Investigation of Chemical Stimuli Responsive Colorimetric Sensors and Enhancing Raman Detection Limit Using SERS

Abstract

Chemical sensors have been paid a lot of attentions for their intrinsic ability of detecting various stimulates, such as composition, coordination, action, pH, and oxidation/reduction potential. In particular, industrial field have invested on such chemical sensors to avoid from severe damage caused by chemical leakages. Detecting very small amount of hazard chemical leakages with early stage of leakage during manufacturing processes should be considered as first priority to guarantee employees to work in safe place. Although various types of precision gas sensors are currently being developed and used, cheap, improved sensitivity & selectivity, fast reacting sensors that can be intuitively recognized by field workers or/and residents near chemical plants are also urgently needed. In this dissertation, colorimetric sensors are discussed for detecting not only biomolecule but also chemicals like toxic gases. Visible colorimetric sensors have been widely accepted to make highly sensitive and selective responses from various stimulation. The technics of detecting organic hazard compounds with colorimetric changes using chemoresponsive dye deposited on substrate are listed in following chapters. Based on molecular recognition mechanisms, these technics are widely used in clinical and environmental analyses with highly selective and sensitive ability. Miniaturization and simplification are also important tasks to achieve on industrial scale. In this thesis, I focus on colorimetric sensors that could identify small amount analyte with acceptable selectivity and sensitivity. Various kind of colorimetric chemical sensors based on SERS are discussed from the next section. Gas phase analytes react directly with solid dye compound which is broadly distributed on substrate with large surface area. To achieve with commercial requirements, most of dye molecules used in system are synthesized in one-step or commercially available dyes such as color changing indicators. By adding additive like ammonium salt, the system can have dual selectivity on both acid and base gas analyte with a single dye through reverse titration. For example, if we use acidic molecule in sensor system, reaction on dye will favorable to basic gas and vice versa. To get low cost, but showing acceptable detection limit, dyes and additives are combined with polymer nanofiber produced by electrospinning. These fibers have advantage of not only increasing surface area to react with analyte, but also reducing detection time All of the reactions between dye and analyte finished within 1 minute after exposed and visualized by color change. In the last chapter, new platform of SERS system which is combination of dielectric spheres and metallic nanorods in solution is introduced. Up to date, metal nanoparticles have been used to enhance Raman signal by interplaying with surface plasmon event. The basic idea has been to develop techniques such as single molecule detection or high resolution imaging through nanogap control. Among the flooding studies using metal nanoparticles, we have been employed dielectric nanoparticles which have sufficient electromagnetic fluctuations that can be utilized in SERS. This Study has been carried out using disordered dielectric particles in solution to achieve light trapping which generate multiple forward scattering to get enhanced SERS. Since intensity of the dipole-dipole coupling among dielectric nanoparticles is much lower than metal nanoparticles, the enhancement factors caused by electronic hot spot should be significantly weaker than those of metal nanoparticles. However, strong magnetic field and electric field in the dielectric system can play important role on contributing scattering. Once the two hotspots of metal's electric hotspot and dielectrics were used simultaneously, they would have a much more interactive enhancement. In this study, we have established effect of SERS on metal nano-rod by dielectric coupling between metal nano-rod and dielectric.