Although the results in Figs. provide strong evidence that the surface charge was altered by the decoration by the quaternary ammonium cation with PS beads, the role of the beads may not be clear. Because the diameter of the beads was 1 μm, which is much larger than the screening length, the FET signal may be attributed to the CRP in proximity to the graphene surface. It is possible that quaternary ammonium cations, apart from the beads, were also adsorbed on the graphene surface through cation-π interactions, leading to surface-charge modulation and signal generation. In the current process, decoration and mAb immobilization were not uniform, as indicated by the fluorescence image in Fig. . This inhomogeneity may account for the variations in the response intensity of the GFETsig over the array.