Ana V. Girão

Shaping Gold Nanocomposites with Tunable Optical Properties

We report the synthesis of morphological uniform composites using miniemulsions of poly(tert-butyl acrylate) or poly(styrene) containing organically capped gold nanocrystals (NCs). The optical features of such hybrid structures are dominated by plasmonic effects and depend critically on the morphology of the resulting nanocomposite. In particular, we demonstrate the ability to tune the overall optical response in the visible spectral region by varying the Au NCs arrangement within the polymer matrix, and therefore the interparticle plasmon coupling, using Au NCs resulting from the same batch of synthesis. This is a consequence of two well-known effects on the optical properties of Au particles: the variation of the surrounding dielectric refractive index and interparticle plasmonic coupling. The research reported here shows a general strategy to produce optical responsive nanocomposites via control of the morphology of submicrometric polymer particles containing metal nanocrystals and thus is an alternative to the more common strategy of size tuning metal nanoparticles used as nanofillers.

Tailoring gold and silver colloidal bimetallic nanoalloys towards SERS detection of rhodamine 6G

Nanoparticles of gold, silver and their bimetallic alloys have been prepared by an adapted method of reduction of the aqueous salts using sodium borohydride. It is demonstrated that the temperature and order of addition of the corresponding salt solutions influence the nanoalloys chemical arrangement giving different surface plasmon resonance behavior. The colloidal dispersions show very good Surface-Enhanced Raman Spectroscopy (SERS) signal towards rhodamine 6G. The obtained nanoparticles were then successfully deposited onto glass supports by evaporation of the respective colloids and also used as functional SERS sensors with efficient detection of trace amounts of the analyzed dye. These spectroscopic studies demonstrated that both mono and bimetallic nanoparticles show very good SERS sensitivity with the advantage of being prepared using a straightforward synthetic method in the absence of an additional stabilizer as needed for commonly used core–shell systems.

Biological synthesis of nanosized sulfide semiconductors: current status and future prospects.

There have been extensive and comprehensive reviews in the field of metal sulfide precipitation in the context of environmental remediation. However, these works have focused mainly on the removal of metals from aqueous solutions—usually, metal-contaminated effluents—with less emphasis on the precipitation process and on the end-products, frequently centering on metal removal efficiencies. Recently, there has been an increasing interest not only in the possible beneficial effects of these bioremediation strategies for metal-rich effluents but also on the formed precipitates. These metal sulfide materials are of special relevance in industry, due to their optical, electronic, and mechanical properties. Hence, identifying new routes for synthesizing these materials, as well as developing methodologies allowing for the control of the shape and size of particulates, is of environmental, economic, and practical importance. Multiple studies have shown proof-of-concept for the biological synthesis of inorganic metallic sulfide nanoparticles (NPs), resorting to varied organisms or cell components, though this information has scarcely been structured and compiled in a systematic manner. In this review, we overview the biological synthesis methodologies of nanosized metal sulfides and the advantages of these strategies when compared to more conventional chemical routes. Furthermore, we highlight the possibility of the use of numerous organisms for the synthesis of different metal sulfide NPs, with emphasis on sulfate-reducing bacteria (SRB). Finally, we put in perspective the potential of these methodologies in the emerging research areas of biohydrometallurgy and nanobiotechnology for the uptake of metals in the form of metal sulfide nanoparticles. A more complete understanding of the principles underlying the (bio)chemistry of formation of solids in these conditions may lead to the large-scale production of such metal sulfides, while simultaneously allowing an enhanced control over the size and shape of these biogenic nanomaterials.

Functionalized magnetite particles for adsorption of colloidal noble metal nanoparticles.

Magnetite (inverse spinel type) particles have been surface-modified with siliceous shells enriched in dithiocarbamate groups. The deposition of colloidal noble metal nanoparticles (Au, Ag, Pt, Pd) onto the modified magnetites can be performed by treating the respective hydrosols with the magnetic sorbents, thus allowing their uptake from water under a magnetic gradient. In particular, for Au colloids, these magnetic particles are very efficient sorbents that we ascribe to the strong affinity of sulfur-containing groups at the magnetite surfaces for this metal. Considering the extensive use of Au colloids in laboratorial and industrial contexts, the approach described here might have an impact on the development of nanotechnologies to recover this precious metal. En route to these findings, we varied several operational parameters in order to investigate this strategy as a new bottom-up assembly method for producing plasmonic-magnetic nanoassemblies.

Electron doping of Ca4Mn3O10 induced by vanadium substitution

In this work, electron doping of the anisotropic phase Ca4Mn3O10 was achieved by substitution of manganese by vanadium. No significant structural modifications were detected to the highest substitution ratio of 10% achieved. The introduction of vanadium decreases the electrical resistivity, described as two-dimensional variable range hopping, and induces the appearance of a ferromagnetic behavior. Consistently negative magnetoresistance develops in the magnetic ordered state. These results are characteristic of the presence of double exchange interactions, indicating that vanadium doping induces mixed valence of manganese.

Biotechnologically obtained nanocomposites: A practical application for photodegradation of Safranin-T under UV-Vis and solar light

This research was undertaken to determine the potential of biologically obtained ZnS-TiO2 nanocomposites to be used as catalysts in the photodegradation of organic pollutants, namely, Safranin-T. The photocatalysts were prepared by modifying the surface of commercial TiO2 particles with naturally produced ZnS, using sulfide species produced by sulfate-reducing bacteria and metal contaminated wastewaters. Comparative studies using powder X-ray diffraction (XRD) and scanning electron microscopy (SEM), prior and after photodegradation, were carried out in order to monitor possible structural and morphological changes on the particles. Adsorption properties and specific areas were determined by the Brunauer–Emmet–Teller (BET) method. The final solutions were characterized by UV-Vis and chemical oxygen demand (COD) content in order to determine Safranin-T concentration and toxicity. The influence of the catalyst amount, initial pH and dye concentration was also evaluated. Finally, the efficiency of the precipitates as catalysts in sunlight-mediated photodegradation was investigated, performing two scale experiments by using different volumes of dye-contaminated water (150 mL and 10 L). All tested composites showed potential to be used as photocatalysts for the degradation of Safranin-T, although the ZnS-TiO2_0.06 composite (0.06 g of TiO2 per 50 mL of the zinc solution) was the most effective. This substantiates the applicability of these biologically obtained materials as efficient photocatalysts for the degradation of organic pollutants, in laboratorial conditions and under direct sunlight.

Electron Doping of Ca4Mn3O10Induced by Vanadium Substitution

In this work, electron doping of the anisotropic phase Ca4Mn3O10 was achieved by substitution of manganese by vanadium. No significant structural modifications were detected to the highest substitution ratio of 10% achieved. The introduction of vanadium decreases the electrical resistivity, described as two-dimensional variable range hopping, and induces the appearance of a ferromagnetic behavior. Consistently negative magnetoresistance develops in the magnetic ordered state. These results are characteristic of the presence of double exchange interactions, indicating that vanadium doping induces mixed valence of manganese.

Electron Doping of Ca 4 Mn 3 O 10 Induced by Vanadium Substitution

In this work, electron doping of the anisotropic phase Ca4Mn3O10 was achieved by substitution of manganese by vanadium. No significant structural modifications were detected to the highest substitution ratio of 10% achieved. The introduction of vanadium decreases the electrical resistivity, described as two-dimensional variable range hopping, and induces the appearance of a ferromagnetic behavior. Consistently negative magnetoresistance develops in the magnetic ordered state. These results are characteristic of the presence of double exchange interactions, indicating that vanadium doping induces mixed valence of manganese.