Rapid Concentration of Nanoparticles with DC Dielectrophoresis in Focused Electric Fields
© to the authors 2009
Received: 24 August 2009
Accepted: 18 September 2009
Published: 1 October 2009
We report a microfluidic device for rapid and efficient concentration of micro/nanoparticles with direct current dielectrophoresis (DC DEP). The concentrator is composed of a series of microchannels constructed with PDMS-insulating microstructures for efficiently focusing the electric field in the flow direction to provide high field strength and gradient. The location of the trapped and concentrated particles depends on the strength of the electric field applied. Both ‘streaming DEP’ and ‘trapping DEP’ simultaneously take place within the concentrator at different regions. The former occurs upstream and is responsible for continuous transport of the particles, whereas the latter occurs downstream and rapidly traps the particles delivered from upstream. The performance of the device is demonstrated by successfully concentrating fluorescent nanoparticles. The described microfluidic concentrator can be implemented in applications where rapid concentration of targets is needed such as concentrating cells for sample preparation and concentrating molecular biomarkers for detection.
The ability to concentrate or extract micro/nanoparticles, such as cells, viruses, bacteria, and DNA, from the background matrix is essential to many biomedical applications. The form of these particles in high concentration facilitates the subsequent analytical and processing steps. For example, current methods in microbial analysis of water quality require subpopulations (e.g.E. coli) sampled in detectable levels of concentration . In the process of gene hybridization, rates can be accelerated by concentration of single-stranded DNA. The sensitivity of fluorescence-based bioassays is greatly improved with pre-concentrated labeled targets. In recent years, more and more biological and chemical assays are conducted in microscale devices with the rapid development of micrototal analysis systems (μ-TAS) . Traditional methods of concentrating samples by centrifuging and subsequently removing the supernatant are not amenable to the format of microchips. A number of methods have been reported concerning on-chip microfluidic concentration and manipulation of micro/nanoparticles such as dielectrophoresis [3–5], optical tweezers , and ultrasonic wave . They are readily integrated into microdevices by patterning micro/nanometal electrodes (in the case of dielectrophoresis) or using remote manipulation with laser or ultrasound.
We report here a direct current dielectrophoresis-based method for rapid concentration of nanoparticles in a microfluidic device. Dielectrophoresis (DEP) is the motion of a particle in a non-uniform electric field due to the unbalanced electrostatic forces on the particle’s induced dipole . This phenomenon has been widely used for concentration, manipulation, separation, sorting, and transport of particles such as beads, bacteria, and cells [3–5, 9–12]. The majority of these applications employ AC electric fields generated by closely spaced microelectrode arrays that are generally constructed with MEMS-based microfabrication techniques. AC fields promote lower electrode polarization and electrophoretic effects. However, AC DEP faces certain issues that limit its applications, such as electrode fouling and electric field decay above the microelectrodes. An alternative to AC dielectrophoresis is the insulator-based DEP (iDEP) or DC DEP [5, 13, 14], in which no metal microelectrodes are embedded in the chip and DC electric fields are applied from an external electrode pair. This simplifies the fabrication of microdevices by eliminating the metal deposition processes. Insulator structures are robust and chemically inert. Effects such as electrochemical reactions and electrolysis observed in AC DEP are less likely to occur in iDEP. Cummings and Singh observed two flow regimes in insulator-based DEP: (1) ‘streaming DEP’ where streams of highly concentrated and rarified particles were created between the insulating posts at relatively low voltages and (2) ‘trapping DEP’ where particles were trapped around the insulating posts at higher voltages . These observations lead to potential applications, for example, streaming DEP can be used to focus and transport particles, and trapping DEP can be used for particle concentration and filtration. The mechanics behind the phenomenon is the competition between electrokinetic (electrophoresis and electroosmosis) and dielectrophoretic forces . The former is linearly proportional to the electric field, while the latter is proportional to the field squared. At low voltages, electrokinetic flow is dominant over DEP and diffusion, resulting in the regime of streaming DEP. At higher voltages, DEP is dominant, resulting in trapping DEP. This letter describes an insulating microstructure that is designed to highly focus and thus ‘amplify’ the electric field. Upon the application of voltage, the generated electric field is focused in the direction of fluid flow. Trapping DEP first occurs at these field-focused areas located at the downstream, while streaming DEP occurs upstream and continuously transports and delivers the particles. The described setup is capable of rapidly concentrating and collecting nanoparticles from continuous flow that is driven by electroosmosis.
Materials and Methods
The Microstructure for Field Focusing
where μek, μeo, and μep are the electrokinetic, electroosmotic, and electrophoretic mobility, respectively. E is the electric field. Equation 1 indicates that the electrokinetic motion of the particles is linearly proportional to the local electric field. Under ideal electrokinetic flow, particles flow along the electric field lines, and no concentration of particles occurs.
where μdep is the dielectrophoretic mobility.
In contrast to electrokinetic flow, dielectrophoretic motion is along the electric field gradient, and the transport can result in concentration, focusing, or trapping of particles in non-uniform fields. Such field gradients are readily produced either by embedded metal electrodes in the case of AC dielectrophoresis or by insulating posts within the channel exposed to an external electric field. Dependent on the polarizability of the particles and suspended medium, positive DEP (drawing particles to field maximum) or negative DEP (repelling particles from field maximum) takes place .
The above equations imply that the resulting particle movement depends on the relative strengths of electrokinetic and dielectrophoretic effects. As discussed in Ref. , above a threshold electric field, DEP becomes dominant over electrokinetic and diffusion effects. Particles will be trapped and concentrated dielectrophoretically, and this regime is called ‘trapping DEP’. In this work, we proposed a field-focusing structure (Fig. 1b) in which rapid electric field gradients are generated to readily induce ‘trapping DEP’. Electrokinetic flow is responsible for particle transport, while dielectrophoresis is responsible for local particle trapping.
Microfabrication and Experimental Setup
Results and Discussions
The performance of the microfluidic concentrator was tested with green fluorescent polystyrene microspheres of diameter of 930 nm (Duke Scientific Co., CA, USA). The microspheres emit green light (508 nm) when they are excited by blue light (468 nm). Before use, the particles were re-suspended at low concentration in deionized water, and the conductivity of the medium was adjusted by adding phosphate buffer solution (PBS, Fisher Scientific, NJ). The conductivity was measured using a conductivity meter with graphite sensor electrodes (Dist3WP, Hanna Instruments Inc., RI). Solutions of 1.0–10.0 mS/m were used in the experiments. To begin, a suspension of fluorescent microspheres was injected into the inlet reservoir of the microfluidic chip with a pipette. To generate the electric field, a DC voltage was applied to the platinum wire electrodes placed in the two reservoirs at the ends of the channel. To investigate the effect of electric field strength on the particle motion, the applied voltage on the two electrodes was increased incrementally from 50 to 1,000 V with a high-voltage power supply (PS350, Stanford Research). The corresponding electric field was ~20–400 V/cm.
We have demonstrated a microfluidic concentrator for rapid and efficient trapping of nanoparticles. The concentrator is composed of a series of microchannels formed by PDMS-insulating microstructures. The applied fields were focused stepwise within the microchannels. Streaming DEP occurred at low electric fields, and trapping DEP occurred at higher electric fields. As the electric field increased, concentration of nanoparticles began to occur at different regions. Both streaming DEP and trapping DEP could simultaneously occur. The concentration was very rapid and efficient as the streaming DEP delivered particles and trapping DEP trapped the delivered particles. The proposed concentrator design can be re-configured into a format with more or less trapping regions, depending on the applications. Furthermore, the microfluidic concentrator can be implemented in applications where concentration of targets are needed, such as the concentration of cells for sample preparation and the concentration of molecular biomarkers for biological assays.
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