Multi-morphology Of Nano-structured Metal Substrate Surface Enhanced Fluorescence Effect

Metallic nanostructures are capable of supporting surface plasmon resonance. The resonant interaction between the free electron oscillation and the electromagnetic field(EM) of the light constitutes the surface plasmon. With the help of excitation of SP, the electron-photon interaction and molecular photonic state can be modulated. The SP distribution and the fluorescence emission from the molecules in the vicinity of nanostructured metallic surface can be modified by controlling the size, properties and shape of the metallic nanostructure. Therefore, the study on the coupling effect among surface plasmon polaiton(SPP) enhancement substrate and surface enhanced fluorescence(SEF) have been one of most active and important topic.In current dissertation, different multidimensional metallic nanostructured substrates with various shapes and morphologies are fabricated through different techniques including electrochemical method, self-assembled method and mechanically polished method, which is for obtaining better SEF. The influence of the morphology and dimension of the nanostructured substrates, as well as the distance between the fluorophore and substrate surface are investigated with laser spectroscopic technique. The important role of the local enhanced EM field in SEF effect, is also explored in this dissertation with help of FDTD numerical method. The results show that nanostructured metal substrate surfaces with fractal or higher dimensions can produce better fluorescence enhancement. The current work should be useful for better understanding of coupling and inter-conversion mechanism among free electrons, surface plasmon, photons and fluorophores in SEF effect. It may pave the way for the development of low-cost, easy fabrication, and high sensitive biological sensor chip, which is important for fluorescence imaging, micro-biological molecule detection and so on.The main results of the dissertation are summarized as follows:1. The SEF from the silver nanostructure with fractal-like morphology is studied. Metal substrates with complex structures, which are made of silver fractal-like structure and nanoparticles(NPs), are prepared through electrochemical reduction followed by self-assembled method. The experimental results show that the fractal-like structure decorated with silver NPs presents stronger fluorescence enhancement, comparing with pure silver fractal-like structure. The larger volume and more hotspots can be produced when the silver NPs deposited on the fractal structure, thus better SEF effect could be observed from the decorated structure.2. The enhanced fluorescence effect of silver dendrites nanostructure substrate is studied. The prepared substrate was manufactured by modified galvanic displacement process between Ag ion and Al/Cu at room temperature. The results show that silver dendrites nanostructures, fabricated with properly controlled experimental condition, such as reaction time and solution concentration, exhibit very good fluorescence enhancement capabilities. The observation highlights the importance of strong coupling effects, such as tip effect and antenna effect between nanobranches in SEF. The variation of SEF efficiency can be qualitatively explained with local surface plasmon resonance model of coupled silver nanostructures.3. The dimension effect of fabricated nanostructure is studied. Multi-dimensional(2D and3D) metallic nanostructures were fabricated by self-assembling silver NPs on APS-modified glass substrate and using PATP molecule as a linker. The experimental observation showed that the3D nanostructure substrate presented stronger fluorescence enhancement, comparing with what was observed on the2D NPs arrays. The FDTD simulation results also show that higher intensity of local electric field and stronger coupling of surface plasmon resonance enhance the excitation and emission of fluorophore molecules more effectively in3D silver nanostructure, leading to a larger fluorescence enhancement on the nanostructure substrate with higher dimension.4. The effect of natural oxidation layer on the metallic nanostructure to the SEF is studied. The metallic nanostructure substrate was fabricated with physically polished method. It was shown that the quenching effect was observed when the dye molecule was directly adsorbed onto the substrate surface without the formative oxide layer. However, the fluorescence was enhanced obviously when the native oxide layer was formed on the substrate surface. The experimental observations were discussed by taking into account the non-radiative energy transfer process and the local surface plasmon resonance on the rough copper surface. The result also explains why rare quenching effect was presented in most of the SEF effects with untreated metal substrate surface. It highlights the importance of the spacer layer formation on the substrate in SEF.In conclusion, better fluorescence enhancement could be achieved with properly designed metal nanostructures of various morphology and dimensions. It was also shown that the suitable distance between molecules and substrate is also important for observing efficient enhancement.