Electrolyte Gradient-Based Modulation of Molecular Transport through Nanoporous Gold Membranes

Abstract

Nanopores, and nanoporous materials in general, are interesting for applications in chemical and biomolecular transport as pore sizes are on the same scale as the dimension of many (bio)chemical species. Many studies have focused on either single pores or small arrays of cylindrical pores, which are convenient in terms of their amenability toward computational modeling of transport phenomenon. However, the limited overall porosity may inhibit transport flux as well as the eventual implementation of these materials as active separation elements. Inspired by its relatively high porosity, we have explored nanoporous gold (NPG) as a membrane across which small molecular species can be transported. NPG offers a random, bicontinuous pore geometry, while also being inherently conductive and readily amenable to surface modification-attributes that may be enabling in the pursuit of size- and charge-based approaches to molecular separations. NPG was fabricated via a free-corrosion process whereby immersion of Au-containing alloys in concentrated nitric acid preferentially dissolves the less noble metals (e.g., Ni, Cu). Average pore diameters of 50 ± 20 nm were obtained as verified under scanning electron microscopy. NPG membranes were sandwiched between two reservoirs, and the selective transport of chemical species across the membrane in the presence of an ionic strength gradient was investigated. The flux of small molecules were monitored by UV-vis absorption spectrometry and found to be dependent upon the direction and magnitude of the ionic strength gradient. Moreover, transport trends underscored the effects of surface charge in a confined environment, considering that the pore diameters were on the same scale as the electrical double layer experienced by molecules transiting the membrane. Under such conditions, the transport of anions and cations through NPG was found to depend on an induced electric field as well as ion advection. Further electrical and surface chemical modulations of transport are expected to engender increased membrane functionality.

Ion Transport in Confined Geometries below the Nanoscale: Access Resistance Dominates Protein Channel Conductance in Diluted Solutions

“…Also, potassium channels have a permeability ratio for K + over Na + above 100:1, and calcium channels select for Ca 2+ over Na + with a ratio over 1000:1 . Such peculiar transport features and others related have stimulated the design of biomimetic nanofluidic devices used in engineering applications like DNA sequencing, desalination, molecular separation, and energy conversion. − In parallel, much effort has been made to elucidate how pore dimensions affect the transport mechanisms, albeit many questions remain unanswered yet: , a variety of specific phenomena found in the nanometer regime cannot be understood simply downsizing the concepts working well at the microscale . Compared to microfluidics, several factors make the difference.…”

Macroscopic Salt Rejection through Electrostatically Gated Nanoporous Graphene

“…For example, an applied bias on a mesoporous carbon membrane, having pores of <5 nm, allows the ion flux to stop once the Debye length of the solution approaches the pore size . Similar experiments using nanoporous gold membranes, however, find that the cation and anion transport is enhanced upon applying a bias . While the former behavior was ascribed to the depletion of the channel from ions upon gating, the latter was a result of enhanced transport by dominating drift currents.…”

“…27 Similar experiments using nanoporous gold membranes, however, find that the cation and anion transport is enhanced upon applying a bias. 29 While the former behavior was ascribed to the depletion of the channel from ions upon gating, the latter was a result of enhanced transport by dominating drift currents.…”

“…The above mechanistic findings, nevertheless, do not fully clarify the experimentally observed reduction of diffusive flux upon gating. Indeed, the same physical mechanism also governs the diode-like ionic transport through an IFET 5,38,39 or a nanoporous metal membrane, 29 in which a nonzero electric potential at the nanopore center usually results in an increase of ionic conductivity. To this end, we further increased the graphene surface potential ψ G considering the same system (details see supplementary section S7).…”

Chemically-Gated and Sustained Molecular Transport through Nanoporous Gold Thin Films in Biofouling Conditions

“…However, there are still obstacles, such as biofouling of the nanostructured material, controlling the release rate, and the unavoidable depletion of drug molecules in a fully implanted system. While there have been some remedies (e.g., antibiofouling coatings [20] and size-exclusion membranes [21]) and stimulus-responsive gating mechanisms [22][23][24][25] for the former two obstacles, the latter has remained non-trivial, especially for non-convective drug delivery platforms. There have been creative solutions, such as replenishment of drug depot via selective capture of pharmaceuticals at the delivery site [26].…”