Three-dimensional nucleation with diffusion controlled growth: a comparative study of electrochemical phase formation from aqueous and deep eutectic solvents

The nucleation of Ag onto vitreous carbon from aqueous 3 M NaCl or 0.6 M NaClO4 and deep eutectic solvent (DES) 1:2 M mixture of choline chloride:urea solutions containing Ag+, has been studied analyzing the chronoamperometric response to single potential steps. From the coordinates of the maxima observed in the current responses, the nucleation frequencies A (s(-1)) and number densities of nucleation sites N-0 (cm(-2)) were obtained from the standard model of nucleation with diffusion-controlled three-dimensional growth. Analysis of the overpotential dependence of nucleation frequencies using the classical electrochemical nucleation theory allowed to calculate the Gibbs free energy of nucleation Delta(G) over tilde (n(c)) and critical nucleus size n(c) as well as the exchange current density j(o), transfer coefficient alpha and surface tension sigma of silver nuclei. The kinetics of Ag+ reduction is two orders of magnitude slower in DES compared to both aqueous systems studied, and values of alpha << 0.5 where found in both aqueous and DES media, indicating either that the intermediate state for metal ion reduction is located close to the initial state, i.e., the solvated or complexed metal ion in solution, or that the metal ion is specifically adsorbed on the surface and the symmetry factor involved requires an alternative electron transfer formalism. The low Delta(G) over tilde (n(c)) and n(c) values observed indicate that the discharge of a single Ag ion on the surface already becomes a supercritical nucleus, involving a very low Gibbs energy barrier, characteristic of a non-activated process. (C) 2016 Elsevier B.V. All rights reserved

Pseudo-single crystal electrochemistry on polycrystalline electrodes : visualizing activity at grains and grain boundaries on platinum for the Fe2+/Fe3+ redox reaction

The influence of electrode surface structure on electrochemical reaction rates and mechanisms is a major theme in electrochemical research, especially as electrodes with inherent structural heterogeneities are used ubiquitously. Yet, probing local electrochemistry and surface structure at complex surfaces is challenging. In this paper, high spatial resolution scanning electrochemical cell microscopy (SECCM) complemented with electron backscatter diffraction (EBSD) is demonstrated as a means of performing ‘pseudo-single-crystal’ electrochemical measurements at individual grains of a polycrystalline platinum electrode, while also allowing grain boundaries to be probed. Using the Fe2+/3+ couple as an illustrative case, a strong correlation is found between local surface structure and electrochemical activity. Variations in electrochemical activity for individual high index grains, visualized in a weakly adsorbing perchlorate medium, show that there is higher activity on grains with a significant (101) orientation contribution, compared to those with (001) and (111) contribution, consistent with findings on single-crystal electrodes. Interestingly, for Fe2+ oxidation in a sulfate medium a different pattern of activity emerges. Here, SECCM reveals only minor variations in activity between individual grains, again consistent with single-crystal studies, with a greatly enhanced activity at grain boundaries. This suggests that these sites may contribute significantly to the overall electrochemical behavior measured on the macroscale

Direct In Vivo Evidence for Tumor Propagation by Glioblastoma Cancer Stem Cells

High-grade gliomas (World Health Organization grade III anaplastic astrocytoma and grade IV glioblastoma multiforme), the most prevalent primary malignant brain tumors, display a cellular hierarchy with self-renewing, tumorigenic cancer stem cells (CSCs) at the apex. While the CSC hypothesis has been an attractive model to describe many aspects of tumor behavior, it remains controversial due to unresolved issues including the use of ex vivo analyses with differential growth conditions. A CSC population has been confirmed in malignant gliomas by preferential tumor formation from cells directly isolated from patient biopsy specimens. However, direct comparison of multiple tumor cell populations with analysis of the resulting phenotypes of each population within a representative tumor environment has not been clearly described. To directly test the relative tumorigenic potential of CSCs and non-stem tumor cells in the same microenvironment, we interrogated matched tumor populations purified from a primary human tumor transplanted into a xenograft mouse model and monitored competitive in vivo tumor growth studies using serial in vivo intravital microscopy. While CSCs were a small minority of the initial transplanted cancer cell population, the CSCs, not the non-stem tumor cells, drove tumor formation and yielded tumors displaying a cellular hierarchy. In the resulting tumors, a fraction of the initial transplanted CSCs maintained expression of stem cell and proliferation markers, which were significantly higher compared to the non-stem tumor cell population and demonstrated that CSCs generated cellular heterogeneity within the tumor. These head-to-head comparisons between matched CSCs and non-stem tumor cells provide the first functional evidence using live imaging that in the same microenvironment, CSCs more than non-stem tumor cells are responsible for tumor propagation, confirming the functional definition of a CSC

Determination of Specific Electrocatalytic Sites in the Oxidation of Small Molecules on Crystalline Metal Surfaces

The identification of active sites in electrocatalytic reactions is part of the elucidation of mechanisms of catalyzed reactions on solid surfaces. However, this is not an easy task, even for apparently simple reactions, as we sometimes think the oxidation of adsorbed CO is. For surfaces consisting of non-equivalent sites, the recognition of specific active sites must consider the influence that facets, as is the steps/defect on the surface of the catalyst, cause in its neighbors; one has to consider the electrochemical environment under which the “active sites” lie on the surface, meaning that defects/steps on the surface do not partake in chemistry by themselves. In this paper, we outline the recent efforts in understanding the close relationships between site-specific and the overall rate and/or selectivity of electrocatalytic reactions. We analyze hydrogen adsorption/desorption, and electro-oxidation of CO, methanol, and ammonia. The classical topic of asymmetric electrocatalysis on kinked surfaces is also addressed for glucose electro-oxidation. The article takes into account selected existing data combined with our original works.M.J.S.F. is grateful to PNPD/CAPES (Brazil). J.M.F. thanks the MCINN (FEDER, Spain) project-CTQ-2016-76221-P

Electrochemically synthesized polymers in molecular imprinting for chemical sensing

This critical review describes a class of polymers prepared by electrochemical polymerization that employs the concept of molecular imprinting for chemical sensing. The principal focus is on both conducting and nonconducting polymers prepared by electropolymerization of electroactive functional monomers, such as pristine and derivatized pyrrole, aminophenylboronic acid, thiophene, porphyrin, aniline, phenylenediamine, phenol, and thiophenol. A critical evaluation of the literature on electrosynthesized molecularly imprinted polymers (MIPs) applied as recognition elements of chemical sensors is presented. The aim of this review is to highlight recent achievements in analytical applications of these MIPs, including present strategies of determination of different analytes as well as identification and solutions for problems encountered

From METS to malaria: RRx-001, a multi-faceted anticancer agent with activity in cerebral malaria

BACKGROUND: The survival of malaria parasites, under substantial haem-induced oxidative stress in the red blood cells (RBCs) is dependent on the pentose phosphate pathway (PPP). The PPP is the only source of NADPH in the RBC, essential for the production of reduced glutathione (GSH) and for protection from oxidative stress. Glucose-6-phosphate dehydrogenase (G6PD) deficiency, therefore, increases the vulnerability of erythrocytes to oxidative stress. In Plasmodium, G6PD is combined with the second enzyme of the PPP to create a unique bifunctional enzyme, named glucose-6-phosphate dehydrogenase–6-phosphogluconolactonase (G6PD-6PGL). RRx-001 is a novel, systemically non-toxic, epigenetic anticancer agent currently in Phase 2 clinical development for multiple tumour types, with activity mediated through increased nitric oxide (NO) production and PPP inhibition. The inhibition of G6PD and NO overproduction induced by RRx-001 suggested its application in cerebral malaria (CM). METHODS: Plasmodium berghei ANKA (PbA) infection in C57BL/6 mice is an experimental model of cerebral malaria (ECM) with several similar pathological features to human CM. This study uses intravital microscopy methods with a closed cranial window model to quantify cerebral haemodynamic changes and leukocyte adhesion to endothelial cells in ECM. RESULTS: RRx-001 had both single agent anti-parasitic activity and significantly increased the efficacy of artemether. In addition, RRx-001 preserved cerebral perfusion and reduced inflammation alone or combined with artemether. RRx-001’s effects were associated with inhibition of PPP (G6PD and G6PD-6PGL) and by improvements in microcirculatory flow, which may be related to the NO donating properties of RRx-001. CONCLUSION: The results indicate that RRx-001 could be used to potentiate the anti-malarial action of artemisinin, particularly on resistant strains, and to prevent infection

Publisher Correction: Marked bias towards spontaneous synaptic inhibition distinguishes non-adapting from adapting layer 5 pyramidal neurons in the barrel cortex

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