Guangzhao Zhang

Current Rectification in Temperature-Responsive Single Nanopores

Herein we demonstrate a fully abiotic smart single-nanopore device that rectifies ionic current in response to the temperature. The temperature-responsive nanopore ionic rectifier can be switched between a rectifying state below 34 degrees C and a non-rectifying state above 38 degrees C actuated by the phase transition of the poly(N-isopropylacrylamide) [PNIPAM] brushes. On the rectifying state, the rectifying efficiency can be enhanced by the dehydration of the attached PNIPAM brushes below the LCST. When the PNIPAM brushes have sufficiently collapsed, the nanopore switches to the non-rectifying state. The concept of the temperature-responsive current rectification in chemically-modified nanopores paves a new way for controlling the preferential direction of the ion transport in nanofluidics by modulating the temperature, which has the potential to build novel nanomachines with smart fluidic communication functions for future lab-on-chip devices.

Nanoscale Tubular and Sheetlike Superstructures from Hierarchical Self‐Assembly of Polymeric Janus Particles

Inspired by hierarchical protein self-assembly in biological systems, the self-assembly of nanoparticles into superstructures has attracted much attention because of potential applications of these superstructures in the fields such as electronic or optical materials and novel nanodevices. Template-free self-assembly of nanoparticles with anisotropic interactions is of particular interest because it can lead to tailor-made complex superstructures. In fact, Janus nanoparticles are nano-objects with anisotropic interactions. The facile preparation, applications, and self-assembly of Janus nanoparticles have attracted extensive interest, and Janus particles with various sizes, structures, and compositions have been recently reported. Several kinds of Janus particles are capable of self-assembling into regular superstructures. For example, M ller and co-workers reported that amphiphilic Janus micelles prepared from ABC triblock copolymers could self-assemble into spherical supermicelles. 20] Granick and co-workers reported that amphiphilic and zwitterionic Janus colloidal spheres could assemble in water to form ordered clusters. In a recent study, we prepared Janus nanoparticles by using hybrid organic/inorganic nanotubes as a desymmetrization tool and the as-prepared amphiphilic Janus nanoparticles self-assembled into narrowly size-distributed flowerlike supermicelles in water. Despite these results, assembly of Janus nanoparticles into superstructures other than spherical supermicelles or clusters, such as nanowires, tubular and sheetlike superstructures, still remains challenging. Herein, we report a novel mechanism for the formation of polymeric Janus particles from mixed-shell micelles (MSMs) and the template-free self-assembly of the Janus particles into tubular superstructures and nanosheets. Micelles with mixed P2 VN/PEO shells were prepared by noncovalent crosslinking of poly(acrylic acid) (PAA) blocks by addition of 1,2-propanediamine (PDA) to a solution of PEO3500-bPAA3800/P2VN38000-b-PAA24000 (1:1 (w/w), PEO = poly(ethylene oxide), P2VN = poly(2-vinyl naphthalene), and the subscripts denote the molecular weights of the respective blocks) in DMF (Figure 1 a). The molar ratio of AA/PDA was 1:5, and the total polymer concentration was 1.0 mgmL . MSMs with PEO/P2VN as the mixed shell and a PDAcross-linked PAA network as the core were thus formed by noncovalent cross-linking. After switching the solvent from DMF to water (pH 7) by using dialysis, P2 VN in the mixed shell collapsed into separated microdomains (Figure 1b). In water, P2VN microdomains were surrounded and protected by solvated PEO chains so that the MSMs were individually dispersed. By decreasing the pH value of the aqueous solution to 3.1, intramicellar complexation occurred between PEO and PAA (Figure 1c), which resulted in an asymmetric intramicellar phase separation between the PEO/PAA complex and P2VN. As a result, amphiphilic Janus nanoparticles with a hydrophobic P2VN domain (formed by aggregation of all the P2VN small domains in a MSM) on one side and a hydrophilic PEO/PAA complex domain on the opposite side were formed (Figure 1d). These Janus nanoparticles were able to self-assemble in water to produce tubular and sheetlike superstructures. The average hydrodynamic radius, hRhi, of MSMs in DMF was 190 nm (Figure 2a, curve 1). In water, MSMs were individually dispersed with an hRhi value of 140 nm (Figure 2a, curve 2) because of the protection of solvated PEO chains in the mixed shell. The decrease in the hRhi value of the MSMs should result from the collapse of P2 VN chains in water. In the transmission electron microscopy (TEM) image of MSMs cast from neutral water stained with RuO4 (Figure 2b), contrast within each MSM was observed. The darker domains were assigned to P2VN microdomains (2–

Role of Chain Interpenetration in Layer-by-Layer Deposition of Polyelectrolytes

The effects of temperature, pH, and salt concentration on the layer-by-layer (LBL) deposition of sodium poly(styrene sulfonate) (PSS)/poly[2-(dimethylamino)ethyl methacrylate] (PDEM) were investigated by use of a quartz crystal microbalance with dissipation (QCM-D). At pH 4, the frequency change (Deltaf) gradually decreased to a constant, indicating that the polyelectrolyte complexes of the layer were not dissolved. As the layer number increased, the -Deltaf oscillatedly increased, indicating that the thickness of the multilayer increased. At the same time, the dissipation change (DeltaD) oscillatedly increased with the layer number, indicating the chain interpenetration or complexation that led to the alternative swelling-and-shrinking of the outermost layer. For the same layer number, as the temperature increased, the amplitude of DeltaD increased, indicating that the chain interpenetration increased. The thickness also increased with temperature. Further increasing the pH to 7 led to a thicker layer, reflected in the larger amplitude of DeltaD. At pH 10, the polyelectrolytes no longer formed multilayers on the surface because of the lack of electrostatic interactions. On the other hand, the addition of NaCl also led to a thickness increase. The amplitude in DeltaD increased with NaCl concentration, indicating that the chain interpenetration increased. Our experiments indicated that the LBL deposition of polyelectrolytes was dominated by the chain interpenetration. Also, the polyelectrolyte complexes in the layer can redissolve into solution from the surface at a high temperature or a high salt concentration.

A zwitterionic gel electrolyte for efficient solid-state supercapacitors

Gel electrolytes have attracted increasing attention for solid-state supercapacitors. An ideal gel electrolyte usually requires a combination of advantages of high ion migration rate, reasonable mechanical strength and robust water retention ability at the solid state for ensuring excellent work durability. Here we report a zwitterionic gel electrolyte that successfully brings the synergic advantages of robust water retention ability and ion migration channels, manifesting in superior electrochemical performance. When applying the zwitterionic gel electrolyte, our graphene-based solid-state supercapacitor reaches a volume capacitance of 300.8 F cm−3 at 0.8 A cm−3 with a rate capacity of only 14.9% capacitance loss as the current density increases from 0.8 to 20 A cm−3, representing the best value among the previously reported graphene-based solid-state supercapacitors, to the best of our knowledge. We anticipate that zwitterionic gel electrolyte may be developed as a gel electrolyte in solid-state supercapacitors.

Soluble graft-like complexes based on poly(4-vinyl pyridine) and carboxy-terminated polystyrene oligomers due to hydrogen bonding

Abstract Mono-carboxy terminated polystyrene oligomer (MCPS) and di-carboxy terminated polystyrene oligomer (DCPS) were prepared by anionic polymerization. Viscometry and dynamic light scattering (DLS) studies show that both MCPS and DCPS can interact with poly(4-vinyl pyridine) (PVPy) via hydrogen bonding to form soluble graft-like complexes in CHCl 3 . The marked differences in the viscosity–composition behaviour, intrinsic viscosities and hydrodynamic radius distributions f ( R h ) between MCPS/PVPy and DCPS/PVPy blend solutions proved that the two carboxyl ends in DCPS provide much stronger ability for complex formation than the mono carboxyl end in MCPS does.

Origin of hysteresis observed in association and dissociation of polymer chains in water.

By choosing poly(N,N-diethylacrylamide) which lacks the possibility to form intra- or inter-chain hydrogen bonds, we studied its chain association and dissociation in water by using laser light scattering (LLS), ultrasensitive differential scanning calorimetry (US-DSC) and Fourier transform infrared spectroscopy (FTIR). As the solution temperature increases, the average hydrodynamic radius (R(h)) and average radius of gyration (R(g)) decrease, indicating the intrachain shrinking. When the temperature is higher than its lower critical solution temperature (LCST, approximately 30 degrees C), the apparent weight-average molar mass (M(w,app)) increases, reflecting the interchain association. At the same time, FTIR study reveals that as the temperature increases, the area ratio of two absorption peaks, respectively, associated to one hydrogen bonded carbonyl >C=O...H-O-H and free carbonyl >C=O groups increases, while that related to two hydrated hydrogen bonded carbonyl groups decreases, indicating heating-induced dehydration. In the reversible cooling process, R(h), R(g), M(w,app) and area ratios of the absorption peak are similar to those in the heating process for each given temperature, indicating that there is no hysteresis in the interchain association and dissociation. This present study confirms that the hysteresis previously observed for a sister polymer, poly(N-isopropylacrylamide), is due to the formation of some additional hydrogen bonds in its collapsed state at temperatures higher than the LCST.

Electrically tunable block copolymer photonic crystals with a full color display

Electrically tunable full color display photonic crystals (PCs) made of a thin film of block copolymer, polystyrene-b-poly(2-vinyl pyridine) (PS-b-P2VP), have been developed. The phase separation of PS and P2VP blocks leads to a one-dimensional periodic lamellar structure parallel to the surface of the thin film. In a solvent mixture of water and ethanol, each P2VP layer is swollen and partially charged. Applying a low voltage (<2.5 V) to such a thin film can lead to the repeatable expansion or contraction of each P2VP layer, depending on the direction and strength of the electrical potential. Such a variation of the lamellar periodicity leads to a full color display over a wide range from ultraviolet to near infrared. The advantages of such PCs are their facile preparation, low driving voltage and good durability, which make them potentially useful in information technologies.

Ion-specific conformational behavior of polyzwitterionic brushes: exploiting it for protein adsorption/desorption control.

The conformation of polyzwitterionic brushes plays a crucial role in the adsorption/desorption of proteins on solid surfaces. By use of quartz crystal microbalance with dissipation (QCM-D) and surface plasmon resonance (SPR), we have systematically investigated the conformational behavior of poly(sulfobetaine methacrylate) (PSBMA) brushes as a function of ionic strength in the presence of different ions. The frequency change demonstrates that the effectiveness of anions to weaken the inter/intrachain association and to enhance the hydration of the grafted chains increases from kosmotrope to chaotrope in the low ionic strength regime, but the ordering of anions is almost reversed at the high ionic strengths. The dissipation change indicates that some heterogeneous structures are formed inside the brushes in the presence of chaotropic anions with the increase of ionic strength. In SPR studies, the change of resonance unit (ΔRU) with ionic strength is determined by the balance between the increase of thickness and the decrease of refractive index of the brushes. No anion specificity is observed in the SPR measurements because ΔRU is insensitive to the coupled water molecules inside the brushes. For the control of protein adsorption/desorption, our studies show that the brushes can more effectively resist the protein adsorption in the presence of a more chaotropic anion and a more chaotropic anion can also more effectively induce the protein desorption from the surface of the brushes. In addition, no obvious cation specificity can be observed in the conformational change of the brushes in either QCM-D or SPR measurements.

Activity and thermal stability improvements of glucose oxidase upon adsorption on core-shell PMMA-BSA nanoparticles.

The interaction and adsorption of enzyme, glucose oxidase (GOx), on poly(methyl methacrylate)-bovine serum albumin (PMMA-BSA) particles were studied by using a quartz crystal microbalance with dissipation (QCM-D) and laser light scattering (LLS). The enzyme was irreversibly immobilized on the PMMA-BSA particle surface. The amount of enzyme immobilized on PMMA-BSA particles and the enzymatic activity were determined by UV/vis measurements. The influences of pH and ionic strength on the adsorption indicate that the electrostatic interaction plays a major role on the immobilization. The adsorbed GOx can retain at least 80% of the free enzyme activity. Thermal stability studies reveal that the adsorbed GOx only losses 28% of its activity in comparison with a 64% activity loss of free GOx when it is incubated at 50 degrees C for 35 h.

Role of methyl in the phase transition of poly(N-isopropylmethacrylamide).

Poly(N-isopropylmethacrylamide) (PiPMA) has one more methyl group at each monomeric unit than poly(N-isopropylacrylamide) (PiPA). By use of laser light scattering (LLS) and ultrasensitive differential scanning calorimetry (US-DSC) we have investigated the association and dissociation of PiPMA chains in water. LLS studies reveal that PiPMA chains form larger aggregates at a temperature above its lower critical solution temperature (LCST) as the chain molar mass (Mw) decreases. In comparison with PiPA aggregates, PiPMA aggregates show a larger ratio of average radius of gyration to average hydrodynamic radius (/), indicating that PiPMA aggregates are looser. US-DSC studies show PiPMA chains have smaller enthalpy change (DeltaH) and entropy change (DeltaS) than PiPA chains during the phase transition, indicating that PiPMA chains have smaller conformational change. Our experiments demonstrate that the additional methyl groups in PiPMA chains restrain the intrachain collapse and interchain association, leading the phase transition to occur at a higher temperature.