Carboxyl-functionalized graphene SGFET: pH sensing mechanism and reliability of anodization

In this work, we fabricated graphene solution-gate field-effect transistor (SGFET) to investigate the pH sensitivity of carboxyl functional groups. The functionalization of graphene with carboxyl was achieved through anodization by applying a sequential potential scan from 0.6 V to 1.3 V in 0.1 V steps. Raman spectroscopy was used to determine the defect density of the graphene caused by the anodization. The sequential anodization in Carmody buffer solution at pH 7 had low defective effect on graphene structure and shows that the original structure of graphene was conserved. Furthermore, we measured pH sensitivity of the carboxyl-functionalized graphene at pH 2 to pH 12. The pH sensitivity was 32.6 mV/pH at low pH region and pH sensitivity measurement was saturated at high pH (basic region). The negatively charged surface of carboxyl-functionalized graphene caused weak pH detection in the basic region. The pH sensing mechanism of carboxyl functionalized graphene in the low and high pH region is discussed in detail. The reliability of the carboxyl-functionalized graphene SGFET device was evaluated by measuring pH sensitivity repetitively after anodization at different potential scans of 0.9 V, 1.3 V, 1.5 V, 1.7 V and 1.9 V which demonstrated in all cases that, the pH sensitivity of carboxyl functionalization on graphene SGFET shows a similar trend. This functionalization method allows the modification of the graphene surface for further uses in biosensing.