Tuning direct current streaming dielectrophoresis of proteins.

Dielectrophoresis (DEP) of biomolecules has large potential to serve as a novel selectivity parameter for bioanalytical methods such as (pre)concentration, fractionation, and separation. However, in contrast to well-characterized biological cells and (nano)particles, the mechanism of protein DEP is poorly understood, limiting bioanalytical applications for proteins. Here, we demonstrate a detailed investigation of factors influencing DEP of diagnostically relevant immunoglobulin G (IgG) molecules using insulator-based DEP (iDEP) under DC conditions. We found that the pH range in which concentration of IgG due to streaming iDEP occurs without aggregate formation matches the pH range suitable for immunoreactions. Numerical simulations of the electrokinetic factors pertaining to DEP streaming in this range further suggested that the protein charge and electroosmotic flow significantly influence iDEP streaming. These predictions are in accordance with the experimentally observed pH-dependent iDEP streaming profiles as well as the determined IgG molecular properties. Moreover, we observed a transition in the streaming behavior caused by a change from positive to negative DEP induced through micelle formation for the first time experimentally, which is in excellent qualitative agreement with numerical simulations. Our study thus relates molecular immunoglobulin properties to observed iDEP, which will be useful for the future development of protein (pre)concentration or separation methods based on DEP.