(a) Measured transient electrolytic current between two neighbouring graphene sites with the application of a voltage step of 1.0 V in 0.005 × PBS. The transient response is complete in ca. 30 ms with a peak current of ~600 nA. From this, we estimate a peak current of ~1.4 μA with a 2.4-V bias difference between two neighbouring graphene sites (Supplementary Note 1). (b) Reference electrode (Ag/AgCl) to graphene voltammogram at a scan rate of 10 mV s−1. From this, the d.c. steady-state leakage current for graphene biased at either +1.2 or −1.2 V is extrapolated to be 1–10 nA (Supplementary Note 2).