Smart membranes by electron beam cross-linking of copolymer microgels.

Poly(N-isopropylacrylamide) (pNIPAM) based copolymer microgels were used to create free-standing, transferable, thermoresponsive membranes. The microgels were synthesized by copolymerization of NIPAM with N-benzylhydrylacrylamide (NBHAM). Monolayers of these colloidal gels were subsequently cross-linked using an electron gun leading to the formation of a connected monolayer. Furthermore, the cross-linked microgel layer is detached from the supporting material by dissolving the substrate. These unique systems can be used as transferable, thermoresponsive coatings and as thermoresponsive membranes. As a proof of principle for the use of such membranes we studied the ion transport through them at different temperatures revealing drastic changes when the lower critical solution temperature of the copolymer microgels is reached.

UV cross-linked smart microgel membranes as free-standing diffusion barriers and nanoparticle bearing catalytic films.

In this study we use poly(N-isopropylacrylamide) (PNIPAM) based copolymer microgels to create free-standing, transferable, thermoresponsive membranes. The microgels are synthesized by copolymerization of NIPAM with 2-hydroxy-4-(methacryloyloxy)-benzophenone (HMABP) and spin-coated on Si wafers. After subsequent cross-linking by UV-irradiation, the formed layers easily detach from the supporting material. We obtain free standing microgel membranes with lateral extensions of several millimetres and an average layer thickness of a few hundred nanometres. They can be transferred to other substrates. As one example for potential applications we investigate the temperature dependent ion transport through the membranes via resistance measurements revealing a sharp reversible increase in resistance when the lower critical solution temperature of the copolymer microgels is reached. In addition, prior to cross-linking, the microgels can be decorated with silver nanoparticles and cross-linked afterwards. Such free-standing nanoparticle hybrid membranes are then used as catalytic systems for the reduction of 4-nitrophenol, which is monitored by UV/Vis spectroscopy.

Controlled translocation of DNA through nanopores in carbon nano-, silicon-nitride- and lipid-coated membranes.

We investigated experimentally and theoretically the translocation forces when a charged polymer is threaded through a solid-state nanopore and found distinct dependencies on the nanopore diameter as well as on the nano membrane material chemistry. For this purpose we utilized dedicated optical tweezers force mechanics capable of probing the insertion of negatively charged double-stranded DNA inside a helium-ion drilled nanopore. We found that both the diameter of the nanopore and the membrane material itself have significant influences on the electroosmotic flow through the nanopore and thus on the threading force. Compared to a bare silicon-nitride membrane, the threading of DNA through only 3 nm thin carbon nano membranes as well as lipid bilayer-coated nanopores increased the threading force by 15% or 85%, respectively. This finding was quantitatively described by our recently developed theoretical model that also incorporates hydrodynamic slip effects on the translocating DNA molecule and the force dependence on the membrane thickness. The additional measurements presented in this paper further support our model.

Video-based and interference-free axial force detection and analysis for optical tweezers.

For measuring the minute forces exerted on single molecules during controlled translocation through nanopores with sub-piconewton precision, we have developed a video-based axial force detection and analysis system for optical tweezers. Since our detection system is equipped with a standard and versatile CCD video camera with a limited bandwidth offering operation at moderate light illumination with minimal sample heating, we integrated Allan variance analysis for trap stiffness calibration. Upon manipulating a microbead in the vicinity of a weakly reflecting surface with simultaneous axial force detection, interference effects have to be considered and minimized. We measured and analyzed the backscattering light properties of polystyrene and silica microbeads with different diameters and propose distinct and optimized experimental configurations (microbead material and diameter) for minimal light backscattering and virtually interference-free microbead position detection. As a proof of principle, we investigated the nanopore threading forces of a single dsDNA strand attached to a microbead with an overall force resolution of ±0.5 pN at a sample rate of 123 Hz.