pH-switchable, ion-permselective gold nanotubule membrane based on chemisorbed cysteine.

An electroless gold plating method was used to deposit Au nanotubules within the pores of a polycarbonate template membrane. pH-switchable ion-transport selectivity was introduced by chemisorbing L-cysteine to the inside tubule walls. At low pH, where both the amino and carboxyl groups of the cysteine are protonated, these membranes preferentially reject cations and transport anions. At high pH, where both the amino and carboxyl groups are deprotonated, these membranes preferentially reject anions and transport cations. At pH = 6.0, near the isoelectric point of the chemisorbed cysteine, these membranes show neither cation nor anion transport selectivity. In addition to this electrostatically based selectivity, because of the small inside diameter of the Au nanotubules (as small as 0.9 nm), these membranes show molecular-size-based selectivity.