Nanocompósitos para SERS: síntese, estudos óticos e analíticos


This thesis describes several strategies as well as characterization of nanocomposites and other nanostructures, for analysis in surface enhanced Raman scattering (SERS). In particular, we studied the incorporation of metal nanoparticles (NPs) such as Ag and Au in polymer matrices to assess the potential of these materials as novel SERS substrates for the detection of molecules. The first chapter consists of a literature review, which emphasized the development of new substrates for SERS using nanoparticles, including Ag, Au and Cu. Firstly; this section presents a brief description of the plasmonic properties of the metals used and some basic concepts of Raman spectroscopy. Posteriorly, the SERS effect is described in more detail, mainly reviewing the materials chemistry described in recent studies in view of their use as new platforms for chemical analysis by SERS. Chapter 2 describes the synthesis and characterization of nanocomposites of poly(butylacrylate) obtained from in situ miniemulsion polymerization in the presence of Ag nanoparticles. Nanocomposites Ag/PtBA were investigated as novel SERS substrates for the development of new versatile platforms for molecular detection. Several studies such as the effect of temperature, pH and pressure were investigated in order to understand the effect of the polymer matrix on the diffusion and adsorption of the analyte (thiosalicylic acid) on the metal surface. In chapter 3, the nanocomposite Ag/PtBA described in Chapter 2 has been investigated in more detail as potential substrates for bio-detection in SERS. These substrates were exposed to various pre-treatment methods for the detection of adenine, a base deoxyribonucleic acid (DNA). A series of experiments were carried out in the Ag/PtBA systems in order to obtain information on the effect of pH variation in the detection of the analyte in dilute aqueous solutions. The obtained polymer nanocomposites have the advantage that they can be processed, using available technology. Thus, the study of their thermal properties is especially relevant. The influence of the incorporation of inorganic NPs in the glass transition temperature (Tg) of the polymer PtBA was investigated by differential scanning calorimetry (DSC) and the results are presented in Chapter 4. These studies describe the effect on the Tg of the PtBA when nanocomposites prepared by different methods (methods in situ and ex situ), containing metal nanoparticles with different size distributions and variable amounts in the polymer matrix are analysed. These studies correlate the influence of the metal NPs in the mobility of the polymer chains with the thermal properties observed. Chapter 5 describes the preparation and characterization of polymer-based composite materials (PtBA) and Au NPs with rod morphology (NRs). These materials were investigated as substrates for SERS, lending to a good SERS signal for 2-2'-dithiodipyridine. The effect of the morphology of the metallic NPs (rods and spheres), the aspect ratio (A.R.) of the rods and the type of polymeric matrix (PtBA and PnBA) on the SERS signal was also investigated. Chapter 6 describes the use of the SERS technique as an alternative method to monitor morphological changes in colloidal Au NRs. The Au NRs were collected at different stages of oxidation promoted by the presence of K2S2O8 and their sensitivity as SERS substrates was evaluated, using the anion diethyldithiocarbamate (DTC) as molecular probe. The studies were performed using excitation lines at 1064 nm and 633 nm. This study demonstrated that the sensitivity of the Au NRs as SERS substrates decreases as their A.R. decreases due to the competitiveness of CTAB (stabilizer) and DTC for the surface of NRs. It is noted that this process is accompanied by a different behaviour in terms of adsorption of the two tautomers of the DTC to the metal surface. Chapter 7 introduces a new type of composite SERS, using biopolymer matrices. Thus, it describes the preparation and characterization of nanocomposites based on carrageenan and Ag NPs. This section evaluates the use of these materials as novel substrates for SERS analysis, using 2-2'- dithiodipyridine as molecular probe. Pioneering studies correlating the dependence of the SERS signal with the gel strength is describe in more detail. Thus, systematic studies were performed on these nanobiocomposites used as SERS substrates in different analytical conditions and their rheological properties investigated. Chapter 8 described the investigation of Ag/gelatine nanocomposites as substrates for SERS, using the diethyldithiocarbamate anion as model analyte. Several experiments were carried out to correlate the variation of the gel strength with the increase of the SERS signal as well as adsorption of the two different tautomers of the DTC on the metal surface. Throughout this work are presented different methods for preparing and obtaining polymer-based nanocomposites (natural or synthetic) and metal NPs (Ag and Au). This research has not only enabled the synthesis and study of new substrates for SERS but also the understanding of matrix/metal NPs effects in the SERS signal and the formation of "hot spots". This work contributes to the enrichment in the area of Nanoscience and Nanotechnology, demonstrating the effectiveness and reproducibility of these polymer-based nanocomposites as new SERS substrates. Although the optical properties of these materials presented here are focussed on the molecular detection by SERS, these materials can be investigated in other technological area.

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Sara Isabel Augusto Fateixa Caria

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Tito Trindade

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