Visualization of Ion Fluxes in Nanopipettes: Detection and Analysis of Electro-osmosis of the Second Kind

Abstract

Nanopipettes are finding increasing use as nano "test tubes", with reactions triggered through application of an electrochemical potential between electrodes in the nanopipette and a bathing solution (bath). Key to this application is an understanding of how the applied potential induces mixing of the reagents from the nanopipette and the bath. Here, we demonstrate a laser scanning confocal microscope (LSCM) approach to tracking the ingress of dye into a nanopipette (20−50 nm diameter end opening). We examine the case of dianionic fluorescein under alkaline conditions (pH 11) and large applied tip potentials (±10 V), with respect to the bath, and surprisingly find that dye ingress from the bath into the nanopipette is not observed under either sign of potential. Finite element method (FEM) simulations indicate this is due to the dominance of electro-osmosis in mass transport, with electro-osmotic flow in the conventional direction at +10 V and electro-osmosis of the second kind acting in the same direction at −10 V, caused by the formation of significant space charge in the center of the orifice. The results highlight the significant deviation in mass transport behavior that emerges at the nanoscale and the utility of the combined LSCM and FEM approach in deepening understanding, which in turn should promote new applications of nanopipettes.

Spatiotemporal Controlled Formation of Synthetic Nanoreactors in Single Living Cells

“…The simulated results indicated that the concentration distribution of 2MeIm was unaffected by the electric field. In contrast, the concentration of Zn 2+ dramatically increased with the potential magnitude under negative potentials, with the distribution concentrated at the tip orifice. − Therefore, when a negative potential was applied, the ZIF-8 nanoparticles were likely to grow inside the nanopipet within a confine space, which was likely to be condensed into clusters. On the contrary, as the distribution of Zn 2+ was mainly outside the nanopipette under positive potentials; it could be deduced that the ZIF-8 nanoparticles were formed at the tip place and then driven outside the nanopipette by the electric field; thus, the ZIF-8 particles could be delivered into the bulk solution outside the nanopipette.…”

Specially Resolved Single Living Cell Perfusion and Targeted Fluorescence Labeling Based on Nanopipettes

“…c 0 was the primary concentration of the dyes flowing from the nanopipette to the cytoplasm edge. Navier–Stokes equations were used to describe the electro-osmotic flow of the fluorescence molecules as shown in the Supporting Information. − …”

The New Era of High-Throughput Nanoelectrochemistry

“…The asymmetric ion transport under diluted electrolyte solution gives rise to the ion current rectification (ICR) effect. , This effect has be further developed into a nanopore sensing approach where the characteristic current–voltage curves can be modulated by the changes on the surface of the nanopore induced by the presence of analytes (Figure B) . Furthermore, recent studies on the time-dependent dynamics of mass transport into nanopipettes and characterization of nanopipette transport physics will benefit a wide range of nanoscale experimentation, including the synthesis of inorganic and organic materials and the stabilization of unusual polymorphs by confinement …”