Enhanced thermal stability and pH behavior of glucose oxidase on electrostatic interaction with polyethylenimine.

Electrostatic interactions, mediated by ionic-exchange, between polyethylenimine (PEI) and glucose oxidase (GOx) were used to form GOx-PEI macro-complex, which were evaluated for pH and thermal stability of GOx. Under the experimental conditions, the complex had a dominant GOx presence on its surface and a hydrodynamic diameter of 205 ± 16 nm. Activity was evaluated from 40 to 75 °C, and at pH from 2 to 12. GOx activity in complex was maintained up to 70 °C and it was lost at 75 °C. In contrast, free GOx showed a maximum activity at 50 °C, which was completely lost at 70 °C. This difference, observed by fluorescence analysis, was associated with the compact unfolded structure of GOx in the complex. This GOx stability was not observed under pH variations, and complex formation was only possible at pH ≥ 5 where enzymatic activity was diminished by the presence of PEI.

Super-stretchable graphene oxide macroscopic fibers with outstanding knotability fabricated by dry film scrolling.

Graphene oxide (GO) has recently become an attractive building block for fabricating graphene-based functional materials. GO films and fibers have been prepared mainly by vacuum filtration and wet spinning. These materials exhibit relatively high Young's moduli but low toughness and a high tendency to tear or break. Here, we report an alternative method, using bar coating and drying of water/GO dispersions, for preparing large-area GO thin films (e.g., 800-1200 cm(2) or larger) with an outstanding mechanical behavior and excellent tear resistance. These dried films were subsequently scrolled to prepare GO fibers with extremely large elongation to fracture (up to 76%), high toughness (up to 17 J/m(3)), and attractive macroscopic properties, such as uniform circular cross section, smooth surface, and great knotability. This method is simple, and after thermal reduction of the GO material, it can render highly electrically conducting graphene-based fibers with values up to 416 S/cm at room temperature. In this context, GO fibers annealed at 2000 °C were also successfully used as electron field emitters operating at low turn on voltages of ca. 0.48 V/µm and high current densities (5.3 A/cm(2)). Robust GO fibers and large-area films with fascinating architectures and outstanding mechanical and electrical properties were prepared with bar coating followed by dry film scrolling.

Tunable protein-resistance of polycation-terminated polyelectrolyte multilayers.

The prevention of nonspecific protein adsorption is a crucial prerequisite for many biomedical and biotechnological applications. Therefore, the design of robust and versatile methods conferring optimal protein-resistance properties to surfaces has become a challenging issue. Here we report the unexpected case of polycation-ending polyelectrolyte multilayers (PEM) that efficiently prevented the adsorption of a negatively charged model protein, glucose oxidase (GOX). PEM films were based on two typical weak poyelectrolytes: poly(allylamine hydrochloride) (PAH) and poly(acrylic acid) (PAA). No chemical modification of the polyelectrolytes was required and tunable GOX adsorption was possible by simply changing the buildup pH conditions. Protein-resistance properties are attributed to high film hydration becoming the predominant factor over electrostatic interactions. We explain this effect by oscillations of the internal PAA ionization state throughout the buildup, which results in an excess of carboxylic acid groups within the film. This excess acts as a reservoir of potential carboxylate groups compensating the outer PAH positive charges. Partial results indicated that the system was also resistant to the adsorption of a positively charged protein, lysozyme. Control of the internal ionization of weak polyelectrolyte multilayers might open a route toward simple tuning of protein adsorption. These results should help to rationalize the design of biomaterials, biosensors, or protein separation devices.

Granular structure of self-assembled PAA/PAH and PSS/PAH nascent films imaged in situ by LC-AFM.

Liquid cell atomic force microscopy (LC-AFM) is used to image self-assembled polyelectrolyte films eliminating any drying effects on the film structure. Weak/weak and strong/weak polyelectrolyte films are formed by the alternated deposition of poly(acrylic acid) [PAA]/poly(allylamine hydrochloride) [PAH], and poly(sodium 4-styrene sulfonate) [PSS]/PAH, respectively, forming a granular surface structure. Number and area of grains (GN, GA) are used to characterize the surface of these films during their build up process. We show that hydrophilic PAA increases GA and decreases GN, while these parameters follow an opposite behavior with PSS. In both cases, GA and GN always have a simple inverse relationship, and then grain surface coverage (GS=GNGA) is nearly constant and independent of polyelectrolyte nature and the substrates used here, but also in the published data as well. The drying of the weak/weak film was also imaged after natural and forced solvent evaporation, and the surface structure is strongly affected, although the GS values keep roughly the same value found for wet films. The set of these results indicates that GS may be considered as a constant parameter during the build-up for the self-nascent assembled polyelectrolytes. The granular structure is still maintained after glucose oxidase adsorption on these films with comparable GS values.

Structure of polyelectrolyte complexes by Brownian dynamics simulation: effects of the bond length asymmetry of the polyelectrolytes.

Brownian dynamics simulations were performed to study the structure of polyelectrolyte complexes formed by two flexible, oppositely charged polyelectrolyte chains. The distribution of monomers in the complex as well as the radius of gyration and structure factor of complexes and individual polyelectrolytes are reported. These structural properties were calculated for polyelectrolyte chains with equal number of monomers, keeping constant the bond length of the negative chain and increasing the bond length of the positive chain. This introduces an asymmetry in the length of the chains that modulates the final structure of the complexes. In the symmetric case the distribution of positive and negative monomers in the complex are identical, producing clusters that are locally and globally neutral. Deviations from the symmetric case lead to nonuniform, asymmetric monomer distributions, producing net charge oscillations inside the complex and large changes in the radius of gyration of the complex and individual chains. From the radius of gyration of the polyelectrolyte chains it is shown that the positive chain is much more folded than the negative chain when the chains are asymmetric, which is also confirmed through the scaling behavior of the structure factors.