Water surety in the face of natural contaminants such as Cryptosporidium parvum and newly realized security threats such as B.anthracis presents a significant analytical chemistry challenge. Pathogenic bacteria can contaminate water at concentrations as low as 1 bacterium/liter. Even if a sensitive detection method is employed, the entire liter must be sampled. By selectively isolating and concentrating the pathogenic bacteria, however, a detectable amount of material could be delivered to an analytical device in a greatly decreased sample volume.
Polarizable particles can be moved in a non-uniform electric field by dielectrophoretic force as first reported by Pohl in 1951. Separation of live and dead cells has been demonstrated by other investigators using pressure-driven flow with AC field in an electrode array. There are a number of drawbacks to this method including fabrication complexity and charging effects, fouling, and gas bubbles at the electrode surface. Insulative (electrodeless) dielectrophoresis (iDEP) avoids these problems by creating the heterogenous electric field between only two electrodes using the geometry of insulating posts etched in channels. Because no thin film electrodes are required, a much larger working area can be fabricated.
Sandia scientists, Blanca Lapizco-Encinas, Blake Simmons, Eric Cummings, and Yolanda Fintschenko, have recently performed the first demonstration of iDEP for the concentration and separation of bacteria. The flow is driven electrokinetically. The point where the dielectrophoretic mobility exceeds the electrokinetic mobility is the point at which species trap. The post geometry, spacing and size affect the strength of the dielectrophoretic traps.
|Fig 1. Image of iDEP Simultaneous Concentration and Separation of Live and Dead E. coli. 10 X magnification, inverted fluorescence microscope. Live cells are green, dead cells are red. A field of 1.3 kV/cm was applied.||Fig 2. Image of Simultaneous Concentration and Separation of Live E. Coli and Inert 1-mm Particles. All conditions were as in Figure 1 except live cells were at a 1:100 dilution while the 1 mm rhodamine labeled polystyrene beads were diluted 1:10. The applied voltage was 2.5 kV over 1.25 cm. The post offset is 0° , and the spacing is 15 mm.|
(Figure 1) shows the separation and concentration of live and dead E. coli. The cells were observed to have a negative dielectrophoretic mobility in a deionized water background, which is consistent with the theory. When particles are negatively dielectrophoretic, the field-induced dipole in the particles is less than that of the background electrolyte, resulting in the particle being repelled from regions of high electric field. The dead bacteria, have a leaky membrane resulting in less negatively dielectrophoretic behavior. This results in a separation of bands of live and dead cells under iDEP trapping conditions
It was important to determine the behavior of E. coli in the presence of inert, non-cellular particles that could be in sample background. Therefore, 1-mm polystyrene beads coated with rhodamine were added to a sample of live E. coli. The polystyrene beads exhibit positive dielectrophoresis, in contrast to the DEP behavior for live cells (Figure 2).
These experiments show the power of iDEP for the selective concentration of bacteria. Potential applications include a front-end device for large volume applications such as water analysis and small volume applications such as medical diagnostics.