Aromatic Surface Enhanced Raman Spectroscopy Of Molecular Characterization And Theoretical Research

Surface-enhanced Raman scattering (SERS) is a special surface opticalphenomena that people found in surface science study by using Raman spectroscopy.In recent years, great progresses have been made for the theoretical research and theapplications of SERS effect. And it has become a very active field in Ramanspectroscopic study. At present, the use of confocal microscopic Raman spectrometercan effectively eliminate the fluorescence background of Raman scattering. SERS,with the high sensitivity and the ability of detection of monolayers or submonolayersof molecules adsorbed on metal surface, can give the molecular structure informationabout surface adsorbates. Therefore, Surface enhanced Raman spectroscopy wasconsidered as a very good technique for surface researches. Self-assembledmonolayers (SAMs) via the artificial design of self-assembled monolayers to attainthe aims of controlling the composition, structure and function of the surface. Thistechnique can provide a favorable platform for the functional interface at basicresearch and application of the project. Spectroelectrochemistry is a techniquecombining the spectroscopy and the conventional electrochemistry. It characterizesand investigates an electrochemical system in situ on the level of molecules. There isan outstanding predominance in investigating the configuration and the bonding ofmolecules adsorbed on an electrode, and identifying the molecular speciesparticipating in the process of electrochemistry. These techniques are applied widelyin the currently research field of chemical modification of electrodes andbioelectrochemistry, and at the same time, these techniques are also as a part of theprimary investigating means for this paper.With the rapid development of computer hardwares, the application of computershas been deeply utilized into almost every field of chemical researches, and it hasbrought dramatical developments for the traditional chemical research means andmethods. Therefore, a science called "computational chemistry" has also been derived.The development of quantum chemistry provides the adequate conditions for the wideapplication of computational chemistry in chemical researches. Density Functional Theory (DFT) is a quantum mechanics method specialized in electronic structures ofmany-electron systems. With so many advantages followed, such as containingelectronic relation, better calculation results than HF method, fast computing speed,and considerably high accuracy, DFT has been widely used in molecular structurecalculation and molecular simulation. Because of its relatively low calculation cost,and its ability to calculate large systems, DFT has been applied more and more widelyto Surface enhanced Raman spectroscopic researches.Researches of the monolayers of mercaptan derivative containing aromaticheterocyclic groups have a good application prospect. Moreover, people haveemployed these compounds to construct monolayers in the fields of chemicalmodification of electrodes and bioelectrochemistry. In this paper,4,4’-dithiodipyridine(PySSPy), histidine (HIS) and azathioprine (AZA) molecules are investigated byspectroelectrochemistry and theoretical calculation techniques.In practical terms, this paper mainly covers the following three aspects:(1) Based on the theoretical calculation of normal Raman spectra of4,4’-dithiodipyridine, Potential energy distribution (PED) techniques were used togain assignments of spectral peaks of PySSPy. The influence of electrode potential onthe molecular structure of PySSPy SAMs used as a linkage layer for the assembly ofgold nanoparticles on a gold electrode surface was discussed. Then, based on thecharacteristics of SERS spectra, the interaction between PySSPy SAMs and thesubstrate was analyzed by combining pseudo potential calculation and molecularorbital theory. In addition, the preliminary analyses of the enhancement mechanism ofSERS were also provided. It was found that the adsorption behavior of PySSPy on agold electrode is similar to that of4-MPy, that is to say, it would form a configurationas PyS-Au and the adsorption is characterized with the pyridine ring perpendicular tothe substrate surface. With the change of potential, the adsorption configuration alsochanges.(2) Based on the theoretical calculation of normal Raman spectra of histidine,PED techniques were used to gain assignments of spectral peaks of histidine. Theinfluence of electrode potential on the molecular structure of histidine adlayer located between a gold nanoparticle film and a gold electrode surface was discussed. Then,based on the characteristics of SERS spectra, the interaction between the histidineadlayer and the substrate was analyzed by combining pseudo potential calculation andmolecular orbital theory. In addition, the preliminary analyses of the enhancementmechanism of SERS were also provided. It was found that the adsorption behavior ofhistidine on a gold electrode surface is characterized by the adsorption of the carboxylgroup or both of the carboxyl group and the amino group from a histidine molecule.As for its adsorption orientation, the molecule is tilted with the gold electrode surface.With the change of potential, its adsorption site is only a carboxyl group.(3) Based on the theoretical calculation of normal Raman spectra ofazathioprine, PED techniques were used to gain assignments of spectral peaks ofazathioprine. The influence of electrode potential on the molecular structure ofazathioprine adlayer located between a gold nanoparticle film and a gold electrodesurface was discussed. Then, based on the characteristics of SERS spectra, theinteraction between the azathioprine adlayer and the substrate was analyzed bycombining pseudo potential calculation and molecular orbital theory. In addition, thepreliminary analyses of the enhancement mechanism of SERS were also provided. Itwas found that the adsorption behavior of azathioprine on a gold electrode surface ischaracterized by the adsorption of azathioprine on a gold electrode surface with thepurine ring perpendicular to it, and nitroimidazole ring slightly away from the surfaceof the gold electrode surface. The inclination between two rings are nearly90degreeby each other. And the adsorption site is N5. With the change of potential, itsadsorption sites also change.All these work about theoretical calculation mentioned above are significantexploration for our future study and will help us to find the adsorption mechanism ofmore and more important molecules by the SERS technique in the near future. It willalso provide effective approach and important information in in-depth study related tothe molecular structure.