Synthesis and self-assembly of plasmonic metal nanoparticles for surface-enhanced raman scattering spectroscopy
Chew, Wee Shern
Date of Issue2017-08-23
School of Physical and Mathematical Sciences
Plasmonic metal nanoparticles (MNPs) exhibit localized surface plasmon resonance, which enables surface-enhanced Raman scattering (SERS) spectroscopy and photothermal effect. SERS is a sensitive vibrational spectroscopic technique capable of providing molecular fingerprint of analytes for qualitative and quantitative studies, whereas photothermal effect allows the conversion of photons into heat. Although SERS is a powerful analytical tool, it can be limited to certain factors such as fluorescence emission, highly dilute samples, and unstable target analytes. In this thesis, we will be discussing the synthesis and self-assembly of MNPs to improve the applications of SERS. For synthesis, we develop a new cubic MNP, known as nanoporous gold nanoframes (NGNs), produced from a seed-mediated method. The NGN synergizes nanoscale porosity and hollow nanostructure in which lower porosity generates greater SERS capability. We also demonstrate the multifunctionality of NGN, which is able to combine photothermal effect and SERS for in-situ heating and SERS analysis. We successfully apply this for SERS monitoring of bovine serum albumin’s denaturation. For self-assembly, we demonstrate the fabrication of a superhydrophobic NIR-SERS platform (S-nIR-NGN) from electrostatic self-assembly of NGNs, in which an optimum static contact angle of 169 ± 1° is obtained. S-nIR-NGN enables the SERS detection of a fluorescent dye, Nile blue A, up to 10-12 M which improves the limit of detection by 50-fold compared to gold nanostars. Finally, we demonstrate the self-assembly of Ag nanocubes on liquid droplet in an organic phase for the fabrication of plasmonic liquid marbles (PLM). The merging of PLMs is performed to produce an isolated environment which protects its encapsulating analytes from degradation, enabling accurate SERS analysis. Using azo dye as a model reaction, we are able to detect bisphenol A up to 1 × 10-15 mole, improving its detection limit by 4 orders of magnitude compared to a SERS-based aptasensor platform. From the works done, we envision the diversification of hollow MNPs, benefiting from the generic synthesis method introduced, which can lead to new discoveries of optical or physical properties for tailored applications. Additionally, the synergy of superhydrophobicity, photothermal effect, and PLM with SERS shows the importance of tailored platform for specific target analytes to refine the field of analytical chemistry.