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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
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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
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