Close-packed nanoparticle monolayers self-assembled from dodecanethiol-ligated gold nanocrystals. TEM image (left) and atomistic simulation of tryptophan transport through a pore.
A recent collaboration between users at the University of Chicago and the University of Illinois at Chicago with the Center for Nanoscale Material’s Electronic and Magnetic Materials and Devices Group has produced the thinnest nanofiltration membrane achieved thus far, at ~30 nm, made of just four layers of nanoparticles.
Close-packed nanoparticle monolayers have recently been shown to form mechanically robust, free-standing membranes. We report the first measurements of molecular transport through such ultrathin sheets, self-assembled from dodecanethiol-ligated gold nanocrystals. For aqueous solutions we find filtration coefficients 2 orders of magnitude larger than those observed in polymer-based filters, sieving of large solutes, and for smaller solutes a pronounced dependence of rejection on being charged. These results open up new possibilities for controlled delivery and separation of nano-objects.
A separation membrane is a key component in both nanofiltration and reverse osmosis filtration systems. Typically they are microns-thick polymer films. Reducing the thickness of the membrane reduces the pressure that needs to be applied across the membrane in order to achieve a certain amount of flux, which is a major operational cost in these devices. The filtration coefficient of this membrane for aqueous solutions is two orders of magnitude larger than for typical polymer-based nanofiltration systems. Near only 80 kPa pressure, the membrane exhibits pronounced charge sensitivity for a variety of dyes and other molecules, while rejecting molecules greater than 1.7 nm in size. Guided by atomistic molecular dynamics simulations, it was found that molecular transport occurs through pore-like regions between close-packed nanoparticles and that dielectric exclusion dominates the charge-dependent rejection.