Nanobubble-controlled nanofluidic transport

Abstract

Nanofluidic platforms offering tunable material transport are applicable in biosensing, chemical detection, and filtration. Prior studies have achieved selective and controllable ion transport through electrical, optical, or chemical gating of complex nanostructures. Here, we mechanically control nanofluidic transport using nanobubbles. When plugging nanochannels, nanobubbles rectify and occasionally enhance ionic currents in a geometry-dependent manner. These conductance effects arise from nanobubbles inducing surface-governed ion transport through interfacial electrolyte films residing between nanobubble surfaces and nanopipette walls. The nanobubbles investigated here are mechanically generated, made metastable by surface pinning, and verified with cryogenic transmission electron microscopy. Our findings are relevant to nanofluidic device engineering, three-phase interface properties, and nanopipette-based applications.

Fig. 1. Nanobubble-induced ion current rectification.

(A to C) Cryogenic transmission electron micrographs and corresponding ionic current measurements for (A) a nanobubble-plugged nanopipette, (B) a nanobubble-free nanopipette, and (C) an air-filled nanopipette. (D) Additional nanobubble micrographs.

Fig. 2. Electronic characterization of a nanobubble-plugged nanochannel.

(A) Ionic currents through a single nanopipette in 3 M KCl, with relative nanobubble sizes. (B) Nanobubbles induce surface-governed ion transport through interfacial electrolyte films (thickness, d_el) enriched with cations by the nanobubble surface charge (σ_NB). (C) Finite element simulation of ion transport in (A). (D) Normalized current noise spectra for nanobubble configurations in (A). (E) Equivalent circuit representation of nanofluidic model in (B). The interfacial electrolyte resembles a voltage-dependent resistor. The nanobubble resembles a shunt capacitor. (F and G) AC impedance measurements (symbols) for nanopipette configurations in (A), fit to single-element parallel R-C circuit transfer functions (lines).

Fig. 3. Nanobubble-induced ion current enhancement.

(A) Ionic currents through a single nanopipette in 3 M KCl. Inset: Nanobubbles enhance current magnitudes. (B) Ionic currents through a single nanopipette in 140 mM KCl. At the lower ionic strength, the nanobubble induces stronger current enhancement and rectification. (C) Ionic currents through a positively charged nanopipette in 140 mM KCl resemble a bipolar nanofluidic diode with polarity determined by the presence or absence of a nanobubble. (D) Ionic currents through a single nanopipette in 5 mM KCl demonstrate further increases in current enhancement and rectification with greater electrolyte dilution.

Fig. 4. Nanobubble metastability.

(A) Ionic currents through an otherwise unperturbed nanobubble-plugged nanopipette. The nanobubble grows for 5 days before settling to a low-conducting state, with dynamic bubble heights estimated (inset). (B) Nanobubble-electrolyte gas exchange (J_gas). Efflux occurs through spherical caps and influx occurs through the interfacial electrolyte. Flux magnitudes depend on the interfacial gas concentration (c_surf) determined by the contact angle (φ_NB) and radius (r_NB). (C) Pressure balances (left axis) describe the electrolyte (black curve) and nanobubble (blue line) pressures according to two-phase pressure differences (green lines). Dissolved gas concentrations (right axis, red dashed curve) determine influx and efflux regimes in (B). (D) Gas oversaturation ratio at the nanobubble surface versus contact angle (left axis, solid line). The dissolved gas concentration in the interfacial electrolyte drives influx by slightly exceeding the surface concentration (right axis) and depends on the interfacial electrolyte thickness (dashed and dotted curves).