(182g) DNA Identification with Symmetry-Breaking Dielectrophoretic Patterns of Genetic Bead Suspensions

Dielectrophoresis (DEP) is a term commonly used to describe the field induced polarization and translational motion of a polarizable particle in a non-uniform AC electric field. Traditional DEP detection, particle sorting and concentration schemes are typically based on the determination and exploitation of particle specific DEP crossover frequencies, in that each particle-type has a specific electric field frequency where the induced DEP particle dipole goes to zero.

In this work we consider the effect that a particle specific DEP force has on the local particle distribution in the vicinity of a local field minimum or maximum. In the presence of a non-uniform field, the particle concentration obeys a Poisson-Boltzmann distribution with a "potential" field that scales as ~exp(a²E²Re(f)/6µD) , where a is the particle size, E is the local electric field intensity, f is the Clausius-Mossoti factor for a spherical particle, D is the effective particle diffusivity, and µ is the fluid viscosity. As the particles achieve maximum packing at a field minimum (for negative DEP) and maximum (for positive DEP), this Boltzmann distribution produces a suspension polarization length, analogous to the Debye screening length for ions, that characterizes the suspension aggregate dimension in the direction normal to the field gradient.

By designing a quadrupole micro-electrode with specific symmetries and length scales comparable to the average suspension polarization length, we are able to overlap the different suspension aggregates into a kalaidescope of two dimensional suspension patterns with different symmetries and anti-symmetries. An open orifice exists at the center of the quadrupole for positive DEP (Fig. A-B) and a closed circular aggregate appears for negative DEP (Fig. C). Unlike previous DEP colloidal results, the reported suspension pattern orifice dimensions are highly dependent on the field frequency and particle surface charge, and unlike traditional crossover frequency measurements, can be used to directly calculate the field induced DEP particle force.

We utilize the above mentioned exponential Boltzmann sensitivity to detect the cross-over frequency of the particles and the frequency dependent DEP force, not from tracking individual particles but by inspecting the changes in the overall positive DEP suspension pattern (Fig. A-B), particularly in the orifice dimension. In fact, we can discern small changes in the particle dielectrophoretic mobility without crossing the crossover frequency. In this manner, we are able to discern whether certain genetic beads, functionalized with DNA probes, are hybridized with specific DNA targets. The hyrbridization reaction changes the surface conductance of the beads and the corresponding DEP force to produce a rich variety of suspension patterns with different symmetries. DNA identification with genetic bead suspensions are hence much more sensitive than single particle tracking due to the Boltzmann exponential dependence and the long-range suspension pattern morphologies. Just like hydrodynamic instabilities can magnify thermal noise, symmetry breaking in the suspension allows us to decipher molecular events on nano-colloids.