CONTACTLESS CONDUCTIVITY DETECTION IN LTCC TECHNOLOGY FOR MICROCHIP

ELECTROPHORESIS Georg Fercher1,2, Walter Smetana1 and Michiel J. Vellekoop1

1Institute of Sensor and Actuator Systems, TU Wien, AUSTRIA 2Integrated Microsystems Austria GmbH, Wiener Neustadt, AUSTRIA

ABSTRACT

In this work we report on a novel contactless conductivity detector produced in Low Temperature Co-fired Ceramic (LTCC) technology for microchip capillary electrophoresis (CE). LTCC is very promising for this detection method because of its high permittivity compared to glass or plastics. The coupling of the detection sig-nal to the fluidic channel is improved which enhances the detection sensitivity. We show the first successful measurements with this setup confirming the feasibility of contactless conductivity detection of inorganic ions after CE separation in a LTCC fabricated microchannel. KEYWORDS: Electrophoresis, Contactless Conductivity Detection, LTCC, Micro-fluidics

INTRODUCTION

Contactless conductivity detection of ion concentrations in liquids is a multi-purpose detection method for bioanalytical applications and has been widely used e.g. for on-chip capillary electrophoresis (CE) devices [1-2]. Small dimensions of both the detector and read-out electronics make it an attractive alternative to fluores-cence- or optical absorbance-based systems since conductive compounds such as in-organic ions can be sensed without prior chemical modification. Furthermore, chip materials do not need to be transparent. The measurement electrodes are located out-side the fluidic channel, avoiding direct contact with the buffer solution and there-fore fouling or damaging of the electrodes.

Low Temperature Co-fired Ceramics (LTCC) as a high-performance material for the fabrication of microfluidic devices is receiving increased interest [3-5]. In its pre-fired state, LTCC is a flexible tape that can be easily structured using stamping, cutting, embossing or laser micromachining techniques. Channels and cavities can so be fabricated in multilayer arrangements. Once fired, electroosmotic flow (EOF) properties of LTCC are similar to those of glass, making this material interesting for capillary electrophoresis (CE) chips. LTCC-tapes are available with high values of material permittivity as compared to plastics or glass. When combined with contact-less conductivity detection, better signal coupling into the microfluidic channel and therefore enhanced detection sensitivities are possible.

Our paper presents a microfluidic device with a combined contactless conductiv-ity detector entirely fabricated in LTCC technology. CE separations of two different inorganic ions were successfully carried out.

978-0-9798064-1-4/µTAS2008/$20©2008CBMS 916

Twelfth International Conference on Miniaturized Systems for Chemistry and Life SciencesOctober 12 - 16, 2008, San Diego, California, USA

DEVICE DESCRIPTION Five layers of LTCC-foils (Heraeus Heratape 707) were combined for the fabri-

cation of the device, each with a thickness of 138 µm in the unfired state (Figure 1 A and B). Two microchannels of different lengths (45 and 140 mm respec-tively), via holes and liquid inlets/outlets were micromachined with a diode pumped NdYAG laser.

Two electrodes facing each other, arranged at opposite sides of the device, form the detector (Figure 1 C and D). They are separated from the microfluidic channel by one single LTCC-layer and were realized by a screen-printing process using sil-ver conductor paste (Heraeus TC 7304A). The tapes were subsequently collated to a stack which was then exposed to a lamination process in an isostatic press (100 bar; 3 minutes). Firing of the laminated stack was conducted in a conventional belt fur-nace with a peak temperature of 850°C and a total cycle time of 90 minutes.

Figure 1. Exploded view (A) and photograph of the CE device (B). (1) Top layer

with fluid inlet/outlet ports, alignment holes and contact pads; (2) structured middle layer with two microchannels; (3) bottom layers for mechanical stability. Insets (C) and (D) show a schematic drawing of the longitudinal section along the channel and

a photograph of the cross-section of the device, respectively.

RESULTS AND DISCUSSIONS Successful CE separations of inorganic ions were carried out with the LTCC de-

vice (Figure 2) at a separation field strength of 200 Vcm-1. The injection procedure was conducted manually by injecting buffer and samples with pipette tips into the channel via the inlet ports. Channels were conditioned with 10 column-volumes of a 100 mM NaOH solution and then rinsed with 10 column-volumes of buffer solution (10 mM MES/Histidine). Platinum wires were positioned in both inlet and outlet ports. A LCR meter (Agilent 4285A) set to a frequency of 200 kHz and a signal voltage of 2 Vp-p was used to record changes in liquid conductivity due to the con-ductive zones passing the detector.

917

Twelfth International Conference on Miniaturized Systems for Chemistry and Life SciencesOctober 12 - 16, 2008, San Diego, California, USA

The optimum measurement frequency where the detector shows maximum re-sponse depends on the value of the liquid conductivity. An equivalent circuit of the detector as depicted in Figure 3 was used to find this frequency of maximum sensi-tivity (200 kHz). An increase of the wall capacitance Cwall contributes to an en-hanced signal coupling into the channel and thus improved detector response. The application of LTCC-tapes with a permittivity of 7 as used for our device is a valu-able approach to achieve this.

Figure 2. Measured electrophero-

grams for sample mixtures containing 0.2 and 1 mM potassium and lithium ions. CE parameters: 10 mM MES/Histidine buffer at pH 6; electric field strength 200 Vcm-1.

Figure 3. Equivalent circuit of the contactless conductivity cell and alloca-tion of the electric elements inside the opposite-electrode detector consisting of the three LTCC layers (A), (B) and (C). E1 and E2 show the electrodes.

CONCLUSIONS

In this work a microfluidic device fully assembled in LTCC-technology is intro-duced. CE separations of two different inorganic ions were carried out using a novel contactless conductivity detector build up in this ceramics technology. The opposite arrangement of the detector electrodes minimizes stray capacitance as compared to planar set-ups. Future chip designs will be produced employing ceramics with even higher values of permittivity, enabling a further increase in detection sensitivity.

ACKNOWLEDGEMENTS

The authors would like to thank Heinz Homolka, Edeltraud Svasek and Peter Svasek for their support in the realization of the device. The authors are very grate-ful to Dr. Annette Kipka and Christina Modes (Heraeus) for the supply of the ce-ramic tapes. This work has been partly financially supported by the EU 4M Project (Contract Number NMP2-CT-2004-500274). REFERENCES [1] F. Laugere, G.W. Lubking et. al., Sens. Act. A, 92, 2001, pp. 109-114. [2] J. Lichtenberg, N.F. de Rooij et. al., Electrophoresis, 23, 2002, pp. 3769-3780. [3] J.A. Fracassi da Silva, C.L. do Lago, Anal. Chem, 70, 1998, pp. 4339-4343. [4] M. Goldbach, H. Axthelm, M. Keusgen, Sens. Act. B, 120, 2006, pp. 346-351. [5] C.S. Henry, M. Zhong et. al., Anal. Commun., 36, 1999, pp. 305-307. [6] P. Wasilek, L.J. Golonka et. al., Proc. 26th ISSE, 2003, pp. 202-206.

918

Twelfth International Conference on Miniaturized Systems for Chemistry and Life SciencesOctober 12 - 16, 2008, San Diego, California, USA

MAIN MENUGo to Previous DocumentCD/DVD HelpSearch CD/DVDSearch ResultsPrint

/ColorImageDict > /JPEG2000ColorACSImageDict > /JPEG2000ColorImageDict > /AntiAliasGrayImages false /CropGrayImages true /GrayImageMinResolution 150 /GrayImageMinResolutionPolicy /OK /DownsampleGrayImages true /GrayImageDownsampleType /Bicubic /GrayImageResolution 300 /GrayImageDepth -1 /GrayImageMinDownsampleDepth 2 /GrayImageDownsampleThreshold 2.00333 /EncodeGrayImages true /GrayImageFilter /DCTEncode /AutoFilterGrayImages true /GrayImageAutoFilterStrategy /JPEG /GrayACSImageDict > /GrayImageDict > /JPEG2000GrayACSImageDict > /JPEG2000GrayImageDict > /AntiAliasMonoImages false /CropMonoImages true /MonoImageMinResolution 1200 /MonoImageMinResolutionPolicy /OK /DownsampleMonoImages true /MonoImageDownsampleType /Bicubic /MonoImageResolution 1200 /MonoImageDepth -1 /MonoImageDownsampleThreshold 1.50000 /EncodeMonoImages true /MonoImageFilter /CCITTFaxEncode /MonoImageDict > /AllowPSXObjects false /CheckCompliance [ /None ] /PDFX1aCheck false /PDFX3Check false /PDFXCompliantPDFOnly false /PDFXNoTrimBoxError true /PDFXTrimBoxToMediaBoxOffset [ 0.00000 0.00000 0.00000 0.00000 ] /PDFXSetBleedBoxToMediaBox true /PDFXBleedBoxToTrimBoxOffset [ 0.00000 0.00000 0.00000 0.00000 ] /PDFXOutputIntentProfile (None) /PDFXOutputConditionIdentifier () /PDFXOutputCondition () /PDFXRegistryName () /PDFXTrapped /False

/Description >>> setdistillerparams> setpagedevice