(a) During the immobilization of Cy3-labelled probe DNA and associated streptavidin, even- (odd-) numbered sites are biased at +1.2 V (−1.2 V) (20 and 120 s for streptavidin and Cy3-labelled probe DNAs, respectively). During the subsequent immobilization of Cy5-labelled probe DNAs and associated streptavidin, the even- (odd-) numbered sites are left unbiased (biased at +1.2 V), with the same timing scheme as before. (b) Confocal fluorescence image (false coloured) of the array with Cy3 and Cy5 probe DNAs shown, respectively, as red and green, and the corresponding normalized fluorescence intensity. The averaged fluorescence intensities across the four even-numbered and four odd-numbered graphene sites with Cy5 intensity are normalized to the maximum Cy3 intensity; *P<0.05; **P<0.01; error bars represent ±1 s.d. Scale bar, 100 μm.