The synthesis and study of so-called "nanoparticles", particles with diameters in the range of 1-20 nm, has become a major interdisciplinary area of research over the past 10 years. Semiconductor nanoparticles promise to play a major role in several new technologies. The intense interest in this area derives from their unique chemical and electronic properties, which gives rise to their potential use in the fields of nonlinear optics, luminescence, electronics, catalysis, solar energy conversion, and optoelectronics, as well as other areas. The small dimensions of these particles result in different physical properties from those observed in the corresponding macrocrystalline, "bulk", material. As particle sizes become smaller, the ratio of surface atoms to those in the interior increase, leading to the surface properties playing an important role in the properties of the material. Semiconductor nanoparticles also exhibit a change in their electronic properties relative to that of the bulk material; as the size of the solid becomes smaller, the band gap becomes larger. This allows chemists and material scientists the unique opportunity to change the electronic and chemical properties of a material simply by controlling its particle size. Research has already led to the fabrication of a number of devices. This review aims to highlight recent advances in the synthesis of compound semiconductor nanoparticle materials and their potential use in areas such as catalysis and electronic device fabrication.

The synthesis of CdS and CdSe nanocrystallites using the thermolysis of several dithioor diselenocarbamato complexes of cadmium in trioctylphosphine oxide (TOPO) is reported. The nanodispersed materials obtained show quantum size effects in their optical spectra and exhibit near band-edge luminescence. The influence of experimental parameters on the properties of the nanocrystallites is discussed. HRTEM images of these materials show well-defined, crystalline nanosized particles. Standard size fractionation procedures can be performed in order to narrow the size dispersion of the samples. The TOPO-capped CdS and CdSe nanocrystallites and simple organic bridging ligands, such as 2,2′-bipyrimidine, are used as the starting materials for the preparation of novel nanocomposites. The optical properties shown by these new nanocomposites are compared with those of the starting nanodispersed materials.

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This review focuses on recent developments in hybrid and nanostructured substrates for SERS (surface-enhanced Raman scattering) studies. Thus substrates composed of at least two distinct types of materials, in which one is a SERS active metal, are considered here aiming at their use as platforms for chemical detection in a variety of contexts. Fundamental aspects related to the SERS effect and plasmonic behaviour of nanometals are briefly introduced. The materials described include polymer nanocomposites containing metal nanoparticles and coupled inorganic nanophases. Chemical approaches to tailor the morphological features of these substrates in order to get high SERS activity are reviewed. Finally, some perspectives for practical applications in the context of chemical detection of analytes using such hybrid platforms are presented.

Recebido em 24/10/03; aceito em 5/2/04; publicado na web em 17/6/04 POLYMER BASED NANOCOMPOSITES: SYNTHETIC STRATEGIES FOR HYBRID MATERIALS. Associating the well known advantages of hybrid materials to the wide potential of nanomaterials, the new and featuring class of polymer nanocomposites turned into one of the most intensively researched areas. This review highlights recent developments in the field of the synthesis of polymer based nanocomposites. Important issues related to the surface modification of fillers, in order to promote the compatibility between the inorganic/organic components, are also reported. The enhancement of the physical properties and the potential applications of polymer nanocomposites are considered in typical examples, given for each synthetic method described.Keywords: nanocomposites; polymers; chemical synthesis. INTRODUÇÃOEncontramos na mitologia grega um dos registos mais antigos da existência de híbridos como seres prodigiosos e fantásticos que povoaram a imaginação dos humanos. O grifo, por exemplo, aliava a força de um corpo de leão à perspicácia e capacidade de voar conferida pela cabeça e asas de uma águia, resultando num temeroso guardião do templo dos Deuses (Figura 1) 1 . Na própria natureza existem materiais híbridos admiráveis, sintetizados através de processos quími-cos em meio aquoso, sob condições de pressão e temperatura ambientes, com mecanismos de "automontagem" envolvendo interacções electroestáticas, ligações de pontes de hidrogénio ou forças de van der Waals 2 . Alguns dos nanocompósitos que ocorrem na natureza apresentam uma arquitectura molecular e sinergia perfeitas entre os seus componentes. Alguns exemplos típicos são o nacre (revestimento das pérolas) que consiste em lamelas nanométricas de aragonite (CaCO 3 ) dispersas numa mistura de proteínas e polissacarídeos, os ossos, os dentes e as conchas de alguns moluscos, que apresentam propriedades mecânicas extraordinárias 2 . Talvez seguindo esta bioinspiração os investigadores começaram por estudar uma forma de reproduzir estas propriedades, tentando aproximar-se do nível de controlo da estrutura e propriedades dos compósitos naturais, numa tentativa de obter materiais avançados. A bio-mineralização é de facto um campo de investigação actual em grande desenvolvimento e com forte repercussão no campo dos materiais híbridos 3,4 . A tendên-cia actual na área dos nanocompósitos é a preparação de materiais em que a interacção entre os componentes ocorre à escala nanométrica ou molecular. Os nanocompósitos obtidos deste modo, apresentam propriedades distintas dos compósitos tradicionais, podendo ser estabelecidas no processo de síntese , tintas e revestimentos 9 e como materiais retardadores de chama 10 . Os nanocompósitos são materiais híbridos em que pelo menos um dos componentes tem dimensões nanométricas. Tal como acontece nos compósitos tradicionais, um dos componentes serve de matriz, na qual as partículas do segundo material se encontram dispersas. Os componentes de um nanocompósito podem ser de natureza...

Nanofibrillated cellulose offers new technological solutions for the development of paper products. Here, composites of nanofibrillated cellulose (NFC) and Ag nanoparticles (NP) were prepared for the first time via the electrostatic assembly of Ag NP (aqueous colloids) onto NFC. Distinct polyelectrolytes have been investigated as macromolecular linkers in order to evaluate their effects on the building-up of Ag modified NFC and also on the final properties of the NFC/Ag composite materials. The NFC/Ag nanocomposites were first investigated for their antibacterial properties towards S. aureus and K. pneumoniae microorganisms as compared to NFC modified by polyelectrolytes linkers without Ag. Subsequently, the antibacterial NFC/Ag nanocomposites were used as fillers in starch based coating formulations for Eucalyptus globulus-based paper sheets. The potential of this approach to produce antimicrobial paper products will be discussed on the basis of complementary optical, air barrier and mechanical data.