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Carbon dots (CDs) have emerged in the last few years as new carbon nanostructures due to their excellent optical and physicochemical properties and thus promising applications in different areas. In this work, we describe the preparation of different CDs obtained from distinct bottom-up methods and also using different low molar mass precursors in order to study advantages and disadvantages of each experimental method used. Different techniques were performed to characterize the nanoparticles, such as Transmission electron microscopy, Potentiometric titration, Fourier transform infrared, Fluorescence and Ultraviolet-visible spectroscopy. The CDs obtained using conventional hydrothermal carbonization showed the highest values of quantum yield (QY), whereas the nanoparticles obtained by the heating reflux reaction method exhibited the lowest QY values mainly due to incomplete polymerization and/or carbonization reactions of the material. In addition, a study to evaluate the use of the CDs as nanoprobes for metallic ions showed that one of the samples exhibited high selective and sensitive response toward Fe3+, with a detection limit of 0.89 µM. Furthermore, the nanoparticles used as sensors were obtained with an environmentally friendly strategy, and the CD-based photoluminescence method is simple, has a short response time and present low-cost.

Carbon dots (CDs) represent a rising class of nanomaterials with numerous optical and physicochemical properties that make them useful for a variety of applications. In this work, three different methods, conventional hydrothermal carbonization (CHC), microwave-assisted irradiation and heating reflux reaction technique, were used as different bottom-up approaches to obtain different CDs. The carbon nanoparticles were prepared using three different carbon precursors (maleic, fumaric and adipic acids) and one nitrogen source (ethylenediamine). The CDs were characterized using different techniques including Fourier transform infrared spectroscopy, Transmission electron microscopy, Ultraviolet-visible, Fluorescence and Zeta potential measurements. Both, the experimental method used to obtain the CDs and the different precursors, presented a considerable influence on the quantum yield values (QY), and also on the fluorescence emission curves. The molar proportion between the carbon and nitrogen source was varied to improve the QY, and one of the samples prepared by CHC method reached QY = 25.4%. The carbon nanoparticles obtained by CHC were used as nanoprobes for metal ions and exhibited great selectivity and sensitivity for Fe3+ ions, reaching a limit of detection of 35 nM, which is lower than most reported values for CD-based PL method used as probes for Fe3+ ions.

This review reports the preparation and characterization of bionanocomposites based on biodegradable polymers reinforced with cellulose nanocrystals (CNC) described in the literature. The outstanding potential of cellulose nanocrystals as reinforcement fillers of biodegradable polymers is presented with an emphasis on the solution casting process, which is an appropriate method to investigate the physico-chemical effects of the incorporation of CNC into the polymeric matrices. Besides solution casting, other small scale methods such as electrospinning and layer-by-layer are also covered.

O comportamento de uma série de novos poliacrilatos líquido-cristalinos de cadeia lateral, no estado puro e em solução, foi estudado através das técnicas de Espalhamento de Luz, Raios-X e Microscopia Ótica de Luz Polarizada. A investigação dos homopolímeros no estado puro deu-se através das técnicas de luz espalhada despolarizada e espalhamento de Raios-X a Baixo e Alto Ângulo. A intensidade de luz espalhada anisotrópica por monodomínios constituídos de poliacrilatos líquido-cristalinos foi relacionada com o arranjo orientacional da cadeia polimérica, possibilitando o estudo das transições de fases e a determinação de parâmetros de ordem em homopolímeros esméticos. A caracterização, o arranjo e a ordenação local de cada mesofase foram obtidos através de Espalhamento de Raios-X tanto em homopolímeros não-orientados como orientados pela ação de um campo magnético. A influência do grupo espaçador e do mesogênico lateral no empacotamento local das camadas esméticas é descrito. A ordenação lamelar de cadeias poliméricas, em função da temperatura, para uma série de homopolímeros esméticos estudados, é discutida. O estudo dos poliacrilatos em solução deu-se em solvente orgânico e em cristais líquidos de baixa massa molar. O comportamento de soluções dos homopolímeros em THF em diferentes regimes de concentração foi investigado por espalhamento de luz estático e dinâmico. A formação de agregados pelo aumento da concentração em solvente orgânico é definida como uma manifestação das forças de interação responsáveis pela formação das fases líquido-cristalinas no estado puro. A organização de um dos homopolímeros em soluções concentradas em THF é discutida através de Espalhamento de Raios-X a Baixo Ângulo. Misturas de homopolímeros nos nematogênicos 5CB e 8CB foram investigadas através de Espalhamento de Raios-X a Baixo Ângulo. Dados complementares foram obtidos através de Microscopia Ótica de Luz Polarizada. O polímero nemático, na presença dos solventes líquido­cristalinos, apresentou uma fase esmética induzida, enquanto que o polímero esmético manteve essa mesofase e sofreu um efeito de inchamento das lamelas. Adicionalmente, microestruturas com arranjo lamelar foram detectadas na matriz isotrópica do solvente. The solution and bulk behaviors of a series of new side chain liquid crystalline polyacrylates have been investigated using Light Scattering, X-Ray Scattering and Polarized Optical Microscopy. Depolarized Light Scattering and X-Ray Scattering have been used to investigate the polyacrylates in the bulk state. Anisotropic light scattering in monodomains of liquid crystalline polyacrylates was related to the orientacional arrangement of the polymeric chains. ln this way, the phase transition temperatures as well as the degree of order of the smectic polymers have been determined. The characterization and arrangement of each mesophase have been obtained using X-Ray Scattering in non-orientated and orientated homopolymers by magnectic field. Toe influence of the lateral spacers and the mesogenic groups on the local packing of the smectic layers is described. The lamellar order of the polymeric chains as a function of temperature to a series of homopolymers is discussed. Toe solution behavior of the homopolymers was studied in THF (organic solvent) and two liquid crystal solvents of low molar mass (5CB and 8CB). Static and Dynamic Light Scattering have been used to study the homopolymers in THF at different concentration regimes. The formation of clusters in organic solvent was observed as a consequence of the interaction forces between liquid crystalline polymer chains, which cause the liquid crystalline mesophases in the bulk state. The arrangement of polyacrylate concentrated solution in THF is discussed on the basis of Small Angle X-Ray Scattering experiments. Mixtures of the homopolymers in the 5CB and 8CB nematogenics have been investigated through Small Angle X-Ray Scattering. Complementary data were obtained using Polarizing Optical Microscopy. ln solution in the liquid crystalline solvents, the nematic polymer exhibits an injected smectic phase, whereas the smectic one maintains the smectic arrangement already observed in the bulk, with a swollen smectic layers. Moreover, microstructures in the isotropic solvent matrix containing the liquid crystalline polymers with a smectic arrangement were detected.

As propriedades de soluções diluídas e semi-diluídas de polímeros líquido cristalinos do tipo poliacrilato foram investigadas através de espalhamento de luz estático e dinâmico. Alguns importantes parâmetros tais como raio de giro, peso molecular ponderal médio, segundo coeficiente virial, coeficiente de difusão e raio hidrodinâmico foram obtidos. Soluções diluídas para os dois diferentes pesos moleculares investigados em ambos os solventes Utilizados, 1HF e tolueno, mostram que o comportamento dos polímeros em relação a sua arquitetura é típico de cadeia linear flexível, enquanto que a conformação dos mesmos em solução é dependente do peso molecular. Para soluções com concentrações relativamente altas, uma maior repulsão entre as cadeias do que para macromoléculas lineares é atribuída à presença dos grupos mesogênicos laterais. Detectou-se a presença de clusters para todas as soluções a partir de concentrações de aproximadamente 50mg/mL, que foi determinada como sendo a concentração caracteristica do início da formação de agregados em solução. Esta concentração foi detenninada através de medidas de luz despolarizada e pelo estudo do coeficiente de difusão dos clusters. Uma forte dependência angular da luz espalhada no espalhamento de luz estático, o aparecimento de um segundo decaimento, de tempo longo, nas funções de correlação temporal obtidas no espalhamento de luz dinâmico, bem como uma queda brusca no valor do módulo osmótico indicam a presença de clusters de tamanho muito grande em todas as soluções com concentrações maiores do que 150mg/mL. A independência em relação ao solvente utilizado e a pouca dependência do peso molecular dos polímeros para a formação dos clusters, indicam que os mesmos são formados devido às interações entre os grupos mesogênicos de diferentes cadeias em solução, independentemente de uma interpenetração das mesmas. The properties of dilute and semi-dilute solutions of polyacrilate liquid crystals have been investigated by static and dynamic light scattering. Some important parameters like radius of giratio:n, average molecular wheight, second virial coefficient, diffusion coefficient and hydrodynamic radius could be determined. Dilute solutions of polyacrilates of two different molecular weights investigated in THF and toluene showed common flexible chain behavior. The form of these polymers in solution showed molecular weight dependence. For solutions with moderately high concentrations a stronger repulsion than linear macromolecules can be explained due to mesogenic groups appended to polymer backbone. The presence of clusters was detected for all solutions at concentrations above 50mg/mL. ln this work, this concentration is related to a characteristic concentration, in which cluster formation begin in solution and was determined by the diffusion coefficient of clusters as well as depolarized light scattering. A strong angular dependence in the static light scattering, the appearance of a second, slow motion in the time correlation function in the dynamic light scattering and a downturn of the osmotic modulus indicated the presence of big clusters in all solutions in concentration above 150mg/mL. The results showed that cluster formation is independent of the solvent used and only a slight molecular weight dependence was observed. These are indications that clusters are formed due to interactions between mesogenic groups of different chains in solution, which are independent of chains entanglement.