Microfluid Nanofluid
DOI 10.1007/s10404-012-0956-0
RESEARCH PAPER
Electric field-induced concentration and capture
of DNA onto microtips
Dinesh Kalyanasundaram • Shinnosuke Inoue • Jong-Hoon Kim •
Hyun-Boo Lee • Zenko Kawabata • Woon-Hong Yeo • Gerard A. Cangelosi
Kieseok Oh • Dayong Gao • Kyong-Hoon Lee • Jae-Hyun Chung
•
Received: 28 November 2011 / Accepted: 11 February 2012
Ó Springer-Verlag 2012
Abstract Simple, high-yield concentration of DNA is
important for high-throughput genetic analysis and disease
diagnosis. Glass-based microfilters are popular but the
process requires centrifugation steps with cumbersome
chemical processes. As an alternative, a concentration
method using an electric field has been explored previously, but with limited efficiency. In this paper, electric
field-induced concentration and capture of DNA are studied by using high-aspect-ratio microtips coated with a gold
layer. The microtips are immersed longitudinally into a
solution of 100 lL containing k-phage DNA. After DNA
concentration using an electric field, the microtips are
withdrawn from the solution. Under AC- and biased AC
fields, DNA is concentrated by electrophoresis (EP),
dielectrophoresis (DEP), and electroosmotic flow (EOF).
To reduce capillary effects in the withdrawal process, the
microtips are coated with positively charged poly-L-lysine
(PLL). The pattern of captured DNA is analyzed by fluorescence microscopy. DEP attracts DNA molecules at the
edges of microtips, where the highest gradient of electric
Electronic supplementary material The online version of this
article (doi:10.1007/s10404-012-0956-0) contains supplementary
material, which is available to authorized users.
D. Kalyanasundaram S. Inoue J.-H. Kim H.-B. Lee Z. Kawabata W.-H. Yeo D. Gao J.-H. Chung (&)
Department of Mechanical Engineering,
University of Washington, Seattle, WA 98195, USA
e-mail: jae71@uw.edu
G. A. Cangelosi
Seattle Biomedical Research Institute,
307 Westlake Ave N, Suite 500, Seattle, WA 98109, USA
K. Oh K.-H. Lee
NanoFacture, Inc., P.O. Box 52651, Bellevue, WA 98015, USA
field exists. EP attracts DNA onto the surface of microtips
following the vectors of an electric field. EOF generates
vortexes that deliver DNA onto microtips. Using this
method, 85% of DNA is captured on the PLL-coated
microtips after three sequential captures. The concentration
mechanism can potentially facilitate rapid and simple
preparation of DNA for downstream analysis.
Keywords DNA concentration Electric field Microtip Frequency response
1 Introduction
Concentration, purification and recovery of DNA is important
for disease diagnosis (Caskey 1993) and genome sequencing
(Feero et al. 2008). Current concentration methods using glass
matrices are cumbersome with multiple centrifugation and
microfiltration steps combined with the use of alcohol or
chaotropic solution. Centrifugation can potentially damage
DNA by hydrodynamic shear force in microfilters (Price et al.
2009). The use of alcohols can denature DNA (Wagner et al.
1993). For example, past studies on centrifugation-based
methods have shown that plasmid DNA of 10–20 kb in length
was fragmented by shear force (Levy et al. 1999), as was
chromosomal DNA (Levy et al. 2000).
For a filter-free concentration method, the manipulation
of DNA using an electric field has been explored by the
following research groups. Washizu and Kurosawa (1990)
studied the stretching of DNA using an electric field. An
electric force was used to hold DNA molecules that were
cut by ultraviolet light (Washizu and Kurosawa 1990), and
laser beam (Washizu et al. 1995).
For DNA concentration, Bakewell and Morgan (2006)
used interdigitated microelectrodes for the concentration of
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Microfluid Nanofluid
12 kb plasmid DNA in solution using DEP. Fluorophorelabeled DNA was observed between planar electrodes on a
glass substrate. The concentration efficiency decreased as
the frequency increased from 100 kHz to 20 MHz. Wong
et al. (2004) combined DEP, EP, and EOF to concentrate
k-DNA. The combined effect of EOF, EP and DEP resulted
in concentration of DNA in the center of ring-shaped
electrodes. The highest concentration performance was
observed at 1 kHz, where DEP force and EOF were at
maximum. Ying et al. (2004) designed nano-pipettes to
concentrate DNA by DEP. Du et al. (2008) and Du and
Wei (2010) studied trapping of T4 DNA molecules at AC
frequencies of 1, 100 kHz and 20 MHz. It was found that
an optimal time for DNA concentration was required
for maximum efficiency. Yokokawa et al. (2010) studied
both AC- and biased AC fields for concentrating k-DNA
molecules. Biased AC fields yielded higher concentration
of DNA than AC fields. Our recent study shows DNA
concentration on a nanotip using an AC electric field at
5 MHz, which demonstrated DNA detection at 7 pg/mL in
a 2-lL sample volume (Yeo et al. 2009). In all the above
experiments, only limited concentration performance of
DNA was observed in sample volumes smaller than 20 lL.
Bioassays frequently require sample volume larger than
100 lL, which can be advantageous to improve the sensitivity of biosensors (Hanselle et al. 2003).
In this paper, a microtip concentrator is uniquely
designed to concentrate and extract DNA on to a microtip
surface using electric fields. The captured DNA can be
used for downstream analysis or potentially stored at room
temperature (Lee et al. 2011; Michalet et al. 1997). As a
fundamental study, this paper presents a microtip-based
concentration of DNA for various electric fields including
DC, AC and biased AC at various frequencies and
immersion times. For DNA concentration, a high electric
field of 7.4 9 105 V/m is applied between gold-coated
microtips and an aluminum well containing 100 lL volume of k-DNA solution. To reduce capillary-induced
effects upon microtip withdrawal, poly-L-lysine (PLL) is
coated on microtips for electrostatic capture. The captured
DNA is dyed with PicoGreenÒand analyzed by a fluorescence microscope. To understand the pattern of attracted
DNA molecules on the microtips, numerical simulation
results were compared with experimental results.
1.1 Experimental configuration
The experimental configuration is composed of an array of
microtips and a rectangular well (Fig. 1) that can hold
100 lL of sample solution. The microtip array is composed
of 5 microtips, each with 1 lm in thickness and 50 lm in
width at the tip part. Both microtips and well are electrically conductive. The microtips are immersed into k-DNA
123
Fig. 1 Schematic of a microtip array in an aluminum well. The inset
shows an image taken by a scanning electron microscope (SEM). The
microtip is divided into three regions, tip, base, and silicon chip parts.
The rectangular trench areas are created during fabrication
solution in z direction. When an electric field is applied to
the microtip array immersed in the well, DEP, EP, and
EOF can be generated. After concentrating DNA on the
microtip surface, the microtip is withdrawn from the well.
This induces capillary action to either retain or release the
concentrated DNA.
1.2 Numerical analysis of electric field-induced
concentration on microtips
To understand the effect of EP, DEP and EOF on the
concentration of DNA, numerical computation was conducted by Comsol Multiphysics (Version 4.1). The purpose
of the numerical study is to understand the attracted pattern
of k-DNA molecules on a microtip surface for EP, DEP
and EOF. The DNA pattern on the microtip surface is
compared to experimental results, yielding a basic understanding of the concentration mechanism to enhance the
capturing yield. For simple analysis, k-DNA was modeled
as a microsphere. A quarter model of Fig. 1 was used in the
numerical study. The model, boundary conditions and
values of various parameters for simulation are described
in the supplementary information.
For electric field-induced phenomena, EP is the electrostatic motion of a charged particle under an electric
field. Negatively charged DNA molecules move toward
positive electrode in an electric field (Lei et al. 2009). For
the numerical results, microspheres at initial locations are
attracted along the vectors of an electric field and deposited
on both edges and surfaces of microtips depending on the
initial position of a microsphere in medium (Fig. 2a, b).
Microfluid Nanofluid
Fig. 2 Trajectories of a 3-lm diameter sphere near the microtip
according to EP, DEP, and EOF. a Initial position of particles at
100 lm distance from a microtip. b Trajectories of spheres under EP.
The inset image shows the final location of spheres on microtip
surface. c Trajectories of spheres under DEP. The inset image shows
that spheres are attracted at the edge of a microtip. d Trajectories of
the spheres under EOF. In the inset image, spheres are finally located
on microtip surface along an electric field
DEP is the movement of a particle in a non-uniform
electrical field due to the induced dipole moment. The
magnitude of DEP force depends on the volume of a
sphere, the polarizability of a sphere and a medium, and the
gradient of squared electric field (Zheng et al. 2004). In the
given geometry of a microtip, microspheres are attracted to
an edge of microtips due to a higher gradient of an electric
field (Fig. 2c).
In the presence of an electric field, a very thin ionic
layer forms on the surface of a microtip. The charged ions
in the electrical double layer between the surface and the
electrolyte experience an electrostatic force when an
electric field is applied. The unbalance of charges on
electrodes generates EOF acting as electrokinetic pumps
(Bown and Meinhart 2006). In the given geometry of microtips, microspheres are transported to the microtip surface by the convective vortexes, but returned to the bulk
fluid without other attractive forces (Fig. 2d). According to
our experimental observation using polystyrene beads
(diameter 19.0 lm) in 19 TE buffer, EOF was clearly
observed at the frequencies between 1 kHz and 5 MHz
(Supplementary Information). At frequencies over 10 MHz,
EOF was negligible. At such high frequencies, the ions could
not respond to an electric field because the change of electric
polarity on electrodes could be higher than ion mobility.
In summary of the numerical results, DNA can be
attracted to both edge and surface of the microtip by EP
and to the edge by DEP. EOF can deliver DNA to the
microtip but carry it away from the microtip due to the
circulation flow.
2 Experimental materials and methods
2.1 Experimental setup
The experimental setup is illustrated in Fig. 1. Microtips
were dipped and withdrawn in the solution along the z axis,
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Microfluid Nanofluid
which was controlled by a linear motor. An electric field
was applied by a signal generator (Agilent 33220A)
between the aluminum well and the microtips.The microtips were designed to increase the efficiency of capture by
increasing the strength of an electric field through the high
aspect ratio. In particular, a saw tooth profile was patterned
at the edge of microtips, which could increase the strength
of an electric field by accumulating electric charges at the
edge. Once attracted, DNA could be captured on the
microtip surface. The fabrication procedure of the microtip
is given in the supplementary information.
A well was made of conductive aluminum foil of
12 mm 9 2 mm 9 2.5 mm that could contain 100 lL of
sample solution. The vibration of the aluminum well was
applied to circulate the DNA solution. The vibration
amplitude was 100 lm in a longitudinal direction at 60 Hz.
This circulation flow was not considered for the numerical
analysis because the flow velocity in the vicinity of the
microtip surface was close to 0 lm/s.
2.2 Experimental method
Bacteriophage k-DNA (48.5 kbp, 31.5 kDa) was purchased
from New England Biolabs (Ipswich, MA). The concentration was 500 lg/mL (16.2 nM) in 19 Tris EDTA (TE)
buffer of 7.5 pH. Further dilutions to 1 nM were made with
19 Tris ETDA buffer of 7.5 pH. A green intercalating dye
(PicoGreenÒ, excitation and emission wavelengths: 480
and 520 nm, respectively) was purchased from Invitrogen
(Carlsbad, CA). After 200-fold dilution with 19 TE buffer,
PicoGreen was mixed with equal volume of 1 nM k-DNA.
The mixture was incubated for 5 min at room temperature
before the experiment.
The captured k-DNA on microtips was imaged by a
fluorescence microscope (Olympus BX-41). The fluorescence images of both front and back sides of microtips
were captured and digitized into black and white pixels.
The threshold value was determined to minimize the
fluorescence signals of negative controls. Using the
threshold, the most pixels in negative control signals were
converted into black dots. The digitization was conducted
using an image processing tool of Matlab 8.0. Through this
image processing, captured DNA could be effectively
located and quantified on microtip surface. After digitization, the white pixels of the images were summed to yield
fluorescence signals showing the location and amount of
DNA.
The first set of experiments was conducted with goldcoated microtips, which are referred as‘non-coated microtips’
in this paper. The effects of frequencies and immersion
time were studied for concentration of k-DNA. The frequencies were varied from 100 Hz to 10 MHz. In addition
123
to an AC field, a bias of 3 V (DC field) was added to
observe the effect of a biased AC field. The test was also
conducted without an electric field to assess DNA capture
by capillary action alone. A negative control experiment
without DNA was conducted only with the PicoGreenÒ
dye.The immersion time for the frequency tests was 1 min.
The fluorescence signals from five microtips were averaged
after the capture.
Based on the results of the frequency study, tests for
immersion time were conducted for selected electric fields.
The electric fields were 100 Hz AC, 10 MHz AC, 1 kHz
biased AC, and 10 MHz biased AC. A DC field was not
chosen because it could have EP similar to 100 Hz. For AC
fields, 100 Hz was chosen because the fluorescence signal
was highest in the frequency study. 10 MHz was also
chosen to observe how DEP affected the concentration
without EOF. For biased AC fields, 1 kHz was chosen
because the highest fluorescence signal was observed in the
frequency test. 10 MHz was also chosen to observe the
DNA capture using DEP and EP. The immersion time
periods were varied from 1 up to 8 min until a steady
decrease of fluorescence signals was measured.
A second set of experiments was conducted with the
microtips that were coated with PLL (0.1% w/v in water;
Sigma-Aldrich P8920) on top of the gold layer of microtips. The microtips were immersed into PLL solution for
5 min and were cured for 2 min at 200°C. The cationic
polymer layer could retain the negatively charged DNA on
microtip surface. The microtips are referred as ‘PLL-coated
microtips’ in the paper. Using an AC field at 10 MHz, the
immersion time was varied from 1 to 10 min, and fluorescence signal was measured. A similar experiment was
also conducted for a biased AC field of 10 MHz varying
the immersion time until the fluorescence signal continuously decreased. To assess the reproducibility using PLLcoated microtips, additional experiments for 10 MHz AC
and 10 MHz biased AC fields were conducted at 4- and
10-min immersion times, respectively. The experimental
parameters for non-coated- and PLL-coated microtips are
summarized in Tables S2 and S3 in the supplementary
information, respectively. The AC voltages for various
frequencies are listed in Table S4. For biased AC fields, a
bias of 3 V DC was added to AC potentials in serial
connection.
To estimate the total amount of captured DNA using
microtips, ten sequential immersions from the same well
were conducted using ten different microtips. A 10 MHz
biased AC field at 20 Vpp was used to capture k-DNA. The
immersion time was 10 min for each DNA capture. Due to
the evaporation of the solution during the experiments,
10 lL of DNA–PicoGreen mixture was refilled into the
well after each capture.
Microfluid Nanofluid
3.1 Experiments on non-coated microtips
3.1.1 Characterization for electric fields
DNA capture was studied for DC-, AC- and biased AC
electric fields. Different frequencies for both AC and
biased AC ranging from 100 Hz to 10 MHz were studied.
When an electric field was applied, DNA was concentrated on the microtips. In the case of DC field, only 3 V
was applicable without causing bubble formation at the
microtips.
Figure 3 shows the digitized fluorescence signals on
non-coated microtips for 1 MHz AC and 1 MHz biased
AC. The signal was observed only on the edges of the base
and in the rectangular trenches. When the microtips were
withdrawn from the solution, the DNA at the tip part was
removed from the microtips due to the capillary force.
Hence, these images may not be directly compared with the
simulation results. On the base part of the microtips, the
attracted DNA was concentrated along the meniscus of the
trapped solution on the microtip surface because of evaporation of the solution drop (Deegan et al. 1997). The
fluorescence signal was also observed at the rectangular
trench on the Si chip part. The rectangular trench was
generated during the RIE step in the fabrication of microtips. The 1 lm-deep trenches generated capillary action to
further concentrate captured DNA along the rectangular
edges. Overall, the bias increased the signal magnitude.
Without an electric field, about 0.5 lL of the solution
containing DNA was captured on the microtips due to
capillary action. The fluorescence amplitude without an
electric field was 2,299 in Fig. 4a. In the frequency study
using AC fields, the highest fluorescence signal was measured at 100 Hz (Fig. 4a). As the frequency increased, the
signal decreased steadily.
In the case of biased AC, the highest fluorescence signal
was observed at 1 kHz (Fig. 4b). Overall, higher fluorescence signals were observed between 100 Hz and 10 kHz,
which was consistent with previous results using planar
electrodes (Wong et al. 2004). In comparison with the
fluorescence signal of an AC field, that of a biased AC field
was increased only at the frequency of 10 MHz (Fig. 4b).
At such a high frequency, DNA concentration could be
driven solely by DEP while the EOF component was
(a) 8,000
7,000
Fluorescence signal
3 Experimental results and discussions
6,000
5,000
4,000
3,000
2,000
1,000
0
Frequency
(b)
8000
Fluorescence signal
7000
6000
5000
4000
3000
2000
1000
0
Frequency
Fig. 3 Digitized fluorescence signals of captured DNA on noncoated microtips. a DNA on a microtip at an AC field (20 Vpp at
1 MHz). b DNA on a microtip at a biased AC field (20 Vpp at 1 MHz
with 3 V bias)
Fig. 4 Fluorescence signals upon various frequencies of AC- and
biased AC fields for non-coated microtips. a Fluorescence signals at
AC fields for the immersion time of 1 min (n = 5). b Fluorescence
signals at biased AC fields for the immersion time of 1 min (n = 5).
Error bars represent standard deviation
123
Microfluid Nanofluid
negligible. By adding a DC field, EP force could attract and
capture more DNA.
3.1.2 Characterization for immersion time
Figure 5 shows the immersion time responses for noncoated microtips. For AC fields, the immersion time was
varied for the selected frequencies of 100 Hz and 10 MHz.
100 Hz was the frequency yielding high-fluorescence signals. 10 MHz was selected to study the effect of DEP
without EOF. The fluorescence signals for AC fields were
fluctuating regardless of immersion time. For biased AC
fields, the fluorescence signal for 1 kHz was decreased
with increase of the immersion time while that for 10 MHz
biased AC was increased up to 4 min. The fluorescence
signal for 10 MHz biased AC was decreased after 4 min.
For both AC- and biased AC fields below the frequency of
10 MHz, the fluorescence signal was reduced when the
immersion time was greater than 1 min. The reduced signal
could be caused by EOF or capillary action that might
remove the attracted DNA from microtip surface. The
increase and subsequent decrease of DNA concentration
according to time were observed by Du et al. (2008).
3.2 Experiments with PLL-coated microtips
To improve the yield of DNA capture, PLL-coated microtips
were used. With PLL-coated microtips, both 10 MHz AC
and 10 MHz biased AC fields were studied as a function of
immersion time. The captured pattern of DNA on noncoated- and PLL-coated microtips was compared.
3.2.1 Characterization for immersion time
Figure 6 shows the immersion time responses of PLLcoated microtips for both 10 MHz AC- and 10 MHz biased
AC fields. Overall, the fluorescence signals of PLL-coated
microtips were significantly greater than those of noncoated microtips. The highest fluorescence signal for a
10 MHz AC field was observed at 4 min of immersion
time. The fluorescence signals dropped significantly after
4 min. For a biased AC field, the signal was saturated at
10 min. Therefore, the bias could accumulate the concentrated DNA by electrostatic force on a PLL layer.
3.2.2 Captured pattern of DNA on non-coated- and PLLcoated microtips
The digitized fluorescence signals for an AC field of 4 min
immersion are shown in Fig. 7a and b for non-coated- and
PLL-coated microtips, respectively. For PLL-coated
microtips, DNA was located along the microtip edges of a
high electric field strength because DNA was attracted by
DEP. DNA was also found at the trenches because DNA
was partially aggregated by capillary action. However, for
non-coated microtips, fluorescence signals were mainly
found at the rectangular trenches but not at the edges due to
capillary action (Fig. 7a).
Overall, the absence of fluorescence signal at the tip part
of the non-coated microtips showed that the capillary
action was dominant in removing the concentrated DNA.
Fluorescence signals were also observed in the rectangular
trenches because of capillary action. PLL-coated microtips
30,000
25,000
Fluorescence signal
100 Hz AC
14,000
10 MHz biased AC
Fluorescence signal
12,000
1 kHz biased AC
10 MHz AC
10,000
8,000
6,000
20,000
15,000
10,000
10 MHz AC
4,000
5,000
10 MHz Biased AC
2,000
0
0
0
0
5
10
5
10
15
20
25
Immersion Time (minutes)
Immersion Time (minutes)
Fig. 5 Immersion time tests of AC- and biased AC fields for noncoated microtips (n = 5). Error bars represent standard deviation
123
Fig. 6 Immersion time tests of PLL-coated microtips for 10 MHz
AC and 10 MHz biased AC (n = 5). Error bars represent standard
deviation
Microfluid Nanofluid
Fig. 7 Fluorescence signal image of non-coated- and PLL-coated
microtips after capture of k-DNA at the immersion time of 4 min.
a Non-coated microtip at 10 MHz AC field (signal magnitude 445).
b PLL-coated microtip at 10 MHz AC field (signal magnitude
13,722). c Non-coated microtip at 10 MHz biased AC field (signal
magnitude 13,314). d PLL-coated microtip at 10 MHz biased AC
field (signal magnitude 13,763)
could hold the captured DNA along the microtip edges
where DEP was highest (Fig. 7b). This is also shown in the
simulation results of DEP in Fig. 2c, where the particles
are attracted to the edges of the tip part.
For biased AC fields, digitized fluorescence images of
non-coated- and PLL-coated microtips are shown in
Fig. 7c and d, respectively. For non-coated microtips,
fluorescence signals were mainly observed at the trenches
and the base part of microtips (Fig. 7c). A DC field
introduced EP in addition to DEP, enabling the retention of
more attracted DNA. For PLL-coated microtips (Fig. 7d),
fluorescence signals were observed both at tip- and base
parts of microtips. As observed earlier, non-coated microtips could not retain the DNA attracted to the tip part while
the PLL-coated tips retained the captured DNA as attracted. For PLL-coated microtips, a significant portion of the
DNA was captured onto the edges of the microtips while
the fluorescence signal was relatively small in the trenches.
Comparing with the simulation results (Fig. 2), a coinciding pattern of EP and DEP could be observed on the tip
part.
Interestingly, when a biased AC field was applied, the
fluorescence amplitudes for both non-coated- and PLLcoated microtips were very similar for 4 min immersion.
The average amplitudes for non-coated- and PLL-coated
microtips for 10 MHz biased AC were 11,270 (Fig. 5) and
11,554 (Fig. 6), respectively. For PLL-coated microtips,
however, the fluorescence signals for a 10 MHz AC field
decreased rapidly after 4 min of immersion. For a
10 MHz biased AC field, the fluorescence signal was
saturated within an error range at immersion time of
10–20 min.
To assess the reproducibility of the results, three sets of
additional experiments were conducted using PLL-coated
microtips (Fig. 8). Both AC- and biased AC fields were
used to capture k-DNA. For 10 MHz AC, the capture was
performed at 4-min immersion time. For 10 MHz biased
AC, the capture was performed at 10-min immersion time
when the fluorescence signal was saturated. On the average,
a biased AC field showed a higher yield than an AC field,
which is consistent with the observation of Yokokawa et al.
(2010).
In summary, a PLL layer was beneficial to retain captured DNA. The capture of DNA was dependent upon
electric fields. DC bias improved the yield of DNA capture.
Under DEP, DNA was attracted toward the edge of PLLcoated microtips (Fig. 7b). Under both DEP and EP, DNA
was attracted to both edge and surface of PLL-coated
microtips (Fig. 7d). These observations were consistent
with the numerical results in Fig. 2.
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Microfluid Nanofluid
16,000
25,000
14,000
12,000
Fluorescence signal
Fluorescence signal
20,000
15,000
10,000
5,000
10,000
8,000
6,000
4,000
2,000
0
10 MHz AC
10 MHz biased AC
Fig. 8 Reproducibility tests for AC- and biased AC fields using PLLcoated microtips. The immersion times are 4 and 10 min for AC- and
biased AC fields, respectively. The error bars are standard deviation
(n = 3)
3.2.3 Consecutive capture of DNA
To estimate the capture yield using PLL-coated microtips,
ten consecutive captures in the same well containing
k-DNA were conducted. A 10-MHz biased AC field at
20 Vpp was applied for immersion time of 10 min. For the
first three captures, the fluorescence signals were significantly higher in comparison with the other consecutive
runs (Fig. 9). Assuming the total signal for 10 runs was
equal to the whole DNA amount in the sample solution,
nearly 85% of the fluorescence signals were measured from
the first three captures. An approximate correlation
between the fluorescence signal and DNA concentration in
Fig. 9 shows that 10,000 fluorescence units correspond to
0.76 lg of k-DNA.
In the microtip test, the concentration mechanism
changes depending on frequencies (Supplementary information). EP is effective between 0 and 1 kHz where the
DNA mobility is greater than the polarity change of the
potential in DC and AC fields. DEP is constantly effective
in the frequency range of 0–10 MHz. EOF is effective
between 1 kHz and 5 MHz according to our experimentaland analytical results. Considering the results, a DC field
can add EP to the phenomena of an AC field. The study of
EP, DEP, and EOF yields a guideline in designing effective
DNA concentration methods. The operational parameters
for capturing DNA from a sample mixture need further
optimization based on these results. The optimization can
depend on integrity of DNA and contaminants for downstream analysis including PCR-based methods and gel
electrophoresis.
123
0
1
2
3
4
5
6
7
8
9
10
Immersion
Fig. 9 Ten consecutive captures using PLL-coated microtips from a
single well containing k-DNA. A 10-MHz biased AC field at 20 Vpp
is applied for immersion time of 10 min. The fluorescence signals are
measured after each capture (n = 5). Error bars represent standard
deviation
4 Conclusions
The concentration of DNA onto microtips using electric
fields was studied by experiment to understand the effect of
EP, DEP and EOF. Using ‘non-coated’ microtips, high
fluorescence signals were observed at a 100-Hz AC field
and a 1-kHz biased AC field for 1 min immersion. DNA
captured on the non-coated microtips was rearranged when
removed from the solution due to capillary action. To
retain DNA on microtip surface as attracted, microtips
coated with a positively charged PLL layer were used.
With increased immersion time, ‘PLL-coated’ microtips
exhibited increased capture yield of DNA at a biased AC
field of 10 MHz. Total 85% of DNA in a 100-lL well was
captured on the PLL-coated microtips with three sequential
captures with 10 min immersion at a biased AC potential
of 20 Vpp at 10 MHz. Numerical simulation was conducted
to understand the pattern of DNA concentration under EP,
DEP and EOF onto the microtips. DNA was attracted to
both surface and edge of microtips by EP while DNA was
attracted to the edge of the microtips by DEP. EOF
transported DNA to the microtips through vortexes. Future
study will focus on rapid and simple extraction of human
genomic DNA from sample mixture for downstream
analysis.
Acknowledgments We acknowledge the support of NSF STTR II
award (0956876). JC, JK, HL, and WY acknowledge the support of
NSF Career (ECCS-0846454).
Microfluid Nanofluid
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