Swelling Behavior of Multiresponsive Poly(methacrylic acid)‐block‐‐poly(N‐isopropylacrylamide) Brushes Synthesized Using Surface‐Initiated Photoiniferter‐Mediated Photopolymerization

Surface‐initiated photoiniferter‐mediated photopolymerization (SI‐PMP) in presence of tetraethylthiuram disulfide is used to directly synthesize surface‐grafted poly(methacrylic acid)‐block‐poly(N‐isopropylacrylamide) (PMAA‐b‐PNIPAM) layers. The response of these PMAA‐b‐PNIPAM bi‐level brushes to changes in pH, temperature and ionic strength is investigated by using in‐situ multi‐angle ellipsometry to measure changes in solvated layer thickness. As expected for a block copolymer architecture, PMAA blocks swell as pH is increased, with the maximum change in the thickness occurring near pH = 5, and PNIPAM blocks exhibit lower critical solution temperature (LCST) behavior, marked by a broad transition between swollen and collapsed states. The response of the bi‐level brushes to changes in added salt at constant pH is complex, as the swelling behaviors of both the weak polyelectrolyte, PMAA, and thermoresponsive PNIPAM are affected by changes in ionic strength. This work demonstrates not only the robustness of SI‐PMP for making novel, bi‐level stimuli‐responsive brushes, but also the complex links between synthesis, structure, and response of these materials.     

Organic/Inorganic Hybrid Nanochannels Based on Polypyrrole‐Embedded Alumina Nanopore Arrays: pH‐ and Light‐Modulated Ion Transport

Inspired by the asymmetric structure and responsive ion transport in biological ion channels, organic/inorganic hybrid artificial nanochannels exhibiting pH‐modulated ion rectification and light‐regulated ion flux have been constructed by introducing conductive polymer into porous nanochannels. The hybrid nanochannels are achieved by partially modifying alumina (Al₂O₃) nanopore arrays with polypyrrole (PPy) layer using electrochemical polymerization, which results in an asymmetric component distribution. The protonation and deprotonation of Al₂O₃ and PPy upon pH variation break the surface charge continuity, which contributes to the pH‐tunable ion rectification. The ionic current rectification ratio is affected substantially by the pH value of electrolyte and the pore size of nanochannels. Furthermore, the holes (positive charges) in PPy layer induced by the cooperative effect of light and protons are used to regulate the ionic flux through the nanochannels, which results in a light‐responsive ion current. The magnitude of responsive ionic current could be amplified by optimizing this cooperation. This new type of stimuli‐responsive PPy/Al₂O₃ hybrid nanochannels features advantages of unique optical and electric properties from conducting PPy and high mechanical performance from porous Al₂O₃ membrane, which provide a platform for creating smart nanochannels system.     

Blood‐Inert Surfaces via Ion‐Pair Anchoring of Zwitterionic Copolymer Brushes in Human Whole Blood

A strategy to create blood‐inert surfaces in human whole blood via ion‐pair anchoring of zwitterionic copolymer brushesand a systematic study of how well‐defined chain lengths and well‐controlled surface packing densities of zwitterionic polymers affect blood compatibility are reported. Well‐defined diblock copolymers, poly(11‐mercaptoundecyl sulfonic acid)‐block‐poly(sulfobetaine methacrylate) (PSA‐b‐PSBMA) with varying zwitterionic PSBMA or negatively charged PSA lengths, are synthesized via atom‐transfer radical polymerization (ATRP). PSA‐b‐PSBMA is grafted onto a surface covered with polycation brushes as a mimic polar/hydrophilic biomaterial surface via ion‐pair anchoring at a range of copolymer concentrations. Protein adsorption from single‐protein solutions, 100% blood serum, and 100% blood plasma onto the surfaces covered with PSA‐b‐PSBMA brushes is evaluated using a surface plasmon resonance sensor. Copolymer brushes containing a high amount of zwitterionic SBMA units are further challenged with human whole blood. Low protein‐fouling surfaces with >90% reduction with respect to uncoated surfaces are achieved with longer PSA blocks and higher concentrations of PSA‐b‐PSBMA copolymers using the ion‐pair anchoring approach. This work provides a platform to achieve the control of various surface parameters and a practical method to create blood‐inert surfaces in whole blood by grafting ionic‐zwitterionic copolymers to charged biomaterials via charge pairing.     

pH‐Responsive Nanoporous Silica Colloidal Membranes

Free‐standing colloidal membranes (nanofrits) with varied thickness and nanopore size are fabricated and modified with pH‐responsive poly(2‐(dimethylamino)ethyl methacrylate) brushes. The polymer‐modified nanofrits demonstrate excellent gating behavior for molecular diffusion: in the presence of acid, the diffusion rate of positively charged species significantly decreases. Increasing the polymer length and membrane thickness and decreasing the nanopore size leads to the complete acid‐controlled gating of the membranes.     

Protein/Polymer‐Based Dual‐Responsive Gold Nanoparticles with pH‐Dependent Thermal Sensitivity

This article presents the synthesis and physicochemical behavior of dual‐responsive plasmonic nanoparticles with reversible optical properties based on protein‐coated gold nanoparticles grafted with thermosensitive polymer brushes by means of surface‐initiated atom transfer radical polymerization (SI‐ATRP) that exhibit pH‐dependent thermo‐responsive behavior. Spherical gold NPs of two different sizes (15 nm and 60 nm) and with different stabilizing agents (citrate and cetyltrimethylammonium bromide (CTAB), respectively) were first capped with bovine serum albumin (BSA). The resulting BSA‐capped NPs (Au@BSA NPs) exhibited not only extremely high colloidal stability under physiological conditions, but also a reversible U‐shaped pH‐responsive behavior, similar to pure BSA. The ?‐amine of the L‐lysine in the protein coating was then used to covalently bind an ATRP‐initiator, allowing for the SI‐ATRP of thermosensitive polymer brushes of oligo(ethylene glycol) methacrylates with an LCST of 42 °C in pure water and around 37 °C under physiological conditions. Such protein coated nanoparticles grafted with thermosensitive polymers exhibit a smart pH‐dependent thermosensitive behavior.     

Multiresponsive PEDOT–Ionic Liquid Materials for the Design of Surfaces with Switchable Wettability

Among the different types of stimuli‐responsive polymers, conjugated polymers reveal unique multiresponsive behavior. In this work, the synthesis and characterization of new functional poly(3,4‐ethylenedioxythiophenes) (PEDOT) bearing imidazolium ionic‐liquid moieties (PEDOT‐Im) is reported. PEDOT‐Im polymers show multiresponsive properties to a variety of stimuli, such as temperature, pH, oxidative doping, and presence of anions. These stimuli provoke different changes in PEDOT‐Im, such as changes in color, oxidation state, and, wetting behavior. In all cases, a reversible effect is observed, and the polymers reveal responsive properties in solution as well as in the form of thin films. Whereas sensitiveness to pH and oxidative doping are known phenomena for other PEDOT derivatives, responsiveness to temperature and to anions is a unique property of PEDOT‐Im. The anion exchange is further investigated by means of the Quartz Crystal Microbalance with dissipation. Anion exchanges induce fast, adjustable, and reversible contact angle changes between 24° and 107°. As a potential application, surfaces with switchable wettability triggered by anion solutions are prepared by spin‐coating PEDOT‐Im films onto different substrates.     

Self‐Assembly Behavior of an Oligothiophene‐Based Conjugated Liquid Crystal and Its Implication for Ionic Conductivity Characteristics

In this work, a joint experimental and computational study on the synthesis, self‐assembly, and ionic conduction characteristics of a new conjugated liquid crystal quaterthiophene/poly(ethylene oxide) (PEO4) consisting of terminal tetraethyleneglycol monomethyl ether groups on both ends of a quaterthiophene core is performed. In agreement with molecular dynamic simulations, temperature‐dependent grazing‐incidence wide angle X‐ray scattering and X‐ray diffraction indicate that the molecule spontaneously forms a smectic phase at ambient temperature as characterized both in bulk and thin film configurations. Significantly, this smectic phase is maintained upon blending with bis(trifluoro‐methanesulfonyl)imide as ion source at a concentration ratio up to r = [Li⁺]/[EO] = 0.05. Nanosegregation between oligothiophene and PEO moieties and π–π stacking of thiophene rings lead to the formation of efficient 2D pathways for ion transport, resulting in thin‐film in‐plane ionic conductivity as high as 5.2 × 10^?4 S cm^?1 at 70 °C and r = 0.05 as measured by electrochemical impedance spectroscopy. Upon heating the samples above a transition temperature around 95 °C, an isotropic phase forms associated with a pronounced drop in ionic conductivity. Upon cooling, partial and local reordering of the conducting smectic domains leads to an ionic conductivity decrease compared to the as‐cast state.     

A High‐Performance Graphene Oxide‐Doped Ion Gel as Gel Polymer Electrolyte for All‐Solid‐State Supercapacitor Applications

A high‐performance graphene oxide (GO)‐doped ion gel (P(VDF‐HFP)‐EMIMBF₄‐GO gel) is prepared by exploiting copolymer (poly(vinylidene fluoride‐hexafluoro propylene), P(VDF‐HFP)) as the polymer matrix, ionic liquid (1‐ethyl‐3‐methylimidazolium tetrafluoroborate, EMIMBF₄) as the supporting electrolyte, and GO as the ionic conducting promoter. This GO‐doped ion gel demonstrates significantly improved ionic conductivity compared with that of pure ion gel without the addition of GO, due to the homogeneously distributed GO as a 3D network throughout the GO‐doped ion gel by acting like a ion “highway” to facilitate the ion transport. With the incorporation of only a small amount of GO (1 wt%) in ion gel, there has been a dramatic improvement in ionic conductivity of about 260% compared with that of pure ion gel. In addition, the all‐solid‐state supercapacitor is fabricated and measured at room temperature using the GO‐doped ion gel as gel polymer electrolyte, which demonstrates more superior electrochemical performance than the all‐solid‐state supercapacitor with pure ion gel and the conventional supercapacitor with neat EMIMBF₄, in the aspect of smaller internal resistance, higher capacitance performance, and better cycle stability. These excellent performances are due to the high ionic conductivity, excellent compatibility with carbon electrodes, and long‐term stability of the GO‐doped ion gel.     

Synthetic Asymmetric‐Shaped Nanodevices with Symmetric pH‐Gating Characteristics

Synthetic stimuli‐gated nanodevices displaying intelligent ion transport properties similar to those observed in biological ion channels have attracted increasing interests for their wide potential applications in biosensors, nanofluidics, and energy conversions. Here, bioinspired asymmetric shaped nanodevices are reported that can exhibit symmetric and linear pH‐gating ion transport features based on polyelectrolyte‐asymmetric‐functionalized asymmetric hourglass‐shaped nanochannels. The pH‐responsive polymer brushes grafted on the inner channel surface are acted as a gate that open and close in response to external pH changing to linearly and symmetrically regulate transmembrane ionic currents of the channel. A complete experimental characterization of the pH‐dependent ion transport behaviors of the nanodevice and a comprehensive discussion of the experimental results in terms of theoretical simulation are also presented. Both experimental and theoretical data shown in this work demonstrate the feasibility of using the asymmetric chemical modification method to achieve symmetric pH gating behaviors inside the asymmetric nanochannels, and lay the foundation to build diverse stimuli‐gated artificial asymmetric shaped ion channels with symmetric gating ion transport features.     

Color Tunable Poly (N‐Isopropylacrylamide)‐co‐Acrylic Acid Microgel–Au Hybrid Assemblies

A new class of materials that are capable of color tunability over 300 nm with a 15 °C temperature change is introduced. The materials are assembled from thermoresponsive poly (N‐isopropylacrylamide)‐co‐acrylic acid (pNIPAm‐co‐AAc) microgels, which are deposited on Au coated glass substrates. The films are also pH responsive; the temperature‐induced color change was suppressed at high pH and is consistent with the behavior of a solution of suspended microgels. The mechanism proposed to account for the observed optical properties suggests that they result from the two Au layers being separated from each other by the “monolithic” microgel film, much like a Fabry‐Pérot etalon or interferometer. It is the modulation of the distance between these two layers, facilitated by the microgel collapse transition at high temperature, that allows the color to be tuned. The sensitivity of the system presented here will be used for future sensing and biosensing applications, as well as for light filtering applications.     

Chain Extension of Stimuli‐Responsive Polymer Brushes: A General Strategy to Overcome the Drawbacks of the “Grafting‐To” Approach

Stimuli‐responsive polymer brushes are smart materials for the design of bio‐interactive and responsive interfaces. The “grafting‐to” approach is a convenient preparation procedure that allows the modification of surfaces with preformed and most notably well‐defined functionalized macromolecules. However, the shortcoming of this approach is an intrinsic limitation of the grafting density, which in turn affects the stimuli‐responsive properties of the brush system. Here, a general strategy to overcome this limitation and to simultaneously improve the switching behavior of a temperature‐responsive poly(N‐isopropylacrylamide) (PNiPAAm) brush is reported. A technically simple processing step is used in combination with the thermal azide–alkyne cycloaddition to perform the chain extension of alkyne‐functionalized PNiPAAm brushes with azide‐functionalized PNiPAAm molecules.     

Electrochemically Triggered Selective Adsorption of Biotemplated Nanoparticles on Self‐Assembled Organometallic Diblock Copolymer Thin Films

The controlled adsorption of the iron‐containing cage protein ferritin at the nanoscale using stimuli‐responsive self‐assembled diblock copolymer thin‐film templates is reported. The diblock copolymer used study consists of a cylinder‐forming polystyrene‐block‐polyferrocenylsilane (PS‐b‐PFS), with PFS as the minor block, and shows reversible redox properties. To prevent any spontaneous protein adsorption on either block, the electrolyte pH is selected to leave the ferritin negatively charged, and the protein concentration and solution ionic strength are carefully tuned. Selective adsorption of ferritin on the PFS domains of the self‐assembled thin films is then triggered in situ by applying a positive potential, simultaneously oxidizing the PFS and attracting the ferritin electrostatically.     

Smart Homopolymer Modification to Single Glass Conical Nanopore Channels: Dual‐Stimuli‐Actuated Highly Efficient Ion Gating

For the first time, the integration of a smart homopolymer poly[2–(dimethylamino) ethyl methacrylate] (PDMAEMA), which undergoes both pH‐ and temperature‐induced conformational transitions, is demonstrated in a single glass conical nanopore channel, to achieve a smart nanodevice that can be reversibly switched between the high conducting “on” state and low conducting “off” state with high gating efficiency due to the conformational changes of the homopolymer brush effected by varying the pH and temperature of the surrounding media.     

Multivalent‐Ion‐Mediated Stabilization of Hydrogen‐Bonded Multilayers

Hydrogen‐bonding interactions are an important alternative to electrostatic interactions for assembling multilayer thin films of uncharged components. Herein, a new method is reported for rendering such films stable at pH values close to physiological conditions. Multilayer films based on hydrogen bonding are assembled by the alternate deposition of poly[(styrene sulfonic acid)‐co‐(maleic acid)] (PSSMA) and poly(N‐isopropylacrylamide) (PNiPAAm) at pH 2.5. The use of PSSMA results in multilayers that contain free styrene sulfonate groups, as these moieties do not interact with the PNiPAAm functional groups. Subsequent infiltration of a multivalent ion (Ce⁴⁺ or Fe³⁺) leads to an increase in the total film mass, with little impact on the film morphology, as determined by using atomic force microscopy. To examine the film stability, the resulting films have been exposed to elevated pH (7.1). While there is substantial swelling of the multilayers (25 % and 55 % for Ce⁴⁺‐ and Fe³⁺‐stabilized films, respectively), film loss is negligible. This provides a stark contrast with non‐stabilized films, which disassemble almost immediately upon exposure to pH 7.1. This method represents a simple and effective strategy for stabilizing hydrogen‐bonded structures non‐covalently. Further, the multivalent ions also render the films responsive to changes in the local redox environment, as demonstrated by film disassembly after exposure of Fe³⁺‐treated films to iodide solutions.     

Supramolecular Self‐Assembly of Nanoconfined Ionic Liquids for Fast Anisotropic Ion Transport

Materials involving nanoconfinement of ionic liquids (ILs) have been pursued for functionalities and ionic devices. However, their complex synthesis, challenges to achieve long‐range order, and laborious tunability limit their practical implementation. Herein, these challenges are addressed by complexing surfactants to ILs, yielding a facile, modular, and scalable approach. Based on structural screening, ionic complexation of di‐n‐nonylamine to the terminal sulfonic acid of 1‐(4‐sulfobutyl)‐3‐methylimidazolium hydrogen sulfate IL is selected as a proof of concept. Spontaneous homeotropic smectic order over micrometers is observed, with alternating ionic and alkyl layers. The 1 nm thick ionic layers involve 2D crystalline internal order up to 150 °C, strongly promoting anisotropic ion transport (σ_||/σ_⊥ > 6500), and curiously, still allowing fluidity. High ionic conductivity of 35 mS cm^?1 and mesoscopic diffusion coefficient of ≈10^?5 cm² s^?1 at 150 °C along the ionic layers are observed. Fast anisotropic ion transport by simply complexing two components open doors to functional materials and applications.     

Stimuli‐Responsive Colloidal Systems from Mixed Brush‐Coated Nanoparticles

We report on the application of mixed polymer brushes to the reversible in situ switching colloidal systems (suspensions of responsive 200 nm in diameter particles in individual solvents and immiscible liquids). We used mixed copolymer brushes to fabricate responsive nanoparticles and employed the particles to prepare responsive colloidal systems, which demonstrated drastic transformation/switching of material properties upon external stimuli. The interaction between the particles themselves and the particles and their environment can be precisely tuned by a change of solvent and pH. We show that this behavior can be used for a reversible formation of particle aggregates, stabilization and switching between w/o and o/w emulsions, and regulation of the particle transport between immiscible liquids across the interface. We demonstrate an example of the application of the responsive colloids for the fabrication of ultrahydrophobic coatings with textured surfaces from aqueous dispersion, and no surfactant application, using the switching properties of the responsive particles.     

DyeIonogels: Proton‐Responsive Ionogels Based on a Dye‐Ionic Liquid Exhibiting Reversible Color Change

Transparent, ion‐conducting, and flexible ionogels based on the room temperature ionic liquid (IL) 1‐butyl‐3‐methylimidazolium bis(trifluoromethane sulfonyl)imide [Bmim][N(Tf)₂], the dye‐IL (DIL) 1‐butyl‐3‐methylimidazolium methyl orange [Bmim][MO], and poly(methylmethacrylate) (PMMA) are prepared. Upon IL incorporation the thermal stability of the PMMA matrix significantly increases from 220 to 280 °C. The ionogels have a relatively high ionic conductivity of 10^?4 S cm^?1 at 373 K. Most importantly, the ionogels exhibit a strong and reversible color change when exposed to aqueous or organic solutions containing protons or hydroxide ions. The resulting material is thus a prototype of soft multifunctional matter featuring ionic conductivity, easy processability, response to changes in the environment, and a strong readout signal, the color change, that could be used in optical data storage or environmental sensing.     

Double Stimuli‐Responsive Isoporous Membranes via Post‐Modification of pH‐Sensitive Self‐Assembled Diblock Copolymer Membranes

Double stimuli‐responsive membranes are prepared by modification of pH‐sensitive integral asymmetric polystyrene‐b‐poly(4‐vinylpyridine) (PS‐b‐P4VP) diblock copolymer membranes with temperature‐responsive poly(N‐isopropylacrylamide) (pNIPAM) by a surface linking reaction. PS‐b‐P4VP membranes are first functionalized with a mild mussel‐inspired polydopamine coating and then reacted via Michael addition with an amine‐terminated pNIPAM‐NH₂ under slightly basic conditions. The membranes are thoroughly characterized by nuclear magnetic resonance (¹H‐NMR), Fourier transform infrared spectroscopy and X‐ray‐induced photoelectron spectroscopy. Additionally dynamic contact angle measurements are performed comparing the sinking rate of water droplets at different temperatures. The pH‐ and thermo‐double sensitivities of the modified membranes are proven by determining the water flux under different temperature and pH conditions.     

Positively K+‐Responsive Membranes with Functional Gates Driven by Host–Guest Molecular Recognition

A novel positively K⁺‐responsive membrane with functional gates driven by host‐guest molecular recognition is prepared by grafting poly(N‐isopropylacrylamide‐co‐acryloylamidobenzo‐15‐crown‐5) (poly(NIPAM‐co‐AAB₁₅C₅)) copolymer chains in the pores of porous nylon‐6 membranes with a two‐step method combining plasma‐induced pore‐filling grafting polymerization and chemical modification. Due to the cooperative interaction of host‐guest complexation and phase transition of the poly(NIPAM‐co‐AAB₁₅C₅), the grafted gates in the membrane pores could spontaneously switch from “closed” state to “open” state by recognizing K⁺ ions in the environment and vice versa; while other ions (e.g., Na⁺, Ca²⁺ or Mg²⁺) can not trigger such an ion‐responsive switching function. The positively K⁺‐responsive gating action of the membrane is rapid, reversible, and reproducible. The proposed K⁺‐responsive gating membrane provide a new mode of behavior for ion‐recognizable “smart” or “intelligent” membrane actuators, which is highly attractive for controlled release, chemical/biomedical separations, tissue engineering, sensors, etc.