As seen in Table 3, the rectification factor dropped to 2 and 3, close to that of the expected as-made membranes. The disappearance of rectification effect provided

supportive evidence that the functional anionically charged dye played as gatekeeper to modulate the ionic flux through DWCNT membranes. Table 3 Summary of ionic find more rectification factor on DWCNT membrane after water plasma oxidation to remove gatekeepers Concentration Rectification factor (mM) Potassium ferricyanide NDS Sodium benzenesulfonate 10 3.2 ± 0.3 1.7 ± 0.2 2.4 ± 0.2 50 2.8 ± 0.3 1.5 ± 0.07 2.0 ± 0.2 100 2.4 ± 0.2 1.4 ± 0.0.02 2.0 ± 0.2 Ferricyanide has a well-known redox potential of 0.17 V (vs. Ag/AgCl), and thus, an important control experiment was AMN-107 ic50 done to make sure that the Selleck C646 observed rectification was not due to faradic current; instead, it was due to transmembrane ionic current. Cyclic voltammetry scans (−0.6 to 0.6 V) showed no redox reaction on both as-made and one-step functionalized DWCNT membranes in 50-mM ferricyanide (Additional file 3: Figure S3). We also did not observe redox reaction on glassy carbon in 2-mM ferricyanide, as seen in the flat curve in Additional file 4: Figure S4A. The much larger conductive

area of the glassy carbon electrode compared to 5% DWCNT membrane requires the use of more diluted (2 mM) ferricyanide solution. However, with the supporting 0.5-M electrolyte KCl solution, the oxidation and reduction peaks were observed at 0.29 and 0.06 V, which

were similar to those found in reports [30, 50]. The experiment was also repeated with both redox species. In Additional file 4: oxyclozanide Figure S4B, no redox peak was found on glassy carbon in 50-mM ferricyanide solution and 25-mM ferricyanide/ferricyanide solution. The control experiments of cyclic voltammetry on DWCNT membrane and glassy carbon ruled out the redox reaction of ferricyanide, which supports the ionic rectification on electrochemically grafted CNT membranes. The non-faradic (EIS) spectra indicated that the functionalized gatekeeper by a single step can be actuated to mimic the protein channel under bias. This functional chemistry was proven to be highly effective on the enhancement of ion rectification. The disappearance of rectification also supported its effectiveness after removing the grafted gatekeeper by plasma etching. Interestingly, no apparent change of rectification was seen for the two-step functionalization. The likely reason is that highly efficient functional density can be obtained by electrografting of amine in one step since the poor yield in the second step (carbodiimide coupling reaction) resulted in a significantly lower gatekeeper density on CNT membranes. To address this question, two- and one-step functionalizations were quantified using dye assay on glassy carbon due to its well-defined area and similar chemical reactivity to CNTs.