| Nanophotonics, especially plasmonics is a kind of very active research area, which deals with the interaction behavior between electromagnetic radiation and metallic nanostructures. It has attracted enormous attention over recent decades due to its great potential of ripple effects on electronics, energy, environmental, and medical industries as well as scientific interests. In particular, noble metal nanoparticles exhibit localized surface plasmon resonance (LSPR), which is the collective oscillating excitation of the free electrons on the surface of metal nanoparticles when light is incident on the particle. The LSPR extinction peak is very sensitive to the dielectric environment near the particle surface and can be tailored by the particle's sizes and shapes. These properties allow LSPR-active substrate using plasmonic gold nanoparticles to be a great transducer for biosensing with real-time and label-free measurement. In addition, the plasmonic gold nanoparticles such as gold nanorod and bipyramid are prepared by the seed-mediated and surfactant-directed method based on the cetyltrimethylammonium bromide (CTAB), which has a great influence on the synthesis. In the growth mechanism, it is believed that CTAB interacts with different facet and defects on the growing nanoparticles to produce different rate of gold ion reduction onto the nanoparticles to generate anisotropic growth. Therefore, CTAB layer is greatly interesting because the modification of nanoparticles surface chemistry is essential to biological targeting, film formation, and assembly of complex structures. Surface enhanced Raman spectroscopy (SERS) of gold nanorods in CTAB solution has been used to analyze a surfactant structural transition based on the distance dependent electromagnetic enhancement. As the surfactant concentration in the gold nanorod solution was reduced, a structural transition in the surfactant layer between 2 mM and 5 mM CTAB solution was observed through a sudden increase in the signal from the alkane chains. A structural transition in the CTAB layer that stabilizes gold nanorods was identified by comparing the intensities of different bands within the CTAB molecule. Therefore, the surface manipulation and analysis of the nanostructures and their interface with controlled environment provide important insight into their structural function and interpretation, and many opportunities for biomedical applications. |
