RESUMO Entre os nanomateriais de carbono, os pontos de carbono (PCs) estão entre os mais atraentes devido as suas interessantes propriedades fotoluminescentes. Os PCs apresentam processo de síntese fácil, possibilidade de funcionalização da superfície, baixa toxicidade, alta solubilidade em água e biocompatibilidade. Devido a isso, suas aplicações na área biomédica e de bioimagem tem se destacado na literatura. Neste trabalho, PCs foram sintetizados por dois métodos diferentes a partir da ovalbumina, o qual é um precursor não tóxico e composto predominantemente por aminoácidos de cadeia curta. Os PCs foram caracterizados por espectroscopias de absorção (UV-Vis), de emissão (fluorescência) e infravermelho (IVTF), além de análise termogravimétrica (ATG). Os PCs foram utilizados como sondas fluorescentes para uma imagem colorida (azul e verde) de células HEK293 e não induziram a morte celular, o que indica que os mesmos são biocompatíveis e não tóxicos para este tipo de células. Dessa forma, os resultados aqui apresentados demonstram que os PCs podem ser considerados potenciais substitutos para corantes comerciais em marcação biológica.

ABSTRACT Among the carbon nanomaterials, carbon dots (CDs) are among the most attractive ones due to their interesting photoluminescent properties. PCs feature easy synthesis process, possibility of surface functionalization, low toxicity, high water solubility and biocompatibility. Because of this, their applications in the biomedical and bioimaging areas has been highlighted in literature. In this work, CDs were synthesized by two different methods using ovalbumin as precursor, which is a nontoxic reactant composed predominantly of short chain amino acids. The CDs were characterized by absorption (UV-Vis), emission (fluorescence), infrared (IRFT) spectroscopies, and thermogravimetric analysis (TGA). The CDs were used as fluorescent probes for multicolor (blue and green) imaging of HEK293 cells and did not induce to cell death, which indicates that the CDs are biocompatible and nontoxic to this type of cells. Therefore, the results presented here demonstrated that CDs can be considered as a potential substitute for commercial dyes in biological images.

Recently, a new class of materials, the Perovskite Nanocrystals, also known as perovskite quantum dots (PQDs) has emerged. Perovskite nanocrystals are promising in optoelectronics due to their quantum confinement properties, narrow photoluminescence bands with short decay times, optically tunable bandgaps, easy surface passivation and high photoluminescent quantum yields. The intense photoluminescence covering the entire visible region of the spectrum can be easily adjusted through the composition and/or the nanocrystals sizes. This paper presents the state-of-the-art of these materials, since their discovery to current research, including their synthesis methods, properties and applications.

RESUMO Este trabalho relata nanopartículas de prata estabilizadas com citrato (AgNPs) sintetizadas pela redução química do íon nitrato com boro-hidreto de sódio, em diferentes pHs (2-9). As AgNPs sintetizadas por este método apresentaram distribuição de tamanho de 5 a 249 nm dependente do pH, conforme determinado por espalhamento dinâmico de luz, além de morfologia esférica, conforme determinado por microscopia eletrônica de transmissão. Na faixa de pH de 3 a 7 ocorreu a agregação das nanopartículas. A distribuição de tamanho, dependendo do pH, foi determinada por espalhamento dinâmico de luz. O potencial Zeta foi determinado e a estabilidade coloidal foi correlacionada com a agregação de nanopartículas em diferentes pHs. A atividade antimicrobiana dependente do tamanho foi avaliada para duas soluções, em que ambas as amostras exibiram atividade antimicrobiana, embora as menores AgNPs sem aglomeração apresentaram propriedades antimicrobianas melhores.

ABSTRACT This paper reports citrate-stabilized silver nanoparticles (AgNPs) synthesized by nitrate ion chemical reduction with sodium borohydride, at different pHs (2–9). The AgNPs synthesized by this method exhibited size distribution from 5 to 249 nm, depending on pH, as determined by dynamic light scattering, and morphology spherical, as determined by transmission electron microscopy. In pH range 3–7 occurred aggregation of the nanoparticles. The size distribution depending on pH was determined by dynamic light scattering. The zeta potential was determined, and the colloidal stability was correlated with nanoparticles aggregation at different pHs. The size-dependent antimicrobial activity was evaluated for two solutions, wherein both samples exhibited antimicrobial activity, although the smallest AgNPs without agglomeration have enhanced antimicrobial properties.

Solar energy has been considered as an important source of clean and safe energy to overcome the problems associate to the burn of fossil fuels (e.g. climate changes, pollution, health problems, etc.). In the area of photovoltaics, devices that convert solar energy into electricity, perovskites solar cells (PSC) have attracted great attention due to their rapid development, high energy conversion efficiency, diversification of the processing methods and different materials. The fantastic properties of the hybrid perovskite materials, such as high absorption coefficient, direct and tunable bandgap, ambipolar charge carrier, simple preparation, etc., have quoted this technology as one the most important in this century. The rapid development of PSC has provided a significant increase in energy conversion efficiency, which was first reported, in 2009, as 3.8% and now reaches up to 22.10%, according to the National Laboratory of Renewable Energy (NREL). Many studies have been carried out to further increase the efficiency of devices and solve problems as the instability of the material, and the presence of lead, so that the PSC can be applied commercially. This paper presents a review on PSC and the major advances reported in device’s architecture, preparation methods, novel materials and interface engineering.

This work reports a simple and reproducible method for silica-coated multiwalled carbon nanotubes (MWCNTs) nanostructures by the sol-gel method using tetraethoxysilane and 3-aminopropyltriethoxysilane as organosilicon precursors of silica. The synthetic method is based on a noncovalent functionalization of the MWCNTs by the adsorption of the monolayers of different polyelectrolytes, such as poly(allylamine hydrochloride) and poly(sodium 4-styrene sulfonate), which are positively and negatively charged, respectively. The role of these polyelectrolytes is the dispersion and interaction with both the nanotubes and the silicon precursors, influencing directly the silica shell growth. Two different morphology types of the silica-coated multiwalled carbon nanotubes composites were obtained on multiwalled carbon nanotubes. In both case the samples displayed uniformity in the silica layer. Finally, the investigation of the photoluminescence spectra of the samples suggested the presence of some incorrect oxygen bond, or vacancy of oxygen or even presence of nitrogen in the network, which is a typical feature of materials prepared by the sol-gel process at room temperature.

Quantum dot solar cells (QDSC) have been subject of extensive research in recent years. QDSC, as a promising alternative to existing solar cells, are among the candidates for next-generation photovoltaic devices that require low cost, high efficiency, so that quantum dots stand out for their unique features and versatile desirable on photovoltaic systems. The rapid development of QDSC provided a significant increase in energy conversion efficiency, which was certified for the first time in 2010 as 2% then being currently as 11.3%, according to the National Renewable Energy Laboratory (NREL). This paper presents a review of the quantum dot-sensitized solar cells and the major advances reported concerning these cells, besides other types of cell architectures involving quantum dots.

In this work a colloidal approach to synthesize water-soluble CdSe quantum dots (QDs) bearing a surface ligand, such as thioglycolic acid (TGA), 3-mercaptopropionic acid (MPA), glutathione (GSH), or thioglycerol (TGH) was applied. The synthesized material was characterized by X-ray diffraction (XRD), Fourier-transform infrared spectroscopy (FT-IR), UV-visible spectroscopy (UV-Vis), and fluorescence spectroscopy (PL). Additionally, a comparative study of the optical properties of different CdSe QDs was performed, demonstrating how the surface ligand affected crystal growth. The particles sizes were calculated from a polynomial function that correlates the particle size with the maximum fluorescence position. Curve resolution methods (EFA and MCR-ALS) were employed to decompose a series of fluorescence spectra to investigate the CdSe QDs size distribution and determine the number of fraction with different particle size. The results for the MPA-capped CdSe sample showed only two main fraction with different particle sizes with maximum emission at 642 and 686 nm. The calculated diameters from these maximum emission were, respectively, 2.74 and 3.05 nm.

The work described in this paper is related to the effect of adding carbon nanotubes (CNT) on the mechanical properties of polymer-cement composites. A full factorial design has been performed on 160 samples to identify the contribution provided by the following factors: polymeric phase addition, CNT weight addition and water/cement ratio. The response parameters of the full factorial design were the bulk density, apparent porosity, compressive strength and elastic modulus of the polymer-cement-based nanocomposites. All the factors considered in this analysis affected significantly the bulk density and apparent porosity of the composites. The compressive strength and elastic modulus were affected primarily by the cross-interactions between polymeric phase and CNT additions, and the water/cement ratio with polymeric phase factors.

The post-preparative size-selective precipitation technique was applied in CdTe and CdSe semiconductor nanocrystals prepared via colloidal route in water. The synthesis of CdTe and CdSe nanoparticles and the effect of the post-preparative size-selective precipitation have been characterized mainly by mean of ultraviolet and visible absorption spectroscopy (UV-Vis). It was demonstrated that the size-selective precipitation are able to isolate particles of different sizes and purify the nanoparticles as well.

Mineral rock wastes are being widely investigated due to possible damage to the environment when discarded indiscriminately in the nature, but also because of their great potential as ceramic raw materials. This work aims to study the effect of steatite particle additions on the mechanical properties of clay composites. A comprehensive series of experiments have been conducted to assess the influence of: steatite particle size, steatite fraction and compacting pressure on the performance of clay composites. The composite of superior properties was manufactured with 20 wt. (%) of steatite, 100-200 US-Tyler of steatite particle size and 30 MPa of compacting pressure.

Colloidal semiconductor nanocrystals, also known as quantum dots, have attracted great attention since they have interesting size-dependent properties due to the quantum confinement effect. These nanoparticles are highly luminescent and have potential applications in different technological areas, including biological labeling, light-emitting diodes and photovoltaic devices. The synthetic methods of semiconductor nanocrystals have progressed in the last 30 years, and several protocols were developed to synthesize monodisperse nanocrystals with good optical properties, different compositions and morphologies. This review describes the main methods used to synthesize nanocrystals in the II-VI and III-V systems, and the recent approaches in this field of research.

Ceramic matrix composites (CMCs), constituted of a silicon boron oxycarbide (SiBCO) matrix and unidirectional carbon fiber rods as a reinforcement phase, were prepared by pyrolysis of carbon fiber rods wrapped in polysiloxane (PS) or poly(borosiloxane) (PBS) matrices. The preparation of the polymeric precursors involved hydrolysis/condensation reactions of alkoxysilanes in the presence and absence of boric acid, with B/Si atomic ratios of 0.2 and 0.5. Infrared spectra of PBS showed evidence of Si-O-B bonds at 880 cm-1, due to the incorporation of the crosslinker trigonal units of BO3 in the polymeric network. X ray diffraction analyses exhibited an amorphous character of the resulting polymer-derived ceramics obtained by pyrolysis up to 1000 °C under inert atmosphere. The C/SiBCO composites showed better thermal stability than the C/SiOC materials. In addition, good adhesion between the carbon fiber and the ceramic phase was observed by SEM microscopy