Rational design of plasmonic nanostructures for surface enhanced spectroscopy
Date of Issue2014
School of Materials Science and Engineering
Austrian Institute of Technology; University of Natural Resources and Life Sciences, Vienna
Plasmonics has emerged as a promising and extensively researched field due to its advantages of confining electromagnetic radiation by metal nanostructures and has found wide applications including sensors, wave guides, solar cells, lithographical methods, etc. However, the current applications related to the biological system are still limited, and possess tremendous possibilities and great potential to improve the performance. This dissertation focuses on the rational design and development of plasmonic nanostructures that can push the limit of its plasmonic application, especially related to surface enhanced spectroscopies, as they are widely adopted for useful analysis of different biological components. Through such approach, improvements can be achieved for the enhanced signal to noise ratio, additional information for spectroscopic interpretation, or potentiometric range for analysis, which builds up the platform to extend to other related systems as well. Firstly, a simple strategy based on the synergistic modulation of inter-particle and substrate-particle interaction has been applied to the large-scale fabrication of two-dimensional (2D) Au and Ag nanoparticle arrays. By controlling the electrical double layer thickness of the colloidal particles and lowering the basicity of the aminosilane modified substrate, the nanoparticle arrays could be formed with a wide range of inter-particle distances, and tunable plasmonic properties. Methylene blue was selected to demonstrate that these arrays can be employed as wavelength-selective substrates for multiplexed acquisition of surface-enhanced Raman scattering (SERS) spectra and metal enhanced fluorescence (MEF) spectra. Cytochrome c is a redox protein that plays a crucial role in the electron transport within the respiratory chain. SERS spectra and surface enhanced infrared absorption spectra (SEIRAS) were both collected for cytochrome c adsorbed on two layers of gold nanoparticle modified surface. External potential was applied to cytochrome c electrochemically and changed its redox state. During this process, the conformational change was monitored and analyzed by two-dimensional correlation spectroscopy which combines both the IR and Raman spectra. The method of 2D correlation analysis is very powerful to convolute spectra with overlapping bands. There are many approaches especially in IR for deconvoluting bands such as phase sensitive detection (PSD), etc., but there is no method yet available to deconvolute both Raman and IR and simultaneously plot them together in one spectrum. This is important since IR and Raman are methods which are complementary to each other. Through 2D correlation analysis, the components that undergo conformational change can be highlighted, and their sequential relationship was also identified. Roughened silver surface prepared by electrochemical oxidation and reduction cycles was also used to study cytochrome c with its B band resonance before. But one limitation for the silver substrate is that it can be easily oxidized in the electrochemical environment and hinders its application. To address this challenge, a thin gold layer was sputtered onto the colloidal assembly of silver nanoparticles on the silver substrate to increase its potentiometric range. Good enhancement factor and resonance condition from the silver structure were also maintained as revealed from the SERS spectra of its potential titration. In conclusion, rational design of plasmonic substrates has been tailor to achieve the targeted application with different biological molecules. Tunable plasmonic resonance, two dimensional correlation analysis, and extended potentiometric range have been realized to extend the existing platforms for better performance, more information that can be obtained, and wide application.