(a) During the immobilization of streptavidin and probe DNA, even- (odd-) numbered sites are biased at +1.2 V (−1.2 V) with respect to VREF=0 V for 20 s. (b) Confocal fluorescence image (false coloured) of the array after the site-specific probe DNA immobilization, and the corresponding spatial profile of the averaged fluorescence intensity shown across the array. The averaged fluorescence intensity across the four even-numbered graphene sites is distinctively stronger than that across the four odd-numbered sites. Error bars represent ±1 s.d. *P<0.05. Scale bar, 100 μm. (c) Simulated electrolytic potential profile and spatial pattern of the corresponding electric field magnitude just above the substrate surface, in the setup mimicking the steady-state drift situation (Supplementary Note 4); in the latter, the electric field magnitude of repulsive fields is set to zero and a moving average and normalization to the maximum are applied. (d) Electrical detection of 1 pM hybridization after site-specific DNA immobilization. ΔV0 statistics for each graphene FET are from 5 VREF–IDS measurements before and after 1 pM hybridization, treating forward and reverse VREF sweeps separately. The averaged ΔV0 values subsume both forward and reverse sweeps. Error bars represent ±1 s.d. *P<0.05.