The DFT And Vibrational Spectroscopy Study On Sevearl SERS Probe Molecules And Their Applications

SERS technology is widely used in chemistry, physics, materialscience, life science and environmental science, et al. SERS technology ishighly potential applicable in food security, homeland security, historicalrelics identification and environmental monitoring et al. For fundamentalSERS studies, the vibrational assignments are basic and importantinformation. The dissertation present a series vibrational assignments ofnew SERS probe molecules of MTT and ninhydrin and widely usedSERS probe molecules of4-mercaptophenol and4-mercaptobenzoic acidwhich are lack of directly vibrational assignments and symmetricalassignments. Base on the vibrational assignments, a guidance SERSapplication of pesticide residue detection was performed. In detail, thedissertation include the following parts:1.4-mercaptophenol (4-MPH) was designed as a model molecule fortheoretical and experimental studies of the molecule structure. Densityfunctional theory (DFT) calculations have been performed to predict theIR and Raman spectra for the molecule. In addition, Fourier transforminfrared (FTIR) and Raman spectra of the compound have been obtainedexperimentally. All FTIR and Raman bands of the compound obtainedexperimentally were assigned based on the modeling results obtained atthe B3LYP/6-311+G**level. Our calculated vibrational frequencies are in good agreement with the experimental vales. The molecularelectrostatic potential surface calculation was performed and the resultsuggested that the4-MPH has two hydrogen bond donors and threehydrogen bond acceptors. HOMO-LUMO gap was also obtainedtheoretically at B3LYP/6-311+G**level.2. The structural parameters of the title molecule (MTT) have beenobtained at the B3LYP/6-311++G**level of theory. The two phenyl ringsand the two heterocyclic rings are found in four different planes. Thetetrazolium ring was connected with other three rings and their distancesare also different in decrease order of1.461(from tetrazolium ring tocarbon connected phenyl)>1.445(from tetrazolium ring to nitrogenconnected phenyl)>1.425(from tetrazolium ring to thiazolyl ring).Fourier transform infrared (FTIR) and Raman spectra of the compoundwere obtained experimentally. All FTIR and Raman bands of thecompound obtained experimentally were assigned based on the modelingresults obtained at the B3LYP/6-311++G**level. The calculatedvibrational frequencies were in good agreement with the experimentalvalues. In addition, the UV-Vis spectra were obtained experimentally andtheoretically. Considering the effect of the PCM modelling error, thecalculated absorbance peaks obtained at the B3LYP/6-311++G**levelwere also in good agreement with the experimental values. TheHOMO-LUMO gap was predicted to be1.83eV at the B3LYP/6-311++G**level.3. Ninhydrin was designed as a model molecule for theoretical andexperimental studies of the molecule structure. Density functional theory(DFT) calculations have been performed to predict the IR and Ramanspectra for the molecule. In addition, Fourier transform infrared (FTIR)and Raman spectra of the compound have been obtained experimentally.Based on the modeling results obtained at the B3LYP/6-311++G**level,all FTIR and Raman bands of the compound obtained experimentallywere assigned. Our calculated vibrational frequencies are in goodagreement with the experimental vales. The molecular electrostaticpotential surface calculation was performed and the result suggested thatthe ninhydrin had two potential hydrogen bond donors and four potentialhydrogen bond acceptors. HOMO-LUMO gap was also obtainedtheoretically at B3LYP/6-311++G**level.4.4-mercaptobenzoic acid (4-MBA/pMBA) was designed as amodel molecule for theoretical and experimental studies of the moleculestructure. Density functional theory (DFT) calculations have beenperformed to predict the IR and Raman spectra for the molecule. Inaddition, Fourier transform infrared (FTIR) and Raman spectra of thecompound have been obtained experimentally. Based on the modelingresults obtained at the B3LYP/6-311++G**level, all FTIR and Ramanbands of the compound obtained experimentally were assigned. Our calculated vibrational frequencies are in good agreement with theexperimental vales. The molecular electrostatic potential surfacecalculation was performed and the result suggested that the4-MBA hadtwo hydrogen bond donors and three hydrogen bond acceptors.HOMO-LUMO gap was also obtained theoretically atB3LYP/6-311++G**level.5. A surface enhanced Raman spectra (SERS) detection platform forthe widely used pesticide of triadimefon (TDF) is described. TDF isdifficult to connect to the gold/silver SERS substrate, but the opticalproperties of the probe molecule4-mercaptobenzoic acid (4-MBA),including the peak intensity and the peak position, are easily altered bythe multiple weak interaction-assisted SERS detection platform. The limitof detection (LOD) of this method is1.0×10-9M and the linear range isfrom1.0×10-6M to1.0×10-9M. The linear region is from1.0×10-6M to1.0×10-9M, described by the equation y=884.01x+2.24with a correlationcoefficient (R2) of0.9990. Interference of foreign metal ions with higherconcentrations than common mineral water is too weak to thedetermination. Furthermore, physical insights into the phenomena and thedetection mechanism were obtained and investigated theoretically. Thepreferential conformation of the complex and reduced density gradient(RDG) calculation results indicated that the interactions between TDFand4-MBA consisted of multiple weak interactions through two hydrogen bonds and one van der Waals interaction. The intermolecularinteraction was negative for the charge transfer from the SERS substrateto the probe molecule, thus the peak intensity decreased. TDF interactsdirectly with the carboxyl of the probe molecule and consequently has aneffect on the ring vibrations.