Temperature
controlled microfluidic
PDMS reactor
M. Chudy, P. Prokaryn, A. Dybko, Z. Brzozka, Department
of Analytical Chemistry, Warsaw Uiziversity of Technology, Noakowskiego 3, 00-664 Warsaw, Poland e-mail:
[email protected], tel. 1-48 22 660 78 73 fax+48 22 660 56 31 Abstract A simple fabrication of a PDMS microfluidic structure with a microreactor temperature control is described in this paper. A Peltier module was covered completely with the liquid prepolymer mixture, which next was cross-linked to form a heated PDMS plate. A temperature sensor (i.e. a p-n diode) was placed in the second PDMS plate. The microchannels were laser engraved. After sealing of the microsystem, the formation of molybdenum blue was performed in the heated (6O’C) microreactor and evaluated using a UV-Vis fibre optic microdetector. This reaction was used for phosphate determination. Keywords: PDMS, polymer micromachining, optical detection
laser engraving,
temperature
control,
1. Introduction Control of fluid temperature within microanalytical systems can be very important especially for electroosmotic separation [ 31, enzyme activated reactions [2], clinical analysis [3] and environmental pollutants determination [4]. Assembling of heating elements inside silicon-based microsystems requires additional steps during their fabrication (another photolitho~aphy or LPCVD process etc.). This causes that the microsystem is more complicated and the total cost of the microsystem is increased. Microfluidic devices can also be based on the use of elastomeric materials, e.g. PDMS [4] but their flexibility practically precludes thin metal heating electrodes deposition on their surface. The aim of this paper is to present the application of a commercially available heater i.e. a Peltier module in a microfluidic structure. 2. Experimental Up to now, to carry out a reaction which required that the solutions should be heated up we have placed a PDMS microfluidic structure with microchannels and 30 cm long mixing meander, which formed a microreactor, in a special plexi housing with a heating wire [4]. The next step in development of our microsystem was the integration of a heater within a PDMS block. We decided to apply a commercially available heater, which could be covered completely with liquid PDMS mixture (Dow Coming, Sylgard 184) without any damage. The cheapest way to reach the goal of the project was to use a Peltier device
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(4 x 4 cm) coupled with a p-n diode used as a temperature heated microreactor fabrication is shown in Fig. 1a. steel bolts
sensor. The scheme of the
temperature
liquid mixture of PDMS and cross-linking
PDMS pouring out and cross-linking
PDMS plates and steel bolts removing
laser engraving
/ ’ O2 plasma
Figure I. a) Fabrication of the microreactor with Peltier device; b) Photograph of fabricated device
oxidizing
UV-Vis
detector irreversible assembling of microsystem
in plain glass beakers. The Peltier device and the temperature sensor were placed on thin PDMS spacers just to let the liquid prepolymer reach the space between the element and the mould wall. Three steel bolts were mounted in one of the moulds for inlets and outlet forming. The moulds were filled with PDMS and left for cross-linking. Then the steel bolts were removed from the structure and two PDMS plates were carefully removed from the moulds. In the next step, V-grooves were laser engraved to form microchannels on the surface of PDMS plate with the temperature sensor. Having done that, surfaces of PDMS plates were modified in oxidising plasma
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(Oxford Instruments 3000, 1 Tr, q(02) - 200 mlimin, 20 s, 20 W, 25*C) and irreversibly bonded to seal the system. The Peltier device’s position was adjusted to tit the microreactor area. To improve the heat exchange a square fragment of PDMS was removed from the outer side of the Peltier module. Prior to use the temperature sensor was calibrated against the temperature changes. The temperature sensor and the Peltier module were connected to a temperature regulator in order to keep the temperature constant in a microreactor. The outlet of the structure was connected to a UV-Vis fibre optic microdetector. The detector was made of a silicon structure containing fluidic microchannels and V-grooves for optical fibres and was covered with a glass plate with bonded inlet/outlet capillaries (Fig. 2). The glass cover and the silicon structure were anodically bonded. Anisotropically etched fluidic microchannel (150 pm in depth and 700 pm in width) was fabricated to obtain detection volume of 900 nl. Two multimode 62.51125 pm optical fibres were positioned in the V-grooves to ensure 1 cm optical path length.
Figure 2. Nanoliter
W-Vis
detector for microfluidic
systems.
4. Results and discussion The reduction of molybdenephosphoric acid to molybdenum blue was performed in the fabricated microreactor. The reaction is widely used for phosphates determination and in a classical spectrophotometric method is carried out at temperature of boiling water bath. The constructed microdevice allowed setting the microreactor temperature within the range of 20 - 60°C and total chemicals flow rate of 30 @+nin. The standard phosphate solutions and complexing-reducing agent [4] were introduced into the system with the help of a peristaltic pump. The dynamic and calibration curves for phosphate determination obtained in designed microsystem at two extreme temperatures are shown in Fig. 3. During the preliminary tests of the reaction zone’s heating the recorded signal was not stable and exhibited high noise level, i.e. incidental intense peaks, which were caused by air bubbles released from the heated solution. To prevent this inconvenient
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h=650
nm
q=30
plimin
60 0
30
60
90
0,o
0,5
I,0
time [min]
I,5
zo
2s
WI
yg P/ml
Figure 3. Dynamic responses and calibration curves for POh3- obtained using temperature controlled PDMS microreactor (20 and 60°C) coupled with UV-Vis detector. phenomenon the analytical reagent and standard solutions were prepared with the use of degassed redistilled water, then the detecting area was not disturbed by the bubbles and correct calibration could be performed. The measurements were performed at 650 nm as the analytical wavelength. It was observed that the reduction reaction took place when the microreactor was heated up (see dynamic curve). The calibration curve presented in Fig. 3 was obtained for 6O’C. The future work will be focused on the integration of optical fibres directly in a polymer block with a microreactor and the application of such a system for human toxin analysis. Acknowledgements Michal Chudy wishe to thank The Foundation for Polish Science for financial support. The authors wishes also to thank Prof. Romuald Beck for his help with plasma oxidation optimization and Dr. Jan Dziuban’s group for the micro UV-Vis detector fabrication. References [l] W.A Gobie, C.F. Ivory, J. Chromatogr., 516, 191-197, (1990). [2] J. Khanduriana, T.E. M&night, S.C. Jacobson, L.C. Waters, R.S. Foote, J.M. Ramsey, Integrated system for rapid PCR-bused DNA analysis in microjluidic devices, Anal. Chem., 72, 2995-3000, (ZOOO), [3] J.D. Artiss, R.E. Karcher, S.L. Collins, B. Zak, Application and evaluation of a new cold-stable kinetic Jaffe reagent to the Hitachi 747fou the determinution qf serum creatinine, Microchem. Journal, 65,277-282, (2000), [4] M. Chudy, P. Prokaryn, W. Wroblewski, A. Dybko, Z. Brzozka, Micrqj7uidic system for phosphate determination, Eurosensors XVI 2002, 445-446, Prague, Czech Republic.
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