Superparamagnetic nanoparticle-based nanobiomolecular detection in a microfluidic channel

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1 Current Applied Physics 6 (2006) Superparamagnetic nanoparticle-based nanobiomolecular detection in a microfluidic channel Kyu Sung Kim, Je-Kyun Park * Department of BioSystems, Korea Advanced Institute of Science and Technology (KAIST), Guseong-dong, Yuseong-gu, Daejeon , Republic of Korea Received 20 September 2004; received in revised form 28 May 2005 Available online 11 August 2005 Abstract A novel biomolecular detection principle based on the magnetic force in a microfluidic channel or a lab-on-a-chip was developed. The microbeads conjugated with superparamagnetic nanoparticles can only switch their own fluidic path in a microfluidic channel due to magnetic fields. To investigate the migration behavior of microbeads, the sample and buffer solutions were injected in the same flow rate to make a laminar flow. The streptavidin conjugated superparamagnetic nanoparticles were reacted with biotinylated fluorescent microbeads. The velocities of microbeads were observed in a microchannel by the CCD camera on an inverted microscope. As expected, the movements of microbeads conjugated with superparamagnetic nanoparticles were dependent upon the concentrations of superparamagnetic nanoparticles. This detection scheme can be useful for various assay systems and multiplexed biological analysis systems using encoded microbeads. Ó 2005 Elsevier B.V. All rights reserved. PACS: a; w; Ha; j; a Keywords: Microbeads; Superparamagnetic nanoparticles; Microfluidic channel; Magnetic field 1. Introduction The development of robust, sensitive and highthroughput biosensor is one of the major issues in the area of nanobiotechnology [1 5]. Until now, some technical achievements for biological detection have been reported such as a diffusion-based immunoassay [6] and nanoparticle-based protein assay [1 3,7]. The significance of these researches is that it will provide an assay platform for nanoscale biological detection system, which enables high-throughput and high-sensitive protein assay. * Corresponding author. Tel.: ; fax: address: jekyun@kaist.ac.kr (J.-K. Park). Recently, some microbead-based analytical applications have been reported [8 10]. Since the microbead has large surface area per unit volume, it can provide not only relatively large binding sites but also short biochemical assay time. Especially, superparamagnetic nanoparticles have single domain magnetic dipoles in an applied magnetic field. Without an external magnetic field, they do not have permanent magnetic dipoles because the magnetic dipole dipole interaction energy is weaker than thermal energy. Therefore, superparamagnetic nanoparticles have been widely used for the construction of biological assay systems, such as DNA hybridization [11] and ligand receptor reactions [12,13]. In this paper we are trying to develop a novel biomolecular detection principle based on the magnetic force in a microfluidic channel or a lab-on-a-chip. The proposed detection scheme is that the specific microbeads /$ - see front matter Ó 2005 Elsevier B.V. All rights reserved. doi: /j.cap

2 K.S. Kim, J.-K. Park / Current Applied Physics 6 (2006) conjugated with superparamagnetic nanoparticles can only switch their own fluidic path in a microchannel due to magnetic fields. As shown in Fig. 1, the buffer solution is injected into the one side of the inlets and the sample solution is injected into another side of the inlets. The sample solution consists of the microbeads and magnetic nanoparticle complex. When antigen molecules (target analytes) simultaneously react with the microbeads and superparamagnetic nanoparticles which are immobilized by a specific antibody, the superparamagnetic nanoparticles will be attached onto the microbeads by the antigen antibody complex. Only the microbeads conjugated with superparamagnetic nanoparticles consequently move to the higher magnetic field gradient under the specific applied magnetic field. In general, the flow in a microfluidic channel remains laminar, and the diffusion effects of the microsized beads are negligible. Therefore, the microbeads conjugated with superparamagnetic nanoparticles can change their fluidic path in an applied magnetic field. If the concentration of superparamagnetic nanoparticles on a microbead increases, the velocity of a microbead will increase because the velocity of microbeads is proportional to the total volume of the magnetic nanoparticles on a microbead, magnetic field strength, and field gradient [14]. This implies that the target analytes in a microfluidic channel are quantified by nanoparticle concentrations or a magnetic force. In addition, since the microbeads have a fluorescent property, the path-changed microbeads can be identified by their own wavelength of fluorescent microbeads which can be encoded [15]. This could be useful to construct a multiplexed assay system. In this study, the above detection scheme was justified using streptavidin biotin conjugated with superparamagnetic nanoparticles as a model assay system. 2. Theory In a microchannel in which either width or height is less than 200 lm, an aqueous flow is generally laminar, not turbulent. The diffusion is an efficient process for mixing the dissolved contents of two or more fluids and the particles can also be separated by diffusion according to their size. At the perpendicular direction against the flow direction there is no force on the microbead. However, when the microbeads are conjugated with the superparamagnetic nanoparticles under the applied magnetic fields, they will move against the flow direction due to the magnetic force on each labeled superparamagnetic nanoparticle on the microbead. The magnetic force, F sm, on a superparamagnetic nanoparticle in the aqueous solution is given by Eq. (1) [14]: ~F sm ¼ 1 V sm Dv sm rb 2 ; ð1þ 2 l 0 where V sm is the volume of the superparamagnetic nanoparticle, Dv sm = v sm v aq is the net magnetic susceptibility of a superparamagnetic nanoparticle in aqueous solution, B is the magnetic field and l 0 is the vacuum permeability. When the superparamagnetic nanoparticles are conjugated with the microbeads, the applied magnetic fields will induce the magnetic forces on superparamagnetic nanoparticles, and then the total magnetic forces of the superparamagnetic nanoparticles on the microbeads will make the movement of the microbeads. The total magnetic force, F tsm, of the superparamagnetic nanoparticles on the microbead is the sum of the magnetic forces acting on each superparamagnetic nanoparticle on the microbead: ~F tsm ¼ N sm ~F sm ¼ 1 2 N V sm Dv sm sm rb 2 ; ð2þ l 0 where N sm is the number of the superparamagnetic nanoparticle conjugated with a microbead. If the superparamagnetic nanoparticles are a specific size, the volume (V sm ) and the magnetic susceptibility (v sm ) of each superparamagnetic nanoparticle have the same value because V sm and v sm are the variables of particle size. Therefore in the specific magnetic field gradient ($B 2 ), the F tsm is determined by the number of the superparamagnetic nanoparticle. When the microbeads are moved by the total magnetic force, the StokesÕ drag force (F D ) is generated against the apposite direction of the moving microbeads. The F D is represented by the following Eq. (3): ~F D ¼ 6pR M g~v; ð3þ Fig. 1. Proposed detection principle. Microbeads conjugated with superparamagnetic nanoparticles (d) and unconjugated microbead (s). where R M is the radius of the microbead, g is the viscosity of the aqueous medium and ~v is the velocity of the microbead. The ~v of the microbead results from the magnetic force. Since the direction of the magnetic force is the perpendicular direction of fluid flow and the

3 978 K.S. Kim, J.-K. Park / Current Applied Physics 6 (2006) The green yellow fluorescent microbeads immobilized with biotin molecules (Fluospheres) were purchased from Molecular Probes (Eugene, OR). The microbeads (1 lm diameter) were brought into solution in 0.4 ml of as aqueous suspensions containing 1% solids and 0.02% Tween Ò 20. The concentration of the microbeads were beads/ml. The binding capacity of the biotin molecules of the fluorescent microbeads was 7.1 nmol/mg. The superparamagnetic nanoparticles (Streptavidin MicroBeads) were obtained from Miltenyi Biotec (Germany). The superparamagnetic nanoparticles consists of iron oxide and their size was 50 nm diameter including polymer coating on a surface. The magnetic nanoparticles were immobilized with streptavidin molecules which can specifically bind with biotin molecules. The superparamagnetic nanoparticles were brought into 1.0 ml solution. All solutions were prepared with Millipore water (Milli-Q, Millipore Co.) and the other chemicals used were of analytical reagent grade Microfabrication Fig. 2. The microbeads conjugated with the superparamagnetic nanoparticles in a microchannel. The total magnetic force (F tsm ) and StokesÕ drag force (F D ) are shown. microbeads move in the laminar flow as shown in Fig. 2, the F D equals the F tsm : ~F D ¼ ~F tsm. ð4þ Combining Eqs. (2) and (3) into Eq. (4), the velocity of the microbeads in the aqueous medium is represented by following Eq. (5): ~v ¼ N smv sm Dv sm rb 2. ð5þ 12pR M gl 0 Therefore, when the size of the superparamagnetic nanoparticle and the microbead is assumed to be uniform respectively, the velocity of the microbead is decided by the number of the superparamagnetic nanoparticle conjugated on the microbead (N sm ) and the magnetic field gradient and strength. 3. Experimental 3.1. Materials The microfluidic device was fabricated by conventional PDMS (polydimethylsiloxane) (Sylgard 184, Dow Corning) molding processes. The fabrication process is simply illustrated at Fig. 3. The positive photoresistor (AZ9260) coated on the bare Si wafer to create molds. The coating speed was set at 6000 rpm for 60 s to make the 5 lm thickness which was the height of the microchannel. The photoresistor was patterned using UV lithography. After the patterning, the prepared mixture of PDMS was degassed under the vacuum, poured onto the mold and cured for 30 min at 100 C on the hot plate. The cured PDMS was peeled from the mold and rinsed in the ethanol. The slide glass was rinsed in the heptane. The rinsed PDMS and slide glass were dried in the dry oven at 80 C and treated by air plasma using an expanded plasma cleaner (Harrick Science, Ossing, NY) for 20 s. Then, the PDMS and slide glass were bonded immediately. Inlet and outlet holes were punched before the PDMS rinsing step. The dimension of the microfabricated channel was Spin coating PR patterning PDMS curing Peeling off Bonding (plasma treatment) Bare Si wafer PR AZ9260 PDMS Slide glass Fig. 3. Microfabrication process of the microfluidic device.

4 K.S. Kim, J.-K. Park / Current Applied Physics 6 (2006) lm height and 100 lm width, except for junction part of the channel Microchannel design and measurement set-up As shown in Fig. 4, the microchannel was primarily H-shape to extract the microbeads which only were conjugated with the superparamagnetic nanoparticles. Two separate channels (100 lm width) were joined and created the junction which was 200 lm width. The junction part of the channel was leaned toward the one side of the device due to have an influence on the high magnetic field gradients. The device was mm 3 with 1/ 16 in. holes of two inlets and two outlets. The tubing was inserted into the holes to connect the microsyringes. The microsyringes were connected with another side of tubing to pump the aqueous medium by two syringe pumps (KDS-100, KD Scientific Inc., PA). The sample solutions consisted of the mixture of the microbeads and the superparamagnetic nanoparticles and they were injected into the one inlet. Simultaneously, the buffer solutions were injected into another inlet in the same flow rate to make a laminar flow. In this study, we used NdFe35 permanent magnet which was mm 3 and B r = 12,000 G. The magnetic flux density on the central axis at the 10 5mm 2 surface was 6000 G. The permanent magnet was fixed at 2 mm apart from the microchannel to maintain a high magnetic field gradient. The extraction of microbeads conjugated with the superparamagnetic nanoparticles was occurred at the H-junction of the microchannel in the magnetic field. Fig. 4. Top view of the PDMS device. The H-junction part of the channel was leaned to the one side to have an influence on the high magnetic field gradients. The permanent magnet was fixed at 2 mm apart from the microchannel. To investigate the velocity of microbeads in the microchannel, the streptavidin conjugated superparamagnetic nanoparticles were reacted with biotinylated fluorescent microbeads in the microchannel. The mixed solutions and the buffer solution were injected using syringe pumps into each side of the inlets at the same flow rate. The movement of the microbeads conjugated with the superparamagnetic nanoparticles was observed in the junction microchannel of the 200 lm width by the CCD camera on an inverted microscope (Carl Zeiss, Germany). 4. Results and discussion 4.1. Migration behavior of microbeads In order to investigate the migration behavior of microbeads, the junction part of the channel was maintained to make a laminar flow by two inlet flows. The solutions of the fluorescent microbeads were diluted from beads/ml to beads/ml in phosphate buffered saline (PBS), ph 7.2 and 2 mm EDTA solution. The concentrations of the microbeads were fixed and the volume of the solution of the superparamagnetic nanoparticles was increased. The volume of the superparamagnetic nanoparticle solution was 0, 1, 2, 4, 6, 7, 8 and 10 ll. The total volume of the mixture solution of the microbeads and superparamagnetic nanoparticles was 100 ll. Therefore, the concentration of the microbeads was beads/ml in the mixture solution. Fig. 5 shows the migration images of the fluorescent microbeads at the H-junction of the microchannel in the magnetic field. It was confirmed that the microbeads conjugated with the superparamagnetic nanoparticles were moved by magnetic fields. In addition, the movement of microbeads conjugated with superparamagnetic nanoparticles was dependent upon the concentration of superparamagnetic nanoparticles as expected by theory. As shown in Fig. 6, the velocity of the microbeads increased as the concentration of the superparamagnetic nanoparticles increased. The velocity was almost saturated at the 10% (v/v) of the superparamagnetic nanoparticles. The reason of the saturated velocity can be interpreted as the limited binding capacity of the microbead surface. The surface area of microbead is p lm 2. When it is assumed that the superparamagnetic nanoparticle of 50 nm diameter occupies nm 2 area, the number of the superparamagnetic nanoparticles that can be conjugated on each microbead is about 1257 particles, geometrically. In 100 ll mixture sample, the number of the microbeads was about beads. We can suppose that the total number of the superparamagnetic nanoparticles which can be conjugated with the microbeads

5 980 K.S. Kim, J.-K. Park / Current Applied Physics 6 (2006) Fig. 5. CCD image of the fluorescent microbeads migration by magnetic field in the channel of 2 mm apart from the magnet. Interval time: 3 s. superparamagnetic nanoparticles was fixed at 10% (v/v). When the concentration of the microbeads was beads/ml, the velocity was about 1.49 lm/s. This result was obtained that the concentration of the microbeads was four times as much as that of the previous experiment. Due to the increase of the microbead concentrations, the surface areas with which the superparamagnetic nanoparticles can be conjugated also increase 4 times. It means that the number of the superparamagnetic nanoparticles per microbead was reduced by one-fourth. We could expect that it was the same condition compared with the previous experiment, which has beads/ml of microbead and 2.5% (v/v) of superparamagnetic nanoparticle. In the curve of Fig. 6, the velocity at the 2.5% (v/v) of superparamagnetic nanoparticles was 1.54 lm/s. This velocity is satisfied with that the same ratio of the microbeads and superparamagnetic nanoparticles have the same velocity. From the result, it seems that the detection ranges can be adjusted by changing the concentration of the microbeads. 5. Conclusion Fig. 6. The relationship between the measured microbead velocity and the volume percent of superparamagnetic particles. is about particles. These geometrical properties are related to the migration of the microbead, the detection ranges and the detection limits of the biomolecules. This is because these properties are variables of the Eq. (5), which determine the velocities of the microbead. Thus, to optimize the experimental conditions, the geometrical considerations are essential Effect of the microbeads and superparamagnetic nanoparticles We carried out the experiment with different concentration of the microbeads while the concentration of the A novel biomolecular detection system based on the magnetic force in microfluidic channels using microbeads and superparamagnetic nanoparticles was developed. In this study, we focused on the investigations of the migrational characteristics of microbeads conjugated with superparamagnetic nanoparticles in a magnetic field. As a model assay system, the streptavidin biotin conjugated with superparamagnetic nanoparticles were used in the experiments. The microbead conjugated with superparamagnetic nanoparticles moved in an applied magnetic field. The movement of fluorescent microbeads conjugated with superparamagnetic nanoparticles was dependent upon the concentration of superparamagnetic nanoparticles. The detection strategy is currently being studied to develop the biological assay systems, such as sandwich immunoassay using IgG and anti-igg reaction, by immobilization of various biomolecules on both microbead and superparamagnetic nanoparticle. It is expected that this platform can be integrated in a lab-on-a-chip and performed biomolecular assay by observing the movement of microbeads. In addition, the fluorescent microbeads can be encoded and the encoded microbeads could be utilized in highthroughput analysis for multiplexed biological assay. Acknowledgement This research was supported by a grant from CHUNG Moon Soul BioInformation and BioElectronics Center, KAIST.

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