Influence of Lanthanum on Stern Layer Conductance in the Nanochannel.

In this report, high-frequency electric impedance spectroscopy was performed to investigate ionic transport through nanochannels. Special attention was focused on (i) conductance behaviors depending on the role of cation valence in three background electrolytes (XCln): monovalent 1-1 (K+ and Cl-), divalent 2-1 (Mg2+ and 2Cl-), and trivalent 3-1 (La3+ and 3Cl-), (ii) the effects of proton and bicarbonate ions on bulk and surface conductance, and (iii) the connected microchannel dimension (surface/height ratio aspect) within the nanochannel apparent conductance. The results highlight a net quantitative increase in surface silanol density and a strong decrease in surface ionization degree when lanthanum cations are employed. The results also demonstrate that La3+ strongly interacts with the silica surface, leading to negative values of standard free energy for ion-site interactions and chemical potential for ion-ion correlations in the Stern layer of -0.8 and -10.2 kT, respectively. We ascribed the evolution of surface charge density to the balance between the mole ratios of water molecules and adsorbed cations at equilibrium. We found that La3+ behaves as an acidic cation (Lewis conceptualization) that neutralizes the negative silica surface accompanying water molecule expulsion due to steric hindrance. This study constitutes a new contribution to ion-site interactions and to ion-ion correlation phenomena on the planar silica surface to explain charge inversion observation in micro-nanofluidic devices.

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/Du≪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 < Lnanoslit < 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.