An Electrochemical Gate Based on a Stimuli-Responsive Membrane Associated with an Electrode Surface

pH Modulated Formation of Complexes with Various Stoichiometry between Polymer Network and Fe(III) in Thermosensitive Gels Modified with Gallic Acid

Thermoresponsive gels based on N-isopropylacrylamide functionalized with amino groups were modified with gallic acid, with gallate (3,4,5-trihydroxybenzoic) groups being introduced into the polymer network. We investigated how the properties of these gels were affected at varying pH, by the formation of complexes between the polymer network of the gels and Fe³⁺ ions (which form stable complexes with gallic acid, exhibiting 1:1, 1:2, or 1:3 stoichiometry, depending on pH). The formation of complexes with varying stoichiometry within the gel was confirmed using UV-Vis spectroscopy, and the influence of such complexes on swelling behavior and volume phase transition temperature were investigated. In the appropriate temperature range, complex stoichiometry was found to strongly affect the swelling state. Changes in the pore structure and mechanical properties of the gel caused by the formation of complexes with varying stoichiometry were investigated using scanning electron microscopy and rheological measurements, respectively. The volume changes exhibited by p(NIPA-5%APMA)-Gal-Fe gel were found to be greatest at close to human body temperature (~38 °C). Modification of thermoresponsive pNIPA gel with gallic acid opens new opportunities for the development of pH- and thermosensitive gel materials.

Biofouling-Resistant Porous Membranes with a Precisely Adjustable Pore Diameter via 3D Polymer Grafting

A facile route to biofouling-resistant porous thin-film membranes that can be fine-tuned for specific needs in diverse bioseparation, mass flow control, sensors, and drug delivery applications is reported. The proposed approach is based on combining two distinct macromolecular systems-a cross-linked poly(2-vinyl pyridine) network and a 3D-grafted polyethylene oxide (PEO) layer-in one robust porous material whose porosity can be adjusted within a wide range, covering the macroporous and mesoporous size regimes. Notably, this reconfigurable material maintains its antifouling properties throughout the entire range of pore size configurations because of a dense surface carpet of PEO chains with self-healing properties that are immobilized both onto the surface and inside the polymer network through what was termed 3D grafting. Experimental results are supplemented by computer simulations of a coarse-grained model of a porous membrane that shows qualitatively similar pore swelling behavior.

Facile Fabrication of Composite Membranes with Dual Thermo- and pH-Responsive Characteristics

Facile fabrication of novel functional membranes with excellent dual thermo- and pH-responsive characteristics has been achieved by simply designing dual-layer composite membranes. pH-Responsive poly(styrene)-block-poly(4-vinylpyridine) (PS-b-P4VP) block copolymers and polystyrene blended with thermoresponsive poly(N-isopropylacrylamide) (PNIPAM) nanogels are respectively used to construct the top layer and bottom layer of composite membranes. The stretching/coiling conformation changes of the P4VP chains around the pK_a (∼3.5-4.5) provide the composite membranes with extraordinary pH-responsive characteristics, and the volume phase transitions of PNIPAM nanogels at the pore/matrix interfaces in the bottom layer around the volume phase transition temperature (VPTT, ∼33 °C) provide the composite membranes with great thermoresponsive characteristics. The microstructures, permeability performances, and dual stimuli-responsive characteristics can be well tuned by adjusting the content of PNIPAM nanogels and the thickness of the PS-b-P4VP top layer. The water fluxes of the composite membranes can be changed in order of magnitude by changing the environment temperature and pH, and the dual thermo- and pH-responsive permeation performances of the composite membranes are satisfactorily reversible and reproducible. The membrane fabrication strategy in this work provides valuable guidance for further development of dual stimuli-responsive membranes or even multi stimuli-responsive membranes.

Estimation of electron transfer properties of ferrocenyl-dicholesteryl-peptide in liquid and gel

Molecular dynamics simulations were carried out to investigate the conformations of ferrocenyl-dicholesteryl N-formamidoformamide (Fc-LS2) molecules in solvents of methanol and 1-propanol. Fc-LS2 comprises ferrocene and cholesteryl units linked by a biocompatible N-formamidoformamide peptide unit. Our results showed that Fc-LS2 formed a gel with 1-propanol but not with methanol. Charge-transfer properties of Fc-LS2/1-propanol gel and Fc-LS2/methanol liquid were also investigated by quantum mechanical (QM) calculations. The QM results indicate that the amino acid linkages contribute to improved charge-transport properties and the transfer integrals of Fc-LS2/1-propanol are larger than those of Fc-LS2/methanol.

Biocomputing, Biosensing and Bioactuation Based on Enzyme Biocatalyzed Reactions

The focus of this review paper is on the design and implementation of smart ‘Sense-and-Treat’ systems using enzyme-biocatalytic systems. These systems were used to perform biomolecular computing and they were functionally integrated with signal responsive materials aiming towards their biomedical use. Electrode interfaces, functionalized with signal-responsive materials, find applications in biocomputing, biosensing, and, specifically, triggered release of bioactive substances. ‘Sense-and-Treat’ systems require multiple components working together, including biosensors, actuators, and filters, in order to achieve closed-loop and autonomous operation. In general, biochemical logic networks were developed to process single biochemical or chemical inputs as well as multiple inputs, responding to nonphysiological (for concept demonstration purposes) and physiological signals (for injury detection or diagnosis). Actuation of drug-mimicking release was performed using the responsive material iron-cross-linked alginate with entrapped biomolecular species, responding to physical, chemical or biochemical signals.

Stimuli-responsive porous hydrogels at interfaces for molecular filtration, separation, controlled release, and gating in capsules and membranes

A continuously growing area of controlled and tunable transport and separation of biomolecules and drugs has recently attracted attention to the structures which can be referred to as stimuli-responsive porous hydrogel thin films. Because of spatial constraints, swelling/shrinking of the hydrogel films results in closing/opening (or vice versa) of the film's pores. Such responsive systems can be used in the configuration of plane films or capsules. The combination of a low thickness (translating into a low hydrodynamic flow resistance and rapid response) with well-defined size and shape of pores (translating into better control of transport and separation), which can be closed, opened, or tuned by an external signal (allowing a large amplitude of changes in diffusivity of solutes in the thin film and a precise control of the pore size), makes these materials very attractive for a range of applications, such as molecular filtration, separation, drug delivery, sensors, and actuators.

Reversible "closing" of an electrode interface functionalized with a polymer brush by an electrochemical signal

The poly(4-vinyl pyridine) (P4VP)-brush-modified indium tin oxide (ITO) electrode was used to switch reversibly the interfacial activity by the electrochemical signal. The application of an external potential (-0.85 V vs Ag|AgCl|KCl, 3M) that electrochemically reduced O(2) resulted in the concomitant consumption of hydrogen ions at the electrode interface, thus yielding a higher pH value and triggering the restructuring of the P4VP brush on the electrode surface. The initial swollen state of the protonated P4VP brush (pH 4.4) was permeable to the anionic [Fe(CN)(6)](4-) redox species, but the electrochemically produced local pH of 9.1 resulted in the deprotonation of the polymer brush. The produced hydrophobic shrunken state of the polymer brush was impermeable to the anionic redox species, thus fully inhibiting its redox process at the electrode surface. The interface's return to the electrochemically active state was achieved by disconnecting the applied potential, followed by stirring the electrolyte solution or by slow diffusional exchange of the electrode-adjacent thin layer with the bulk solution. The developed approach allowed the electrochemically triggered inhibition ("closing") of the electrode interface. The application of this approach to different interfacial systems will allow the use of various switchable electrodes that are useful for biosensors and biofuel cells with externally controlled activity. Further use of this concept was suggested for electrochemically controlled chemical actuators (e.g. operating as electroswitchable drug releasers).

Switchable selectivity for gating ion transport with mixed polyelectrolyte brushes: approaching 'smart' drug delivery systems

A pH-responsive mixed polyelectrolyte brush from tethered polyacrylic acid (PAA) and poly(2-vinylpyridine) (P2VP) (PAA:P2VP = 69:31 by weight) was prepared and used for selective gating transport of anions and cations across the thin film. An ITO glass electrode was modified with the polymer brush and used to study the switchable permeability of the mixed brush triggered by changes in pH of the aqueous environment in the presence of two soluble redox probes: [Fe(CN)(6)](4-) and [Ru(NH(3))(6)](3+). The responsive behavior of the brush was also investigated using the in situ ellipsometric measurements of the brush swelling, examination of the brush morphology with atomic force microscopy (AFM), and contact angle measurements of the brush samples extracted from aqueous solutions at different pH values. The mixed brush demonstrated a bipolar permselective behavior. At pH<3 the positively charged P2VP chains enabled the electrochemical process for the negatively charged redox probe, [Fe(CN)(6)](4-), while the redox process for the positively charged redox probe was effectively inhibited. On the contrary, at pH>6 a reversible redox process for the positively charged redox probe, [Ru(NH(3))(6)](3+), was observed, while the redox process for the negatively charged redox species, [Fe(CN)(6)](4-), was fully inhibited. Stepwise changing the pH value and recording cyclic voltammograms for the intermediate states of the polymer brush allowed electrochemical observation of the brush transition from the positively charged state, permeable for the negatively charged species, to the negatively charged state, permeable for the positively charged species. The data of ellipsometric, AFM and contact angle measurements are in accord with the electrochemical study. The discovered properties of the brush could be used for the development of 'smart' sensors and drug delivery systems, for example, a smart drug delivery capsule which could release negatively charged molecules of drugs in acidic conditions, while positively charged molecules of drugs will be released in neutral conditions.