Electrochemical Study of Dopamine at Electrode Fabricated by Cellulose-Assisted Aqueous Dispersion of Long-Length Carbon Nanotube

A long-length (hundred micrometers) carbon nanotube is successfully dispersed in aqueous solution with surfactant cellulose while maintaining its length. An electrochemical study of the synthetic pathway of dopamine (DA), dopamine-o-quinone (DAQ), leucodopaminechrome (LDAC), and dopaminechrome (DAC) at the electrode fabricated by the long-length carbon nanotube dispersed solution is presented. The sequence DA reversible arrow DAQ -> LDAC reversible arrow DAC for the reaction is electron transfer-chemical reaction-electron transfer (ECE)-type, which is a chemical reaction (DAQ -> LDAC, C) interposed between two electron transfer reactions (DA reversible arrow DAQ and LDAC reversible arrow DAC, E). The salient electrochemical signals due to both DA reversible arrow DAQ and LDAC reversible arrow DAC can be obtained at the long-length carbon nanotube electrode, unlike other carbon electrodes such as carbon paste, graphene, fullerene, nanofiber, and graphite. The overall reaction is dominated by step DAQ -> LDAC and is sensitive to pH. With cyclic voltammetry in acidic media, the peak current due to LDAC -> DAC disappeared at a higher scan rate because the reaction rate for DAQ -> LDAC was so slow that DAQ was completely consumed in the electron transfer of DAQ -> DA before the chemical reaction of DAQ -> LDAC could go forward. In alkaline media, the peak height due to DAC -> LDAC became as high as that due to DA -> DAQ because the DAQ -> LDAC rate became fast enough that a sufficient amount of LDAC was generated for the subsequent reaction of LDAC -> DAC. Concomitantly, the reaction DAQ + LDAC reversible arrow DA + DAC was generated. Quantitative and selective detection of dopamine based on the signal due to LDAC -> DAC is possible just as in the conventional strategy of direct oxidation of dopamine (DA -> DAQ).