(8f) Temperature Measurement in a Microfluidic Device for Insulator-Based Dielectrophoretic Applications

Temperature Measurement
in a Microfluidic Device for Insulator-based Dielectrophoretic Applications

Asuka Nakano, Kathleen
Bush, Alexandra Ros

Department of Chemistry and Biochemistry, Arizona
State University, Tempe AZ, 85287

Direct current (DC) insulator-based dielectrophoresis
(iDEP) has been used with cells and biomolecules such as DNA and proteins for
separation, pre-concentration, and fractionation. Unlike the other existing
analytical techniques, DEP response is governed by a particle's polarizability
in an inhomogeneous electric field. This additional parameter space facilitates
improved separation in a gel-free environment, which is of particular
importance for more complex samples such as disease markers found in body
fluids. DC iDEP has potential to be used as an alternative to AC iDEP since DC
iDEP does not require electrokinetic and/or pressure pumps necessary in AC iDEP
experiments.

The application of large DC
voltage in iDEP results in heat generation known as Joule heating within the microfluidic
device.  This phenomenon is of great interest due to its influence on protein
migration¹ as
well as protein stability.  In this work we present a means to measure fluid
temperature in microfluidic systems by implementing fluorescent microscopy that
enables dual color detection with an optical splitter and a CCD camera. Our
previous work demonstrated DEP streaming of monomeric immunoglobulin G (IgG) due
to positive DEP, however, unlike DNA and cells, the
mechanisms of protein polarization remain less understood. Furthermore, additional
electroosmotic and electrophoretic forces interplay with DEP resulting in complex
protein migration behavior.  Fluid temperature due to Joule heating is another
factor, which has not been thoroughly investigated experimentally in iDEP. Therefore
our study provides novel information to develop iDEP devices for separation,
concentration, and fractionation.

In this work, we demonstrate a way to quantify the
fluorescence emission ratio of two dyes²: Rhodamine B (RhB), a temperature sensitive
dye, and Rhodamine 110 (Rh110), a temperature insensitive dye, using the same
device developed for our IgG DEP experiments. While the emission intensity of RhB
is proportional to the local temperature, it is biased by variations in the
illuminating fluorescence light intensity. To eliminate the potential bias, we
employed the two dye system and performed quantitative analysis by referencing
the RhB fluorescence intensity with the Rh110 fluorescence intensity. Experiments
were performed under the same buffer conditions and applied potentials to mimic
the conditions of successful iDEP protein streaming. Over a period of 30
minutes, we tracked the change in temperature as well as temperature variations
in different locations around the post regions and within the reservoirs.

Our preliminary experiments show that there is no
significant temperature rise either within the channel or the reservoir despite
minor challenges associated with these dyes such as photobleaching and non-specific
adsorption of RhB to the PDMS surface. Our results demonstrate that the effects
of Joule heating are small, which can be explained by two reasons. First, the
temperature does not increase significantly within the channel since the bulk
liquid is consistently refreshed by electrokinetic flow thus minimizing an increase
in temperature. Second, the temperature within the reservoir does not vary to a
large extent owing to the large solution volumes (~70 µL). Even though Joule heating
is present, heat is dissipated causing the solution to remain at room
temperature.

In addition to DC iDEP, our scope can be further expanded to
temperature measurement within a channel with smaller structures and
constrictions (i.e. nano-posts). With combination of AC iDEP, the
nanostructures have potential to create larger DEP force and to immobilize
proteins via DEP (DEP trapping). Thus, it is of importance to measure
temperature fluctuations since a larger Joule heating effect is expected with such
small constrictions.  Our study therefore provides valuable information of the
micro- and nano-environment, in which protein iDEP experiments are performed
leading to more profound understanding of protein iDEP.

References

(1)    Chaurey, V.; Polanco, C.; Chou, C.-F.; Swami, N. S. Biomicrofluidics 2012, 6,
012806?14.

(2)    Ross, D.; Locascio,
L. E. Anal. Chem. 2002, 74, 2556?2564.