Concentration of Magnetic Beads Utilizing Light-Induced Electro-Osmosis Flow

Entry by Yuhang Jin, AP225 Fall 2011

Reference

Shih-Mo Yang, Punde Tushar Harishchandra, Tung-Ming Yu, Ming-Huei Liu, Long Hsu, and Cheng-Hsien Liu, IEEE Trans. Magn., 2011, 47, 2418.

Introduction

Magnetic beads have wide applications in the separation of biomolecules. Traditional magnetic separation technology involves the use of bulk magnets, which makes scaling down of the device rather inefficient. Other techniques for the manipulation and separation of microparticles, such as optical tweezers and dielectrophoresis, are also limited in their flexibility. Therefore optoelectronic tweezers featuring light-induced method and nonuniform electric field were developed. The simplest design of an optoelectronic tweezer modulates the conductivity of amorphous silicon with dynamic light pattern and hence enables trapping and manipulation of particles. In addition, another approach of microparticle concentration via light-induced electro-osmosis flow was also reported. However, the chips required for those means are generally difficult to fabricate, impeding their convenient implementation in biology.

Previously, the authors presented an easier method for chip fabrication by using organic photoconductive material Y-type TiOPc for the light-induced electro-osmosis flow chip with a region of minimum flow velocity for the trapping and collection of magnetic beads. In this paper, they integrate the TiOPc-based substrate and the light-induced electro-osmosis flow (LEOF) phenomenon. The fabrication process in single and simple, the illumination light power is small, and the low-frequency kHz region for manipulating magnetic beads widens the scope of TiOPc-based optoelectronic dielectrophoresis chip. The device has been demonstrated to be capable of high-efficiency concentration and enrichment of magnetic microparticles.

Chip fabrication, system and theory

Fig.1 TiOPc-based LEOF chip. A thin TiOPc layer, blue color, is fabricated on ITO glass substrate with a simple fabrication process.
Fig.2 Optical system setup for on-chip manipulation of magnetic beads by LEOF.

The TiOPc-based LEOF is designed with simple fabrication process. TiOPc is spin-coated onto the ITO glass, yielding a thin TiOPc layer of 500 nm, whose properties remain stable under normal operation. An ITO glass is then placed on the TiOPc substrate with a 100 μm gap to obtain the TiOPc-based LEOF chip. The prototype chip is shown in Fig. 1.

The optical system of TiOPc-based LEOF chip is illustrated in Fig. 2. The light source is concentrated through a pair of focusing lenses and illuminated on the digital micromirror display (DMD). The reflecting image is projected on the TiOPc surface through a 1/20X optical system which consists of two lenses. The dynamic image is controlled by controlling the DMD surface in real time. An XYZ translation stage is utilized to tune the chip position and a function generator is used to provide external ac voltage. The microparticle manipulation process is recorded by a CCD.

For electro-osmosis flow, the dielectrophoresis (DEP) force acting on a spherical particle of radius suspended in a medium with relative permittivity is given as

$F_{DEP}=2\pi\epsilon_m Re[f_{CW}(\omega)]\nabla E_{rms}^2$

where $E_{rms}$ is the root mean square of the ac electric field and $f_{CW}(\omega)$ is

$f_{CW}(\omega)=\frac{\epsilon_p^*-\epsilon_m^*}{\epsilon_p^*+2\epsilon_m^*}$.

The DEP force is proportional to the third power of the particle radius, too small to drive the magnetic beads. Therefore electro-osmosis flow should be considered especially in the presence of low applied frequency. When external ac voltage is applied to LEOF chip, free cations in the electrolyte are strongly attracted toward the illuminated TiOPc surface, forming a compact immobile layer with the thickness of about several angstroms. The region with a higher concentration of cations from the compact layer to the uniform bulk liquid is named as the diffuse layer. These two layers together are called the electrical double layer (EDL). When a tangential electric field component is applied in the EDL region, the ions in this region get driven at the slip velocity:

$V_{EOF}=\frac{\lambda_D\rho_{DL}E_t}{\eta}$

where $\lambda_D$ is the Debye length, $\rho_{DL}$ is the charge per unit area of induced charge of the double layer, $E_t$ is the tangential component of the electric field, and $\eta$ is the bulk liquid viscosity. Hence, when light image is projected on the TiOPc layer, the electric field near the image edge is nonuniform. The light-induced electro-osmosis flow could be utilized to concentrate the magnetic beads.

Experimental results

Fig.3 Schematic diagram of TiOPc-based LEOF chip utilized for concentration of magnetic beads.
Fig.4 Flow direction simulation of the light-induced EOF.
Fig.5 Simulation results of flow magnitude of the light-induced electro-osmosis flow.

Concentration of magnetic beads on TiOPc chip is illustrated in Fig. 3. The charges transport through the organic photoconductive material layer within illuminating region and induce LEOF on the surface. The induced liquid flows at the edge of illuminating area concentrate the magnetic beads toward the center of light pattern. Microparticle concentration is increased via dynamic light pattern.

To verify the EOF effect induced by the electric field gradients above the TiOPc virtual electrode, simulation of the light-induced flow is performed with its results shown in Figs. 4 and 5. In Fig. 4, the region within red dotted line is the projected light pattern. The arrows indicate the liquid flow direction on the TiOPc surface. The center of the pattern cross has the minimum liquid flow to trap magnetic beads. In Fig. 5, height represents the magnitude of the flow utilized to drive the magnetic beads toward the illuminating region. The maximum slip velocity is at the edge of the light pattern. Minimum flow velocity is at the center where the light pattern crosses wherein the magnetic beads are trapped.

In this study, light-induced electro-osmosis flow is applied to concentrate magnetic beads with a two-step process. First, the magnetic beads are attracted from the nonilluminating region to the light pattern region due to the light-induced electro-osmosis flow. Second, two groups of magnetic beads are combined together by moving the cross light image. Fig. 6 demonstrates the concentration process of magnetic beads. The results demonstrate that our LEOF approach has better flexibility than the traditional approach.

Single magnetic bead driving and collection by light-induced EOF is shown in Fig. 7. The liquid flow is generated to drive the magnetic bead indicated with a red arrow along the center of light pattern.

Fig.6 Large numbers of magnetic beads are concentrated by the light-induced EOF. When the light bar is moved downward, the light-induced liquid flow would drive the magnetic beads and keep their position within the center of light pattern.
Fig.7 Single magnetic bead is driven toward the center of the light pattern. The magnetic bead indicated by a red arrow within the light pattern is attracted to the minimum flow velocity region.