From proton exchange membranes in fuel cells to ion channels in biological membranes, the well-specified control of ionic interactions in confined geometries profoundly influences the transport and selectivity of porous materials. Here we outline a versatile new approach to control a membrane's electrostatic interactions with ions by depositing ligand-coated nanoparticles around the pore entrances.

Leveraging the flexibility and control by which ligated nanoparticles can be synthesized, we demonstrated how ligand terminal groups such as methyl, carboxyl and amine can be used to tune the membrane charge density and control ion transport. Further functionality, exploiting the ligands as binding sites, was demonstrated for sulfonate groups resulting in an enhancement of the membrane charge density. We then extended these results to smaller dimensions by systematically varying the underlying pore diameter.

These results outline a previously unexplored method for the nanoparticle functionalization of membranes using ligated nanoparticles to control ion transport.

1. •Edward Barry, Sean P. McBride, Heinrich M. Jaeger, Xiao-Min Lin, “Ion Transport Controlled by Nanoparticle-Functionalized Membranes”, Nature Communications 5, 5847 (2014). link
   

By utilizing particles encoded a priori with the desired functionality, we are able to bypass the requirement of a favourable surface chemistry in order to bind the molecules to the surface of the porous substrate itself, or any additional steps coating the substrate with a metal such as gold. Owing to the flexibility by which ligated nanoparticles can be synthesized, including both the core materials themselves and the encapsulating ligands, we believe the approach outlined here can be generalized to a similar set of functionalities as their SAM counterparts for different core materials. Previous work has already demonstrated that strong ligand–ligand interactions required for NP film stability are attainable not only for alkanethiol-ligated gold nanoparticles, but other core materials such as cobalt oxide and iron oxide with oleylamine and oleic acid ligands, as well as DNA-coated nanoparticles.

The end result is a potentially powerful route towards engineering a set of desired interactions in which ligated nanoparticles serve as fully functionalized building blocks for a type of bottom-up assembly for the active layer of membranes. By extension, successive deposition of ligated NP films opens up exciting possibilities including the fabrication of composite layers, each with a specific chemical function.