Insight into the surface composition of bimetallic nanocatalysts obtained from microemulsions.

The enhancement of catalysts efficiency of bimetallic nanoparticles depends on the ability to exert control over surface composition. However, results relating surface composition and feeding solution of bimetallic nanoparticles synthesized in microemulsions are controversial and apparently contradictory. In order to comprehend how the resulting surface can be modified under different synthesis conditions and for different pairs of metals, a computer simulation study was carried out. The resulting surface compositions are explained based on the relative rates of deposition of the two metals, which depend on the particular metal pair, the concentration of reactants and the microemulsion composition. This study provides a satisfactory understanding of experimental results and allows us to identify the main factors affecting the nanoparticle's surface composition. Consequently, concrete and practical guidelines can be established to facilitate the experimental synthesis of bimetallic nanoparticles with tailored surfaces.

Cage-like effect in Au-Pt nanoparticle synthesis in microemulsions: a simulation study.

The different distributions of metals in bimetallic nanoparticles synthesized in microemulsions were studied by computer simulation. The simulations demonstrated that if the difference between the reduction potentials of both metals is about 0.15-0.3 V, the compartmentalization of the reaction media causes the accumulation of slower reduction reactants in the microemulsions droplets, which favours the chemical reaction like a cage effect: increasing the local concentration of the slower reduction metal salt gives rise to a faster reduction, so the differences in reduction rates of both metals are attenuated. A more coincidental reduction of both metals deeply affects the nanoparticle structure, causing a better mixed alloy. This effect will be more pronounced when the concentration is higher and the intermicellar exchange rate is faster. This means that for any fixed microemulsion the nanoparticle structure can be modified by changing the reactant concentration: the core can be enriched in the faster reduction metal by lower concentrations, and the shell can be enriched in the slower metal by higher concentrations. Based on these observations, this study suggests a route to help experimentalists better create nanoparticles with a pre-defined structure.

Modelling of nano-alloying and structural evolution of bimetallic core-shell nanoparticles obtained via the microemulsion route.

A Monte Carlo model has been developed to describe the formation of bimetallic nanoparticles via the microemulsion route. The motivation stems from the need to understand the kinetics of nanoparticle formation in microemulsion droplets in order to determine the best experimental conditions to synthesize a nanoparticle with a given structure. We focus our study on the influence of the homogeneous and heterogeneous critical nucleus sizes of both metals on nanoparticle structure, as well as the role played by the surfactant film flexibility. The study reveals that the final structure is sensitive to changes in the critical nucleus numbers, because these parameters determine the rate of nucleation. An increase in the difference between nucleation rates of both metals gives rise to a better segregation of metals in the final nanoparticle. Likewise, as long as the formation of heterogeneous seeds is faster, the degree of alloying is greater. Finally, a fast material intermicellar exchange leads to a better mixture of metals, so the influence of the critical nucleus sizes on nanoparticle structure becomes less pronounced as the flexibility of surfactant film is increased.

Simulation of the kinetics of nanoparticle formation in microemulsions.

Monte Carlo simulations were carried out to explain experimental results concerning the different sizes obtained for Ag and Au nanoparticles synthesized in microemulsions. Computer simulations allowed to study the interplay between the chemical reaction rate and the material interdroplet exchange, and their consequences on the mechanism and size distribution of nanoparticles synthesized in microemulsions. It has been shown that, although the material interdroplet exchange depends primarily on the flexibility of the surfactant film, a slow reaction rate leads to a more effective material interdroplet exchange for a given microemulsion. Two factors contribute to this result. Firstly, a slow reaction implies that autocatalytic growth takes place for a longer period of time, because there are available reactants. If the reaction is faster, the reactants are almost exhausted at early stages of the process. As a consequence, autocatalytic growth is only possible at the beginning. Secondly, a slow reaction rate implies the continuous production of seed nuclei, which can be exchanged between micelles due to their small size, allowing the coagulation of two nanoparticles (growth by ripening). Once again, this exchange can only take place at early stages of the synthesis when the reaction is faster. Both factors, autocatalysis and ripening, favour the slow growth of the biggest nanoparticles leading to the production of larger particles when the reaction is slower.

Critical nucleus size effects on nanoparticle formation in microemulsions: a comparison study between experimental and simulation results.

Monte Carlo simulations were performed to study the influence of critical nucleus size on nanoparticle formation in microemulsions. It was found that critical nucleus size strongly affected nucleation and growth rates, as well as final nanoparticle sizes. An increase of critical nucleus leads to a slower nucleation process. In contrast, it gives rise to acceleration of the growth process. Final nanoparticle sizes increase as the critical nucleus value increases. It is predicted that this dependence will be less pronounced when a high reactant concentration is used. We have compared the simulation results with experimental data taken from different authors. Good agreement between the two kinds of results supports the conclusions of this paper.

Increased adrenocorticotropin responses to acute stress in Otsuka Long-Evans Tokushima Fatty (type 2 diabetic) rats.

The Otsuka Long-Evans Tokushima Fatty (OLETF) rat is a new diabetic strain of rats whose disease closely resembles human type 2 diabetes. We measured plasma adrenocorticotropic hormone (ACTH) and corticostrone levels, and iodine-125-labeled ovine corticotropin-releasing factor ([125I]oCRF) binding in the anterior pituitary after ether-laparotomy stress in OLETF rats to examine the alteration of the hypothalamic-pituitary-adrenal (HPA) axis. In addition, we examined ACTH secretion following CRF administration in vivo and in vitro to characterize the mechanisms regulating the HPA axis in OLETF rats. Body weight, plasma glucose and insulin levels in OLETF rats were significantly higher than that in Long-Evans Tokushima Otsuka (LETO) rats. Basal plasma ACTH levels tended to be higher in OLETF rats than in LETO but it did not reach statistical significance. Ether-laparotomy stress dramatically increased plasma ACTH levels at 2 h after the stress both in either OLETF and LETO rats; the peak plasma ACTH level in OLETF rats following the stress was significantly greater than in LETO rats. Plasma ACTH levels following CRF (2 microg/kg, i.v.) in OLETF and LETO rats showed statistically significant increases at 10 and 30 min after CRF administration compared to ACTH levels at 0 min, however, the peak plasma ACTH level in OLETF rats at 10 min after CRF administration was significantly greater than in LETO rats. In contrast to ACTH levels, no significant differences in corticosterone levels between OLETF and LETO were observed at any of the time points. CRF (10 ng/ml) significantly increased ACTH secretion in pituitary cultures from OLETF compared to LETO rats. These data reveal a complex regulation of the endocrine system in this diabetic condition and suggest that HPA axis may be more stimulated during acute stress in diabetes mellitus than in unaffected subjects.

Age-related reciprocal modulation of interleukin-1beta and interleukin-1 receptors in the mouse brain-endocrine-immune axis.

The cytokine interleukin-1 (IL-1) alters a variety of immune, central nervous system and neuroendocrine activities characteristic of an integrator of the brain-endocrine-immune response to stress. In an attempt to define the regulation of IL-1 and IL-1 receptors in the mouse brain-endocrine-immune axis during maturation, we measured tissue levels of IL-1beta using an ELISA and iodine-125-labeled recombinant human interleukin-1alpha ([125I]IL-1alpha) binding in hippocampus, pituitary, testis and spleen following intraperitoneal injection of saline or the bacterial endotoxin, lipopolysaccharide (LPS) in 3- and 24-week-old C57BL/6 mice. Basal IL-1beta levels were detectable in all the tissues examined. Basal levels of IL-1beta in the hippocampus, spleen and testis in 24-week-old mice were significantly higher than those in 3-week-old mice. [125I]IL-1alpha binding was detectable in all the mouse tissues examined and [125I]IL-1alpha binding levels in the pituitary, spleen and testis in 3-week-old mice were significantly higher than those in 24-week-old mice. To further evaluate the modulation of IL-1 receptors in aging, we measured [125I]IL-1alpha binding in 2-, 5-, 10-, 18-, and 24-month-old mice. [125I]IL-1alpha binding in testis in 24-month-old mice was higher than the other groups; [125I]IL-1alpha binding in the hippocampus and spleen was unchanged during these periods. Dramatic increases in IL-1beta concentrations were observed at 2 h after LPS injection in the pituitary, spleen, testis and plasma in both 3- and 24-week-old mice groups. In contrast, IL-1beta levels in the hippocampus increased in response to LPS injection only in 24-week-old mice. [125I]IL-1alpha binding in hippocampus was significantly decreased in 24- but not in 3-week-old mice after LPS injection. [125I]IL-1alpha binding in the spleen and testis were significantly decreased in both age groups following LPS administration. These data demonstrate differential regulation of IL-1 and its receptors in the brain-endocrine-immune axis during maturation and in response to endotoxin challenge.

Hypothalamic-pituitary-adrenal axis in WBN/Kob rats with non-insulin dependent diabetes mellitus.

In an attempt to define the hypothalamic-pituitary-adrenal (HPA) axis in non-insulin dependent diabetic rats (WBN/Kob), we measured plasma corticotropin releasing factor (CRF), as well as arginine vasopressin (AVP), ACTH, and corticosterone (B), CRF concentrations in the median eminence (ME), the remainder of the hypothalamus (rHY) and the neurointermediate lobe of the pituitary (NIL). We also measured Iodine-125-labeled ovine CRF ([125I]oCRF) binding in the brain and peripheral tissues. Body and thymus weight in WBN/Kob rats were significantly lower than in control Wistar rats, but adrenal weight was higher in WBN/Kob rats. Plasma ACTH levels were significantly higher in WBN/Kob rats than in the control rats. However, plasma CRF, AVP and B levels in WBN/Kob rats were not different from those in the control rats. The CRF concentration was significantly decreased in the ME of the diabetic rats, compared with control rats, but CRF concentrations in the rHY and NIL were unchanged in the two groups. A significant reduction in [125I]oCRF binding in the anterior pituitary was demonstrated in WBN/Kob rats, but no significant difference between the diabetic and control rats in CRF binding was observed in the frontal cortex, spleen or adrenal gland. These findings suggest that the HPA axis is chronically stimulated in the non-insulin dependent diabetic rats.

Corticotropin-releasing factor treatment upregulates interleukin-1 receptors in the mouse pituitary: reversal by dexamethasone.

Intraperitoneal injection of rat/human corticotropin-releasing factor (CRF) (40 micrograms/kg/0.2 ml of saline) resulted in a dramatic increase in specific iodine-125-labeled human interleukin-1 alpha ([125I]IL-1 alpha) binding in the male C57BL/6 mouse pituitary at 2 and 6 h after the injection although it did not affect [125I]IL-1 alpha binding in the mouse hippocampus, spleen and testis at any time after the injection. [125I]IL-1 alpha binding was unchanged at 2 h following dexamethasone (DEX) treatment (1 mg/kg/0.2 ml of 4% ethanol-saline) in the mouse pituitary and the hippocampus. In contrast, DEX inhibited CRF-induced upregulation of IL-1 receptors in the pituitary at 2 h after the injection. These data demonstrate complex interactions between CRF and DEX on IL-1 receptors during stress.

Nitric oxide plays no role in ACTH release induced by interleukin-1 beta, corticotropin-releasing hormone, arginine vasopressin and phorbol myristate acetate in rat pituitary cell cultures.

To investigate the role of nitric oxide (NO) in pituitary ACTH secretion, the effect of a nitric oxide synthase (NOS) inhibitor, N omega-Nitro-L-arginine (Nitro-Arg), on ACTH secretion induced by interleukin (IL)-1 beta, corticotropin releasing hormone (CRH), arginine vasopressin (AVP) and phorbol myristate acetate (PMA) was examined in rat anterior pituitary cell cultures. Nitro-Arg did not affect IL-1 beta-induced ACTH release in pituitary cell cultures incubated with Dulbecco modified Eagle's medium, Basal Medium Eagle or Krebs Ringer bicarbonate-glucose buffer. It did not affect CRH-, AVP- or PMA-induced ACTH release either. These results suggest that NO does not play an important role in ACTH release at the pituitary level.

Reciprocal modulation of corticotropin-releasing factor and interleukin-1 receptors following ether-laparotomy stress in the mouse.

Ether-laparotomy stress resulted in a dramatic decrease in specific iodine-125-labeled ovine CRF binding ([125I]oCRF) in the pituitary at 6 h after the onset of the stress although it did not affect [125I]oCRF binding in the pituitary at 2 h after the stress. [125I]oCRF binding was unchanged in the frontal cortex after the stress. In contrast, [125I]interleukin-1 (IL-1)alpha binding was significantly increased in the pituitary at 2 h after the stress and tended to be higher than non-stressed levels at 6 h after the stress but was not statistically significant. Ether-laparotomy stress did not affect [125I]IL-1 alpha binding in hippocampus, spleen and testis at any time after the stress. Plasma adrenocorticotropic hormone (ACTH) and corticosterone were increased at 2 h after the stress. These data demonstrate complex interactions between CRF and IL-1 receptors on HPA axis during stress.

Regulation of interleukin-1 receptors and hypothalamic-pituitary-adrenal axis by lipopolysaccharide treatment in the mouse.

We measured iodine-125-labeled recombinant human interleukin-1 alpha (125I-IL-1 alpha) binding in the hippocampus, pituitary, liver, spleen and testis, and plasma adrenocorticotropic hormone (ACTH) and corticosterone levels after i.p. injection of various dose and treatment regimens of the bacterial endotoxin, lipopolysaccharide (LPS). Plasma ACTH and corticosterone levels were significantly increased at 2 h after acute administration of LPS (60 or 300 micrograms/mouse). 125I-IL-1 alpha binding in all peripheral tissues examined was significantly and comparably decreased at 2 h after a single injection of 30 micrograms or 300 micrograms LPS/mouse. On the other hand, 125I-IL-1 alpha binding in hippocampus was significantly decreased only after high dose administration of LPS (300 micrograms/mouse). In order to evaluate if activation of IL-1 in brain resulting in the observed decrease in 125I-IL-1 alpha binding may require more sustained exposure to endotoxin, we compared the effects of a single injection (60 micrograms/mouse) and two injections of LPS (30 micrograms/mouse each at 0 and 12 h). A single injection of LPS (60 micrograms/mouse) decreased 125I-IL-1 alpha binding in the testis but not in the hippocampus, while two LPS injections (30 micrograms/mouse each at 0 and 12 h) caused dramatic reductions in 125I-IL-1 alpha binding in both the hippocampus and testis.(ABSTRACT TRUNCATED AT 250 WORDS)