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.
Highlights
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
To use the applied potential to control reaction conditions within a nanopore, it is essential that the local electric field and ion distribution are understood. Most understanding in this area comes from finite element method (FEM) simulation of the continuum Poisson−Nernst−Planck model,[1,3,4,13,16] with the Navier−Stokes equation added to account for electroosmotic flow (EOF)[17] and speciation reactions to account for solution equilibria.[16]
Our results support the idea that the walls exhibit significant capacitance at large applied potentials and that electro-osmosis of the second kind can have a significant effect on the overall flow at the orifice
Summary
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). N anopores and nanopipettes are gaining in popularity as nanoscale electrochemical devices, from use as nanosized “test tubes” to carry out reactions in confinement,[1−6] to nanoscale sensing,[7−9] and nanoparticle analysis.[10,11] For conical pores, the geometry ensures that usually but not always[4] (depending on the solvent/electrolyte system) most of the potential applied across the nanopore drops inside the orifice, with a resulting intense electric field in this region In principle, this field can be used to accumulate or deplete ionic species within the pore, providing a fascinating way of controlling the composition of microscopic regions of solution. Our results support the idea that the walls exhibit significant capacitance at large applied potentials and that electro-osmosis of the second kind can have a significant effect on the overall flow at the orifice
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