Avanços na síntese e caracterização de nanocompositos multifuncionais de níquel/óxido de grafeno reduzido

Doutoramento em Engenharia MecânicaO grafeno é constituído por uma monocamada de átomos de carbono dispostos numa espécie de rede hexagonal perfeita. Devido às suas propriedades extraordinárias, este nanomaterial tem suscitado um grande interesse tanto no setor científico como no industrial. A este respeito, a investigação em torno do grafeno mostrou um aumento exponencial em áreas tão diferentes como a energia, biomedicina, eletrónica, entre outras. O óxido de grafeno (GO), um dos derivados de grafeno, foi considerado como um substrato interessante para o desenvolvimento de nanocompositos. Isto deve-se fundamentalmente à presença de grupos funcionais de oxigénio na superfície do grafeno, os quais proporcionam locais reativos para a nucleação e o crescimento de outras estruturas. O níquel (Ni) é um metal de transição muito abundante na terra, possui uma superfície brilhante comum à maioria dos metais e é dúctil e maleável possuindo propriedades magnéticas e catalíticas superiores, condutividade térmica e elétrica razoáveis sendo muito utilizado em diferentes aplicações. As nanopartículas (NPs) de Ni são utilizadas como catalisadores heterogéneos e receberam atenção notável devido ao seu baixo custo, reduzida toxicidade, baixa corrosão, entre outras características. Desta forma, a funcionalização do GO com NPs de Ni pode constituir uma nova família de nanocompósitos com propriedades sinérgicas. Esta tese está focada no controlo da síntese de nanocompósitos Ni/GO, uma vez que o tamanho, a morfologia e a dispersão de NPs de Ni no grafeno afetam as suas funcionalidades e estão em dependência direta com as metodologias de síntese. Em primeiro lugar, foi usado um método hidrotérmico de fácil implementação e execução num passo único. Foram estudados vários parâmetros de síntese, incluindo temperatura, tempo de reação e agente redutor. O controlo destes parâmetros influenciou efetivamente o tamanho das NPs de Ni, variando estas de 150 a 900 nm, a morfologia variou de forma esférica a formato em espiga e de partículas finas bem distribuídas para agregados. Em seguida, o controlo do tamanho das NPs de Ni para valores inferiores a 10 nm e com distribuição de tamanho reduzido no substrato foi conseguido através de um procedimento de síntese em dois passos com base num método solvotérmico seguido por tratamento térmico sob atmosfera redutora de H2. O tempo de reação mostrou ser um fator chave para controlar a distribuição e o tamanho das NPs de Ni simultaneamente com a redução do GO (rGO). O aquecimento em atmosfera de H2 foi crucial para formar as NPs de Ni metálicas cristalinas. A influência de um tratamento térmico adicional em atmosferas redutora e inerte sobre a estrutura do nanocompósito Ni/rGO foi também investigada. Diferentes nanocompósitos apresentaram boa estabilidade térmica sob H2 até à temperatura de 450 °C durante 2 horas. O tratamento a 900 °C sob o fluxo de árgon alterou a estrutura do Ni/rGO por formação de “sulcos” através da rede de carbono e coalescência das NPs de Ni com formação de partículas maiores. O estudo das propriedades eletrofisicas dos nanocompositos Ni/rGO mostrou que estas são dependentes do tamanho e estrutura das NPs de Ni nas folhas de rGO. Esta é uma potencial vantagem do método de síntese desenvolvido para o design de diferentes nanocompositos de Ni/rGO que poderão ser materiais favoráveis para aplicação em dispositivos eletrónicos integrados.Graphene, the world thinnest material made of carbon atoms in a dense honeycomb network has captured a great interest in both scientific and industry sectors due to its remarkable properties. In this regard, the graphene research is facing an incredible rise in different areas such as energy, biomedical, sensor and electronic applications, between others. Graphene oxide (GO), one of the graphene derivatives, has been considered as an interesting substrate to build nanocomposites. This is due to the presence of oxygen functionalities at the graphene surface which provides reactive sites for the nucleation and growth of other structures. Nickel (Ni) is a transition metal very abundant on earth, it has a shiny surface common to most metals and is both ductile and malleable possessing different properties such as superior magnetic and catalysis properties, a fairly good heat and electrical conductivity and is widely used in different areas of application. Ni nanoparticles (NPs) find use as heterogeneous catalyst and received noteworthy attention because of its inexpensive, non-toxic, low corrosion, waste minimization, between other characteristics. In this way, the functionalization of GO with Ni NPs can establish a new family of nanocomposites with synergic properties. This thesis is focused on the control of the synthesis of Ni/GO nanocomposites, since the size, morphology and dispersion of Ni NPs on graphene affect their functionalities and are in direct dependence with the synthesis methodologies. First, a facile one pot hydrothermal method was introduced and various synthesis parameters including temperature, reaction time and reducing agent were investigated. The control of these parameters effectively influenced the Ni size, ranging from 150 to 900 nm, the morphology from spherical to spiky shape and from well distributed fine particles to the big aggregation. Then, the control of the Ni NPs size to values of less than 10 nm with narrow size distribution on the substrate was achieved via a two-step synthesis procedure based on a solvothermal method followed by a heat treatment under H2 reducing atmosphere. The reaction time was shown to be a key factor to control the size and size distribution of Ni NPs simultaneously through the reduction of GO (rGO). Heating treatment under H2 was crucial to form the crystalized metallic Ni NPs. The influence of further thermal treatment under reducing and inert atmospheres on the structure of Ni/rGO nanocomposite was also investigated. Different nanocomposites showed a good thermal stability under H2 up to 450°C during 2 hours’ treatment. Higher temperature (900°C) under Argon flow changed the structure of Ni/rGO by formation of trenches through the carbon etching and coalescence of Ni NPs to form bigger particles. The study of the electrophysical properties of Ni/rGO showed that these properties are dependent on the size and structure of Ni NPs on rGO nanosheets. This is the potential advantage of the synthesis method developed for designing different matrix of Ni/rGO nanocomposites which could be a favorable material for integrated electronic devices application

A novel shiga based immunotoxin against Fn-14 receptor on colorectal and lung cancer

Immunotoxins are regarded as a type of targeted therapy for killing cells by highly potent bacterial, fungal or plant toxins. Shiga like toxins (SLTs) are a group of bacterial AB5 protein toxins that inhibit host cell protein synthesis through the removal of a single adenine residue from the 28S rRNA and lead to apoptosis. Here, we described the design and usage of a Stx-based immunotoxin that can induce the selective cytotoxicity and apoptosis in Fn-14-positive cells related to the colon and lung cancer. In the present study, the Stx2a-PE15-P4A8 fusion protein was expressed efficiently in E. coli (DE3) system when driven from inclusion bodies by 8 M urea. The Stx2a-PE15-P4A8 fusion protein was expressed efficiently in E. coli (DE3) system and then purified. The purified fusion protein could specifically target Fn-14 receptor existed on colon and lung cancer cell lines and suppress these cells in a dose-dependent manner. In addition, the protein was able to nearly 50 % of apoptotic cell death and maintains about 54 % of its stability after 24 h of incubation in mouse serum at 37 °C. Compared to PE38-P4A8 construct in our previous study, these results showed that the Stx2a-PE15-P4A8 construct can be an efficient therapeutic candidate for cancer immunotherapy

Nanoengineered Nickel/reduced graphene oxide composites: control of interfacial nanostructure for tunable electrophysical properties

Here we present a novel solvothermal synthesis approach for the accurate control over the structural features of nickel/reduced graphene oxide (Ni/rGO) nanocomposites for tuneable electrical properties. We discovered that the dynamic chemical structure of GO during reaction, acts as an active template for the controlled nanostructured growth of nickel nanoparticles (Ni NPs). Therefore, the precise control of reaction time offered the possibility to modulate nucleation and coalescence phenomena of Ni NPs, allowing in this way to precisely adjust their size, density and NiO@Ni structure on the final Ni/rGO nanocomposites. The electrophysical properties (work function and conductivity) of different Ni/rGO nanocomposites were determined and found to be directly dependent on the Ni NPs radius and also on the NiO buffer layer width. We confirmed a crucial role of the NiO buffer layer thickness at the Pt-NiO-Ni-NiO-rGO interface changing the conductivity from metallic to those specific to a Schottky contact or to a p-n heterojunction. These new findings reveal a relevant potential for using Ni/rGO nanocomposites as a versatile and promising material for micro-, nano- and optoelectronics as well as for energy storage technologies.publishe