Electropreconcentration diagrams to optimize molecular enrichment with low counter pressure in a nanofluidic device

Concentration polarization (CP)‐based focusing electrokinetics nanofluidic devices have been developed in order to simultaneously detect and enrich highly diluted analytes on‐a‐chip. However, stabilization of focal points over long time under the application of the electric field remains as a technical bottleneck. If pressure‐assisted preconcentration methods have been proposed to stabilize propagating modes at low inverse Dukhin number ( 1 / D u ≪ 1 ) , these recent protocols remain laborious for optimizing experimental parameters. In this paper, “electric field E/counter‐pressure P” diagrams have been established during pressure‐assisted electro‐preconcentration of fluorescein as a model molecule. Such E/P diagram allows direct observation of the region for which the optimal counter‐pressure P leads to a stable focusing regime. This region of stable focusing is shown to vary depending of the nanoslit length (100 μm < L nanoslit < 500 μm) and the nature of the background electrolyte (KCl and NaCl). Longer nanoslits (500 μm) produce stabilization at low counter‐pressure P, whereas NaCl offers a narrower region of stable focusing in the E/P diagram compared to KCl. Finally, the ability of such pressure‐assisted protocol to concentrate negatively charged proteins has been tested with a more applicative protein, i.e., ovalbumin. The corresponding E/P diagram confirms the existence of the stable focusing regime at both low electric field E (≤20 V) and counter‐pressure P (≤0.4 bar). With an enrichment factor as high as 70 after 2 min for ovalbumin at a concentration of 10 μM, such pressure‐assisted nanofluidic electro‐preconcentration protocol appears very promising to concentrate and detect biomolecules.