We developed a graphene nanoplatelet-modified fluorine-doped tin oxide electrode (GNP/FTO) for the simultaneous detection of dopamine (DA) and uric acid (UA) in the presence of ascorbic acid (AA) and investigated the interaction mechanisms of DA, UA, and AA with GNPs considering their charging states at different pH values. Owing to the unique structure and properties originating from the oxygen and nitrogen functional groups at the edges, GNPs showed high electrocatalytic activity for the electrochemical oxidations of AA, DA, and UA with peak-to-peak potential separations (Delta EP) between AA-DA and DA-UA of ca. 0.23 and 0.17 V, respectively. These values are sufficiently high to allow the simultaneous detection of DA and UA without interference from AA. The highly sensitive and stable GNP/FTO sensor showed sensitivities of ca. 0.15 +/- 0.004 and 0.14 +/- 0.007 mu A/mu M, respectively, with detection limits of ca. 0.22 +/- 0.009 and 0.28 +/- 0.009 mu M, respectively, for DA and UA. The sensor could detect DA and UA concentrations in human serum samples with excellent recoveries. (C) 2017 Elsevier B.V. All rights reserved.