Catalina Haro-Pérez

Exposure to silver nanoparticles produces oxidative stress and affects macromolecular and metabolic biomarkers in the goodeid fish Chapalichthys pardalis

Silver nanoparticles (AgNPs) are the most commercialized nanomaterial worldwide, mainly due to their microbicidal activity. Although, AgNPs have been shown to be toxic to aquatic species, their effect on endemic fish, like Goodeidae, has not been demonstrated. Endemic species are under strong pressures by anthropogenic contamination and destruction of their habitat; therefore, we studied adult Chapalichthys pardalis, an endemic fish of Mexico. We evaluated the toxic effect of AgNPs through oxidative stress, macromolecular and metabolic biomarkers. We determined the LC50 (96h) and performed subchronic tests (21days) using sublethal AgNPs concentrations (equivalent to CL1 and CL10). At the end of the bioassay, we quantified 10 stress biomarkers in the liver, gills, and muscle, including the antioxidant enzymes (superoxide dismutase [SOD], catalase [CAT], and glutathione [GPx]), thiobarbituric acid reactive species (TBARS), protein oxidation (CO), macromolecules (proteins, lipids, and carbohydrates), and metabolites (glucose and lactate). In addition, we determined the integrated biomarkers response (IBR). LC50 was of 10.32mgL-1. Results of subchronic exposure (21days) revealed that AgNPs produce oxidative stress in C. pardalis adults, as evidenced by a diminution in antioxidant enzymes activity and an increase in TBARS and oxidized proteins. AgNPs also diminished levels of macromolecules and generated a high-energy consumption, reflected in the reduction of glucose levels, although lactate levels were not altered. The IBR analysis evidenced that the largest effect was produced in organisms exposed to LC10, being the liver and gills the organs with the greatest damage. Results demonstrated that exposure to AgNPs induces acute and chronic toxic effects on C. pardalis and forewarns about the impact that these nanomaterials can exert on these ecologically relevant aquatic organisms.

Simultaneous characterization of rotational and translational diffusion of optically anisotropic particles by optical microscopy

We probe the roto-translational Brownian motion of optically anisotropic particles suspended in water with a simple and straightforward optical microscopy experiment that does not require positional or rotational particle tracking. We acquire a movie of the suspension placed between two polarizing elements and we extract the translational diffusion coefficient D T and the rotational diffusion coefficient D R from the analysis of the temporal correlation properties of the spatial Fourier modes of the intensity fluctuations in the movie. Our method is successfully tested with a dilute suspension of birefringent spherical colloidal particles obtained by polymerizing an emulsion of droplets of liquid crystal in a nematic phase, whose roto-translational dynamics is found to be well described by theory. The simplicity of our approach makes our method a viable alternative to particle tracking and depolarized dynamic light scattering.

Three dimensional cross-correlation dynamic light scattering by non-ergodic turbid media

We investigate dynamic light scattering by non-ergodic turbid media with an adapted version of the method proposed by Pusey and van Megen [Physica A 157, 705 (1989)]. Our formulation follows the derivation of the original method by extending it to the three dimensional cross-correlation scheme (3DDLS). The main finding is an expression to obtain the dynamic structure factor from light scattering that takes into account the system turbidity and the peculiarities of the 3D geometry. From 3DDLS measurements in well-controlled solid-like systems of different turbidity, we confirm that our results can be interpreted reasonably well by the theoretical approach described here. Good agreement is found with earlier reported results on similar systems.

Study of translational and rotational dynamics of birefringent colloidal particles by depolarized light scattering in the far- and near-field regimes.

We characterize the translational and rotational dynamics of birefringent spherical colloidal particles by depolarized light scattering in the far- and near-field regimes. For this purpose, we use depolarized dynamic light scattering and propose an extension of dynamic heterodyne near-field light scattering that takes into account the polarization state of the light. Such a combination of methods allows to access colloidal dynamics in an extended q-range and permits to evaluate different modes of particles motion in suspension. Furthermore, we obtain a good agreement between results from the far- and near-field approaches thus validating our proposal and opening the possibility to investigate simultaneously the subtle interplay between translational and rotational motions of anisotropic colloidal particles in length-scales from the order of the particle size to several interparticle distances.

Characterization of slow dynamics in turbid colloidal systems by a cross-correlation scheme based on echo dynamic light scattering.

We describe the implementation of echo dynamic light scattering in a cross-correlation detection scheme, which enables the study of slow dynamics in moderately turbid colloidal systems by adapting a commercial light scattering device. Our setup combines a 3D cross-correlation detection scheme (3DDLS), which allows for suppression of multiple scattering, with the speckle echo technique for dynamic light scattering. The recorded cross-correlation echoes provide precise ensemble-averaged results that appropriately describe sample dynamics of ergodic and non-ergodic colloidal systems of different turbidities. Additionally, the high mechanical stability achieved in our setup makes possible an absolute estimation of the scattering intensity correlation function (ICF) directly from the height of echoes, thus making unnecessary any correction for imperfect rotation of the sample or of any ad hoc assumption regarding the correspondence between the absolute values of echo height and ICF. Furthermore, we find that zeroth-order echo height represents the coherence factor of the 3DDLS experiment.

Using a Parallel Genetic Algorithm to Fit a Pulsed Townsend Discharge Simulation to Experiments

A genetic algorithm has been used to obtain the transport/reaction coefficients which describe the experimental ionic avalanches as obtained in a pulsed discharge operated in the Townsend regime. The calculation of the avalanche currents involves solving the continuity equations for the charged species present in the discharge by means of an explicit finite difference method. This is done by taking into account the drift and reactions of ions and electrons. In this work we apply this procedure to study the discharge of two pure gases, namely, \(SF_6\) and \(N_2\). In both cases the algorithm deals with a large number of transport/reaction coefficients (fitting parameters) and produces a solution in a small number of steps. For this purpose the algorithm generates a population of 480 individuals each one defined by a large number of genes (transport/reaction coefficients). Each individual determines a solution of the ionic avalanches which is compared to the experiment and a fitness function is defined to sort the individuals. This sorted population is then employed to obtained a new generation of individuals by using five evolution rules. The algorithm can be easily parallelized and produces an excellent agreement with the experimental data.

Investigating the time dynamics of photon sequences scattered by tracer particles immersed in a polymeric gel

By dynamic light scattering, we study the effects of spatial confinement of tracer colloidal particles immersed in polyacrylamide networks on the time dynamics of sequences of elapsed times of scattered photons. We vary the degree of particles spatial confinement by changing the concentration of crosslinker in solution, from 0 to 2% in monomer mass fraction, which allows us to change the elasticity of the polymer gel. By calculating the Allan factor of the point processes of scattered photons arrival times, we are able to characterize the statistical properties of the scattering process. For all the samples we find that: i) the statistics of the photon arrival times is not Poissonian but clusterized in a certain counting time interval; ii) the time dynamics of the photon sequences is fractal in an ample range of counting times, from ~10−3 to ~10−1 s, with scaling exponents ranging between ~1.7 and ~1.0; iii) the crosslinking density of the polymer network, and thus, the system elasticity is correlated with the value of the fractal exponent. The value of the fractal exponent decreases as the system elasticity, or tracers spatial confinement, increases.

Crystal-Liquid Transition in Binary Mixtures of Charged Colloidal Particles

In this work we present experimental results on the melting of binary colloidal crystals composed by two different sized charged colloidal particles. As the number fraction of the smallest particles is increased, we observe a crystal-liquid transition. The melting line is characterized by studying the structural and dynamic properties of the colloidal suspensions by means of cross-correlation dynamic light scattering. The initial crystal structure consists of a body centered cubic lattice and after reaching a certain concentration of small particles, the system melts. At the melting point the resulting structure measured by static light scattering represents that of a strongly correlated fluid.