A three-dimensional graphene-carbon nanotube (G-CNT) composite-coated screen printed electrode (SPE) is developed as the substrate for bismuth-modified electrodes to measure trace lead ions in this work. This G-CNT/SPE is modified with Nafion and then deposited with Bi nanoparticles, denoted as Bi/Nafion/G-CNT/SPE. Due to smooth electron pathways and effective exposure of Bi nanoparticles on G-CNT and the negative charge of Nafion, the Pb2+-sensing ability of Bi/Nafion/G-CNT/SPE is over 50 times of that of a Bi-deposited SPE electrode, which can be further optimized by the experimental design strategy combining the fractional factorial design (FFD) and the steepest ascent path (SAP) studies. In the FFD study, the concentration of Nafion and pH of the stripping buffer solution are identified to be the key factors affecting the stripping current of lead pre-deposited onto Bi/Nafion/G-CNT/SPE. From the SAP study, a simple but reliable model for increasing the lead-ion sensitivity of Bi/Nafion/G-CNT/SPE has been constructed. The highest stripping current response of lead-ion sensing on Bi/Nafion/G-CNT/SPE can be obtained when 0.3% Nafion and the stripping buffer solution with pH = 4.75 are employed. (C) 2013 The Electrochemical Society.