Durable Hydrophilic Microchannels with Controlled Morphology by the

ARTICLE pubs.acs.org/ac

Durable Hydrophilic Microchannels with Controlled Morphology by the Direct Molding Method Tae-Ho Yoon,†,‡ Ming Li,† Lan-Young Hong,† Jinkee Lee,§ and Dong-Pyo Kim*,†,|| )

†

Center of Applied Microfluidic Chemistry and Graduate School of Analytical Science and Technology, Chungnam National University, Daejeon, 305-764, South Korea ‡ Convergence Technology Development Center, Electronics and Telecommunications Research Institute, Daejeon, 305-700, South Korea § School of Engineering, Brown University, Box D, 184 Hope Street, Providence, Rhode Island 02912, United States

bS Supporting Information ABSTRACT: We present a laminated hydrophilic microchannel fabricated by a direct micromolding and facile bonding technique with a hydrophilic organic-inorganic hybrid (HP) resin, which is built into two supporting parallel polydimethylsiloxane (PDMS) substrates. This approach allows one to shape the cross-section of the hydrophilic channel into a regular rectangular or geometrically complicated mouth-throat structure. Spontaneous self-wetting flow of water occurs when water is introduced to the HP microchannel with rectangular cross-section without the aid of mechanical pumping or electric field. The capillary flow velocity can be controlled by varying the channel size. The channel surface behaves like a glass surface in terms of zeta potential for pH larger than 3, and the electro-osmotic flow (EOF) velocity can be as high as 4.7
10-4 cm2/V 3 sec at pH 9.0. The capillary electrophoresis (CE) module made of the HP resin for amino acid or DNA separation is demonstrated to yield a higher resolution in a shorter retention time with repeatability and long-lasting durable performance with antifouling property, when compared with PDMS CE.

E

lastic silicone resin, in particular polydimethylsiloxane (PDMS), has widely been used in the field of microfluidics because of the ease with which microchannels can be fabricated by soft lithography. However, the hydrophobic nature of PDMS with water contact angle over 100
makes it difficult to transport polar solutions into microchannels or nanochannels.1,2 Proteins, such as lysozyme, are easily adsorbed on a hydrophobic surface by nonspecific binding, which causes fouling on the channel surface, especially in the field of capillary electrophoresis or electrochromatography.3-5 In the fields of biology and fuel cell, hydrophilic surface is preferred for good transportation of proton ions.6,7 It is not surprising, therefore, that various approaches have been taken to promote hydrophilicity. Oxygen plasma treatment of the PDMS surface has been the most frequently used approach; however, the drawback is low durability with only temporary relief for several minutes up to a day.8,9 Surface coating methods with an ultrathin layer have also been reported such as silanization by self-assembly,10 bulky coating of proteins or lipid,11 thermal coating of hydrophilic organic polymer,12,13 and sol-gel condensation of alkoxide precursors in the PDMS micropore,14,15 but these methods are difficult to optimize and induce channel deformation. Furthermore, hydrophilic glass-like microchannel fabricated by thick silica sol-gel wet coating on PDMS microchannel was intrinsically shrunken to result in insufficient durability for practical use.16 The hydrophilic r 2011 American Chemical Society

material coating causes deformation of the premade rectangular cross-section of channel to round shape due to preferential filling at the edges. This problem makes it difficult to fabricate sophisticated channel pattern on nano- and microscale.17 In the field of bioengineering, monolithic polyethylene glycol (PEG) microchannel fabricated by soft lithography is widely used because of hydrophilicity, biocompatibility, and the capability of preserving moisture, but it has low mechanical stability and is vulnerable to swelling in an aqueous environment.18 Herein, we report novel hydrophilic microchannels with controlled cross-sectional shape that are fabricated by direct micromolding of organic-inorganic hybrid resin with antifouling property. The hydrophilic organic-inorganic hybrid (HP) microchannel supported on two parallel PDMS slabs allows spontaneous self-wetting capillary flow and electro-osmotic flow (EOF). The capillary electrophoresis (CE) was carried out for the separation of an amino acid mixture in a rectangular channel and DNA in a microstructured channel, respectively, for comparison with the PDMS-based system. The results suggest that the durable HP microchannels are a useful microfluidic module with reliable CE performance and they could become an alternative to costly glass microfluidics. Received: August 17, 2010 Accepted: January 18, 2011 Published: February 24, 2011 1901

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Figure 2. Model diagram for capillary flow behavior in a rectangular microchannel.

Figure 1. Fabrication scheme of laminated HP-PDMS microfluidic chip.

’ MATERIALS AND METHODS Preparation and Pattern Replicability of Hydrophilic Resin. The HP resin was synthesized using tetraethoxysilane Si(OC2H5)4 (TEOS), titanium chloride TiCl4, 4,5-dihydroxy-m-benzenedisulfonic acid disodium salt, (Tiron, (OH)2C6H2(SO3Na)2), 3-(methacryloyloxy)propyltrimethoxy-silane (MPTMS), and polyethylene glycol dimethacrylate (PEG-DMA) with the ratio of TEOS/TiCl4/ (OH)2C6H2(SO3Na)2/MPTMS/PEG-DMA = 1.0:0.1:0.025:0.88: 0.416 (molar ratio) by following the protocol reported in our previous report (see Figure S1 of Supporting Information).19,20 In order to evaluate the nanoscale replicability of the HP resin, blue-ray disk (BD, 23G, Maxwell) was used as a nano master. A nano patterned master with 100 and 900 nm height and 2.5 μm pitch on Si wafer (3-in. P-type), fabricated by two step lithography, was also used.21 The diluted HP resin was spin-coated on the masters silanized with trichloro(perfluorooctyl)silane (Sigma Aldrich, USA) at 1000 rpm to render 5 μm thickness. A PDMS mixture (prepolymer/curing agent = 10:1 in weight) was poured on the uncured HP resin layer to 5 mm thickness in forming a supporting PDMS slab. They were then cured together at 80
C for 6 h to obtain a double layered HP-PDMS composite. After consolidation, the laminated HP-PDMS composite was peeled off from the master cautiously and then postcured at 100
C for 5 h. The resulting morphologies were examined by scanning electron microscopy (SEM, XL30SFEG, PHILIPS) and atomic force microscropy (AFM, X-100, PSIA). HP Microchannel in Laminated PDMS by Direct Molding. The hydrophilic microchannel with rectangular cross-section between two supporting PDMS slabs was fabricated by a stepwise process including preparation of master, molding, and channel sealing steps as shown in Figure 1. First, the patterned masters with convex relief structures were prepared by conventional photolithography that was conducted using SU8-50 photoresist (Micro Chem, USA).22 The dimensions of convex relief structure on Si wafer were controlled to render different heights of 2, 5, and 10 μm and identical width (50 μm) and length (30 mm) by varying the viscosity (range 50-310 cP) of SU8 photoresist diluted with propyleneglycolmethyletheracetate (Sigma-Aldrich, USA) and the rotational frequency in the range of 1000-2000 rpm. Second, the HP resin was spun-coated on the fabricated SU-8 master to become a 15 μm thickness coating. Subsequently, PDMS precursor was added on HP resin with a total thickness of 5 mm in forming a supporting slab. They were then cured together at

80
C for 6 h to obtain a double layered HP-PDMS composite. After consolidation, the HP-PDMS composite structure was peeled off from the master cautiously,then holed for tube connection, and postcured at 100
C for 5 h. Another flat and smooth HPPDMS slab was also prepared to use as a lid with the identical protocol for spin-coating and curing. Prior to the bonding, each HP surface was treated with oxygen plasma (Tesla coil, Western Electrics, USA) for 20 s at ambient condition. Then, they were aligned and placed on a hot plate at 80
C for 2 h, which is the identical condition as for the PDMS to PDMS bonding process.23 Finally, the leak-free and transparent HP-PDMS composite microfluidic device was built by bonding the HP-PDMS channel with the cured HP-PDMS lid. The holes for inlet and outlet were 1 mm. For the comparison, the conventional PDMS microchannel was also fabricated using the identical master. In order to fabricate alternative HP-PDMS composite microchannel with a mouth-throat microstructure, the hemispherical SU-8 master was fabricated by overexposure photolithography with a photomask having a series of 20
10 μm2 sized square windows (See Figure S2 of Supporting Information).24 Eventually, a series of hemispherical microchannel interconnected mouth-throat microstructure on PDMS substrate was obtained by direct molding and a consecutive bonding step with the identical protocol for rectangular HP-PDMS microchannel as shown in Figure 1. The zeta potential of the smooth HP resin film with 10 μm thickness (cured at 100
C for 5 h) was measured by an electrophoresis machine (ELS-Z, Otsuka Electronics, Co. LTD, Korea) for various pH values. EOF was measured by the current-monitoring method.25 Reservoirs at both sides of 20 mm long straight channel were filled with 20 mM phosphate buffer solution (PBS). Upon applying 120 V/cm, the buffer in the reservoir at higher voltage was replaced by 19 mM PBS when the current became stable. Average EOF values were obtained from the values measured three times. To investigate the capillary flow of the HP-PDMS laminated composite microchannel both experimentally and theoretically, a simple rectangular channel model as shown in Figure 2 was adopted from the literature with the equations that follow.26 The capillary pressure, ΔP, in rectangular channel of width w and height h is given by the following Young-Laplace equation:
 2 2 þ ΔP ¼ σ 3 cos θ ð1Þ w h where σ is the surface tension between air and water and θ is the contact angle of meniscus (at 20
C). Total capillary force, Fcap, is given by
 1 1 þ wh ¼ 2σcos θðw þ hÞ Fcap ¼ ΔPwh ¼ 2σ 3 cos θ w h ð2Þ 1902

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The volumetric flow rate for the rectangular channel Q is given by Q ¼ ÆuæA ¼






0

¥ X

1

wh ΔP @ 48h tan hðiπw=2hÞA 1- 5 s π w i ¼ 1, 3, 5: : : i5 12μ 3

ð3Þ where, Æuæ is the average velocity, A is the cross-sectional area, μ is the viscosity, which is taken as that of water, and s is the distance from the flow entrance. It follows from eq 3 that the average velocity is given by
 ds h2 σ 3 cos θ 1 1 ¼ þ Æuæ ¼ dt w h s 6μ 0 1 ¥ X 48h tan hðiπw=2hÞ @1 A ð4Þ π5 w i ¼ 1, 3, 5::: i5 Capillary Electrophoresis with HP Microchannels. For CE separation performance of the HP-PDMS composite microchip with rectangular cross-section, a mixture of fluorescein isothiocyanate (FITC) labeled amino acids, arginine (Arg), phenylalanine (Phe), glutamine acid (Glu), and aspartic acid (Asp), were used. To label the FITC in each amino acid, 1 mL of a 20 mM solution of each amino acid in carbonate buffer (pH 9.0, 0.2 M) was added to 250 μL of 30 mM FITC solution in acetone containing 0.001% (v/v) pyridine. The mixture was allowed to react in darkness at room temperature for 24 h and then diluted in separation buffer to obtain the desired concentration of labeled product. For the CE analysis, electrophoresis buffer was prepared with 2 mM borate buffer solution, pH 8.8. The separation channel is (gated injection in cross channel chip) 25 mm long, 20 μm wide, and 10 μm high. Applied field strength was 300 V/cm. For the detection, laser-induced fluorescence was measured by a 473 nm diode-pumped solid-state laser (20 mW, Shanghai Dream Laser) and photomultiplier tube module (H5784, Hamamatsu). For the CE experiment of the mouth-throat HPPDMS microchannel, a DNA mixture of 5 different sizes (A: 3530 bp; B: 5400 bp; C: 11 500 bp; D: 21 226 bp; E: 48 502 bp of λ-DNAs conjugated with fluorescence probe YOYO (Molecular probe, Invitrogen)) was injected into the HP-PDMS microchannel. The separated DNAs by CE were detected by fluorescence microscopy (Leica, IL-2, Germany) under exposure of 491 nm wavelength of light (See Figure S3 of Supporting Information).

’ RESULTS AND DISCUSSION Characteristics and Pattern Replicability of Hydrophilic Resin. The HP resin was prepared by a binary sol-gel reaction

of TEOS-TiCl4 and subsequent sulfonation with a hydrophilic generator, (HO)2C6H2(SO3Na)2, utilizing simple chelating chemistry between the catecholic group (dihydroxy benzene) and surface titania ion of the inorganic ordered mesoporous structure as recently reported by our own group.27 It was found that the hydrophilicity is associated with the presence of sulfonated functional groups exposed on benzene ring which is conjugated on titanium ion. The incorporation of the organic polymer portion lessened the brittleness of SiO2-TiO2SO3-Naþ inorganic network and improved the interfacial adhesion to substrates. In this work, the low viscosity inorganic-

Figure 3. (a) AFM images of HP resin pattern replicated from BD (pitch 360 nm, height 23 nm), (b) scheme of nanoscale Si master with weir structure (width 30 μm, pitch 2.5 μm), (c) SEM image of replicated HP-PDMS pattern, (d) open rectangular HP-PDMS composite channel with the interface between HP resin and PDMS substrate (width 20 μm, height 10 μm), and open mouth-throat HP-PDMS composite channels replicated from the masters prepared by UV overexposure with intensity of (e) 600 mJ/cm2 and (f) 1200 mJ/cm2, respectively.

organic hybrid HP resin with excellent processability was used with the following composition: TEOS/TiCl4/(OH)2C6H2(SO3Na)2/MPTMS/PEG-DMA = 1:0.1:0.025:0.88:0.416; the contact angle 61
on HP-PDMS substrate was much lower than 105
on plain PDMS.17 The HP resin could readily be coated on PDMS as well as other substrates such as polycarbonate or glass slide in the thickness range of 20 nm to 2 μm. The cured HP resin film displayed excellent optical transparency as good as PDMS, chemical stability with no swelling behavior against various organic solvents, and higher mechanical strength (tensile strength and Young’s modulus of 71 MPa and 1.76 GPa, respectively) than that of PDMS.28 In order to test the replicability of HP resin on a nanoscale by a direct molding method, we fabricated nano patterns using BD as a nanoscale master. As shown in the AFM images of Figure 3a, the HP resin duplicated well the line patterns of the BD master (pitch 360 nm and depth 23 nm) with excellent fidelity. In the case of the more complex nanomaster having 100 and 900 nm of height of weir structure (Figure 3b), the HP resin also successfully duplicated the nanoscale pattern with no defect in contrast to the PDMS that did not completely replicate the relief structure of the nano master, as shown in Figure 3c (See Figure S4 of Supporting Information). This superior replicability of HP resin to PDMS resin is attributed to easy infiltration of the resin into fine relief structures due to its low viscosity (4 cP) compared to PDMS (3900 cP). It is worth pointing out that the mechanical combination of patterned hard HP layer and soft PDMS supporting substrate allows a reliable release from the master. Furthermore, the strong interfacial adhesion between HP and PDMS was achieved by liquid-liquid phase conformal contact as seen in Figure 3d. This HP-PDMS laminated composite mold 1903

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Figure 5. (a) Zeta potential of HP-PDMS and glass as a function of pH (applied voltage = 60 V, 10 mM NaCl buffer) and (b) comparison of EOF between glass and HP-PDMS for various pH values.

Figure 4. Images of meniscus in (a) PDMS and (b) HP-PDMS composite microchannels that are 50 μm wide and 10 μm high, and (c) comparison between theory (lines) and experiment (symbols) for capillary flow velocity as a function of the distance from the entrance for various channel height (O = 10 μm, 0 = 5 μm, and Δ = 2 μm).

is similar to the soft PDMS-hard PDMS composite mold that enhanced the release behavior and fine replicability.28 One advantage of this direct molding is that it can shape the crosssection of channels as shown in Figure 3d, unlike the postcoating method that caused the deformation of channel shape into circular cross-section as reported.16 The rectangular channel shape was prepared from the master obtained by conventional photolithography. Moreover, the HP-PDMS composite channel with a series of mouth-throat microstructures was also made from the alternative master with the controlled dimension produced by UV overexposure photolithography, as demonstrated in Figure 3f,g. More geometrically sophisticated composite channels could be enabled by adopting various masters and HP resin with high replicability. Flow in Rectangular HP-PDMS Microchannels. It is important for practical applications that two pieces of HP-PDMS laminates are simply bonded to seal the channel with no leak. Silicone containing surfaces of both HP sides were activated by oxygen plasma and subsequently bonded by annealing at elevated temperature. In general, it is expected that hydrophilic HP resin would show a wetting behavior different from hydrophobic PDMS. As shown in Figure 4a, the PDMS microchannel, 50 μm wide and 10 μm high, showed a convex meniscus at the interface between water and air when primed by mechanical flow forced by a syringe pump. On the contrary, the HP-PDMS composite microchannel with the same dimension exhibited spontaneous flow with a concave meniscus after water was dropped into the reservoir as shown in Figure 4b due to the capillary force and strong interaction between water and the hydrophilic surface.26 Therefore, the migration velocities of the meniscus front were thoroughly measured in the HP channels for three different heights of 2, 5, and 10 μm, and these values were compared with

the theoretical values given by eq 4. The velocities were determined by an image analysis software (Image J) with 30 snapshot images taken in a second. To calculate average velocity, the following parameters were used contact angle of meniscus = 61
, surface tension between air and water at 20
C = 71
10-3 N/m, and viscosity of water at 20
C = 1 cP.29 As shown in Figure 4c, the average velocity of capillary flow depended on channel height, as well as the passage distance along the channel. As indicated in eq 4, velocity of capillary flow is mainly governed by channel height for a given channel width when gravity is neglected. In the case of 10 μm high channel, the average velocity was 18.5 mm/s at a distance 2.5 mm away from the reservoir. The average velocity decreased with decreasing channel height: 10.4 mm/s in the 5 μm high channel and 4.8 mm/s in the 2 μm high channel, respectively. This flow behavior is well matched with previous report with ethanol in PDMS microchannel.26 As the meniscus front moved into the microchannel, capillary flow became slower due to increased viscous resistance. It is very useful for the design of simple microfluidic systems that the hydrophilic surface of the microchannel induces spontaneous flow without the mechanical or electrical pumping system and that the channel dimension can be used to control the capillary flow velocity. Another interesting feature of the HP microchannel is an electrokinetic behavior due to a negatively charged channel surface with SiO2-TiO2-PEG-SO3-Naþ chemistry. It is readily implied that the ionization behavior of -SO3-Naþ can change with pH conditions. Therefore, the zeta potential of the cured HP-PDMS film was measured for different pH values as shown in Figure 5a with a bias of 60 V between electrodes. The zeta potential at pH 9 reached -42 mV, which is close to -48 mV for the glass surface. This behavior implies that the sulfonate ion on the HP resin channel can release or receive Hþ ion from water, depending on pH. Because the pKa value of (OH)2C6H2(SO3Na)2 and glass is 4.0 and 2.0, respectively, EOF of HPPDMS showed a slightly lower value than that of glass, as shown Figure 5b,30,31 and EOF velocity of HP-PDMS increased with higher pH up to 4.7
10-4 cm2/V 3 sec at pH 9. However, the lowest EOF was at a pH range of 3-5. Therefore, this HP microchannel is a very useful microfluidic module for realizing liquid transportation into microchannel by capillary driven flow 1904

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Figure 6. CE separation of rectangular PDMS and HP-PDMS channels with a mixture of FITC labeled amino acids; (a) fresh PDMS chip right after oxygen plasma bonding (1: Arg, 2: Phe, 3: Glu, 4: Asp, and *: FITC hydrolysis product), (b) PDMS chip aged for 3 days, (c) fresh HP-PDMS chip, (d) HP-PDMS chip aged for 6 days (9 repetitive injections), and detailed elution peaks of (e) the 2nd injection and (f) the 9th injection of (d).

Table 1. Repeatability of Migration Times, Peak Intensity and Areas, Comparison of Theoretical Plate Numbers (N) and Height of Equivalent Theoretical Plates (HETP) between HP-PDMS and PDMS CE Modules N (HETP, μm)

repeatability RSDa, % a

a

migration time (s)

migration time

intensity (area)

HP-PDMS (Fresh)

PDMS (Fresh)

HP-PDMS (6 days aging)

PDMS (3 days aging)

Arg Phe

17.67 20.08

1.5 2.3

3.8 (3.5) 4.0 (5.0)

30 000 (0.83) 25 000 (1.0)

8000 (3.1) 8500 (2.9)

30 000 (0.83) 25 000 (1.0)

-

Glu

26.05

3.4

9.0 (9.7)

10 000 (2.5)

4100 (6.1)

10 000 (2.5)

-

Asp

28.89

3.6

9.0 (9.8)

10 000 (2.5)

4100 (6.1)

10 000 (2.5)

-

Calculated from nine repeated injection peaks.

and also electrically driven EOF, which makes the HP microfluidics an attractive alternative to costly glass microfluidics. CE Separation Performance of HP Microchannels. CE separation performance of the HP-PDMS chip with rectangular cross-section was tested with a mixture of four FITC labeled amino acids in comparison to native PDMS chip. When buffer solution was introduced into the reservoir, it filled the channels completely within 5 s due to spontaneous capillary flow. Fresh PDMS chip right after plasma treatment showed unstable baseline and low resolution performance (Figure 6a). Furthermore, the PDMS, aged for 3 days, lost the separation efficiency presumably due to the unstable hydrophilic surface and the nonspecific binding on hydrophobic surface as shown in Figure 6b. In the case of the HP channel, all peaks eluted faster than the native PDMS chip because of better EOF behavior (Figure 6c). In addition, the antifouling capability might enhance the CE separation performance, as the nonspecific adsorption of HP resin for fibrinogen protein and pathogen bacteria was recently reported by our

own work.27 The total plate numbers measured by four FITClabeled amino acids were all over 104, in particular, 3
104 plates for the FITC-Arg peak. With PDMS, the plate number was only 8
103 for Arg and 4.1
103 for Glu and Asp, as shown in Table 1, and the heights of equivalent theoretical plates were less than 2.5 μm in a 2.5 cm long separation channel. Furthermore, the HP microchannel as a CE module still maintained its stable separation capability with consistent theoretical plate numbers even after aging for 6 days as shown in Figure 6d-f, while the plain PDMS channel completely lost EOF behavior after 3 days of aging, and the migration times of four FITC-labeled amino acids showed little variation with less than 4% of relative standard deviations when repeatedly injected 9 times, resulting in high reproducibility of HP-PDMS channels. In addition, the grooved HP-PDMS composite channel with 25 μm of mouth and 5 μm of throat structure in 25 mm of separation region was also used to test as a CE module for DNA separation by injecting five different sizes of λ-DNA mixture 1905

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nanoscale patterns that is superior to PDMS and led to direct fabrication of a hydrophilic microfluidic module without a cumbersome coating step and channel deformation problem. The rectangular HP-PDMS composite channel induced a spontaneous flow by capillary force, and the capillary flow velocities were well matched with theoretical predictions. The simple microfluidic system requires no mechanical or electrical pump, and the flow velocity can be controlled at the desired level by changing the channel size. The zeta potential of HP film at pH 9 reached -42 mV, and the enhanced EOF velocity was up to 4.7
10-4 cm2/ V 3 sec, which is comparable to the performance of a glass microfluidic system. For CE separation performance, the rectangular HP-PDMS microchannel yielded a separation efficiency of amino acid mixture that is superior to that of the PDMS channel even when aged for 6 days. The mouth-throat HP-PDMS channel also showed meaningful DNA separation performance when compared to gel electrophoresis. These results showed that this hydrophilic HP-PDMS microchannel could be an attractive alternative to costly glass microfluidic devices.

’ ASSOCIATED CONTENT Figure 7. (a) Optical and SEM images of mouth-throat HP-PDMS microchannel assembled with four reservoirs as a CE module and (b) fluorescence image of separated DNAs with lapse time of 0.5 s at 25 mm away from the injection port.

ranging from 3530 bp to 48 502 bp under an applied DC electric field of 200 V/cm. Here, the diameter of a λ-DNA is expressed by twice the radius of gyration Rg = (1/3 lp L)1/2 derived from the worm-like chain model, where lp and L denote persistence and contour length of the DNA molecules, respectively. The λ-DNA stained by YOYO was increased from 50 to 66 nm in persistence length and from 16 to 22 μm in contour length. Thus, the conformation diameter of λ-DNA stained by YOYO was estimated to become about 1.4 μm, that is smaller than the throat size of 5 μm, but a series of mouth-throat structures along the channel could cause hindrance to size-dependent DNA movement by topographical effect. Eventually, the different mobility resulted in DNA separation efficiency as shown in Figure 7b. This hindrance effect of DNA occurred by a narrow structure was similarly observed by Ros et al.32 Note that the entropic-based DNA separation in the nanoscale channel showed reversed order of elution phenomena, i.e., large DNA molecules migrated with faster mobility than smaller DNA.33 In addition, this work is comparable to gel electrophoresis induced DNA separation, where DNA moves through a microporous gel matrix under an applied electric field at different velocity depending on DNA size. It is noteworthy that the separation time was 11 s and the channel length was 25 mm, which means a shorter separation time and shorter channel length than is possible with gel electrophoresis and a glass CE module. These results amply demonstrate the usefulness of the HP CE module with controlled channel shape.

’ CONCLUSIONS A hydrophilic and low viscosity inorganic-organic hybrid resin of SiO2-TiO2-PEG-SO3-Naþ composition was used to fabricate two types of laminated HP-PDMS composite microchannels by direct molding and facile bonding methods. The resin also allows exact shaping of the cross-section of the microchannels. The HP resin with water contact angle of 61
showed replicability of

bS

Supporting Information. Synthesis scheme of hydrophilic organic-inorganic hybrid, fabrication process microstructured master by overexposure photolithography; experimental setup of CE separation module; pattern replicability of PDMS using nanoscale master with weir structure. This material is available free of charge via the Internet at http://pubs.acs.org.

’ AUTHOR INFORMATION Corresponding Author

*E-mail: [email protected]. Tel:þ42-42-821-7684. Fax: þ82-42823-6665.

’ ACKNOWLEDGMENT This work was supported by the National Research Foundation of Korea (NRF) grant funded by the Korea government (MEST) (20100000722). ’ REFERENCES (1) Zhang, X.; Haswell, S. J. MRS Bull. 2006, 31, 95–99. (2) Zhou, J.; Ellis, A. V.; Voelcker, N. H. Electrophoresis 2010, 31, 2–16. (3) Lu, J. R.; Su, T. J.; Thirtle, P. N.; Thomas, R. K.; Rennie, A. R.; Cubitt, R. J. Colloid Interface Sci. 1998, 206, 212–223. (4) Ocvirk, G.; Munroe, M.; Tang, T.; Oleschuk, R.; Westra, K.; Harrison, D. J. Electrophoresis 2000, 21, 107–115. (5) G€ubitz, G.; Schmid, M. G. Electrophoresis 2004, 23, 3981–3996. (6) Khademhosseini, A.; Yeh, J.; Eng, G.; Karp, J.; Kaji, H.; Borenstein, J.; Farokhzad, O. C.; Langer, R. Lab Chip 2005, 5, 1380–1386. (7) Cai, Y. H.; Hu, J.; Ma, H. P.; Yi, B. L.; Zhang, H. M. J. Power Sources 2006, 161, 843–848. (8) Hillborg, H.; Ankner, J. F.; Gedde, U. W.; Smith, G. D.; Yasuda, H. K.; Wikstrom, K. Polymer 2000, 41, 6851–6863. (9) Berdichevsky, Y.; Khandurian, J.; Guttman, A.; Lo, Y. H. Sens. Actuators, B 2003, 97, 402–408. (10) Wong, I.; Ho, C.-M. Microfluid. Nanofluid. 2009, 7, 291–306. (11) Dong, Y.; Phillips, K. S.; Cheng, Q. Lab Chip 2006, 6, 675–681. (12) Chung, C. K.; Chen, Y. S.; Shih, T. R. Microfluid. Nanofluid. 2009, 6, 853–857. (13) Eddington, D. T.; Puccinelli, J. P.; Beebe, D. J. Sens. Actuators, B: Chem. 2006, 114, 170–172. 1906

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Analytical Chemistry

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dx.doi.org/10.1021/ac102160b |Anal. Chem. 2011, 83, 1901–1907