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ISSN 1759-9660
Analytical
Methods
www.rsc.org/methods Volume 5 | Number 7 | 7 April 2013 | Pages 1631–1888
1759-9660(2013)5:7;1-Z
Volume 5 | Number 7 | 2013 Analytical Methods Pages 1631–1888
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PAPER
Garcia et al.
Microfab-less micro uidic capillary electrophoresis devices
Microfab-less microfluidic capillary electrophoresis
devices†
Thiago P. Segato,
a
Samir A. Bhakta,
b
Matthew T. Gordon,
b
Emanuel Carrilho,
a
Peter A. Willis,
c
Hong Jiao
d
and Carlos D. Garcia*
b
Compared to conventional benchtop instruments, microfluidic devices possess advantageous
characteristics including great portability potential, reduced analysis time (minutes), and relatively
inexpensive production, putting them on the forefront of modern analytical chemistry. Fabrication of
these devices, however, often involves polymeric materials with less-than-ideal surface properties,
specific instrumentation, and cumbersome fabrication procedures. In order to overcome such
drawbacks, a new hybrid platform is proposed. The platform is centered on the use of 5 interconnecting
microfluidic components that serve as either the injector or reservoirs. These plastic units are
interconnected using standard capillary tubing, enabling in-channel detection by a wide variety of
standard techniques, including capacitively coupled contactless conductivity detection (C
4
D). Due to the
minimum impact on the separation efficiency, the plastic microfluidic components used for the
experiments discussed herein were fabricated using an inexpensive engraving tool and standard
Plexiglas. The presented approach (named 5
2
-platform) offers a previously unseen versatility, enabling
the assembly of the platform within minutes using capillary tubing that differs in length, diameter, or
material. The advantages of the proposed design are demonstrated by performing the analysis of
inorganic cations by capillary electrophoresis on soil samples from the Atacama Desert.
1 Introduction
Microchip –capillary electrophoresis (mchip-CE) devices are
part of a trend combining portability, miniaturization, and low
cost with high analytical performance. Considering a variety of
potentially customizable parameters including separation
media, material substrate, fabrication method, and detection
scheme, these small devices are capable of handling chemical
analyses across a broad spectrum of disciplines.
1–4
Additionally,
mchip-CE offers a number of advantages over traditional bench-
top instrumentation such as lower volumes of sample and
reagents, shorter analysis times, and the capacity to operate in a
fully automated fashion.
5,6
Microchips were initially developed from glass substrates
through photolithography and a variety of etching tech-
niques.
7–9
Although glass has almost ideal optical properties
and well-known surface chemistry, the fabrication protocols are
expensive, lengthy, and typically yield rather fragile chips that
can be ruined even by small particles clogging a channel.
Among other materials (most oen polymers) that have been
extensively utilized for fabrication,
10,11
it is worth mentioning
poly(methyl methacrylate) (PMMA),
12
polycarbonate,
13
and
poly(dimethylsiloxane).
14,15
One of the main advantages of these
polymeric materials is that they allow fast and cost-efficient
fabrication of devices by a variety of techniques including laser
ablation,
16
hot embossing,
17,18
and microwave bonding.
19
Addi-
tionally, a variety of procedures are currently available to modify
the surface of these materials.
20–25
More recently, polyester-
toner
26
and paper-based microuidic devices
27–29
have emerged
as promising platforms for microuidic applications. In both
cases, the devices can be produced by a direct-printing process
and represent one of the simplest available technologies for
microchip production (less than $0.10 per device).
Although all of these methods have yielded examples of
functioning microuidic devices, it is clear that there is a trade-
offbetween the fabrication procedure, the material, and the
microdevice performance. In other words, high-performing
devices are still expensive and low-cost devices only offer limited
analytical performance. There are also a variety of standard chips
commercially available, but these items are expensive and
inherently non-recongurable. For analysis in remote areas or
locations where microfabrication facilities are unavailable, on-
site reconguration could be required, limiting the versatility of
the standard approach utilizing glass microchips.
a
Instituto de Quimica de S~
ao Carlos, Universidade de S~
ao Paulo, S~
ao Carlos, SP, Brazil
b
Department of Chemistry, UT San Antonio, One UTSA Circle, San Antonio, TX 78249,
USA. E-mail: carlos.garcia@utsa.edu; Fax: +1 210 458-7428; Tel: +1 210 458-5774
c
NASA/Jet Propulsion Laboratory, Pasadena, CA, USA
d
HJ Science & Technology, Santa Clara, CA, USA
†Electronic supplementary information (ESI) available. See DOI:
10.1039/c3ay26392d
Cite this: Anal. Methods, 2013, 5, 1652
Received 13th November 2012
Accepted 27th January 2013
DOI: 10.1039/c3ay26392d
www.rsc.org/methods
1652 |Anal. Methods, 2013, 5, 1652–1657 This journal is ªThe Royal Society of Chemistry 2013
Analytical
Methods
PAPER
Aiming to overcome such drawbacks, a series of modular
(plug-n-play) microuidic systems have been proposed.
30–33
These devices add tremendous exibility to the design but are
typically limited to hydrodynamic pumping and most oen
require microfabrication facilities. Alternatively, this manu-
script describes a microchip-inspired platform based on 5
plastic microuidic components that serve as the injector
(1 cm 1cm0.4 cm) or reservoirs (1.9 cm 1.9 cm
0.6 cm). These components are interconnected using standard
capillary tubing, enabling in-channel detection by a wide variety
of standard techniques, including C
4
D (demonstrated in this
manuscript), as well as electrochemical or optical methods. The
resulting devices are suitable for capillary electrophoresis, avoid
the use of specic machinery or microfabrication facilities, are
inexpensive (less than $70 per re-usable setup), and are
assembled (or recongured) in just a few minutes. Such features
make this platform a worthy candidate to have a high impact in
society because it could be replicable for didactic purposes, and
it could make the eld of microuidics accessible to low-
resource communities. The capabilities of the resulting device
were demonstrated by performing an analysis of representative
inorganic cations in soil samples from the Atacama Desert.
2 Materials and methods
Reagents and solutions
All chemicals were analytical reagent grade and used as
received. The analytes (KCl, NaCl, LiCl, CaCl
2
, MgCl
2
) and
NaOH were purchased from Sigma-Aldrich (Saint Louis, MO);
(NH
4
)
2
SO
4
was purchased from MCB (Darmstadt, Germany).
Aqueous solutions were prepared using 18 MU-cm water
(NANOpure Diamond, Barnstead; Dubuque, Iowa) and were
ltered using a hollow ber lter (0.2 mm, Barnstead). The pH of
the solutions was adjusted when necessary, using either 1 mol
L
1
NaOH or 1 mol L
1
HCl (Fisher Scientic; Fair Lawn, NJ)
and measured using a glass electrode and a digital pH meter
(Orion 420A+, Thermo; Waltham, MA). The background elec-
trolyte (BGE) used for all the experiments was prepared from a
stock solution of 100 mmol L
1
2-(N-morpholino)ethane-
sulfonic acid (MES) and 100 mmol L
1
L-histidine (HIS). Stock
solutions of each analyte (10 mmol L
1
each) were prepared
daily in DI water and then diluted in the running buffer prior to
analysis.
Electrophoretic system
The system was assembled by connecting 4 PMMA reservoirs to
a central interconnect (UltemCross C360-204, Labsmith;
Livermore, CA) via standard silica capillary tubing (50 mm ID,
360 mm OD; Polymicro Tech; Phoenix, AZ). The solution reser-
voirs were fabricated by cutting squares of 1.9 cm 1.9 cm from
standard layers of PMMA (1/1600 thick) using a computer-
controlled engraver (Gravograph IS400, Gravotech; Duluth,
GA).‡These squares were denoted as “top”and “bottom”. While
the “bottom”layer consists of a at piece of PMMA, the “top”
unit has a hole drilled into the PMMA that serves as the well for
sample/buffer/waste and also contains a ne channel to connect
the capillary tubing. In order to avoid leaks, the capillary tubing
was rst glued to the “top”piece with “PMMA glue”(PMMA
dissolved in chloroform) and then thermally sealed to the
“bottom”piece at 120 3C for 15 min. The reservoirs fabri-
cated in this manner were connected to one another via an
interconnect (1 cm 1cm0.4 cm), forming the microchip-
inspired platform schematically shown in Fig. 1. Connection
between the central square and the capillaries was performed
using four PEEK ttings (360 mm, Labsmith; Livermore, CA).
The system was assembled under water to prevent formation of
air bubbles during the application of the electrophoretic
potential. In order to calculate the volume of the inter-
connecting square, one of the pieces was sanded to half height
and visualized using a 3D laser microscope (Olympus LEXT).
The picture inset in Fig. 1 shows that the connector comprises
inner channels of approximately 250 mm, which are larger than
standard injectors specically designed for microchip applica-
tions. The dead volume of the interconnect (according to the
manufacturer) is 38 nL.
The system was washed daily with 0.1 mol L
1
NaOH,
ultrapure water, and running buffer for 30 min each. This
procedure was adopted to activate the fused silica surface and
promote higher and stable electro-osmotic ow (EOF). Between
each injection, the capillary was rinsed with running buffer for
20 min. The sample injection was performed by applying
vacuum of 70 kPa on the sample waste reservoir for a selected
period of time. Aer the application of the vacuum, the reser-
voir was replenished with running buffer. To perform the
electrophoretic separation, a selected potential was applied to
the buffer reservoir, with respect to the ground electrode, which
was placed in the buffer waste reservoir. For all experiments
involving electrophoresis, a high-voltage rack (HV-RACK-4-250,
Ultravolt; Ronkonkoma, NY) was used. The openC
4
D (https://
sites.google.com/site/openc4d/) detector was obtained from
the University of Sao Paulo in Brazil and used in the format
described by Francisco and do Lago.
34
The electronic circuitry of
the C
4
D includes a signal generator, a detection cell, a tran-
simpedance amplier, a rectier, a low-pass lter, and an
Fig. 1 Pictureof the 5
2
platformassembled from the 5 squares and capillaries.Inset
showing a microphotograph of the central interconnect (1.28 mm 1.28 mm).
‡Alternatively, these pieces can be fabricated with a standard saw and drill set.
This journal is ªThe Royal Society of Chemistry 2013 Anal. Methods, 2013, 5, 1652–1657 | 1653
Paper Analytical Methods
analog-to-digital converter. The arrangement includes two
2 mm coiled copper electrodes separated by a gap of 0.51 mm.
Data acquisition was obtained using the Swing CE soware
supplied with the openC
4
D and the experimental conditions for
the detector include using a sine wave with a frequency of
1.1 MHz with an amplitude of 4 V (peak-to-peak).
Soil samples
Soil samples were collected from the Atacama Desert (northern
Chile) in June 2005. Due to the extreme aridity of this region
(experiencing less than a centimeter of precipitation per
decade) and the chemical/mineralogical composition of the
surface materials present, these samples are well-known
analogues to Martian regolith. All samples were GPS-coded,
cached on site, placed in sealed vials, and maintained in a
sterile desiccator until used. Details related to the collection
sites for the samples used in this manuscript are included in
Table 1. For sample preparation using our proposed platform,
an aliquot of 10 mg of soil was added to 10 mL of running buffer
and stirred in an ultrasonic bath for 10 min. One mL of this was
centrifuged at 13 400 rpm for 15 min and the supernatant was
injected hydrodynamically in the electrophoretic system. Addi-
tional information related to these samples, the collection sites,
and corresponding mchip-CE analysis for organic species can be
found elsewhere.
35
In order to verify the results obtained with the proposed
platform, the elemental composition of the soil samples was
analyzed by energy dispersive X-ray spectroscopy (EDX). The
experiments were performed by placing an aliquot of the
sample in a Hitachi High Resolution 5500 SEM Scanning elec-
tron microscope, equipped with an XFlash 4010 Si dridetector
(Bruker AXS; Billerica, MA) and operated at 30 kV. The data,
collected over an approximate area of 50 mm
2
, was analyzed with
built-in soware (Quantax Espirit 1.9).
Safety considerations
The high voltage power supply and associated open electrical
connections should be handled with extreme care to prevent
electrical shock.
3 Results and discussion
Although we foresee a wide number of potential applications,
the goal of this manuscript was to demonstrate the character-
istics and advantages of the proposed platform through the
analysis of inorganic cations in soil samples. Key factors
affecting the performance of the platform were investigated and
are discussed.
Effect of buffer solution
Similar to conventional CE, the buffer solution has a signicant
effect on the analysis because it inuences the total charge of
analytes, the magnitude of the EOF, and the generation of Joule
heating (which could affect resolution). Furthermore, as previ-
ously reported, the buffer system also has a considerable effect
on the signal/noise obtained in C
4
D.
36,37
Therefore, an equi-
molar MES and HIS buffer, pH ¼6.1 + 2 mmol L
1
18-crown-6
was selected based on previous literature reports.
38–40
Although
this background electrolyte was selected as a simple solution to
demonstrate the functionality of the system, alternative condi-
tions
41,42
could be selected to provide improved the resolution, if
needed.
The effect of the buffer concentration on the separation
and detection was evaluated in the 10–50 mmol L
1
range (for
each component) by injecting a standard solution containing
100 mmol L
1
of the six cations diluted in the same buffer. As
observed in Fig. 2, concentrations $30 mmol L
1
MES and
30 mmol L
1
HIS yielded signicant increases in the overall
analysis time but enabled the identication of all six selected
cations. This behavior can be attributed to a decrease in the
effective charge of the surface of the capillary, shielded by the
increasing concentration of ions in the background electro-
lyte. It is also important to note that, within the investigated
range of buffer concentrations, the signal/noise was not
adversely affected. Considering these results, and as a balance
between resolution and analysis time, 30 mmol L
1
MES and
30 mmol L
1
HIS pH ¼6.1 (+3 mmol L
1
18-crown-6, vide
infra) was selected as the optimum background electrolyte
and used for the rest of experiments described in this
manuscript.
Table 1 Information related to the mineralogy and location sites of soil samples collected from the Atacama Desert
Label Mineralogy Depth Latitude Longitude Elevation
AT40B1-08 Exposed duracrust <1 cm S2403.6290W6952.09201081 m
AT44B1-08 Exposed duracrust <1 cm S2403.6510W6952.10201075 m
AT54A1-08 Duracrust 2–3 cm S2403.6800W6952.09801055 m
Fig. 2 Effect of the concentration of equimolar MES and HIS buffer (pH ¼6.1) on
the separation of the selected cations, at 100 mmol L
1
each. Other conditions:
3 mmol L
1
18-crown-6, E
SEP
¼10 kV, capillary length ¼60 cm, effective length ¼
56 cm, 5 s hydrodynamic injection.
1654 |Anal. Methods, 2013, 5, 1652–1657 This journal is ªThe Royal Society of Chemistry 2013
Analytical Methods Paper
Effect of buffer additives
It is well-known that the separation of some cations can be
optimized by the addition of 18-crown-6 to the running elec-
trolyte.
43
The main reason for this is that 18-crown-6 is able to
form inclusion complexes with several inorganic cations, which
affects the effective electrophoretic mobility of the cations and
imparts selectivity to the separation step.
44
Consequently, the
effect of the concentration of 18-crown-6 on the separation was
investigated in the 0–5 mmol L
1
range, using 30 mmol L
1
MES and 30 mmol L
1
HIS buffer as the running electrolyte. The
results are summarized in Fig. 3. In line with previous reports,
where the stability complex constant of K
+
with 18-crown-6 (log
K
s
¼2.1) was reported to be signicantly higher than that of
NH
4+
(log K
s
¼1.01),
43
sequential additions of 18-crown-6 only
inuenced the migration time of the peak corresponding to
potassium, enabling its separation from NH
4+
with as little as
1 mmol L
1
. In order to maximize the separation and minimize
the possibility of co-migration with other species present in the
target samples, a concentration of 3.0 mmol L
1
18-crown-6 was
selected and used for all the experiments described in this
manuscript.
It is also important to highlight that Tanyanyiwa and
Hauser
38
stated that although it is possible to achieve complete
resolution of the ammonium and potassium peaks using long
capillaries and concentrations of 18-crown-6 as low as 1 mmol
L
1
, it would not be possible to resolve them on glass chips with
less than 2 mmol L
1
. In such cases,
45
concentrations as high as
7.5 mmol L
1
would be required. The results shown in Fig. 3
(where baseline separation of the ammonium and potassium
peaks was achieved) strongly indicate that the proposed plat-
form is able to offer not only the advantages of most micro-
uidic systems but also a performance that is comparable to
standard bench-top instruments.
Effect of capillary length
Generally, increasing the effective length of the capillary is
benecial to separation efficiency and resolution of separations
under diffusion-limited conditions.
46,47
Although replacing the
capillary in most commercial bench-top systems is not
complicated, the operation must be manually performed
(reassembling the capillary cartridge) and is oen limited to
xed increments.
47
At the microchip-scale, changing the length
of the separation channel is signicantly more challenging. For
that reason, most designs include separation channels in the
range of a few centimeters or require the implementation of
serpentine geometries which can induce dispersion.
48
There-
fore, in order to demonstrate the possibility to change and
customize the capillary length in the proposed design, the effect
of four different capillary lengths on the separation was inves-
tigated: 15 cm, 30 cm, 45 cm and 60 cm (effective lengths of
11 cm, 26 cm, 41 cm, and 56 cm, respectively). As observed in
Fig. 4, signicant increases in the analysis times and separation
efficiencies were obtained when the separation was performed
using longer capillaries. In the case of the 60 cm-long capillary
(using the conditions described in Fig. 4), an average of 17 000
plates per m was obtained (ranging from 7 300 plates per m for
NH
4+
to 27 000 plates per m for Na
+
). The resolution, calculated
for the 60 cm capillary and the conditions described in Fig. 4,
ranged from 1.1 (for the peaks corresponding to Ca
2+
and Na
+
)
to 3.4 (for the peaks corresponding to Mg
2+
and Li
+
). Based on
these results, 60 cm was selected as the optimum length and
was used for all the subsequent experiments described in this
manuscript.
Effect of injection time
At any scale, obtaining a reproducible and representative
sample injection has been deemed paramount for quantitative
analytical applications.
49
Although injections in microchips
routinely rely on some variant of electrokinetic injection
2,50
(due
to its simplicity), its performance can be signicantly affected
by EOF velocity, sample bias, sample conductivity, and elec-
trolysis effects. Since these issues are particularly important for
the analysis of small ions with high electrophoretic mobility,
hydrodynamic injection (applying vacuum on the sample waste
reservoir, for example) was selected for the experiments
described in this manuscript. Besides being reproducible, this
method avoids the use of additional hardware and is signi-
cantly simpler than previously proposed sample injection
methods.
51,52
Although preliminary experiments were per-
formed using a soldering iron pump (1700, Paladin Tools, USA),
Fig. 3 Effect of the concentration of 18-crown-6 on the separation of the
selected cations at 100 mmol L
1
each. Conditions: 30 mmol L
1
MES and 30 mmol
L
1
HIS, E
SEP
¼10 kV, capillary length ¼60 cm, effective length ¼56 cm, 5 s
hydrodynamic injection.
Fig. 4 Effect of the capillary length on the separation of the selected cations at
100 mmol L
1
each. Conditions:E
SEP
¼10 kV, 30 mmol L
1
MES and 30 mmol L
1
HIS + 3 mmol L
1
18-crown-6 as running buffer; 5 s hydrodynamic injection.
Original electropherograms included as ESI.†
This journal is ªThe Royal Society of Chemistry 2013 Anal. Methods, 2013, 5, 1652–1657 | 1655
Paper Analytical Methods
the house vacuum line (70 kPa) was used for the experiments
herein described. The selected method yielded comparable
results while enabling the control of the injection time. Next,
the effect of injection time on signal magnitude was investi-
gated over a range of 1–8 s. As it can be observed in Fig. 5,
signicant increases in signal (proportional to the injection
time) were obtained in the 1–6 s range. As further increases in
injection time (in the 6–8 s range) did not yield improvements
in signal/noise, 6 s was adopted as the optimal time for injec-
tion. Notably, no signicant peak distortion was observed
within the selected times, suggesting that only the center of the
interconnect is being lled and that the sample plug is being
pinched with ow from the separation channel and buffer
reservoir.
Analytical gures of merit
Using the optimized conditions for the separation and detec-
tion (10 kV as the separation potential, 30 mmol L
1
MES and
30 mmol L
1
HIS pH ¼6.1 + 2 mmol L
1
18-crown-6 as running
buffer; 6 s hydrodynamic injection, and 60 cm capillary), linear
relationships between the concentration and the C
4
D signal
were obtained for the six cations analyzed up to 500 mM. At
higher concentrations, signicant co-migration of the ions was
observed, precluding the analysis. The limit of detection for
each cation was estimated using a signal/noise ratio of at least 3,
obtained upon the injection of samples under the optimum
conditions. The results corresponding to each calibration curve
are summarized in Table 2.
The proposed system provided similar sensitivity to other
microuidic systems coupled to C
4
D
53
and conventional capil-
lary electrophoresis systems when coupled to either indirect UV-
Vis
54
or conductivity detection.
55
Although these values were
considered appropriate for the target application, alternative
congurations can be selected to further improve the
sensitivity.
56
Analysis of soil samples
The identication and quantication of the components of
each sample were performed by comparing the electrophero-
grams obtained with standard solutions to those obtained with
the corresponding samples under the optimal conditions. A
main peak at 8.9 min was observed in all samples (data available
in the ESI†), with a migration time matching that of Ca
2+
. In two
samples (AT40B1-44 and AT40B1-54), it was also possible to
identify a second peak with much lower intensity that was
assigned to Na
+
. Based on the peak intensity, the amount of
Ca
2+
was 20.4, 44.1, and 78.2 mg of Ca
2+
per gram of soil in the
samples marked as ATB1-40, ATB1-44, and ATB1-54, respec-
tively. These ndings are in agreement not only with previous
reports describing the abundance of CaSO
4
in such samples,
but also with the results obtained by EDX (see ESI†).
4 Conclusions
A new hybrid device, based on the use of 5 plastic microuidic
components, was fabricated quickly and inexpensively. Addi-
tionally, the new platform bypasses some of the traditional
problems involving microchip fabrication, including large/
specic machineries and lengthy assembly times. The platform
itself is highly versatile and can be coupled with a number of
inline detection methods, such as C
4
D or UV-Vis. The variable
length of the separation channel adds another advantage in that
the separations can be adjusted if necessary. The simplicity of
the platform allows for customization in terms detection,
capillary length, injection type (gated and pinched electroki-
netic or hydrodynamic), and reservoir volumes. This device is
an attractive approach for portable analytical instrumentation
capable of performing rapid analyses, as demonstrated through
the conductometric detection of inorganic cations.
Acknowledgements
The authors gratefully acknowledge the nancial support
provided by STTN/NASA (NNX12CG20P-1), The University of
Texas at San Antonio and the National Institutes of Health
through the National Institute of General Medical Sciences
(1SC3GM081085, 2SC3GM081085), the NASA Astrobiology
Science and Technology Development (ASTID) Program
(104320), and the Research Centers at Minority Institutions
(G12MD007591).
Fig. 5 Effect of the injection time on the signal magnitude. Hydrodynamic
injections were performed applying vacuum (70 kPa) on the SW reservoir for the
selected times. Migration order is as shown in previous figures.
Table 2 Migration time (t
M
), sensitivity, coefficient of determination (R
2
), and
calculated limit of detection (LOD) corresponding to the analysis of the selected
inorganic cations under optimal conditions
Cation t
M
(min)
Sensitivity
(AU mmol
1
L) R
2
LOD
(mmol L
1
)
NH
4+
7.1 0.1 7.7 0.2 0.99 7
K
+
8.2 0.1 4.1 0.1 0.99 53
Ca
2+
8.9 0.1 4.1 0.1 0.99 38
Na
+
9.3 0.1 4.9 0.2 0.99 57
Mg
2+
9.6 0.1 9.9 0.5 0.98 45
Li
+
11.0 0.1 3.2 0.2 0.98 91
1656 |Anal. Methods, 2013, 5, 1652–1657 This journal is ªThe Royal Society of Chemistry 2013
Analytical Methods Paper
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