The sulfonate Ro 61-8048 mw density as a function of one-step amine grafting time is shown in Figure 8. The sulfonate density reached its saturated level at 0.9 ×

1015 molecules/cm2 after 2 min of grafting. Since each Direct Blue 71 dye molecule contains four PSI-7977 clinical trial sulfonate groups, the dye molecule density was calculated as 2 × 1014 molecules/cm2, nearly one-half of the ideal monolayer density of 3.8 × 1014 molecules/cm2. The amine grafting density was less efficient than diazonium grafting density, which is consistent with that in the report [49]. Comparison of the total surface charge density by the two grafting methods is shown in Table 4. In the first step of the two-step functionalization, the carboxyl density reached up to 1.3 × 1015 molecules/cm2 after 8 min of grafting, showing an efficient process. After carbodiimide coupling

of dye in the second step, the charged density increased to 2.0 × 1015 molecules/cm2. With each carboxyl site being replaced with one dye molecule containing four sulfonate groups Belnacasan after coupling, each reacted site will have a net gain of three more charges. Going from 1.3 × 1015 to 2.0 × 1015 charges/cm2, with 3 charges/added dye, resulted in a sulfonate density of 0.93 × 1015 charges/cm2 after the two-step functionalization. The dye density was calculated as 0.233 × 1015 molecules/cm2 (one-fourth of the sulfonate density). This resulted in a carbodiimide coupling efficiency of 18% on glassy carbon. The net sulfonate density for the one- and two-step reactions is both comparable at 0.9 × 1015 charges/cm2, where the less efficient electrochemical either oxidation of amine is similar to the loss in efficiency for the carbodiimide coupling reaction. However, in the case of the DWCNT membranes, the two-step modification was not effective at showing rectification (Table 2). There are two possible reasons for the poor rectification on the membrane with two-step modification. The first possible reason is that dye molecules were directly conjugated on the CNT surface via the C-N bond in single-step modification. In two-step modification, the dye molecules were anchored on the diazonium-grafted layer, which is less conductive than glassy

carbon. Therefore, the directly grafted dye molecules in a single step are more responsive to the applied electric field. Another possible reason is that the actual yield of the second step in the two-step modification on CNT membranes may be significantly below the 18% yield seen on glassy carbon. The CNT surfaces interfere in the coupling reaction, presumably through the absorption of intermediates. Figure 7 Schematic illustration of dye assay quantification. (A) Quantification of carboxylic density on glassy carbon by pH-dependent adsorption/desorption. (B) Quantification of sulfonate density by ionic screening effect. (assumed charge/dye = 1:1). Figure 8 Quantification of sulfonate density as a function of grafting time using dye assay.