Also, due to the relatively large size of DWCNTs (approximately 2.0-nm i.d.) compared to single-walled CNTs (SWCNTs, 1.4 nm), the rectification of small ion pairs (i.e., KCl) was not seen, as was for the case of SWCNTs [42]. However, larger mobile anions such as ferricyanide, 2,6-naphthalenedisulfonic acid (NDS), and benzenesulfonate showed rectification (Table 1). The ionic current of potassium ferricyanide vs. transmembrane bias for as-made and modified DWCNT membranes is shown in Figure 6, with a summary of rectification factors in Table 2. The highest observed experimental rectification factor of ferricyanide was
14.4 for single-step grafting, which was 3.7 times as that of as-made membrane. Luminespib manufacturer The rectification factor dropped with increasing ionic concentration, which was expected for the screening of charge on the gatekeepers at high ionic strength. The rectification factor dropped to 9.8 when the ferricyanide concentration increased from 10 to 50 mM. With the concentration increasing up to 100 mM, the rectification factor further dropped to 8.0. It seemed that rectification
was attributed to both charge and steric effects at low concentration. The steric effect was dominant at the high-concentration region. Table 1 Summary of ionic rectification factor on single-step modified DWCNT-dye membrane Concentration Rectification factor (mM) Potassium ferricyanide NDS Sodium benzenesulfonate 10 7.2 ± 0.3 3.1 ± 0.3 2.4 ± 0.2 50 6.4 ± 1 2.0 ± 0.1 STI571 in vitro 2.0 ± 0.1 100 5.6 ± 1 2.3 ± 0.1 1.7 ± 0.1 Rectification factor was calculated by the ratio of ionic transport current at ±0.6-V bias. Linear scan was from −0.60 to +0.60 V with the scan rate at 50 mV/s. Figure 6 Ionic rectification curves Carbohydrate on (A) as-made and (B) modified DWCNT membranes with potassium ferricyanide. Table 2 Comparison of ionic current rectification factor in K 3 Fe(CN) 6 solution Concentration of K3Fe (CN)6 Rectification factor (mM) As-made Single-step electrooxidation
of amine Electrochemical grafting of diazonium and coupling of dye Chemical grafting of diazonium and coupling of dye 10 3.9 ± 0.8 14.4 ± 0.6 2.9 ± 0.2 4.0 ± 0.4 50 4.4 ± 0.9 9.8 ± 0.3 2.9 ± 0.2 3.3 ± 0.07 100 3.4 ± 0.1 8.0 ± 0.4 3.2 ± 0.3 3.6 ± 0.2 Rectification factor was calculated by the ratio of ionic transport current at ±0.6-V bias. Linear scan was from −0.60 to + 0.60 V with the scan rate at 50 mV/s. On another modified membrane with one-step amine grafting, we compared the rectification factor of three different ions, namely ferricyanide, NDS, and sodium benzenesulfonate, to examine the role of anion size in being repelled by the modification of CNT tips. In Table 1, we saw that as the ion size was reduced, smaller rectification factors were seen, which were consistent with those of partially blocked ion channels. Similar to Table 2, as ionic strength was increased, the rectification factor decreased for all of the anions. It indicated that the rectification was partially attributed to the charge effect.