Espalhamento Raman intensificado por superfície de uma única molécula do ácido 4-mercaptobenzoico e de corantes catiônicos e aniônicos

Abstract

This thesis addresses the study of efficient hot spot generation using 4-mercaptobenzoic acid (4-HMB) to induce the formation of AgNP aggregates through the molecular coupling of the carboxyl group of 4-HMB adsorbed on different AgNP by thiol group to increase SERS sensitivity to obtain spectra at a concentration limit of 3.0×10-9 mol L-1 . We also investigated whether the selectivity of adsorption of cationic and anionic dyes on AgNP is promoted by electrostatic interactions on SM-SERS. We worked with spherical AgNP stabilized by BO3- ions because they do not present vibrational signatures in the same region of the analytes under study. For the inference of SM-SERS using confocal Raman microscopy, the confocal volume probed by the Raman spectrometer was determined for incident radiation 632.8 nm and 785 nm. The sample concentration for the occurrence of SM was 1.59×10-9 mol L-1 and 2.72×10-11 mol L-1 for the confocal aperture 50×1000 µm and for 25×1000 µm the concentration is 4.35×10-10 mol L-1 and 2.36×10-11 mol L-1 , respectively in the laser lines at 632.8 nm and 785 nm. The SM-SERS and SERS studies were performed with exciting radiation 632.8 nm and confocal aperture 50×1000 μm due to localized surface plasmon resonance of AgNP. The 4- HMB, linked to AgNP by a thiol-Ag bond, was able to trigger AgNP aggregation via H-bond between the carboxyl group of 4-HMB adsorbed on different AgNP at pH=4, below pKa=4.79 of the carboxyl group. AgNP aggregates were formed from the dimeric structures of 4-HMB adsorbed on different AgNP, AgNP-(4-HMB)2-AgNP. As tools for the characterization of AgNP-(4-HMB)2-AgNP, UV-VIS, DLS, and SERS spectroscopy techniques were used. DFTbased calculation of the Ag4-(4-HMB)2-Ag4 model was performed for comparison between the SERS spectra calculated for the monomeric structure of 4-HMB adsorbed on Ag of the 4-HMBAg4 model and experimental. We saw two structures by observing the vibrational mode (CO) at 1682 cm-1 of 4-HMB-Ag4, which changes to 1604 cm-1 with the formation of Ag4-(4-HMB)2- Ag4. It was not observed below 1.0×10-6 mol L-1 the vibrational signature of 4-HMB adsorbed on AgNP at pH above pKa, however, at pH=4 the signal is drastically intensified. The hypothesis that SERS enhancement of the 4-HMB is due to AgNP self-assembly triggered by 4-HMB H-bonds was reinforced by the non-observation of the SERS signal of 4- bromobezenothiol (4-BrBT) at 5.0×10-7 mol L-1 . Detection of 4-HMB by SM-SERS was experimentally demonstrated and the results were associated with a greater number of hot spots formed. In addition, four dyes that differ in total charge were analyzed. The SERS spectra of the cationic dyes (Rhodamine-6G and Crystal Violet) were obtained in the SM regime (3.0×10- 9 mol L-1 ). In this regime, fluctuations in SERS intensities are observed, associated with the absence of cationic dye in the region of hot spots and hot spots in the confocal volume. The anionic dyes (IR-820 and Congo red) did not show SERS signal at concentrations lower than 3.0x10-6 mol L-1 . This indicated that the electrostatic interaction of the dye with the Ag surface was dominant for the SERS performance.