MAGNETIC CORE SHELL NANOPARTICLES TRAPPING USING IRONBEADS MAGNETIC CHAMBER

B. Teste1, F. Malloggi2 , A.L. Gassner3, T. Georgelin4, H.H. Girault3,J.M. Siaugue4, P. Tabeling2 ,S. Descroix1.

1Physicochimie des Electrolytes, Colloïdes et Sciences Analytiques (PECSA) UMR 7195 CNRS-ESPCI, 10 rue Vauquelin, 75231 Paris Cedex 05, France

2 MMN - UMR 7083 - Gulliver CNRS-ESPCI, 10 rue Vauquelin, 75231 Paris Cedex 05, France3Laboratoire d'Electrochimie Physique et Analytique, EPFL SB ISIC LEPA, Station 6, CH-1015, Lausanne, Switzerland.

4Physicochimie des Electrolytes, Colloïdes et Sciences Analytiques (PECSA) UMR 7195 CNRS-Paris6-ESPCI, 4 place Jussieu, 75252 Paris Cedex 05, France

ABSTRACT

We report on the development of a PDMS microdevice integrating a magnetic chamber dedicated to magnetic nanoparticles trapping / release. This magnetic chamber consists in ferromagnetic iron beads, packed by a physical restriction, that present the capability of concentrating magnetic field lines from external permanent magnet. It results an enhanced magnetic flux density and therefore high magnetic force within the magnetic chamber. The microdevice has been characterized with finite element numerical simulation and fluorescent microscope imaging. Setting the flow rate at 100 μl/h, nanoparticles enrichment about 4000-fold has been achieved with a trapping / release sequence of a few seconds. Keywords: Magnetic core shell nanoparticles, magnetic trapping, microsystem, preconcentration.

INTRODUCTIONIn the field of analytical and bioanalytical developments, magnetic particles appeared as very interesting tool and have

been integrated in microdevice for various functionalities: mixing, separation, solid support for bioassay, transporting and detection [1]. Among these functionalities, magnetic particles trapping appeared as a crucial issue of analytical sequence as it should allow efficient separation, washing and preconcentration step.In that way two main strategies can be followed to perform an efficient trapping of magnetic particles. The first one consists in placing an external permanent magnet or electromagnet outside the channel, the magnetic flux density (B) and consequently the magnetic force (Fmag) rapidly decrease when the distance from the magnet increases. As the magnetic force is proportional to the particles volume, only above 300 nm beads can be easily trapped in this format. In order to trap more efficiently magnetic nanoparticles, more complicated strategies have to be carried out. One interesting approach consists in increasing B and therefore Fmag. In that way, magnetic structures of iron, nickel or permalloy have been integrated within the microdevice in order to locally increase B and Fmag. These ferromagnetic materials generate high magnetic field when an external magnetic excitation is applied and loose it when the external permanent magnet is removed. Different microdevices have been developed integrating iron tracks surrounding the microchannel [2] or nickel posts into microchannel [3]. Electromagnets have also been developed into microfluidic system [4]. In counterpart, this microfabrication approach requires several complex fabrication steps using different techniques like polymer photo-ablation [1] or combination of lithography and etching [5]. We report here a simple method to fabricate internal magnetic chamber which avoids complicated and time consuming process for a simple and efficient trapping of 30 nm magnetic core shell nanoparticles (MCSNP) whose magnetic weight represents only 9.5%. Micrometric iron beads (6-8 μm) capable of concentrating magnetic field lines were used to develop a magnetic chamber into microchannel which offers very small dead volume and very high B value. This magnetic chamber has been described through two different approaches: finite element numerical simulations and fluorescent MCSNP imaging. THEORY

Numerical simulations have been performed using Flux expert software (Astek, France) to study the attraction between MCSNP and iron beads magnetized material. The magnetic field lines from external permanent magnet are distributed through iron beads and thus locally increase B and Fmag compared to a system without magnetic chamber. The resulting B gradient exerts a magnetic attraction or repulsion on the MCSNP. The Fmag is given by equation 1

BBFmag )(�����

0µV (1)

Fmag depends on the volume of the particle (V), the difference of magnetic susceptibility ( �� ) between the particle and the surrounding medium as well as B magnitude and gradient. Homogeneous B will align the particles in the direction of the magnetic field but to exert a translational force on nanoparticles a B gradient is required

978-0-9798064-3-8/µTAS 2010/$20©2010 CBMS 1151 14th International Conference onMiniaturized Systems for Chemistry and Life Sciences

3 - 7 October 2010, Groningen, The Netherlands

EXPERIMENTAL The magnetic chamber developed in our microsystem is depicted in Figure 1a. The magnetic iron beads (6-8 μm) are

packed against a physical restriction. The latter was made of tunnel (3 μm high and 25 μm width) thus allowing fluids to pass through it. The MCSNP trapping was visualized using fluorescent MCSNP excited with mercury lamp. The fluorescent signal was collected with a CCD camera and analyzed by HIRIS software (RD Vision, France). Sample and flow handling were performed with syringe pump. Experimental conditions were not directly compatible with numerical parameters range due to software limitations. At microchannel scale the magnetic field lines are parallel (on y axis) using a magnet of 2 cm in diameter and 1 cm in high. In order to reflect our experimental conditions, simulations performed with two magnets in attraction (Figure 1b) appeared as the most appropriate approach since the magnetic field lines between the two magnets are parallel. Due to this configuration, quantitative results from simulation cannot be directly compared to experimental results. Simulations were used to evaluate and explain Fmag gain, to depict B and Fmag spatial distribution and to optimize the magnetic chamber development.

(a) (b)Figure 1. (a) Experimental set up used for fluorescent MCSNP trapping microscope imaging. (b) Scheme of the system used for numerical simulations. The system is composed of microchannel (light grey) closed by insulating layers (blue) and surrounding by two permanent magnets (dark grey). Iron beads column (dark spot) is placed in the centre of each magnet. The whole system is placed in an air box (dark lines). H, L, h and l represent respectively the high of the channel, the length of the channel, the high of the magnets and their length.

RESULTS AND DISCUSSION The B and Fmag spatial distribution calculated by numerical simulation are shown on Figure 2 and 3. In the system

without iron beads the magnetic field lines go straight from one magnet toward the other and B is principally definite on y component in the channel zone between the two magnets. The presence of iron beads is responsible of magnetic field lines concentration trough beads due to their high magnetic permeability. The B and Fmag are improved 6 and 1700 times respectively compared to a system without iron beads. B and Fmag maxima are located on iron beads magnetic pole where the MCSNP should be preferentially trapped.The feasibility of using such a system has been experimentally proved. As expected, the presence of iron beads allows the trapping and release (Figure 4) of fluorescent MCSNP. First, fluorescent MCSNP were flowed through the channel in absence of permanent magnet. No fluorescence signal has been observed within the iron beads plug highlighting the absence of non-specific adsorption of MSCNP on iron beads. When an external magnet was placed on the top of the microdevice (Figure 4 a), MCSNP were progressively trapped in the magnetic chamber. The magnetic chamber saturation was reached after 25 s, the non-trapped MCSNP flow trough the magnetic plug and were detected in the channel behind the physical restriction. The capture capacity of such magnetic chamber was evaluated to be 1.1 � 104

MCSNP/ μm3 and the preconcentration factor achieved was about 4000 times.

1152

Figure 2. Mapping of B on y axis without (a) or with (b) iron beads

Figure 3. Mapping of Fmag acting on 30 nm magnetic nanoparticles on x component without (a) or with (b) iron beads.

Concerning the release step (Figure 4 b), 2 seconds and a volume of 2.5nL were necessary to elute all the MCSNP. The absence of residual fluorescent signal in the magnetic chamber after release confirmed that MCSNP were not adsorbed into iron beads even at very high concentration and that no remnant magnetization persists in the iron beads magnetic plug. These experimental results correlate the numerical simulations as in absence of iron beads fluorescent MCSNP cannot be trapped with a permanent magnet even at very low flow rate (data not shown). The integration of packed iron beads allows a very fast and efficient trapping of MCSNP leading to high preconcentration factor with a preconcentration time of 25 s.

CONCLUSION A very simple and efficient microdevice dedicated to magnetic nanoparticles trapping has been developed. This device

is based on high magnetic gradient generation using permanent external magnet and iron beads packed within the microchannel. The ability of such a magnetic chamber to trap MCSNP has been successfully demonstrated by numerical simulation and experimental study. A preconcentration factor of 4000 was achieved in less than one minute for a complete sequence of nanoparticles trapping and release. Future work will consist in performing immunoassay with MCSNP as substrate support dedicated to allergy diagnosis.

ACKNOWLEDGEMENTS The authors thank the agence national pour la recherche (ANR) for financial support

REFERENCES [1] M.A.M. Gijs, F. Lacharme, U. Lehmann, Microfluidic Applications of Magnetic Particles for Biological Analysis and Catalysis, Chemical Reviews, 110, 3, 1518-1563, (2010). [2] M. Abonnenc, A. L. Gassner, J. Morandini, J. Josserand, H. H. Girault, Magnetic track array for efficient bead capture in microchannels, Anal Bioanal Chem 395, 747-757, (2009).[3] T. Deng, M. Prentiss, G. M. Whitesides, Fabrication of magnetic microfiltration systems using soft lithography, Appl. Phys. Lett. 80, 3, 461-463 (2002). [4] A. C. Siegel, S. S. Shevkoplyas, D. B. Weibel, D. A. Bruzewicz, A. W. Martinez, G. M. Whitesides, Cofabrication of electromagnets and microfluidic systems in polydimethylsiloxane, Angewandte 45, 6877-6882, (2006). [5] K. Smistrup, O. Hansen, H. Bruus, M. F. Hansen, Magnetic separation in microfluidic systems using microfabricated electromagnets—experiments and simulations, j. magn. magn. matter, 293, 597-604, (2005).

Figure 4. Experimental trapping (a) and release (b) sequence of fluorescent MCSNP under a 50 µL/h flow rate.

1153