Influence Mechanism Of Noble Metal Nanoparticles On The Fluorescence Of Quantum Dots And Porphyrins

In recent years, noble metal nanoparticles have been widely used in development of analytical chemistry, and biomedicine due to the significant enhancement of surface photoelectric signals. Plenty of research achievements have been gained in investigating the influence mechanism of noble metal nanoparticles on the fluorescence properties of fluorescent materials. Some results indicated that fluorescence emission was increased by surface plasmon resonance, and others indicated that fluorescence emission was quenched by energy transfer to the metal. The discussions of interaction mechanism have not come to an agreement. Semiconductor quantum dots and organic dyes were chosen to be generally simplified to radiative dipole, and the particularity of the fluorescent energy level and process was neglected, which made a gap between these research achievements and practical applications. So it has become an important branch of nanotechnology to research the influence mechanism of noble metal nanoparticles on the specific fluorescent energy level and process, which can provide better theoretical and experimental support for more extensive application of noble metal nanoparticles.In this thesis, quantum dots and porphyrins derivatives were chosen as two typical and widely used fluorescent materials. By means of steady state and time-resolved spectral techniques, the influence mechanisms of noble metal nanoparticles on specific fluorescence emission level and fluorescence process were studied. Some preliminary research on metal enhanced two-photon-excited fluorescence was investigated, which has important application background but rarely been studied so far.Firstly, in order to match the surface plasmon resonance peak of noble metal nanoparticles with fluorescent level, vacuum thermal evaporation and seed-mediated growth method were utilized to prepare spherical and rodlike noble metal nanoparticles. Deposition amount of the vacuum thermal evaporation method and reactants'concentration of the seed-mediated growth method were adjusted to controll the size and shape of noble metal nanoparticles and adjust the surface plasmon resonance peaks.Secondly, the interaction theoretical model in quantum dot/noble metal nanoparticles compound system was detailed and an experimental proving was given. The model of quantum dots was simplistic as radiative dipole in researching this kind of compound system, and covered the actual physical process. Excitons states of quantum dots were further refined. Interactions between gold nanoparticles and intrinsic states and surface states of quantum dots were investigated. By time-resolved fluorescence spectroscopy, the interaction between intrinsic states of quantum dots and gold nanoparticles was discovered, while the interaction between surface states of quantum dots and gold nanoparticles was not discovered even though the spectral overlap condition was satisfied. It was observed that the fluorescence intensity of quantum dots was quenched, the peak position showed a redshift and the fluorescence process was influenced since these reasons.Thirdly, the influence of noble metal nanoparticles on the specific fluorescence level in organic dye/noble metal nanoparticles compound system was investigated. The model of organic dye was unified as radiative dipole before, but it tends to focus on the particular fluorescence level of organic dyes during its application, such as the nature of porphyrins Q-band lifetime. Therefore, Q-band lifetime of porphyrin derivatives adsorbed on noble metal nanoparticles'surface was studied. By adjusting surface plasmon resonance absorption peaks of noble metal nanoparticles to resonate with porphyrins'Q-band absorption, lifetime of Q-band was extended because of the stabilization of excited porphyrins'polarity resulting from light field enhancement near noble metal nanoparticles. Energy transfer occurred when adjusting surface plasmon resonance absorption peaks of noble metal nanoparticles to resonate with porphyrins'Q-band emission, which shortened the lifetime of Q-band. The lifetime of Q-band directly determines the population of porphyrins'first triple state, so this kind of modulation is significant for porphyrin as a photosensitizer in photo-dynamic therapy.Finally, noble metal nanoparticles enhanced two-photon-excited fluorescence of quantum dots was theoretically studied. Surface plasmon resonant enhancement effect was thought to be the only origin of two-photon-excited fluorescence enhancement before, and energy transfer effect was neglected, thus there was not a unified conclusion on adjusting the surface plasmon resonant peak to the excitation region or emission region, and it is not good enough to guide practicle applications. Energy transfer effect was taken into account for the first time, only dipole approximation was calculated and the enhancement curve of two-photon-excited fluorescence by noble metal nanoparticle was obtained. The influence of the energy transfer effect near the resonant peak was observed. By adjusting surface plasmon resonance peak of noble metal nanoparticles to the excitation region, the energy transfer effect in the emission region will be depressed and it brings a positive impact to metal enhanced two-photon-excited fluorescence. We successfully explained the reported experimental results using this theoretical analysis. It has practical significance in noble metal nanoparticle enhanced two-photon-excited fluorescence.