Investigations On The Mechanism Of Nanoparticle Film Growth And SERS-based Optical Fiber PH Sensor

Because of its unique physical and chemical properties, the assembly of nanoparticles has great potential applications in medicine, sensing, chemical separation and so on. Therefore, developing new strategies for assembling nanoparticles into thin films has long been a matter of great concerns. However, most previous researches pained concentration on developing new assembling methods and technology, relative less effort was devoted to understand the assembling mechanism.Fiber optical sensors are now intensively studied because the sensors have many unique advantages such as anti-electromagnetism interference, electric insulation, corrosion resistance, high sensitivity, essential safety, light weight, small and flexible size, applicability to a wide go objects, low coast and so on, and are widely used for measuring rotation, vibration, temperature, electric current, electric field, magnetic field, pressure and other physical parameters. According to the difference in working mechanisms, the fiber sensors have different structures and working modes. This thesis is devoted to understand the mechanism of nanoparticle film growth and to develop SERS-based fiber pH sensor. Some main achievements and conclusions are outlined as follows.First, we improved the synthesis method of spherical Au nanoparticles. By finely tuning the ratio of reduce reagent to HAuCl4 and the feeding rate of reagents, we optimize and improve the previously reported seed-mediated growth strategy. The improved strategy allows easy synthesis of 70 nm Au spherical with improved uniform in shape and size.Second, we investigate the mechanisms of nanoparticle film growth and film climbing up wall by emulsion drop coalescence. It is revealed that upon the formation of emulsions by sonicating Au colloids and water phase, the kinetics of Au nanoparticle adsorption to the water-oil drop interface switches to a sorption barrier-free, diffusion-controlled adsorption. Therefore, sonication and increase in total water-oil interface (by the formation of emulsions) both largely improve the transfer parameter of nanoparticles that are adsorbed from bulk colloids to the water-oil drop interface. The particle-laden emulsion drops are unstable. Natural breaking emulsions occurs during standing the emulsions, which releases nanoparticles from the oil drop surfaces to the flat water-oil interfaces, and results nanoparticle film growth therein, During the emulsion drop coalescing with the flat water-oil interface, the additional pressure on the curved oil drop surface would more compress the nanoparticle monolayer on the drop surface. Thus, a pressure gradient in the flat interface-located nanoparticle films is generated and dives the films climbing up the inner wall of the vial.Third, we designed and developed a fiber optical pH sensor based on SERS. The sensor was prepared by depositing highly SERS-active silver nanoparticles on the fiber end surface, which allows SERS activation of fiber end, then by adsorbing pH-sensitive 4-mpy on the silver nanoparticle surfaces.4-mpy molecule will change its molecule structure when the environmental pH changes, as can be notably revealed by the SERS spectra of 4-mpy. Therefore, the characteristic of the SERS spectrum of 4-mpy also singles environmental pH. At the region of pH=0-3, the integrated area ratio of peak 1610 cm-1 to peak 1575 cm-1, both are sensitive to pH, decreases with increase in pH, while the logarithm has a good linear range of pH 0-2. Moreover, the sensor is nearly immune to the ionic strength of the detected solution. Therefore, the sensor is expected to find wide applications for on-line, in situ, and remote pH sensing of acidic solutions.