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Based on the experimental strategy of fractional factorial design (FFD) and path of the steepest descent/ascent (PSD/PSA), the preferred orientation ratio of Bi(110)/Bi(012) facets of Bi deposits electroplated under a direct-current (dc) mode could be precisely controlled and predicted. The intensity ratio of Bi(110)/Bi(012) facets (i.e., the preferred orientation ratio which is denoted as f) was employed as the response variable since this variable was found to be one of the key factors determining the sensitive ability of bismuth-film electrodes (BFEs) to Sn(II). In the FFD study, temperature of the plating bath was identified to be the key factor affecting the preferred orientation ratio of Bi deposits meanwhile f only weakly depended on pH, current density, and stirring rate. From the PSD/PSA study, a simple but reliable model for changing the preferred orientation ratio was constructed and the deposit plated at 28 • C and pH = 4.25 showed the highest f value. Moreover, BFEs with various f values could be easily prepared and controlled, meanwhile the morphologies of Bi deposits with different f values were also examined in this work.

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.

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