# Potentiometric Studies on Ion-Transport Selectivity in Charged Gold Nanotubes

## TL;DR

This paper investigates how different cations affect ion transport in gold nanotubes, finding that the electrostatic model holds true for gold but not for polymer membranes.

## Contribution

The study experimentally confirms the double-layer overlap model's validity for gold nanotubes, resolving a key discrepancy with polymer nanopores.

## Key findings

- Ccrit values for all tested cations in gold nanotubes were identical and matched DLOM predictions.
- The success of DLOM in gold nanotubes contrasts with its failure in polymeric nanopores due to differences in fixed negative charges.
- Gold nanotubes maintain ideal cation permselectivity under conditions predicted by the double-layer overlap model.

## Abstract

Under ideal conditions, nanotubes with a fixed negative tube-wall charge will reject anions and transport-only cations. Because many proposed nanofluidic devices are optimized in this ideally cation-permselective state, it is important to know the experimental conditions that produce ideal responses. A parameter called Ccrit, the highest salt concentration in a contacting solution that still produces ideal cation permselectivity, is of particular importance. Pioneering potentiometric studies on gold nanotubes were interpreted using an electrostatic model that states that Ccrit should occur when the Debye length in the contacting salt solution becomes equivalent to the tube radius. Since this “double-layer overlap model” (DLOM), treats all same-charge ions as identical point charges, it predicts that all same-charged cations should produce the same Ccrit. However, the effect of cation on Ccrit in gold nanotubes was never investigated. This knowledge gap has become important because recent studies with a polymeric cation-permselective nanopore membrane showed that DLOM failed for every cation studied. To resolve this issue, we conducted potentiometric studies on the effect of salt cation on Ccrit for a 10 nm diameter gold nanotube membrane. Ccrit for all cations studied were, within experimental error, the same and identical, with values predicted by DLOM. The reason DLOM prevailed for the gold nanotubes but failed for the polymeric nanopores stems from the chemical difference between the fixed negative charges of these two membranes.