Publications

Abstract Graphene and its derivatives are generally portrayed as electron transfer enhancers that effectively boost the electrochemical response of classic electrodes for applications in renewable energy, electronics, or analysis (amongst others). However, a number of fundamental studies have challenged this view. In certain reports, not only could no beneficial effect be demonstrated, but the opposite was concluded. If we want to advance towards a more rational design of high-performance electrode devices, these discrepancies need to be cleared and the fundamental aspects of electron transfer reactions through graphene-electrodes further understood. The present study contributes to this cause by exploring the relationships between the structure and morphological appearance of graphene films and their electrochemical performance in fundamental proof-of-concept experiments. The results unveil that important differences in the structure and morphology of the films (which are tightly related to the composition and load of graphene materials) govern the electrochemical response of the modified electrodes. Thereby, a possible explanation for the apparently contradictory conclusions reported in the literature is provided.

Abstract Cubic nanoparticles are referred to as the best shaped particles for magnetic hyperthermia applications. In this work, the best set of values for obtaining optimized shape and size of magnetic particles (namely: reagents quantities and proportions, type of solvents, temperature, etc.) is determined. A full industrial implementation study is also performed, including production system design and technical and economic viability.

Abstract A hybrid conjugate of reduced graphene oxide/ferrous-ferric oxide nanoparticles (rGO-Fe3O4 NPs) is characterized and assembled with chitosan and laccase to form a layered functional superstructure. After its characterization by field-effect scanning electron microscopy, energy-dispersive X-ray analysis, X-ray photoelectron spectroscopy, attenuated total reflectance Fourier transform infrared, cyclic voltammetry (CV), and electrochemical impedance spectroscopy (EIS), the nanocomposite has been deposited on glassy carbon for the enzyme-mediated electrochemical determination of the endocrine disruptor bisphenol A (BPA). Proof-of-concept assays conducted by using CV, EIS, and square wave voltammetry reveal that the enzymatic biosensor provides linear response in a wide range of BPA concentrations (6-228 ppb), very high sensitivities, and excellent durability (over 1-month storage). Using amperometric detection, remarkable sensitivities (2080 mu A mu M-1 cm(-2)) and detection limits (18 nM) are attained. Applications to real samples of bottled water proved feasible with recoveries in the range 107-124%.

Abstract Chitosan and chitosan-nanoparticles were combined to prepare biobased and unplasticized film blends displaying antimicrobial activity. Nanosized chitosans obtained by sonication for 5, 15, or 30 min were combined with chitosan at 3:7, 1:1, and 7:3 ratios, in order to adjust blend film mechanical properties and permeability. The incorporation of nanosized chitosans led to improvements in the interfacial interaction with chitosan microfibers, positively affecting film mechanical strength and stiffness, evidenced by scanning electron microscopy. Nanosized or blend chitosan film sensitivity to moisture was significantly decreased with the drop in biocomposite molecular masses, evidenced by increased water solubility and decreased water vapor permeability. Nanosized and chitosan interactions gave rise to light biobased films presenting discrete opacity and color changes, since red-green and yellow-blue colorations were affected. All chitosan blend films exhibited antimicrobial activity against both Gram-positive and Gram-negative bacteria. The performance of green unplasticized chitosan blend films displaying diverse morphologies has, thus, been proven as a potential step towards the design of nontoxic food packaging biobased films, protecting against spoilage microorganisms, while also minimizing environmental impacts.

Abstract Hybrid conjugates of graphene with metallic/semiconducting nanostructures can improve the sensitivity of electrochemical sensors due to their combination of well-balanced electrical/electrocatalytic properties and superior surface-to-volume ratio. In this study, the synthesis and physical characterization of a hybrid conjugate of reduced graphene oxide and nickel nanoparticles (rGO-Ni NPs) is presented. The conjugate was further deposited onto a glassy carbon electrode as a nanocomposite film of chitosan and glucose oxidase. The electrochemical response and morphology of the films were investigated using SEM, CV, and EIS, and their applications as a glucose biosensor explored for the first time in proof-of-concept tests. The low operating potential along with the good linearity and sensitivity (up to 129 mu A cm(-2) mM(-1)) found in the sub-millimolar range suggest potential applications in the self-management of hypoglycemia from blood samples or in the development of non-invasive assays for body fluids such as saliva, tears or breath.

Abstract Compared to the oil-derived plastics typically used in food packaging, biofilms of pure chitosan present serious moisture issues. The physical degradation of the polysaccharide with ultrasound effectively reduces the water vapor permeability in these films but, unfortunately, they also turn more brittle. Blending chitosans of different morphology and molecular mass (M) is an unexplored strategy that could bring balance without the need of incorporating toxic or non-biodegradable plasticizers. To this end, we prepared and characterized the mixtures of a high-M chitosan with the products of its own ultrasonic fragmentation. Biopolymer degradation was followed by dynamic light scattering (DLS) and the mechanical and structural characteristics of the mixtures were evaluated from different rheological methods and atomic force microscopy (AFM). The results indicate that, through the control of the sonication time and mixture ratio, it is possible to adjust the viscoelasticity and morphological aspect of the mixtures at intermediate levels relative to their individual components. In a more general sense, it is emphasized the importance of design and materials processing for the development of a novel generation of additive-free sustainable but functional bioplastics.

Abstract The use of biodegradable natural polymers is a suitable alternative for the preparation of more environmentally-friendly plastics and biocompatible nanoparticulated systems. Chitosan is an abundant and inexpensive candidate. However, its transparent films present poor mechanical response and high sensitivity to moisture. Moreover, the findings made by different researchers on the effects of molecular mass and degree of deacetylation (DD) on these properties are still controversial. This paper aims to unveil the separate effects of these parameters on biofilm properties. For these purposes, two aqueous solutions of chitosan (DD = 90 and 95%) were submitted to controlled fragmentation by ultrasonication. The resulting solutions were characterized by rheological techniques and the nanoparticles formed were studied ex-situ by atomic force microscopy (AFM) and scanning electron microscopy (SEM). Irrespective of DD, the application of longer sonication times reduced the viscoelasticity of the solutions and yielded nanoparticles with lower size (and molecular mass). The mechanical strength and stiffness of transparent biofilms fabricated from these solutions, without plasticizers, were determined in stress tests. Sensitivity to moisture was also evaluated through water vapor permeability measurements and water sorption isothermal data. The results showed a significant decrease in the permeability with decreasing the molecular mass. However, the mechanical properties were adversely affected. These findings may be useful for the future design of bioplastics with improved properties but also for the development of biocompatible nanoparticles with tunable size and molecular mass.

Abstract Protein adsorption is a delicate process, which results from the balance between the properties of proteins and their solid supports. Although the relevance of some of these parameters has been already unveiled, the precise involvement of electrostatics and other weaker intermolecular forces requires further comprehension. Aiming to contribute to this task, this work explores the attachment, rearrangement, and surface aggregation of a model spheroprotein, such as bovine beta-lactoglobulin (beta-LG), onto hydrophilic substrates prefunctionalized with different alkylthiol films. Thereby, a variety of electrostatic scenarios for the adsorption of beta-LG could be recreated through the variation of the pH and the functional chemistry of the surfaces. The changes in surface mass density (plus associated water) and film flexibility were followed in situ with quartz crystal microbalance with dissipation monitoring. Film packing and aggregation were assessed by faradaic electrochemical measurements and ex situ atomic force microscopy and field effect scanning electron microscopy. In contrast to previous hypotheses arguing that electrostatic interactions between charged substrates and proteins would be the only driving force, a complex interplay between Coulombic and non-Coulombic intermolecular forces (which would depend upon the experimental conditions) has been suggested to explain the results.

Abstract The limited stability and random orientation of antibodies passively adsorbed onto solid supports are some of the main factors limiting the analytical performance of immune-based assay systems. Although the use of specific antibody-binding proteins led to significant enhancements in sensitivity, several uncertainties related to the orientation of these layers and the stability of the complexes formed with the antibodies need to be addressed for further progress. This paper introduces an alternative strategy based on the use of a charged polysaccharide layer for the stabilized and oriented assembly of antibody fragments. About one monolayer of F(ab')(2) fragments of anti-human immunoglobulin G was passively adsorbed onto mercaptopropionic acid (MPA) and MPA/chitosan modified Au surfaces showing very good conformational stability. However, interrogation tests in the presence of human immunoglobulin G showed a piezoelectrical antibody binding signal about two times higher when the fragments were adsorbed onto chitosan. Given the similar coverage and conformational stability found in both cases and considering the different electrostatic scenarios, it is strongly suggested that the enhanced recognition of antigens may arise from the assembly of F(ab')(2) mostly oriented in a hinge down end-on phase. Supporting this view, a limit of detection of about 3 mu g mL(-1) was obtained from electrochemical methods. Although high, this is one of the best results reported (to the best of our knowledge) in proof-of-concept experiments using 2D electrically insulating immobilization layers with such a limited antibody loading capacity.

Abstract The use of inexpensive and biodegradable deep-eutectic ionic mixtures as solvents for the electrochemical synthesis of conducting polymers could potentially improve the sustainability of these processes and reduce their economic cost. Such an unexplored approach was investigated in this communication by growing a model polymer such as polyaniline from a 1:2 mixture of choline chloride and 1,2-ethanediol (the so-called Propeline) using potentiodynamic and potentiostatic electrochemical procedures. Beyond a preparation method, cyclic voltammetry was also used to characterize the growth of the polymers. The morphology of the films, and their optical properties, were assessed ex-situ by means of scanning electron microscopy and spectroscopic measurements in the UV-vis. The polyanilines thus prepared exhibited nanoparticulated morphology and high reversibility to doping/dedoping which evidences fast charge transport across the films. Excellent conductivities higher than 50 S cm(-1) were found under this approach.