Towards Rapid Analysis Of Environmental Organic Pollutants By Surface-Enhanced Raman Spectroscopy

Surface-enhanced Raman spectroscopy (SERS) is deemed a powerful tool of chemical and biological analysis, which could give fingerprint spectra for most molecules. Raman signal enhancement can reach to six orders of magnitude or more in SERS. The groundwork in SERS is fabricating replicable substrates with desired localized surface plasmon resonance and large average enhancement factor. The Raman enhancement mainly comes from localized surface plasmon resonance which is excited by the interaction between electromagnetic radiation and nanometer-sized metallic structures. However, the most used silver nano-structures suffer from oxidation in air and water, which can damp their SERS activity. The temporal stability of the SERS substrates under intense laser radiation also hinders the further practical applications of SERS. As a surface-sensitive technique, SERS performance decays exponentially according to the distance between target molecules and SERS substrates. Therefore, target molecules must be efficiently enriched on the surface of SERS substrates for high-performance analysis. But it generally takes one hour or even several hours to enrich samples by routine immersion method in which SERS substrates were immersed in sample solution to adsorb target molecules through diffusion. A more rapid method should be proposed to shorten the analysis time in SERS analysis. Much work has been done to detect environmental organic pollutants by Raman spectroscopy, while the lack of Raman spectra of environmental organic pollutants greatly hinders this convenient and powerful technique. To promote the progress of environmental organic pollutants monitoring by Raman spectroscopy, a Raman spectra database of environmental organic pollutants is needed. It is difficult to gather Raman spectra of environmental organic pollutants by measuring standard samples as it could not get standard samples of some pollutants. In this paper, we aim to improve the temporal stability of the SERS substrates under intense laser radiation and the enrichment efficiency on the surface of SERS substrates and obtain a Raman spectra database of environmental organic pollutants. The detailed information of the dissertation is listed as follows.1. Silver nanoplates prepared by modified galvanic displacement for reliable surface-enhanced Raman spectroscopy.The research of SERS substrate mainly focused on improving the enhancement factor and spatial uniformity, while ignoring the time stability in practical application. We have prepared stable SERS substrates on commercial copper foil via a modified galvanic displacement. The process is simple and easy to scale up. We carried out molecular dynamics simulation to study the adsorptions of2-nitrobenzoic acid on Ag (100),(110) and (111). The relatively large interaction energy between2-nitrobenzoic acid molecules and Ag (111) suggests that the2-nitrobenzoic acid molecules prefer to adsorb on Ag (111) rather than Ag (100) and Ag (110). Those dense silver nanoplates, covering the substrates, are free from oxidation with the help of metallic copper. The substrates shows strong surface plasmon resonance in visible light region and have large surface enhancement for the Raman spectra of4-aminothiophenol. Under a continuous laser radiation, the substrates can still give stable SERS signals. The SERS substrates may have more applications for SERS detections.2. Combination of solid phase extraction and surface-enhanced Raman spectroscopy for rapid analysis.As a surface sensitive spectroscopy, intensive Raman signal could be only obtained when analytes are close enough to the surface of SERS substrates. The effective enrichment of samples on the SERS substrates is particularly important for high performance SERS analysis. It typically takes several hours to enrich of samples by routine immersion method. A more rapid method should be proposed to shorten the SERS analysis time. Herein, solid phase extraction (SPE) technology was employed to endow SERS substrates with enrichment capacity. Silver dendrites were prepared and filled in quartz capillary as high sensitive SERS substrates. Porous structures in this SERS-active extraction column enable high diffuse efficiency of samples in the column. Propanethiol which self-assemble on the surface of silver dendrites serves as an adsorbent for environmental pollutants. The column demonstrates high SERS temporal stability and excellent SERS reproducibility. As an example, quantitative analysis of fluoranthene was accomplished in the concentration range of0.01-100μg/mL with this column. The time of extraction process could be accomplished in10s and the total analysis time of one sample including extraction, elution, spectral acquisition and intermediate process will be less than30s. The method for rapid analysis of environmental pollutants in this paper, which combines the advantage of SPE and SERS, can greatly shorten the time of total analysis and may find further practical application in more widespread fields.3. Theoretical evaluation of the configurations and Raman spectra of209polychlorinated biphenyl congeners.Though polychlorinated biphenyls (PCBs) have distributed as threats in the environment to human beings for several decades, monitoring of trace level PCBs in-field is still a challenge. As a potential method for monitoring PCBs at trace levels, Raman spectroscopy has been used to detect several PCBs in the laboratory. To facilitate the development of rapid detection of PCBs by Raman spectroscopy, it is essential to investigate the Raman spectra of all PCB congeners. The stable configurations and vibrational spectra of all the PCB congeners were calculated by Gaussian03program package. Based on molecular symmetry, PCBs are classified into seven groups. The structural features and the normal vibration modes for each group are discussed. Taking the C2-2group as an example, the wavenumber ranges of the various normal vibration modes in the Raman spectra of PCBs were analyzed. The accuracy of calculated results was verified by experimental Raman spectra of PCB77standard. This study can elucidate further information to promote the development of Raman spectroscopy in environmental monitoring.