Pressure-sensitive ion transport measurements

The chip with formed one graphene nanopore was mounted in the custom-made PMMA chambers (see Fig. 1) as described before (25). After mounting, the nanopore was wetted with degassed water:ethanol (v/v, 1:1) solution for at least 45 min to remove bubbles trapped in the chambers. After changing to the degassed salt solutions (various ion types, ionic concentrations, and pH values; Sigma-Aldrich), two PMMA chambers on both sides of the chip were sealed by threaded tee joints containing a pair of chlorinated Ag/AgCl electrode, and a gas pipe was connected to a voltage-controlled pressure regulator (SMC Corp.). The regulator used to apply pressure was connected to an air compressor via filters (SMC Corp.). A pair of Ag/AgCl electrode was used to apply the voltage, and the current between the two electrodes was detected using an Axopatch 200B patch-clamp amplifier (Molecular Devices). We used a NI PXI-4461 card for data digitalization. The ionic current was recorded by applying a step-like pressure sweep (varying typically from 0 to 2.5 bar in 0.25-bar steps for 5-s intervals) that was executed at least 3 cycles at a constant voltage bias (ranging typically from −100 to 100 mV with a step of 50 mV), and then we analyze the SD of current time trace as the error bars of the data from three experimental repetitions. The typical current time trace was shown in figs. S5 and S6.

The ionic conductance under different applied pressures was obtained by fitting the I-V data. We defined the streaming conductance change ratio, G_str/G₀, as the increased conductance with applied pressure divided by the conductance without applied pressure to determine the pressure-sensitive response.