Elías Pérez

Luminescence Concentration Quenching Mechanism in Gd2O3:Eu3+.

Luminescence concentration quenching in Gd2O3:Eu(3+) nanocrystals results from strong interactions among O(2-) ions and Eu(3+) ions. Because all synthesized Gd2O3:Eu(3+) nanocrystals present the same cubic crystalline phase regardless of Eu(3+) concentration, it is possible to study the optical properties as a function of the dopant concentration. The emission intensities and lifetime curves for Gd2O3:Eu(3+) were analyzed by a simple rate equation model to study the interaction between the O(2-) ions and Eu(3+) ions. The rate equation model considers that such interaction is driven by the following energy transfer processes: the direct energy transfer (O(2-) → Eu(3+)), back-transfer (Eu(3+) → O(2-)), and direct energy migration (Eu(3+) → Eu(3+)). The exact solution of this model agrees with the experimental results, luminescence concentration quenching is reproduced and the corresponding energy transfer rates are reported. Quantitative results suggest that the direct energy transfer and direct energy migration processes are the main responsible for the luminescence concentration quenching, whereas the back-transfer process promotes the Eu(3+) emission.

Biosensing and protein fluorescence enhancement by functionalized porous silicon devices.

Porous silicon (PSi) is a promising biomaterial presenting the advantage of being biocompatible and bioresorbable. Due to the large specific surface area and unique optical features, these microporous structures are excellent candidates for biosensing applications. Investigating device functionality and developing simple Si-based transducers need to be addressed in novel biological detection. Our work demonstrates that, among the various PSi configurations for molecular detection, PSi microcavity structure demonstrates the best biosensing performance, reflected through the enhanced luminescence response and the changes in the refractive index. For successful immobilization, molecular infiltration and confinement are the two key factors that are controlled by the pore size distribution of the PSi microcavities and by the surface modification obtained by silane-glutaraldehyde chemistry. Enhancement of the fluorescence emission of confined fluorescent biomolecules in the active layer of PSi microcavities was observed for a nonlabeled protein with a natural green fluorescence, the glucose oxidase enzyme (GOX). An increase in the fluorescence emission was also observed when functionalized PSi material was used to detect specific binding between biotin and a low concentration of labeled streptavidin. Evidence for the enzymatic activity of GOX in its adsorbed form is also presented. Use of smart silicon devices, enabling enhancement of fluorescence emission of biomolecules, offers easy-to-use biosensing, based on the luminescence response of the molecules to be detected.

Hemocompatibility assessment of poly(2-dimethylamino ethylmethacrylate) (PDMAEMA)-based polymers

Poly(2-dimethylamino-ethylmethacrylate) (PDMAEMA), a cationic polymer, has been widely reported as a nonviral carrier. Despite the fact that the cytotoxicity of this polymer has been extensively studied, there is a lack of information about its blood compatibility. Hence, this work evaluates the hemocompatibility of free-form PDMAEMA homopolymers differing in molecular weight (Mw) with or without a poly(ethylene glycol) (PEG) sequence in the form of a palm tree-like structure. Poly(ethylenimine) (PEI) was used as a reference in order to compare its hemoreactivity. Hemagglutination, hemolysis, platelet number, blood coagulation, and the complement systems were assessed in normal human whole blood according to the ISO 10993-4. Results showed that Mw, concentration, and incubation time strongly affected the hemocompatibility of the polymers evaluated. Our in vitro observations highlight that PDMAEMA homopolymers interacted strongly with the surface of the red blood cells but not with the inner structure of the membrane, while PEI behaved in the opposite way. No clear correlation has been evidenced between PDMAEMA-induced hemagglutination, PEI-induced hemagglutination, and hemolysis. Interestingly, if these polyelectrolytes strongly affect the platelets and blood coagulation cascades in a dose dependent way, none of them significantly affects the complement system. Our work reveals new knowledge on the toxicology of 2 families of polycations largely explored for gene delivery and on their mechanisms of cellular and humoral interactions.

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.

In situ surface structure study of polyelectrolyte multilayers by liquid-cell AFM

Abstract Liquid cell Atomic Force Microscopy (AFM) is used to image in situ self-assembly polyelectrolyte films (SAPFs). We show that the technique is appropriate to study surface structure of these systems. In this work, we report images of layer-by-layer deposition for negative poly(sodium 4-styrenesulfonate) (PSS) and positive poly(allylamine hydrochloride) (PAH), when an initial layer of positive poly(ethylenimine) (PEI) starts the multilayer polymer film on a glass surface. This work is addressed to study first layers of SAPFs. Height AFM images are obtained using contact mode at different pH buffer where polyelectrolytes are initially dissolved. The pH values were: 3.5, 6.8 and 10.5. In all the cases, the polyelectrolyte film surfaces are not flat, they show rough surfaces with average grain domains ranging from 50 to 90 nm in diameter. The roughness and grain domains slightly grow as a function of deposited layer. In order to understand the origin of grain structure observed by AFM, size of positive–negative polyelectrolyte complex was determined by Dynamic Light Scattering. The results suggest that grains of granulate surface are formed by polyelectrolyte complexes. In the present work, we also discuss effects of kinetics and preparation on the surface structure.

Coarse-grained simulations of the salt dependence of the radius of gyration of polyelectrolytes as models for biomolecules in aqueous solution.

The salt dependent radius of gyration of a polyelectrolyte in aqueous solution is calculated in an environment where the polyelectrolyte is surrounded by a permeable membrane that exchanges only solvent particles with the bulk. We obtain additionally the scaling exponent of the gyration radius as a function of the polymerization degree, and find that the polyelectrolyte retains a stretched conformation during the condensation and re-expansion process, indicating that these effects are of an electrostatic nature. The solvent quality is also shown to affect the polyelectrolyte conformation, especially for the poor solvent case. These results are obtained using a hybridized Monte Carlo technique with the coarse-grained, dissipative particle dynamics method with fluctuating number of solvent particles. The full range of the electrostatic interactions is included in the simulations, using the Ewald sum method, and the counterions and solvent molecules are included explicitly. In the complex systems mentioned above, the electrostatic interactions and the solvent quality play a key role in understanding phenomena that do not occur in uncharged systems. Our results are compared and validated with the behavior of some biomolecules under similar environments.

Dynamic light scattering by optically anisotropic colloidal particles in polyacrylamide gels.

The translational and rotational motions of optically anisotropic spherical particles embedded in cross-linked polyacrylamide gels is studied by dynamic light scattering. The particles are liquid crystal droplets solidified in the nematic phase. The amount of cross linkers is varied to cross the sol-gel transition where the system becomes nonergodic for both translational and rotational diffusion modes of the probes. The translational and rotational dynamic correlation functions are obtained by measuring the intensity correlation function between crossed polarizers in the parallel and perpendicular geometries. Data from nonergodic systems is analyzed using an extension, to include rotations, of the method of Pusey and van Megen [Physica A 157, 705 (1989)]. Both diffusion modes are observed to be arrested as the rigidity of the gel increases.

Fluorescence tuning of confined molecules in porous silicon mirrors

Emission signal from fluorescent molecules (fluorescein-5-maleimide) in a porous silicon mirror is enhanced by tuning the pore size and reflectance spectrum of the porous silicon multilayer structure. This is achieved when the reflectance spectrum of the silicon mirror overlaps the fluorescent excitation and emission wavelengths of the fluorescent molecule, and chemical linkers assure the molecular confinement.

On dynamical behaviour of two-dimensional biological reactors

The dynamical behaviour for a generic two-dimensional model of a continuous bioreactor is studied in this article. The state variables for the bio-reacting system are restricted to concentrations of substrate and biomass, where the specific growth rate is a smooth function of the substrate concentration, which can be a kinetic function, monotone or non-monotone (as Monod, Haldane, Teissier, etc.). The effect of input (dilution rate) on multiplicity and bifurcation of equilibrium is shown in open-loop configuration. The absence of limit cycles on open-loop configuration and through state-feedback on the dilution rate are demonstrated. The aim of analysing oscillations under state feedback control is related to the possible improvement of reactor yields under this operation regime. An example for a cell-producing bioreactor illustrates the analytical results.

Coherent optical reflectance from a monolayer of large particles adsorbed on a glass surface

We develop a coherent-scattering model for the reflection of light from a monolayer of large particles and low surface coverage. The model takes into account multiple scattering between particles of the monolayer and with the substrate, and it can be used around the critical angle in an internal reflection configuration. We compare the results of the model with our own reflectivity data taken with latex particles adsorbed on a glass-water interface and with a simpler effective-medium model.