Hiléia K.S. Souza

Electrospinning of agar/PVA aqueous solutions and its relation with rheological properties

In this work, we report the successful fabrication of agar-based nanofibers by electrospinning technique, using water as solvent media. A tubeless spinneret was attached inside the electrospinning chamber, operating at 50°C, to avoid agar gelation. Agar pure solution (1 wt%) showed inadequate spinnability regardless of the used electrospinning conditions. The addition of a co-blending polymer such as PVA (10 wt% starting solution) improved the solutions viscoelasticity and hence, the solutions spinnability. Agar/PVA solutions were prepared with different mass ratios (100/0, 50/50, 40/60, 30/70, 20/80 and 0/100) and electrospun at various sets of electrospinning conditions. Best nanofibers were obtained with 30/70 and 20/80 agar/PVA blends while samples with higher agar contents (50/50 and 40/60 agar/PVA) were harder to process and led to discontinuous fibrous mats. This first set of encouraging results can open a new window of opportunities for agar-based biomaterials in the form of nanofibers.

Choline chloride based ionic liquid analogues as tool for the fabrication of agar films with improved mechanical properties.

In the present paper, we test the suitability of ChCl/urea (DES-U) and ChCl/glycerol (DES-G) eutectic mixtures, each one prepared at 1:2 molar ratio, for the production of agar films. A three-step process is proposed: pre-solubilization of polymer in DES followed by compression-molding and subsequent drying. The mechanical properties, water resistance and microstructure of the films were evaluated at different polymer concentrations (i.e. 2-6%, w/w). DES-U showed by far, the best film forming ability. Agreeing with the diffusion and SEM data, films with the best mechanical properties were found at the lowest and highest agar concentrations (tensile strengths of 24.2-42 MPa and elongations of 15.4-38.9%). The water sorption and contact angle studies suggested increased hydrophilicity for the film containing the lowest concentration of agar. The use of choline chloride based ionic liquid analogues as solvent and plasticizer might be a promising tool for the development of new non-aqueous materials based on seaweed polysaccharides.

Aggregation-induced conformational transitions in bovine β-lactoglobulin adsorbed onto open chitosan structures

Chitosan is a natural polysaccharide which strongly interacts with whey proteins in solution. Certain protein–polyelectrolyte complexes and electrostatically assembled thin films have been shown to be good platforms for the preservation of globular and small proteins in their native state mainly due to the hydrophilic nature of these polyelectrolytes and to the lack of physical space for embedded proteins to relax. As a consequence, the use of natural polyelectrolytes in new nanocomposite materials for medicine, chemical analysis, catalysis, etc. has exploded in the last few years. Nevertheless, in many of these applications proteins are immobilized in more open structures without physical restrictions to undergo conformational changes. In this work, we investigate the nature of these transitions for a model whey protein, β-LG, on a chitosan-decorated Au surface in a scenario for which protein–surface interactions compete with boosted protein–protein non-coulombic forces. The adsorption kinetics, protein mass uptake (plus associated water), and flexibility of the adsorbed layers have been followed in situ by the quartz crystal microbalance with dissipation monitoring (QCM-D). Further ex situ characterization has been performed by atomic force microscopy (AFM) and non-invasive scanning electron microscopy (SEM). The balance between both types of interactions yielded surfaces heavily loaded with protein and water in which orientational transitions seemed restricted. The kinetics of the process was registered in a wide range of concentrations and successfully fitted to a double exponential equation derived from the RSA theory accounting for the establishment of slow post-adsorption conformational transitions.

Thermo-compression molding of chitosan with a deep eutectic mixture for biofilms development

A eutectic mixture of choline chloride (ChCl) and citric acid (CA) was successfully used for the preparation of chitosan (Chit) bio-films by thermo-compression molding. Optimization of the film preparation conditions was carried out using response surface methodology and a Box-Behnken design based on the best mechanical properties and lowest energetic requirements (lower compression load and time). The optimum film, made with chitosan and ChCl–CA (eutectic mixture), was compared with films prepared using only CA in their formulation. Chit–ChCl–CA films presented higher elasticity, opacity and total color difference, and a lower tensile strength in relation to the Chit–CA films. Films prepared with the eutectic mixture presented higher water vapor permeability values. These results were associated with the films microstructures. FTIR analysis confirms the occurrence of chemical changes in the processed films. The thermo-compression process significantly affects the crystallinity of the pristine chitosan. The films thermal stability is depressed when compared to the stability of each component.

Complexation of WPI and microwave-assisted extracted agars with different physicochemical properties

The complex formation between whey protein isolate (WPI) and agar has been investigated and their interactions were monitored as a function of the physicochemical properties of agar, the pH and the ionic strength of the medium. Agars from Gracilaria vermiculophylla were extracted under different MAE conditions and characterized according to their physicochemical properties. By using microwave irradiation a wide variety of agars was obtained, as different MAE conditions results in polyelectrolytes with distinct properties. UV-vis (in optical dispersion (O.D.) model) spectroscopy and isothermal titration calorimetry (ITC) have been used to study the formation of insoluble (coacervate) complexes. MAE agars revealed excellent properties for complex formation with WPI. The binding of WPI to MAE agar samples has been shown to be the result of different contributions. O.D. and ITC results showed that the molecular mass and the sulfate content of different agars had a determinant effect on coacervate formation.

Improving agar electrospinnability with choline-based deep eutectic solvents.

Very recently our group has produced novel agar-based fibers by an electrospinning technique using water as solvent and polyvinyl alcohol (PVA) as co-blending polymer. Here, we tested the deep eutectic solvent (DES), (2-hydroxyethyl)trimethylammonium chloride/urea prepared at 1:2 molar ratio, as an alternative solvent medium for agar electrospinning. The electrospun materials were collected with an ethanol bath adapted to a previous electrospinning set-up. One weight percent agar-in-DES showed improved viscoelasticity and hence, spinnability, when compared to 1 wt% agar-in-water and pure agar nanofibers were successfully electrospun if working above the temperature of sol-gel transition (∼80 °C). By changing the solvent medium we decreased the PVA concentration (5 wt% starting solution) and successfully produced composite fibers with high agar contents (50/50 agar/PVA). Best composite fibers were formed with the 50/50 and 30/70 agar/PVA solutions. These fibers were mechanically resistant, showed tailorable surface roughness and diverse size distributions, with most of the diameters falling in the sub-micron range. Both nano and micro forms of agar fibers (used separately or combined) may have potential for the design of new and highly functional agar-based materials.

Alternative plasticizers for the production of thermo-compressed agar films

Agar films were produced by thermo-compression using choline chloride (ChCl) as a plasticizer with urea. The three solid components were mixed together with the salt and urea (minor components) added to agar (main component) according to a fixed mass ratio of, respectively, 1.16:1:5. A central composite rotatable design (CCRD) with three parameters, 2(3), was used to evaluate the effects of temperature (X1; °C), time (X2; min) and applied load (X3; kN) of heat-pressing on the maximum tensile strength (TS) of the films (Y; MPa). Mixtures of urea and agar prepared at a mass ratio of 1:5 did not form homogeneous films suggesting the important plasticizing role of the salt. Heat-pressing the mixtures at more draconian conditions led to much darker and opaque films, with better mechanical resistance (higher values of TS). The most resistant film (∼ 15 MPa) was obtained at 140°C, 20 min and 176 kN. Selected films, including the optimal, showed similar water sorption profiles and close values of water vapor permeability (∼ 2.5-3.7 × 10(-9)gm(-1)s(-1)Pa(-1)). The fracture behavior and mechanical properties of the films were greatly affected by additional water plasticization when the films were stored at different conditions of relative humidity.

Efeitos ópticos - eletrônicos de solventes nas polarizabilidades eletrônicas do íon Ni2+ no complexo [Ni(C3H4N2)6](CF3SO3)2

Electronic polarizabilities were obtained for the salt of nickel trifluorometanesulfonate and for the complex with imidazole, in methanol and acetonitrile solutions. Starting from the electronic spectra, registered in a concentration of 10-3 mol L-1 and using the program SIMP2FOS to calculate the force of the experimental oscillator and POLAZ-F to calculate the electronic polarizabilities, it was possible to notice the different influence of the solvents in the deformation of the electronic cloud of the nickel (II).