon 15 absorption of a single photon, while the overall rate of free radical formation is not controlled by the reverse electron transfer17. Referring to the work of Kabatc17, a 10-fold molar excess of the alkoxypyridinium salt was used in this study to ensure that the photoinitiated polymerisation of the monolithic 20 stationary phases was as efficient as possible. The absorption properties of the cyanine (λmax = 660 nm) allow the use of red light which easily penetrates the polyimide coating to the internal cavity where the polymerisation mixture is held (Fig. 1).  25  A standard polyimide coated fused silica capillary (Tab. 1) was used as the mold for the synthesis of the organic polymer monolith by photoinitiated polymerisation. Before  polymerisation within the mold the internal walls were pretreated with a silanising agent, 3-(trimethoxysilyl)propyl 30 methacrylate (TMSPM) to ensure that the growing monolith is covalently anchored to the walls18,19. Glycidyl methacrylate (GMA) and ethylene dimethacrylate (EDMA) were chosen as the monomer and cross-linker, respectively, with a mixture of acetonitrile, isopropanol and decanol as porogenic solvents†. 35 After the pretreated capillaries were filled with the polymerisation solution, the capillaries were masked using black electrical tape to ensure that only certain sections of the capillary were exposed to visible light to show that spatial control of the monolith formation within the polyimide coated 40 capillary was possible.  The masked capillary was placed on a flat surface with the LED positioned perpendicular to it at a distance of 15 mm. A 660 nm LED (Soanar, Australia) with a forward current of 30 mA was used and the polymerisation was allowed to proceed 45 for 30 min. The resulting monolithic polymer was more completely formed on the side of the mold adjacent to the LED, while the opposite side had a thinner layer of polymer. The lens-like properties of the capillary walls and the high absorbancy of the dye sensitiser (εmax660 = 230,000 L mol–1 50 cm–1) are believed to be the reason for this occurrence. When the LED is perpendicular to the capillary the majority of light passes through the polyimide coating and the fused silica wall in a straight line with little scattering of the light rays. On reaching the cavity, the transmission of light through the 55 polymerisation solution is hindered by the high absorbancy of the dye. Using the Beer-Lambert law to make an approximation of the light transmitted through the cavity, it is estimated that while approx. 48 % of light is absorbed at the adjacent wall (within 1 µm of the wall) only 1.5 % can be 60 absorbed at the opposite wall (100 µm distance)†. While diffusion of radicals and some light scattering do occur, the more common outcome is an inhomogeneous wall coating.  Keeping all other conditions constant, the LED was shifted incrementally towards the parallel. Moving the LED away 65 from the perpendicular significantly improved the quantity and homogeneity of the polymer formed within the cavity. The optimum degree of polymerisation was achieved when the LED was positioned at approximately 60o to the normal (Fig. 2). At this angle there is increased light scattering within the 70 walls of the capillary, which allows more radicals to be generated giving a more homogeneous monolith. A small motor was also used to rotate the capillary at a rate of 17 rpm so light can penetrate more evenly through the capillary, this has previously been discussed by Eeltink et al20. 75  Fig. 2 Position of LED relative to the capillary during polymerisation   A summary of the optimum conditions used to synthesise poly(GMA-co-EDMA) monoliths by visible light induced polymerisation are outlined in Table 1. 80 Table 1 Optimum conditions for synthesis of organic monolithic materials by visible light induced polymerisation Monolith synthesis conditions LED 660 nm Angle approx. 60o to the normal Distance 15 mm Current  30 mA Reaction time 30 min Mold PI coated FS 375 µm o.d., 100 µm i.d. Composition of polymerisation solution See ESI†  To characterise the monoliths synthesised under these conditions, optical microscopy, scanning electron microscopy and the measurement of flow resistance with respect to flow 85 rate were used.  Scanning Electron Microscopy was used to show that the monoliths had completely filled the capillary and were well attached to the walls (Fig.3a).   Optical microscopy using blue LEDs to back light the 90 capillary at a magnification of 10x (Fig.3b) showed that the plug of monolith has straight, sharp edges.  Fig. 3 (a) Scanning electron micrograph of a poly(GMA-co-EDMA) monolith synthesised under the optimum conditions and (b) light 95 microscope image (10x) showing the sharp edges of the monolith within the capillary  This journal is © The Royal Society of Chemistry [year] Journal Name, [year], [vol], 00–00  |  3  Finally, the flow resistance of the monoliths was measured using a S-100 HPLC pump (Knauer, Germany) with HPLC grade methanol (Sigma-Aldrich, Ireland) as the eluent. These measurements were carried out on a batch of monoliths synthesised under the optimum conditions and were found to 5 be reproducible, Fig. 4 (n = 3).  Fig. 4 Plot of flow rate (µL min–1) vs. flow resistance (MPa cm–1) for a poly(GMA-co-EDMA) monolith synthesised by visible light initiated polymerisation, error bars show an error of ±1σ 10  The flow resistance at 5 µL min–1 is 0.16 MPa cm–1, which shows that these monoliths are suitable for low pressure separations applications such as those involving microfluidic chips and Lab-on-a-Chip systems.  The monoliths were encased in a poly(methyl methacrylate) 15 chip similar to those used by Nie et al21, 22 to be used as electroosmotic pumps (EOPs). The monolith was flushed with 1M NaOH for 3 h to attack the epoxy ring of the GMA and produce some negative charge on the surface which should create electroosmotic flow. Doing this produced a maximum 20 flow rate of 274 nL min-1 using 2 mM phosphate buffer at pH 11 when the applied voltage was 2 kV, for comparison an unmodified commercial silica C18 monolith as EOP have been shown to give a flow rate of approx. 160 nL min-1 with 2 mM NaCl buffer at 2 kV22. This value obtained from the 25 modified GMA-EDMA copolymer shows an acceptable performance as an EOP. When the flow rate was measured without modification of the monolith a negligible flow rate was obtained regardless of the voltage applied.  To conclude, for the first time the synthesis of organic 30 polymers has been demonstrated by photoinitiated polymerisation in polyimide coated fused silica capillaries using visible region LEDs as the light source. In comparison to thermal initiation, polymerisation time is significantly reduced with polymer generated in 30 min. Using LEDs as the 35 light source instead of classical high powered light sources interference from thermal effects during polymerisation is minimised. The monoliths synthesised here have been demonstrated as EOPs for microfluidic devices.  The here presented visible light photoinitiated 40 polymerisation has a potential for use in other polyimide-encased microfluidic devices such as polyimide chips. In addition to photoinduced polymerisation within polyimide coated capillary, there is potential for this method to be used for the grafting of species onto monolithic supports, which are 45 currently impossible to photo-graft using conventional UV-initiatied grafting methods, such as chromophoric monomers with strong absorbance in the UV region. This application is currently being examined.  Z.W., S.A. and M.M. would like to thank the Marie Curie 50 Excellence Grants funding (MC EXT FP6/MEXT-CT-2004-014361). D.H and P.K acknowledge support from the Czech Ministry of Education, Youth and Sport (MSM0021622413). Special thanks go to M. Ryvolová for fruitful discussions on EOF and buffer composition. 55 Notes and references  a National Centre for Sensor Research and School of Chemical Sciences, Dublin City University, Glasnevin, Dublin 9, Ireland . Fax: 00353 1 8360830; Tel: 00353 1 700 5611; E-mail: mirek.macka@dcu.ie b School of Physical Sciences, Dublin City University, Glasnevin, Dublin 60 9, Ireland.  c Department of Chemistry, Faculty of Science, Masaryk University, Kamenice 5/A8, 625 00 Brno, Czech Republic d Australian Centre for Research on Separation Science, Private Bag 75, Hobart TAS 7001, Australia 65 † Electronic Supplementary Information (ESI) available: See DOI: 10.1039/b000000x/  1 F. Svec and J. M. J. Frechet, Analytical Chemistry, 1992, 64, 820. 2 M. Al-Bokari, D. Cherrak and G. Guichon, Journal of 70 Chromatography A, 2002, 975, 275. 3 C. Viklund, E. Ponten, B. Glad, K. Irgum, P. Horstedt, and F. Svec, Chem. Mater., 1997, 9, 463. 4 E. A. Moschou, A.D. Nicholson, G. Jia, J.V. Zoval, M.J. Madou, L.G. Bachas and S. Daunert, Analytical and Bioanalytical Chemistry, 75 2006, 285, 596. 5 B. Beiler and A. Safrany, Radiation Physics and Chemistry, 2007, 76, 1351. 6 R. Bandari, W. Knolle, A. Prager-Duschke, H-J. Gläsel and M.R. Buchmeiser, Macromolecular Chemistry and Physics, 2007, 208, 80 1428. 7 S. Abele, F-Q. Nie, F. Foret, B. Paull and M. Macka, The Analyst 2008, 133, 864. 8 D. J. Throckmorton, T.J. Shepodd and A.K.  Singh, Analytical Chemistry, 2002, 74, 784. 85 9 G. Chen, F. Svec and D.R. Knapp, Lab on a Chip, 2008, 8, 1198. 10 M. T. Dulay, H.N. Choi and R.N. Zare, Journal of Separation Science, 2007, 30, 2699. 11 S. L. McDermott, J. E. Walsh and R. G. Howard, Optics & Laser Technology, 2008, 40, 487. 90 12 P. K. Dasgupta, I-Y. Eom, K.J. Morris and J. Li, Analytica Chimica Acta, 2003, 500, 337. 13 S. Chatterjee, P. Gottschalk, P.D. Davis and G.B. Schuster, Journal of the American Chemical Society, 1988, 110, 2326. 14 S. Murphy, X. Yang and G.B. Schuster, Journal of Organic 95 Chemistry, 1995, 60, 2411. 15 G. B. Schuster, X. Yang, C. Zou and B. Sauerwein, Journal of Photochemistry and Photobiology A: Chemistry, 1992, 65, 191. 16 I. R. Gould, D. Shukla, D. Giesen and S. Farid, Helvetica Chimica Acta, 2001, 84, 2796. 100 17 J. Kabatc and J. Paczkowski, Journal of Photochemistry and Photobiology A: Chemistry, 2006, 184, 184. 18 F. Svec, T. B. Tennikova and Z. Deyl, 'Monolithic materials; Preparation, properties and applications ', Elsevier, 2003. 19 F. M. Okanda and Z. El Rassi, Electrophoresis, 2006, 27, 1020. 105 20 S. Eeltink, F. Svec and J.M.J. Frechet, Electrophoresis, 2006, 27, 4249. 21 F. Q. Nie, M. Macka, L. Barron, D. Connolly, N. Kent and B. Paull, Analyst, 2007, 132, 417. 22 F. Q. Nie, M. Macka and B. Paull, Lab on a Chip, 2007, 7, 1597. 110  4  |  Journal Name, [year], [vol], 00–00 This journal is © The Royal Society of Chemistry [year]   

Photoinitiated polymerisation of monolithic stationary phases in polyimide coated capillaries using visible region LEDs

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Photoinitiated polymerisation of monolithic stationary phases in polyimide coated capillaries using visible region LEDs

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