Dimension-reconfigurable bubble film nanochannel for wetting based sensing

Abstract

Dimensions and surface properties are the predominant factors for the applications of nanofluidic devices. Here we use a thin liquid film as a nanochannel by inserting a gas bubble in a glass capillary, a technique we name bubble-based film nanofluidics. The height of the film nanochannel can be regulated by the Debye length and wettability, while the length independently changed by applied pressure. The film nanochannel behaves functionally identically to classical solid state nanochannels, as ion concentration polarizations. Furthermore, the film nanochannels can be used for label-free immunosensing, by principle of wettability change at the solid interface. The optimal sensitivity for the biotin-streptavidin reaction is two orders of magnitude higher than for the solid state nanochannel, suitable for a full range of electrolyte concentrations. We believe that the film nanochannel represents a class of nanofluidic devices that is of interest for fundamental studies and also can be widely applied, due to its reconfigurable dimensions, low cost, ease of fabrication and multiphase interfaces.

Fig. 1. The principle of film nanochannel and setup.

a Schematic of the principle. The film nanochannel was formed by inserting a gas bubble in a capillary. The film has length L and height h. Two connected electrodes were used for the electrical characterizations. b A snapshot of bubble formation within a PDMS microfluidic flow-focusing structure by high-speed camera (Photron, FASTCAM WX50). c A single bubble was conducted to a glass capillary and remained static in microscopic view, where it appeared circular from the cross-sectional view (d). e Picture of setup. Two electrodes were inserted in the pinched holes, and connected to a potentiostat for electrochemical measurements. f Wave-like patterns were found by AFM at the inner surface of capillary, creating an average roughness of 2.6 nm. g Typical I–V curves of a system with bubble (green) and without bubble (red). h An equivalent circuit of the system, consisting of resistances for the microchannel and film nanochannel and capacitances for the bubble meniscus (C_b,cap, gas/liquid interface) and EDL of film nanochannel (C_fch, liquid/solid interface).

Fig. 2. The electrical characterization of the film nanochannel.

a The amplitude of film nanochannel conductance measured by cyclic voltammetry decreased in time, becoming saturated after a certain number of cycles, at pH values of 8.5 (top) and 4 (bottom), respectively. b Typical I–V curves at the saturated states at pH values of 8.5 and 4 (dashed), showing the Ohmic behavior of the system. c The evolution of system conductance measured by CV (solid lines) gradually reached the value measured by EIS (dashed lines). Inset figure shows snapshot of a single bubble before (top) and after (bottom) the electrical measurements, exhibiting a smoother surface after the measurements.

Fig. 3. The tunable height and length of film nanochannel with characterizations.

a The measured conductance of film nanochannel per unit length at pH 8.5 (dots), with theoretical predictions (black solid line). The colored lines are theoretical conductance of a fictitious solid-state nanochannel with different surface charge densities and a height of 11 nm. b The calculated film thickness from the normalized conductance as a function of KCl concentration at pH 8.5 by theoretical approach (circle dots) and simulations by PNP equations (square dots). Dashed line shows the dependence of the Debye length on concentration. c The normalized conductance with 10 mM KCl solution. d The calculated film thickness as a function of pH at 10 mM solutions. Solid lines are the theoretical predictions described in Methods section. e Bubble shrinkage by applied pressure. The length of the film channel decreases with increasing external pressure of liquid phase. Snapshot of a single gas bubble when the applied external pressure increases from 0 to 1 bar. The length decreases to nearly half value when 1 bar pressure applied on the microchannel. f The decrease of bubble-length (upper graph, green squares)-induced decrease of film nanochannel resistance (upper graph, orange squares) as a function of applied pressure. Solid lines were derived from Eq. 1 with length predicted by Ideal Gas Law. The normalized resistance of film nanochannel (lower graph, dots) and channel height (lower graph, dots) remain constant due to the persistence of capillary pressure (contact angle of bubble). The error bars are the standard deviation of five or more individual results.

Fig. 4. Ion concentration polarization by film nanochannel.

a–f ICP effects were observed in 70 μM STB solution, while not in 3 mM STB solution (g–l), with 30 μM fluorescein used as fluorescence dye. The images were recorded every 2 min, to prevent the photobleaching of fluorescence. White solid lines were drawn to outline the capillary and the position of the bubble.

Fig. 5. Label-free immunosensing by film nanochannel.

a Schematic picture illustrating the principle of sensing the biotin-SAv reaction. b The functionalization with biotin (top) and subsequent successful binding of avidin on the capillary surface (bottom) was demonstrated by the fluorescence of FITC-SAv. c A change of contact angle in the capillary was observed after the immobilization of SAv. d The I–V curves show a strong change of the conductance induced by the biotin-SAv binding reaction (19 μM SAv). e The bubble length-normalized conductance of the film nanochannel G_fch^* was derived by linear fitting in 10⁻³× to 10× PBS solutions. The dashed lines are the theoretical conductance of a ‘fictitious’ solid-state nanochannel neglecting the surface conductance. f Sensitivity (G_SAv/G_bio) of film nanochannels (green) as function of salt concentration, compared to that of a polysilicon nanochannel (orange column). The blue square dots represent the contribution from the change of zeta potential, while the red circular dots represent the contribution from the wettability change. g The kinetics of the biotin-SAv reaction in 1 nM (orange square dots) and 10 nM (green circle dots) SAv solutions, with predicted ratio (0 to 1) of occupied biotin sites (right axis, solid lines). h The reaction kinetics in 50 nM (dark blue dots) and 200 nM (light blue dots) SAv solutions, with predicted ratio of biotin-SAv binding (right axis, solid lines). i The bubble-length normalized film nanochannel conductance G_fch^* as a function of SAv concentration for 0.01× and 1× PBS solutions. The dashed lines represent the measured conductance before biotin-SAv binding at 0.01× (blue) and 1× (red) PBS, while the dots are measured G_fch^* after SAv binding. j Calculated sensitivity as a function of SAv concentrations in 0.01× and 1× PBS solutions. The solid line represents data from a solid-state nanochannel. The error bars are the standard deviation obtained from five or more individual measurements.