Layer-by-Layer Assembly of Polyelectrolytes into Ionic Current Rectifying Solid-State Nanopores: Insights from Theory and Experiment

摘要

Abstract: Molecular design of ionic current rectifiers created on the basis of single conical nanopores isnreceiving increasing attention by the scientific community. Part of the appeal of this topic relies on theninterest in sensors and fluidic nanoactuators based on the transport of ions and molecules through nanoporenarchitectures that can readily be integrated into functional systems. The chemical modification of the porenwalls controls not only the diameter of these nanoarchitectures but also their selectivity and transportnproperties. In order to confer selectivity to solid-state nanopores, it is necessary to develop and explorennew methods for functionalizing the pore walls. Hence, the creation of functional nanopores capable ofnacting as selective ion channels or smart nanofluidic sensors depends critically on our ability to assemblenand build up molecular architectures in a predictable manner within confined geometries with dimensionsncomparable to the size of the building blocks themselves. In this context, layer-by-layer deposition ofnpolyelectrolytes offers a straightforward process for creating nanoscopic supramolecular assembliesndisplaying a wide variety of functional features. In this work, we describe for the first time the integrationnof layer-by-layer polyelectrolyte assemblies into single conical nanopores in order to study and explore thenfunctional features arising from the creation of charged supramolecular assemblies within the constrainedngeometry of the nanofluidic device. To address this challenging topic, we used a combined experimentalnand theoretical approach to elucidate and quantify the electrostatic changes taking place inside the nanoporenduring the supramolecular assembly process. The multilayered films were built up through consecutivenlayer-by-layer adsorption of poly(allylamine hydrochloride) (PAH) and poly(styrenesulfonate) (PSS) on thenpore surface. Our results show that the charge transport properties of single conical nanopores functionalizednwith PAH/PSS assemblies are highly dependent on the number of layers assembled on the pore wall. Inncontrast to what happens with PAH/PSS films deposited on planar surfaces (quantitative charge reversal),nthe surface charge of the pore walls decreases dramatically with the number of PAH/PSS layers assembledninto the nanopore. This behavior was attributed to the nanoconfinement-induced structural reorganizationnof the polyelectrolyte layers, leading to the efficient formation of ion pairs and promoting a marked decreasenin the net fixed charges on the nanopore walls. We consider that these results are of paramount relevancenfor the modification of nanopores, nanopipets, and nanoelectrodes using charged supramolecularnassemblies, as well as of importance in “soft nanotechnology” provided that structural complexity, inducednby nanoconfinement, can define the functional properties of self-assembled polymeric nanostructures.