Detection of organochlorine pesticides by surface-enhaced Raman scattering on dithiol-functionalized plasmon nanoparticles

Resumen

Surface-enhanced Raman spectroscopy (SERS) is one of the most powerful analytical techniques for detection of trace amount of chemicals. It is due to the coupling of the Raman scattering of a molecular system to a localized plasmon resonance of silver or gold nanostructures. However, many molecules of great interest lack affinity for metal surfaces and their SERS detection is not possible. On the other hand, the affinity of an analyte towards metal surface can be increased by modifying or functionalizing the surface with the organic molecules. In this thesis we have used linkers, aliphatic dithiols, to functionalize silver and gold nanoparticles for the SERS detection of low concentrations of organochlorine pesticides (aldrin, ¿-endosulfan, lindane and dieldrin). Using various types of metal colloids and aliphatic ¿,¿-dithiols as bifunctional linkers it was found that the most suitable substrate for SERS detection of these pesticides are citrate silver colloid nanoparticles covered by 1,8-octanedithiol (10-5 M). It should be also noted that functionalization of the metal surface by aromatic dithiols does not lead to a SERS substrate suitable for the detection of the pesticides. Furthermore, the fingerprint region (300¿400 cm-1) for the SERS detection of the pesticides was determined. The studied pesticides follow a Langmuir adsorption model from which the adsorption constant and the limit of detection for individual pesticides were determined. The obtained results confirm the high sensitivity of SERS for the detection of low quantities (~10-8M) of organochlorine pesticides which provide solid basis for the construction of suitable nano-sensors for the identification and quantitative analysis of this type of chemicals. Moreover, the adsorption mechanism of aromatic and linear aliphatic ¿,¿-dithiols on silver and gold nanoparticles has been studied in this thesis by UV-VIS absorption spectroscopy, SERS and transmission electron microscopy. All these experimental techniques were employed to obtain information about the adsorption and coordination mechanism, the orientation, conformational order, and packing of the dithiols on the metal nanoparticle surface.