Controlling ion-transport selectivity in gold nanotubule membranes

We have developed a new class of synthetic membranes that consist of a poro us polymeric support. This support contains an ensemble of gold nanotubules that span the complete thickness of the support membrane. The support is a commercially available microporous polycarbonate filter with cylindrical n anoscopic pores. The gold nanotubules are prepared via electroless depositi on of Au onto the pore walls, and tubules that have inside diameters of mol ecular dimensions (<1 nm) call be prepared. Hence, these membranes are a ne w class of molecular sieves. We review in this paper the ion-transport prop erties of these Au nanotubule membranes. We will show that these membranes call be cation-permselective or anion-permselective, and that the permselec tivity can be reversibly switched between these two states. Ion permselecti vity can be introduced by two different routes. The first entails chemisorp tion of all ionizable thiol, e.g., a carboxylated ammonium-containing thiol to the Au tubule walls. If the thiol contains both of these functionalitie s (e.g., the amino acid cysteine), the permselectivity call be reversibly s witched by varying the pH of the contacting solution phase. Ion permselecti vity, can also he introduced by, potentiostatically charging the membrane i n all electrolyte solution. By applying excess negative charge, cation perm selective membranes are obtained, and excess positive large yields anion pe rmselective membranes. In this case the permselectivity can be reversibly, switched by changing the potential applied to the membrane.