Microreactor synthesis of unimodal molecularly imprinted polymer (MIP) beads used for explosives detection
D. Roeseling, T. Tuercke, S. Loebbecke, H. Krause
Fraunhofer Institute for Chemical Technology, 76327 Pfinztal, Germany
Molecularly imprinted polymers (MIP) have received an increasing interest in the field of extraction, enantiomer separation, product isolation or enrichment of species from diluted mixtures [1-2]. A quite new field is the use of MIP's as sensing materials for the detection of organic compounds and especially of explosives like trinitrotoluene (TNT), dinitrotoluene (DNT) and dinitrodimethylbutane (DMNB) [3-4]. Sensor networks based on MIP technology are currently developed as a new kind of monitoring devices in complex areas surveillance.
The synthesis of molecular imprinted polymers is usually carried out by suspension polymerization in batch vessels in presence of a template . Subsequent removal of the template reveals binding sites that are specific towards the analyte via either covalent or non-covalent interactions. For the use as a chromatographic material MIP beads must have a narrow size distribution to avoid diffusion problems. Especially inefficient mixing during suspension polymerization often leads to a broad particle size distribution of the polymer beads. Some methods for preparing uniform spherical MIP's, using either expensive solvents, monomers or complex shape polymerizations, are described in literature [5-6].
In recent years microfluidic structures have received an increasing interest as reactors for the synthesis of monodisperse polymer particles [6-7].
This paper reports on the use of microreaction technology for the synthesis of MIP particles with narrow size distribution to be utilized as sensors for the gas phase detection of TNT.
The syntheses of particulate MIP particles have been performed in a T-junction microreactor made of glass (micronit microfluidics, The Netherlands) by generating a segmented flow of droplets in a polyvinylalcohol/water mixture used as mobile phase. The droplets are fed with all relevant reagents for the polymerization reaction via a microchannel of 50 µm x 50 µm size (internal diameter) and teared off by the mobile phase in a 200 µm x 200 µm microchannel (Fig. 1). Both fluids (droplet phase and mobile phase) are introduced by syringe pumps (Sykam S1610).
Once the droplets are formed, the segmented flow is released into a wider channel of
500 µm x 500 µm diameter in which either thermally induced polymerization via heating or photochemically induced polymerization is conducted, in the latter case by employing an UV lamp (DYMAX 50 Blue).
The droplet phase consists of a combination of the following functional monomers: acrylamide, methacrylamide, methacrylic acid, 2-hydroxypropylmethacrylate or buntanediol-monoacrylate (all purchased from Aldrich), ethylene glycol dimethacrylate (Fluka) as cross linker, chloroform or toluene (Roth) as porogene and solvent and TNT (Dynamite Nobel) as template molecule. In case of photochemical initiation Irgacure 819 (Ciba Speciality Chemicals) is used as initiator whereas V65 (Wako Chemicals) is used as thermal initiator.
The formed MIP beads were filtrated, dried and the particle size distribution was measured by laser scanning spectrometry (Mastersize S, Malvern Instruments).
The TNT or DNT content of the MIP particles was analyzed by GC (HP 5890) before and after treatment with explosives test vapours, containing typically 500 ppm TNT.
Results and Discussion
We were able to generate MIP particles with a variable size between 3 µm and 150 µm. Control on particle size was achieved by deliberately varying flow rates of the droplet and mobile phase within a range from 4 µL/min up to 5000 µL/min. In comparison to previous suspension polymerizations performed in batch vessels (Fig. 2) we were able to synthesize MIP particles of uniform shape and narrow size distribution (Fig. 3). For the use as an explosives sensor two different particle sizes are of great importance. For example, particles even or smaller than 3 µm can be incorporated onto the surface of a mass sensitive quartz crystal microbalance (QCM) to detect explosives vapour directly from gas phase. Larger MIP particles with a mean size of 20 µm can be used as a stationary phase in columns either to extract explosives out of liquid solutions or to enrich traces of explosives from larger gas volumes.
Unimodal molecularly imprinted polymer beads were successfully produced in a continuously operated microreactor. By varying the flow conditions different particle sizes were obtained. The application of the generated MIP particles for the detection of explosives was shown by GC analysis. Both the synthesis of MIP particles suitable for the detection of other explosives and the generation of smaller MIP particles are currently under further investigation.
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