Temperature-Controlled Flow Switching in Nanocapillary Array Membranes Mediated by Poly(N-isopropylacrylamide) Polymer Brushes Grafted by Atom Transfer Radical Polymerization

Abstract

We report actively controlled transport that is thermally switchable and size-selective in a nanocapillary array membrane (NCAM) prepared by grafting poly(N-isopropylacrylamide) (PNIPAAm) brushes onto the exterior surface of a Au-coated polycarbonate track-etched membrane. A smooth Au layer on the membrane surface, which is key to obtaining a uniform polymer film, was prepared by thermal evaporation of approximately 50 nm Au on both exterior surfaces. After evaporation, the inner diameter of the pore is reduced slightly, but the NCAM retains a narrow pore size distribution. PNIPPAm brushes with 10-30 nm (dry film) thickness were grafted onto the Au surface through surface-initiated atom transfer radical polymerization (ATRP) using a disulfide initiator, (BrC(CH3)2COO(CH2)11S)2. Molecular transport through the PNIPAAm polymer brush-modified NCAMs was investigated by real-time fluorescence measurements using fluorescein isothiocyanate (FITC)-labeled dextrans ranging from 4.4 to 282 kDa in membranes with variable initial pore diameters (80, 100, and 200 nm) and different PNIPAAm thicknesses. Manipulating the temperature of the NCAM through the PNIPAAm lower critical solution temperature (LCST) causes large, size-dependent changes in the transport rates. Over specific ranges of probe size, transport is completely blocked below the LCST but strongly allowed above the LCST. The combination of the highly uniform PNIPAAm brush and the monodisperse pore size distribution is critical in producing highly reproducible switching behavior. Furthermore, the reversible nature of the switching raises the possibility of using them as actively controlled filtration devices.

Current Rectification in Temperature‐Responsive Single Nanopores

“…On the one hand, these smart systems can be switched between high and low conducting states by changing conformations of the attached functional molecules in response to the environmental stimuli, including pH, [20] light, [21] ionic strength, [22] and temperature, [23] to control the amount of the fluid transport. [24][25][26][27][28] On the other hand, the preferential direction of the ion transport can be also controlled by introducing broken symmetry to the nanofluidic system, [29] either in spatial or in chemical composition, [18][19]30] known as ionic rectification, which is essentially important and ubiquitous in physiological process. [31] Therefore, in the recent years, growing interests are focusing on constructing smart nanofluidic systems with responsive ionic rectifying functions.…”

Molecular Aspects of Transport in Thin Films of Controlled Architecture

“…Thermally-switchable and size-selective transport was achieved by grafting poly(JV-isopropylacrylamide) (PNIPAAm) brushes onto the exterior surface of a Au-coated polycarbonate track-etched membrane, while poly(hydroxyethylmethacrylate) (PHEMA) has been used for voltage-gated applications. [5,10] • Molecular recognition (DNAzyme) motifs have been incorporated in the interior of nanopores, and their interfacial chemistry has been characterized. These experiments open the way for "flow-through" chemical sensing which time-integrates the analyte signal allowing ultralow limits of detection and operation in a dosimeter format.…”

Three-dimensional integrated microfluidic architectures enabled through electrically switchable nanocapillary array membranes

“…26 A similar paradigm has been employed in our laboratory. We have grafted voltage, 27 pH, and temperature 28 responsive polymer brushes to membrane surfaces by atom transfer radical polymerization ͑ATRP͒ and studied their ability to modulate nanofluidic transport as shown in Fig. 6.…”