Integration of monolithic porous polymer with droplet-based microfluidics on a chip for nano/picoliter volume sample analysis
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In this paper, a porous polymer nanostructure has been integrated with droplet-based microfluidics in a single planar format. Monolithic porous polymer (MPP) was formed selectively within a microfluidic channel. The resulting analyte bands were sequentially comartmentalised into droplets. This device reduces band broadening and the effects of post-column dead volume by the combination of the two techniques. Moreover it offers the precise control of nano/picoliter volume samples.
KeywordsPDMS Microfluidic Channel Methyl Acrylate Droplet Generation Butyl Methacrylate
Microfabricated systems have emerged as promising tools for chemical and biological analysis due to the speed, throughput and control of minute samples that they make possible [1, 2, 3, 4, 5, 6, 7, 8]. In recent years, there has been considerable interest in droplet-based microfluidics because flow segmentation enables compartmentalisation of reagent volumes from fL to μL within a continuous and immiscible fluid, as well as the generation of monodisperse droplets and the precise control of them [9, 10, 11].
MPP potentially offers the advantages of simple control of permeability and surface areas as well as easy preparation within a micro-fluidic channel for various applications of lab-on-a-chip (LOC) [12, 13, 14, 15]. Various trials have been conducted to combine separation and droplet functions on a chip using beads-packed channels or capillary electrophoresis (CE) separation [16, 17, 18, 19, 20]. However, integration of MPP with droplet microfluidics has not been reported yet. Integration of MPP with droplet-based microfluidics on a single chip will provide various benefits. For example, MPP can be used as a nanostructure for a column or membrane to separate and filter analytes in the microfluidic channel. Subsequent compartmentalisation by droplets following MPP dramatically reduces Taylor dispersion of the effluent and minimizes dead volume effects; in addition, it allows further analysis downstream or offline with the encapsulated analytes. Furthermore, it is easier (in comparison to the widely-used beads) to form a porous nanostructure in a channel using selective UV polymerization and to control properties such as permeability or porosity by adjusting the composition of the MPP solution.
Our device consists of two parts: an MPP-filled channel and a droplet-generation zone. Thermoset polyester (TPE) was chosen as a device material because of its excellent mechanical properties, allowing it to withstand pressures of up to 18 Mpa . It makes possible a broad range of device applications including membrane, separation column and high-frequency droplet generation; these require operational stability under high pressure, which is not feasible when using typical materials, such as polydimethylsiloxane (PDMS) for the microfluidic device.
TPE (CFS Fibreglass, UK) was prepared by mixing it with its polymerization catalyst, methyl ethyl ketone peroxide (MEKP), and polyethylene terephthalate, PET, (Daedong Polymer, South Korea) was used as substrate. The mixture of ethylene diacrylate (EDA, monomer) 0.485 g, methyl methacrylate (MMA, cross-linker) 0.485 g and benzophenone (BP, photo-initiator) 0.03 g was prepared for the grafting layer. Butyl methacrylate (BuMA, monomer) 0.6 g, ethylene dimethacrylate (EDMA, cross-linker) 0.4 g, 1-dodecanol (progen) 1.5 g and 2,2’-dimethoxy-2-phenylacetophenone (DMPAP, photo-initiator) 0.01 g were mixed for the monolithic porous polymer.
For droplet experiments, a 10% (v/v) mixture of fluorocarbon oil, FC-70 (3 M Fluorinert, USA), and 1H, 1H, 2H, 2H-perfluorooctanol (PFO, Sigma-Aldrich) was used for the continuous phase. The mobile phase was a mixture of 26 mM phosphate buffer (pH 7) and methanol (HiPerSolve for HPLC, BDH Prolabo) in a ratio of 5: 95 (v/v)
Fluorescein isothiocyanate, FITC, (Sigma-Aldrich, USA) and Alexa Fluor® 488, AF 488, (Invitrogen, USA) were diluted to 0.02 μM and 50 μM concentration in the mobile phase as the analytes.
Next, as shown in Figure 1(b) and (c), the separation zone of the channel was selectively packed with poly(methyl acrylate) MPP. The TPE channel was selectively exposed to a UV light through a film mask during the polymerisation of the grafting layer and MPP. N2 gas was blown for 10 min into the grafting layer mixture (See section 2.1.) before filling the channel in order to remove oxygen and avoid expansion and subsequent heat-induced voids during polymerisation. Then it was radiated to UV light (broadband 290-385 nm, 12.22 mW/cm2) for 10 mins. The channel was flushed with 10 volumes of cleaning solvent (methanol : DI water = 1:1 (v/v)) to remove the unreacted polymer and was then dried at 40C. The monolithic polymer mixture was prepared and irradiated in a similar fashion as the grafting layer. Subsequently, it was cleaned by flowing the solvent at 10ul/min for 1 h to remove the remaining porogenic solvent and photo-initiators. It was then left to dry at 40°C overnight. UV irradiation was conducted from below since PET has much higher transmittance in the UV range than TPE does.
The mobile phase was introduced into the MPP-filled channel at 10 μL/min by HP 1050 HPLC pump (Agilent, USA). 5 μL of the mixture of two dyes in the mobile phase was injected; then the effluent was segmented by the oil injected at 100 μL/min using precision syringe pumps (Harvard Apparatus, USA). Droplets containing the dyes were recorded by a high speed camera (Phantom, USA) and detected by laser-induced fluorescence (LIF) detection with a beam from 488 nm Ar+ diode laser (Omnichrome, Melles Griot, Cambridge, UK).
Results and discussion
To investigate diffusional band broadening, the chromatograms in (b) and (c) were quantitatively compared by extracting the maximum fluorescence of each droplet.
MPP has been integrated with droplet microfluidic channel on a monolithic chip. Straightforward fabrication of monolithic porous nanostructure within a microfluidic channel has been demonstrated. Band broadening and dead volume issues have been decreased as well by sequential compartmentalisation of analytes at the MPP outflow, encapsulating them in defined droplet volumes. This confirms the potential of MPP for separation or filtering function in a microfluidic chip and the ability of droplets to act as a fraction collector for the handling of nano/picoliter volume samples.
This work was supported by the Global Research Laboratory Program of the National Research Foundation of Korea (NRF) grant (Grant Number K20904000004-13A0500-00410) and Global Frontier Project (Grant Number H-GUARD_2014M3A6B2078957) of BioNano Health-Guard Research Center, which are funded by the Ministry of Science, ICT & Future Planning (MSIP) of Korea.
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