“…It has been recognized that (1) at pH 6.0, AA (pK a = 4.10) and DA (pK a = 8.87) exist in the anionic and cationic forms, respectively [7,15,17]; (2) oxidized DA can be chemically reduced by AA [5,15]; (3) an oxidized acid can introduce carboxyl and hydroxyl groups on carbon nanotubes, and, these groups, in turn, increase the negativity of the carbon nanotubes [9,11]. From the point of view of electrostatic interaction, the negatively charged surface of CAW, especially the HNO 3 treated CAW modified electrode, due to the existence of a number of hydroxyl and carboxyl groups [6], would tend to adsorb the positively charged DA and retard the oxidation of AA negatively charged, resulting in the positive shift of E pa and the decrease of i pa of AA in the presence of DA, compared to the case without the presence of DA, as shown in Table 1.…”