Conformation and Dielectrophoresis of Single Weak Polyelectrolyte under AC Electric Field

The application of AC-electric fields, when integrated with current microarray and ``lab-on-chip'' platforms, has emerged as a useful tool to manipulate and assemble supramolecular aggregates, such as AC-field induced protein crystallization and DNA hybridization, yet the detailed mechanism remains unclear. To understand the AC-polarization mechanism of biomacromolecules, we examine the conformational and dielectrophoretic behavior of polyelectrolytes under AC-electric field using fluorescence correlation spectroscopy (FCS) at single molecular level. We focus on poly (vinyl pyridine) (PVP) as a model polyelectrolyte whose conformation can be tuned by adjusting the pH and ionic strength. We observe that in the non-uniform AC electric field, PVP in a coil conformation experiences the positive DEP force at applied frequency below 500 kHz; it is also intriguing to observe the elongation of PVP coils at a characteristic AC-frequency of $\sim $ 200 kHz. In sharp contrast, PVP in a globule conformation shows insensitive to imposed AC-fields. We also examine the conformational change in a uniform AC-field where the DEP force is absent; the result suggests a redistribution of counterions with an induced dipole of polyelectrolyte. The behavior is further investigated with varied medium conductivity and microelectrode geometry.