# Difference between revisions of "Concentration of Magnetic Beads Utilizing Light-Induced Electro-Osmosis Flow"

Jump to: navigation, search

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.

## Keywords

electro-osmosis flow, light-induced dielectrophoresis, magnetic beads, TiOPc

## 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:

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