Sergiy Minko

Stimuli-responsive hydrogel thin films

In this brief review we address a range of interesting applications and prospects of responsive hydrogel thin films for the fabrication of “smart” responsive surfaces, membranes, sensors with various transduction mechanisms, micro/nanoactuators, and capsules. We show that hydrogel thin films compete with grafted polymers and demonstrate strong advantages for the fabrication of robust multifunctional and multiresponsive surfaces. This article reviews recent publications on the synthesis of responsive hydrogel thin films and hybrid films with entrapped nanoparticles and reagents by the chemical crosslinking of reactive polymers, layer-by-layer deposition, and block-copolymer self-assembly, as well as examining those publications to determine a range of applications.

Responsive Polymer Brushes

The review focuses on responsive/switching behavior of polymer brushes. The structure of the polymer brushes is analyzed in terms of the constitution and conformation of polymer chains. Responsive properties are widely discussed based on phase transition mechanisms in a controlled environment when changes are induced by solvent quality, temperature, concentration of ions, and interactions with liquids and solids. Potential applications of polymer brushes are also discussed.

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 films 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.

Interaction of nanoparticles with lipid membrane.

A nanoscale range of surface feature curvatures where lipid membranes lose integrity and form pores has been found experimentally. The pores were experimentally observed in the l-alpha-dimyristoyl phosphatidylcholine membrane around 1.2-22 nm polar nanoparticles deposited on mica surface. Lipid bilayer envelops or closely follows surface features with the curvatures outside of that region. This finding provides essential information for the understanding of nanoparticle-lipid membrane interaction, cytotoxicity, preparation of biomolecular templates and supported lipid membranes on rough and patterned surfaces.

Responsive brush layers: from tailored gradients to reversibly assembled nanoparticles

We present a condensed overview of the recent developments of novel responsive thin polymer films from end-tethered chains (polymer brushes), which are different from conventional, uniform, and planar brush layers. For this discussion, we selected two types of recently introduced surface layers: binary brush layers with variable chemical composition forming a controllable gradient of composition and properties in a selected direction and brush layers either grafted directly to inorganic nanoparticles to form hybrid core-shell structures or combined with inorganic nanoparticles embedded into this layer. Unlike traditional brush layers, such a design brings a novel set of responsive surface properties allowing for capillary-driven microfluidic motion, combinatorial-like multiplexing response, reversible aggregation and dis-assembly of nanoparticles, fabrication of ultrahydrophobic coatings, and switchable mass transport across interfaces.

Chemical Gating with Nanostructured Responsive Polymer Brushes: Mixed Brush versus Homopolymer Brush

In this report, we describe a novel approach to create an electrochemical gating system using mixed polymer brushes grafted to an electrode surface, and we explore the switchable properties of these mixed polymer brushes. The morphological transitions in the mixed polymer brushes associated with the electrode surface result in the opening, closing, or precise tuning of their permeability for ion transport through the channels formed in the nanostructured thin film in response to an external stimulus (pH change). The gating mechanism was studied by atomic force microscopy, ellipsometry, contact angle measurements, force-distance measurements, and electrochemical impedance spectroscopy. In comparison to a homopolymer brush system, the mixed brush demonstrates much broader variation of ion transport through the thin film. We suggest that this approach could find important applications in electrochemical sensors and devices with tunable/switchable access to the electrode surface.

Molecular-engineered stimuli-responsive thin polymer film: a platform for the development of integrated multifunctional intelligent materials

The area of stimuli-responsive thin polymer films (brushes, layer-by-layer multilayered structures, networks, and hybrid systems with inorganic particles) is discussed in this article with the major focus on the properties and potential applications for sensors, smart coatings, miniaturized devices, and hierarchically assembled multifunctional systems. We suggest directions for further expansion of this area using all aspects of the mechanism of response (transduction, dynamics, selectivity, sensitivity, and amplifications) and combinations of different thin film structural designs.

Biochemically controlled bioelectrocatalytic interface.

A switchable bioelectrocatalytic system for glucose oxidation controlled by external biochemical signals exemplifies interfacing between bioelectronic and biochemical ensembles.

Electrochemically controlled drug-mimicking protein release from iron-alginate thin-films associated with an electrode.

Novel biocompatible hybrid-material composed of iron-ion-cross-linked alginate with embedded protein molecules has been designed for the signal-triggered drug release. Electrochemically controlled oxidation of Fe(2+) ions in the presence of soluble natural alginate polymer and drug-mimicking protein (bovine serum albumin, BSA) results in the formation of an alginate-based thin-film cross-linked by Fe(3+) ions at the electrode interface with the entrapped protein. The electrochemically generated composite thin-film was characterized by electrochemistry and atomic force microscopy (AFM). Preliminary experiments demonstrated that the electrochemically controlled deposition of the protein-containing thin-film can be performed at microscale using scanning electrochemical microscopy (SECM) as the deposition tool producing polymer-patterned spots potentially containing various entrapped drugs. Application of reductive potentials on the modified electrode produced Fe(2+) cations which do not keep complexation with alginate, thus resulting in the electrochemically triggered thin-film dissolution and the protein release. Different experimental parameters, such as the film-deposition time, concentrations of compounds and applied potentials, were varied in order to demonstrate that the electrodepositon and electrodissolution of the alginate composite film can be tuned to the optimum performance. A statistical modeling technique was applied to find optimal conditions for the formation of the composite thin-film for the maximal encapsulation and release of the drug-mimicking protein at the lowest possible potential.

Gold‐Nanoparticle‐Enhanced Plasmonic Effects in a Responsive Polymer Gel

Localized surface plasmon resonance excited in gold nanoparticles coupled with a responsive polymer gel is explored. A specially designed structure (vertically aligned cylindrical pores decorated with gold nanoparticles) of responsive polymer gel thin films allows for the transduction of external signal/ stimuli into a strong optical effect enhanced by interactions of gold nanoparticles.