Optimization, Modification and Applications of Gold Nanoparticles as the Substrates of Surface Enhanced Raman Spectroscopy

Surface enhanced Raman spectroscopy (SERS) is one of the techniques that overcomes the poor intensity of Raman scattering by utilizing the metal surface to enhance the Raman scattering. So far, silver (Ag), gold (Au) and copper (Cu) have been demonstrated to provide good enhancement for Raman signals. Many studies have proved that SERS is a powerful technique. However, the origin of the enhancement still needs clarification. More importantly, how to further improve the SERS enhancement through optimization of the SERS substrates and technique is a long and enduring challenge.;Chapter 3 is dedicated to find out the optimal size of AuNPs. Au NPs with different sizes were synthesized from 17 nm to 80 nm. The SERS activities of AuNPs were tested using target molecules of 4-aminothiophenol and 4-nitrothiophenol. The experiments were performed under three different conditions: same number of AuNPs, same surface area of AuNPs, and same concentration of AuNPs. For the same number of AuNPs, it showed linear relationship between the enhancement factor and sizes of AuNPs. However, in case of same surface area and concentration, the maximum enhancement was achieved around 50 nm AuNPs. These results were identical for both molecules, which indicate that the conclusions might be also applicable to other analytes. More importantly, the highest SERS enhancement can be achieved with AuNPs of 50 nm while they introduce minimum toxicity to the biological samples.;Once the optimal size of AuNPs was found, the work in chapter 4 was dedicated to find the way to improve the SERS enhancement of molecules that do not have strong affinity toward the surface of AuNPs. The affinity was improved by using 2-mercaptoethanol as a linking molecule (2MELM) since the thiol group of 2ME can be strongly adsorbed on the surface of AuNPs, and the other end of 2ME has a hydroxyl group that can induce intermolecular forces toward the molecules of interest. Three molecules were chosen as the target molecule (TM): benzoic acid, cyclohexanol and 1,3-cyclohexanediol. When the results were compared between the substrates with LM and without LM, the spectra of benzoic acid did not display any difference. However, the spectra of the other two TMs show higher enhancement with presence of LM than that without. The results from this simple modification method shine light on the possible characterization or detection of molecules that have low affinity toward the metal surface using SERS.;SERS was also applied to study the intermolecular interaction in the field of Material Science. For the first part of the chapter 5, the interaction between vitamin B12 and metal organic frame (MOF) was studied. The results confirmed the encapsulation and also, strong interactions between the two components were observed from the shift of vibrational modes of both VB12 and Tb-MOF upon encapsulation. Second part of the chapter 5 was dedicated to study the functionalization of single wall carbon nanotubes in polymer network. The results confirm the successfully functionalized carbon nanotube.;SERS was used to detect different biomarkers through collaborations and the results are shown in Chapter 6. The first part was neurotransmitters, dopamin, melatonin and serotonin. All of them showed low muM range as their detection limit. Other two parts are the priliminary results from studying caffeine and nicotine. The results were promsing exhibiting detection limits in low nM to sub nM range. None of these results were performed with optimized condition, therefore, when the conditions are optimized, it is very probable to have even lower detection limits.