Sviluppo e caratterizzazione di substrati SERS ( Surface Enhanced Raman Scattering ) per applicazioni analitiche

This thesis deals with Raman Spectroscopy and SERS (Surface Enhanced Raman Spectroscopy) techniques, used for the detection of analytes at a low concentration level by means of silver and gold colloids. A synthetic description of the colloidal state is given, to understand their physical-chemical properties. The analysis of UV-VIS absorption spectra has shown the difficulty to obtain good reproducibility in terms of wavelengths and FWHH (full width half height) of plasmonic peaks for silver colloids synthesized with the Lee-Meisel (1982) method. As for the gold colloids, they have been synthesized with the Turkevic (1951) method, showing a very sharp diameter distribution of the nanoparticles in accordance with the original procedure. The silver colloids were centrifugated in order to demonstrate the possibility both to separate specific diameter fractions starting from the overall distribution and to increase the nanoparticles concentration. Centrifugation allows to obtain SERS enhancement factors higher than those achieved with as-prepared colloids not subjected to this process. Immobilization of nanoparticles on glass slides has been investigated as a mean to produce solid SERS substrates. By employing these concentrated colloids and solid substrates the detection of apomorphine in aqueous solutions is possible even at very low concentrations. For the solid substrates the detection limit achieved is 10^-8 M, while for the concentrated colloids the limit reaches 10^-9 M. Further studies on the reproducibility of malachite-green SERS signals imply that semi-quantitative analysis is not prohibitive. Finally the detection of apomorphine in human plasma at a concentration level achieved in high dosage conditions [(10^-4 M) Manson (2001)] is possible combining the SERS effect with TLC (Thin Layer Chromatography) technique. In fact, after separation of apomorphine from plasma along the stationary phase, SERS signals of the drug molecule were registered by depositing some drops of concentrated colloids on a TLC slab.