Graduation Year

2015

Document Type

Dissertation

Degree

Ph.D.

Degree Name

Doctor of Philosophy (Ph.D.)

Department

Chemistry

Degree Granting Department

Chemistry

Major Professor

Xiao (Sheryl) Li, Ph.D.

Committee Member

Abdul Malik, Ph.D.

Committee Member

Li-June Ming, Ph.D.

Committee Member

Jianfeng Cai, Ph.D.

Keywords

Cotinine, Raman, Silica core and Silver shell nanoparticles, Trans 3'-hydroxycotinine

Abstract

This dissertation mainly focuses on applications of Surface Enhance Raman Scattering (SERS) to detect tobacco-related biomarkers with optimized experimental conditions (pH and aggregating agents) and SERS substrates (silica core and silver shell nanoparticles). Cotinine (COT) and trans 3-hydroxycotinine (3HC), metabolized from nicotine as one of main chemicals of tobacco, have been used as tobacco biomarkers because their half-life are longer than that of nicotine, which enable to monitor the tobacco exposure. The effects of aggregating agents and pH on SERS detection of COT and 3HC were investigated. Aggregating agents play an important role in SERS detection of target molecules since the strong SERS enhancements are observed from junctions of nanoparticles which can be induced by aggregating agents, and so called "hot spot". That is, the more hot spots are created among the nanoparticles by aggregating agents, the higher the SERS enhancement is. Five cationic (K+, Na+, Mg2+, Li+, Ca2+) and three anionic (Cl-, Br-, I-) aggregating agents were tested. Interestingly but not surprisingly, optimal concentrations of 11 kinds of aggregating agents for COT and 3HC detections vary dramatically within two orders of magnitude. In addition, the effect of pH conditions on SERS intensity of COT and 3HC was investigated since the protonated or deprotonated molecules induced by various ranges of pH values produces change in SERS intensity of the molecule. The highest SERS enhancement is obtained using 1.5 mM MgCl2 for COT at pH 7 and 50 mM NaBr for 3HC at pH 3. Both cations and anions strongly influence the SERS enhancement.

SERS enhancement depends also significantly on the type of metallic substrates. This indicates the choice of metallic substrate is critically important to achieve strong SERS enhancement. While Ag is the most commonly used materials for SERS substrates and has been demonstrated to exhibit high enhancement. It has the disadvantage of limited selection of excited wavelengths, which prevents to apply Ag SERS substrates to biological field. Dielectric core and metallic shell structure has been theoretically studied and it has been proposed that silica core and silver shell (SiO2@Ag) nanoparticles produces higher plasmon resonance than that of silver nanoparticles and their surface plasmon are tunable by controlling shell thickness. Here, SiO2@Ag nanoparticles were successfully fabricated and their activity as substrates for surface-enhanced Raman scattering (SERS) were examined. Both the core and the shell thickness exhibit strong effect on the SERS activity. Using Rhodamin 6 G (R6G) as a probe molecule, it was observed that SERS intensities of R6G were susceptible to change in Ag shell thickness and the size of core-shell nanoparticles. The 76 nm SiO2@ 23 nm Ag shell nanoparticles shows highest SERS intensity of R6G. Moreover, 76nm SiO2@ 23 nm Ag nanoparticles have higher SERS enhancements of R6G, 4-aminothiophenol (4-ATP), and cotinine (COT) than that of both silver nanoparticles and SiO2@Ag nanoparticles of previous studies. Also, the tuneability of surface plasmon of core-shell structure is flexible by changing in the size of either core or shell.

In addition, three Raman spectroscopy application in material science fields were studied: MP-11 encapsulated inside of Tb-mesoMOFs, poly(methyl methacrylate) composites of copper-4,4'-trimethylenedipyridein, and surfactant-free TiO2 surface hydroxyl groups. For the first study, the interaction between the ligands of Tb-meso MOFs and MP-11 was examined. Individual Raman bands of MP-11 and the ligands of Tb-mesoMOFs were distinguished and some of bands were shifted from the complex of MP-11@Tb-mesoMOFs. It is turned out that the interactions is involved through π•••π interactions between the heme and the conjugated triazine and benzene rings of TATB ligand. Next, Raman was used to study the interaction between poly methyl methacrylate (PMMAP) composites and copper-4,4'-trimethylenedipyridein (CU-TMDP). Copper contained in polymer materials has shown improvement performance (thermal and mechanical stability). The Raman results reveal a red-shift of vibrational peaks associated with pyridine ring of CU-TMDP when CU-TMDP is dispersed into PMMA. This interaction, a dipole-dipole interaction or London dispersion force, may produce the stability improvement of metal-containing polymer. The last application is about the effect of pH levels on the phase of TiO2 crystalline. TiO2 crystal has attractive advantage of self-cleaning property. The efficiency of self-cleaning of TiO2 is dependent on the phases (anatase, rutile, and brookite) of TiO2. Raman study revealed that the formation of the anatase phase of TiO2 is interrupted as the pH level increases.

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