PNIPAAM-Modified Nanoporous Colloidal Films with Positive and Negative Temperature Gating

Abstract

The surface of nanopores in colloidal films, assembled from 205 nm silica spheres, was modified with poly(N-isopropylacrylamide), PNIPAAM, brushes using surface-initiated ATRP. The polymer thickness inside nanopores was controlled by the polymerization time. The diffusion through PNIPAAM-modified colloidal films was measured using cyclic voltammetry and studied as a function of temperature and polymer brush thickness. Nanopores modified with a thin PNIPAAM brush exhibited a positive gating behavior, where diffusion rates increased with increasing temperature. Nanopores modified with a thick PNIPAAM layer showed a negative gating behavior where diffusion rates decreased with increasing temperature. The observed temperature response is consistent with two transport mechanisms, one in which molecules diffuse through the nanopores whose volume increases with increasing temperature as the PNIPAAM brush collapses onto the nanopore surface (positive gating) and the second one where molecules diffuse through the porous PNIPAAM that fills the entire nanopore opening and collapses onto itself, becoming hydrophobic and impermeable (negative gating).

Current Rectification in Temperature‐Responsive Single Nanopores

“…This trend could not be observed in control experiments for the gold-coated nanopores without PNIPAM modification. Similar positive thermal gating behavior [47] has been reported in previous works. [23][24][25][26][27] The results showed here present strong evidence for the successful assembly of the thoil-terminated PNIPAM brushes into the nanopore.…”

pH‐Responsive Nanoporous Silica Colloidal Membranes

“…We were also able to prepare polymer-modified colloidal films where molecular transport has been controlled by sterics in response to temperature and pH. [37][38][39] Recently, we developed a free-standing colloidal membrane (nanofrit) with large surface area. [40] These membranes are prepared by sintering, [41,42] which causes the silica spheres to fuse to one another.…”

Molecular transport in thiacalix[4]arene-modified nanoporous colloidal films

“…Colloidal films possess a number of attractive features, including formation via self-assembly of sub-micron-sized silica spheres [4], highly ordered arrays of three-dimensional interconnected nanopores whose size can be controlled by changing the size of the silica spheres used to assemble the films, and facile surface modification [5]. We demonstrated that transport selectivity in colloidal films can be: (i) based on electrostatic interactions with chargeable surface-bound functional groups [6][7][8], (ii) utilize polymer brushes grown inside the nanopores [9,10], and (iii) based on hydrogen bonding and p-p staking non-covalent interactions with surface-immobilized organic receptors [11,12]. In the latter case we were able to demonstrate that the transport through the chiral colloidal films occurs on the surface of the nanopores in a way similar to that demonstrated in porous polymer membranes [13,14] and surface-modified nanotubes [15][16][17][18][19][20][21][22][23][24].…”