Enzyme activity control by responsive redoxpolymers

Poly[glycidyl oligo(oxyethylene)carbamate]s (PGn-EOmR′ and R-PGn-EOmR′): controlled synthesis and effects of molecular parameters (n and m), side groups (R′), and end-groups (R) on thermoresponsive properties

The organocatalytic ROP and the post-modification reaction produced glycidol-based polymers with a variety of structural characteristics, which changed their shapes over a wide range of desired temperatures.

Thermoresponsive Polymers of Poly(2-(N-alkylacrylamide)ethyl acetate)s

Thermoresponsive poly(2-(N-alkylacrylamide) ethyl acetate)s with different N-alkyl groups, including poly(2-(N-methylacrylamide) ethyl acetate) (PNMAAEA), poly(2-(N-ethylacrylamide) ethyl acetate) (PNEAAEA), and poly(2-(N-propylacrylamide) ethyl acetate) (PNPAAEA), as well as poly(N-acetoxylethylacrylamide) (PNAEAA), were synthesized by solution RAFT polymerization. Unexpectedly, it was found that there are induction periods in the RAFT polymerization of these monomers, and the induction time correlates with the length of the N-alkyl groups in the monomers and follows the order of NAEAA < NMAAEA < NEAAEA < NPAAEA. The solubility of poly(2-(N-alkylacrylamide) ethyl acetate)s in water is also firmly dependent on the length of the N-alkyl groups. PNPAAEA including the largest N-propyl group is insoluble in water, whereas PNMAAEA and PNEAAEA are thermoresponsive in water and undergo the reversible soluble-to-insoluble transition at a critical solution temperature. The cloud point temperature (Tcp) of the thermoresponsive polymers is in the order of PNEAAEA < PNAEAA < PNMAAEA. The parameters affecting the Tcp of thermoresponsive polymers, e.g., degree of polymerization (DP), polymer concentration, salt, urea, and phenol, are investigated. Thermoresponsive PNMAAEA-b-PNEAAEA block copolymer and PNMAAEA-co-PNEAAEA random copolymers with different PNMAAEA and/or PNEAAEA fractions are synthesized, and their thermoresponse is checked.

Dynamic mechanism of halide salts on the phase transition of protein models, poly(N-isopropylacrylamide) and poly(N,N-diethylacrylamide)

The effects of salts on protein systems are not yet fully understood. We investigated the ionic dynamics of three halide salts (NaI, NaBr, and NaCl) with two protein models, namely poly(N-isopropylacrylamide) (PNIPAM) and poly(N,N-diethylacrylamide) (PDEA), using multinuclear NMR, dispersion corrected density functional theory (DFT-D) calculations and dynamic light scattering (DLS) methods. The variation in ionic line-widths and chemical shifts induced by the polymers clearly illustrates that anions rather than cations interact directly with the polymers. From the variable temperature measurements of the NMR transverse relaxation rates of anions, which characterize the polymer-anion interaction intensities, the evolution behaviors of Cl^-/Br^-/I^- during phase transitions are similar in each polymer system but differ between the two polymer systems. The NMR transverse relaxation rates of anions change synchronously with the phase transition of PNIPAM upon heating, but they drop rapidly and vanish about 3-4.5 °C before the phase transition of PDEA. By combining the DFT-D and DLS data, the relaxation results imply that anions escape from the interacting sites with PDEA prior to full polymer dehydration or collapse, which can be attributed to the lack of anion-NH interactions. The different dynamic evolutions of the anions in the PNIPAM and PDEA systems give us an important clue for understanding the micro-mechanism of protein folding in a complex salt aqueous solvent.

PQQ-GDH - Structure, function and application in bioelectrochemistry

This review summarizes the basic features of the PQQ-GDH enzyme as one of the sugar converting biocatalysts. Focus is on the membrane -bound and the soluble form. Furthermore, the main principles of enzymatic catalysis as well as studies on the physiological importance are reviewed. A short overview is given on developments in protein engineering. The major part, however, deals with the different fields of application in bioelectrochemistry. This includes approaches for enzyme-electrode communication such as direct electron transfer, mediator-based systems, redox polymers or conducting polymers and holoenzyme reconstitution, and covers applied areas such as biosensing, biofuel cells, recycling schemes, enzyme competition, light-directed sensing, switchable detection schemes, logical operations by enzyme electrodes and immune sensing.

CO2-Responsive Nano-Objects with Assembly-Related Aggregation-Induced Emission and Tunable Morphologies

CO₂-responsive polymeric nano-objects with assembly-related aggregation-induced emission (AIE) are obtained via polymerization-induced self-assembly (PISA) of 2-(dimethylamino)ethyl methacrylate (DMAEMA), 2-(4-formylphenoxy)ethyl methacrylate (MAEBA), and 4-(1,2,2-triphenylvinyl)phenyl methacrylate (TPEMA). These nano-objects exhibit, depending on the feed of MAEBA, a morphology evolution process from spherical micelles to vesicles. Due to the presence of DMAEMA units, CO₂ promotes morphology transformation of the nano-objects from spheres to a mixture of "jellyfish" and vesicles and vesicles to complex vesicles. Moreover, TPEMA endows the AIE feature to these nano-objects, offering a strategy to monitor the morphology evolution process in real time. Thus, this approach is significant for exploring the assembly mechanism of copolymer in polymerization-induced self-assembly and designing multistimuli-responsive polymeric nanomaterials with tunable morphologies and sizes.

Design of binary polymer brushes with tuneable functionality

Using coarse grained molecular dynamics simulations, we study how functionalized binary brushes may be used to create surfaces whose functionality can be tuned. Our model brushes consist of a mixture of nonresponsive polymers with functionalized responsive polymers. The functional groups switch from an exposed to a hidden state when the conformations of the responsive polymers change from extended to collapsed. We investigate quantitatively which sets of brush parameters result in optimal switching in functionality, by analyzing to which extent the brush conformation allows an external object to interact with the functional groups. It is demonstrated that brushes with species of comparable polymer lengths, or with longer responsive polymers than nonresponsive polymers, can show significant differences in their functionality. In the latter case, either the fraction of responsive polymers or the total grafting density has to be reduced. Among these possibilities, a reduction of the fraction of responsive polymers is shown to be most effective.

Multi-modulation for self-assemblies of amphiphilic rigid-soft compounds through alteration of solution polarity and temperature

A new class of small molecule-based amphiphilic carbazole-containing compounds has been designed and synthesized. Detailed analysis of the temperature- and solvent-dependent UV-vis absorption spectra has provided insights into the cooperative self-assembly mechanism of the carbazole-containing compounds. Interestingly, the prepared amphiphilic rigid-soft compounds were also found to display a lower critical solution temperature (LCST) behavior in aqueous solution, which is relatively less explored in small molecule-based materials, leading to promising candidates for the design of a new class of thermo-responsive materials.

Triple-Stimuli-Responsive Ferrocene-Containing PEGs in Water and on the Surface

Triple-stimuli-responsive PEG-based materials are prepared by living anionic ring-opening copolymerization of ethylene oxide and vinyl ferrocenyl glycidyl ether and subsequent thiol-ene postpolymerization modification with cysteamine. The hydrophilicity of these materials can be tuned by three stimuli: (i) temperature (depending on the comonomer ratio), (ii) oxidation state of iron centers in the ferrocene moieties, and (iii) pH-value (through amino groups), both in aqueous solution and at the interface after covalent attachment to a glass surface. In such materials, the cloud point temperatures are adjustable in solution by changing oxidation state and/or pH. On the surface, the contact angle increases with increasing pH and temperature and after oxidation, making these smart surfaces interesting for catalytic applications. Also, their redox response can be switched by temperature and pH, making this material useful for catalysis and electrochemistry applications. Exemplarily, the temperature-dependent catalysis of the chemiluminescence of luminol (a typical blood analysis tool in forensics) was investigated with these polymers.

Switchable Bioelectrocatalysis Controlled by Dual Stimuli-Responsive Polymeric Interface

The engineering of bionanointerfaces using stimuli-responsive polymers offers a new dimension in the design of novel bioelectronic interfaces. The integration of electrode surfaces with stimuli-responsive molecular cues provides a direct control and ability to switch and tune physical and chemical properties of bioelectronic interfaces in various biodevices. Here, we report a dual-responsive biointerface employing a positively responding dual-switchable polymer, poly(NIPAAm-co-DEAEMA)-b-HEAAm, to control and regulate enzyme-based bioelectrocatalysis. The design interface exhibits reversible activation-deactivation of bioelectrocatalytic reactions in response to change in temperature and in pH, which allows manipulation of biomolecular interactions to produce on/off switchable conditions. Using electrochemical measurements, we demonstrate that interfacial bioelectrochemical properties can be tuned over a modest range of temperature (i.e., 20-60 °C) and pH (i.e., pH 4-8) of the medium. The resulting dual-switchable interface may have important implications not only for the design of responsive biocatalysis and on-demand operation of biosensors, but also as an aid to elucidating electron-transport pathways and mechanisms in living organisms by mimicking the dynamic properties of complex biological environments and processes.

Controlled decoration of the surface with macromolecules: polymerization on a self-assembled monolayer (SAM)

Polymer functionalized surfaces are important components of various sensors, solar cells and molecular electronic devices. In this context, the use of self-assembled monolayer (SAM) formation and subsequent reactions on the surface have attracted a lot of interest due to its stability, reliability and excellent control over orientation of functional groups. The chemical reactions to be employed on a SAM must ensure an effective functional group conversion while the reaction conditions must be mild enough to retain the structural integrity. This synthetic constraint has no universal solution; specific strategies such as "graft from", "graft to", "graft through" or "direct" immobilization approaches are employed depending on the nature of the substrate, polymer and its area of applications. We have reviewed current developments in the methodology of immobilization of a polymer in the first part of the article. Special emphasis has been given to the merits and demerits of certain methods. Another issue concerns the utility - demonstrated or perceived - of conjugated or non-conjugated macromolecules anchored on a functionally decorated SAM in the areas of material science and biotechnology. In the last part of the review article, we looked at the collective research efforts towards SAM-based polymer devices and identified major pointers of progress (236 references).

Organometallic polymers for electrode decoration in sensing applications

Macromolecules containing metals combine the processing advantages of polymers with the functionality offered by the metal centers. The developments in the area of electrochemical chemo/biosensors based on organometallic polymers are reviewed.

Multi-stimuli responsive polymers – the all-in-one talents

The integration of several responsive moieties within one polymer yields smart polymers exhibiting a multifaceted responsive behaviour.

Control the wettability of poly(n-isopropylacrylamide-co-1-adamantan-1-ylmethyl acrylate) modified surfaces: the more Ada, the bigger impact?

Surface-initiated SET-LRP was used to synthesize polymer brush containing N-isopropylacrylamide and adamantyl acrylate using Cu(I)Cl/Me6-TREN as precursor catalyst and isopropanol/H2O as solvent. Different reaction conditions were explored to investigate the influence of different parameters (reaction time, catalyst concentration, monomer concentration) on the polymerization. Copolymers with variable 1-adamantan-1-ylmethyl acrylate (Ada) content and comparable thickness were synthesized onto silicon surfaces. Furthermore, the hydrophilic and bioactive molecule β-cyclodextrin-(mannose)7 (CDm) was synthesized and complexed with adamantane via host-guest interaction. The effect of adamantane alone and the effect of CDm together with adamantane on the wettability and thermoresponsive property of surface were investigated in detail. Experimental and molecular structure analysis showed that Ada at certain content together with CDm has the greatest impact on surface wettability. When Ada content was high (20%), copolymer-CDm surfaces showed almost no CDm complexed with Ada as the result of steric hindrance.

Functional polymers in protein detection platforms: optical, electrochemical, electrical, mass-sensitive, and magnetic biosensors

The rapidly growing field of proteomics and related applied sectors in the life sciences demands convenient methodologies for detecting and measuring the levels of specific proteins as well as for screening and analyzing for interacting protein systems. Materials utilized for such protein detection and measurement platforms should meet particular specifications which include ease-of-mass manufacture, biological stability, chemical functionality, cost effectiveness, and portability. Polymers can satisfy many of these requirements and are often considered as choice materials in various biological detection platforms. Therefore, tremendous research efforts have been made for developing new polymers both in macroscopic and nanoscopic length scales as well as applying existing polymeric materials for protein measurements. In this review article, both conventional and alternative techniques for protein detection are overviewed while focusing on the use of various polymeric materials in different protein sensing technologies. Among many available detection mechanisms, most common approaches such as optical, electrochemical, electrical, mass-sensitive, and magnetic methods are comprehensively discussed in this article. Desired properties of polymers exploited for each type of protein detection approach are summarized. Current challenges associated with the application of polymeric materials are examined in each protein detection category. Difficulties facing both quantitative and qualitative protein measurements are also identified. The latest efforts on the development and evaluation of nanoscale polymeric systems for improved protein detection are also discussed from the standpoint of quantitative and qualitative measurements. Finally, future research directions towards further advancements in the field are considered.

Temperature-responsive polymer/carbon nanotube hybrids: smart conductive nanocomposite films for modulating the bioelectrocatalysis of NADH

A temperature-sensitive polymer/carbon nanotube interface with switchable bioelectrocatalytic capability was fabricated by self-assembly of poly(N-isopropylacrylamide)-grafted multiwalled carbon nanotubes (MWNT-g-PNIPAm) onto the PNIPAm-modified substrate. Electron microscopy and electrochemical measurements revealed that these fairly thick (>6 μm) and highly porous nanocomposite films exhibited high conductivity and electrocatalytic activity. The morphological transitions in both the tethered PNIPAm chains on a substrate and those polymers wrapping around the MWNT surface resulted in the opening, closing, or tuning of its permeability, and simultaneously an electron-transfer process took place through the channels formed in the nanostructure in response to temperature change. By combining the good electron-transfer and electrochemical catalysis capabilities, the large surface area, and good biocompatibility of MWNTs with the responsive features of PNIPAm, reversible temperature-controlled bioelectrocatalysis of 1,4-dihydro-β-nicotinamide adenine dinucleotide with improved sensitivity has been demonstrated by cyclic voltammetry and electrochemical impedance spectroscopy measurements. The mechanism behind this approach was studied by Raman spectroscopy, in situ attenuated total reflection FTIR spectroscopy, and contact angle measurements. The results also suggested that the synergetic or cooperative interactions of PNIPAm with MWNTs gave rise not only to an increase in surface wettability, but also to the enhancement of the interfacial thermoresponsive behavior. This bioelectrocatalytic "smart" system has potential applications in the design of biosensors and biofuel cells with externally controlled activity. Furthermore, this concept might be proposed for biomimetics, interfacial engineering, bioelectronic devices, and so forth.

Stimuli responsive polymers for nanoengineering of biointerfaces

There is an increasing demand on the development of "smart" switchable interfaces since controlling surface topography and chemical functionality on a nanometer scale is crucial for numerous biomedical applications. Those surfaces, which are based on stimuli responsive polymers (SRPs), are able to modify their interactions with cells, biomolecules responding to different physical (e.g., temperature) or chemical (e.g., pH) stimuli. Such behavior may partially mimic complex dynamic properties of natural systems that are regulated by many biological stimuli. This paper reviews major studies and applications of SRPs as biointerfaces in a form of thin polymeric films (gels) and surface tethered polymers (brushes).

Effect of substrate inhibition and cooperativity on the electrochemical responses of glucose dehydrogenase. Kinetic characterization of wild and mutant types

Thanks to its insensitivity to dioxygen and to its good catalytic reactivity, and in spite of its poor substrate selectivity, quinoprotein glucose dehydrogenase (PQQ-GDH) plays a prominent role among the redox enzymes that can be used for analytical purposes, such as glucose detection, enzyme-based bioaffinity assays, and the design of biofuel cells. A detailed kinetic analysis of the electrochemical catalytic responses, leading to an unambiguous characterization of each individual steps, seems a priori intractable in view of the interference, on top of the usual ping-pong mechanism, of substrate inhibition and of cooperativity effects between the two identical subunits of the enzyme. Based on simplifications suggested by extended knowledge previously acquired by standard homogeneous kinetics, it is shown that analysis of the catalytic responses obtained by means of electrochemical nondestructive techniques, such as cyclic voltammetry, with ferrocene methanol as a mediator, does allow a full characterization of all individual steps of the catalytic reaction, including substrate inhibition and cooperativity and, thus, allows to decipher the reason that makes the enzyme more efficient when the neighboring subunit is filled with a glucose molecule. As a first practical illustration of this electrochemical approach, comparison of the native enzyme responses with those of a mutant (in which the asparagine amino acid in position 428 has been replaced by a cysteine residue) allowed identification of the elementary steps that makes the mutant type more efficient than the wild type when cooperativity between the two subunits takes place, which is observed at large mediator and substrate concentrations. A route is thus opened to structure-reactivity relationships and therefore to mutagenesis strategies aiming at better performances in terms of catalytic responses and/or substrate selectivity.

Redox-responsive gels with tunable hydrophobicity for controlled solubilization and release of organics

The hydrophobicity of the chemical environment within a redox-responsive polymer gel synthesized by copolymerization of hydroxybutyl methacrylate (HBMA) and vinylferrocene (VF) can be controlled by tuning the oxidation state of the redox-responsive moiety, ferrocene. When ferrocene is in the uncharged reduced state, the gel is hydrophobic and selectively extracts butanol from aqueous solution. Upon oxidation to ferricenium ions, charge is induced at the ferrocene sites making the gel hydrophilic, with a reduced capacity for butanol relative to water. Equilibrium distribution coefficients and separation factors provide quantitative evidence for this changing preference for butanol depending on oxidation state. The selection of the monomer constituting the polymer backbone, HBMA, was based on an initial screening using the Hansen solubility parameters of commercially available monomers. The effect of the various constituents of the gel on the gel's butanol extraction ability has been ascertained experimentally.

One-pot preparation of ferrocene-functionalized polymer brushes on gold substrates by combined surface-initiated atom transfer radical polymerization and "click chemistry"

A gold substrate with surface-grafted ferrocene functional polymer brushes, or Au-g-PFTMA surface [PFTMA = poly(5-ferrocene-triazolyl methacrylate)], was prepared by a combination of surface-initiated atom transfer radical polymerization (SI-ATRP) and "click chemistry" in one pot, in the presence of 2-azidoethyl methacrylate (AzEMA), ethynyl ferrocene, CuBr catalyst, CuBr(2) deactivator, and pentamethyldiethylenetriamine ligand. Thus, SI-ATRP of AzEMA from the Au substrate (the "grafting from" process) and click chemistry of the ethynyl ferrocene to the azide functional group of AzEMA (the "grafting to" process) proceeded simultaneously to produce the functional PFTMA brushes on the Au surface. Kinetic studies suggest that the reaction involving simultaneous SI-ATRP and click chemistry is still consistent with a controlled/"living" process. The composition and physical properties of the modified gold surface were analyzed by X-ray photoelectron spectroscopy, water contact angle measurement, and cyclic voltammetry. The redox-responsive properties of the ferrocene-functionalized polymer brushes on the Au-g-PFTMA surface were demonstrated in the reversible loading-unloading step of the β-cyclodextrin polymer via host-guest interaction.

Ionic topochemical tuned biosensor interface

Two new hydrophilic, poly(ethylene glycol) (PEG)-based redox copolymers bearing electrochemically active ferrocene (Fc) and thiol/disulfide anchoring functionalities were synthesized. These copolymers are shown to adsorb on gold surfaces causing polymeric self-assembled monolayers (pSAMs) that possess triple functions: "redox-active", "ionic-tunable", and "bio-inert". Both immobilized polymers showed redox potentials at +400 mV (Ag|AgCl), and facilitate the electrocatalytical oxidation of NADH. Additionally, interfacial architecture of the polymers is affected by an increase in Ca(2+) concentration, which leads to an amplification of the electrochemical response. The electrode current, measured for NADH-oxidation, increased by 80% after addition of 10 mM Ca(2+) ions. Considering the Ca(2+) influence on the heterogeneous electron transfer a structural model of the immobilized polymers is proposed based on the strong chelating ability of noncyclic PEG moieties.

Reactive surface coatings based on polysilsesquioxanes: defined adjustment of surface wettability

We have investigated a generally applicable protocol for a substrate-independent reactive polymer coating that offers interesting possibilities for further molecular tailoring via simple wet chemical derivatization reactions. Poly(methylsilsesquioxane)-poly(pentafluorophenyl acrylate) hybrid polymers have been synthesized by RAFT polymerization, and stable reactive surface coatings have been prepared by spin-coating on the following substrates: Si, glass, gold, PMMA, PDMS, and steel. These coatings have been used for a defined adjustment of surface wettability by surface-analogous reaction with various amines (e.g., glutamic acid to obtain hydrophilic surfaces (Theta(a) = 18 degrees) or perfluorinated amines to obtain hydrophobic surfaces (Theta(a) = 138 degrees)). Besides the successful covalent attachment of small molecules and polymers, amino-functionalized nanoparticles could also be deposited on the surface, resulting in nanostructured coatings, thereby expanding the accessible contact angle of hydrophobic surfaces further to Theta(a) = 152 degrees. The surface-analogous conversion of the reactive coating with isopropyl amine produced in situ temperature-responsive coatings. Using the presented simple, generally applicable protocol for substrate-independent reactive polymer coatings, the contact angle of water could be switched reversibly by almost 60 degrees.

Stimuli-responsive surfaces for bio-applications

The development of surfaces that have switchable properties, also known as smart surfaces, have been actively pursued in the past few years. The recent surge of interest in these switchable systems stems from the widespread number of applications to many areas in science and technology ranging from environmental cleanup to data storage, micro- and nanofluidic devices. Moreover, the ability to modulate biomolecule activity, protein immobilisation, and cell adhesion at the liquid-solid interface is important in a variety of biological and medical applications, including biofouling, chromatography, cell culture, regenerative medicine and tissue engineering. Different materials have been exploited to induce such changes in surface biological properties that are mostly based on self-assembled monolayers or polymer films. This critical review focuses on the recent progress in the preparation of these switchable surfaces, and highlights their applications in biological environments. The review is organized according to the external stimuli used to manipulate the properties of the substrate-chemical/biochemical, thermal, electric and optical stimuli. Current and future challenges in the field of smart biological surfaces are addressed (189 references).